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

Article abstract of DOI:10.1246/bcsj.65.971

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 84percent and 98percent (E)-selectivities for 1a and 1b, while the meso isomer, 97percent and 98percent (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 deg 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 (24percent d.e.), while the (Z)-isomer, the racemic ketone (ca. 70percent d.e.) upon protonation with concd. hydrochloric acid in THF.