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Asymmetric Michael Additions to Nitroalkenes

“…Also an oxazoline-containing amino acid ester was prepared by transformation of the nitro group into a nitrile oxide, followed by a dipolar cycloaddition and hydrolysis. Scheme 14 Seebach and co-workers [28] reported on the asymmetric Michael addition of enolates derived from acyloxazolidinones to various aliphatic and aromatic nitro olefins, and the application of this reaction as the key step in the synthesis of differently substituted pharmacologically active γ-lactams and γ-amino acid derivatives [29] (Scheme 15). The best results concerning the stereoselectivity were obtained using Ti enolates, prepared by treating 46 with TiCl 4 and Hünig's base.…”

“…Unfortunately, partial racemization occurred at this stage but the enantiomeric excess of the final products could be increased through recrystallization. Scheme 15 A very interesting extension of this method involves the addition of the oxazolidinones 46a to 1,2-disubstituted nitroethene derivatives [28] to generate 47a and thus three stereogenic centers in a single step. In this process two stereocenters are formed by differentiation between the Re and Si faces of the enolate and the nitroalkene and the third one by diastereoselective protonation of the nitronate intermediate.…”

Determination of Enantiomer Purity ofβ- andγ-Amino Acids by NMR Analysis of Diastereoisomeric Palladium Complexes

“…b 2,3 -Amino acids are accessible by a-alkylation of enantiomerically pure b 3 -amino acids through doubly lithiated intermediates, a method developed by us more than a decade ago [3] [16] [17]. gAmino acids have been prepared by double homologation of a-amino acids [8] [18] or by addition of Evanstype enolates to nitro olefins, and hydrogenation [10]. g 2 -amino acids) 4 ), so that we expected that their NMR spectra would show sizeable chemical-shift differences for diastereoisomeric complexes.…”

Crystal Structures– AManifesto for the Superiority of the Valine-Derived 5,5-Diphenyloxazolidinone as an Auxiliary in Enantioselective Organic Synthesis

“…10 ) For other diphenyl-oxazolidinones of this type, derived from phenylalanine, phenylglycine, and tert-leucine, see, e.g., [ 1 Ph groups introduced by reaction of (S)-or (R)-valine ester with commercial phenyl Grignard reagent geminal diphenyl substitution leads to high crystallinity of all derivatives -DIOZ itself (m.p. 251°) precipitates from reaction mixtures, is filtered, washed, dried, and reused Ph group in cis position to the i-Pr group has a buttressing effect, fixing the conformation around the CH-CHMe 2 bond to mimic a t-Bu group's effect upon substituents on the N-atom nucleophile attack on the C=O group (Bürgi-Dunitz trajectory) is sterically blocked by a Ph group on one face and by an i-Pr group on the other face of the ring plane deprotonation on the N-atom or on groups attached to it can be achieved directly with BuLi, due to steric protection of the C=O group, to give amine-and salt-free solutions of Li derivatives; low temperatures are not always required the exocyclic C=O group is much more reactive than the endocyclic one, which allows for simple cleavage of N-acyl groups become evident only recently, as we [9 ± 18] 11 ) and others [24 ± 28] have employed it in an ever increasing number of different types of reactions 12 ). Some of the most prominent features of the oxazolidinone A and its derivatives B and C are listed in Fig.…”