<P> Multiple chemical approaches exist to encourage selectivity of α - and β - glycosidic bonds . The highly substrate specific nature of the selectivity and the overall activity of the pyranoside can provide major synthetic difficulties . The overall specificity of the glycosylation can be improved by utilizing approaches which take into account the relative transition states that the anomeric carbon can undergo during a typical glycosylation . Most notably, recognition and incorporation of Felkin - Ahn - Eisenstein models into rationale chemical design can generally provide reliable results provided the transformation can undergo this type of conformational control in the transition state . </P> <P> Fluorine directed glycosylations represent an encouraging handle for both B selectivity and introduction of a non-natural biomimetic C2 functionality on the carbohydrate . One innovative example provided by Bucher et al. provides a way to utilize a fluoro oxonium ion and the trichloroacetimidate to encourage B stereoselectivity through the gauche effect . This reasonable stereoselectivity is clear through visualization of the Felkin - Ahn models of the possible chair forms . </P> <P> This method represents an encouraging way to selectivity incorporate B - ethyl, isopropyl and other glycosides with typical trichloroacetimidate chemistry . </P> <P> O - linked glycopeptides recently have been shown to exhibit excellent CNS permeability and efficacy in multiple animal models with disease states . In addition one of the most intriguing aspects thereof is the capability of O - glycosylation to extend half life, decrease clearance, and improve PK / PD thereof the active peptide beyond increasing CNS penetration . The innate utilization of sugars as solubilizing moieties in Phase II and III metabolism (glucuronic acids) has remarkably allowed an evolutionary advantage in that mammalian enzymes are not directly evolved to degrade O glycosylated products on larger moieties . </P>

Where is a glycosidic bond located in dna