Ketene dithioacetals readily reacted with aldehydes in the presence of a Lewis acid such as BF3 . OEt2 or TMSOTf to generate the cationic adducts, which in turn reacted with NaA1H(2)(OCH2CH2OCH3)(2), leading to the corresponding beta-hydroxy dithioacetals, masked cross-aldols between aldehydes. Each diastereomer was selectively obtained by choosing the appropriate Lewis acid and substituents on the sulfur atoms of the ketene dithioacetals.