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  • 1995-1999  (33)
  • 1985-1989  (30)
  • 1960-1964  (10)
  • 1920-1924
  • 1820-1829
  • Synthetic methods
  • 1
    ISSN: 1434-193X
    Keywords: Homogenous catalysis ; Palladium ; Cross-coupling ; Heterocycles ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 5-acceptor-substituted 2,3-dibromofurans 1 and 2 underwent a regioselective Pd0-catalyzed coupling reaction at the C-2 carbon atom. With alkynes the corresponding 2-alkynylfurans 4 and 5 were accessible (49-97% yield). Alkyl-, aryl-, and alkenylzinc reagents gave the 2-substituted furans 8 starting from compound 2 (66-84% yield). The 2-allylfurans 8e and 8f were obtained by a regioselective Stille coupling in 79% and 73% yield. The latter reaction was also applied to the parent 2,3-dibromofuran (27) and yielded the substitution product 28 (60% yield). Subsequent Pd0-catalyzed reactions to introduce a methyl group in 3-position by a methyldebromination were successfully conducted for 2-alkynyl-3-bromofurans with MeZnCl and PdCl2(PPh3)2 as the catalyst in THF (reflux) to yield compounds 13-16 and 24 (67-76%) and with SnMe4 and PdCl2[P(o-Tol)3]2 as the catalyst in DMA (90 °C) for the 2-allyl-3-bromofuran 8e to yield 18 (70%). The more facile reaction of the 2-alkynylfurans relative to those of furans bearing an sp3-carbon atom at C-2 appears to be due to steric reasons. Studies on the 2-alkyl-3-bromofuran 20 supported this notion. With the regioselective coupling methodology the terpene rosefuran (22) was prepared in four steps starting from furan 2 (35% yield overall). The F5 furan fatty acid (26) was synthesized from furan 1 in five steps (29% yield overall).
    Additional Material: 4 Tab.
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  • 2
    ISSN: 1434-193X
    Keywords: Homogenous catalysis ; Iron ; Imidation ; Sulfur ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The imidation of sulfides and sulfoxides to the corresponding sulfimides and sulfoximides was carried out with N-tert-butyloxycarbonyl azide (BocN3) in the presence of FeCl2. Sulfoxides 1 reacted at room temperature in CH2Cl2 to give the corresponding sulfoximides 3 in 40-95% yield. The imidation of the sterically congested substrate tert-butyl methyl sulfoxide (1f) proceeded sluggishly (10% yield). The sterospecificity of the reaction was demonstrated with the enantiomerically enriched substrates (R)-(+)-1b and (S)-(-)-1d which yielded the sulfoximides (R)-(+)-3b and (S)-(-)-3d with retention of configuration. Mechanistically, an intermediate (nitrene)FeIV complex is postulated as the reactive nitrene transfer reagent which is formed from FeCl2 and BocN3. The more nucleophilic sulfides 2 reacted more readily in the imidation than sulfoxides. Their conversion to the corresponding sulfimides 4 was conducted with BocN3 and a substoichiometric amount of FeCl2 (0.25 equiv.). Yields ranged between 44 and 92%. In an alternative reaction mode, BocN3 was utilized at 0°C in the presence of FeCl2 and acetyl acetone. The sulfimidation, which did not otherwise occur at this temperature, was accelerated by the ligand (36-90% yield).
    Additional Material: 4 Tab.
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2315-2321 
    ISSN: 1434-193X
    Keywords: Asymmetric catalysis ; Transfer hydrogenations ; Ruthenium ; Coordination chemistry ; Nitrogen heterocycles ; Synthetic methods ; Hydrogenations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chiral 1,2,3,4-tetrahydroisoquinoline intermediates in the Rice and Beyerman routes to morphine, (+)-(R)-1-(3-hydroxy-4-methoxybenzyl)-6-methoxy-1,2,3,4-tetrahydroisoquinoline (6) and (+)-(R)-1-(3,5-dibenzyloxy-4-methoxybenzyl)-6-methoxy-1,2,3,4-tetrahydroisoquinoline (5), were prepared in high ee by ruthenium-catalyzed asymmetric transfer hydrogenation of the corresponding imine precursors (Noyori method). The yield of the key raw material in the Beyerman route, 3,5-dibenzyloxy-4-methoxyphenylacetic acid (1), starting from gallic acid methyl ester (7) was improved by a factor of 5 over previously described syntheses. Key steps in the new procedure are the selective formation of methyl 3,5-dihydroxy-4-methoxybenzoate (9) via the 3,5-diacetate and an improved benzylation of the hydroxyl groups in 9.
    Additional Material: 1 Tab.
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 751-756 
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Streptenol A ; Synthetic methods ; Natural products ; SAMP/RAMP hydrazones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The asymmetric synthesis of (+)-streptenol A was carried out in ten steps and with high enantioselectivity (ee ≥ 96%). The key steps are the α-alkylation of 2,2-dimethyl-1,3-dioxan-5-one RAMP hydrazone A (1), subsequent deoxygenation and elaboration of the side chain via aldehyde B to furnish (+)-streptenol A in 23% overall yield. In analogy, the enantiomer (-)-streptenol A was synthesized using the corresponding SAMP hydrazone in 18% overall yield.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1434-193X
    Keywords: Organocerium reagents ; Phosphane oxides ; Phosphonates ; Electrophiles ; Synthetic methods ; Electrophilic Additions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The addition of organocerium reagents 2a-g to phosphinoyl chloride 1a or chlorophosphates 1b leads to the synthesis of phosphane oxides 3aa-ag and phosphonates 3bb, be in good to high yield. The reaction can be extended to cerium enolates 4 (of ketones) and 6 (of nitriles) except when a benzyl group bound to the carbonyl moiety should be metallated. The latter reaction is the first reported synthesis of β-oxophosphane oxides by a simple reaction between enolates and a phosphorus(V) halide.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 2769-2773 
    ISSN: 1434-193X
    Keywords: Rubicenes ; Cyclization ; Polycyclic aromatic hydrocarbons ; Arenes ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of novel disubstituted rubicenes 1a-k is described. Starting from 1,5-dichloroanthraquinone and aryllithium reagents 3, the diol adducts 4 are reduced and the resulting diarylanthracenes 5 are cyclized to afford the title compounds in fair to good overall yield.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 885-887 
    ISSN: 1434-1948
    Keywords: Chirality ; P ligands ; Synthetic methods ; Triphosgene ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Treatment of (1S,2S)-C5H8(PH2)2 (1) with OC(OCCl3)2 gave (1S,2S)-C5H8(PCl2)2 (2) as the first example of an optically active bis(phosphonous dichloride). 2 was converted to (1S,2S)-C5H8(PX2)2 [PX2 = P(OMe)2 (3), P(OPh)2 (4), P[OCH-(Me)Et-(S)]2 (5), P[2-OC10H6C10H6O-2′-(R)] (6), P(NC4H8O-cyclo)2 (7), P(C6H11-cyclo)2 (8)] by reactions with oxygen, nitrogen, and carbon nucleophiles.
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  • 8
    ISSN: 1434-193X
    Keywords: Sulfur heterocycles ; Synthetic methods ; Conducting materials ; Desulfurization ; C-C coupling ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two types of naphtho-fused 1,3-dithiole-2-thiones (2a-f and 4a-d) were obtained by the reaction of 3,4,7,8-tetrachloro-naphtho[1,8-cd:5,6-c′d′]bis(1,2-dithiole) (1) and sodium trithiocarbonate. The dechalcogenization of the thiones, using mercury acetate, leads to the corresponding naphtho-1,3-dithiol-2-ones (3a-d and 5a-d). The structures of 3a and 4a were confirmed by X-ray investigation. The tetrathiafulvalenes (TTFs) 6 and 7 were synthesized from 2d and 3c by coupling of the starting compounds with the help of triethyl phosphite. Using 2,3-dichloro-5,6-dicyano-benzoquinone (DDQ) the TTFs were converted into semiconducting charge-transfer complexes.
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 1645-1652 
    ISSN: 1434-193X
    Keywords: Azomethine ylides ; Nitrogen heterocycles ; Ring expansion ; Small ring systems ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Mechanistic investigations by means of proton spectroscopy detected intermediates and uncovered the course of reactions in acetate-buffered [D4]methanol of primary cyclopropanediamines cis- and trans-2a with benzaldehyde (3a) or 2,2-dimethylpropanal (3b), of secondary cyclopropanediamines cis- and trans-2b with 3b, and of the ring-methylated cyclopropanediamine trans-14a and the aromatic aldehydes 3a and c. This study provided the basis of an expedient synthesis of pyrroles which takes place under exceptionally mild conditions. Irrespective of the configuration, primary (2a·2HBr) and secondary cyclopropanediammonium dibromides 2b and c·2HBr that are devoid of ring substituents react with aromatic aldehydes 3a, e-h, cinnamic aldehyde (3i), and 3b to afford 2-substituted (8a, b) and 1,2-disubstituted pyrroles (8c-i), respectively. The 3-substituted secondary trans-cyclopropanediammonium dibromides 24·2HBr furnish 1,2,4-trisubstituted pyrroles 25. While the primary 1-methylcyclopropanediammonium dibromide trans-14a·2HBr reacts regioselectively with 3a and c to produce only 2,3-substituted pyrroles 19a, c, the corresponding secondary dibromide trans-14c·2HBr gives rise to the formation of mixtures of 1,2,3- (22) and 1,2,5-trisubstituted pyrroles 23. The key step of pyrrole formation from 1,2-cyclopropanediamines and aldehydes is the ring expansion of intermediate monoiminium ions of type 5 via azomethine ylides (E, Z)-6 to yield dihydropyrrolium ions 7.
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  • 10
    ISSN: 1434-193X
    Keywords: Epoxides ; Regioselectivity ; Nucleophilic additions ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The regiochemical outcome of the ring-opening of epoxides bearing remote polar functionalities has been established in the case of carbocyclic (1 and 2) and the corresponding furanosidic (3 and 4) title epoxides. Under standard conditions, the regioisomeric C-1 products are the sole (from trans epoxides 2 and 4) or predominant (from cis epoxides 1 and 3) ring-opening products. However, under chelating conditions, and only in the case of the cis epoxides 1 and 3, a consistent increase in C-2 selectivity is unexpectedly observed.
