Approaches related to the synthesis of Fmoc-Ser/Thr[GalNAc(Ac)3-α-D]-OH

Protein O-glycosylation, particularly mucin-type glycosylation, plays important biological roles. However, the limited access to homogeneously glycosylated protein isoforms (glycoforms) has impeded the establishment of correlations between diverse glycan structures and protein properties, restricting therapeutic and diagnostic applications. Chemical synthesis of glycopeptides utilizing properly protected glycoamino acids as building blocks has emerged as a valuable approach to address the gap in glycoform acquisition. This review focuses on past efforts for the chemical synthesis of two fundamental glycoamino acid building blocks, Fmoc-Ser[GalNAc(Ac)₃-α-D]-OH and Fmoc-Thr[GalNAc(Ac)₃-α-D]-OH, by categorizing various methods based on glycosylation donors. The emphasis is on stereoselectivity, synthetic route length, and overall yield, with the goal of inspiring and guiding future research to further improve their synthetic efficiency and promote advancements in protein glycosylation studies and applications.