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  • 11
    Electronic Resource
    Electronic Resource
    Intramolekulare Cycloadditionen mit Nitriloxiden: Untersuchungen zur Synthese von Heterophanen (1989)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 122 (1989), S. 2147-2157 
    Details
    ISSN: 0009-2940
    Keywords: 1,3-Dipolar cycloadditions, intramolecular ; Nitrile oxides ; Regioselectivity ; Furanophanes ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Intramolecular Cycloadditions with Nitrile Oxides: Investigtions towards the Synthesis of HeterophanesThe intramolecular cycloaddition reactions of the nitrile oxides 30a-d, 35, 36, and 47, obtained in situ from the 2,5-difunctional furan hydroximoyl chlorides 12a, b, 18 and the nitro compounds 9c, d, 21, 29, are described. With 30c the expected isoxazolofuranophane of type 31 is formed in acceptable yield; the additionally formed regioisomer 32c and the dimer 34c are very minor products in this case. Whereas the highest homologue 30d gives a complex product mixture with small amounts of 31 d and the macrocycle 34d as the only identified compounds, the derivatives bearing a shorter side chain (30a, b; n = 1, 2) give rise to the exclusive reaction of the dipole with a double bond of the furan system (to give 33a, b). The same behavior is observed in the case of the electronically and sterically modified dipole systems 35/36 and 47, resp.; in spite of the much more favorable conditions for the formation of cyclophanes the cycloadditions result in the exclusive formation of the heterotricycles 41, 42 and 49. Possible preparative applications of the isomeric cycloaddition compounds are discussed.
    Notes: Die intramolekularen Cycloadditionsreaktionen der aus 2,5-difunktionellen Furan-Hydroximsäurechloriden (12a, b und 18) bzw. den Nitroverbindungen 9c, d, 21 und 29 in situ hergestellten Nitriloxide 30a-d, 35, 36 und 47 werden beschrieben. Eine Reaktion zu den angestrebten Isoxazolo-furanophanen des Typs 31 verläuft für 30c in brauchbarer Ausbeute; das Regioisomere 32c sowie das Dimere 34c treten hierbei nur in geringem Maße auf. Während das höchste Homologe 30d neben 31d und dem Makrocyclus 34d hauptsächlich Zersetzungsprodukte bildet, reagieren die Derivate mit kürzerer Seitenkette (30a, b; n = 1, 2) ausschließlich unter dipolarer Addition an eine Furandoppelbindung zu 33a, b. Für n = 1 entsprechen diesem Verhalten auch die elektronisch bzw. sterisch modifizierten Dipolsysteme 35/36 und 47, die trotz günstigerer Voraussetzungen für die Cyclophanbildung nur zu den Tricyclen 41, 42 bzw. 49 reagieren. Die präparativen Anwendungsmöglichkeiten der isomeren Cycloadditionsverbindungen als Synthesezwischenprodukte werden diskutiert.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19891221116
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  • 12
    Electronic Resource
    Electronic Resource
    Intramolekulare Cycloadditionen Mit Azomethin-Yliden: Synthese und NMR-Untersuchungen einiger Paracyclophan-Derivate vom Makrolid-Typ (1988)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 71 (1988), S. 404-418 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Intramolecular Cycloaddition Reactions with Azomethine Ylides: Synthesis and NMR Investigations of Some Macrolide-Type ParacyclophanesSeveral aziridine derivatives bearing a para-substituted phenyl group have been prepared as precursors of azomethine ylides for intramolecular 1,3-dipolar cycloadditions. Upon heating in refluxing toluene, the expected reactions take place resulting in the regioselective formations of the pairs of diastereoisomers 14/15 and 16/17, respectively. As indicated by the sharply decreasing yield in the a-series (〈 5%), a useful application of this method for preparing paracyclophane compounds is restricted to derivatives with more than 11 bridging atoms. The results of detailed 1H- and 13C-NMR investigations with the pyrrolidino-annulated cyclophanes are discussed with respect to the configuration at the five-membered heterocycles and to the conformational flexibility of the bridges.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19880710214
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  • 13
    Electronic Resource
    Electronic Resource
    Intramolekulare Cycloadditionen mit Carbonyl-Yliden: Von Oxiranen zu großen Ringen (1984)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 117 (1984), S. 2157-2192 
    Details
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Intramolecular Cycloadditions with Carbonyl Ylides: From Oxiranes to Large Ring SystemsWithin the classes of 1,3-dipoles intramolecular cycloaddition reactions with carbonyl ylides, accessible by heating of the oxirane precursors 4 - 10, are studied for the first time in a systematic manner. Although the formation of the corresponding fused and/or bridged tetrahydrofuran derivatives (32 - 34, 44 - 46, 50 - 63) has been established in all cases with the exception of 4e and 6a, the rate of formation as well as the yield of the cycloadducts depend strongly on the dipolarophilicity of the olefinic component: Whereas non-activated C = C bonds undergo the cycloaddition step slowly and only when being arranged in a relatively close position to the dipole unit (the maximum size of the second ring, formed by “tieing”: 9-membered), ene-oxiranes with an electron deficient double bond are reacting much faster and (in contrast to cycloadditions so far known for other 1,3-dipoles) also at a large distance between the π-centers. In this way macro-cycles with up to 34 ring members have been obtained. The high regio- and stereoselectivity observed in most cases of the cycloadditions is explained mainly by steric factors.
