Bridgehead nitrogen heterocycles. Synthesis of [1,2,4]triazolo[1,5-a]pyridin-1-ium and pyrido[2,1-f][1,2,4]triazin-9-ium derivatives

Abstract

A number of derivatives of bridgehead nitrogen heterocycles from pyrylium salts and amidrazones have been prepared. 2-Methylthio-4,6-diphenylpyrylium tetrafluoroborate (13) reacts with N³-aryl substituted amidrazones to yield derivatives of [1,2,4]triazolo[1,5-a]pyridin-1-ium tetrafluoroborates [(17)–(21)]. 2-Ethoxycarbonyl-4,6-diphenylpyrylium (14) reacts with N³-aryl substituted amidrazones to give 4-oxopyrido[2,1-f][1,2,4]triazin-9-ium derivatives [(23)–(27)]. Compound (14) also reacts under neutral conditions with unsubstituted amidrazones to yield the corresponding pyridinium salts [(29)–(33)]; these cyclised on treatment with base giving the zwitterionic 4-oxidopyrido[2,1-f][1,2,4]triazin-9-ium compounds [(34)–(39)].