Stereoselective Synthesis Of ?-Glucans Related To Fuzi Polysaccharide

With the development of modern biotechnology and bioindustry,a lot of new generation vaccines have been developed and successfully applied in clinics.Compared to traditional vaccines,the safety of novel vaccines has been significantly improved but with the declined immunogenicity,which usually require adjuvants to enhance or elicit the appropriate immune response.Presently,only a few vaccine adjuvants have been licensed by the FDA for human use,however,the related adverse effects and disadvantages are serious and limit the clinical effects of vaccines.Consequently,the development of new-type,high efficient,and low-toxic vaccine adjuvants have attracted increasing attention around the world.Polysaccharides,indispensable biomacromolecules for organisms,possess a range of biological activities.It was demonstrated that the Fuzi a-glucan that derived from the traditional Chinese medicine Aconitum carmichaeli Debx exhibited low toxicity and strong immunostimulanting properties,such as inducing lymphocyte proliferation and cytokine secretion and enhancing antibody production.Thus,the Fuzi a-glucan has great potential for exploitation as novel vaccine adjuvant.Based on the above research background,we here designed and synthesized a series of Fuzi a-glucan oligosaccharides including the a-pentaglucan repeating unit,its dimer,trimer,as well as the bi-,tri-and tetra-valent glycoclusters.The structurally well-defined and homogeneous molecules could facilitate not only structure-activity relationship analyses but also in-depth biological mechanism studies,both of which would be very useful for development of carbohydrate adjuvant.This text includes:In the first chapter,we briefly reviewed the advantages,disadvantages,and biological mechanism of human vaccine adjuvants with a hope to find inspiration for developing new adjuvants.The research progress of immune-enhancing activities of polysaccharides from the traditional Chinese medicine,especially the Fuzi ?-glucan,was extensively summarized.Accordingly,we proposed the research basis,objective,and experimental program based on the Fuzi a-glucan.In the second chapter,the stereoselective syntheses of a variety of a-glucan oligosaccharides and glycoclusters corresponding to Fuzi polysaccharide was introduced.All of the glucosyl linkages were constructed with excellent or exclusive a-specificities,which were achieved based on the a-directing effects of a AgOTf/TolSCl catalytic system to promote the reaction and the ?-shielding properties of large carbohydrate residues or the remote participation of acyl groups at the donor 6-O-position.(1)Synthesis of the Fuzi a-pentaglucan repeating unit.The fully protected pentasaccharide intermediate was constructed with exclusive cc-selectivity and 41%yield by a[1+1+2+1+1]one-pot glycosylation.Thereafter,this intermediate was smoothly converted to the target Fuzi a-pentaglucan via three-step reactions to deprotect the silyl ether,benzoyl,benzyl and azido groups.(2)Synthesis of the Fuzi a-pentaglucan dimer.Initially,we attempted a[5+5]synthetic strategy to assemble the fully protected decasaccharide.Even under the diluted reaction concentration,e.g.5 mM,only a moderate selectivity(?:?=5:1)was obtained.Next,we turned to an alternative[4+6]strategy,and the desired decasaccharide was obtained as the a-only isomer in an 81%yield,which was possibly owing to the steric assistance of the ?-(1?3)branch in tetrasaccharide donor.By using the above-mentioned deprotection protocols,the target a-decaglucan was readily obtained with high yield.(3)Synthesis of the Fuzi a-pentaglucan trimer.Encouraged by the efficient synthesis of the Fuzi ?-pentaglucan dimer,its trimer was facilely assembled with 36%yield by a[4+5+6]one-pot glycosylation strategy to construct the fully protected pentadecasaccharide and the followed deprotection procedures.The glycoclusters corresponding to Fuzi a-glucan were synthesized by the "Click Reaction" using the protected a-pentaglucan containing an azido group functionalized linker in the presence of copper sulfate and sodium ascorbate.The target divalent glycocluster was conveniently obtained in 63%yield via the "Click" condensation and subsequent Pd(OH)2-catalyzed hydrogenation to remove all the benzyl groups.However,under the same reaction conditions,the more sterically congested trivalent glycocluster was generated in a very low yield.After screening various reaction conditions,the desired glycocluster was produced with 68%yield in the DCM-MeOH-H2O(v:v:v=3:3:1)mixed solvent when 0.1 equiv of copper sulfate and 0.5 equiv of sodium ascorbate per alkynyl group were employed for each "Click Reaction".At last,the fully protected tetravalent a-pentaglucan cluster was obtained with 64%yield by taking advantaging of the above-described conditions.Hydrogenation of the protected tri-and tetravalent a-glucan clusters are on going in our laboratory.