Total synthesis of (+)-albicanol and (+)-albicanyl acetate via a highly diastereoselective intramolecular [3 + 2] cycloaddition

Abstract

The enantioselective total syntheses of the drimane-type sesquiterpenes albicanol (1) and albicanyl acetate(2) starting from (+)-Wieland–Miescher ketone (8) have been accomplished. The key step in the synthetic strategy involves a highly diastereoselective intramolecular [3 + 2] dipolar cycloaddition reaction of the nitrile oxide (6), which affords the isoxazoline (5) as the sole product in high yield. Reductive hydrolysis of compound (5) followed by methylenation of the resulting β-hydroxy ketone (4) by application of the Nozaki–Lombardo procedure provides (+)-albicanol (1), which is then converted into (+)-albicanyl acetate (2) by acetylation. The olefinic acetal (15), an intermediate of the present total synthesis, has been transformed by sequential Jones oxidation and methylation into a secosesquiterpene (17), one of the components of sun-cured Greek tobacco.