Indolenines oxindoles

Under indole alkaloids we include naturally occurring bases or their derivatives which contain a true indole nucleus or a modification thereof, indoline (dihydroindole), N-acyl-indoline, indolenine, oxindole etc. 

Traditional methods for bromination of indoles are not very satisfactory. For example, use of NBS in aqueous acetic acid generally gives rise to oxindoles or indolenines rather than to bromoindoles. Bromination is better in aprotic solvents but yields are still often poor and/or conditions vigorous. By contrast, NBS-silica brominated skatole rapidly at room temperature in... 

Cabucine (10-methoxyajmalicine) (135) and tetraphylline (11-methoxyaj-malicine) (136) have been converted into the corresponding oxindole A and B analogues (137-140) via the corresponding 7-chloro-indolenine derivatives.82... 

The reaction of yohimbine hydrochloride with m-chloroperbenzoic acid also results in the formation of a 7-chloro-indolenine derivative hence the product, following normal Ab-oxidation, is the JVb-oxide (141). In contrast to the 7-chloro-indolenine derivatives, no conversion into the corresponding oxindole analogue... 

Partial syntheses of oxindoles and pseudoindoxyls were already known at the time of publication of Volume VIII. Exposure of yohimbines or heteroyohimbines to Z-butyl hypochlorite (46) or lead-IV salts of strong carboxylic acids (4) afforded the corresponding indolenines (Cl, X = Cl or RCO2 ), which upon acid treatment rearranged to oxindoles, whereas base treatment led to pseudoindoxyls. The method has been used in the partial syntheses of mitraphylline and isomitraphylline (46). 

Reductive cleavage of indole alkaloids. Le Men et al. have reported the reductive cleavage of indolenine alkaloids (equation I) and of oxindole alkaloids (equation 11) with formic acid and formamide. 

This reaction proceeds by capture of the 3-bromo-3ET-indolenine intermediate by water, leading to formation of an oxindole which is further brominated. By the use of a limited amount of NBS, the subsequent bromination can be avoided. 

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