A novel [2 + 2] photodimerization of N-[(E)-3,4-methylenedioxycinnamoyl]dopamine in the solid state

[reaction: see text] To give photoreactivity to catecholamines, N-cinnamoyl-substituted dopamines (E)-5a-5c and O,O'-dimethyldopamines (E)-3a-3c were prepared under solvent-free conditions. A particularly interesting observed photoreactivity was that of N-[(E)-3,4-methylenedioxycinnamoyl]dopamine ((E)-5c), which underwent a novel type of [2 + 2] photodimerization in the solid state, giving exclusively a tricyclic product 8. This reaction is the first example of solid-state photoaddition of an alkene to the benzene ring.