[3,5]-sigmatropic shifts in the photochemistry of 1-allyl-naphthalen-2(1H)- ones

Although the formation of a 2a,3,4,8b-tetrahydro-1,4-methanocyclobuta[a] naphthalen-2(1H)-one (2) in the photolysis of 1-methyl-1-(prop-2-enyl)- naphthalen-2(1H)-one (5) is confirmed by X-ray crystallography of the corresponding 7-bromo photoproduct (3), the available evidence does not support the previously made suggestion that this occurs via a [3,5]-sigmatropic shift; an alternative mechanism involving an arene-alkene exciplex is suggested.