(f)The p-nitrophenyl ester prepared in part (e) is an “active”ester. The p-nitrophenyl group is agood leaving group and can be displaced by the amino nitrogen of valine ethyl ester to form anew peptide bond.(g)Hydrogenolysis of the Z-protected tetrapeptide ester formed in part (f) removes theZ protecting group.27.32Consider, for example, the reaction of hydrazine with a very simple dipeptide such as Gly-Ala.Hydrazine cleaves the peptide by nucleophilic attack on the carbonyl group of glycine.It is the C-terminal residue that is cleaved as the free amino acid and identified in the hydrazinoly-sis of peptides.NHCHCO2H2NNH2H3NCH2CH3NCHCO2OH2NCH2CNHNH2OCH3CH3Gly-Ala
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