合成路线1

N-Boc-N-Methyl-L-valine (XII) was condensed with N,O-dimethylhydroxylamine by means of PyBOP as the coupling reagent. The resultant Weinreb amide (XIII) was then reduced to aldehyde (XIV) using LiAlH4. Wittig condensation of the valinal derivative (XIV) with (carbethoxyethylidene)triphenylphosphorane provided the unsaturated amino ester (XV). The N-Boc protecting group of (XV) was subsequently removed by treatment with trifluoroacetic acid, giving (XVI). Coupling of amino ester (XVI) with N-Boc-tert-leucine (XVII) was carried out via formation of the mixed anhydride of (XVII) with pivaloyl chloride. The resultant N-Boc dipeptide (XVIII) was further deprotected with trifluoroacetic acid, yielding (XIX). Coupling of dipeptide (XIX) with the N-Boc amino acid (IX) provided tripeptide (XX). The title compound was then obtained by sequential deprotection of the ethyl ester and N-Boc groups of (XX) using LiOH and trifluoroacetic acid, respectively.