Highly Enantioenriched Homoenolate Reagents by Asymmetric γ-Deprotonation of Achiral 1-Silyl-Substituted 1-Alkenyl Carbamates

Abstract

1-trimethylsilyl-1-alkenyl carbamates 1 are deprotonated by n-butyllithium/(-)-sparteine (2) with a high degree of enantiotopic differentiation in the gamma-position to form the enantiomerically enriched allyllithium derivatives 3. Trapping these with several electrophiles proceeds stereospecifically in an anti-S(E)' or syn-S(E)' substitution to form products 4 or ent-4, respectively. Compounds 3a (R = Me) and 3b (R = Ph) exhibit toward carbonyl electrophiles opposite senses of almost complete stereospecificity, thus for 3b.2 the involvement of a eta(3)-complex is suggested. [reaction: see text]

Asymmetric γ‐Deprotonation and Substitution Reactions of (Z)‐1,3‐Diphenyl‐1‐propenyl N,N‐Diisopropylcarbamate

“…All carbonyl additions of aldehydes, ketones and acid chlorides onto lithium intermediate (S)-7 proceed as highly γ-selective syn-S E Ј processes. This fact implies the involvement of a η 3 -ion pair 7B, since in all reactions, where an allylic η 3 -intermediate is likely to contribute, similar stereochemical characteristics had been recorded by Hoppe et al [6,12] and Beak et al [13,14] It seems that η 3 -allyllithium compounds (e.g. 7B) have a higher Lewis acidity than the η 1 -isomers (e.g.…”

“…Excess Ti(OiPr) 4 might retard the retro-homoaldol reaction by formation of the less reactive titanium alkoxide of 13. [6,12] and Beak et al [13,14] It seems that η 3 -allyllithium compounds (e.g. 7B) have a higher Lewis acidity than the η 1 -isomers (e.g.…”

“…[4] Previously, a surprising and efficient approach to highly enantioenriched 1,3-disubstituted (-)-sparteine complexes was found through γ-deprotonation of achiral 1-alkenyl carbamates 6a-c (Scheme 2). [5,6] Scheme 2.…”

“…[4] Previously, a surprising and efficient approach to highly enantioenriched 1,3-disubstituted (-)-sparteine complexes was found through γ-deprotonation of achiral 1-alkenyl carbamates 6a-c (Scheme 2). [5,6] In most cases, diastereomeric ratios of the carbanionic intermediates 7a-c:8a-c of greater than 95:5 were produced, as determined by trapping experiments. [5,6] The question if the 1,3-diphenyl-substituted 7d has considerable configurational stability, due to its high mesomeric stabilization, is now addressed.…”

“…[5,6] In most cases, diastereomeric ratios of the carbanionic intermediates 7a-c:8a-c of greater than 95:5 were produced, as determined by trapping experiments. [5,6] The question if the 1,3-diphenyl-substituted 7d has considerable configurational stability, due to its high mesomeric stabilization, is now addressed. This issue was questionable since the (-)-αisosparteine complex 9 (Figure 1) and the appropriate (-)sparteine complex, of which an X-ray structure analysis could be obtained, is configurationally labile even at -78°C.…”