The Wittig Synthesis and Photocyclodehydrogenation of 3-(2-Aryl)Vinylindole Derivatives

Wittig condensation of indole-1-(p-toluenesulfonyl)-3-methyltriphenylphosphonium iodide (8) with aromatic and heterocyclic aldehydes gives good yields of the 3-(2-aryl)-vinylindole derivatives 9a–g (Scheme 1). Compounds 11a and 11b are obtained similarly starting with the known indole-1-methyl-3-methyltriphenylphosphonium salt 10 and pyridine-3- and -4-aldehydes. An alternate Perkin-type condensation route provides the trisubstituted 3-vinylindoles 15a and 15b (Scheme 2) which cannot be synthesized by the Wittig route.Compounds 9a and 9b; 9e and 11b; 13, 15a, and 15b; as well as the previously known 12a-c undergo photocyclodehydrogenation to give benzo- and pyridocarbazole derivatives 16a and 16b; 18a and 19a, 18b and 19b; 17a, 17b, and 17c; and 16c–e, respectively in fair to good yields (Scheme 3). Compounds 9d, 9f, and 11a fail to photocyclize. The reasons for these failures are briefly explored and discussed.