Sugars enediol forms

The acyclic, enolic compounds 7 and 9 may exist in either cis or trans forms. Methyl ethers of 7 have been isolated in the cis form,8 but it is not known whether the trans forms, which must be acyclic, exist. The relative proportion of isomers is controlled by the geometry of the parent sugar enediol. Although the acyclic forms are readily interconvertible tautomers, it appears that, in acidic medium, further reaction occurs much more rapidly than any equilibrating reactions. Compound 7 undergoes rapid elimination of a second hydroxyl group to give 11. This acyclic product, also, may exist as either a cis or a trans isomer, both forms of which have been isolated.8 The loss of a third molecule of water per molecule occurs after, or simultaneously with, the cyclization of 11 (see Section II, 3 p. 171), and results in formation of 5-(hydroxymethyl)-2-furaldehyde (5). 

When sugars are treated with aqueous ammonia for a short time at low temperature in the absence of a catalyst, the reaction is arrested before heterocyclic compounds can be formed in appreciable proportion, and the products are mainly epimerization products of the sugars, probably formed by way of their 2,3-enediols. These epimerization products are summarized in Table I which shows the reactions of D-glucose, D-fructose, lactose, maltose, and melibiose with aqueous ammonia for a short time at low temperature. A dark-colored, high polymer is also formed in some instances (the browning reaction). In the ammoniacal solution, the monosaccharides are epimerized the disaccharides are epimerized and, in addition, may be hydrolyzed to monosaccharides that can also be epimerized hence, the variety of products obtained may be considerable. 

In acidic degradation, 1,2-enediol forms from the aldose or ketose after a series of dehydration reactions. If the initial sugar is a hexose, 1,2-enediol is converted to HMF. If it is a pentose, 1,2-enediol is converted to 2-furaldehyde. 3-Deoxyaldose-2-ene, 3-deoxyosulose, and osulos-3-ene are intermediates in the acidic degradation of fructose. The last series of reactions include both fragmentation reactions (flavor production) and polymerization reactions (color production). 

In some instances, the reactive forms or the intermediates themselves, show polarographic waves which are separated from those of reactants and products. The formation of the thiol form of thiamine, < ) the bicyclic form of protopine< > and the enediol forms of some sugars< > and pyridoine< ) are examples of the formation and detection of reactive forms. A few examples of the detection of intermediates, are the diketone and enediol produced during the alkaline degradation of phenylglyoxal< > and of a, -unsaturated ketone as an intermediate in the cleavage< ) of... 

The mechanisms by which D-sorbose and L-sugars are formed are not known. Conceivably 3,4- and 4,5-enediols might be formed. On the other hand, the possibility exists of the cleavage of the carbon chain through a reversed aldol condensation. If a hexose was cleaved to two glyceraldehyde molecules, they would be expected partially to isomerize immediately to dihydroxyacetone. A very rapid aldol condensation of glyceraldehyde and dihydroxy acetone occurs (p. 113) with the formation of fructose and sorbose. These ketohexoses have a trans arrangement of the hydroxyls at the new asymmetric centers 92 93). 

Enediol forms of a-hydroxycarbonyl compounds react with a-dicarbonyl compounds with interconversion, in which a-hydroxycarbonyl compounds are transformed into a-dicarbonyl compounds and vice versa (Figure 4.66). The reaction explains a number of oxidation-reduction reactions that occur in degradation products of sugars and in the Maillard reaction. The transfer of two protons takes place within the complex of the a-dicarbonyl compound, with the endiol in either the basic energy singlet state or in the excited triplet state, through the more advantageous biradical mechanism. 

Aldoses are reducing sugars because they possess an aldehyde function m then-open chain form Ketoses are also reducing sugars Under the conditions of the test ketoses equilibrate with aldoses by way of enediol intermediates and the aldoses are oxidized by the reagent... 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

The first dehydration products formed by general, acid-base catalysis are represented by the enolic forms (7, 9, and 10) of the deoxydicarbonyl sugars 7a, 9a, and 10a. The enolic compounds are formed from enediols by the removal of a molecule of water through -elimination of a hydroxyl group. For example, from the 1,2-enediol (6) derived from D-glucose or D-fructose, the enolic form (7)of3-deoxy-D-eryf/iro-hexosulose (7a) is produced, whereas from the 2,3-enediol... 

Kinetics of oxidation of four pentoses by bromamide-T were conducted in alkaline medium at different temperatures and the overall activation parameters have been calculated.52 Aldonic acids were the oxidation products, and a mechanism was suggested in which formation of the enediol anion of the sugar is the rate-limiting step. As aldoses may undergo epimerization in alkaline solutions, the oxidation of monosaccharides with bromamide-T was also performed in hydrochloric acid solution.53 Kinetic parameters revealed a low reactivity of ketoses relative to aldoses, and indicated that the cyclic forms of the latter are involved in the oxidations. 

The proposed intermediate, 2-0-(/3-D-glucopyranosyl)-D-erythrose (XC), hydrolyzes, supposedly via the enediol, to D-glucose and D-erythrose (XCI). This D-glucose, under the action of alkali, then produces lactic acid. The other products shown above (formaldehyde, glycolaldehyde) are known to be formed by alkaline fragmentation of the carbon chain, and are ordinarily present when alkali acts on a reducing sugar.68... 

Furanones and pyranones constitute a family of compounds that derive from a common a-dicarbonyl intermediate, namely, CHj-CO-CO-(CHOHJn-CHjR (where R-H or OH). Acetylformoin, which has a fruity-caramel aroma, has been isolated from browned instant orange juice (43) and from a sugar model system containing a catalyst (38). It can be formed by dehydration and cyclization of methyl dihydroxy-ethyl reductone (enediol of the methyl dicarbonyl intermediate), but it decomposes easily in air to pungent compounds (44). 

A test for reducing sugars, employing the same silver-ammonia complex used as a test for aldehydes. A positive test gives a silver precipitate, often in the form of a silver mirror. Tollens reagent is basic, and it promotes enediol rearrangements that interconvert ketoses and aldoses. Therefore, both aldoses and ketoses give positive Tollens tests if they are in their hemiacetal forms, in equilibrium with open-chain carbonyl structures, (p. 1118)... 

When D-glucose 6-phosphate and D-fructose 6-phosphate are inter-converted by D-glucose 6-phosphate ketol isomerase in either deuterium oxide33 or water-1 (Ref. 34), isotope is incorporated at C-l of D-fructose 6-phosphate and C-2 of D-glucose 6-phosphate, indicating that the interconversion involves an enediol intermediate, which may arise from an open-chain (37) or cyclic (38) form of the sugar,... 

Sugars in weak alkaline solutions undergo isomerization to form 1,2-enediol followed by the formation of a mixture of sugars. 

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. 

---