The reaction of the protected L-glutamine (I) with N,O-dimethylhydroxylamine (II) by means of isobutyl chloroformate and NMM in dichloromethane gives the methoxyamide (III), which is reduced with DIBAL in THF yielding the glutaminal (IV). The condensation of (IV) with phosphonate (V) by means of NaN(SiMe3)2 in THF affords the carbamoylhexenoate (VI), which is deprotected with TFA and HCl and condensed with N-(tert-butoxycabonyl)-4-methyl-L-phenylalanine (VII) by means of HOBT, NMM and EDC in dichloromethane to give the dipeptide (VIII), which is finally deprotected with HCl and condensed with N-(cyclopentylsulfanylcarbonyl)-L-tert-leucine (IX) by means of HOBT as before.