Benzopyrans. Part 44. Synthesis of 3-(4-oxo-4H-1-benzopyran-2yl)indolizines

摘要

The pyridinium methylid (5), generated from 1-(4-oxo-4H-1-benzopyran-2-yl)methylpyridinium iodide (4) in the presence of a base, undergoes [3+2] cycloaddition with dimethyl acetylenedicarboxylate and ethyl propiolate, the cycloadducts rapidly aromatising to the indolizines (8) and (9), respecively. Similar cycloaddition of 5 with ethyl acrylate and acrylonitrile is also followed by dehydrogenation giving respectively the indolizines (9) and (10).