Synthetic study on several eremophilane sesquiterpenes using a common intermediate

The syntheses of racemic ishwarane (6), dehydrofukinone (7), hydroxyeremophilone (8), 9,10-dehydrofuranoeremophilane (9), 10α-furanoeremophilone (10), and 9,10-dehydrofuranoeremophil-1-one (11) from a common intermediate, cis-4,4a,5,6,7,8-hexahydro-4a,5-dimethylnaphthalen-2(3H)-one (5), are described. For the synthesis of ishwarane (6) the key step is sequential Michael addition and displacement of the enolate of the isomeric octalone, cis-4a,5,6,7,8,8a-hexahydro-4a,5-dimethylnaphthalen-1(4H)-one (4), prepared from the octalone (5), with methyl α-bromoacrylate; for the syntheses of the other terpenes (7)–(11) the key step is the zinc chloride-assisted aldol condensation of the enolates of the octalone (5) and the suitably functionalised derivatives with acetone or acetonyl tetrahydropyranyl ether.