The Methyl Ethers of L-Rhamnose

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. For example, trimethyl-L-rhamnopyranose has been isolated from methylated gum arabic86 and from the methylated polysaccharide from the somatic portion of the cells of Mycobacterium tuberculosis,26 and in both these polysaccharides L-rham-nopyranose residues evidently occupy terminal positions in the molecule. 

2-Methyl-L-rhamnose (L) has been prepared from the 3,4-diacetyl-L-rhamnose l,2-(methyl orthoacetate), (XLVIII of Haworth, Hirst and Samuels67) which on treatment with methanolic hydrogen chloride gave a mixture of 3,4-diacetyl-L-rhamnopyranose (XLIX) and methyl 2,3,4-triacetyl-/J-L-rhamnopyranoside. Methylation of XLIX followed by 

The only recorded derivative of 3-methyl-L-rhamnose is the phenyl-osazone, which was prepared from 2,3-dimethyl-L-rhamnose.69 

4-Methyl-L-rhamnose (LII) has been identified among the products of hydrolysis of methylated slippery elm mucilage.80 This sugar had been 

5-Methyl-L-rhamnose has been synthesized from the reducing product, 2,3-isopropylidene-L-rhamnofuranose (LIV), which is obtained by the condensation of L-rhamnose with acetone in the presence of sulfuric acid and copper sulfate. Methylation with silver oxide and methyl iodide in the presence of sodium sulfate gave methyl 2,3-isopropylidene-5-methyl-/ -L-rhamnofuranoside (LV), from which was obtained by hydrolysis 5-methyl-L-rhamnose (LVI).62 

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Apart from their intrinsic interest the methyl ethers of certain of the pentoses, especially those of L-arabinose and D-xylose, are of great importance in structural studies of the naturally occurring pentosans, and of plant gums and mucilages.1 This also applies to the methyl ethers of L-rhamnose and L-fucose. 

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