Dedicated to Prof. Jean-Marie Lehn on the occasion of his 70th birthday 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-1,3,5triene 18.
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