Synthesis of L-Iduronic Acid Derivatives: Crystal Structure of Methyl (Methyl 2,3,4-Tri-O-Acetyl-β-L-Idopyranosid)Uronate

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Abstract
Several L-iduronic acid derivatives were prepared by chemical synthesis including the reducing sugar, the α– and ß-methyl glycosides and L-iduronolactone. The ß-methyl glycoside, as well as two isomeric orthoesters, were the unexpected products of glycosylation of methyl (2,3,4-tri-O-acetyl-α-L-idopyranosyl bromide) uronate. EI-MS was used to distinguish between the orthoesters and the glycosides. The crystal structure of the ß-methyl glycoside was determined by direct methods and refined by full-matrix least squares to a final value of R = 0.067 for 1739 reflections. The pyran ring was found to occur in the 1C4 conformation, with the three acetoxy substituents in axial orientations. In aqueous solution, the α-anomer of the reducing sodium salt is almost entirely in a 2SO ring conformation and the α-methyl glycoside is an equilibrium mixture of conformations which is sensitive to pH: The ß-anomers all have spectral data consistent with predominant or only slightly distorted 1C4 chairs.