Rapid Synthesis of l ‐Idosyl Glycosyl Donors from α‐Thioglucosides for the Preparation of Heparin Disaccharides

A new methodology for the synthesis of the most challenging heparin building block has been developed. Orthogonally protected l ‐idosyl glycosyl donors were prepared by C5 epimerization of the corresponding thioglucosides using the hydroboration/oxidation method followed by a 4,6‐acetal formation. The α‐anomeric configuration was crucial, and the bulky C4 substituent was advantageous for the high l ‐ ido diastereoselectivity. The 4,6‐arylmethylene group proved to be a directing element in glycosylation, whereby stereoselective α‐idosylation could be achieved by using idosyl donors without a C‐2 participating group.