Efficient Selective Preparation of Methyl-1,2,4-tri-O-acetyl-3-O-benzyl-beta-L-idopyranuronate from Methyl 3-O-benzyl-L-iduronate

Methyl 1,2,4-tri-O-acetyl-3-O-benzyl-L-idopyranuronate 6beta/6alpha, prepared from methyl 3-O-benzyl-L-iduronate (4), is a key synthon in heparin/heparan sulfate synthesis. The 1H and 13C NMR spectra of the furanose-pyranose mixture of 4, after dissolution and equilibration in d(4)-methanol, were fully assigned allowing to expect that 4 could crystallise in the beta-pyranose form. New acetylation conditions able to trap this form were subsequently devised, allowing the isolation of 83% of pure 6beta by simple crystallisation, along with 9% of the 6beta/6alpha mixture. This represents a major advantage over the previously published procedure, especially on multigram scales.