An enantioselective synthesis of alpha-methylene-beta-hydroxy carboxylic acid derivatives via a highly diastereoselective, one-pot syn-aldol and beta-elimination sequence utilizing the chiral beta-(phenylselenyl)propionyl imide 15 is described. This new method, which constitutes an alternative to the Baylis-Hillman reaction, has been applied to the synthesis of the C(15)-C(21) fragment of tedanolide C.