Synthesis of methyl 3,6-dibromo-3,6-dideoxy-β-d-allopyranoside and -glucopyranoside, and their interconversion in the presence of bromide ion

The bromination of methyl β-d-glucopyranoside with tribromoimidazole and triphenylphosphine in toluene-acetonitrile (2:1 v/v) for 4 h at 110°C gave methyl 3,6-dibromo-3,6-dideoxy-β-d-alloside in low yield, while the bromination in toluene under otherwise comparable conditions gave methyl 3,6-dibromo-3,6-dideoxy-β-d-glucoside. Examination of the time course of the reaction in toluene showed that the dibromoalloside was initially formed and was converted into the dibromoglucoside during the bromination. The rate constants of the interconversion between the dibromoglycosides in LiBr-N,N-dimethylacetamide (DMA) were about 103 times as large as those of the interconversion of the corresponding dichloroglycosides in LiCl-DMA. The results are discussed in relation to the presence of 3,6-dibromo-3,6-dideoxyglucose residues, in addition to 3,6-dibromo-3,6-dideoxyallose and 6-bromo-6-deoxyglucose residues, in bromodeoxycellulose samples having high ds values obtained by the bromination of cellulose with tribromoimidazoletriphenylphosphine.