    Notes: Am Beispiel der aus den Oxiranvorstufen 4-10 durch Erhitzen zugänglichen Carbonyl-Ylide werden erstmals intramolekulare Cycloadditionsreaktionen dieser 1,3-Dipol-Klasse systematisch untersucht. Obwohl mit Ausnahme von 4e und 6a in allen Fällen die Bildung der entsprechenden anellierten und/oder überbrückten Tetrahydrofuran-Derivate (32 - 34, 44 - 46, 50 - 63) nachgewiesen wird, hängen die Bildungsgeschwindigkeit und die Ausbeute der Cycloaddukte ganz entscheidend von der dipolarophilen Qualität der Olefin-Komponente ab: Während nicht-aktivierte C = C-Bindungen diese Cycloadditionen nur langsam und bei relativ geringer Entfernung zu der Dipol-Einheit eingehen (maximale Größe des durch „Einschnürung“ entstandenen zweiten Ringes: 9gliedrig), reagieren En-Oxirane mit elektronenarmen Doppelbindungen deutlich schneller und - im Gegensatz zu bisher bekannten Cycloadditionen anderer 1,3-Dipole - auch noch bei großem Abstand der π-Zentren. Auf diese Weise ist es gelungen, Makrocyclen mit bis zu 34 Ringgliedern herzustellen. Die zumeist hohe Regio- und Stereoselektivität bei den intramolekularen Cycloadditionen wird vor allem auf sterische Faktoren zurückgeführt.
    Additional Material: 10 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19841170612
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  • 14
    Electronic Resource
    Electronic Resource
    Die stereoselektive Bildung von Polycyclen durch intramolekulare Cycloaddition von Carbonyl-Yliden an cyclische C=C-Bindungen (1983)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 116 (1983), S. 2383-2389 
    Details
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselective Formation of Polycyclic Systems by Intramolecular Cycloaddition of Carbonyl Ylides to Cyclic C—C-BondsIntramolecular cycloadditions of carbonyl ylides 2, generated by thermolysis of the ene-oxiranes 1, are investigated with special regard to the stereochemical course of the reaction. On heating up to 180°C the expected transformations of 1a-d take place leading predominantly to one cycloadduct in each case(structures 8-11), accompanied by a minor isomer; the ratio being 3:1, 25:1, 10:1 and 2:1 for the reactions of 1a, 1b, 1c and 1d, respectively. In the case of 1a two further products are formed to which the structures of 6 and 7 are tentatively ascribed. The rate increasing influence of the bridged ene-unit on the cyclisation step is explained in terms of entropic factors.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19831160629
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  • 15
    Electronic Resource
    Electronic Resource
    Photochemische Umwandlungen, 64. Die 3σ → 3π-Route zu Oxepinen/Benzoloxiden (1986)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 119 (1986), S. 589-615 
    Details
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Photochemical Transformations, 64. The 3σ → 3π-Route to Oxepines/Benzene OxidesThe 7-oxanorbornadiene skeleton (8a), its 2,3-bis(trifluoromethyl) derivative (8b), and several 2,3-dicarboxylic esters with potential dipolarophilic groups at C-1 (16a - d, 17a - c) were synthesized and selectively transformed into the 3-oxaquadricyclanes (18a,b, 19a - d, and 20a - c) through sensitized (8a,b) or direct (16a - d, 17a - c) photoexcitation. In the thermolysis of these oxaquadricyclanes [18a(b): Ea(C6D6) = 32.6 ± 0.3 kcal/mol (32.2 ± 1.4 kcal/mol); lg A = 15.8 (14.5)] CF3(CH3CO2) substituents at C-2(3) favor the scission of the opposite (neighbouring) cyclopropane bonds. Competition experiments with dimethyl acetylenedicarboxylate as external dipolarophile support the stepwise mechanism of the oxaquadricyclane → oxepine conversion. The intramolecular carbonyl ylide-interception cannot compete with oxepine formation in 19a - d but is efficient in 20a - c. The unusually low activation barrier (Ea(C6D6) = 23.5 ± 1 kcal/mol; lg A = 11.6) and singular specifity in case of 20a is interpreted in terms of a concerted [π2 + σ2 + σ2]-cycloaddition to the bishomofuran unit of the oxaquadricyclane skeleton.
