Dimethyldioxirane oxidation of N-substituted-2-methylindoles to indoxyls and bisindoxyls

Oxidation of 2-methylindoles 12a-f with dimethyldioxirane (DMD) revealed that N-unprotected 12b,f and protected N-carbamate 2-methylindole 12c afforded indoxyls 16b,c and o-(N-propionyl)aminobenzoic acid 17f as the main products, while N-alkyl- or N-aryl-2-methylindoles 12a,d,e gave 1,5?-diphenyl-4?,5?-dihydro-3?H-spiro[indole-2,2?-pyrano[3,2-b]indole]-3(1H)-one (10), 2,3'-bisindolin-3-ones13, dispiro[indole-2,2?-furan-5?,2?-indole]diones 14 or 2-[(3-oxoindolin-2-yl)methyl]-2-hydroxyindolin-3-ones 15. The structure of dimers10, 13a,d,e, 14a,d and 15a,d followed from NMR measurements, X-ray diffraction analysis confirmed those structures of 13a,d,e,14a,d and 15d, and some mechanistic implications are discussed.