DDQ Oxidations in the Indole Area. Synthesis of 4-Alkoxy-β-carbolines Including the Natural Products Crenatine and 1-Methoxycanthin-6-one

Article abstract of DOI:10.1021/jo00270a029

The seven-step synthesis of the cytotoxic, antileukemic alkaloid 1-methoxycanthin-6-one (2b) is described.The pivotal steps are represented by the oxidation (DDQ, aqueous THF, room temperature) of 1-(methoxycarbonyl)-1,2,3,4-tetrahydro-β-carboline (10) to provide the 4-oxo-substituted derivative 14 in 78percent yield, and conversion of the 4-oxo analogue 7 into 4-methoxy-1-alkyl-β-carboline (23) via a methoxylation-oxidation process .This four-step, one-pot reaction has been shown to be general; 4-oxo-1,2,3,4-tetrahydro-β-carboline (18) was converted into the corresponding 4-methoxy-, 4-ethoxy-, 4-(allyloxy)-, and 4-(benzyloxy)-β-carbolines (19a-d, respectively) on heating in the appropriate alcohol in the presence of pTSA and a trialkyl orthoformate (Table II).The proposed mechanism for this intriguing transformation is outlined in Scheme IV.Execution of this process has also resulted in a four-step preparation of crenatine (1a), a 4-methoxy-1-ethyl-β-carboline alkaloid.Finally, steric and electronic parameters have also been successfully manipulated to direct the DDQ oxidation of 1,2,3,4-tetrahydro-β-carbolines to position 1, regiospecifically.The conversion of tetrahydro-β-carboline 25 into 2-acylindole 38 and benzamide 26 into 1-oxotetrahydro-β-carboline 27 (Table I), respectively, is in agreement with the proposed mechanism for this process.