An Fmoc-protected L-serine building block carrying a β-D-GlcNAc residue, designed for solid-phase peptide synthesis (SPPS) of glycopeptides. The Nα-Fmoc group is removed under standard base conditions (piperidine) during chain elongation, while the free α-carboxylic acid is available for coupling to the growing resin-bound peptide. The sugar hydroxyl groups are protected as three acetate esters (Ac3), which keep the glycan inert during synthesis and are removed under mild base (e.g., sodium methoxide / hydrazine) after assembly. The installed glycan is O-GlcNAc (GlcNAcβ-O-Ser/Thr), the intracellular monosaccharide modification central to O-GlcNAcylation signaling, used to build O-GlcNAc peptides for signaling, nutrient-sensing, and antibody research.