Sensitized photooxygenations of 3-vinylindole derivatives

Article abstract of DOI:10.1021/jo00079a032

A series of 1-methyl- and 1-(phenylsulfonyl)-substituted 3-vinylindoles with different electronic and steric features has been synthesized and their sensitized photooxygenation in aprotic solvents investigated. 1-Methyl-3- vinyl-(1a), 1,2-dimethyl-3-vinyl- (1b), 1-methyl-3-(β-methoxyvinyl)- (4-Z and 4-E), 1-(phenylsulfonyl)-3-vinyl- (8a), 1-(phenylsulfonyl)-3-(α- methylvinyl) (8b), 1-(phenylsulfonyl)-3-(β-methoxyvinyl)- (8c and 8d), and cis-1-(phenylsulfonyl)-3-(α-methyl-β-methoxyvinyl) indoles (15-Z) react with ¹O₂ predominantly to give endoperoxides via [4 + 2] cycloadditon. However, 1,2-dimethyl-3-(β-methoxyvinyl)indole (1c) gives [2 + 2] cycloaddition with the 3-double bond to give 1,2-dimethyl-3-formylindole (3c); trans-1-(phenylsulfonyl)-3-(α-methyl-β-methoxyvinyl)indole (15-E) gives the 3-indolyl allylic hydroperoxide (17) via ene reaction, along with a small amount of isomerization of the 3-vinyl double bond. A zwitterionic intermediate in the isomerization is proposed. Most of the resulting dioxacarbazole endoperoxides are isolable and inert to reduction by trimethyl phosphite and thiourea except for N-methyldioxacarbazole 5, which undergoes clean rearrangement to indolin-2-one epoxide 7 at -20 °C.