Photochemical formation of 3,4,4a,5,6,8a-hexahydro-2H-naphthalen-1-ones from cyclohex-2-enones and 2-methylbut-1-en-3-yne

Irradiation of 2-alkynylcyclohex-2-enones 1a and 1b in the presence of 2-me thylbut-1-en-3-yne in benzene affords 6-methylenehexahydronaph-thalen-1-one s 2 as main products while photolysis in methanol gives predominantly 1:1 d iastereomeric mixtures of 6-methoxy-6-methylnaphthalenones 4 and 5, respect ively, The key step in this stepwise [4 + 2]-cycloaddition is the 1,6-cycli zation of biradical 6 to 1,2-cyclohexadiene 7.