Silver Oxide Promoted\nSynthesis of Alpha <i>O</i>‑GalNAc Containing Glyco-Amino\nAcids: Synthesis of Core 2\nContaining Glyco-Amino Acids for Solid Phase Synthesis of Glycopeptides

<i>O</i>-GalNAc glycans\non glycoproteins with\neight different\ncore structures sharing a common α-glycosidic linkage (<i>O</i>-GalNAc-α-Ser/Thr) are critical in various physiological\nand pathological processes. Among the eight <i>O</i>-GalNAc\nglycan cores, core 2 characterized by a GlcNAcβ1–6(Galβ1–3)GalNAc\nstructural motif plays a significant role in regulating diverse biological\nprocesses, such as immune response modulation, adhesive properties\nof selectins, and gastrointestinal tract protection. However, the\nlarge-quantity synthesis of core 2 containing glyco-amino acids for\ndownstream solid-phase peptide synthesis is challenging. In this work,\nwe successfully employed a silver oxide for coupling a 2-azido-galactosyl\nchloride donor with two acceptors, Fmoc-Ser/Thr-O<sup>t</sup>Bu, respectively,\nfor the large-scale synthesis of the two important intermediates,\nα-GalN<sub>3</sub>–Fmoc-Ser/Thr-O<sup>t</sup>Bu, which\ncan be further utilized for the large-scale synthesis of core 2 containing\nglyco-amino acids. The two intermediates, α-GalN<sub>3</sub>–Fmoc-Ser/Thr-O<sup>t</sup>Bu, were utilized for synthesizing\ncore 2 containing Fmoc-Ser/Thr-COOH. The synthesis of core 2 containing\nFmoc-Ser-COOH was achieved on a 1.95 g scale, while the synthesis\nof core 2 containing Fmoc-Thr-COOH was achieved on a 0.38 g scale.\nAdditionally, the synthesis of the 2-azido-galactosyl chloride donor\nwas optimized into a three-step process with only one column chromatography\npurification. Finally, core 2 containing Fmoc-Ser/Thr-COOH were applied\nfor the synthesis of glycosylated CCR1 and CCR5 N-terminal peptides.