P-Nitrophcnyl ester

I Antanovic, GT Young. Amino-acids and peptides. Part XXV. The mechanism of base catalysed racemisation of the p-nitrophcnyl esters of acylpeptides. J Chem Soc C 595, 1967. 

The p-nitrophcnyl ester prepared in part (e) is an active ester. The p-nitrophenyl group is a good leaving group and can be displaced by the amino nitrogen of valine ethyl ester to form a new peptide bond. 

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