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  • 11
    ISSN: 1434-193X
    Keywords: Synthetic methods ; Vinyloxiranes ; Chloroallylation ; Thioallylation ; DIBAL-H ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A practical and straightforward procedure is described for the diastereoselective synthesis of functionalized cis- and trans-vinyloxiranes. Readily available esters and lactones were sequentially treated in a one-pot procedure with DIBAL-H and (Z)-γ-chloroallyl-BBN or (E)-phenylthioallyltitanium reagents to give syn-β-chlorohydrins or anti-β-hydroxy thioethers in good yields with high regio- and diastereoselectivities. The β-hydroxy intermediates were stereoselectively converted into the corresponding cis- or trans-vinyloxiranes upon treatment with DBU (for halohydrins) or by alkylation/elimination (for β-hydroxy thioethers). The protocol tolerates several functional groups and promises to be of general applicability.
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  • 12
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 2839-2849 
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Hydrazones ; Synthetic methods ; Alkylations ; Deoxygenation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An efficient asymmetric synthesis of protected anti-1,3-diols 5 (de ≥ 98%, ee = 92-98%) from 2,2-dimethyl-1,3-dioxan-5-one SAMP hydrazone 1 is described. The key steps are the diastereo- and enantioselective α,α′-bisalkylation followed by reduction of the ketones 2 and a variant of the Barton-McCombie deoxygenation. The new method allows the synthesis of acetonide-protected anti-1,3-diols with a broad range of substituents in good overall yields (31-69%).
    Additional Material: 1 Tab.
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  • 13
    ISSN: 1434-193X
    Keywords: Thia-aza heterocycles ; Bis(azolyl)alkanes ; 1-(Haloalkyl)pyridinium halides ; Synthetic methods ; Cyclization reactions ; Ab initio calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Novel 5/6/5 heterocycles, 1,3,4-thiadiazolo[3,2-a]-1,3,4-thiadiazolo[3,2-d]-1,3,5-triazinium halides 7, have been synthesized by the reaction of 2-amino-1,3,4-thiadiazoles 6 with either 1-(haloalkyl)pyridinium halides 4 or N,N′-methylenebis(pyridinium) dibromides 5. The tricyclic compounds 7 are generated in the course of several successive reaction steps in which specific proton migrations, bond-breaking and bond-forming processes occur. The structures 7 have been verified by spectral data (1H and 13C NMR, MS), X-ray analysis and ab initio calculations. The latter show that both sp2-C atoms C(10) and C(12) of 7 are significantly positively charged and, therefore, exhibit electrophilic properties towards the primary amino group of the amino-thiadiazoles 6. In the course of a multi-step reaction cascade of the 6/7 mixture, novel multi-aza/thia heterocycles 8 are formed. The structures of the latter compounds have been confirmed by X-ray analysis as well as by detailed experimental and theoretical NMR-spectroscopic studies.
    Additional Material: 3 Ill.
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  • 14
    ISSN: 0009-2940
    Keywords: Bis(tetramethylipiperidino)aluminum halides ; Alkoxy((tetramethylpiperidino)aluminum halides ; 27Al-NMR spectra ; Aluminum ; Amides ; Synthetic methods ; Bridging ligands ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: N-Lithio-2,2,6,6-tetramethylpiperidine [Li(tmp)] reacts with AlX3 (X = Cl, Br) in diethyl ether/ n-hexane solution to generate the products of substitution and ether cleavage, [tmpAl(X)(μ-OEt)], (la, X = Cl; 1b, X = Br). However, when the reaction is allowed to proceed in n-hexane alone, an almost quantitative yield of compounds tmp2AlX (2a, X = Cl; 2b, X = Br; 2c, X = I) is obtained. According to 27AI-NMR spectroscopy, mass spectroscopy, cryoscopy, and X-ray crystal structure determinations, these compounds are monomeric in the solid state, in solution, and in the gas phase. 2b reacts with AgBF4 yielding the fluoride-bridged dimer (tmp2AlF), 2d, as shown by X-ray crystal structure determination.
    Additional Material: 3 Tab.
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  • 15
    ISSN: 0947-3440
    Keywords: Cholesterol ; Steroid side chain construction ; Alkylations ; Synthetic methods ; Steroids ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 21-(3-hydroxy-3-methylbutyl)- and 21-(2-hydroxy-2-methylpropoxy)cholestane derivatives 3 and 5, formally double chain hybrid (20R)- and (20S)-cholesterol analogues, were synthesized. The C-22 to C-27 section of the 25-hydroxycholestane side chain was established by stereoselective alkylation with 5-bromo-2-methyl-2-(triethylsilyl)oxypentane of the pregnanoic ester 8, derived from 3β-acetoxyandrost-5-en-17-one (7). Reduction of the ester 9 to the 21-alcohol 10 permitted elaboration of a second hydroxylated side chain, either via the tosylate 11 by alkynation/hydrogenation (yielding intermediate 13) or by alkylation of 10 with bromoacetate (yielding 14) followed by reaction with methyl magnesium bromide.
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  • 16
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 2581-2585 
    ISSN: 0947-3440
    Keywords: Ozonides, functionalized ; Substitutions ; Alcohols ; Aldehydes ; Synthetic methods ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Substitution of 3-chloro-3,5-bis(chloromethyl)-5-methyl-1,2,4-trioxolane (3) with allyl alcohol gave the corresponding diastereomeric allyloxy-substituted ozonides 4, which were converted into diozonides 7 by ozone treatment. Substitutions of 3 with ethanediol or with 1,3-propanediol gave the corresponding hydroxyalkoxy-substituted ozonides 8, 14, which were oxidized to the corresponding aldehydes 10, 16. Reaction of 3,5-dichloro-3,5-bis(chloromethyl)-1,2,4-trioxolane (1a) with ethanediol gave the corresponding bis(hydroxy)-substituted ozonide 19 as well as a bicyclic ozonide 18 by reaction of 1a with ethanediol in a ratio of 1:1.
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  • 17
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 623-635 
    ISSN: 0947-3440
    Keywords: High-pressure chemistry ; Nucleophilic aromatic substitution ; Cycloadditions ; Synthetic methods ; Synthesis under neutral, mild conditions ; Supramolecular chemistry ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The most recent successes achieved in organic synthesis by use of high-pressure technique are reviewed.
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  • 18
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 961-965 
    ISSN: 0947-3440
    Keywords: C—C coupling ; Dienediynes ; McMurry reaction ; Neocarzinostatin ; Synthetic methods ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The monocyclo-diendiyne → bicyclo-trienediyne strategy for obtaining dienediyne models of the antitumor antibiotic neocarzinostatin chromophore (1) was extended from generating 6-/11-membered bicyclotrienediynes (e.g. 2 → 4) to generating the 6-/10-membered bicyclotrienediyne 5. In the preparatory steps (Scheme 2), the bistriflate 10 was successively coupled with alkynes 9 and 11. Via the dienediynediol and its subsequent oxidation with the Dess-Martin periodinane, we obtained the monocyclic dienediyne keto aldehyde 13. Cyclization of 13 with low-valent titanium afforded the bicyclotrienediyne 5 in 42% yield (Scheme 3). Compound 5 was converted in five steps - namely ketal cleavage, monosilylation of the resulting diol 14, asymmetric Sharpless epoxidation, desilylation and carbonate formation - into the dienediyne epoxycarbonate 3. The structure of the latter is more closely related to the neocarzinostatin chromophore than our earlier analogues.
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  • 19
    ISSN: 0947-3440
    Keywords: C-C coupling ; Dienediynes ; Enol triflates ; McMurry reaction ; Neocarzinostatin ; Synthetic methods ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel synthetic strategy leading to bicyclic dienediyne models of the chromophore 1 of the anti-tumor antibiotic neocarzinostatin is described. Its key step is a ring-closing McMurry reaction of the dienediyne keto aldehydes 17 or 23. It leads to dienediynediols (compounds 19 and 24, respectively) or to trienediynes (compounds 18 and 25, respectively). Low temperatures favor the formation of dienediynediols while high temperatures favor the formation of trienediynes, so that the McMurry reactions of keto aldehyde 23 show an almost perfect temperature-dependent chemoselectivity (Scheme 6). The trienediyne 25 contains a ketal group which was removed by acid-catalyzed methanolysis (Scheme 8). The resulting diol 31 was mono-tert-butylsilylated to provide the allyl alcohol 36 (Scheme 10). It was epoxidized regio- and stereoselectively with Sharpless' asymmetric epoxidation reagent. The resulting epoxide 37 was converted into the dienediyne epoxycarbonate syn-33 in the final step of a sequence totaling seven steps and 5% yield starting from the bistriflate 3b; 3b itself is accessible from 2-formylcyclohexa none in two steps and 47% overall yield. The dienediyne epoxycarbonate syn-33 is the first synthetic congener of the neocarzinostatin chromophore furnished both with the - correctly configured - epoxide and carbonate rings.
    Additional Material: 1 Tab.
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  • 20
    ISSN: 0947-3440
    Keywords: Photochemistry ; Synthetic methods ; Oxetanes ; Diols ; Amino alcohols ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preparation of 3-heteroatom-substituted oxetanes by the Paternò-Büchi reaction, and their application in synthesis are reviewed. 3-Oxetanols and 3-aminooxetanes are the two most important oxetanes in this respect. By tuning the electronic properties of the enol and enamine substrates, a successful photocycloaddition to carbonyl compounds, with high yield, is possible. Since the oxetane formation proceeds stereoselectively, diastereomerically pure products are readily accessible, which can then be used in further transformations. To this end, regioselective ring-opening reactions have been developed, some of which will be discussed in this account.
