The synthesis of (+)-matatabiether and related methylcyclopentane monoterpenes

The syntheses of(+)-matatabiether (1), neonepetalactone (15) and related alcohols3 and5, and dihydropyran2 are described, 5-Isopropenyl-2-methyl-1-cyclopentene-1-carboxaldehyde (6) was converted by LAH reduction and acetylation to 2-acetoxymethyl-3-isopropenyl-2-methylcyclopentene (8) which was hydroborated to give 3(β-hydroxyisopropyl2-acetoxymethyl-2-methylcyclopentene (9). Hydrolysis of9 and cyclization with acid gave dihydropyran2 and a small amount of matatabiether (1). Oxymercuration demercuration of acetate9 gave a readily separable mixture of 1-acetoxymethyl-4,8-dimethyl-2-oxabicyclo[3.3.0]octane (13) and 8-acetoxymethyl-1,4-dimethyl-2-oxabicyclo[3.2.1]octane (12). Pyrolysis of acetate12 afforded (+)-matatabiether (1), while hydrolysis of12 gave alcohol3. Oxidation and esterification of aldehyde6 gave methyl 5-isopropenyl-2-methyl-1-cyclopentene-1-carboxylate (18). Hydroboration of18 and heating the resulting hydroxyester to 200° gave neonepetalactone15 contaminated by small amounts of dihydronepetalactone20 and an isomeric dihydronepetalactone21. Catalytic hydrogenation of neonepetalactone yields isodihydronepetalactone (22). These conversions establish thecomplete stereochemistry of