Chemical Biology Studies Based On Sugar Conjugates

Carbohydrates play a vital role in many physiological processes including cell adhesion, cell differentiation, infected agent that make us ill, as well as in major diseases such as cancer, cardiovascular disease and inflammatory diseases. Glycoconjugate(glycopeptides/glycolipid) are the major bioactive form of oligosaccharide. Scientists have paid more and more attention on glycoconjugate since the huge diversity in it. But the further understanding the functions of glycoproteins and glycopeptides and analyzing their structure-activity relationships (SARs) is restricted by their limited supplemental resources. Therefore exploring rapid and efficiently artificial synthetic approaches to access the target glycopeptides, and finding the novel bioactive glycocongate for vaccine and drug discovery is very important. This thesis pursued the studies on two aspects:(1) A new approach for the synthesis of O-glycopeptides through a combination of solid-phase glycosylation and fluorous tagging (2) Reasonable design and total synthesis of analogues of TTSG from Morinda citrifolia, which act as highly selective antibiotic agent and target the membrane of bacteria.1) A new approach for the synthesis of O-glycopeptides through a combination of solid-phase glycosylation and fluorous taggingAs we all know, it is difficult to separate glycopeptides from natural glycoforms, by means of microheterogeneity at carbohydrate portions and uncontrollable glycosylation in the biosynthetic approach. To overcome these obstacles, there is a need for rapid and efficiently artificial synthetic approaches to access the target glycopeptides, especially methods for preparing pure and structurally well-defined glycopeptides libraries for vaccine and drug discovery.Using preformed glycosylated amino acids or oligosaccharides as building blocks during solid-phase synthesis of glycopeptide and purified by the HPLC is currently the most popular method for synthesizing glycopeptides. However, it suffers from cumbersome operation, low efficiency and an apparently variable synthetic outcome from one target product to another.Herein, we report the first attempt of solid-phase synthesis of O-linked glycopeptides through the combination of a solid-phase hybrid glycopeptidation procedure and fluorous tagging strategy in this thesis. In this strategy, a TentaGel resin with an aryl hydrazine "safety-catch" linker was selected as the solid phase support, after constructing the peptides portion by the SPPS strategy, the saccharine motif was introduced step by step with the help of the solid-phase glycosylation. the stable aryl hydrazine linker under reactions conditions eliminates the risk of destruction the structure of the desired glycopeptide because of its acidic and/or basic liability to the glycomoiety and side-protection groups of peptides. Finally F-SPE protocol allowed us to purify the target product simply.This project report a new and efficient hybrid strategy to synthesize and purify O-glycopeptides was developed via a combination of the advantages of solid-phase glycosylation strategy with the superiority of fluorous chemistry. This approach not only introduces a fluorous-tagged glycosyl donor and aryl hydrazine safety-catch linker into the solid-phase synthesis of O-linked glycopeptides for the first time, but also avoids generation of extra costs with the help of solid-phase glycosylation and F-SPE chemical recycling. This strategy will be a significant advance in realizing automated O-linked glycopeptide synthesis for nonspecialists.2) Reasonable design and total synthesis of analogues of TTSG from Morinda citrifolia, which act as highly selective antibiotic agent and target the membrane of bacteria.Streptococcus is a kind of Gram-positive bacteria including many pathogenic bacteria which could induce nasosinusitis, bacterial meningitis, otitis media, bacterial pneumonia, endocarditis et. al., it is a huge threat to the health of human being. The first medical choice in clinic is the penicillins’antibiotics. However, the emerging of penicillin resistance streptococcus make it difficult to cure the disease caused by the streptococcus. But combined the broad-spectrum antibiotic could destroy the normal balance of bacteria in human body, which may be even more harmful for the public health. Develop an antibiotic agent which not only have specific killing effect to the streptococcus, but also is sensitive to the clinical drug resistance streptococcus for the harmless of the normal balance of bacteria in human body would be the optimal choice.This project syntheses19analogues of a novel glycolipid natural product which was extracted from the fruit of Morinda citrifolia with a wisely designed route, including the IAD reaction for preparing the1,1-α,α trehaloside and the highly stereoselectively constructing the P-mannoside. Besides that, there are still some other valuable results in this part of the thesis:1) Largely expanded the application of IAD reaction to establish the trehaloside and the feasibility of using the oxonium which generated in situ to highly stereoselectively construct theβ-mannoside in the synthesis of glycolipid motif.2) Rational designed a few analogues of TTSG, and prepare19compounds by means of optimized routes.3) Discovering some rapidly and specifically anti-streptococcus bioactive agents. 4) Identify the sensitivity of our synthetic analogue compound54to the multidrug-resistance streptococcus.5) Using the hemolysis assay to identify the safety of our bioactive compounds.6) Preliminary demonstrated the mechanism of action is targeting the cell membrane of the bacteria by taking the active compound54as an example. And identify that the reason of the genus selectivity of the killing effect may be the different permeability between the various bacteria according to the protoplast assay.