Studies On ?-selective Glycosylation Of 3,5-trans-3-amino-2,3,6-trideoxyhexopyranoses

Challenges for stereoselective glycosylation of deoxy sugars are notorious in carbohydrate chemistry.3,5-trans-3-amino-2,3,6-trideoxypyranoses(3,5-trans-3-ADSs)which are important components of a few bio-important antibiotics,and they form ?-glycosidic bonds with other sugars or aglycons consistently.Chemical construction ?-O-glycosidic bonds of 3,5-trans-3-ADSs is essentially an extreme challenge,not only because of the lack of C-2 participation group,but also due to the particular instability of the glycosidic bonds and anomeric effect in favor of ?-configuration.At present,there is no glycosylation method with wide applicability and good ? selectivity.Therefore,our study on ?-selective glycosylation of rare 3-amino sugars is very important.This strategy took the advantage of the C-3 axial amide group of 3,5-trans-3-ADSs which served as a hydrogen-bond(H-bond)donor.By introducing sub-stoichiometric amount of phosphine oxide as exogenous nucleophilic reagent(exNu)to the glycosylation system,an intramolecular H-bond was formed between the C-3 amide group and the ?-oxyphosphonium ion.This interaction stabilized the ?-face covered intermediate and limited the formation of the corresponding more reactive ?-intermediate.Therefore,a reversed ?-selectivity was obtained that is distinct from the conventional exNu mediated glycosylation.Wide range of substrates were examined and good to excellent ?-selectivities were guaranteed by the H-bonding assisted exNu effect.The effectiveness of this strategy was further demonstrated by the structure modification of natural products and drugs with 3,5-trans-3-ADSs.