Heparan sulfate (HS) is an important class of polysaccharides that plays roles in a wide range of biological events, including blood coagulation, cell differentiation, inflammatory responses, tumor metastasis and viral infections [1,2]. A thorough understanding of the biological activities of HS can lead to the development of novel therapeutics. HS consists of repeating disaccharide units of glucosamine (GlcN)-α-1→4 linked to a uronic acid, which can be either D-glucuronic acid (GlcA) or L-iduronic acid (IdoA) [2]. In addition, the 2-O on uronic acid, 6-O and 3-O on GlcN can be sulfated. However, these sulfate modifications are often incomplete, thus resulting in high heterogeneity of naturally-existing HS polymers [3,4]. This structural complexity and heterogeneity have hampered efforts to understand the detailed structure–activity relationships (SAR) of HS. The interactions of HS with its biological receptors can be highly structurally specific [5, ∗6]. Hence, it is critical to obtain pure HS containing a variety of sulfation sequences and backbone structures to advance the understanding of HS activities. As it is challenging to isolate sufficient quantities of defined HS from natural sources due to its heterogeneity, synthesis is the preferred method to access HS sequences.

Significant breakthroughs in the synthesis of HS oligosaccharides have been accomplished over the last two decades [7, 8, 9, ∗10]. HS sequences having the length approaching those of the HS polysaccharides can now be prepared [11, 12, 13]. On the other hand, to accelerate SAR studies, the availability of libraries of HS structures becomes critical. This is highlighted by recent studies where through screening of libraries of diverse HS sequences, key structural features of HS such as 3-O sulfation and the length of non-sulfated N-acetylation domain have been identified for interactions with a variety of HS binding proteins including tau, high mobility group 1 protein, the receptor binding domain of SARS-CoV-2 spike protein, and chemokines [14, 15, 16, 17, 18].

Construction of HS libraries is challenging owing to the numerous sulfation alternatives (N-, 3-O, and 6-O of GlcN and 2-O position of GlcA/IdoA) and variability in backbone architecture. Given that the synthesis of a single HS oligosaccharide can take more than 40 steps, innovative methods are needed to enable the preparation of large HS libraries. There have been many excellent reviews on the synthesis of HS oligosaccharides [7, 8, 9, ∗10]. Rather than providing a comprehensive summary on HS synthesis, we focus our discussion on synthetic strategies toward HS libraries with emphasis on the advances achieved during the last two years, including the synthesis of comprehensive libraries of HS tetrasaccharides.