Total synthesis of complex oligosaccharides by iterative one-pot glycosylation based on pre-activation of thioglycosyl donors

The rapid expansion of the glycobiology field requires robust methods for producing oligosaccharides. Due to the vast heterogeneity of carbohydrates existing in nature, synthesis is essential for providing access to sufficient quantities of these molecules in a pure form. However, tremendous challenges are associated with the assembly of oligosaccharides with their vast regio- and stereo-chemical complexities. During the past two decades, remarkable progress has been achieved in oligosaccharide syntheses with many novel methodologies and reagents being developed. In the first part of the thesis, a systematic overview of common synthetic strategies in oligosaccharide synthesis is provided. These synthetic approaches are grouped into four categories: (1) differential activation of donors, which include active-latent glycosylation, selective donor activation, orthogonal glycosylation, armed-disarmed chemoselective glycosylation and pre-activation based glycosylation; (2) selective reaction with acceptors; (3) two-directional glycosylation and convergent modular synthesis where strategies involving differential donor activation and selective reaction with acceptors are combined; and (4) chemoenzymatic synthesis. The advantages and disadvantages of these approaches are discussed with pertinent examples from the literature.;In the second part, the total synthesis of the tumor-associated carbohydrate antigens Globo-H hexasaccharide and SSEA-3 pentasaccharide has been accomplished by the pre-activation based iterative one-pot glycosylation. The fully protected Globo-H hexasaccharide was first assembled by sequential coupling of four glycosyl building blocks, leading to the desired hexasaccharide in 47% overall yield in one-pot. Although model study on constructing the challenging Gal-alpha-1→4-Gal linkage in Gb3 trisaccharide yielded the desired alpha linkage almost exclusively, similar approach to assemble the hexasaccharide led to the formation of significant amount of beta anomer. As an alternative, the second synthesis utilized three components in one-pot with the Gal-alpha-1→4-Gal linkage pre-formed, producing the desired hexasaccharide in a similar overall yield as the four component approach. Both approaches demonstrate that oligosaccharides containing alpha and beta linkages within the same molecule can be constructed in one-pot via the pre-activation based approach with higher glyco-assembly efficiencies than the automated solid phase synthesis strategy. Similarly, the fully protected SSEA-3 pentasaccharide was obtained in 37% yield in one-pot, along with the beta anomer isolated. The stereoselectivity in formation of the challenging Gal-alpha-1→4-Gal linkage was then investigated and the results demonstrated that the alpha selectivity decreased when larger oligosaccharide donors were used. This was rationalized by a steric buttressing effect due to the glycosyl unit(s) linked to 3-O of the reducing end of galactose in the donor. Globo-H and SSEA-4 can also be obtained by enzymatic synthesis using SSEA-3 pentasaccharide as the common precursor, which demonstrated that chemoenzymatic synthesis, through the combination of the power of enzymatic synthesis with the flexibility of chemical synthesis, presents a potent approach to rapid access of complex oligosaccharides.;In the third part, the total synthesis of the heparin-like hexasaccharide has been accomplished by the pre-activation based iterative one-pot glycosylation. The fully protected heparin-like hexasaccharide was obtained in excellent yield by sequential coupling of three disaccharide glycosyl building blocks in one-pot. The formation of glucuronic acid moiety was accomplished by efficient oxidation after the assembly of the desired hexasaccharide. The successful assembly of heparin-like hexasaccharide not only paves the road for efficient one-pot modular synthesis of heparin fragments but also is promising for developing a chemoenzymatic approach for a facile synthesis of complex heparin and heparan sulfate fragment libraries.