Pyrido[1,2-a]pyrimidinium salts. Part I. Synthesis from 2-aminopyridines and interconversion with 2-(2-acylvinylamino)pyridines

Abstract

The scope and steric restrictions of a route for the preparation of pyrido[1,2-a]pyrimidinium salts from 2-aminopyridines and 1,3-dicarbonyl compounds, or their acetals, have been investigated. 2-(2-Acylvinylamino)pyridines and not, as formerly suggested, acylethylidenaminopyridines, are shown to be the intermediates in this type of reaction, and the previous claim for the synthesis of a 1,8-naphthyridine under similar conditions is disproved. Good yields of twenty-six pyrido[1,2-a]pyrimidinium perchlorates and bromides have thus been obtained from the 2-aminopyridines in one step by reactions carried out at room temperature. Spectroscopic evidence is presented for all the products, including those where the formation of positional isomers is theoretically possible.