The chemistry of santonene. Part XI. Products of photolysis of 4-hydroxysantonene and its 4-epimer, their structures, and some pyrolysis studies

Abstract

Photolysis of 4-hydroxysantonene [4-hydroxy-3-oxo-4αH-eudesma-1,5,7(11)-trien-12,6-olactone](3) affords the A-nor-6-acetyl compound (4)(triplet state intermediate), 4-hydroxyphotosantonene {a spiro(bicyclo[3.1.0]hexane)-6,1′-cyclopentane}(5)(singlet state) and a dilactone (6a){a rearrangement product of (5) having a spiro(bicyclo[3.1.0]hexane)-6,1′-cyclopropane skeleton}. Photolysis of 4-hydroxy-4βH-santonene (7) affords the same A-nor-6-acetyl compound (4), 4-hydroxy-4βH-photosantonene (8), and a dilactone (9a). The structure of the A-nor-6-acetyl compound follows from its conversion [by hydrogenation to (13) followed by removal of the 6-acetyl group with base] into the epimeric lactones (14) and (15). The n.m.r. and c.d. spectra of the 3-alcohols derived from (14) and (15) support the suggested stereochemistry of (4). Photolysis of 1,2-dihydro-4-hydroxy-santonene and its 4-epimer in the presence of acetophenone gives the dihydro-A-nor-6-acetyl compound (13). The structures of the hydroxy-photocompounds follow from comparison of their spectra with those of the photosantonenes and photopyrosantonins.

The dilactones are formed from the hydroxyphotosantonenes via hydroxy-keten intermediates. Pyrolysis of the dilactones leads to a ‘Woodward-Hoffman forbidden rupture of the spiropentane skeleton, to give compounds containing a methylenecyclobutane system.