Synthetic photochemistry. Elaboration of the tricyclo[6.3.0.0]-undecane (‘hirsutane’) carbon skeleton by intramolecular photocyclisations of dicyclopent-1-enylmethanes

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Abstract
Irradiation of the dicylopent-1-enylmethane (5) in methanol is shown to lead to intramolecular [2 + 2] cycloaddition, followed by in situ addition of methanol to the presumed intermediate bicyclo[2.1.0]pentane (11), producing the cis,syn,cis-tricyclo[6.3.0.0]undecane (12) in >90% yield. The general method is applied in a synthesis of the hirsutane carbon skeleton [viz. (19)→(20)] found in hirsutic acid (7) and related natural sesquiterpenes. Attempts to expand the scope of the photoprocess to other fused polycyclic systems met with mixed success. Thus, although irradiation of the cyclopentenone (24) in methanol led to the bicyclo[3.3.0]octenone (25), the cyclopentenone (29) instead produced the isomeric alkene (30) resulting from a 1,3-H shift, and irradiation of (31) led only to decomposition.

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