Phosphine-Catalyzed Formal Oxa-[4 + 2] Annulation Employing Nitroethylene and Enones: Enantioselective Synthesis of Dihydropyrans.

The first phosphine-catalyzed enantioselective formal oxa-[4 + 2] reaction between nitroethylene and α-cyano-α,β-unsaturated ketones has been developed. In the presence of a dipeptide-based phosphine catalyst and achiral Brønsted acid additives, highly functionalized 3,4-dihydro-2 H-pyrans were obtained with excellent enantio- and diastereoselectivities.