7, 7-Dimethyltricyclo[3.3.0.02, 8]octan-3-ones as Synthetic Intermediates. II. : A Total Synthesis of (±)-Pentalenene, an Angular Triquinane Sesquiterpene

A total synthesis of (±)-pentalenene (3), one of the least-oxidized triquinane sesquiterpenes, is described. By a several-step sequence, 4, 4-dimethyl-2-cyclopentenone (5) was derived to a tricyclooctanone (4), which was subjected to a reductive C2-C8 bond opening reaction to give 15. The triquinane derivative (20), obtained from 15, was transformed to (±)-pentalenene (3) and (±)-epi-pentalenene (21) in a 1.8 : 1 ratio.On the other hand, hydroboration-oxidation of 4 followed by tosylation gave easily separable products, 23 and 24, which were transformed to 20A and 20B, precuesors for (±)-3 and (±)-21, respectively.