Preparation of New Nitrogen-Bridged Heterocycles. 40. Synthesis of 1,4-Dihydropyrido[2,3-b]indolizin-4-one Derivatives

The title compounds, 1,4-dihydropyrido[2,3-b]indolizin-4-one derivatives, were prepared in moderate to good yields via the N-formylation of some 3-acetyl-2-(alkylamino)indolizines with triethyl orthoformate and acetic anhydride in the presence of zinc chloride followed by the intramolecular dehydration of the resulting 3-acetyl-2-(N-alkylformamido)indolizines under strong alkaline conditions.