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  • 21
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 36 (1997), S. 1164-1170 
    ISSN: 0570-0833
    Keywords: carbenes ; carbenoids ; cyclizations ; rearrangements ; synthetic methods ; Carbenes ; Carbenoids ; Cyclization ; Rearrangement ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Alkenylidenes R2C=C: (= alkylidene carbenes) undergo regio- and stereoselective intramolecular C—H insertion reactions that are excellently suited for the synthesis of cyclopentenes. The 1, 2-shifts occurring with R=H and R=Ar are useful for the preparation of alkynes. Alkenylidenes are efficiently generated from carbonyl compounds by diazomethylation, from vinyl halides by α-elimination, from alkynyliodonium salts by addition of nucleophiles, and from alkynes by retro-1, 2-shifts. Specific applications of the various methods, particularly in the synthesis of natural products, are discussed.
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  • 22
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 36 (1997), S. 186-204 
    ISSN: 0570-0833
    Keywords: chirality ; catalysis ; copper ; reagents ; synthetic methods ; Homogeneous catalysis ; Chirality ; Copper ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Of all of the organometallic reagents currently used to form carbon-carbon bonds, organocopper reagents rank amongst the most important. Interest in these reagents centers not only on their regioselectivity, but also increasingly on their application in stereoselective transformations (principally Michael additions and SN2′ reactions); the use of suitable substrates or chirally modified cuprates can lead to highly diastereo- and enantioselective reactions. Simultaneously, extensions of methods for the preparation and application of these reagents (for example functionalized organocopper species and Lewis acid catalysis, respectively) have opened up new horizons for organocopper reagents. Mechanistically, the reactions are well-documented and understood, but this aspect of the subject has not kept pace with the many rapid developments in preparative chemistry. Organocopper ragents have proved to be indispensable in the synthesis of complex natural products and pharmaceuticals, chiral auxiliaries, and molecules with interesting structural features. In this review we will discuss some of the more recent important developments in this area; the organization will follow the type of selectivity (regio-, diastereo-, and enantioselectivity).
    Additional Material: 1 Tab.
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  • 23
    ISSN: 0570-0833
    Keywords: carbonyl complexes ; cations ; superacidic systems ; synthetic methods ; transition metals ; Carbonyl ligands ; Cations ; Superacidic systems ; Synthetic methods ; Transition metals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Homoleptic carbonyl cations of the electron-rich metals in Groups 8 through 12 are the newest members of the large family of transition metal carbonyls. They can be distinguished from typical metal carbonyl complexes in several respects. Their synthesis entails carbonylation of metal salts in such superacids as fluorosulfuric acid and “magic acid” HSO3F—SbF5. Thermally stable salts with [Sb2F11]- as counterion are obtained with antimony pentafluoride as reaction medium. Both the [Sb2F11]- anion and superacid reaction media have previously found little application in the organometallic chemistry of d-block elements. Also unprecedented in metal carbonyl chemistry are the coordination geometries with coordination numbers 4 (square-planar coordination) and 2 (linear coordination) for the cation. Formal oxidation states of the metals, and the charges of the complex cations, extend from + 1 to +3: thus CO is largely σ-bonded to the metal, and the CO bond is strongly polarized. Minimal metal → CO π-backbonding and a positive partial charge on carbon are manifested in long M—C bonds, short C—O bonds, high frequencies for C—O stretching vibrations (up to 2300 cm-1), and small 13C NMR chemical shifts (up to δc, = 121). Prominent examples of these unusual homoleptic carbonyl cations, which were recently the subject of a Highlight in this journal, include the first carbonyl cation of a p-block metal [Hg(CO)2]2+, the first trivalent carbonyl cation [Ir(CO)6]3+, and the first multiply charged carbonyl cation of a 3d metal [Fe(CO)6]2+. In this overview we propose to (a) outline the historical origins of cationic metal carbonyls and their methods of synthesis; (b) present a summary of the general field of carbonyl cations, which has developed over a yery short period of time; (c) discuss the structural and spectroscopic characteritics of metal-CO bonding; (d) discuss the special significance associated with reaction media and the [Sb2F11]- anion; and (e) point to the most recent results and anticipated future developments.
    Additional Material: 8 Ill.
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  • 24
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    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 35 (1996), S. 241-270 
    ISSN: 0570-0833
    Keywords: alkenes ; asymmetric syntheses ; Horner-Wittig reactions ; phosphane oxides ; synthetic methods ; Alkenes ; Asymmetric synthesis ; Horner-Wittig reaction ; Phosphane oxides ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In 1959, Horner showed that metalated alkyldiphenylphosphane oxides react with aldehydes or ketones to give alkenes. With this reaction, the diphenylphosphoryl (Ph2PO) group made its entrance into synthetic organic chemistry. In the thirty-six years since that date, extensive research has shown that this olefination, the Horner-Wittig reaction, has unique properties that make it much more than simply the phosphane oxide cousin of the more famous phosphorus-based olefinations - the Wittig reaction (based on phosphonium salts) and the Wadsworth-Emmons reaction (based on phosphonate esters). Early work on the Horner-Wittig reaction concentrated on the reactivity of phosphane oxides and the regioselectivity of their reactions, but more recently the power of the Ph2PO group to control the stereochemistry of alkenes, and to produce “on demand” either stereoisomer in high stereochemical purity, has emerged. From the study of these stereocontrolled Horner-Wittig reactions arose the realization that the Ph2PO group is useful not only for the control of the two-dimensional stereochemistry of alkenes, but also of three-dimensional stereochemistry in general.After a brief introduction to phosphane oxide chemistry, this review will examine the Horner-Wittig reaction, in both its original and “stereocontrolled” varieties. From there, we will move on to an account of the stereoselective construction of molecules containing the Ph2PO group, concentrating on the stereochemical directing effects of the Ph2PO group and on the role of its unique combination of attributes - steric bulk, electronegativity, and Lewis basicity - in controlling these reactions. Finally, we will present what is intended as a practical guide to this chemistry, covering the type of functionalized alkenes that have been made with the help of the Ph2PO group and giving guidelines that we hope will help the organic chemist to make the most of the chemistry the Ph2PO group has to offer.
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  • 25
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    Angewandte Chemie International Edition in English 35 (1996), S. 1050-1066 
    ISSN: 0570-0833
    Keywords: carbon-carbon coupling ; carbon monoxide ; carbonylation ; radicals ; synthetic methods ; C-C coupling ; Carbonylation ; Radicals ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Although known since the 1950s, free-radical carbonylation has not received much attention until only recently. In the last few years the application of modern free-radical techniques has revealed the high synthetic potential of this reaction as a tool for introducing CO into organic molecules. Clearly now is the time for a renaissance of this chemistry. Under standard conditions (tributyltin hydride/CO) primary, secondary, as well as tertiary alkyl bromides and iodides can be efficiently converted into the corresponding aldehydes. Aromatic and α,β-unsaturated aldehydes can also be prepared from the parent aromatic and vinylic iodides. If the reaction is carried out in the presence of alkenes containing an electron-withdrawing substituent, the initially formed acyl radical subsequently adds to the alkene, leading to a general method for the synthesis of unsymmetrical ketones. This three-component coupling reaction can be extended successfully to allyltin-mediated reactions. Thus, β,γ-enones can be prepared from organic halides, CO, and allyltributylstannanes. In a remarkable one-pot procedure alkyl halides can be treated with a mixture of alkene, allyltributylstannane, and carbon monoxide in a four-component coupling reaction that provides β-functionalized δ,∊-unsaturated ketones by the formation of three new C—C bonds. The reaction of 4-pentenyl radicals with CO leads to acyl radical cyclization, which provides a useful method for the synthesis of cyclopentanones. Certain useful one-electron oxidations can be combined efficiently with free-radical carbonylations. These findings and others discussed in this article clearly demonstrate that free-radical carbonylation can now be considered a practical alternative to transition metal mediated carbonylation.
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  • 26
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    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 35 (1996), S. 2056-2083 
    ISSN: 0570-0833
    Keywords: protecting groups ; retrosynthetic analyses ; synthetic methods ; total syntheses ; Protecting groups ; Retro reactions ; Synthetic methods ; Total synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The choice of protecting groups is one of the decisive factors in the successful realization of a complex, demanding synthetic project. The protecting groups used influence the length and efficiency of the synthesis and are often responsible for its success or failure. A wide range of blocking groups are currently available for the different functional groups; however, an overall strategy combining these different masking techniques in an advantageous and reliable manner has never been proposed or at best only for individual cases. This review attempts to make a contribution to filling this gap. First a very short overview of the most commonly used protecting groups will be given, in which they are classified according to their lability and not according to the functional group they protect. This classification clarifies coherent concepts for the development of blocking strategies. On the basis of this brief summary reliable strategies will then be illustrated and developed with selected examples from the recent literature by which protecting groups may be combined successfully and advantageously in synthetic projects of differing degrees of complexity and difficulty.
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  • 27
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    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 35 (1996), S. 1380-1419 
    ISSN: 0570-0833
    Keywords: glycals ; glycoconjugates ; glycosylations ; oligosaccharides ; synthetic methods ; Glycals ; Glycoconjugates ; Glycosylation ; Oligosaccharides ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This review provides a personal account of the explorations of a research group in oligosaccharide and glycoconjugate construction. The journey began twenty years ago with the study of Diels-Alder reactions of complex dienes. By extending this methodology to aldehydo-type heterodienophile equivalents, access to unnatural glycals was gained (LACDAC reaction). From this point a broad-ranging investigation of the use of glycals in the synthesis of oligosaccharides and other glycoconjugates was begun. Mobilization of glycals both as glycosyl donors and glycosyl acceptors led to the strategy of glycal assembly. Several new glycosylation techniques were developed to provide practical underpinning for this logic of glycal assembly. Glycal-based paradigms have been shown to be nicely adaptable to solid phase supported synthesis. Moreover, glycal assembly - both in solution and on solid phases - has been used to gain relatively concise and efficient entry to a variety of biologically interesting and potentially valuable constructs. Some of these syntheses, particularly in the field of tumor antigens, have led to novel compounds which are in the final stages of preclinical assessment. This review presents an account of the chemical reasoning at the center of the program.
