A Novel Method For 1,2-cis-Glucosidation And Its Application To One-pot Multistep Synthesis Of Complex Oligosaccharides

Carbohydrate distributes widely in the world,and it is known as one of the three basic substances of lives.Carbohydrate as the most important energy substance in living organisms provides energies required for material exchange and transmission,neural activities,metabolism,and various other biological functions.Carbohydrate is also very important signal molecule in organisms to play a critical role in various activities,such as cell recognition and adhesion,signal transmission.Furthermore,polysaccharides have important medicinal values as well.a-Glucans are composed of a-linked D-glucose residues via ?-(1?2),?-(1?3),a-(1?4)and ?-(1?6)linkages and often with branches,exemplified by starches,animal glycogens and active ingredients of some traditional Chinese medicine.a-Glucans have been demonstrated to possess diverse and excellent biological activities,such as anti-aging,anti-oxidation,anti-virus,anti-tumor,immunostimulating and so on,and have been used in health products and medicines.a-Linked 2-amino-2-deoxy-D-glucose and its N-acetyl derivative are the key components in many natural polysaccharides and glycoconjugates,such as chitin,heparin,glycosylphosphatidylinositol and so on,which are important constituents of bacterial polysaccharides playing key roles in bacterial invasion,pathogenicity and interaction with the host immune system.Therefore,these polysaccharides are useful target antigens for the development of antibacterial vaccines.Due to the inherent microheterogeneity of polysaccharides,it is difficult to derive structurally uniform polysaccharides from natural sources for detailed biological study.Chemical synthesis has thus attracted significant attentions as a method for obtaining homogeneous and structurally well-defined oligosaccharides as natural polysaccharide surrogates.For the efficient synthesis of oligosaccharides,it is important to have highly stereoselective methods for the construction of a-glucosidic and 2-amino-2-deoxy-a-glucosidic bonds.In contrast to the construction of 1,2-trans ?-glucosides and ?-aminoglucosides,which can be easily achieved by taking advantage of the neighboring participation effects of 2-acyl substituents,stereoselective synthesis of the 1,2-cis a-D-glucosides or 2-deoxy,2-amino-?-D-glucosides is rather difficult.To date,many methods have been explored for the generation of 1,2-cis-glycosidic linkages by such as altering the leaving group,promoter,protecting group,temperature,solvent,glycosyl acceptor structure and so on.However,most methods usually result in anomeric mixtures that need time-consuming purification,which affect the synthetic efficiency.This drawback becomes more apparent in the synthesis of complex oligosaccharides as these molecules contain multiple glucosidic linkages and their synthesis requires complete stereocontrol in each glycosylation step;otherwise,very complex mixtures will be obtained after several steps of glycosylation,which could not only reduce the yield but also make the product purification nearly impossible.There have been a number of reported methods for a-glycosidation of glucose and glucosamine.However,developing more universal methods still has practical significance.In this dessertation,we planned to develop a new,robust,and generally applicable a-glycosylation method to construct a-glucosides and 2-amino-2-deoxy-a-glucosides with high stereoselectivity and efficiency and apply it to the synthesis of complex oligosaccharides with important biological activities.Previously,we observed that the glucosylation reaction of a galactose acceptor at low temperature,with 2,3,4-tri-O-benzyl-6-O-tert-butyldimethylsilyl-thioglucoside as donor and toluenesulfenyl chloride(TolSCl)and silver triflate(AgOTf)as promotors,was a-specific.It was proved to be especially facile and efficient and could be used for preactivation-based one-pot assembly of oligosaccharides.These results suggested that the combination of the TolSCl/AgOTf promoter system with properly protected donors might be used to futher enhance the a-directing effect to create a general a-specific glucosylation method.The research of this dissertation was based on this hypothesis,and it includes the following three parts.In chapter 1,we have provided a review about the commonly used glycosylation methods and their mechanisms.Especially,this chapter has provided a detailed account of available methods for the generation of 1,2-cis-glycosidic linkages.In chapter 2,we have described a novel method for 1,2-cis-glucosidation.For the exploration of this method,we designed,synthesized and studied four different donors to react with acceptors.We found that after donor preactivation with TolSCl and AgOTf at-78? and addition of acceptors at the same temperature,complete a-selectivity and high yields were achieved by using 6-O-TBS and 6-O-Bz-protected 2,3,4-tri-O-benzyl-thioglycosides as donors.Combined with previous reports,we suggested that exclusive a-stereoselectivity might be caused by synergistic effects of multiple factors,including steric ?