Transformation of Monomeric Saccharides

Transformation of Monomeric Saccharides 1. Aldopentoses, Aldotetroses, and Aldotrioses 

D-xylose was converted into 2-furaldehyde in acidified, tritiated water, no carbon-bound isotope was detected. This suggested that the 1,2-enediol (2) reacted immediately, as otherwise, tritium would have been detected at the aldehydic carbon atom of 2-furaldehyde, as a result of aldose-ketose interconversion.An acidic dehydration performed with d-[2- H]xylose showed that an intramolecular C-2-C-1 hydrogen transfer had actually occurred. Thus, these data indicated that an intramolecular hydride shift is more probable than the previously accepted step involving a 1,2-enediol intermediate. 

Although small proportions of other products are formed when D-xylose is exposed to rather high acid concentrations, arabinose, lyxose, and ribose form considerably more of alternative products (generally reductic acid) than of 2-furaldehyde under these conditions. Reductic acid (2,3-dihydroxy-2-cyclopenten-l-one, 47) has been detected as a product after acid exposure of D-xylose or its major dehydration product, 2-furalde-hyde. Further work performed with D-[l- C]xylose and [a- C]2-fural-dehyde showed that reductic acid having identical label distribution was obtained from both starting materials. This indicated that a common primary source was involved, probably 2-furaldehyde, as it is readily formed from D-xylose under acidic conditions. 

D-Xylose and D-arabinose have been treated with a 0.5 M acetate buffer (pH 4.5) at reflux. Besides 2-furaldehyde, some catechols and unique chromones were isolated from the reaction mixture in small proportions. These included 3,8-dihydroxy-2-methylchromone and its precursor, 5,6,7,8-tetrahydro-3,5-dihydroxy-2-methyl-8-oxochromone. A trihydroxy-2-methylchromone was also isolated from the D-xylose reac- 

Both D-[l- C]xylose and D-[5- C]arabinose were exposed to a concentrated phosphate buffer solution (pH 6.7). 1-Hydroxy-2-propanone (ace-tol) was distilled from the heated solution. Radioassay indicated that similar labeling [3- C] occurred in the acetol from both pentoses, with loss of the configurational difference thus, a 3-ketopentose or its enediol was suggested as an intermediate. Further work with 3-0- and 6-0-methyl-D-glucose and with 1-0-methyl-D-fructose indicated that /3-elimination from a 3-ketose or, in the case of a hexose, from a 3-ketose or a 4-ketose, or both, tautomerization of the resulting a-diketone to a /3-diketone, and hydrolytic cleavage are essential steps in the formation of acetol. 

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