Chemistry of santonene. Part IX. Photolysis products of santonene, and their chemical transformations

Abstract

Irradiation of santonene [3-oxo-4αH-eudesma-1,5,7(11)-trien-12,6-olactone](3) affords photosantonenes A and B [(4) and (5)] which both possess a spiro(bicyclo[3.1.0]hexane)-6,1′-cyclopentane skeleton, and are in thermal equilibrium. Both give 1,2-dihydro-derivatives, (6) and (7). These in turn can be equilibrated (at 110°) with their 6,10-isomers, (11) and (12) respectively.

Treatment of dihydro-A (6) with acetic acid–hydrogen chloride affords the cyclopentenone (13a). This can also be obtained from the keto-acid (14), which itself is derived from the known photopyrosantonins A and D. The cyclopentenone (13a) and an isomer (13b) can also be prepared from photosantonene A by reduction with zinc. Treatment of (13b) with base affords (13a). Over-reduction of photosantonene A affords a hexahydro-derivative (16a) with opening of the cyclopropane ring. The photo-isomer A also undergoes acid catalysed methanolysis to the methoxy-lactone (17b). This equilibrates with the isomer (17a) in trifluoroacetic acid–benzene. Pyrolysis of photosantonene A affords partially racemised santonene.