Ascorbic acids, preparation

There is good evidence for the presence of a five-membered ring in D-araboascorbic acid (XXXIX). This analog of L-ascorbic acid, prepared by method 2 (page 87) from methyl 2-keto-D-gluconate shows the same chemical properties and the same absorption characteristics as L-ascorbic acid. Titration of XXXIX with diazomethane affords, as in the case of L-ascorbic acid, a 3-methyl derivative LXXXII which upon further action with diazomethane gives rise to the 2,3-dimethyl-D-arabo-ascorbic acid (LXXXIII)3 ... 

L-Ascorbic acid is found in all living tissues, both animal and plant matter, and as such has been consumed by humans for thousands of years, thus giving encouragement that the compound is physiologically acceptable and safe. Furthermore, extensive testing of L-ascorbic acid prepared by chemical synthesis confirms its relative safety. Large-scale manufacture, coupled with high standards of purity and relatively low cost, makes application to food products economically feasible. 

Prepare 5- ig/ j,l standards (Sigma) in deionized water of each of the following water-soluble vitamins thiamine-HCl cyanocobalamin (B12) riboflavin (B2) nicotinamide pyridoxine-HCl (Bj) nicotinic acid, ascorbic acid. Prepare a mixed standard containing the seven water-soluble vitamins at respective concentrations of 6, 2, 1, 6, 6, 6, 6 Xg/ 0.1. 

Carboxylic Acid Derivatives.—Calcium D-glucarate, D-glucaro-l,4-lactone, calcium sodium -D-galacturonate, methyl (l,2,3,4-tetra-0-acetyl-j8-i>galacto-pyranosyl)uronate, calcium D-araW o-hexulopyranosonate, the 2-C-benzyl derivative (326) from L-ascorbic acid (prepared by treating L-ascorbic acid with benzyl chloride and then with methanol), and 2-(4-bromophenylhydrazono)-dehydro-L-ascorbic acid. ... 

Finally mention should be made of the crystcilline enolic substances (CXV-CXVII) resembling ascorbic acid, prepared by the enolisation by alkali of D-mannosaccharo-i4 3,6-(CXIV),D-glucosaccharo-i,5 3,6-(CXVI), and 1,4 3,6- (CXVIII) dilactones . 

M.p. 190-192 C. The enolic form of 3-oxo-L-gulofuranolactone. It can be prepared by synthesis from glucose, or extracted from plant sources such as rose hips, blackcurrants or citrus fruits. Easily oxidized. It is essential for the formation of collagen and intercellular material, bone and teeth, and for the healing of wounds. It is used in the treatment of scurvy. Man is one of the few mammals unable to manufacture ascorbic acid in his liver. Used as a photographic developing agent in alkaline solution. 

Chemical Properties. The most significant chemical property of L-ascorbic acid is its reversible oxidation to dehydro-L-ascorbic acid. Dehydro-L-ascorbic acid has been prepared by uv irradiation and by oxidation with air and charcoal, halogens, ferric chloride, hydrogen peroxide, 2,6-dichlorophenolindophenol, neutral potassium permanganate, selenium oxide, and many other compounds. Dehydro-L-ascorbic acid has been reduced to L-ascorbic acid by hydrogen iodide, hydrogen sulfide, 1,4-dithiothreitol (l,4-dimercapto-2,3-butanediol), and the like (33). 

The L-xylosone pathway to L-ascorbic acid was never commercialized because L-xylosone was not teaddy available and was too expensive to prepare. The route, however, was valuable for L-ascorbic acid stmcture determination and for the preparation of derivatives. 

Fermentation. Much time and effort has been spent in undertaking to find fermentation processes for vitamin C (47). One such approach is now practiced on an industrial scale, primarily in China. It is not certain, however, whether these processes will ultimately supplant the optimized Reichstein synthesis. One important problem is the instabiUty of ascorbic acid in water in the presence of oxygen it is thus highly unlikely that direct fermentation to ascorbic acid will be economically viable. The successful approaches to date involve fermentative preparation of an intermediate, which is then converted chemically to ascorbic acid. 

Industrial uses of L-ascorbic acid relate to its antioxidant and reducing properties. It is used as an antioxidant in the commercial preparation of beer, fmit juices, cereals, and caimed and frozen foods, etc. 

Ascorbic acid has a variety of reactive positions that can be used to synthesize derivatives (99). Various derivatives and analogues have been prepared in attempts to find substances with increased biological activity (100,101). 

Methods for the preparation of L-ascorbic acids having isotopic C, H, O in various positions have been described and reviewed (104,105). Labeled L-ascorbic acid has played an important role in the elucidation of the metaboHc pathway of L-ascorbic acid in plants and animals. 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

The calcium form of EDTA instead of free EDTA is used in many food preparations to stabilize against such deleterious effects as rancidity, loss of ascorbic acid, loss of flavor, development of cloudiness, and discoloration. The causative metal ions are sequestered by displacing calcium from the chelate, and possible problems, such as depletion of body calcium from ingestion of any excess of the free chelant, had it been used, are avoided. 

Salads. A combination of citric acid and ascorbic acid is used as an alternative to sulfites in prevention of enzymatic browning in fresh prepared vegetables (56). 

DETERMINATION OF ASCORBIC ACID IN POLYVITAMIN PREPARATIONS BY ION-PAIR THIN LAYER CHROMATOGRAPHY... 

Ascorbic acid, HQHt06, also known as vitamin C, is a weak acid. It is an essential vitamin and an antioxidant. A solution of ascorbic add is prepared by dissolving 2.00 g in enough water to make 100.0 mL of solution. The resulting solution has a pH of 2.54. What is Ka for ascorbic acid ... 

Procedure. Prepare a standard solution of ascorbic acid (ca 10 3M) by dissolving about 0.18 g (accurately weighed) in a 1 L graduated flask using oxygen-free water. 

Ascorbic acid. Dissolve 20 g of the solid in 100 mL of de-ionised water. This reagent must be freshly prepared. 

Benzenediamine (358) and 5-(l,2-dihydroxyethyl)tetrahydro-2,3,4-furane-trione (359) (prepared in situ by oxidation of ascorbic acid with p-benzoqui-none) gave either 3-(2,3,4-trihydroxybutyryl)-2(17i)-quinoxalinone (360)... 

Deoxy-6-fluoro-L-ascorbic acid ° (314) was prepared from methyl 2,3-<9-isopropylidene-6-0-tosyl-a-L-gulosonate (313) by reaction with KP followed by isomerization of the product (with H" " cation-exchange resin). 

CuCl, especially in a single crystal form, is extensively used as an optical material for its special optical properties. Orel et al. [2] first proposed a new method to obtain CuCl particles by the reduction of Cu with ascorbic acid. Several dispersants were used in the reduction and monodispersed CuCl particles can be obtained by selecting the proper dispersant and reduction conditions. In this work, the above method was used to modify the traditional process of CuCl preparation, namely, by reducing the Cu " with sodium sulfite to obtain the highly active CuCl catalyst to be used in the direct process of methylchlorosilane synthesis. 

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