The reaction of N-(benzyloxycarbonyl)-L-asparagine (I) with bis(trifluoroacetyl)iodobenzene (BTI) in DMF/water gives 3-amino-2(S)-(benzyloxycarbonylamino)propionic acid (II), wehich is protected at the amino group with tert-butoxycarbonyl anhydride yielding carbamate (III). The esterification of (III) with ethanol WSC and DMAP affords the expected ethyl ester (IV), which is selectively deprotected with methanesulfonic acid in acetonitrile providing 3-amino-2-(benzyloxycarbonylamino)propionic acid ethyl ester (V). The condensation of (V) with N-(tert-butoxycarbonyl)-beta-alanine (VI) by means of HOBT, WSC and triethylamine in DMF gives the protected propionamidopropionic ester (VII), which is selectively deprotected by hydrogenation with H2 over Pd/C in ethanol/ethyl acetate giving the 2-aminopropionic ester (VIII). The condensation of (VIII) with 4-ethylbenzenesulfonyl chloride (IX) by means of triethylamine in dichloromethane yields the sulfonamide (X), which is deprotected with methanesulfonic acid as before afording intermediate (XI) with a free amino group, which is condensed with 4-(tert-butoxycarbonylamino)benzoic acid (XII) by means of HOBT and WSC in DMF providing the fully protected target compound (XIII).

The deprotetion of (XIII) first with trifluoroacetic acid gives the ethyl ester (XIV), which is finally hydrolyzed with HCl in hot acetic acid.

The intermediate 4-(tert-butoxycarbonylamindino)benzoic acid (XII) has been obtained as follows: Comerciaally available 4-amidinobenzamide (XV) is hydrolyzed with aqueous HCl to give 4-amidinobenzoic acid (XVI), which is then treated with tert-butoxycarbonyl anhydride and N,O-bis(trimethylsilyl)acetamide in dichloromethane to afford the target intermediate (XII).