Oxidative Cyclization to Indoles

Indole alkaloids represent a fundamental class of natural products and have important biological properties. Therefore, a large number of different methods for indole 

The tricarbonyliron-coordinated cyclohexadienylium salts are readily available on a large scale by azadiene-catalyzed complexation of the corresponding cyclo-hexadienes with pentacarbonyliron [23] and subsequent hydride abstraction using trityl tetrafluoroborate [24]. Alkylation of methyl lithioacetate with the iron complex 

Selective reduction of the methyl ester to the corresponding aldehyde using DIBAL at low temperature and subsequent reductive amination with iodopiperonyl-ammonium chloride affords the tricarbonyliron-cyclohexadiene complex with the secondary alkylamine in the side chain. Iron(O)-mediated oxidative cydization 

Electrophilic aromatic substitution of 2,3-dimethyl-4-methoxyani]ine by reaction with the tricarbonyliron-coordinated cyclohexadienylium salt generates the aryl-substituted tricarbonyliron-cyclohexadiene complex. Treatment of this complex with very active manganese dioxide results in oxidative cyclization and aromatization with concomitant demetallation to afford directly 4-deoxycarbazomycin B, a degradation product of the antibiotic carbazomycin B [32]. Using ferricenium hexafluorophos- 

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