Synthesis of enantiopure azetidine 2-carboxylic acids and their incorporation into peptides
摘要:
Enantiopure azetidine 2-carboxylic acids were prepared by hydrolysis of the corresponding 2-cyano azetidines, without ring cleavage of the azetidine or epimerization. The produced amino acids, which are conformationally constrained analogues of phenylalanine. can be cleanly debenzylated and used for the synthesis of tripeptides. In the course of the synthesis of new enantiopure 2-cyano azetidines through intramolecular alkylation of a metallated amino nitrile, it was found that the involved anionic cyclisation can be thermodynamically controlled, thus enhancing its diastereoselectivity. (C) 2003 Elsevier Ltd. All rights reserved.
alkylation of the corresponding azetidine with methyl trifluoromethanesulfonate. These compounds were treated with an array of nitrogen (azide anion or benzylamine) and oxygen (acetate anion or alkoxides) nucleophiles which provided interesting insights into the regioselectivity of the ring-opening process. Nucleophilic opening occurred in most cases regioselectively at C-4 with azetidinium ions possessing
N-Alkyl azetidines react with triphosgene (BTC) following two possible pathways: N-alkyl ring scission or ring cleavage, to give cyclic or acyclic N-carbamoyl chlorides. Predominance of one pathway over the other is governed by the nature of the substituents on the azetidine ring and on the nitrogen atom as well as by the relative stereochemistry of the ring substituents, and is examined in detail
Upon treatment with DAST (diethylaminosulfur trifluoride) enantiopure 2-hydroxyalkylazetidines rearrange into 3-fluoropyrrolidines. The reaction is stereospecific and involves a bicyclic 1-azoniabicyclo[2.1.0]pentane intermediate which is regioselectively opened by a fluoride anion.
Meisenheimer rearrangement. This ring expansion leading to isoxazolidines occurs readily, without trapping of the transient N-oxide. Starting with azetidines bearing a nitrile or an ester group at C-2, the rearrangement is regioselective. However, a varying amount of epimerization on the migrating radical is observed, which can also be observed with the related [1,2] Stevens rearrangement.