Efficient preparations of thioglycoside derivatives of L-idose and L-iduronic acid are described. The method avoids the tedious chromatographic separations of furanose and pyranose anomeric mixtures, and affords the thioglycosides in a stereoselective manner. The L-idose and L-iduronic acid thioglycosides having combinations of different protecting groups proved to be efficient glycosyl donors in the synthesis of heparin disaccharides.