Triarylpyrylium tetrafluoroborates are very effective sensitizers for the photoinduced (450 W Xenon arc lamp or 250 W slide projector with halogen lamp), electron-transfer-catalyzed Diels-Alder reaction between indole and substituted 1,3-cyclohexadienes, if acyl chlorides are used as trapping agents. Thus 9-acyl- 4,4a,9,9a-tetrahydro-1,4-ethano-1H-carbazoles are generated in one step (6 examples) with practically total regioselectivity, such that a substituent in the 1-position of the 1,3-cyclohexadiene is always found in the 1-position of the tetrahydrocarbazole.
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Gieseler et al. (1990) studied this question.