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  • 28
    ISSN: 0570-0833
    Keywords: computer chemistry ; synthesis planning ; WODCA ; Synthetic methods ; Computer chemistry ; Synthesis design ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The planning of syntheses in organic chemistry has continuously been given more solid foundations during the last decades. Widely applicable rules have been formulated. Nearly parallel with the systematization of this field the potential for the automation of synthesis planning by the use of computers has been promulgated. However, after more than two decades of continuing efforts by a number of groups, computer-assisted synthesis planning, which relies on large libraries of synthons and transforms, has failed to establish itself firmly. This is in marked contrast to the acceptance of reaction databases. Their use has become routine with a surprisingly short time. Apparently the classical approaches to computer-assisted synthesis planning do not satisfy the needs of the preparative chemist. However, this lack of success does not yet mean the complete demise of the whole field. The conceptual shortcomings and problems of the first generation of programs are both of technical and psychological nature and need to be critically analyzed. Meanwhile, work has begun on systems of the second generation, which try to support the chemist in the synthesis laboratory with new methods in novel ways. Care is taken to mimic and support the typical planning style of the human chemist, who often on the spur of the moment switches direction in planning as well as his (sub) goals and methods. New tools that correspond better to the thinking patterns and working habits of chemists empower the user to plan syntheses of organic molecules in an interactive and innovative fashion. We feel that this field is far from dead and continue our research activities. In this article we introduce a system with a number of novel concepts and demonstrate its capabilities with some practical examples.
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  • 29
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    Angewandte Chemie International Edition in English 34 (1995), S. 259-281 
    ISSN: 0570-0833
    Keywords: carbon-carbon coupling ; catalysis ; cycloadditions ; synthetic methods ; C-C coupling ; Atom economy ; Homogeneous catalysis ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enhancing the efficiency of the synthesis of complex organic products constitutes one of the most exciting challenges to the synthetic chemist. Increasing the catalogue of reactions that are simple additions or that minimize waste production is the necessary first step. Transition metal complexes, which can be tunable both electronically and sterically by varying the metal and/or ligands, are a focal point for such invention. Except for catalytic hydrogenation, such methods have been rare in complex synthesis and virtually unknown for C—C bond formation until the advent of cross-coupling reactions. These complexes may orchestrate a variety of C—C bond-forming processes, important for creation of the basic skeleton of the organic structure. Their ability to insert into C—H bonds primes a number of different types of additions to relatively nonpolar π-electron systems. Besides imparting selectivity, they make feasible reactions that uncatalyzed were previously unknown. The ability of these complexes to preorganize π-electron systems serves as the basis both of simple additions usually accompanied by subsequent hydrogen shifts and of cycloadditions. The ability to generate “reactive” intermediates under mild conditions also provides prospects for new types of C—C bond-forming reactions. While the examples reveal a diverse array of successes, the opportunities for new invention are vast and largely untapped.
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  • 30
    ISSN: 0570-0833
    Keywords: carbohydrates ; enzymes ; organic synthesis ; Synthetic methods ; Enzymes ; Carbohydrates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Carbohydrates on cell surfaces are information molecules. Although only seven or eight monosaccharides are commonly used as building blocks in mammalian systems, the multifunctionality of these monomers can lead to the assembly of an immense variety of complex structures. Millions of different tetrasaccharide structures, for example, can be constructed from this small number of building blocks, if branching, the stereochemistry of glycosidic linkages, and the modification of hydroxyl and amino groups are taken into consideration. Oligosaccharides therefore represent an effective class of biomolecules that code for a vast amount of information required in various biological recognition processes, such as intercellular communication, signal transduction, cell adhesion, infection, cell differentiation, development and metastasis. The pace of development of pharmaceuticals based on carbohydrates has, however, been slower than that based on other classes of biomolecules. Part of the reason is the lack of technologies for the study of complex carbohydrates. There is no method to amplify oligosaccharides for sequence analysis. There is no machine available for automated synthesis of oligosaccharides. In addition, the possibly poor bioavailability and difficulties in the large-scale synthesis of carbohydrates have undoubtedly contributed to this slow pace. The enzymatic and chemoenzymatic methods, especially those based on aldolases and glycosyltransferases, described here appear to be useful for the synthesis of mono- and oligosaccaharides and related molecules. Further advances in glycobiology will probably lead to the development of new technologies for the study of carbohydrate recognition and for the synthesis of bioactive carbohydrates and mimetics to control the recognition processes.
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  • 31
    ISSN: 0570-0833
    Keywords: carbohydrates ; enzymes ; organic synthesis ; Synthetic methods ; Enzymes ; Carbohydrates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Recognition of carbohydrates by proteins and nucleic acids is highly specific, but the dissociation constants are relatively high (generally in the mM to high μM range) because of the lack of hydrophobic groups in the carbohydrates. The high specificity of this weak binding often comes from many hydrogen bonds and the coordination of metal ions as bridge between sugars and receptors. Though weak hydrophobic interactions between sugars and proteins have also been identified, the unique shape of a complex carbohydrate under the influence of anomeric and exo anomeric effects (the glycosidic torsion angles are therefore often not flexible but are typically somewhat restricted) and the topographic orientation of the hydroxyl and charged groups contribute most significantly to the recognition process. Studies on the structure-function relationship of a complex carbohydrate therefore require deliberate manipulation of its shape and functional groups, and synthesis of oligosaccharide analogs from modified monosaccharides is often useful to address the problem. The availability of various monosaccharides and their analogs for the synthesis of complex carbohydrates together with the information resulting from structural studies (such a NMR or X-ray studies on sugar-protein complexes) will certainly provide a basic understanding of complex carbohydrate recognition. An ultimate goal is to develop simple and easy-to-make non-carbohydrate molecules that resemble the active structure involved in carbohydrate-receptor interaction or the transition-state of an enzyme-catalyzed transformation (for example, glycosidase or glycosyltransferase reactions) and have the approprite bioavailability to be used to control the carbohydrate function in a specific manner. In part one of this review we described various enzymatic approaches to the synthesis of monosaccharides, analogs, and related structures. We describe in this part enzymatic and chemoenzymatic approaches to the synthesis of oligosaccarides and analogs, including those involved in E-selectin recognition, and strategies to inhibit glycosidases and glycosyltransferases.
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  • 32
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    Angewandte Chemie International Edition in English 33 (1995), S. 2379-2411 
    ISSN: 0570-0833
    Keywords: C-C coupling ; Synthetic methods ; Heck reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The palladium-catalyzed coupling of haloarenes and haloalkenes with alkenes, which was discovered by Richard F. Heck in the late sixties, has been a topic of fluctuating interest; however, in the last six years activity in this area has flourished. Careful choice of substrates and skillful tailoring of reaction conditions lead to impressive sequences consisting of even different reaction types that occur not only in a consecutive mode, but also in a single operation. The wellestablished Heck reaction, together with other mechanistically related palladiumcatalyzed transformations with arene, alkene, and alkyne derivatives, opens the door to a tremendous variety of elegant and highly convergent routes to structurally complex molecules. The reaction is not disturbed by heteroatoms such as oxygen and nitrogen (nor by sulfur and phosphorus with some limitations). The spectrum of recent achievements starts with a range of chemoselective and regioselective monocouplings of highly functionalized substrates with unsymmetrical and multisubstituted reaction partners. Other advances include cascade reactions in which three, four, five, and even eight new C—C bonds are formed to yield oligofunctional and oligocyclic products with impressive molecular complexity. Even the enantioselective construction of complex natural products with quaternary stereocenters has been achieved with Heck reactions in key steps, as exemplified by the synthesis of crinan, picrotoxinin, and morphine. Today, the Heck reaction is indispensible in the arsenal of synthetic methods available to organic chemists. Certainly it is only a matter of time before the Heck reaction is applied in industrial syntheses.
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  • 33
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    Angewandte Chemie International Edition in English 34 (1995), S. 2443-2465 
    ISSN: 0570-0833
    Keywords: catalysis ; oxidations ; synthetic methods ; Amines ; Homogeneous catalysis ; Oxidation ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The metabolism of amines is governed by a variety of enzymes such as amine oxidase, flavoenzyme, and cytochrome P-450. A wide variety of compounds are produced such as ammonia and alkaloids in selective and clean oxidation reactions that proceed under mild reaction conditions. Simulation of the functions of these enzymes with simple transition metal complex catalysts may lead to the discovery of biomimetic, catalytic oxidations of amines and related compounds. Indeed, metal complex catalyzed oxidations have been found to proceed with high efficiency. The first section of this review discusses the dehydrogenative oxidations of amines with transition metal catalysts by transition metal catalysts that simulate amine oxidase. The second section highlights the catalytic oxidation of secondary amines to nitrones by simulation of flavoenzymes. The third section describes the simulation of the function of cytochrome P-450 with lowvalent ruthenium complexes and peroxides. Biomimetic ruthenium-catalyzed oxidations of tertiary amines, secondary amines, and other substrates such as amides, β-lactams, nitriles, alcohols, alkenes, ketones, and even nonactivated hydrocarbons can be performed selectively under mild conditions. These three general approaches provide highly useful strategies for synthesis of fine chemicals and biologically active compounds such as alkaloids, amino acids, and β-lactams.
