ASYMMETRIC ALDOL REACTION BETWEEN ACHIRAL SILYL ENOL ETHERS AND ACHIRAL ALDEHYDES BY USE OF A CHIRAL PROMOTER SYSTEM

In the presence of a chiral promoter consisting of tin(II) triflate, a chiral diamine, and tributyltin fluoride, the silyl enol ether of S-ethyl ethanethioate or S-tert-butyl ethanethioate reacts with aldehydes to afford the corresponding aldol-type adducts in good yields with high enantioselectivities. In the reaction of silyl enol ether of S-ethyl propanethioate with aldehydes, perfect stereochemical control is attained by the combined use of tin(II) triflate, a chiral diamine, and dibutyltin diacetate. A wide variety of aldehydes, including aliphatic, alpha,beta-unsaturated, and aromatic aldehydes, are applicable to this reaction. The chiral promoters are characterized by Sn-119 NMR spectra and proved to form the three-component complexes, which behave as chiral Lewis acids. The reaction proceeds directly from silyl enol ethers without any accompanying metal exchange on the enolates.