PHOTOCHEMICALLY INDUCED RADICAL CATION DIELS-ALDER REACTION OF INDOLE AND ELECTRON-RICH DIENES

Diels-Alder reactions between indole (3) and substituted cyclohexa-1,3-dienes (2) can be effected by a photoinduced catalyzed electron transfer reaction using catalytic amounts of triarylpyrylium tetrafluoroborates (1) as sensitizers and an acid chloride as a trapping agent. Irradiation generates N-acyl-1,4,4a,9a-tetrahydro-1,4-ethanocarbazoles in one step. The products are formed with nearly total regioselectivity, such that a substituent in the 1-position of the cyclohexa-1,3-diene is always found in the 1-position of the tetrahydrocarbazole, and a substituent in the 2-position of the diene always appears in the 3-position of the product.