Studies On The Construction Of Highly Stereoselective 2-deoxyglycosidic Linkages

2-Deoxyglycosides are important structural motifs for many natural products and drugs such as antibiotics,anti-cancer and cardiotonic agents.However,stereoselective synthesis of these oligosaccharides remains challenging:1).The absence of a C2 hydroxyl group results in the difficulty in stereoselective glycosylation with an aglycon due to the loss of neighboring-group participation;2).The glycosidic linkages of 2-deoxyglycosides are much more labile to acidic conditions.A variety of methods have been reported to construct 2-deoxyglycosides,which can be divided into two strategies:1).Indirect strategy.The glycosylation is conducted with the use of a donor having a stereodirecting group at C2,followed by removal of the C2 directing group to provide the desired glycosylated product;2).Direct strategy.The glycosylation undergoes by directly using a 2-deoxy carbohydrate donor.The indirect strategy often ensures high stereoselectivity.The direct strategy is more straightforward,however is somehow difficult in governing the stereoselectivity.In this thesis,we proposed two methods for synthesis of 2-deoxyglycosides:1).a-selective construction of 2-deoxyglycosides based on unusual axial rich comformation of Bis-TBDPS protected donor;2).I2-catalyzed glycosylation with ortho-alkynylphenyl thioglycosides as donors.1.The acid-catalyzed glycosylation between alcohols and glycals is investigated.TBDPS-protected L-rhamnal glycal underwent glycosylation smoothly with various acceptors to generate the 2-deoxyglycosides with high ?-selectivity.We also found that Ferrier rearrangement occurred exclusively for the rhamnal glycal in the presence of different alcohols with the use of more acidic catalysts or at high reaction temperature.These findings implies a new route to the synthesis of 2,3-unsaturated O-glycosides.2.A new type of ortho-alkynylphenyl 2-deoxy thioglycoside has been synthesized.Treatment of the thioglycoside with I2 results in the in-situ formation of 2-deoxy glycosyl iodides,which can be used directly for glycosylation to provide 2-deoxyglycosides.Iodine is an inexpensive,environmentally benign and low-toxic promoter and the reaction condition is mild.The gold-catalyzed cyclization of ortho-alkynylphenyl thioglycoside provides 2-phenyl-3-glycosyl benzothiophenein excellent yield in the absence of an acceptor.