OXIDATION OF N-ACYLINDOLES BY DIMETHYLDIOXIRANE AND SINGLET OXYGEN - SUBSTITUENT EFFECTS ON THERMALLY PERSISTENT INDOLE EPOXIDES AND DIOXETANES

Photooxygenation of the N-acylindoles 1a-d afforded the labile indole dioxetanes 2 and the relatively stable allylic hydroperoxides 3. The dioxetanes 2a,c,d were sufficiently stable for isolation and spectral characterization. Additionally, they were characterized by chemical transformations, namely reduction to the indole epoxides 5 by dimethyl sulfide, acid-catalyzed rearrangement to allylic hydroperoxides 3c,d, and thermolysis to the cleavage products 4. Oxidation of the N-acylindoles I with dimethyldioxirane afforded the epoxides 5, which were characterized on the basis of their spectral data and chemical transformations to the 2-indolinones 6 and 2-methyleneindoline 7. These decomposition products (except 7e) were fully characterized. The stability of the epoxides 5 depended on the substitution type, i.e., the carbazole epoxide 5c decomposed already at temperatures above ca. -50 degrees C, whereas the cyclopentindole epoxide 5b was stable at 20 degrees C for days. The latter was unequivocally characterized by X-ray analysis.