    Notes: Das 7-Oxanorbornadien-Grundgerüst (8a), das 2,3-Bis(trifluormethyl)-Derivat (8b) und diverse 2,3-Dicarbonester mit potentiell dipolarophilen Gruppen an C-1 (16a - d, 17a - c) wurden synthetisiert und durch sensibilisierte (8a,b) bzw. direkte Lichtanregung (16a - d, 17a - c) durchweg hochselektiv in die 3-Oxaquadricyclane (18a,b, 19a - d, 20a - c) umgewandelt. Bei der Thermolyse dieser Oxaquadricyclane [18a(b): Ea(C6D6) = 32.6 ± 0.3 kcal/mol (32.2 ± 1.4 kcal/mol); lg A = 15.8 (14.5)] begünstigen CF3(CH3CO2)-Reste an C-2(3) die Spaltung der gegenüberliegenden (benachbarten) Cyclopropanbindungen. Durch Konkurrenzversuche mit Acetylendicarbonsäure-dimethylester als externem Dipolarophil wird der Mehrstufen-Mechanismus der Oxaquadricyclan → Oxepin-Isomerisierung untermauert. Die intramolekulare Carbonylylid-Abfangreaktion kann in 19a - d nicht, wohl aber in 20a - c mit der Oxepinbildung konkurrieren. Deren im Falle von 20a außergewöhnlich niedrige Aktivierungsbarriere (Ea(C6D6) = 23.5 ± 1 kcal/mol; lg A = 11.6) und singuläre Spezifität werden im Sinne einer konzertierten [π2 + σ2 + σ2]-Cycloaddition an die Bishomofuran-Einheit des Oxaquadricyclan-Gerüstes interpretiert.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19861190222
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  • 16
    Electronic Resource
    Electronic Resource
    Photochemische Umwandlungen, 651). Die 3σ → 3π-Route zu 1H-Azepinen/Benzoliminen (1986)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 119 (1986), S. 616-644 
    Details
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Photochemical Transformations, 65. The 3σ → 3π-Route to 1H-Azepines/Benzene IminesWith several newly prepared substrates the influence of substituents upon the individual steps in the 3σ → 3π-route to 1H-azepines is more precisely defined: The C-unsubstituted 7-azanorbornadiene 2a, its 2,3-dichloro derivative 2b, the dimethyl 5,6-dichloro-2,3-dicarboxylate 2c, and the diesters 2d,e with dipolarophilic groups at C-1/N-7 are selectively isomerized by sensitized/direct photoexcitation into the azaquadricyclanes 29a - e, some of which are highly unstable. For the thermal conversion of the basic skeleton (N-Tos) 29a the kinetic parameters have been determined (benzene): Ea = 28.0 ± 0.2 kcal/mol, lg A = 15.7; ΔH≠ = 27.3 ± 0.2 kcal/mol, ΔS≠ = 11.1 ± 0.7 e.u. This barrier is lowered more efficiently by the chloro (29b,c) than by the methoxycarbonyl substituents (29f), with the former (latter) causing exclusive scission of the opposite (neighbouring) cyclopropane bonds. The intermediate azomethine ylides are captured with dipolarophilic reagents more or less efficiently depending on their substitution pattern. In the case of 29d(28d) the intramolecular addition of the unactivated yne component (37) at -30°C is so fast, that azepine formation is almost totally suppressed ([π2 + σ2 + σ2], 36?). The azepine/benzene imine equilibrium mixture 31c⇄32c (ca. 90:10) crystallizes as 31c (X-ray crystal structure analysis).
