Photocycloaddition of cyclohex-2-enones to 2-methylbut-1-en-3-yne

Irradiation (350 nm) of 2-alkynylcyclohex-2-enones 1 in benzene in the pres ence of an excess of 2-methylbut-1-en-3-yne (2) affords in each case a mixt ure of a cis-fused 3,4,4a,5,6,8a-hexahydronaphthalen-1(2H)-one 3 and a bicy clo[4.2.0]octan-2-one 4 (Scheme 2), the former being formed as main product via 1,6-cyclization of the common biradical intermediate. The (parent) cyc lohex-2-enone and other alkylcyclohex-2-enones 7 also give naphthalenones 8 , albeit in lower yields, the major products being bicyclo[4.2.0]octan-2-on es (Scheme 4). No product derived from such a 1,6-cyclization is observed i n the irradiation of 3-alkynylcyclohexenone 9 in the presence of 2 (Scheme 4). Irradiation of the 2-cyano-substituted cyclohexenone 12 under these con ditions again affords only traces of naphthalenone 13, the main product now being the substituted bicyclo[4.2.0]oct-7-ene 16 (Scheme 5), resulting fro m [2 + 2] cycloaddition of the acetylenic C-C bond of 2 to excited 12.