Total synthesis of (±)-pentalenene, (±)-pentalenic acid, and (±)deoxypentalenic acid through an intramolecular double Michael reaction

The angular triquinane sesquiterpenes, (±)-pentalenene (1), (±)-pentalenic acid (2), and (±)-deoxypentalenic acid (4), were synthesized via the intramolecular double Michael reaction as the key step. Heating the bis-enone (10), prepared from 4,4-dimethylcyclopent-2-enone (11) in six steps, with chlorotrimethylsilane, triethylamine, and zinc chloride gave the tricyclo[7.3.0.0]dodecanedione (9), which was converted into the above triquinanes after ring contraction.