Fused mesoionic heterocycles: synthesis of 1,3,4-oxadiazolo[3,2-a]pyridine and 1,3,4-thiadiazolo[3,2-a]pyridine derivatives

Abstract

Two general methods are reported for the preparation of mesoionic derivatives of 1,3,4-oxadiazolo[3,2-a] pyridine and 1,3,4-thiadiazolo[3,2-a]pyridine from 1-amino-4,6-diphenyl-2-pyridone (3) and 1-amino-4,6-diphenylpyridine-2-thione (4), respectively. The first involves the initial formation of N,N′-disubstituted ureas or thioureas (7)–(11), which undergo cyclization either by thermal treatment [to give (14) and (15)] or by the action of Ph₃P–CCl₄[to give (16)–(20)]. The second is based in the reaction of the iminophosphoranes (12) and (13) with carbon dioxide or carbon disulphide and isocyanates or isothiocyanates to give 2-olates [(14) and (15)], 2-thiolates [(21) and (22)], or 2-aminides [(16)-(20)]. The reactions of the N-amino-heterocycle (4) with phenyl isothiocyanate and ethoxycarbonyl isothiocyanate led directly to the mesoionic compounds (19) and (20), respectively.