An Fmoc-protected L-serine building block carrying a GlcNAcβ1-2Manα disaccharide, 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 acetate esters (peracetate), which keep the glycan inert during synthesis and are removed under mild base (e.g., sodium methoxide / hydrazine) after assembly. The installed glycan is GlcNAcβ1-2Man, a biosynthetic extension of O-mannose (core M3-type) glycans relevant to α-dystroglycan glycosylation, used to build O-mannosyl glycopeptides.