Regio- and Diastereoselective Synthesis of Dihydropyrans and Pyranopyrans via Oxonium-Ene Reaction of β-Allenols and Aldehydes.

Abstract

Bismuth trifluoromethanesulfonate can be efficiently used for the preparation of dihydropyrans from β-allenols and aldehydes by oxonium-ene reaction in good yields. The reaction is highly regioselective. On the other hand, the same reaction with trimethylsilyl trifluoromethanesulfonate at -45 °C affords the hexahydropyrano[4,3- b]pyran skeleton in moderate yields.