Magnesium cation-induced anti-aldol selective tandem Michael/aldol reaction

A mixture of magnesium thiolate or selenolate, β-substituted-α,β-unsaturated ester and aldehyde affords a Michael/aldol tandem adduct, α-phenylthio- or α-phenylselenoalkyl-β-hydroxyester, in a good yield. The reaction proceeded in anti-aldol selective manner, which is contrast to the products from a similar reaction in the presence of lithium cation. An NMR study and an experiment for trapping the reaction intermediates suggest that magnesium thiolate, which forms precipitate in the reaction mixture, first attacks the aldehyde, not the unsaturated ester, to give α-alkoxysulfide.