The chemistry of santonene. Part VIII. Photoreaction of 4αH-pyrosantonin

Abstract

Irradiation of 4αH-pyrosantonin [(11S)-3-oxo-4αH-eudesma-1,5-dien-12,6-olactone](7) affords four isomeric photopyrosantonins A–D [(9)–(12)], each possessing a spiro(bicyclo[3.1.0]hexane)-6,1′-cyclopentaneskeleton. The two trans-fused lactones A and D [(9) and (12)] are readily hydrolysed to give the same diketo-acid (17a) with opening of the lactone and cyclopropane rings. The cis-fused lactones, B and C [(10) and (11)] are more stable but on treatment with acid afford the diketo-acid (18a). The cis-lactones can be equilibrated photochemically and by acid. The trans-lactones can be equilibrated photochemically. Pyrolysis of dihydro-derivatives of the trans-lactones affords the corresponding derivatives of the cis-lactones. The mechanism of the photochemical reaction is discussed; the pathway from the trans-lactone to the diketo-acid (17) has been followed by use of a deuterium-labelling technique.