EFFECTS OF LITHIUM-SALTS ON THE ENANTIOSELECTIVITY OF PROTONATION OF ENOLATES WITH CHIRAL IMIDE

An increase in enantioselectivity was observed in the asymmetric proto nation 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 tr eated with n-BuLi in the presence of 5 equiv of LiBr in Et2O and the r esulting lithium enolate 5 was then protonated by a solution of (S,S)- imide 1 in THF. In contrast, the product 6 obtained without LiBr exhib ited a lower enantiomeric excess (74% ee).