Publisher Summary This chapter discusses the properties of the known methylated pentoses that helps in structural studies on polysaccharides containing pentose residues. The derivatives of rhamnose and fucose have also been included in the chapter because of the importance of the former as a building unit in plant gums and mucilages and because fucose is a component of gum tragacanth, of the seaweed polysaccharide sulfate fucoidin, of the jelly coat of sea urchin eggs, of blood-group polysaccharides, and of frog-spawn mucin. Many polysaccharides contain L -rhamnose as a constituent and various methylated L -rhamnoses have, therefore, been isolated by the hydrolysis of methylated gums and mucilages. By oxidizing the free sugar with silver oxide the silver salt of 2- O -methyl-4-deoxy- L -erythronic acid has been obtained. The monomethylrhamnose isolated from methylated slippery elm mucilage was oxidized to give a crystalline lactone that from its negative rotation and rapid hydrolysis in water was undoubtedly a pyranolactone. 5-Methyl- L -rhamnose has been synthesized from the reducing product, 2,3-isopropylidene- L -rhamnofuranos, which is obtained by the condensation of L -rhamnose with acetone in the presence of sulfuric acid and copper sulfate.