α-Selective glycosylation affords mucin-related GalNAc amino acids and diketopiperazines active on Trypanosoma cruzi

This work addresses the synthesis and biological evaluation of glycosyl diketopiperazines (DKPs) cyclo[Asp-(αGalNAc)Ser] 3 and cyclo[Asp-(αGalNAc)Thr] 4 for the development of novel anti-trypanosomal agents and Trypanosoma cruzi trans-sialidase (TcTS) inhibitors. The target compounds were synthetized by coupling reactions between glycosyl Amino acids αGalNAc-Ser 7 or αGalNAc-Thr 8 and the amino acid (O-tBu)-Asp 17, followed by one-pot deprotection-cyclisation reaction in the presence of 20% piperidine in DMF. The protected glycosyl amino acid intermediates 7 and 8 were, in turn, obtained by α-selective, HgBr2-catalysed glycosylation reactions of Fmoc-Ser/Thr benzyl esters 12/14 with αGalN3Cl 11, being, subsequently, fully deprotected for comparative biological assays. The DKPs 3 and 4 showed relevant anti-trypanosomal effects (IC50 282-124 μM), whereas glycosyl Amino acids 1 and 2 showed better TcTS inhibition (57-79%) than the corresponding DKPs (13-25%).