The condensation of 1-(D-3-acetylthio-2-methylpropanoyl)-L-proline (I) with L-phenylalanine tert-butyl ester (II) by means of N-methylmorpholine and phenyl chloroformate in THF gives 1-(D-3-acetyl-thio-2-methylpropanoyl)-L-prolyl-L-phenylalanine tert-butyl ester (III), which is hydrolyzed with trifluoroacetic acid in anisole.