REGIOSELECTIVE INTRAMOLECULAR BASE-INDUCED SYNTHESIS OF ALPHA,BETA-UNSATURATED ACYLDECALINS FROM DECALONES VIA CARBON HOMOLOGATION WITH TOSMIC - SYNTHESIS OF (+/-)-6-EUDESMEN-4-ALPHA-OL AND (+/-)-VETISELINENE

Reaction of decalones 6a-e with TosMIC gives adducts 7a-e which can be methylated concomitant with regioselective migration of the double bond to an endocyclic position. The regioselectivity of the double bond migration is determined by the presence or absence of a free hydroxyl group at C(4). Base-catalyzed methylation of TosMIC adducts 7b-d, possessing a free hydroxyl group at C(4), and subsequent acid-catalyzed hydrolysis led preferentially to the C(6)-C(7) double bond isomers 10b-d, respectively. An intramolecular deprotonation of H-6 is considered to be responsible for this regioselectivity. If the hydroxyl group at C(4) is absent or protected as in 7a and 7e, respectively, the C(7)-C(8) double bond isomers 9a and 9c are obtained via an intermolecular abstraction of a sterically less shielded proton at C(8). The usefulness of this methodology is illustrated by the total synthesis of (+/-)-6-eudesmen-4 alpha-ol (18) and (+/-)-vetiselinene (22).