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

Bismuth trifluoromethanesulfonate can be efficiently used for the preparation of dihydropyrans from beta-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 degrees C affords the hexahydropyrano[4,3-b]pyran skeleton in moderate yields.