The condensation of tert-butoxycarbonyltyrosine (I) with methionylglycine methyl ester S-oxide (II) gives the protected tripeptide (III), which is saponified to tert-butoxycarbonyltyrosyl-methionylglycine S-oxide (IV). The condensation of (IV) with 4-nitrophenylalanyl-prolinamide (V) yields the protected tripeptide (VI), which is finally deprotected as usual.