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  • 34
    ISSN: 0570-0833
    Keywords: Metallo-ene reaction ; Ene reaction ; Synthetic methods ; Catalysis ; Natural products ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Metallo-ene reactions, hardly recognized until very recently, have experienced a breathtaking development when applied in an intramolecular sense. Efficient regio- and stereoselective magnesium-ene cyclizations have served as a cornerstone for numerous syntheses of structurally diverse natural products (e.g., sesquiterpenes of marine or plant origin, alkaloids, fragrances, insect defense compounds, and a fungitoxin). A brilliant example is the synthesis of the elusive odorant (+)-khusimone which outshines 20 years of work in the field of tricyclovetivane synthesis. Palladium-, platinum-, and nickel-catalyzed versions of the metallo-ene reaction are in a comparatively early stage of exploration, but, nevertheless, reveal intriguing potential. Hence an almost 100% stereospecific C—O→C—;Pd-→ C—C chirality transfer permits simple and selective, cis- or trans-annelation processes. The mild cyclization conditions are compatible with various functional groups, such as nitrogen moieties, which offer interesting perspectives for the preparation of heterocycles (e.g., alkaloids) difficult to obtain by other methods. Carbon monoxide insertion reactions of the cyclized σ-metal intermediates were shown to afford annelated cyclopentanones and cyclopentenones with concomitant stereocontrolled formation of four carbon-carbon bonds. These and other observations, highlighted in this article, provide a platform for further extensions and applications of this powerful method in organic synthesis.
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  • 35
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    Angewandte Chemie International Edition in English 28 (1989), S. 716-734 
    ISSN: 0570-0833
    Keywords: Gene technology ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oligonucleotide synthesis, until a few years ago the rather exotic preserve of a few experts, has become an integral part of the arsenal of molecular-biological techniques. The last decade, in particular, has seen unbelievably rapid development in this area. DNA synthesis has been automated and can now produce genes greater than 1000 base pairs in length. Tailor-made synthetic genes also permit the synthesis of altered or even novel proteins (de novo protein design) by gene-technological methods. Together with modern methods of gene isolation, sequencing, and expression, gene synthesis has played a major part in the enormous advances achieved in gene technology.
    Additional Material: 12 Ill.
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  • 36
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    Angewandte Chemie International Edition in English 28 (1989), S. 1173-1192 
    ISSN: 0570-0833
    Keywords: Cyclization ; Allylic alkylation ; Alkylation ; Synthetic methods ; Palladium ; Catalysis ; C-C coupling ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The history of ring systems in organic chemistry parallels their synthetic accessibility. Transition-metal-catalyzed cyclizations offer a new opportunity to create carbo- and heterocyclic compounds with great facility. Among these methods, allylic alkylations catalyzed by palladium have proven unusually productive because of the extraordinary chemo-, regio-, and diastereoselectivity and the continuing possibility for the development of enantioselectivity. The rules for ring closure differ from those for non-transition-metal-catalyzed reactions. A major benefit is the ability to generate medium (eight-, nine-, ten-, and eleven-membered) and large rings in preference to normal (five-, six- and seven-membered) rings. With the appropriate substrate, efficient macrocyclizations are possible under conditions of normal concentrations. A second major benefit derives from the complementary stereochemistry of the metalcatalyzed substitution (net retention of configuration) compared to non-metal-catalyzed reactions (inversion of configuration). Further, the requirement for the substrate to conform to the transition-metal template may impose a stereochemical preference in the intermediate that ultimately translates into the thermodynamically less stable organic product regardless of the stereochemistry of the starting material. While more work has focused on carbocyclic synthesis, the possibilities for heterocyclic synthesis are just beginning to be tapped. In addition to forming heterocycles by C—C bond formation, use of a heteroatom as a nucleophile has already proven effective for oxygen and nitrogen, with other nucleophiles awaiting investigation. New dimensions for cyclization via allylic alkylation arise by generating the requisite π-allylpalladium intermediates by methods other than palladium(0)-initiated allylic ionizations. In addition, metals other than palladium will clearly expand the possibilities, but as yet remain untapped.
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  • 37
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    Angewandte Chemie International Edition in English 28 (1989), S. 695-707 
    ISSN: 0570-0833
    Keywords: Enantioselectivity ; Lipases ; Synthetic methods ; Enzyme catalysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantioselective biocatalysis in nonaqueous media is becoming increasingly important in preparative synthetic chemistry. This article discusses (1) the general catalytic properties of enzymes in nonaqueous environments, (2) the basic principles that govern lipase-catalyzed enantioselective esterification and transesterification reactions in organic media for the preparation of optically active acids and alcohols, (3) the determination of kinetic and thermodynamic parameters, and (4) the quantitative analysis of published data.
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  • 38
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    Angewandte Chemie International Edition in English 28 (1989), S. 969-980 
    ISSN: 0570-0833
    Keywords: Stereoselectivity ; Reaction mechanisms ; Synthetic methods ; Free radicals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The regio- and chemoselectivities of free radical reactions are often high and largely predictable; systematic studies have now shown that the stereoselectivity of free radical reactions can also be directed. Examples involving five- and six-membered cyclic radicals will be used to show how steric and stereoelectronic effects influence the diastereoselectivity of reactions of cyclic radicals with olefins. The temperature, the solvent, and the nature of the radical scavenger used also play a role, so that, if the correct reaction conditions are used, the stereoselectivity of reactions for cyclic reactants can be very high. Lower stereoselectivities are often observed for reactions between acyclic radicals and acyclic alkenes. However, preliminary experiments have indicated that under certain conditions such systems can also react in a stereoselective manner.
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  • 39
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    Angewandte Chemie International Edition in English 27 (1988), S. 679-682 
    ISSN: 0570-0833
    Keywords: Reduction ; Hydrogenation ; Hydrocarbons ; Carbon ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For the synthesis of organic compounds, reduction is an indispensible reaction type which is also widely used on an industrial scale. In industrial processes hydrogen is usually used as reducing agent, since strong reducing agents like alkali metals and hydrides can only be used to a limited extent for safety and economic reasons. Very economical reducing agents that are convenient to handle and have high potential application are hydrocarbons in presence of carbon. Hydrocarbon/carbon systems can be readily used instead of molecular hydrogen and expensive metal catalysts for the hydrogenation of compounds containing, for example, CC-, CO-, or NO-double bonds. Furthermore, these systems can be used for carrying out reductions which hitherto required strong reducing agents such as zinc, tin, alkali metals and hydrides. Especially suitable as economic sources of hydrogen are refinery products such as vacuum gas oil, fuel oil S or vacuum residue oil. Hydrocarbons are dehydrogenated to unsaturated systems and finally to carbon.
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  • 40
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    Angewandte Chemie International Edition in English 27 (1988), S. 797-827 
    ISSN: 0570-0833
    Keywords: Cyclobutanes ; Cyclobutenes ; Synthetic methods ; Natural products ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chemical reactivity of cyclobutanones and cyclobutenones is considerably different from that of cyclic ketones with larger rings; this is due to their ring strain of ca. 25 kcal/mol. Detailed knowledge regarding the influence of this ring strain on regio-, chemo- and stereoselective transformations of four-membered ring ketones is of particular importance. While several reactions, such as the Baeyer-Villiger reaction, the Beckmann and Favorskii rearrangements and cine-substitution often proceed in a manner specific to four-membered rings, other reactions such as the facile ring-opening by nucleophiles, the rearrangement to tropolones, the thermal [2+2]-cycloreversion, the isomerization to vinylketenes and the photochemical formation of oxacarbenes are rather specific to cyclobutanones and cyclobutenones. The remarkable selectivity and the excellent yields of such transformations, which are favored or caused by ring strain as the inherent driving force, offer the synthetic chemist fascinating possibilities for the development of new strategies for the synthesis of natural products and biologically active compounds.
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  • 41
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    Angewandte Chemie International Edition in English 27 (1988), S. 1113-1126 
    ISSN: 0570-0833
    Keywords: Indoles ; Synthetic methods ; Transition metals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The use of transition-metal complexes as reagents for the synthesis of complex organic compounds has been under development for at least several decades, and many extraordinary organic transformations of profound potential have been realized. However, adoption of this chemistry by the practicing synthetic organic chemist has been inordinately slow, and only now are transition-metal reagents beginning to achieve their rightful place in the arsenal of organic synthesis. Several factors contributed to the initial reluctance of synthetic organic chemists to use organometallic reagents. Lacking education and experience in the ways of elements having d electrons, synthetic chemists viewed organometallic processes as something mysterious and unpredictable, and not to be discussed in polite society. Organometallic chemists did not help matters by advertising their latest advances as useful synthetic methodology, but restricting their studies to very simple organic systems lacking any serious functionality (e.g., the “methyl, ethyl, butyl, futile” syndrome). Happily, things have changed. Organometallic chemists have turned their attention to more complex systems, and more recently trained organic chemists have benefited from exposure to the application of transition metals. This combination has set the stage for major advances in the use of transition metals in the synthesis of complex organic compounds. This review deals with one aspect of this area, the use of transition metals in the synthesis of indoles.
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  • 42
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    Angewandte Chemie International Edition in English 27 (1988), S. 226-246 
    ISSN: 0570-0833
    Keywords: Zeolites ; Catalysis ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Zeolites have proved to be valuable technical catalysts in petrochemistry and in oil processing. The characteristic properties of zeolites, such as acidity, shape-selectivity and thermal stability also enable them to be used for highly selective synthesis in the fields of chemical intermediates and fine chemicals. This interesting area of application has grown continuously in recent years. The present article summarizes the various standard types of reaction involved in organic syntheses which can be catalyzed by zeolites; these include, inter alia, electrophilic and nucleophilic substitution reactions, isomerization of double bonds and carbon skeletons, as well as addition, elimination and hydrogenation reactions.
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  • 43
    ISSN: 0570-0833
    Keywords: Ketenimines ; Carbene ligands ; Isocyanides ; Carbocycles ; Heterocycles ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ketenimine complexes are readily available in great variety by reaction of isocyanides with carbene complexes. They have proven to be useful building blocks in new synthetic approaches to carbocyclic and N-heterocyclic four-, five-, and six-membered rings. The reactions involve new metal-induced bond formation patterns of the ketenimine ligands, which can be influenced across a wide range by varying the following five parameters: the metal, the ligands, and the three substituents on the N=C=C unit.
    Additional Material: 8 Tab.
