Efficient isomerization of methyl arabinofuranosides into corresponding arabinopyranosides in presence of pyridine.

Fisher glycosylation, the oldest but efficient reaction towards alkyl glycosides, suffers nonetheless from lack of selectivity, especially when dealing with pentoses. In this case, a mixture of the four isomers, namely the furanosides and the pyranosides, is formed. According to previous studies, the rate and selectivity of the reaction depend greatly on the reaction time and the temperature. In this report, another factor was evaluated, the introduction of a weak nucleophilic base. Interestingly, addition of pyridine few hours after the reaction has started allowed rapid isomerization of the methyl pentofuranosides into its pyranoside counterparts. The reaction proceeds with great diastereoselectivity using arabinose, ribose, xylose and lyxose as starting pentoses. Corresponding methyl pyranosides were obtained as the sole isomers with yields ranging from 65% to 75%.