Pyranoindole and thiopyranoindole. Oxidative cyclization of 3-indolepropanol and 3-indolepropanethiol with N-bromosuccinimide, N-chlorosuccinimide, and singlet oxygen.
Oxidative cyclization of 3-indolepropanethiol (6) and 3-indolepropanol (9) with N-bromosuccinimide or N-chlorosuccinimide (NCS) gave the corresponding thiopyrano-[2, 3-b] indole (7) and pyrano [2, 3-b] indole (10) in good yields. Reaction of 7 with NCS in carbon tetrachloride gave an unstable chloroindolenine (18) which transformed to a spirooxindole (19) on treatment with ethanolic hydrochloric acid. Reaction of 7 with NBS in carbon tetrachloride gave 7-bromo derivative (20) via the 3-bromoindolenine. Similarly pyranoindole (10) gave a spirooxindole (23) in good yield on treatment with NCS in methylene chloride followed by the addition of ethanolic hydrochloric acid. On the other hand reaction of 9 with two equivalents of NCS gave 23 and dichlorooxindole (24). Dye-sensitized photooxygenation of 3-indolepropanol (9) in benzene gave 4a-hydroxypyrano [2, 3-b] indole instead of 10.