Reaktionen von 3‐Dimethylamino‐2,2‐dimethyl‐2H‐azirin mit. NH‐aciden Heterocyclen; Synthese von 4H‐imidazolen

Abstract

Reactions of 3‐Dimethylamino‐2,2‐dimethyl‐2H‐azirine with NH‐Acidic Heterocycles; Synthesis of 4H‐ImidazolesIn this paper, reactions of 3‐dimethylamino‐2,2‐dimethyl‐2H‐azirine (1) with heterocyclic compounds containing the structure unit CONHCONH are described. 5,5‐Diethylbarbituric acid (5) reacts with 1 in refluxing 2‐propanol to give the 4H‐imidazole derivative 6 (Scheme 2) in 80% yield. The structure of 6 has been established by X‐ray crystallography. Under similar conditions 1 and isopropyl uracil‐6‐carboxylate (7) yield the 4H‐imidazole 8 (Scheme 3), the structure of which is deduced from spectral data and the degradation reactions shown in Scheme 3. Hydrolysis of 8 with 3N HCl at room temperature leads to the α‐ketoester derivative 9, which in refluxing methanol gives dimethyl oxalate and 5‐dimethyl‐amino‐2,4,4‐trimethyl‐4H‐imidazole (10). On hydrolysis the latter is converted to the known 2,4,4‐trimethyl‐2‐imidazolin‐5‐one (11) [6]. Quinazolin‐2,4 (1H, 3H)‐dione (12) and imidazolidinetrione (parabanic acid, 14) undergo with 1 a similar reaction to give the 4H‐imidazoles 13 and 15, respectively (Schemes 4 and 5).In Scheme 6 two possible mechanisms for the formation of 4H‐imidazoles from 1 and heterocycles of type 16 are formulated. The zwitterionic intermediate f corresponds to b in Scheme 1. Instead of dehydration as in the case of the reaction of 1 with phthalohydrazide [3], or ring expansion as with saccharin and cyclic imides [1] [2], f, undergoes ring opening (way A or B). Decarboxylation then leads to the 4H‐imidazoles 17.