Synthesis of an Fmoc-threonine bearing core-2 glycan: a building block for PSGL-1 via Fmoc-assisted solid-phase peptide synthesis

Abstract

Selectins (L, E, and P) are vascular endothelial molecules that play an important role in the recruitment of leukocytes to inflamed tissue. In this regard, P-Selectin glycoprotein-1 (PSGL-1) has been identified as a ligand for P-Selectin. PSGL-1 binds to P-Selectin through the interaction of core-2 O-glycan expressing sialyl Lewis(x) oligosaccharide and the three tyrosine sulfate residues. Herein, we report the synthesis of threonine-linked core-2 O-glycan as an amino acid building block for the synthesis of PSGL-1. This building block was further incorporated in the Fmoc-assisted solid-phase peptide synthesis to provide a portion of the PSGL-1 glycopeptide.

Figure 1

Fmoc-Threonine bearing core-2-O-glycan as a building block for PSGL-1.

Scheme 1

Synthesis of Donor 5 Reagents and conditions. a. p-anisaldehyde 4 M NaOH, 1 h, 71% b. Ac₂O, DMAP (cat.), 16 h, 80% c. 5 M HCl, acetone, 0.5 h, 92% d. TrocCl, pyridine, DIPEA, 16 h, 80%. 3. EtSH, BF₃OEt₂, 4Å MS, CH₂Cl₂, 2 h, 80%.

Scheme 2

Synthesis of disaccharide 10. Reagents and conditions. a. NaOMe, MeOH, 3 h b. benzaldehyde dimethyl acetal, PTSA, MeNO₂, 2 h, 53%, steps c. TMSOTf, 8, CH₂Cl₂, 4Å MS, CH₂Cl₂, 0 °C, 1 h, 61% d. 80% HOAc, 70 °C, 2 h, (10: 54%, 9: 28%).

Scheme 3

Synthesis of trisaccharide-linked Fmoc-threonine 1. Reagents and conditions. a. 5, NIS, TfOH, 4Å MS, CH₂Cl₂, 0 °C, 12 h (11: 51%, 11a: 19%) b. Ac₂O, DMAP, Pyridine, 1 h, 47%. C. Zn dust, Ac₂O–AcOH–THF, 3 h, 47% d. TFA, CH₂Cl₂, 2 h, 72%.

Scheme 4

Incorporation of trisaccharide linked threonine 1 into Fmoc-assisted solid-phase peptide synthesis. Reagents and Conditions. a. Fmoc-AA-OH; HBTU/HOBt b. 20% piperidine in DMF c. Repeat steps a and b for subsequent amino acid coupling d. 9.5:0.1:0.2:0.2 TFA–H₂O–EDT–Et₃SiH, 2 h.