Photochemical transformations of some neoclerodane and labdane diterpene ketones

Some neoclerodane (2-4) and labdane (5 and 6) diterpene ketones having a beta- or gamma-hydroxy function, an a-epoxy group, or an a-olefinic double bond have been irradiated at lambda = 313 nm in dry solvents (benzene or methanol). Fruticolone (2) yielded the naturally occurring 5,6-seco-neoclerodan-6,1 alpha-olide derivative fruticolide (1), while hispanolone (5) gave the delta-lactone 8, both transformations involving a Norrish type I photoreaction. The alpha,beta-unsaturated ketone derivative 4 was transformed into 7 by a stereoselective intramolecular [2+2] cycloaddition reaction involving its furanic 13(16)-double bond. Finally, the or-epoxy ketone 6 was rearranged to the 10(9-->8)abeo-labda-7,9-dione derivative 10. Apart from the interest in these reactions for synthetic purposes, the photochemical transformation of fruticolone (2) into fruticolide (1) suggests that the latter might originate from the former as an artefact during the extraction and isolation procedure, rather than from any biogenetic pathway.