Synthesis of the O-linked hexasaccharide containing β-d-Galf-(1→2)-β-d-Galf in Trypanosoma cruzi mucins

Abstract

The hexasaccharide β-D-Galp-(1→2)-[β-D-Galp-(1→3)]-β-D-Galp-(1→6)-[β-D-Galf(1→2)-β-D-Galf(1→4)]-D-GlcNAc (1) is the largest carbohydrate structure released as alditol by reductive β-elimination from mucins of some strains of T. cruzi. The terminal β-D-Galp units are sites of sialylation by trans-sialidase which transfers sialic acid from the host to the parasite. Hexasaccharide 1 was synthesized by a [3 + 3]-convergent strategy based on a nitrile assisted glycosylation, using the trichloroacetimidate method. The β-D-Galf-(1→2)-β-D-Galf-D-GlcNAc synthon was sequentially constructed from the reducing end to the non-reducing end employing benzyl α-D-galactofuranoside as starting material for the internal Galf unit. The choice of this novel precursor, obtained in one-reaction step from galactose, allowed the introduction of an orthogonal and participating levulinoyl group at O-2. Thus, the diastereoselective construction of the Galf-β(1→4)-GlcNAc linkage by the trichloroacetimidate method of glycosylation was achieved. The (1)H NMR spectrum of alditol 2 was identical to the product released by β-elimination from the parasite mucin.

Intrinsic Issues in the Assembly of 1,2‐Linked Oligosaccharides

“…Table presents archetypical examples to generate 1,2‐ trans linkages ‐either gluco or manno ‐ using an ester type (CB‐, Lev‐, Piv‐, ClAc‐) participating group as an orthogonal group . To obtain 1,2‐ cis linkages‐ either gluco or manno ‐, ether type non‐participating group (Bn‐, PMB‐, Nap‐) was required.…”

“…With Et 2 O, the yield reached 57% but the ratio β/α reversed to 1 : 1.4 in favor of the undesired isomer. Only a change of strategy to a sequential elongation of the non‐reducing end gave to the trisaccharide 20β with total control of stereoselectivity in each step …”

“…Moreover, the assembly of 1,2‐linked oligosaccharides could be considered as the “driving force” to achieve both stereocontrolled chemical 1,2‐ cis and 1,2‐ trans glycosylations . The motivation of the present review arises from distinctive issues that emerged from previous experiences in the synthesis of oligosaccharides with internal 2‐ O ‐glycosylated units …”

Synthesis of the O-linked hexasaccharide containing β-d-Galp-(1→2)-d-Galf in Trypanosoma cruzi mucins. Differences on sialylation by trans-sialidase of the two constituent hexasaccharides

“…Interestingly, in the preparation of the Galf(1→2)Galf hexasaccharide 7, similar reaction conditions gave higher yields (56 %). 28 When the terminal tetra-O-benzoyl-Galf was replaced by a tetra-O-acetyl-Galp in the trisaccharide acceptor 14, the glycosylation yield diminished but the β-stereochemistry was maintained. The 1 H and 13 C NMR spectra assignments for 15 were performed on the basis of the COSY and HSQC experiments.…”

“…We have recently reported the synthesis of the hexasaccharide 7. 28 We describe now the synthesis of (10) and the corresponding benzyl glycoside 11 and alditol 12 ( Figure 2). were analyzed as acceptors in the trans-sialidase reaction catalyzed by a recombinant TcTS in order to establish potential differences in specificity.…”