An increase in enantioselectivity was observed in the asymmetric protonation of prochiral enolates with a chiral imide using lithium salt as an additive. For example, (R)-enriched 2-n-pentylcyclopentanone 6 was obtained in high yield with 90% ee when the silyl enol ether 4 was treated with n-BuLi in the presence of 5 equiv of LiBr in Et2O and the resulting lithium enolate 5 was then protonated by a solution of (S,S)-imide 1 in THF. In contrast, the product 6 obtained without LiBr exhibited a lower enantiomeric excess (74% ee)
Organocatalyzed Enantioselective Protonation of Silyl Enol Ethers: Scope, Limitations, and Application to the Preparation of Enantioenriched Homoisoflavones
In the present work, enantioselective protonation of silylenolethers is reported by means of a variety of chiral nitrogen bases as catalysts, mainly derived from cinchonaalkaloids, in the presence of various protic nucleophiles as proton source. A detailed study of the most relevant reaction parameters is disclosed allowing high enantioselectivities of up to 92% ee with excellent yields to be achieved
The reaction of lithium enolates of α-amino acid derivatives with chiral amides, easily synthesized from L-tert-leucine, gives corresponding optically active unnatural α-amino acid derivatives with up to 87% ee.
The asymmetric protonation of silylenolates derived from tetralone, benzosuberone, and cyclohexanone has been successfully achieved by using simple and original betaine catalysts derived from Cinchona alkaloids (quinine and quinidine series) to afford the desired α-substituted ketones in high yields and moderate enantioselectivities. The ease of implementation of this approach along with the easy
Enantioselective deprotonation of two racemic cyclic carbonyl compounds by a chiral lithium amide
作者:M.B. Eleveld、H. Hogeveen
DOI:10.1016/s0040-4039(00)84059-9
日期:1986.1
cyclic carbonyl compounds and have been obtained in optical active form (o. y. 46% and 36%, respectively) from the racemic compounds by a deprotonation/protonation sequence using chirallithiumamide . The optical activity of is caused by a difference in the deprotonation rates of the enantiomers.