Aldoses chloride method

Dialkyl dithioacetal derivatives of ketoses, such as D-fiuctose and L-sorbose, me inaccessible directly from the parent sugars, the ketose undergoing extensive decomposition under the conditions employed for mercaptaladon of aldoses. Such derivatives can, however, be prepared by indirect methods. Acetylation of D-fiuctose [40] and L-soibose with acetic adiydride and zinc chloride [41] leads to good yields of acyclic pentaacetates in which foe ketose carbonyl is not involved in a cyclic acetal. Subsequent treatment of these acetylated derivatives with thiols affords foe acetylated dialkyl dithioacetals in satisfactory yields, and conventional deacetylation affords foe unprotected dialkyl dithioacetals [40,41]... 

The methods routinely used for preparing hydrazones of aldoses yield only a gel-like, orange mass from D-fructose and (2,4-dinitrophenyl)-hydrazine. i However, by use of p-dioxane containing catalytic proportions of water (3%) and hydrogen chloride (0.3%), small needles of the p-dioxane solvate are obtained after 5 minutes. Dissolution of these crystals in 1 1 pyridine-96% ethanol gives the pyridine solvate. These crystals (m.p. 173-175°) were found to belong to the monoclinie system, elongated parallel to the 5-axis. ... 

This method for the synthesis of higher-carbon ketoses is based on the reaction of diazomethane with an acid chloride to give a diazomethyl ketone which, on hydrolysis (or acetolysis), furnishes a hydroxy (or acetoxy) methyl ketone. The reaction was first applied in the sugar field in 1938 and has since been widely used in the synthesis of ketoses by Wolfrom and coworkers. As developed by Wolfrom, the synthesis uses fully acety-lated derivatives in the following stages aldose — acetylated aldonic acid acetylated aldonyl chloride acetylated diazomethyl ketose — acetylated ketose — ketose. The method is illustrated in the synthesis of D-galacto-heptulose (10). ... 

Reduction of the sugar acid lactones to aldose sugars also may be accomplished by catalytic hydrogenation (I40) or, very conveniently, with sodium borohydride in aqueous solution ) Other methods of reduction include the catalytic hydrogenation of the acetylated aldonyl chlorides (14 ) or thio esters (I4S). 

Two new methods for the reduction of aldonolactones to aldoses have been developed for use in small-scale syntheses either the lactone itself was reduced with diborane in THF, or an 0-tetrahydropyranyl derivative was reduced with a 1 1 mixture of lithium aluminium hydride and aluminium chloride in ether. The yield of aldose depends on a number of factors and may be low due to the ease of reduction of the aldose to the alditol. The catalytic hydrogenation of D-glucose in the temperature range 100—170 °C and at pressures of 20—80 atmospheres has been examined the effects of pH and promoters (e.g. magnesium, barium chloride, calcium sulphate) were also examined. The rate of hydrogenation was enhanced at pH 8, and calcium sulphate was the most effective promoter at pH 6.8. 

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