Methylation, of carbohydrates

C. Schwarzinger, On the mechanism of thermally assisted hydrolysis and methylation of carbohydrates the contribution of aldol and retroaldol reactions, J. Anal. Appl. Pyrol., 68 69, 137 149 (2003). 

Before we can go on to the pext aspect of the structure of D-(+)-glucose, determination of ring size, we must first learn a little more about the methylation of carbohydrates. 

Purdie introduced the combination methyl iodide-silver oxide for exhaustive methylation of carbohydrates. In an improved procedure" a solution of the carbohydrate in methanol is shaken at 25° with methyl iodide and silver oxide, and four further additions are made at 12-hr. intervals, with occasional filtration from silver salts. 

Methylation of alcohols [I, 193, after citation of ref. 18]. Diazomcthane-boron trifluoride ethcrate is useful for the methylation of carbohydrates containing base-... 

Methylation of carbohydrates. Anderson and Cree21 used the combination methyl iodide-sodium hydride—DMSO and achieved complete methylation of monosaccharides and acidic disaccharides in one step, but acidic polysaccharides were methylated only partially. Stacey et al.a used methyl bromide, sodium hydride, and DM F or N-methyl-2-pyrrolidone for methylation of glucopyranosides. 

The intentional modification of food constituents to improve quality has long been practiced. In food manufacture lipids and carbohydrates are often intentionally modified to impart certain desired characteristics to foods. Important examples are the hydrogenation of polyunsaturated lipids to increase the melting point, and the acetylation and carboxy-methylation of carbohydrates to change their textural characteristics. Similar examples are not available for food proteins. 

Methylation of carbohydrates by dimethyl sulfate and alkali has been much used for preparative purposes. According to Haworth and Leitch698 monosaccharides can be methylated as follows ... 

In Purdie and Irvine s method778 hydroxy compounds are alkylated by alkyl iodides in the presence of silver oxide. This method is of particular significance for methylation of carbohydrates, where, to avoid its oxidation, the reducing group is protected by glycosidation. 

Methylation of carbohydrates. Carbohydrates can be methylated in >80% yield by methyl triflate and a sterically hindered, weak base such as 2,6-di-t-butylpyridine and 2,6-di-r-butyl-4-methylpyridine. The method is useful for sugars that are sensitive to acids or bases. 

Ciucanu I, Costello CE. Elimination of oxidative degradation during per-O-methylation of carbohydrates. J Am Chem Soc 2003 125 16213-16219. 

P. Prehm, Methylation of carbohydrates by methyl trifluoromethanesulfonate in trimethyl phosphate, Carbohydr. Res., 78 (1980) 372-374. 

The Hakomori method has also been used extensively in the methylation of carbohydrates however, yields were typically low and non-sugar products were observed. Subsequent studies by Ciucanu and Kerek [40] led to the conclusion that OH and H were more effective bases than the methylsulfinyl carbanion. These authors developed a new method for the permethylation of carbohydrates using CH3I and solid NaOH in DMSO. This reaction is relatively clean, proceeds to completion in about 6-7 min, and gives very high yields ( 98%). The derivatization may also be performed with KOH, NaH (excess) and sodium t-butoxide. Ciucanu and Kerek have extensively reviewed the experimental parameters and have published a table of procedures, reagents, reaction times and yeilds for various other versions of Ihe permethylation reaction [40]. 

Methylation of carbohydrates containing base-labile substituents using a combination of diazomethane, di-chloromethane, and boron trifluoride etherate is knovm to proceed without concomitant migration of acyl groups (17, 18). Consequently, this method was chosen for methylation of various, partially acylated derivatives of sucrose (19) Methylation of 1, 2,3,3, 4, 6 -hexa-0-acetylsucrose (G) with a freshly prepared solution of Jiazomethane in drchloromethane and boron trifluoride etherate for 0.5 hr at -5° after chromatographic separ-... 

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