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

Methyl tetra-O-acetyl-beta-D-glucopyranuronate (1) and methyl tetra-O-acetyl-alpha-D-glucopyranuronate (3) were isolated as crystalline solids and their crystal structures were obtained. That of the beta anomer (1) was the same as that reported by Root et al., while anomer (3) was found to crystallise in the orthorhombic space group P2(1)2(1)2(1) 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 a anomer (3) was found to be an impurity in initially precipitated batches of beta-anomer (1) in quantities <3%; however, it was possible to remove the alpha impurity either by recrystallisation or by efficient washing, i.e. the a anomer is not incorporated inside the beta anomer crystals. The beta anomer (1) was found to grow as prisms or needles elongated in the a crystallographic direction in the absence of the alpha impurity, while the presence of the alpha anomer (3) enhanced this elongation. (C) 2016 Elsevier Ltd. All rights reserved.