-shielding or remote participation of the TBS or Bz groups at the donor 6-0-position,TolSCl/AgOTf catalytic system and Et2O solvent effect.Encouraged by this discovery,next,we examined its application scope by using various acceptors,which also gave high a-selectivity and yields.These results clearly demonstrated the general applicability of the method.To further demonstrate the practicability of this method in the synthesis of complex oligosaccharides,we applied it to the prepared of a heptaglucan,which was significant synthetic challenge.This oligosaccharide represents a fragment of Pseudallescheria boydii polysaccharide with immunostimulatory activity,containing an ?-(1?4)-linked pentasaccharide backbone and an ?-(1?4)-linked disaccharide branch a-linked to the backbone O-6-position.We chose to employ the remote functional group participation effect of the 6-O-benzoyl thioglucoside donor and the steric ?-shielding effect of the disaccharide branch at 6-0-position combined with preactivation-based glycosylation for exclusive stereoselectivity to eventually achieve one-pot synthesis of the synthetic tragets in 46%yield.These molecules contained a free amino group at the reducing end,which would facilitate their glycoconjugate with other molecules for various biological studies and applications.The highly efficient synthesis of the heptasaccharide illustrated the high practicality of this glycosylation method in the synthesis of complex oligosaccharides containing a-glucosidic linkages.In chapter 3,we have described a novel method for stereoselective 2-amino-2-deoxy-a-glucosylation.Based on the efficient method for a-glucosylation as described above,a method for ?-stereoselective synthesis of 2-amino-2-deoxy-D-glucosides was developed,which was also achieved by the synergetic a-directing effects of the steric?-shielding or the remote functional group participation at the donor 6-O-position in combination with the TolSCl/AgOTf promotor system to facilitate glycosylation at low temperature.The 2-amino group of glucosamine was protected with the azido group,as it not only lacks participation ability but also is stable under both acidic and basic conditions to be compatible with various glycosylation methods and protecting tactics.In addition,the azido group can be easily transformed into amino and acetamido groups via selective reduction or reductive N-acylation methods.at a later stage of the synthesis.To probe the applicability of this method to various linkages in natural polysaccharides and oligosaccharides,we designed a series of glycosyl acceptors and examined their reactions.The results showed that acceptors with a secondary hydroxyl group afforded high a-selectivity and yields,but acceptors with a primary hydroxyl group gave low a-selectivity and yields.This might be due to the higher reactivity and nucleophilicity of primary alcohols.To improve the stereoselectivity of reactions with primary alcohols,we then used 2,3,4-tri-O-benzoyl-protected acceptors,which could reduce the reactivity and nucleophilicity of the acceptor.To our delight,the reaction of these acceptors with the same donors gave much improved ?-selectivity,suggesting the significant impact of the acceptor reactivity on the stereochemical outcome of the reactions.To verify the applicability of this a-aminoglucosylation method in the synthesis of complex oligosaccharides,we used it to preparae of a branched pentasaccharide,which is the repeating unit of Acinetobacter baumannii K47 capsular polysaccharide.This oligosaccharide has a tetrasaccharide backbone composed of three a-acetylglucosamine and one a-acetylgalactosamine residues with ? ?-glucose branch.We achieved first a linear trisaccharide containing the P-glucose and a-acetylgalactosamine residue under the promotion of TolSCl/AgOTf in high yields and complete stereoselectivity.Then,preactivation-based sequential[1+3+1]one-pot glycosylation under aforementioned conditions using TolSCl/AgOTf as the promoter was used to achieve the protected pentasaccharide in 65%yield and the high stereoselectivity.Finally,the target pentasaccharide was obtained in 44%yield for 5 steps after global deprotection to remove tert-butyldimethylsilyl and acyl groups,convert azido groups to acetamido groups,and remove benzyloxycarbonyl and benzyl groups via hydrogenolysis.The synthetic targets contained a free amino group at the reducing end,which could be used for covalent coupling with protein to prepare related glycoconjugate vaccines.