Highly Enantioselective Synthesis of 3,4-Dihydropyrans through a Phosphine-Catalyzed [4+2] Annulation of Allenones and β,γ-Unsaturated α‑Keto Esters

A phosphine-catalyzed novel [4+2] annulation process was devised employing allene ketones as C₂ synthons and β,γ-unsaturated α-keto esters as C₄ synthons. In the presence of an l-threonine-derived bifunctional phosphine, 3,4-dihydro­pyrans were obtained in high yields and with virtually perfect enantio­selectivities. The synthetic value of the dihydro­pyran motif was demonstrated by a concise preparation of an anti-hyper­cholesterol­emic agent.