Methyl tetra-O-acetyl-α-d-glucopyranuronate: crystal structure and influence on the crystallisation of the β anomer

•Purification and crystal structure of methyl tetra-O-acetyl-α-d-glucopyranuronate.•Occurrence of the α anomer in methyl tetra-O-acetyl-β-d-glucopyranuronate.•α Anomer effect on crystallisation of methyl tetra-O-acetyl-β-d-glucopyranuronate. Methyl tetra-O-acetyl-β-d-glucopyranuronate (1) and methyl tetra-O-acetyl-α-d-glucopyranuronate (3) were isolated as crystalline solids and their crystal structures were obtained. That of the β anomer (1) was the same as that reported by Root et al., while anomer (3) was found to crystallise in the orthorhombic space group P212121 with two independent molecules in the asymmetric unit. No other crystal forms were found for either compound upon recrystallisation from a range of solvents. The α anomer (3) was found to be an impurity in initially precipitated batches of β-anomer (1) in quantities