Stereoselective reduction of benzoin by the NADH model 3-(dimethylcarbamoyl)-1,2,4-trimethyl-1,4-dihydropyridine

The Mg(ClO4)2-induced reduction of racemic benzoin by the racemic NADH model compound 3-(dimethylcarbamoyl)-1,2,4-trimethyl-1,4-dihydropyridine (1), in accordance to Cram's rule, leads exclusively to meso-1,2-diphenyl-1,2-ethanediol. However, while R-1 equally reduces 5-benzoin to afford meso-1,2-diphenyl-1,2-ethanediol, S-1 is reluctant to react with S-benzoin. The intermediacy of a strictly organized transition state, composed of chelated substrate, dihydropyridine and magnesium ion is involved.