合成路线1

Protection of the carboxyl group of Boc-L-threonine (I) with [2-(chloromethoxy)ethyl]trimethylsilane (SEM-Cl) (II) by means of Li2CO3 affords protected threonine (III). Separately, Z-L-tyrosine (IV) is treated with dimethyl sulfate and KOH in THF with tetrabutylammonium hydrogen sulfate as a catalyst to furnish N,O-dimethyl-L-tyrosine (V), which is then converted into secondary amine (VI) by coupling with protected threonine (III) either by means of 2,4,6-trichlorobenzoyl chloride in THF and DMAP in benzene or with isopropenyl chloroformate, Et3N and DMAP, followed by elimination of the carbobenzyloxy group by hydrogenation over Pd/C. Coupling of Z-L-leucine (IX) to L-proline methyl ester (X) by means of DCC, HOBt and NMM in CH2Cl2, followed by hydrolysis with LiOH, affords Z-leucylproline (VII), which is then coupled to amine (VI) by means of BOPCl and Et3N in CH2Cl2, and then subjected to deprotection with hydrofluoric acid in acetonitrile affording intermediate (VIII).