    Notes: Der Substituenteneinfluß auf die einzelnen Schritte der 3σ → 3π-Route zu 1H-Azepinen wird an Hand mehrerer neu synthetisierter Edukte näher definiert: Das C-unsubstituierte 7-Azanorbornadien 2a, das 2,3-Dichlorderivat 2b, der 5,6-Dichlor-2,3-dicarbonester 2c und die an C-1/N-7 mit einem dipolarophilen Rest versehenen 2,3-Dicarbonester 2d,e lassen sich durch sensibilisierte/direkte Lichtanregung mehr oder weniger einheitlich in die zum Teil hochlabilen Azaquadricyclane 29a - e isomerisieren. Für die thermische Umwandlung des Grundgerüstes (N-Tos) 29a werden die kinetischen Parameter bestimmt (Benzol): Ea = 28.0 ± 0.2 kcal/mol, lg A = 15.7; ΔH≠ = 27.3 ± 0.2 kcal/mol, ΔS≠ = 11.1 ± 0.7 e.u. Diese Barriere wird durch die Halogenreste in 29b(c) noch stärker als durch die Esterreste (29f) herabgesetzt - wobei die +M- im Gegensatz zu den - M-Resten ausschließlich die Spaltung der gegenüberliegenden Cyclopropanbindungen bewirken. Die intermediären Azomethinylide können je nach Substitution mit dipolarophilen Reagentien unterschiedlich gut abgefangen werden. Im Fall von 29d(28d) ist die intramolekulare Addition der nichtaktivierten In-Seitenkette (37) bei -30°C so rasch, daß Azepinbildung fast vollständig unterdrückt wird ([π2 + σ2 + σ2], 36?). Das Azepin/Benzolimin-Gleichgewichtsgemisch 31c⇄32c (ca. 90:10) kristallisiert als 31c (Röntgenstrukturanalyse).
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19861190223
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  • 17
    Electronic Resource
    Electronic Resource
    Die Dipol-Route zu Furo-/Thieno-Pyrrolen und α-Pyridonen, sowie deren Benzo-Derivaten (1994)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 127 (1994), S. 247-259 
    Details
    ISSN: 0009-2940
    Keywords: Pyrroles, heteroannulated ; α-Pyridones, heteroannulated ; Pentalenes, dihetero- ; Nitrones ; Heterocyclizations, 1,7-dipolar ; Diels-Alder reactions ; Singlet oxygen ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A Novel Route to Isoannulated Heteroaroamtic Compounds, 2[1].  -  The Dipole Route to Furo/Thieno Pyrroles and α-Pyridones, and Their Benzo DerivativesA new methodology involving 1,7-dipolar cyclizations of conjugated dipoles followed by a multistep rearrangement affording ring-contracted products has been employed for the synthesis of various examples of the title compounds. Under different thermolysis conditions the annulated alkynyl nitrones 15-19, 45, and 46 are converted into isoannulated pyrroles and α-pyridones, respectively, with yields up to 90% (general reaction A → B + C). The product distribution is strongly influenced by the nature of the terminal substituent R, hence supporting a mechanism with oxo carbenes as central intermediates which undergo either 6-π cyclization to the pyrrole systems or Wolff rearrangement leading to conjugated ketenes, the precursors of the observed α-pyridones (see Scheme 1). Diels-Alder reactions were performed with benzofuropyridone 40a, napthopyridone 62 as well as with the corresponding dihydronaphtho compounds 64a, b. Whereas the tert-butyl derivatives 22b, 23b and 41b are rather unreactive against dienophiles, they are easily transformed into the cyclic imides 31, 32 and 52, respectively, under the influence of oxygen and light suggesting the formation and subsequent [4 + 2] cycloaddition of 1O2 as important steps.
