INVESTIGATION OF THE DIASTEREOSELECTIVITY IN THE ADDITION OF ORGANOMETALLICS TO THE ALPHA-KETO ESTERS OF AXIALLY CHIRAL 1,1'-BINAPHTHALEN-2-OL DERIVATIVES

The inherent chiral induction abilities of axially chiral 2'-substituted 1,1'-binaphthalen-2-ols 2a-f as the chiral auxiliary for the addition of organometallics to their cc-keto acid esters were examined as a function of the following reaction variables: size of the 2'-substituent, nature of the organometallic reagent, solvent, and temperature. In the addition of MeMgl to the phenylglyoxylates 3a-f of the alcohols 2a-f, the corresponding atrolactic acid esters 5a-f and 6a-f were obtained with up to 52% diastereoisomeric excess (d.e.). The preferred diastereoisomer depended on the size of the 2'-substituent, and thus could not solely be determined by the helicity of the 1,1'-binaphthalene framework. By using MeZnl as the nucleophile, the selectivity increased up to 84% d.e. with the same diastereofacial selectivity as that of MeMgl. On the other hand, the diastereofacial selection was reversed when MeTiCl(3) was employed as the nucleophile, with low selectivity (14% d.e.). It is concluded that MeMgl or MeZnl, as a nucleophile of low Lewis acidity, adds to the keto ester moiety in the s-trans conformation, while the strong Lewis acid MeTiCl(3) mainly adds to the s-cis conformer from the same direction as that of the Grignard addition, thus giving the opposite diastereoisomer.