Highly Stereoselective Synthesis Of 2-Deoxyglycosides And Efficient Preparation Of Steviol Glycosides

This paper mainly includes two parts:“Highly stereoselective synthesis of 2-deoxyglycosides" and "Efficient preparation of steviol glycosides".1)Highly stereoselective synthesis of 2-deoxyglycosides.2-Deoxy-sugars widely exist in natural or artificial preparations,involved antibiotics,antimicrobials,and anticancer agents,and which recognized as the necessary unit for the biological activity of the whole molecule.Considering the low content and the complex formation of 2-deoxy-sugars in natural products.It is necessary to construct 2-deoxyglycosides by chemical method.Owing to 2-deoxyglycosides lack the directing group at C-2 to control the anomeric selectivity,the stereoconfiguration of newly formed glycosidic bonds is difficult to control effectively.Another consideration is the stability of newly formed 2-deoxy-glycosidic bonds,the absence of electron-withdrawing groups at C-2 often leads their greater susceptibility to acids.In recent years,numerous methods have been developed to the synthesis of 2-Deoxyglycosides,but the methods also have some drawbacks which limit the wide application of them,such as harsh reaction conditions.expensive catalysts and give poor stereoselective.So,it is particularly important to develope new glycosylation reactions that permit the mild,more efficient,rapid and selective construction of 2-deoxyglycosides.Mercury(Ⅱ)trifluoromethanesulfonate[Hg(OTf)2]as a metal triflate plays an important role in many reactions with its strong and efficient catalytic ability.Notably,Hg(OTf)2 has the advantages of less moisture absorption,easy preservation and cheaper compared with most similar metal triflate..In this chapter,we describe a new method for the stereoselective synthesis of 2-deoxy-a-glycosides using Hg(OTf)2 as a powerful catalyst.This method has mild conditions,high efficiency,high reaction rate,wide application range and good stereoselective.The value of this method was well demonstrated by the synthesis of the 2-deoxy trisaccharide and 2-deoxy tetrasaccharides in good yields and with excellent stereoselectivity.2)Efficient preparation of steviol glycosides natural products.Steviol glycosides are a group of highly sweet diterpene glycosides.As new natural sweeteners which can replace sucrose,steviol glycosides are widely consumed due to their obvious advantages such as natural origin,high sweetness,noncalorific,noncariogenic,safety and generally healthy.Moreover,the potential pharmacological activity of these compounds has also attracted extensive attention from researchers.However,some factors limit the development in industry:a)Steviol glycosides only discovered in a few plant species,and the content of some high-value glycosides is extremely low.b)An important part of obtaining SvGls is their extraction and purification from the plant material,but this process is very cumbersome.Thus,chemical synthesis of is considered as an efficient way to obtain steviol glycosides with a definite chemical structure.In this chapter,we have designed new synthetic routes and synthesized several stevioside natural products from the rebaudioside A family.Among them,the gold(Ⅲ)chloride—tert-butyl cyanide catalytic system could construct the key β-glycosidic bonds with high stereoselectivity.The method showing good application potential in the synthesis of steviol glycosides,and providing a new method to access these valuable natural products.Hence,a firm basis is now available to promote the synthesis,production and research of steviol glycosides.