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 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.