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  • 1
    Electronic Resource
    Electronic Resource
    Addition of Chiral Glycine, Methionine, and Vinylglycine Enolate Derivatives to Aldehydes and Ketones in the Preparation of Enantiomerically Pure α-Amino-β-Hydroxy Acids (1987)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 70 (1987), S. 237-261 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chiral enolates of imidazolidinones and oxazolidinones from the title amino acids react with carbonyl compounds to afford the corresponding alcohols in excellent yields (see Scheme 5). Furthermore, the addition to aldehydes proceeds with high diastereoselectivity to give, after acid hydrolysis, threo-α-amino-β-hydroxy acids of high enantiomeric purity. Some of the threo-α-amino-β-hydroxy acids prepared in this work are the proteinogenic (S)-threonine (26), the naturally occurring (S)-3-phenylserine (28), and (S)-3-hydroxyleucine (27) as well as the unnatural (S)-4,4,4-trifluorothreonine (30) and (S)-3-(4-pyridyl)serine (31). The N-methylamide of (2S,3R,4R,6E)-3-hydroxy-4-methyl-2-(methylamino)-6-octenoic acid (32), the unique amino acid in the immunosuppressive cyclosporine, was prepared by the new method. This report presents also information suggesting that both steric and stereoelectronic effects are responsible for the good stereoselectivities observed.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19870700129
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  • 2
    Electronic Resource
    Electronic Resource
    Structure and Reactivity of Five- and Six-Ring N, N-, N, O-, and O, O-acetals: A lesson in allylic 1, 3-strain (A1, 3 strain) (1992)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 75 (1992), S. 913-934 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The X-ray structures of fifteen 1, 3-imidazolidine, 1, 3-oxazolidine, 1, 3-dioxan-4-one, and hydropyrimidine-4(1H)-one derivatives are described (Table 2) and compared with known structures of similar compounds (Figs. 1-20). The differences between structures containing exocyclic N-acyl groups and those lacking this structural element arise from the A1,3 effect of the amide moieties. Even t-Bu groups are forced into axial positions of six-ring half-chair or into flag-pole positions of six-ring twist-boat conformers by this effect (Figs. 16-20). In the N-acylated five-membered heterocycles, a combination of ring strain and A1, 3 strain leads to strong pyramidalizations of the amide N-atoms (Table 1) such that the acyl groups wind up on one side and the other substituents on the opposite side of the rings (Figs. 4-9 and Scheme 3). Thus, the acyl (protecting!) groups strongly contribute to the steric bias between the two faces of the rings. Observed, at first glance surprizing stereoselectivities of reactions of these heterocycles (Schemes 1 and 2) are interpreted (Scheme 3) as an indirect consequence of the amide A1, 3 strain effect. The conclusions drawn are considered relvant for a better understanding of the ever increasing role which amide groups play in stereoselective syntheses.
