Novel synthetic routes to nitrogen-bridged tricyclic derivatives of pyrrolo[2,1,5-cd]indolizine and pyrrolo[2,1,5-de]quinolizine derived from 2-acyl-N-(acylmethyl)pyridinium halides

Abstract

Novel synthetic routes to nitrogen-bridged derivatives of pyrrolo[2,1,5-cd]indolizine and pyrrolo[2,1,5-de]quinolizine were developed starting from 2-acyl-N-(acylmethyl)pyridinium halides. Thus, 2-benzoyl-N-phenacylpyridinium bromide (1) afforded 3,4-diphenylpyrrolo[2,1,5-cd]indolizines (4) via 1,3-dipolar cycloaddition, to yield 3,5-dibenzoylindolizines (3), followed by intramolecular McMurry coupling. Similarly, 2-(1,3-dioxolan-2-yl)-N-phenacylpyridinium bromide (5) gave 3-phenylpyrrolo[2,1,5-cd]indolizine (7) together with the unexpected product 3-phenyl-4-hydroxypyrrolo[2,1,5-cd]indolizines (8). However, 2-acetyl-N-phenacylpyridinium bromide (13) or 2-benzoyl-N-acetonylpyridinium bromide (16) underwent a tandem reaction of aldol condensation and 1,3-dipolar cycloaddition to form 3-phenyl-5H-pyrrolo[2,1,5-de]quinolizin-5-ones (15) or 5-phenyl-3H-pyrrolo[2,1,5-de]quinolizin-3-ones (17) in a single step. These novel procedures are general and can be carried out under convenient conditions.