Fucosyl-Auxiliary Group Assisted Enzymatic Oligosaccharide Synthesis

Carbohydrate chains are widely distributed on the outermost position of cell-surface and play important roles in physiological and pathological processes,such as molecular recognition,intercellular interaction,immune response,and biological information transmission.Carbohydrate chains also have important applications in modern medicine.For example,accumulated evidence showed that carbohydrates containing a terminal GalNAc?1,3Gal,which is a new prognostic marker for cervical cancer.Moreover,pigs are considered the best organ donor candidates.However,human antibody-mediated hyperacute rejection presents a formidable barrier.The major xenoactive antigens on porcine endothelial cells are carbohydrate structures bearing a Gal?l,3Gal terminusTherefore,the synthesis of a large number of homogeneous and pure related carbohydrate chains has a great significance to explore its potential physiological activities and related functions.However,the relative carbohydrate chains from natural sources are scarce and difficult to extract.The construction of natural 1,2-cis glycosidic bond has been a difficult and active area in the synthetic carbohydrate community.For chemical synthesis,1,2-cis glycoside bond is different from 1,2-trans glycoside bond,which can be well-controlled through the participation of adjacent groups.Chemical synthesis requires the introduction of auxiliary groups and repeated protection and deprotection manipulation,which increases the difficulty of purification with low overall yield.For enzymatic synthesis,there is no glycosyltransferase available that can catalyze the synthesis of 1,2-cis GalNAc?l,3Gal glycosidic linkage.Moreover,the glycosyltransferase for the synthesis of 1,2-cis Galal,3Gal glycosidic linkage can only recognize a limited number of substratesTo overcome this challenge,a novel chemoenzymatic approach was developed in this thesis that using fucose as an auxiliary group to expand the substrate specificities of two glycosyltransferases for the synthesis of a series of complex glycans bearing a GalNAc?1,3Gal or Gal?1,3Gal at the non-reducing terminus.This thesis mainly contains the following aspects.(1)Chemical or chemoenzymatic synthesis of 5 different disaccharide precursors including Gal?1,3GlcNAc?ProN3,Ga1?1,4GlcNAc?ProN3,Gal?1,3GalNAc?ProN3,Ga1?1,3GalNAc?ProN3 and Gal?1,4Glc?ProN3(2)Enzymatic modular assembly of trisaccharide 6-10 by five disaccharide precursors terminus ?1,2-fucosylation through a step of parallel enzymatic reaction.(3)Enzymatic modular assembly of tetrasaccharide 11-15,by trisaccharide 6-10 terminus ?1,3-N-acetylgalactosylation through a step of parallel enzymatic reaction.(4)Enzymatic modular assembly of tetrasaccharide 16-18 by trisaccharide 7-9 terminus al,3-galactosylation through a step of parallel enzymatic reaction.(5)Using Microwave method,we get trisaccharide 19-26 by fucose acidolysis through a step of parallel reaction.The main novelties of this paper were listed as follows:(1)A novel chemoenzymatic approach was developed in this thesis that using fucose as an auxiliary group to expand the substrate specificities of two glycosyltransferases for the synthesis of a series of complex glycans bearing a GaINAc?1,3Gal or Gal?1,3Gal at the non-reducing terminus.(2)The chemoenzymatic strategy developed in this thesis combines the flexibility of the chemical synthesis and the high efficiency and selectivity of enzymatic synthesis,which provides a novel approach to expand the substrate specificity of given glycosyltransferases.