A Novel Configuration-Controlled Stereoselectivity in Enol Silylation of Ketones. Stereochemistry of Amine-Promoted Enol Silylation of Meso and Racemic α,α′-Dichloro Ketones and of Ketonization of the Resultant Enol Silyl Ethers

Abstract Stereochemistry in amine-promoted enol trimethylsilylation of meso- and dl-α,α′-dichloro ketones, RCHClCOCHClR (1a; R = Me and 1b; R = i-Pr), and in ketonization of the resultant enol silyl ethers (2a and 2b) has been studied. The Et3N-promoted silylation of 1 in benzene shows a marked diastereoselectivity; the racemic isomer exhibits 84% and 98% (E)-selectivities for 1a and 1b, while the meso isomer, 97% and 98% (Z)-selectivities respectively. Both stereo- and diastereo-selectivities markedly depend upon solvent polarity and base strength. For example, both meso- and dl-1a showed small (Z)-selectivities in DMF. The diastereoselectivity markedly decreased with increasing base strength; both 1a and 1b exclusively gave the (E)-isomer of 2 irrespective of the configuration of the ketone when treated with lithium diisopropylamide in the presence of chlorotrimethylsilane at −78 °C. The ketonization of 2a and 2b was also diastereoselective in the direction opposite that observed in the forward reaction; the (E)-isomer predominantly gave the meso ketone (24% d.e.), while the (Z)-isomer, the racemic ketone (ca. 70% d.e.) upon protonation with concd hydrochloric acid in THF.