    Additional Material: 22 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19920750326
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  • 3
    Electronic Resource
    Electronic Resource
    Herstellung chiraler elektrophiler Glycin- und (E)-2,3-Dehydroaminosäure-Derivate aus 2-t-Butyl-3-methyl-4-oxo-1-imidazolidincarbonsäure-t-butylester (Boc-BMI) (1991)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1991 (1991), S. 655-668 
    Details
    ISSN: 0170-2041
    Keywords: Imidazolidinones, bromination of ; Michaelis-Arbusov reaction ; α-Aminoacrylic acid derivatives, chiral, nonracemic ; Horner olefination with DBU/LiBr ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Preparation of Chiral Electrophilic Glycine and (E)-2,3-Dehydroamino Acid Derivatives from t-Butyl 2-t-Butyl-3-methyl-4-oxo-1-imidazolidinecarboxylate (Boc-BMI)Boc-BMI, the compound specified in the title, is brominated under radical-chain conditions (NBS/AIBN) to give trans-bromo-BMI 2, which is highly reactive in nucleophilic substitutions [see the 5-carbo- and -hetero-substituted (N, O, S, P) 2-t-butyl-3-methylimidazolidinones 4 - 23 and Tab. 1]. The substitutions occur preferentially with inversion of the configuration, the mechanism being not clear at all. - The Boc-BMI dimethyl phosphonate 23 (Michaelis-Arbusov product from 2) reacts with aldehydes (15 examples) to give preferentially (E)-5-alkylidene-imidazolidinones, chiral α-aminoacrylic acid derivatives which are available in either the R or the S form, see 24 - 38 (Tab. 2). Best yields and highest (E) selectivities (〉 25:1), with short reaction times, are obtained by using 1,8-diazabicyclo[5.4.0]undec-7-ene. (DBU)/LiBr in dry THF in a very special variant of the Horner olefination. α-Branched, racemic aldehydes may give alkylidene derivatives with efficient kinetic resolution (38). Some speculations are made about the mechanism and the observed stereochemical outcome of the reaction (I, J, K). A comparison is made with other chiral electrophilic glycine (A - C) and α,β-dehydroamino acid derivatives (D, E), as well as with other methods for the preparation of enantiomerically pure methylenedioxolan-, -oxazolidin-, and -imidazolidinones (H), containing an α,β-unsaturated carbonyl system.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.1991199101118
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  • 4
    Electronic Resource
    Electronic Resource
    Stereoselektive Umsetzungen von rac-, (R), oder (S)-5-Alkyliden-2-t-butyl-3-methyl-4-oxo-1-imidazolidincarbonsäure-t-butylestern (chirale 2,3-Dehydroaminosäure-Derivate) und Herstellung einiger nichtproteinogener Aminosäuren (1991)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1991 (1991), S. 669-684 
    Details
    ISSN: 0170-2041
    Keywords: Amino acids, dehydro-, chiral ; Amino acids, nonproteinogenic ; Michael additions of cuprates ; a5-Reactivity of doubly unsaturated carbonyl compounds ; Kinetic d2-versus thermodynamic d4-reactivity of Li dienolates ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselective Conversions of t-Butyl rac-, (R)-, or (S)-5-Alkylidene-2-t-butyl-3-methyl-4-oxo-1-imidazolidinecarboxylates (Chiral 2,3-Dehydroamino Acid Derivatives) and Preparation of Some Nonproteinogenic Amino AcidsThe reacivity of the alkylidene derivatives 1-11 specified in the title and prepared in two operational steps from commercially available Boc-BMI and aldehydes is investigated. - According to high-field 1H-NMR analysis of the crude products, additions of H—H and D—D (catalytic hydrogenation, products 12-20, of R—H (BF3-activated R2CuLi/LiI and protonation, products 21-29), and of Cl2C (CHCl3/NaOH, products 33, 34) occur with complete selectivity from the face opposite to the t-butyl group. The doubly unsaturated carbonyl derivative 11 reacts with dibutyl cuprate in the δ position, again with formation of a single diastereomer 32, while trifluoroethylidene-Boc-BMI 3a is reduced to the difluoro analogue 30 by this cuprate. - Li dienolates are generated by deprotonation with LDA of the ethylidene (2a) and butylidene compound 4a; subsequent alkylations (with primary and secondary halides, products 35-41) and hydroxyalkylations (with aldehydes, products 42-44) lead to single products of electrophilic attack in the a carbonyl position (C-5 of the imidazolidinone ring) under kinetic control. On the other hand, reaction of the 2a-derived dienolate with benzaldehyde under equilibrating conditions furnishes the four possible (E/R, E/S, Z/R, and Z/S) γ adducts 45-48. - A combination of methods - 1H- and 13C-NMR spectroscopy, NOE measurements, spectroscopic analogies, chemical correlation (with authentic samples or by spectroscopic or optical comparison), and X-ray analysis (Table 1, Figure 1, Schemes 1 and 2) - is used to assign the product configurations and thus the stereochemical course of the reactions. Some unusual amino acids (50-52, 54, 55) are prepared by hydrolysis of the corresponding imidazolidinones.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.1991199101119
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