Haworth reagent

The hemiacetal hydroxyl is more readily alkylated than the true alcoholic hydroxyls and can be methylated selectively by treatment of the sugar with one equivalent of the Haworth reagent (see Chapter IV). When the hemiacetal hydroxyl of an aldohexose is blocked, some selective methylation of the hydroxyls at carbons 2 and/or 6 may be achieved with alkali and methyl iodide 22), With starch and cellulose, it has been reported 23) that alkali and methyl iodide lead preponderantly to the 2-0-methyl derivative, the hydroxyl group at carbon 6 presumably being rendered less... 

The aim of the methylation is to achieve an etherification of all of the free hydroxyl groups in the polysaccharide. The original procedure, as used by Denham and Woodhouse and by Haworth, in which the polysaccharide in 25-30% aqueous sodium hydroxide is treated with dimethyl is still the standard method. Fresh reagents are usually added to the reaction mixture after the first methylation period, with or without removal of the sodium sulfate formed, and the process is repeated until there is no further increase in methoxyl content. The many slight modifications of this method seem mostly to be matters of habit or convenience. The use of more concentrated sodium hydroxide solutions has been suggested, and occasionally seems to be essential for a satisfactory result. A practical point is that an aqueous solution of 42.2 g. of sodium hydroxide per 100 ml of solution is of exactly the concentration that 2 ml. is equiva-... 

Acylation by acid anhydrides also prevents somerization, but Ihis is wastelut ot reagent, since half of If IS converted into the cartao> ylic acid, and is therefore economicafly unacceptable. Nevertheless, cyclic anhydrides are viable reagents, as m the Haworth reaction (see Chapter 12). 

There are two major routes to phenanthrene, both of which can be used to prepare substituted derivatives. In the Haworth synthesis (Scheme 12.11), reaction of naphthalene with succinic anhydride yields an oxobut-anoic acid which is reduced under Clemmensen conditions to the butanoic acid. Cyclization in sulfuric acid and reduction of the resulting ketone is followed by dehydrogenation over palladium-on-carbon to phenanthrene. Alkyl or aryl derivatives can be obtained by treatment of the intermediate ketone with a Grignard reagent prior to dehydration and oxidation. 

It is possible that methanolysis or ethanolysis of cyclic acetals occurs by a mechanism similar to that for the previous reagent. Haworth and co-workers " found that both a- and 8-D-glucofuranosides result on treating 1,2-0-isopropylidene-a-D-glucofuranose 5,6-carbonate with methanolic or ethanolic hydrogen chloride. However, methanolysis of l,2 3,4-di-0-... 

Monosaccharides are sweet-tasting solids that are very soluble in water. Noncarbohydrate low-calorie sweeteners such as aspartame have been developed as sugar substitutes. Pentoses and hexoses form cyclic hemiacetals or hemiketals whose structures can be represented by Haworth structures. Two isomers referred to as anomers (the a and p forms) are produced in the cyclization reaction. All monosaccharides are oxidized by Benedict s reagent and are called reducing sugars. Monosaccharides can react with alcohols to produce acetals or ketals that are called glycosides. 

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