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.