Treatment of penicillamine (I) with formaldehyde produced thiazolidine (II), which was then protected with Boc2O to give carbamate (III). Coupling of (III) with 2-methylbenzylamine (IV) by means of either diphenylphosphoryl chloride (DPPCl), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide-HCl (EDC) or dicyclohexylcarbodiimide (DCC) afforded the corresponding amide (V). After acid cleavage of the Boc group of (V), the deprotected thiazolidine (VI) was coupled with N-Boc-(S,S)-3-amino-2-hydroxy-4-phenylbutyric acid (VII) using EDC to give diamide (VIII). Further acid deprotection of the Boc group of (VIII) yielded (IX). This was finally coupled with 3-hydroxy-2-methylbenzoic acid (X) to furnish the title compound. Alternatively, amine (IX) was coupled with 3-acetoxy-2-methylbenzoic acid (XI) to afford (XII), which was then deacetylated using NaOH in MeOH.
Treatment of penicillamine (I) with formaldehyde produced thiazolidine (II), which was then protected with Boc2O to give carbamate (III). Coupling of (III) with 2-methylbenzylamine (IV) by means of either diphenylphosphoryl chloride (DPP-Cl) or 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide-HCl (EDC) afforded the corresponding amide (V). After acid cleavage of the Boc group of (V), the deprotected thiazolidine (VI) was coupled with N-Boc-(S,S)-3-amino-2-hydroxy-4-phenylbutyric acid (VII) using EDC to give diamide (VIII). Further acid deprotection of the Boc group of (VIII) yielded (IX). This was finally coupled with 2-ethyl-3-hydroxybenzoic acid (X) to furnish the title compound. Alternatively, amine (IX) was coupled with 3-acetoxy-2-ethylbenzoic acid (XI) to afford (XII), which was then deacetylated using NaOH in MeOH.