Photoannelation Reactions of 3-(Alk-1-ynyl)cyclohept-2-en-1-ones

Irradiation (350 nm) of the newly synthesized 3‐(alk‐1‐ynyl)cyclohept‐2‐en‐1‐ones 1 and 2 leads to the selective formation of tricyclic head‐to‐head dimers. In the presence of 2,3‐dimethylbuta‐1,3‐diene, the (monocyclic) enone 1 affords trans‐fused 7‐alkynyl‐bicyclo[5.2.0]nonan‐2‐ones as major photoproducts, whereas photocycloaddition of benzocyclohept‐5‐en‐7‐one 2 to the same diene gives preferentially the eight‐membered cyclic allene 16 via ‘end‐to‐end’ cyclization of the intermediate allyl‐propargyl biradical 22. On contact with acid, cycloocta‐1,2,5‐triene 16 isomerizes to cycloocta‐1,3,5‐triene 18.