The chemistry of santonene. Part XII. Photolysis of 4-acetoxysantonene and its 4-epimer: an example of photodecarboxylation of acetates

Abstract

Photolysis of 4-acetoxysantonene [4-acetoxy-3-oxo-4αH-eudesma-1,5,7(11)-trien-12,6-olactone](1b) or its 4-epimer (2b) affords a mixture of 4-methylsantonene (1c) and its 11-methyl-Δ^1,4,6-isomer (5a), 4-methyl-photosantonene (6), and, in methanol, the corresponding 4-acetoxyphotosantonene (7) or its 4-epimer (8). The reaction is believed to proceed via santonenyl and acetoxyl radicals, of which the latter undergo decarboxylation and recombine intermolecularly with the santonenyl radicals. The decarboxylation proceeds via a triplet state, whereas the 4-acetoxyphotosantonenes are formed via a singlet state.

Photolysis of 1,2-dihydro-4-acetoxysantonene (10a) or its 4-epimer (11a) affords 1,2-dihydro-4-methylsantonene (10b), its 11-methyl-Δ^4,6-isomer (14), and both the 6α- and 6β-methyl-Δ^4,7(11)-isomers (12) and (13). The Δ^1,4,7(11)-isomer of 6β-(but not 6α-) acetoxysantonene affords traces of 4- and 11-methyl compounds. Photolysis of 1,2-dihydro-4-methylsantonene (10b) in the presence of a triplet sensitiser affords the stereoisomeric A-norcyclopropanes (18) and (19). The latter is unstable to acid, and rearranges to the 6-isopropenyl derivative (20).