Stereoselective synthesis of cyclic ether compounds via reductive cleavage of bicyclic ketals

Bicyclic ketals in the 6,8-dioxabicyclo[3.2.1]octane system readily reacted with triethylsilane-boron trifluoride etherate at room temperature to give only the cis-tetrahydropyranol derivatives via C-5-O-6 bond cleavage. Threo- and erythro-alcohols were prepared selectively from the endo- and exo-ketal, respectively. Dehydrated products also formed when the tertiary alcohol was involved under these reaction conditions. However, the cleavage reaction with aluminum hydride gave the trans-tetrahydropyranol as the major product without dehydration.