Approach to the Synthesis of Diterpenes with the Bicyclo[5.3.0]decane System: (±) 10-epi-tormesol

The synthesis of (±) 10-epi-tormesol, 27, has been achieved from (±) 1-acetyl-3a,6 dimethylhexahydroazulene 1, by coupling of 5, a dehydroderivative of 1, with 23. The same synthetic procedure afforded a series of diterpenes 24–26 and 28 with the same biannular system. Direct reduction of 1 with different methods does not give the desired spatial relationship (trans/cis) between Me-C7/H-6/H-10 on the substrate, affording instead trans/trans (compounds 5 and 18) and cis/cis (compound 9). Indirect reduction: epoxydation followed by catalytic hydrogenation afforded the desired stereochemistry on the intermediate but deoxygenation caused epimerization at C-10. This synthetic achievement confirmed the original structure assigned to tormesol some years ago.