Terpenoids-LVII: synthesis of some germacrane derivatives

Abstract

Both liquid and solid hexahydrocostunolides obtained by the pressure hydrogenation of costunolide (V) and solid dihydrocostunolide (VI) respectively, give the same optically active hexahydrogermacol (IIIa), when subjected to controlled reduction with LiAlH₄, followed by Huang-Minlon reduction. The alcohol (IIIa) on oxidation with chromic oxide gives optically active hexahydrogermacrone (IIa). Dehydration of IIIa with KHSO₄ followed by catalytic hydrogenation gives the optically inactive saturated hydrocarbon, germacrane (IV). Since the total synthesis of solid dihydrocostunolide (VI) has already been achieved the preparation of IIIa, IIa and IV from VI constitutes a total synthesis of the above three compounds.