Synthesis Of Fluorocarrier-Supported Oligomeric Hyaluronic Acid

The rapid assembly of oligosaccharides is of great importance in the field of carbohydrate.Concerns are mainly raised in three issues:Fast synthesis of carbohydrate building blocks;Efficient construction of glycosidic linkages;Easy purification of intermediates and products involved in the synthetic process.In recent decades,great progress has been made in the field of carbohydrate chemistry,which leads to the possibility of fast assembly of oligosaccharides.Hyaluronic acid(HA),a nonsulfated glycosaminoglycan polysaccharide,is widely distributed throughout connective,epithelial,and neural tissues.HA plays important roles in many key processes,such as cell signaling,wound reparation,tissue regeneration,morphogenesis,matrix organization and pathobiology.Studies showed that the activities of HA are determined by the molecular weight of HA saccharides.HA with high molecular weight has the effects of space filling,immunosuppressive,suppress angiogenesis.However,HA oligomers have recently been identified as potent activators of immunocompetent cells and angiogenic.HA oligomer,which is often prepared by enzymatic degradation of HA polymer.The enzymatic degradation is out of control to a degree that a mixtures of HA oligosaccharides with different molecular weight are obtained.To well understand the structure-activity relationship of HA oligomer,a chemical method is necessary to be developed for the preparation of a pure HA oligomer with well-defined structure.Recently,a feasible and convenient strategy was developed in our group for oligosaccharide synthesis,which realizes reaction in solution,while product purification occurs only by solid-liquid filtration.In combination of the above-mentioned strategy and our newly-published glycosylation method,a protocol is proposed to achieve efficient preparation of hyaluronic acid oligosaccharides with well-defined structure.The following topics are studied and discussed:(1)Starting from D-glucose,a glucuronic trichloroacetimide donor G1 was synthesized via 9 steps.(2)Synthesis of glucosamine acceptors were finished and the coupling reaction of donor G1 with different acceptors were investigated.Large amount of glucuronicthioglycoside was isolated during the glycosylation between donor G1 and the thioglycoside acceptor,which is presumably formed from the transfer of the thio group from acceptor to the glucuronic donor G1.However,the glycosylation of donor G1 with cis-1,5-enyne ether acceptor went smoothly,providing the desired disaccharide G3.(3)Preparation of a fluorous-tagged disaccharide acceptor S3 was investigated.It turns out fluorous-tagged benzyl alcohol would undergo self-etherification in the presence of Hg(NTf2)2 rather than the glycosylation between the alcohol and G3.The synthesis of the desired acceptor S3 was achieved via the glycosylation of the fluorous-tagged benzyl alcohol with a glucosamine donor and a glucuronic donor step by step.The strategy has both advantages of solution-phase reaction and solid-phase seperation,and polytetrafluoroethylene particle-assisted syntheses of hyaluronic acid disaccharide with fluorous tag.(4)Assembly of hyaluronic acid tetrasaccharide was studied using G3 as donor and S3 as acceptor.The glycosylation reaction was conducted between HA disaccharide donor G3 and the disaccharide acceptor S3 using catalytic amount of Hg(NTf2)2 as promoter.And the corresponding fluorous-tagged HA tetrasaccharide G5 was obtained in 17%yield.It seems that the donor G3 is a super disarmed donor with low activity probably due to the electro-drawing effect of the ester group at C-5 position.We are currently optimizing the conditions of the coupling reaction in order to increase the yield.In summary,the glycosylation between glucuronic acid donor and glucosamine acceptor with different C-1 leaving groups was studied and two HA disaccharides were synthesized.The construction of fluorous-tagged HA tetrasaccharide was achieved under the activation of a catalytic amount of Hg(NTf2)2.The glycosylation will be deactivated using glucuronic acid either as a donor or as a acceptor due to the electro-drawing ester group at C-5 position.Thus,the modified route will be applied to the synthesis of HA oligosaccharides according to the results in hand.