An Fmoc-protected L-serine building block carrying an α-D-mannose 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 four acetate esters (Ac4), which keep the glycan inert during synthesis and are removed under mild base (e.g., sodium methoxide / hydrazine) after assembly. The installed glycan is a mannose residue (O-mannosylation), a biologically important Ser/Thr modification found in α-dystroglycan and other mannosylated proteins, used to build O-mannosyl glycopeptides.