    Additional Material: 5 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19941270135
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  • 18
    Electronic Resource
    Electronic Resource
    Zur Reaktivität geometrisch fixierter Vinylaziridine: Synthese und Thermolyse einiger 3,4-Epiminocyclohexene (1983)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 116 (1983), S. 563-586 
    Details
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Reactivity of Geometrically Fixed Vinylaziridines: Synthesis and Thermolysis of Several 3,4-EpiminocyclohexenesThe aminoalcohols 13-16, readily accessible by aminolysis of the oxiranes 5 and 6, are transformed into the epiminocyclohexenes 17-20 using triphenylphosphane/tetrachloromethane as cyclisation reagent. While the N-benzyl-monocarboxylate 17 is thermally quite unstable and undergoes a homodienyl-H-shift to give the imine 23 already at 100°C, the N-phenyl derivative 18 reacts only at 370°C under short-time pyrolysis conditions leading to the isomeric pyrroles 24 and 25. The formation of the fragmentation products is explained by a competitive C/N- and C/C-ring cleavage as the first step. On heating the dimethyldicarboxylate-substituted vinylaziridines 19 and 20 at about 100°C ring expansion reactions take place affording the 2,3-dihydroazepines 32 and 34, respectively; in the case of 19 also the imine 33 is formed to a minor extent. As the mechanism of the transformations 19(20) → 32(34) a direct (1,5)-H-shift has to be excluded on the basis of H/D-exchange experiments. An alternative reaction sequence is proposed with the ester enol 35 as central intermediate, which may be formed either by a homotrienyl-H-shift from 19(20) or - after C/C-ring opening to the respective conjugated azomethine ylides - by (1,8)-H-migration. Trapping experiments with 19 and 20 point to the last mentioned possibility because the 1,3-dipolar-cycloadducts 47 and 48 are formed four times faster than the isomerisation products. Addition of dimethyl acetylenedicarboxylate to the monosubstituted epiminocyclohexenes 17/18 affords the homo-Diels-alder-adducts 44/45.
    Notes: Die durch Aminolyse der Oxirane 5 und 6 zugänglichen Aminoalkohole 13-16 werden unter Verwendung von Triphenylphosphan/Tetrachlormethan als Cyclisierungsreagenz in die Epiminocyclohexene 17-20 übergeführt. Während der N-Benzyl-monoester 17 thermisch sehr instabil ist und schon bei ca. 100°C durch Homodienyl-H-Verschiebung in das Imin 23 übergeht, reagiert der N-Phenyl-monoester 18 erst unter Kurzzeitthermolysebedingungen bei 370°C unter Bildung der Pyrrole 24 und 25. Die Entstehung der Fragmentierungsprodukte wird über eine einleitende kompetitive C/N- und C/C-Ringspaltung erklärt. Beim Erhitzen der diestersubstituierten Vinylaziridine 19 und 20 findet bei ca. 100°C eine Ringerweiterung zu den 2,3-Dihydroazepinen 32 bzw. 34 statt sowie im Falle von 19 noch zu geringem Anteil die Bildung des Imins 33. Als Mechanismus der Umwandlungen 19(20) → 32(34) ist eine direkte (1,5)-H-Wanderung auf Grund von H/D-Austauschexperimenten auszuschließen. Als Alternativen werden Reaktionswege über das Ester-Enol 35 diskutiert, das entweder über eine Homotrienyl-H-Verschiebung aus 19(20) entstanden ist oder nach einleitender C/C-Ringspaltung aus den entsprechenden konjugierten Azomethin-Yliden durch (1,8)-H-Wanderung. Zugunsten der letzteren Möglichkeit sprechen Abfangversuche, bei denen die 1,3-dipolaren Cycloaddukte 47 und 48 ca. viermal rascher als die Umlagerungsprodukte gebildet werden. Die Epiminocyclohexen-monoester 17, 18 reagieren mit Acetylendicarbonsäure-dimethylester unter Bildung der Homo-Diels-Alder-Addukte 44/45.
    Additional Material: 8 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19831160218
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  • 19
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    Electronic Resource
    Butadienyloxiran-Dihydrooxepin-Isomerisierung. Ringerweiterungsreaktionen von sterisch fixierten und benzoanellierten Epoxyhexadienen durch 1,7-Elektrocyclisierung konjugierter Carbonyl-Ylide (1985)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 118 (1985), S. 4035-4058 
    Details
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Butadienyloxirane-Dihydrooxepine Isomerisation. Ring Expansion Reactions of Sterically Fixed and Benzoannelated Epoxyhexadienes by 1,7-Electrocyclisation of Conjugated Carbonyl YlidesThermal ring cleavage of specifically substituted oxiranes results in the formation of conjugated carbonyl ylide intermediates which react by 1,5- and/or 1,7-electrocyclisation to give ring-expanded products. While the spirooxiranes 5t, 6t, and 12t are transformed, as expected, only into 2,3-dihydrofurans (21, 22, 50), the geometrically fixed compounds give rise to the formation of seven-membered ring isomers either partially (6c), or predominantly (5c), or exclusively (7 - 11, 12c). 1,7-Dipolar cyclisations also take place with participation of one or even two aromatic double bonds (7 → 52, 8 → 53, 9 → 54, 12c → 57, and 10 → 55, 11 → 56, resp.) affording mono- and dibenzo-dihydrooxepines, respectively. The rearrangements of 5c to 27 and 6c to 28 show for the first time that the 8π-ring closure of 2-oxaheptatrienyl dipoles proceeds in the theoretically predicted conrotatory manner. The exclusive, i.e. periselective formation of dihydrooxepines during the thermolysis of compounds 7 - 11, 12c is explained by assuming a helical geometry of the dipolar intermediates in which the stereoelectronic situation is specially suitable for the - entropically less advantageous - 1,7-cyclisation process.
