| Popis: |
High yield, highly stereoselective methods for the synthesis of β-mannopyranosides of primary, secondary, and tertiary alcohols are presented. Activation of mannosyl sulfoxides or mannosyl thioglycosides with trifluoromethanesulfonic anhydride or benzenesulfenyl triflate, respectively, leads to the efficient formation of α-mannosyl triflates at −78 °C in dichloromethane, in the presence of 2,6-di- tert -butyl-4-methylpyridine. These triflates then react S N 2-like with alcohols to give the β-mannosides. The use of a 4,6- O -benzylidene protected mannose is required for high selectivity, as is the use of non-participating protecting groups on O -2 and O -3 of the donors. It is further demonstrated that the thioglycoside/benzenesulfenyl triflate activation is applicable in the glucoside series, when both armed and disarmed protecting groups are tolerated. |
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