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  • 44
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    Angewandte Chemie International Edition in English 27 (1988), S. 1484-1508 
    ISSN: 0570-0833
    Keywords: Phosphaalkynes ; Synthetic methods ; Phosphorus-carbon compounds ; Multiple bonds ; Heterocycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Organophosphorus compounds have been applied in two ways in chemical synthesis. They can either be used as a reagent in a step of the synthesis (for example, in the Wittig reaction) or they can be incorporated directly into the target molecule. This second application, in particular, has expanded greatly in the last few years with the preparation of low-coordination phosphorus compounds. These include the phosphaalkynes, which are of great interest to organic and inorganic chemists. Phosphaalkynes have been employed in the synthesis of heterocyclic compounds, phosphaarenes and their valence isomers, and polycyclic compounds. Further applications have been the use of phosphaalkynes as new ligand systems in complex chemistry and their cyclooligomerization with organometallic reagents. While the chemical properties of phosphaalkynes have little in common with those of nitriles, they are in many ways very similar to those of the isoelectronic acetylenes.
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  • 45
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    Angewandte Chemie International Edition in English 27 (1988), S. 622-642 
    ISSN: 0570-0833
    Keywords: Synthetic methods ; Enzyme catalysis ; Fermentation ; Biotechnology ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In recent years, the most significant development in the field of synthetic organic chemistry has been the application of biological systems to chemical reactions. Reactions catalyzed by enzymes and enzyme systems display far greater specificities than more conventional organic reactions. Biological and/or enzymatic syntheses and transformations, that is, “microbial transformations,” have great potential. Some of these reactions have already been shown to have useful applications in the fields of synthetic organic chemistry and biotechnology. This article reviews the current status of the rapidly developing field of microbial transformation, the methodology, available technological procedures, and fields of application being described especially in relation to conventional organic synthesis methods.
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  • 46
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    Angewandte Chemie International Edition in English 26 (1987), S. 15-23 
    ISSN: 0570-0833
    Keywords: Monosaccharides ; Synthetic methods ; Carbohydrates ; Heptoses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The evolution of a strategy to synthesize the title compounds is described. Three principal developments allowed realization of this goal: (1) the attainment of high margins of diastereofacial selectivity and regioselectivity in the construction of pyranoid systems via the Lewis acid-catalyzed cyclocondensation reaction of activated dienes and aldehydes; (2) the exploitation of stereoselective reactions for functionalization of the pyranoid matrix; and (3) the discovery of stereoselective reactions for extending the chiral biases of pyranoid systems to newly emerging stereogenic centers on side chains. The coordination of these components in the synthesis of target systems of high biological interest is described.
    Additional Material: 18 Ill.
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  • 47
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    Angewandte Chemie International Edition in English 26 (1987), S. 190-203 
    ISSN: 0570-0833
    Keywords: Homogeneous hydrogenation ; Hydrogenation ; Synthetic methods ; Catalysis ; Stereoselectivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereochemical control is a major concern in the application of homogeneous catalysis to organic chemistry. In this context, the directed hydrogenation of olefins employing cationic rhodium or iridium catalysts has considerable potential, for very high selectivity can be attained under mild reaction conditions. The only requirement is a polar functional group in proximity to the double-bond which remains bound to the metal during the catalytic cycle and thereby controls the Stereochemical course of hydrogen delivery through the constraints of chelation. The substituent is most frequently a hydroxy group OH but can also be an ester, amide or carbamate group; other groups remain to be scrutinized. In cyclic compounds, directed hydrogenation can lead to face-selectivity, and the polar substituent may be in the β-, γ-, or δ-position to the double-bond. Acyclic stereoselection ensues with β- or γ-substituents in appropriate compounds, and the configuration of reduced product is predictable on the basis of simple rules. The application of optically active rhodium complexes leads to useful kinetic resolution procedures.
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  • 48
    ISSN: 0570-0833
    Keywords: Glycopeptides ; Molecular recognition ; Molecular recognition ; Synthetic methods ; Carbohydrates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Glycopeptides are partial structures of the connecting regions of glycoproteins and, like these, always contain glycosidic bonds between the carbohydrate and peptide parts. Glycoproteins are not only widely distributed but are also decisive factors in post-translational biological selectivity, especially in biological recognition. Targeted syntheses of glycopeptides require stereoselective formation of the glycosidic bonds between the carbohydrate and the peptide parts and protective group methods that enable selective deblocking of only one functional group in these polyfunctional molecules. These heavy demands have been met by the well-established use of benzylic protective groups, which can be removed by hydrogenolysis, combined with the use of base-labile 2-phosphonioethoxycarbonyl (Peoc) or 9-fluorenylmethoxycarbonyl (Fmoc) protective groups or of bromoethyl esters, which can be removed under neutral conditions. The acidolysis of tert-butyloxycarbonyl (Boc) groups and of tert-butyl esters has also been successfully used, although, under acidic conditions, anomerization or rupture of the glycosidic bonds may occur, especially when nucleophiles are present. The stable, two-stage 2-(pyridyl)ethoxycarbonyl (Pyoc) protective groups allow a more reliable synthesis of complex glycopeptides since they can be removed, after modifications, under mild conditions. Particularly suitable for the synthesis of sensitive glycopeptides are the stable allyl protective groups. They can be removed from the complex glycopeptides in a highly selective and effective manner by means of noble-metal catalysts under practically neutral conditions. These methods have been employed to synthesize glycopeptides corresponding to partial structures of interesting glycoproteins. Deprotected glyopeptides representing tumor-associated antigen structures can be coupled to bovine serum albumin, which serves as a biological carrier molecule, without the necessity of using an artificial coupling component (spacer).
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  • 49
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    Angewandte Chemie International Edition in English 26 (1987), S. 24-37 
    ISSN: 0570-0833
    Keywords: Stereoselective aldol reactions ; Aldol reaction ; Chiral enolates ; Enolates ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The aldol reaction is among the most important methods of forming carbon-carbon bonds. The addition of an enolate to an aldehyde leads to the formation of at least one chiral center. In the case of α-substituted enolates it has to a large extent been possible to control the product stereochemistry, while the aldol reaction of α-unsubstituted chiral enolates was for many years a “problem child” for synthetic chemists because of its insufficient stereoselectivity. Progress in this area has only been made in the last few years using either new chiral auxiliaries or alternatives to the aldol reaction.
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  • 50
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    Angewandte Chemie International Edition in English 26 (1987), S. 204-217 
    ISSN: 0570-0833
    Keywords: Synthetic methods ; Reduction ; Carbanions ; Cycloannelation ; Cycloalkenes ; Polycycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The successive reduction of fully conjugated cyclic hydrocarbons leads to singly and multiply charged ions with unusual bonding. The charge distribution in these ions can be determined spectroscopically, and the information so obtained is then used in kinetically controlled trapping reactions for the regioselective introduction of electrophilic groups. When non-benzenoid substrates are used, syntheses become possible which can either not be carried out or can only be carried out with great difficulty in other ways. Examples of new preparative applications are cycloannelation and bridging reactions as well as polymerization reactions. The ion pair structure of the intermediate and the type of electrophile used are of paramount importance in controlling the mechanism of these reductive transformations.
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  • 51
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    Angewandte Chemie International Edition in English 26 (1987), S. 489-503 
    ISSN: 0570-0833
    Keywords: Natural products ; Synthetic methods ; Polyketides ; Stereoselectivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Sequences of directly adjacent stereogenic centers were first discovered for the monosaccharides, which therefore constituted the first focus of interest in Stereoselective synthesis. The structures of many polyketide-derived natural products that have been elucidated in recent years often exhibit longer sequences of stereogenic centers in which hydroxy-substituted secondary carbon atoms alternate with tertiary carbon atoms. This finding inspired the development of methods allowing the specific construction of first two and then - more importantly-three neighboring stereogenic centers. The diverse methodologies that have thereby emerged reveal common principles and stress the variety of approaches possible in stereoselective synthesis.
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  • 52
    ISSN: 0570-0833
    Keywords: Glycosides ; Oligosaccharides ; Koenigs-Knorr method ; Synthetic methods ; Carbohydrates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Glycoproteins, glycolipids, and glycophospholipids (glycoconjugates) are components of membranes. The oligosaccharide residue is responsible for intercellular recognition and interaction; it acts as a receptor for proteins, hormones, and viruses and governs immune reactions. These significant activities have stimulated interest in oligosaccharides and glycoconjugates. With their help it should be possible to clarify the molecular basis of these phenomena and to derive new principles of physiological activity. Major advances in the synthesis of oligosaccharides have been made by the use of the Koenigs-Knorr method, in which glycosyl halides in the presence of heavy-metal salts are employed to transfer the glycosyl group to nucleophiles. The disadvantages of this procedure have led to an intensive search for new methods. Such methods will be discussed in this article. Emphasis is placed on glycoside and saccharide formation by 1-O-alkylation, on the trichloroacetimidate method, and on activation through the formation of glycosylsulfonium salts and glycosyl fluorides.
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  • 53
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    Angewandte Chemie International Edition in English 25 (1986), S. 414-430 
    ISSN: 0570-0833
    Keywords: Gas-phase flow thermolysis ; Thermolysis ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preference for carrying out synthetic organic reactions at the lowest possible temperature is due to the expectation that the selectivity often increases with decreasing temperature, as is confirmed by many examples and also theoretically justified. Selectivity, however, is not the only problem at high temperatures; further factors include the frequently limited thermal stability of the functional groups and structural elements not directly involved in the transformation. In spite of these limitations, the advantages of high temperatures and the greatly improved knowledge of the mechanisms of dynamic gas-phase processes accumulated in recent years can be exploited in directed organic synthesis. In this review the synthetic potential of gas-phase flow thermolysis will be described from the viewpoint of the synthetic chemist with the aid of typical examples of application.
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  • 54
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    Angewandte Chemie International Edition in English 25 (1986), S. 508-524 
    ISSN: 0570-0833
    Keywords: Cross-coupling ; Organotin compounds ; Electrophilicity ; Palladium ; Catalysis ; C-C coupling ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cross-coupling of organotin reagents with a variety of organic electrophiles, catalyzed by palladium, provides a novel method for generating a carbon-carbon bond. Because this mild, versatile reaction is tolerant of a wide variety of functional groups on either coupling partner, is stereospecific and regioselective, and gives high yields of product, it is ideal for use in the synthesis of elaborate organic molecules. When the coupling reaction is carried out in the presence of carbon monoxide, instead of a direct coupling, carbon monoxide insertion takes place, stitching the two coupling partners together and generating a ketone.
