Synthetic studies on terpenoids. Part 19. Synthesis of 3β,10α,14β-trimethyl-1βH,11βH-tricyclo[9.3.0.0]tetradec-6-en-5-one, a tricyclic ketone related to the ophiobolins

The tricarbocyclic ketone (36) with well defined stereochemistry at each of the four contiguous asymmetric centres has been synthesised, and the stereochemistry of these centres has been deduced mechanistically and confirmed by X-ray crystallographic analysis of the bicyclic acidic precursor (22). The orientation of the tertiary methyl group in structure (36) has been deduced from force-field calculations and confirmed by model studies. The configuration of the two ring junction hydrogen atoms and of the two methyl groups in structure (36) are the same as found in the ophiobolins. The conformations of substituted cis-perhydroindanones, bicyclo[3.3.1 ]nonanes, cyclo-octenes and cyclo-octanones, are described.