A mild one-step selective conversion of primary hydroxyl groups into azides in mono- and oligo-saccharides

The direct azidation reaction of several monosaccharide methyl glycopyranosides, sucrose, α,α-trehalose, cyclomaltohexaose and cyclomaltoheptaose with sodium azide in the presence of triphenylphosphine-carbon tetrabromide is reported. The optimal reaction conditions require pre-formation of the reactive species before addition of the sugar substrate. Formation of the primary azidodeoxy compound is accompanied by simultaneous formation of the corresponding primary bromodeoxy and 3,6-anhydro derivatives in the glycopyranoside series, the former being transformed in situ into the azide by quenching of the reaction mixture with methanol before increasing the temperature. Interestingly, good selectivity towards the primary C-6 position of the glucopyranosyl moiety as compared to the fructofuranosyl one was observed in the case of sucrose, advantage of which has been taken in an improved preparation of 2,3,4,1′,3′,4′,6′-hepta-O-acetyl-6-azido-6-deoxysucrose (45% yield from sucrose). Sodium or lithium azide reagents were found equally effective. The azide functionality could be reduced without previous purification and the resulting amino sugar isolated by cation-exchange column chromatography, as illustrated for the preparation of 61-amino-61-deoxycyclomaltoheptaose.