Dehydroascorbic acid structure

J. Hvoslef and B. Pedersen, The Structure of Dehydroascorbic Acid in Solution, Acta Chem. Scand. 1979, B33, 503, and D. T. Sawyer, G. Chiericato, Jr., and T. Tsuchiya, Oxidation of Ascorbic Acid and Dehydroascorbic Acid by Superoxide in Aprotic Media, J. Am. Chem. Soc. 1982,104, 6273. 

The structure of 138 was elucidated, shortly after its isolation, in extensive contributions by many groups. Several important discoveries relating to its constitution were reported in 1933, the same year in which the first successful synthesis was described (see Refs. 274, 324). Thus, the group at the University of Birmingham335 found that the primary oxidation product of 138, the 2,3-diulosono-lactone 148 (dehydroascorbic acid), could be quantitatively oxidized by sodium hypoiodite to generate oxalic acid and L-threonic acid (Scheme 16), which was identified as the crystalline tri-O-methyl-L-threonamide. These results established the stereochemical relationship between 138 and L-gulonic (and L-idonic) acid. The work that provided support for the structure of L-ascorbic acid has been summarized in several reviews.274,336... 

The structure of the bicyclic oxidation product of dehydroascorbic acid bis(phenyl-hydrazone) has been confirmed by detailed high-resolution mass spectroscopy as (34) (80JHC1181). 

The term vtiamin C refers to ascorbic acid (the fully reduced form of the vitamin) and to dchydroascorbic acid- Removal of one electron from ascorbic acid yields semidehydroascorbic acid (ascorbate radical). This form of the vitamin is a free radical it contains an unpaired electron- The structures of free radicals are written with large dots. The removal of a second electron yields dehydroascorbic acid. Conversion of ascorbate to dehydroascorbate, via the removal of two electrons, can occur under two conditions (1) with use of ascorbic add by ascorbate-dependent enzymes and (2) with the spontaneous reaction of ascorbate with oxygen. Semidehydroascorbate is an intermediate in this conversion palhway... 

On the other hand, a monocyclic derivative was obtained when a solution of dehydroascorbic acid and p-bromophenylhydrazine in di-methylacetamide was allowed to stand overnight at room temperature (25). The compound was precipitated with water and recrystallized from absolute alcohol its molecular structure is shown in Figure 11. The lactone ring is of the envelope type with C4 deviating by 0.126 A from the plane defined by Cl, Ol, C2, and 04. The side-chain is very nearly staggered and with 05 and C6 anti,... 

During the last 20 years a better understanding of the structure and chemical nature of DHA and the free radical intermediate that may be formed during the oxidation of AA has developed. These developments were based on modem instrumental techniques including NMR and NMR spectroscopies and pulsed radiation electron spin resonance (ESR) spectroscopy. The chemistry and properties of mono-dehydroascorbic acid (AA ), a free radical intermediate that may be formed in the oxidation of AA, is covered elsewhere in this volume. This chapter concerns DHA, its reactions, structure, and physiological chemistry. 

The nomenclature of the oxidized forms of AA is badly in need of revision. Not only is dehydroascorbic acid a long and cumbersome name, but it is also confusing in inferring that the compound is an acid. As is discussed later in this chapter, the principle structure is a bicyclic compound containing both lactone and hemiketal groups. Names such as ascorbitone or dehydroascorbitone would be better trivial representations. 

Dehydroascorbic acid is now readily prepared in pure form by the Ohmori-Takagi method in which ascorbic acid is dissolved in ethanol and oxygenated in the presence of charcoal. The structure of dehydro-L-ascorbic acid is solvent dependent. In water, dehydro-L-ascorbic acid exists almost exclusively in the bicyclic hydrated monomer in which 6-OH... 

Reactions of cyclic and acyclic 1,2,3-triones with phenylhydrazine give rise to mono- and bis-(phenylhydrazones). The central carbonyl group of a vicinal tricarbonyl system is electron-deficient and highly electrophilic,255 which is why treatment with an aryldiazonium ion affords 2-phenylhydrazones. The structure of such bis(phenylhydrazones) as cyclopentane-1,2,3-trione 1,3-bis(phenylhydrazone), dehydroascorbic acid 2,3-bis(phenylhydrazone), and cy-clobutanetetraone l,3-bis(phenylhydrazone) has been studied by UV, IR,1H-, 13C-, and 15N-NMR spectroscopy.265-268 Quantum-mechanical calculations to predict the most stable tautomeric forms of some 1,2- and l,3-bis(phenylhydrazones) revealed that the chelated bis(hydrazone) structure was usually more stable than the azoene-hydrazine structure.269-271 This does not mean that such structures do not exist, for example cyclobutanetetraone l,2,3-tris(phenylhydrazone) exists in a stable phenylazoene-hydrazine structure (see 49 Scheme 8).257-263... 

