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

Reactions of 3‐Dimethylamino‐2,2‐dimethyl‐2 H ‐azirine with NH‐Acidic Heterocycles; Synthesis of 4 H ‐Imidazoles In this paper, reactions of 3‐dimethylamino‐2,2‐dimethyl‐2 H ‐azirine ( 1 ) with heterocyclic compounds containing the structure unit CONHCONH are described. 5,5‐Diethylbarbituric acid ( 5 ) reacts with 1 in refluxing 2‐propanol to give the 4 H ‐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 4 H ‐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 3 N 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‐4 H ‐imidazole ( 10 ). On hydrolysis the latter is converted to the known 2,4,4‐trimethyl‐2‐imidazolin‐5‐one ( 11 ) [6]. Quinazolin‐2,4 (1 H , 3 H )‐dione ( 12 ) and imidazolidinetrione (parabanic acid, 14 ) undergo with 1 a similar reaction to give the 4 H ‐imidazoles 13 and 15 , respectively (Schemes 4 and 5 ). In Scheme 6 two possible mechanisms for the formation of 4 H ‐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 4 H ‐imidazoles 17 .