The trityl resin derivatized with 1,6-hexanediamine (I) is reacted with 2 nitrobenzenesulfonyl chloride (II) to give the resin-bound sulfonamide (III). Acylation of 5-amino-1-pentanol (V) with 2-(trimethylsilyl)ethyl 4-nitrophenyl carbonate (IV) affords the N-protected aminoalcohol (VI), which is subjected to Mitsunobu coupling with the sulfonamide resin (III) in the presence of 1,1' (azadicarbonyl)dipiperidine (ADDP) and tributylphosphine producing resin (VII). The N-Teoc protecting group of (VII) is selectively removed by means of tetrabutylammonium fluoride to give the amine resin (VIII), which is further coupled to N-Fmoc-O-t-butyl-L-tyrosine (IX) in the presence of HATU and collidine to furnish amide (X).
Selective removal of the N-Fmoc group from the fully protected resin (X) by treatment with piperidine in DMF leads to amine (XI), which is then acylated by butyric acid (XII) yielding the butyramide (XIII). The o-nitrobenzenesulfonyl group in (XIII) is then removed with mercaptoethanol and DBU to afford (XIV). Finally, cleavage from the resin and removal of the O-tert-butyl protecting group of (XIV) by means of trifluoroacetic acid and triisopropylsilane provides the title compound.