Chemical and photochemical oxidation of tetrahydrobetacarboline

The photophysics and the mechanisms of the photochemical and chemical aroma tization of 1,2,3,4-tetrahydro-7H-pyrido/3,4-b/indole (THBC) in 40% v/v met hanol-water media have been investigated. The primary photophysical process es of THBC resemble those of indoles and related alkaloids. The photochemic al oxidation has been carried out in the presence of atmospheric oxygen and light. The chemical oxidation has been studied using sodium peroxodisulpha te (PDS) as the electrophilic agent. In both cases, strong acid media, i.e. , sulphuric acid concentrations higher than 0.5 mol dm(-3) are needed for t he dehydroderivative (DR) to be formed. In the photochemical oxidation, the rate constants for the disappearance of THBC increase linearly with the co ncentration of acid and the intensity of the exciting radiation. However, t he formation of DH only depends on the acidity of the media. In the chemica l oxidation a similar behaviour is observed. In this case, the rate constan ts for the disappearance of THBC increase linearly with both PDS and acid c oncentrations, and the appearance of DH solely varies with the acid concent ration. A two step mechanism is proposed for these oxidation reactions. In the first step, excited or ground state THBC reacts with ground state oxyge n or PDS, respectively, to give an indolenine intermediate. This intermedia te slowly rearranges, in a second acid catalysed step to yield DH. (C) 2000 Elsevier Science S.A. All rights reserved.