A fully unprotected (free amine, free acid) L-serine glycoamino acid carrying a β-D-GlcNAc residue. This deprotected building block is suited to use as a reference standard, an enzymatic substrate/acceptor, or for conjugation and analytical work rather than direct Fmoc-SPPS. 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.