Synthesis of functionalized amino acids by ring‐opening reactions of aliphatically substituted aziridine‐2‐carboxylic esters

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Abstract
Nucleophilic ring‐opening reactions of 3‐alkylaziridine‐2‐carboxylic esters are described. Without ring activation at nitrogen, ring opening could only be accomplished with ethereal hydrogen chloride. Introduction of electron‐withdrawing activating groups at the nitrogen atom was necessary. Three types of activating groups were used, viz. acyl, alkoxycarbonyl and sulfonyl groups. In the presence of a Lewis‐acid catalyst, ring opening with two nucleophiles, namely indole and benzenethiol, could be accomplished. SN2‐type attack at C3 was observed exclusively. With Brönsted acids (hydrogen chloride, formic acid, acetic acid), ring opening could also be effected. 3‐Formyloxy derivatives could also be prepared by reaction of the activated aziridines with N,N‐dimethyl‐formamide in the presence of boron trifluoride etherate. Reaction with sodium azide caused some difficulties; complex product mixtures were usually obtained. N‐Sulfonylaziridine‐2‐carboxylic esters gave a mixture of the two possible regioisomers, resulting from attack at C2 and C3. Treatment of N‐sulfonylaziridine‐2‐carboxylic esters with trimethylsilyl azide, however, gave products exclusively ring‐opened at C2. On reaction of the activated aziridines with acetonitrile in the presence of boron trifluoride etherate, a ring‐expansion reaction was observed, leading to imidazolines. On standing, these heterocyclic compounds slowly hydrolyzed to α,β‐diamino carboxylic acid derivatives.

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