One-pot synthesis of β-d-Gal>(1 → 4)[β-d-Galp(1 → 6)]-d-GlcNAc, a ‘core’ trisaccharide linked O-glycosidically in glycoproteins of Trypanosoma cruzi

Tin(IV) chloride-promoted condensation of benzyl 2-acetamido-3-O-benzoyl-2-deoxy-α-d-glucopyranoside (4) with penta-O-benzoyl-β-d-galactopyranose (6) gave the derivative of β-d-Galp-(1 → 6)-α-d-GlcNAc7 in 80% yield. This was glycosylated with penta-O-benzoyl-α,β-d-galactofuranose (5), employing the same catalyst, to afford the protected benzyl per-O-benzoyl-β-d-Gal>(1 → 4)[β-d-Galp(1 → 6)]d-GlcNAc 10 in 41% yield. Alternatively, compound 10 was obtained directly in a one-pot reaction from 4, by sequential addition of 6 and 5 (34% yield). β-Glycosidic linkages were diastereoselectively formed. De-O-benzoylation of 10, followed by heterogeneous catalytic transfer hydrogenolysis of the benzyl group afforded the free trisaccharide β-d-Gal>(1 → 4)[β-d-Galp(1 → 6)]-d-GlcNAc (14) in 98% yield from 10. Sodium borohydride reduction of 14 gave the corresponding alditol, whose spectral data were identical to those reported for the alditol obtained from the 38–43 kDa cell-surface glycoprotein of Trypanosoma cruzi.