    Additional Material: 1 Ill.
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  • 55
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    Angewandte Chemie International Edition in English 25 (1986), S. 960-970 
    ISSN: 0570-0833
    Keywords: Phase-transfer catalysis ; Catalysis ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The development of Phase Transfer Catalysis (PTC) represents a major step forward in the employment of many organic reactions and renders them very convenient and useful processes. These reactions involve the application of nucleophiles in general, anions and bases in particular, in reactions carried out in a water-organic solvent system. They can be performed both in the laboratory and on an industrial scale. The ease of application of PTC processes is the main reason for their increasing utilization in industry. An outstanding achievement of this technique is the employment of aqueous bases in reactions which traditionally would otherwise require a strong base in a nonaqueous medium. The classical procedures that require severe anhydrous conditions, expensive solvents and dangerous bases such as metal hydrides and organometallic reagents are now replaced by aqueous solutions of, e.g., sodium or potassium hydroxides (PTC/OH processes). In contrast to the extensive synthetic applications of PTC/OH systems, the detailed mechanisms of these processes have been the subject of a great deal of controversy and various mechanisms have been suggested. However, it would seem that our knowledge concerning the mechanistic aspects of such reactions has now reached the stage where it can be used to advantage in synthesis planning. A better understanding of the various factors which influence the reaction would undoubtedly help to optimize PTC/OH processes such as to enable higher yields in shorter reaction times at lower temperatures. The importance of, inter alia, the catalyst will be pointed out and it is highly recommended that such catalysts be always available in the laboratory, for the range of organic reactions that they can efficiently, conveniently and safely catalyze is vast indeed.
    Additional Material: 5 Ill.
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  • 56
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    Angewandte Chemie International Edition in English 25 (1986), S. 1-20 
    ISSN: 0570-0833
    Keywords: Cycloaddition ; Trimethylenemethane ; Synthetic methods ; Cyclopentanes ; Heterocycles ; Natural products ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The power of cycloadditions for ring formation derives from the potential for chemo-, regio-, diastereo-, and enantioselectivity. Trimethylenemethane (TMM) and its equivalents offer the possibility of extending such benefits to the synthesis of five-membered rings. Three types of conjunctive reagents appear to be promising candidates: (1) selected 4-alky-lidene-4,5-dihydro-3H-pyrazoles, (2) 2-[(trimethylsilyl)methyl]allyl esters and halides, and (3) alkylidenecyclopropanes. Thermal reactions, especially intramolecular ones, effect the cycloaddition of the pyrazoles to olefins-especially electron-poor olefins. The latter two precursors require a catalyst, notably a palladium complex, to unlock the synthetic potential. With the 2-[(trimethylsilyl)methyl]allyl esters, a general chemo-, regio-, and diastereoselective cycloaddition with typical Diels-Alder dienophiles forms methylenecyclopentanes in both inter- and intramolecular fashion. A tin analogue allows extension of this cycloaddition to aldehydes, and, in a related sequence, to ketones and imines to form 3-methylenete-trahydrofurans and 3-methylenepyrrolidines with excellent diastereoselectivity. The 2-[(tri-methylsilyl)methyl]allyl esters also serve to effect net methylenecyclopentane annulation to cyclic enols. Such adducts can further undergo three-carbon intercalation or combined ring contraction-spiroannulation simply by the use of either a nucleophilic or electrophilic trigger. Alkylidenecyclopropanes undergo cycloadditions to both electron-rich and electron-poor olefins when stimulated by metal catalysts, especially palladium. A complementary regiochemistry to the approach based upon silicon conjunctive reagents is observed. The availability of these cycloadditions provides new strategies for the synthesis of natural products.
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  • 57
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    Angewandte Chemie International Edition in English 25 (1986), S. 683-701 
    ISSN: 0570-0833
    Keywords: Electroorganic synthesis ; Electrochemistry ; Synthetic methods ; Redox chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electrochemical formation and regeneration of redox agents for organic syntheses (indirect electrolysis) widens the potential of electrochemistry, as higher or totally different selectivities can often be obtained while at the same time the energy input can be lowered significantly. Higher current densities can also be obtained by preventing otherwise often encountered electrode inhibition. New types of redox catalysts can be formed in-situ and can be regenerated after reaction with the substrates. This principle is of increasing importance also for the application of already known redox agents with regard to environmental protection, since large amounts of a product can be generated in a closed circuit using only relatively small amounts of the redox reagent. Consequently the operation of such a process can be greatly simplified, and the release of ecologically objectionable spent reagents into the environment can be prevented. The broad spectrum of redox catalysts currently in use includes, inter alia, metal salts in very low or high oxidation states, halogens in various oxidation states, and, in particular, a wide variety of transition-metal complexes. A great deal of progress has recently been made in the application of organic electron transfer agents, since compounds have been found that are sufficiently stable in both the reduced as well as the oxidized state.
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  • 58
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    Angewandte Chemie International Edition in English 25 (1986), S. 790-807 
    ISSN: 0570-0833
    Keywords: Anthracyclinones ; Synthetic methods ; Antibiotics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The remarkable antitumor properties of anthracycline antibiotics have motivated synthetic chemists, and a large number of groups have been active recently in this field. Initially, there was very little hope for this class of substances due to their extreme toxicity. However, the dividing line between toxicity and desired pharmacological effect has broadened to such an extent, partly through microbiological methods (new strains, mutants) and partly through semisynthetic modifications, that today anthracyclines take number one place (over 20% of the market) in the chemotherapy of (human) cancer. The demands made on modern cytostatica are extremely high, as may be seen from the fact that of the more than 600000 drugs tested in animals, only about 40 are in clinical use. In the meantime, some anthracyclines obtained by total synthesis are also in clinical testing. In this review it will hopefully be demonstrated that all those concerned in the chemotherapeutic use of these substances have also contributed to the general progress made in the solution of regio- and stereo-chemical problems of linearly annelated ring systems.
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  • 59
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    Angewandte Chemie International Edition in English 25 (1986), S. 947-959 
    ISSN: 0570-0833
    Keywords: Lewis acids ; Synthetic methods ; Selectivity ; Organocopper compounds ; Addition ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Organometallic compounds with pronounced nucleophilicity such as organocopper and organolithium compounds, and powerful electrophiles, such as BF3 and AlCl3, are generally thought to be incompatible under normal, homogeneous reaction conditions. As a matter of course, it is anticipated that the two species cannot coexist and undergo transmetalation reactions. To our surprise, however, RCu and BF3 are compatible at low temperature. RCu·BF3 and related organocopper-Lewis acid reagents exhibit new and unique reactivities and selectivities: in conjugate additions, e.g., not only activation but also high regio-, stereo-, and chemo-selectivity as well as very high asymmetric induction is observed; allylic derivatives are alkylated regioselectively, and reaction towards carbonyl groups, imines, epoxides, aziridines, ethers, acetals, orthoformates, and pyridinium salts is in each case found to be selective. RLi·BF3 and related organolithium-Lewis acid reagents also exhibit noteworthy selectivities and reactivities. These complex reagents are utilized in the key steps of the total synthesis of many important natural products, such as certain terpenes, steroids, and alkaloids.
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  • 60
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    Angewandte Chemie International Edition in English 24 (1985), S. 539-553 
    ISSN: 0570-0833
    Keywords: Chirality ; Reduction ; Synthetic methods ; Enzymes ; Biotechnology ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It has been known for many decades that chiral compounds can be obtained by stereospecific biocatalytic reduction. Further significant methodological developments in this field have, however, only been made during the past ten years; they include the application of previously unused microorganisms and electron donors, the discovery of additional substrates for the known reductases, the development of methods for regenerating reduced pyridine nucleotides, and the discovery of new reductases which were sought for specific preparative purposes. Many chiral compounds can now be synthesized by microbial hydrogenation using H2 and hydrogenase-containing microorganisms as well as by electromicrobial or electroenzymatic reduction. In the two latter methods, anaerobic or aerobic organisms are supplied with electrons from electrochemically reduced, artificial mediators, e.g., methyl viologen. Reductases that do not require pyridine nucleotides and can accept electrons directly from reduced viologens are especially useful. Two examples of this type of enzyme are described which are of preparative interest. Many cells contain methyl viologen-dependent NAD(P) reductases, a large number of which have still not been characterized. A productivity number is proposed which allows different methods of bioconversion with microorganisms to be compared. The productivity numbers of compounds synthesized by the methods described in this review are often 10- to 100-fold higher than those of substances obtained by conventional techniques.
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  • 61
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    Angewandte Chemie International Edition in English 24 (1985), S. 553-565 
    ISSN: 0570-0833
    Keywords: Free radicals ; Organomercury compounds ; Synthetic methods ; Organotin compounds ; C-C coupling ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: C—C bond formation is one of the most important synthetic steps in the construction of organic molecules. In the last few years it has been increasingly achieved by radical addition to alkenes. In such reactions the adduct radicals have to be trapped by an donor subsequent to the C—C bond formation in order to prevent polymerization. This task can be accomplished with organotin and organomercury hydrides, the use of which has led to new synthetic methods. The occurrence of radical chain reactions in which reactions take place between radicals and nonradicals is decisive for the success of the synthesis. In these cases small amounts of radical initiators suffice and numerous functional groups may be used in the C—C bond-forming reactions. The yields and selectivities of these radical reactions are often very high.
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  • 62
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    Angewandte Chemie International Edition in English 24 (1985), S. 617-638 
    ISSN: 0570-0833
    Keywords: Enzyme catalysis ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enzymes have great potential as catalysts for use in synthetic organic chemistry. Applications of enzymes in synthesis have so far been limited to a relatively small number of largescale hydrolytic processes used in industry, and to a large number of small-scale syntheses of materials used in analytical procedures and in research. Changes in the technology for production of enzymes (in part attributable to improved methods from classical microbiology, and in part to the promise of genetic engineering) and for their stabilization and manipulation now make these catalysts practical for wider use in large-scale synthetic organic chemistry. This paper reviews the status of the rapidly developing field of enzyme-catalyzed organic synthesis, and outlines both present opportunities and probable future developments in this field.
