SYNTHESIS OF CYCLOHEXENOLS AND CYCLOHEPTENOLS VIA THE REGIOSELECTIVE REDUCTIVE RING-OPENING OF OXABICYCLIC COMPOUNDS

The reductive ring opening of oxabicyclic compounds has been achieved. While RMgBr/MgBr2 works in a few Limited substrates, diisobutylalumin um hydride reacts with oxabicyclic[3.2.1]- and -[2.2.1]alkenes to prov ide cycloheptenols and cyclohexenols in good yield and in some cases i n good regioselectivity. With some substrates further reduction of the alkene was observed which led us to examine transition metals such as nickel which are known to accelerate the hydroalumination reaction. T he reaction with Ni(COD)(2) (COD = cyclooctadiene) gave the best react ivity-selectivity profile, and significantly higher yields were obtain ed with minimal overreduction of the alkene. Phosphine ligands were sh own to dramatically improve the regioselectivity of ring opening of br idgehead-substituted oxabicyclic compounds. The best ligand was 1,4-bi s(diphenylphosphino)butane which gave up to 380:1 selectivity. A serie s of deuterium quenching experiments showed that the selectivity of th e hydroalumination varies according to the reaction protocol and ligan d-metal ratio.