Synthesis and Autoxidation of New Tetracyclic 9H,10H-Indolizino[1,2-b]indole-1-ones

The new tetracyclic 9H,10H-indolizino[1,2-b]indole-1-one derivatives (7a − d, 7ea, 7eb) have been synthesized by modified Fischer indole synthesis from the enol ether of 2,5-dihydroxy-7-methyl-6-cyano-indolizine (3) and arylhydrazines (4a − g). Attempted N-methylation of 7a − d produced a series of autoxidized products including 10-hydroperoxy-1-methoxyindolizino[1,2-b]indole (9a − d) as the major product accompanied with methylperoxides (10a − d and 11a − d) and 2-formyl-3-(pyridine-2-yl)indole (12a, 12c) derivatives as the minor products. A plausible mechanism of the autoxidation is postulated based on the isolation of some intermediates. The reaction is thought to proceed through azaenolate/enamine intermediates following a novel type of autoxidation.