Ribonuclease T1 Peptides. II. Synthesis of a Protected Pentapeptide Corresponding to Sequence 12–16

A protected pentapeptide corresponding to sequence 12–16 of ribonuclease T1 namely benzyloxycarbonyl-l-seryl-l-seryl-l-seryl-β-t-butyl-l-aspartyl-l-valine ethyl ester (XI), was synthesized by coupling of benzyloxycarbonyl-seryl-serine azide with a tripeptide ester, seryl-β-t-butylaspartyl-valine ethyl ester, which is derived from benzyloxycarbonyl-seryl-β-t-butyl-aspartyl-valine ethyl ester by the hydrogenolysis. The hydrogenolysis and the subsequent treatment with trifluoroacetic acid of the protected pentapeptide ester (XI) yielded a neutral pentapeptide ester, seryl-seryl-seryl-aspartyl-valine ethyl ester, as a crystalline product.