    Notes: Durch thermische Ringöffnung von speziell substituierten Oxiran-Derivaten werden konjugierte Carbonyl-Ylid-Zwischenstufen erzeugt, die in 1,5-und/oder 1,7-Elektrocyclisierungsreaktionen zu ringerweiterten Produkten reagieren. Während aus den Spirooxiranen 5t, 6t und 12t erwartungsgemäß nur 2,3-Dihydrofurane entstehen (21, 22, 50), wandeln sich die sterisch partiell bzw. vollständig fixierten Verbindungen entweder teilweise (6c), ganz überwiegend (5c) oder ausschließlich (7 - 11, 12c) in Siebenringisomere um. 1,7-Dipolare Cyclisierungen finden auch unter Beteiligung von einer oder sogar zwei aromatischen C = C-Bindungen statt unter Bildung von Mono- oder Dibenzo-dihydrooxepinen (7 → 52, 8 → 53, 9 → 54, 12c → 57 bzw. 10 → 55, 11 → 56). Am Beispiel der Umwandlungen 5c → 27 und 6c → 28 konnte erstmals gezeigt werden, daß der 8π-Ringschluß von 2-Oxaheptatrienyl-Dipolen in Übereinstimmung mit der Theorie conrotatorisch erfolgt. Die exklusive, d.h. periselektive Bildung von Dihydrooxepinen bei der Thermolyse von 7 - 11 und 12c wird mit einer helikalen Geometrie der dipolaren Zwischenstufe gedeutet, welche nur für die - entropisch nachteiligere - 1,7-Cyclisierung günstige stereoelektronische Voraussetzungen bietet.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19851181016
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  • 20
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    Electronic Resource
    Ein neuer Weg zu isoanellierten Heteroaromaten, 1. Synthese und Reaktionen von Furofuranen, Thienofuranen, Furobenzofuranen und Benzothienofuranen (1993)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 126 (1993), S. 975-995 
    Details
    ISSN: 0009-2940
    Keywords: Pentalenes, dihetero- ; Cyclizations, 1,7-dipolar ; Carbonyl ylides ; Cycloallenes ; Diels-Alder reactions ; o1-Quinodimethanes, hetero analogues ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A Novel Route to Isoannulated Heteroaromatic Compounds, 1. - Synthesis and Reactions of Furofurans, Thienofurans, Furobenzofurans, and BenzothienofuransA general method for the synthesis of furo-and thienofurans 4-7 has been developed. The reaction principle is based on the thermal transformation of suitably structured epoxyhexenynes (15-19) following the general reaction sequence 1→2→3 (annulation type A). Derivatives of the so far unknown diheteropentalenes furo[3,4-b]furan (4b, c, d, g, h), furo-[3,4-b]benzofuran (6b, d, e, 45) as well as benzo[4,5]thieno-[2,3-c]furan (7b, d) are obtained by short-time thermolysis. Likewise two representatives of the previously reported thieno[2,3-c]furan system are prepared (5b, c). By flash vacuum thermolysis the benzo-annulated epoxyhexenyne 19b rearranges in poor yield to the isobenzofuran 52 (identified as dimethyl acetylenedicarboxylate adduct 47) and 2-cyano-α1-naphthol (46). The structure of the furo[3,4-b]furan 4b has been established by X-ray structural analysis. A mechanistic explanation of the transformation of the epoxyhexenynes to diheteropentalenes is proposed. Indications for the occurrence of carbenes as product-determining species are obtained with the phenylcyano-substituted oxiranes 15c, h and 17e which - in additon to the furofurans 4c, h and 6e - lead to the furylindenes 31c, h/32c, h and 41, resp. The Diels-Alder reactivity of the furo-and thienofurans 4g, 5b, c, and 6b has been examined.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19931260419
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