合成路线1

Intermediate (VI) has been obtained as follows: The condensation of benzyloxycarbonyl-L-isoleucine (IX) with malonic acid monomethyl ester potassium salt (X) by means of carbonyldiimidazole (CDI) and MgCl2 in THF gives the ketoester (XI), which is reduced to the hydroxyester (XII) with NaBH4 in methanol. The methylation of (XII) with methyl iodide and silver oxide in DMF affords the N-methyl methoxyester (XIII). The hydrolysis of (XIII) with NaOH in dioxane/water followed by reesterification with isobutylene gives the tert-butyl ester (XIV), which is deprotected by hydrogenation over Pd/C, yielding the amino acid (XV). The condensation of (XV) with benzyloxycarbonyl-L-valine (XVI) by means of DCC in dichloromethane gives the protected dipeptide (XVII), which is debenzylated as usual, yielding (XVIII). The condensation of (XVIII) with N,N-dimethyl-L-valine (XIX) affords the tripeptide ter-butyl ester (XX). Finally, (XX) is hydrolyzed by treatment with trifluoroacetic acid to the free acid intermediate (VI).