NOVEL-APPROACH TO REMOTE ASYMMETRIC INDUCTION IN CARBONYL ADDITION AND RELATED REACTIONS

Neighboring group participation is postulated to be responsible for high asymmetric induction in the addition of various nucleophiles (e.g., Me3SiCN, allylsilanes, and allylstannanes) to chiral alkoxy acetals and aldehydes. Thus, oxocarbenium ions generated by treatment of these substrates with various Lewis acids suffer intramolecular solvation by a neighboring alkoxy group. This establishes a conformationally defined, cyclic oxonium ion intermediate which can be attacked by the nucleophile, providing high diastereoselectivity in the process. The strategy employed is particularly effective for 1,4-asymmetric induction, and it has been extended to 1,2- and 1,3-asymmetric induction as well. The scope of the reaction with regard to the nature of the acetal and participating group was revealed through a series of reactions of appropriate gamma-alkoxy acetals with various nucleophiles under Lewis acid catalysis.