Aldonic acids enolization

Two free-radical chain reactions, in addition to the ionic enolate mechanism, seem reasonable for the oxidation of the sugars by oxygen. With an aldose-2-f one of the free-radical mechanisms would yield non-labeled formic acid and the next lower aldonic acid the other would yield labeled formic acid and the same aldonic acid. 

Chemistry. Ascorbic acid is derived from the aldonic acid form of L-gulose (Figs. 8.56 and 8.57). There are two enolic proton-donor groups, with the one at position 3 being the most acidic with a value of 4.1. Ascor-... 

Both aldoses and ketoses are oxidized to aldonic acids by Tollens reagent (Ag, NH3, HO ), so that reagent cannot be used to distinguish between aldoses and ketoses. Recall from Section 20.3, however, that Tollens reagent oxidizes aldehydes but not ketones. Why, then, are ketoses oxidized by Tollens reagent, while ketones are not Ketoses are oxidized because the reaction is carried out under basic conditions, and in a basic solution, ketoses are converted into aldoses by enolization (Section 19.2). For example, the ketose D-fructose is in equilibrium with its enol. However, the enol of D-fructose is also the enol of D-glucose, as well as the enol of D-maimose. Therefore, when the enol reketonizes, all three carbonyl compounds are formed. 

Aldol condensation of 2,2-diethyl-l,3-dioxolan-4-one lithium or zirconium enolates with aldehydo sugars has afforded higher carbon aldonic acid derivatives, e.g. 1. The synthesis of L-ribono-1,4-lactone has been achieved from d-isoascorbic acid by way of the tetrose and pentitol derivatives 2 and 3 and the d-ribonolactone derivative 4 has been efficiently epimerized to the L-lyxonolactone 5 (Scheme 1). A selective i yn-epoxidation of racemic 2-0-benzyl-4-alkenamides followed by hydrolysis has afforded 3-deoxy-pentono-1,4-lactones. 

Keto and 5-keto aldonic acids also give carbon dioxide and furfural (see below) in yields similar to those for the uronic acids. However, ascorbic acid, as discussed later, gives a very high yield (above 80%) of furfural. Reductic acid, an enolic substance similar in structure to the ascorbic... 

The ascorbic acids can be considered as enolic lactones of the 2-keto and 3-keto aldonic acids. For vitamin C, there apparently exists an equilibrium between the ascorbic acid and the 3-keto acid. The latter has not been isolated, but hydrazone and osazone derivatives have been prepared... 

Cyanohydrin Synthesis from Osones (14 )- For this method, 3-keto aldonic acids are formed as intermediates which are not isolated. The first product, the nitrile, immediately enolizes with simultaneous ring formation to an imino analog (XI) of the ascorbic acid, and the latter is formed by... 

Redox glycosylation via thionoester intermediates. Barrett [112, 113] introduced the concept of redox glycosylation , where the usual glycosylation reaction is replaced by the esterification of an aldonic acid 164, followed by reduction to a reactive intermediate (enol ether from an ester or 0,S-ace al 166 from the thionoester... 

Enolization and Lactonization of 2-Keto Aldonic Adds (140), By the action of sodium methylate on the methyl esters, 2-keto acids are transformed into ascorbic acids (see (VII) and (VIII)). The reaction is almost quantitative. Lactonization and enolization take place simultaneously. Heat treatment of an aqueous solution of the free acid causes only a limited amount of conversion. Acids also act as catalysts (see above). From the acid hydroly-zate of the methyl glycoside of 3,4-0-isopropylidene-2-keto-L-ribonic acid, the 2-keto-L-ribonic acid could not be isolated, because L-erythroascorbic acid was formed very rapidly (14 )-... 

---