An Fmoc-protected L-serine building block carrying a Neu5Acα2-3Galβ1-3GalNAcα trisaccharide, 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 sialic acid carboxyl is protected as a benzyl ester to prevent side reactions during synthesis; it is removed by hydrogenolysis or basic hydrolysis during global deprotection. The installed glycan is the sialyl-T antigen (Neu5Acα2-3Galβ1-3GalNAcα-O-Ser/Thr), a sialylated core 1 mucin O-glycan relevant to tumor glycobiology and selectin/Siglec recognition studies.