Internal Lewis Acid Coordination as a Powerful Tool To Promote Highly Stereoselective Alkylation ofα-Alkyl-β-Hydroxy Ketones with Grignard Reagents

An efficient and highly diastereoselective protocol is described for the alkylation of β-hydroxy ketones that contain an α-stereocenter. This method is based on the preliminary transformation of the β-hydroxy group into a titanium alcoholate by means of the facile transmetalation of the corresponding β-silyloxy derivative with TiCl4 (Method A) or by reaction of the lithium alcoholate with TiCl4 (Method B). On account of the strong internal coordinating action of the Lewis acid, this intermediate assumes a rigid half-chair conformation with the α-alkyl substituent in a pseudoaxial position. This geometrical arrangement facilitates the attack of the entering carbanion on the carbonylic function opposite to the α-substituent. The method uses simple Grignard reagents as the alkylating agents and allows the addition of a wide variety of carbon frameworks to the carbonyl function, including primary and secondary alkyl chains, arylic, alkynylic, vinylic, and benzylic moieties, with high efficiency and stereoselectivity.