Tandem Chain Extension−Aldol Reaction:  Syn Selectivity with a Zinc Enolate

A tandem chain extension-aldol reaction was developed in which beta-keto esters are transformed to alpha-substituted-gamma-keto esters in an efficient zinc-mediated, one-pot reaction. The diastereoselectivity of the reaction ranged from good to excellent with syn stereochemistry observed for beta-keto ester and amide substrates and anti-stereochemistry observed for a beta-keto imide. [reaction: see text]