Spiroketal Reductive Ring Opening

Both the direction and mechanism of reductive ring opening with the spiroketal 9,9-dimethyl-1,6-dioxaspiro[4.5]decane (8) have been explored. With dimedone (3) as precursor, synthesis of spiroketal 8 was realized by way of intermediates 4,5,6, and 7. Lithium aluminum hydride–aluminum chloride catalyzed reduction of spiroketal 8 was found to yield tetrahydropyran 9 in contrast to the steroidal sapogenins which undergo reductive cleavage of the tetrahydropyran ring. The mechanism of ring opening was examined using deuterium labeling (8 → 9b) combined with mass and proton magnetic resonance spectral measurements. Reductive ring opening of spiroketal 8 was thereby found to proceed by transfer of reagent hydride directly to the spirocarbon. The experimental results also suggested that reductive ring opening of relatively nonhindered spiroketals related to 8 may offer a new synthetic route to certain substituted tetrahydropyrans.