Study On Stereoselective Synthesis Of 2-Deoxy-α-Glycosides

2-Deoxyglycosides,ubiquitously existing in the natural prouducts and medicinal milecules,represent an important class of saccharide motifs with a broad spectrum of bioactivies.Absence of directing group at C-2 position renders the stereoselective synthesis of deoxyglucosides greatly challenging.As a result,stereoselective construction of 2-deoxyglycosidic bonds is of high value in glycochemistry.Herein,we present two new methodologies for 2-deoxyglycosidation.1.2-Bromo-α-glycoside chlorides,obtained from the addition of glycals and electrophilic N-bromosuccinimide（NBS）together with nucleophilic Ti（Oi-Pr）₃Cl,are the novel intermediates for the preparation of 2-deoxy-α-glycosides.The dihalogenated sugar derivatives were smoothly activated under the Koenigs-Knorr conditions to afford 2-bromoglycosides.Subsequently,the bromine atom was removed by a radical reaction to obtain title 2-deoxy-α-glycosides.2.Based on the upfront study,we continued to explore the nucleophilic addition of glycals and alcohols to obtain 2,6-dideoxy-α-glycosides in the presence of Lewis acid/base pair composed of B（C₆F₅）₃ and tetrabutyl ammonium bromide（TBAB）.The plausible mechanism were futher proposed by NMR.In summary,excellentα-stereoselectivity and the broad scope of substrates were both featured in these two methods.