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  • 63
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    Angewandte Chemie International Edition in English 24 (1985), S. 668-682 
    ISSN: 0570-0833
    Keywords: Organoaluminum compounds ; Synthetic methods ; Selectivity ; Beckmann rearrangement ; Rearrangement ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In addition to their high oxyphilicity, organoaluminum compounds are endowed with ambiphilic character. These properties can be successfully utilized in developing new synthetic reactions with unique selectivities. Especially noteworthy are the organoaluminum-promoted Beckmann rearrangement of oxime sulfonates, new syntheses of polyamino macrocycles via reductive cleavage of aminals and amidines by diisobutylaluminum hydride, the diastereoselective cleavage of chiral acetals by organoaluminum compounds leading to optically active secondary alcohols, allylic alcohols, and β-substituted carbonyl compounds, and biomimetic terpene syntheses. These and other examples, which illustrate the characteristics of organoaluminum chemistry, are used to demonstrate the distinct advantages of organoaluminum reagents in selective organic synthesis.
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  • 64
    ISSN: 0570-0833
    Keywords: Quinones ; Quinones ; Michael addition ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: If catechol or other hydroquinones are dehydrogenated in the presence of a nucleophilic reagent, the latter reacts, usually by Michael addition, with the quinone formed in situ. The wide range of possible variations offered by this synthetic method is comprehensively described in the present paper. procedures are given for carrying out typical reactions.
    Type of Medium: Electronic Resource
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  • 65
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 3 (1964), S. 250-260 
    ISSN: 0570-0833
    Keywords: Unsaturated fatty acids ; Fatty acids ; Carbonyl olefination ; Synthetic methods ; Wittig reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that carbonyl olefination, i.e. the reaction of carbonyl compounds with alkylidene phosphoranes, can be sterically controlled. In some cases cis-olefins can be obtained by proper choice of the reaction conditions; trans-olefins are formed only in the presence of suitable structural factors. These cis-and trans-olefination reactions have opened new, stereo-specific routes to unsaturated and polyunsaturated fatty acids with differing positions and sequences of cis- and trans-double bonds and to ω-hydroxy and branched chain fatty acids.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
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  • 66
    ISSN: 0570-0833
    Keywords: Cyclization ; Dialdehydes ; Nitromethane ; Carbohydrates ; Polyalcohols ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Condensation of nitromethane with suitable diadehydes in alkaline medium provides a general method of cyclization, in which the methyl group of the nitromethane is incorporated into the ring. This method leads to 5-, 6-, and 7-membered rings and is equally applicable to aliphatic, aromatic, and sugar dialdehydes. For example, glyoxal is converted into 1,4-dideoxy-1,4-dinitro-neo-inositol, and glutaraldehyde into trans-2-nitrocyclohexane-1,3-diol, while the corresponding cyclization of xylo-trihydroxyglutaraldehyde leads to deoxy-nitroinositols having the scyllo, myo-1, and muco-3 configurations.-In the case of aromatic dialdehydes, the cyclization is accompanied by elimination of water. Thus, phthalaldehyde, naphthalene-2,3-dicarboxaldehyde, and homophthalaldehyde yield, respectively, 2-nitroindenol, 2-nitrobenzindenol, and 2-nitronaphthalene. -Application of the method to sugar dialdehydes (aldehydic diglycol derivatives of monosaccharides formed by periodate oxidation) constitutes an excellent synthesis of 3-amino sugars, since 3-deoxy-3-nitropyranoses are formed smoothly on cyclization, and the corresponding 3-amino derivatives are obtained by hydrogenation. Thus, the reaction sequence: periodate oxidation → cyclization with nitromethane → hydrogenation, leads in the case of α-and β-D-pentosides to 3-amino-3-deoxy-D-and-L-pentosides, respectively, with ribo, xylo, and arabino configurations. α-D-hexosides give derivatives with gluco, manno, and galacto configurations. 3-Amino-3,6-dideoxyglucosides of the D-and L-series are obtained from 6-deoxy-D-or-L-hexosides, respectively, and 3-aminohexosans with gulo, ido, and altro configurations are obtained from 1,6-anhydro sugars. Cyclization of the dialdehydes obtained from sedoheptulose and methyl 4,6-O-ethylidene-α-D-glucoside by periodate oxidation, leads to 3-nitro and, after hydrogenation, to 3-amino derivatives of 3-deoxyheptopyranoses.
    Type of Medium: Electronic Resource
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  • 67
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 2 (1963), S. 24-29 
    ISSN: 0570-0833
    Keywords: 3-Aminoquinazol-4-one ; 3-Aminobenzotriazin-4-one ; Quinazolones ; Benzotriazinones ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Among the derivatives of 3-aminoquinazol-4-one and 3-aminobenzotriazin-4-one are compounds with narcotic, sedative, analgetic, and muscle-relaxant properties. Of special interest are compounds with one or several urethane groups and those substituted in the 3-position by a morpholine moiety.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
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  • 68
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 2 (1963), S. 225-238 
    ISSN: 0570-0833
    Keywords: Pyridinium salts ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This review discusses syntheses leading to a new type of N-vinylpyridinium salts, to novel anionocyanines, to aminophenazines and alloxazines, as well as to (aza-) dehydroquinolizinium, acridizinium, and morphanthridizinium salts. In addition, Mannich reactions and aminoalkylations at active methylene groups of cycloimmonium salts, and their reactions with amines are treated. The possibilities offered by the “Ortoleva-King” reaction are demonstrated using numerous examples, such as a new route leading to the yohimbine skeleton.
    Type of Medium: Electronic Resource
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  • 69
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 2 (1963), S. 309-323 
    ISSN: 0570-0833
    Keywords: s-Triazines ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preparation and properties of s-triazine are discussed. Nucleophilic reagents (even water) readily attack s-triazine. Thus, open-chain and heterocyclic compounds, e.g. formamidines, aminomethylene compounds, and triazole, pyridine, and pyrimidine deravatives are accessible from s-triazine. With amidines and imido ethers substituted triazines are obtained (by trans-triazination).The latter are formed via open-chain intermediates.
    Additional Material: 11 Tab.
    Type of Medium: Electronic Resource
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  • 70
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 2 (1963), S. 380-393 
    ISSN: 0570-0833
    Keywords: Pyridinium salts ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cleavage of N-phenylpyridinium or N-vinylpyridinium salts with secondary aliphatic amines leads to aromatic amino compounds or enamines which are often unobtainable by other routes. The other fragment, glutacondialdehyde or its monoanil, gives azulenes, as shown by Ziegler and Hafner, as well as König; these compounds can also be prepared from alkylpyridinium salts with cyclopentadienylsodium (Hafner). Syntheses of labile aldehydes which are otherwise difficult to obtain and of α-ketocarboxylic acids, starting from pyridinium salts and proceeding via nitrones or α-cyanoanils, are illustrated by many new examples. Benzimidazoles, benzothiazoles, and amidines are accessible from pyridinium salts via nitrones or α-cyanoanils; cyanoanils can be transformed into quinoxalines (including xanthopterine), perimidines, etc. Furthermore, nitrosonaphthols and nitrosonaphthylamines react with pyridinium salts containing an active methylene group on the nitrogen, leading to new ring systems. Further synthetic possibilities arise from nucleophilic displacements of the pyridine in pyridinium salts by SN 2 reactions. Combination of the route via nitrones with the methods of Arndt-Eistert, Ortoleva-King, or with chloromethylation constitutes a significant enlargement of its range of application. The formation of α-cyanoanils from diazoketones permits synthesis of α-keto-β-amino acids from α-amino acids, or of phenylglyoxylic acids from benzoic acids by chain extension.
    Type of Medium: Electronic Resource
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  • 71
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 1 (1962), S. 8-21 
    ISSN: 0570-0833
    Keywords: Addition ; Immonium ions ; Isocyanides ; Amino acids ; Synthetic methods ; Alkylidene ammonium ions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The α-addition of immonium ions and anions (OH-, SeH-, S2O3 2-, N3- NCO- NCS-, R—CO2 -, RO—CO2-) to isonitriles, accompanied by secondary reactions provides a means for the one-stage synthesis of organic nitrogen compounds starting with two to five different components. Thus, by the condensations of amines (ammonia, primary, and secondary aliphatic and aromatic amines, hydrazines) and aldehydes or ketones with isonitriles and acids, a number of α-aminocarboxylic acid amides, thioamides, selenoamides, 1,5-disubstituted retrazoles, hydantoin imides, thiohydantoin imides, α-acylamino carboxylic acid amides, oligopeptide derivatives, β-lactams, derivatives of penicillanic acid, urethanes, diacylimides, and various hydrazine derivatives, can be prepared. The reactions are easily carried out and take place under mild conditions. Yields of more than 90% are frequently encountered.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
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  • 72
    ISSN: 0570-0833
    Keywords: Amides ; Azolides ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 73
    ISSN: 0570-0833
    Keywords: Michael addition ; Pyridinium salts ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: By Michael addition of the active methylene groups in pyridinium salts onto suitable acceptor compounds, α-pyridones, substituted pyridines, particularly pyridinecarboxylic acids and pyridylpyridines, including the minor alkaloid of tobacco nicotelline and annelized pyridines, can be prepared by a simple procedure and generally in good yields. From the Michael adducts, polycyclic aromatic hydrocarbons, e.g. substituted fluoranthenes and “bisfluoranthenes” can be prepared; internal Michael addition leads to pyrrolinopyrdinium salts. For example, Michael addition of pyridinium salts onto quinones gives phenacyl substituted quinones, from which benzofurans and cinnolines can be readily obtained.
    Type of Medium: Electronic Resource
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