Experiments Directed toward the Total Synthesis of Polycyclic Terpenes

A synthetic sequence for the conversion of 2-methoxycarbonylmethyl-6-methoxy-1, 2, 3, 4tetrahydronaphthalenone (II) to (•})-3, 16-dioxo-20-nor-hiba-5, (6)-ene is described.The synthetic route to the bridged tetracy clic compound (I) described in the preceding paper1) seemed to be a promising route to the compound having bicyclo [3, 2, 1] octane C D ring and also having trans relationship between 4 b-H and D ring, such as the ketone (XIV).In this paper the stereoselective synthesis of the ketone (XIV) and its further transforma tion into (•}) 3, 16-dioxo-20-norhiba-5, (6)-ene (XXI) are described.The previously described keto ester (II)1) was treated with methyl isopropenyl ketone in dimethy sulfoxide and methanol using sodi um methoxide as a base to give the keto acid (III) in 66% yield.This keto acid was reflux ed in acetic acid and acetic anhydride with borontrifluoride to give the bridged tetracyclic diketone (IV) in 92% yield which was ketaliz ed in dichloroethane with ethylene glycol and 1) T.