Sugar acids metasaccharinic acid

Three structurally isomeric forms have been established for the six-carbon saccharinic acids. In the order of their discovery, these are the sac-charinic or 2-C -methylpentonic acids, the isosaccharinic or 3-deoxy-2-C -(hydroxymethyl)-pentonic acids, and the metasaccharinic or 3-deoxy-hexonic acids. Although none of these six-carbon, deoxyaldonic acids has been crystallized, six are known in the form of crystalline lactones (saccharins). All the possible metasaccharinic acids of less than six-carbon content have been obtained, in the form of crystalline derivatives, by the sugar-alkali reaction. Only one example of a branched-chain deoxyaldonic acid (the racemic, five-carbon isosaccharinic acid) of other than six-carbon content has been so obtained. The formation of saccharinic acids containing more than six carbon atoms remains to be explored. 

Only the racemic form of this acid is obtained from the sugar-alkali reaction. As in the formation of lactic acid, a non-asymmetric enediol is an intermediate in its production (see Section III), and hence the racemate is the sole representative of the four-carbon metasaccharinic acid class. 

D-glucosaccharinic acid but also D-isosaccharinic acid and Kiliani s para-saccharinic acid might be formed by recondensation of appropriate alde-hydic fragmentation products with a lower-carbon metasaccharinic acid. He proposed that the unbranched metasaccharinic acids, in contrast, are formed by direct dismutation of the isomeric sugars. 

Nef s theory of the mechanism of formation of the saccharinic acids is outlined, in its original form, in a paper published in 1907 and, in its final form, in his comprehensive article of 1910. The theory proposes that the reaction takes place in two major steps (a) the isomerization of the sugar, with loss of water, to an a-dicarbonyl compound, and (b) a benzilic acid type of rearrangement of the latter, with hydration, to the saccharinic acid. The second step involves chain rearrangement in the production of the saccharinic, isosaccharinic, and Kiliani s parasaccharinic acids, but not in the production of the metasaccharinic acids. 

Dihydroxybutanoic acid, a 3-deoxytetronic acid, is the only theoretically possible four-carbon metasaccharinic acid it was isolated by Nef in the dl form (Ilab) in the course of his work on the action of sodium hydroxide on D-arabinose. It therefore constitutes the only four-carbon saccharinic acid isolated to date from a sugar-alkali reaction. Resolution of this on acid (Ilab) was accomplished by Nef, who showed that the dextrorotatory acid (Ila) upon oxidation gave rise to 2-deoxy-L-f/Z2/cero-tetraric acid [(-)-)-d-malic acid XV]. During the course of this work, several derivatives of the L and DL saccharinic acids (lib and Ilab) were prepared, but only the brucine salt of the d acid (Ila) was reported. 

The formation of metasaccharinic acids from reducing sugars involves 1,2-enolization (43), elimination of the functional group at C-3 to give (44), which tautomerizes to the glycosulose (45), and a benzilic acid rearrangement of (49) to (50). ... 

Figure 4.31). Metasaccharinic and other sugar acids are, however, formed, preferably in alkaline solutions. Pyranoses also dominate in (Z)-3,4-dideoxy-D-g iycero-hex-3-enos-2-ulose (4-196) solutions. The existence of a six-membered ring in (E)-3,4-dideoxy-D-g/ycero-hex-3-enos-2-ulose is not possible therefore in aqueous solutions this isomer occurs as a straight chain form or as its hydrate (4-197), respectively. 

A 2-ketose may enolize in two ways, namely, by the loss of a proton from either C-1 or C-3. A jS-elimination can then take place from C-3 in the first case, and, in the second, from either C-1 or C-4. Under alkaline conditions, the enolizations are rapid and reversible, so that the direction of the over-all reaction depends on the relative ease of elimination of the functional groups at C-1, C-3, and C-4. The saccharinic, metasaccharinic, and isosaccharinic acids are derived by the elimination of a group from C-1, C-3, or C-4, respectively, from a reducing sugar in alkaline solution. - ... 

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