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Abstract
Thioglycosides and thiosugars both have a long history in bioorganic and medicinal chemistry. As presented in Chapter 1, thioglycosides and thiosugars are well-documented and well-studied mimetics of O-glycosides, and, to date, a considerable number of synthetic methods have been developed to access them. Despite several known examples in which thioglycosides or thiosugars showed higher inhibitory activity than the parent O-glycosides, these mimetics are still widely considered to be less “active” than the parent sugars.Chapter 2 details the design and synthesis of tetravalent glycoconstructs containing 1,5-dithialaminaribiose and -triose mimetics, and evaluation of their inhibitory activity against CR3 and Dectin-1 in comparison to the appropriate analogous O-glycosides. The synthesized compounds showed inhibitory activity against CR3 comparable to that of monovalent 1,5-dithio mimetics, but no clear pattern emerged regarding the number of carbohydrate units in the epitope nor the usefulness of the multivalent construct.
Chapter 3 explores the impact of S-π interactions occurring between ring sulfur atoms or glycosidic sulfur atoms and aromatic amino acid residues on binding of thiosugars and thioglycosides to lectins in an attempt to improve predictions of the value of 5-thio or thioglycosidic mimetics of parent O-glycosides. After an extensive PDB database search and analysis of the search output two representative examples were selected for experimental validation. Accordingly, mimetics were designed, synthesized and evaluated for their binding affinity with target lectins, and the thermodynamic parameters of binding were determined and compared with those of parent O-glycosides.
Chapter 4 presents an extensive variable temperature NMR study of generation and stability of reactive intermediates formed on activation of 5-thioglycosyl donors in attempt to shed light on differences in reactivity between 5-thiosugars and the corresponding parent glycosides. The tetrahydrothiopyrilium cation was successfully generated and characterized spectroscopically, and was found to be marginally more stable than the corresponding oxocarbenium cation. However, no evidence for the generation of a thienium cation was found in VT NMR experiments with 5-thioglycosyl donors, but rather the formation of dioxalenium ions and covalent triflates was observed.