Vinylogous Mannich reactions: some theoretical studies on the origins of diastereoselectivity

reaction: see text] Ab initio calculations at the RHF/3-21G level were used to investigate the limiting transition states in the addition of 2-methoxyfuran to a pyrrolinium ion. Four stationary points were found on the potential energy surface with relative energies of threo Diels-Alder, 0.0 kcal/mol; erythro open, 0.9 kcal/mol; erythro Diels-Alder, 1.3 kcal/mol; and threo open, 1.8 kcal/mol.