Aspartic acid beta-tert-butyl ester (II) was silylated using bis(trimethylsilyl)acetamide and subsequently coupled with the hydroxysuccinimidyl ester of Z-valine (I) to furnish dipeptide (III) (1). This was converted to the corresponding mixed anhydride (IV) with isobutyl chloroformate and N-methylmorpholine. Reaction of anhydride (IV) with ethereal diazomethane, followed by treatment of the intermediate diazo ketone with HBr in HOAc, gave rise to the bromo ketone (V). Tetrafluorophenol (VI) was alkylated with bromide (V) to yield ether (VII). The ketone function of (VII) was then reduced to alcohol (VIII) employing NaBH4. Further hydrogenolysis of the benzyloxycarbonyl group of (VIII) provided amine (IX).
Acylation of tetrachloroaniline (X) with methyl oxalyl chloride (XI) provided the oxamate ester (XII), which was further hydrolyzed to the N-(tetrachlorophenyl)oxamic acid (XIII) by using LiOH. Coupling of acid (XIII) with the intermediate amine (IX) in the presence of HATU furnished the oxalic diamide (XIV). The alcohol function of (XIV) was then reoxidized to ketone (XV) employing the Dess-Martin periodinane reagent. Finally, the tert-butyl ester group of (XV) was cleaved by treatment with trifluoroacetic acid.