A-Mannosides

An a-triflate is formed because this anomer is stabilized by a strong mdo-anomeric effect. Upon addition of an alcohol, the triflate is displaced in an SN2 fashion resulting in the formation of a a-mannoside. A mixture of anomers is obtained when triflic anhydride is added to a mixture of sulfoxide and alcohol. In... 

Access to P-mannosides [209] is illustrated by the preparation of 179 from P-glucoside 178 by oxidation of the equatorial 2-OH followed by stereoselective reduction to give the axial alcohol an efficient indirect route to the a-mannosides [206] utilizes the P-thioglucoside 182, readily obtained from epoxide 173, proceeding via an oxidation-reduction protection sequence to give P-thiomannoside glycosyl donor 184, from which a-mannoside 185 can be stereoselectively prepared. 

Scheme 6.8 Formation of a-mannosides on cyclic mannosyl phosphate activation.

Different mannosyltransferases transfer mannose and 4-deoxymannose from their respective GDP adducts to acceptors. al,2-Mannosyltransferase was employed to transfer mannose to the 2-position of various derivatized a-mannosides and a-mannosyl peptides to produce the Manal,2Man structural unit [29,30]. A recent report indicates that mannosyltransferases from pig liver accept GlcNAcf)l,4GlcNAc phytanyl pyrophosphate, an analogue in which the dolichol chain of the natural substrate is replaced by the phytanyl moiety [31],... 

Results of the conformational analysis of the biantennary tetrasaccharide by NMR and GESA calculations showed no changes for the a-(l-3) and the a-(l-6) bonds in comparison to the corresponding linear trisaccharides, which proves that the two a-mannosidic linkages are independent of each other (Table 6) [30],... 

Fig. 1. a-Mannosidase-catalyzed synthesis of elymodavine a-mannoside (- -), ergometrine a-mannoside (-A-) and chanodavine a-mannoside (- -). The initial concentration of mannose and respective alkaloid were 3 and 0.2 M, respectively. The enzyme concentration was 10 U/ml [21]... 

Ergometrine 0-a-mannoside Enzyme a-mannosidase from Canavalia ensiformis... 

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