Studies On The Glycosylation Of Novel Glycosyl Donors With Dual Roles And The Effect Of Anions On The Stereoselectivity Of Glycosidic Linkages

Carbohydrates, one of the three important natural biological macromolecules, not only serve as major sources of energy and structural materials of organism, but also play critical roles in cell-cell interaction, growth, polarization, recognition and immunoresponse and so forth.Mainly, there are four pathways to obtain carbohydrates:(1) extraction and purification from natural sources; (2) enzymatic synthesis; (3) chemical synthesis; (4) chemo-enzymatic synthesis. Chemical synthesis is one of the major approaches to gain well-defined carbohydrates, in which three imrortant issues have great effects on the efficiency of carbohydrate availability:(i) preparation of glycosyl building blocks; (ii) efficiency of glycosylation; (iii) rapid purification during the synthetic process.In this thesis, we will focus on the development of a new glycosylation method, which includes two major topics:(1) development of a new type glycosyl donors for glycosylation which can be also used as protecting group for C-1 hydroxy group; (2) investigation of anion effect on stereoselectivity of glycosylation.In the first section, the followings have been done:1) synthesis of a new cis-l,5-enynyl alcohol; 2) preparation of a number of cis-1,5-enyne glycosyl donors with different hydroxyl protecting groups; 3) preparation of a series of glycosyl acceptors; 4) preparation of gold catalysts; 5) development of an efficient glycosylation method and exploration of the substrate scopes.In the second section, the followings have been studied and discussed:1) preparation of two kinds of donors:glycosyl trichloroacetimidates and glycosyl hexynoates; 2) preparation of TMSNTf2; 3) exploration of anion effects of NTf2? anion and OTf? on the glycosyl stereoselectivity; 4) study of the substrate scopes.In the first section, a series of "armed" or "disarmed" cis-1,5-enynyl ether donors have been prepared. The stability and orthogonality of the anomeric enynyl group have been investigated under the manipulations of protection and deprotection in carbohydrate chemistry. Those cis-1,5-enynyl donors afforded the desired products in high yield with different acceptors in the presence of catalytic amount of PPh3AuCl/AgNTf2. Thus a novel and efficient glycosylation method-enynyl glycosylation, where the donors are activated by a catalytic amount of Au(?) catalysts has been established.The second part of study is based on the first one. The influene of promoters with different anions on the glycosylation has been discussed. Comparation of the effect of NTf2? and OTf? on stereoselectivity of glycosylation reveals that NTf2? provides higher ?-selectivity than OTf? does.