合成路线1

Removal of the N-(benzyloxycarbonyl) group from the protected dipeptide (I) by catalytic hydrogenolysis provided the dipeptide Val-Pro-O-t-Bu (II), which was N-acylated with anisoyl chloride (III) to afford anisoyl dipeptide (IV). The tert-butyl ester group of (IV) was then cleaved using trifluoroacetic acid to afford (VI). Alternatively, the intermediate (VI) could be obtained from unprotected Val-Pro-OH (V) by treatment with trimethylsilyl chloride and subsequent acylation of the silyl ester with acid chloride (III). The anisoyl dipeptide (VI) was further condensed with racemic threo-aminoalcohol (VII), either in the presence of EDC and HOBt or via conversion to the mixed anhydride with isobutyl chloroformate, to furnish the corresponding peptide amide (VIII) as a diastereomeric mixture. Subsequent oxidation of the alcohol group of (VIII) to the trifluoromethyl ketone (IX) was carried out using a modified Pfitzner-Moffat procedure in the presence of DMSO, EDC and dichloroacetic acid or with KMnO4 in basic medium. The desired S,S,S-isomer was isolated from the epimeric mixture after repeated crystallizations.