The curtius reaction in the synthesis of noncondensed polynuclear azoles

Abstract

Polynuclear noncondensed systems in which the azole rings are bound by urethane or carbamide fragments have been synthesized by the reaction of azole carboxylic acid azides with heterocyclic alcohols.With the aim to synthesize noncondensed polynuclear azole containing systems we have previously [1, 2] used methods of adding heterocycles to the core structure having an active substituent in the form of an azide, nitrile, or chloromethyl group, halogen, or acetylenic bond. The methods allow us to connect a number of heterocycles separated by a methylene group or by other fragments.In this study continuing our work on the synthesis of polynuclear heterocyclic structures we have examined the possible use of the Curtius rearrangement of heterocyclic acid azides to isocyanates and their subsequent reaction with azole-containing alcohols to yield polynuclear structures in which the azole rings are separated by carbamide or urethane fragments.It is known that acyl and aroyl azides undergo the Curtius rearrangement upon heating to give alkyl and aryl isocyanates. The latters subsequently react with compounds having an active hydrogen atom, e.g. alcohols, amines, etc to give urethanes, ureas, and other compounds [3,4]. However the behavior of triazoles and tetrazoles, unsubstituted at the nitrogen atom, and of other heterocyclic alcohols has remained unclear to this time.In order to assess the reactivity of the azole-containing alcohols 1a-d and glycol 2 with isocyanates we have carried out their reaction with benzoyl azide (Scheme 1). Since isocyanates are readily formed from acyl azides upon heating in an anhydrous medium [3,4] their initial separation was not necessary. Reactions of compounds 1a-d, 2 with benzoyl azide were carried out on heating in absolute dioxane. It was found that alcohols 1a-d and glycol 2 react without particular complications with the phenyl isocyanate formed during the Curtius rearrangement to yield the N-phenylurethanes 3a-d and 4 (Table 1).