As the essential and most abundant biomolecules, carbohydrates are involved in the numerous biological processes with a variety of crucial roles [1∗,2, 3]. Unlike nucleic acids and proteins, which are usually linear and involve gene-regulated biosynthesis, glycans are non-template driven products, resulting in the highly diverse, complex and heteregenous structures. Plant polysaccharides are macromolecular biopolymers with numerous different or identical monosaccharides units, including starch, cellulose, hemicelluloses, pectin, arabinogalactans, and so on [4,5]. During the recent decades, glycans from plants, especially medicinal plants, have attracted a lot of interests, mainly because of their non-toxic, no-side effects and important biological functions, including anti-tumor, immunoregulatory, anti-diabetes, anti-coagulant, anti-viral, anti-oxidant, hypolipidemic and anti-hypertension activities [6∗,7, 8]. However, the accessibility of homogeneous and well-defined plants glycans remains the bottleneck for the development of carbohydrates drugs and decipherment of their functions [9,10]. Chemical synthesis provides a reliable and scalable means to produce homogenous and pure glycans for functional studies [11, 12, 13, 14, 15, 16, 17, 18, 19]. However, due to regioselective and stereoselective issues [20,21∗], glycans synthesis remains a tedious and time-consuming process. Efficient synthesis of long, branched, and complicated glycans with many 1,2-cis glycosidic linkages over ten units remains one of the most challenging tasks in chemical synthesis.

Here, we highlight the recent chemical synthesis of long, branched, and complex glycans over ten units from plants, including the branched amylopectin 20-mer 1 from starch [22∗∗], highly branched 17-mer 3 from carthamus tinctorius [23∗∗], the linear α-glucan 30-mer 4 from Longan [24∗∗], highly branched 19-mer 7 from psidium guajava [25∗∗] and highly branched 11-mer 8 from dendrobium huoshanense [24∗∗] (Figure 1). The basic strategies of glycans assembly as well as the utilization of protecting groups and glycosylation methods are discussed.