In addition to these equilibria, in aqueous solution, dehydroascorbic acid rapidly forms a hemiacetal structure. 

Dehydroascorbic Acid. L-rftreo-2,3-Hexodiulo-sonic acid -/-lactone. CtH Of mol wt 174.11. C 41.39%, H 3.47%, O 55.14%. The reversibly oxidized form of ascorbic acid. Prepd by the action of benzoquinone on ascorbic acid Ohle, Erl bach, Ber. 67, 555 (1934) Moll, Wieters, E. Merck s Jahresber, 50, 65 (1936) by the action of iodine Herbert et al, J. Chem. Soc. 1933, 1270 Kenyon, Munro, ibid. 1948, 158 by oxidn with perr-naphthindan -2,3,4-trione hydrate Moubasher, J. Biot. Chem. 176, 533 (1948) see also Muller-Mulot, Z. Physiol Chem. 351, 52 (1970). Isomerization and formn of derivs Egge, Tetrahedron Letters 1969, SOI. Structure studies Teichmann, Ziebarth, Z. Prakt. 

Oxidation of ascorbate produces a relatively stable anion radical (directly detectable by EPR spectroscopy), and dehydroascorbic acid (398) the latter is capable of complexation with Cr(V) (G in Scheme 4) (21, 383). Carbon-based radicals have been detected in Cr(VI) + ascorbate reaction mixtures using spin-trapping techniques (399) but the structures of these radicals are still unclear (298). 

Ascorbic acid oxidase oxidizes ascorbic acid to dehydroascorbic acid. In a pumpkin, two His, Cys and Met coordinate to type I copper, two His and OH (or H2O) to type II copper, and three His to each two type III copper (Fig. 2-24) [46]. Type II and Type III coppers form a triangular cluster, which presumably plays an important role in the four electron reduction of O2. Ascorbic acid oxidase and laccase have similar structures composed of three domains, but celluloplasmin is composed of six domains. 

The structure of the osazone derived from dehydroascorbic acid remains in dispute. Recently (Vol. 13, p. 95) it was described as the phenylhydrazine-phenylazo-structure (34), but it is now claimed that the bis(hydrazone) formula (35) fits the n.m.r. and u.v. data more satisfactorily. The structure of the bi-cyclic oxidation product of (35) has been shown to be a 3,6-anhydride (36) by n.m.r. and m.s. A n.m.r. study of the formation of dehydroascorbic acid hydrazones revealed that the two 2-phenylhydrazides (37) were formed initially and subsequently underwent dehydration to give the two rotationally isomeric 2-phenyIhydrazones. Reaction of 6-bromo-6-deoxy-isoascorbic acid (D-ery thro) with phenyl hydrazine afforded the cyclized bis(hydrazone) (38). Treatment of D-threo-ascorbic acid bis(phenylhydrazone) with caustic soda afforded the cyclized product (39). The corresponding D-erythro- and L-threo-isomers were also prepared and the side-chains have been modified in various ways. 

As a consequence of this structural feature, L-ascorbic acid is easily oxidized to dehydroascorbic acid. Both forms are biologically effective as a vitamin. There is no... 

FIGURE 1. Molecular structure of ascorbic acid (a) and dehydroascorbic acid (b). 

When dehydroascorbic acid is treated with hydroxide ion in ethanol, a yellow precipitate is produced. This was characterized as the pyranoid structure (53), formed by ring opening, tautomerization, and ring closure." ... 

The strong reducing behaviour of vitamin C is based on its enediol structure (Fig. 16.1). The oxidation of ascorbic acid is a two-step process in which semi-dehydroascorbic acid, a strong acid (dimmer form), is involved. This intermediate acts as a radical scavenger. The autoxidation of ascorbic acid is catalysed by transition metal ions" and dehydroascorbic acid is formed in the presence of oxygen. ... 

The structure of vitamin C is shown in Figure 11.27 both ascorbic acid and dehydroascorbic acid have vitamin activity. 

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