The reaction of N-(cyclohexanecarbonyl)-D-alanine (I) with 3-mercapto-2-methylpropanoic acid (II) by means of carbonyldiimidazole (III) in THF gives 3-[N-(cyclohexanecarbonyl)-D alanilthio]-2-methyl-propanoic acid (IV), which is then condensed with L-protine (V) by means of triethylamine and ethyl chloroformate (VI) o THF.