Cleavage of the Boc group from the protected peptide (I) by means of HCl in dioxan gave (II). To this was coupled the carboxylic acid (III) using EDC and HOBt to afford the corresponding amide (IV).

Hydrogenolysis of the benzyl protecting groups provided (V). The desired pentasulfate analogue was obtained by treatment of (V) with SO3 in pyridine, followed by formation of the triethylamine salt.