Condensation of the Weinreb amide of N-Boc-proline (I) with the lithium anion of benzothiazole (II) in THF at -78 C gave benzothiazolyl ketone (III), which was reduced with NaBH4 to afford the diastereomeric mixture of alcohols (IV). Deprotection of the Boc group of (IV) by means of HCl in EtOAc provided pyrrolidine (V). This was then coupled with Z-leucyl-glycine (VI) in the presence of PyBOP to give the peptidyl alcohol (VII). Finally, Swern oxidation of (VII) furnished the corresponding ketone.

Prolinamide (I) was coupled with Z-leucylglycine (II) in the presence of DCC and HOBt to give the tripeptide amide (III). The terminal carboxamido group of (III) was then dehydrated to the corresponding nitrile by treatment with SOCl2 and N-methylmorpholine.