Oxidation of Indoles with Oxodiperoxomolybdenum (VI), MoO5·HMPA. Preparation of 2-Hydroxyindoxyl and Isatogen Derivatives

Oxidation of 1-acetylindoles 1 with (hexamethylphosphoramide) oxodiperoxomolybdenum (VI), MoO5·HMPA, in dry methylene chloride gave 1-acetyl-2-hydroxyindoxyls 4. 2-Phenylindole (8) was similarly treated with MoO5·HMPA to give a dimeric product 11, while oxidation of 8 with three mol eq of MoO5·HMPA gave 2-phenylisatogen (12). The oxidation of other indoles, 14, 19, 21, and 22, with MoO5·HMPA in methylene chloride is also described. A possible mechanism for these oxidations is presented.