Enantiopure formaldehyde N,N-dialkylhydrazones 1 smoothly react with prochiral alkylidene malonates 2 in the presence of MgI₂ to afford the corresponding Michael adducts 3 in excellent yields and good diastereoselectivities; direct racemization-free BF₃·OEt₂-catalyzed thiolysis of the hydrazone CN bond affords the corresponding dithioketals 7 in optically pure or enantiomerically enriched form.