Synthetic photochemistry. A new synthesis of (±)-zizaene via an intramolecular variant of the de Mayo reaction

Irradiation of a 1:1 mixture of the enol acetates (10) and (31) derived from the cyclopentane-1,3-dione (30), is shown to produce a 7:3 mixture of the cycloadducts (32) and (33), resulting from intramolecular [2 + 2] photocycloaddition in the enol acetate (10).Fragmentation of the mesylate (34b) derived from the cycloadduct (32) then leads to the alkene (35), which on hydrogenation produces an unresolved mixture of α-(minor) and β-(major) methyl epimers, (37) and (39) respectively, of the tricyclic ketone (36). The tricyclic ketone (37) has been converted previously into (±)-zizaene (1).The new approach to (37) formally completes a new total synthesis of (±)-zizaene (1) and its methyl epimer.