A solution to the cyclic aldol problem.

[formula: see text] A protocol for achieving stereoselective aldol reactions with cyclic ketones is presented. In terms of yield, the process is particularly effective when a quaternary center at the alpha-carbon of the beta-hydroxy ketone product is created. The stereochemical outcome, anti or syn, is achieved by the Lewis acid-mediated ring expansion of stereochemically homogeneous epoxides in a reaction related to the pinacol rearrangement.