Ascorbic acid esterification

L-ascorbic acid and, 25 751 catalytic esterification of, 10 482 in cationic polymerization of cyclic siloxanes, 22 560 cellulose as, 11 266 a-chiral and homologated, 13 669 control methods for, 26 687-690 derived from halogen fluorides,... 

L-ascorbic acid and, 25 751 as chelating agent, 5 731 in cocoa shell from roasted beans, 6 357t Oxalic-acid-catalyzed novolacs, in molding compounds, 75 786 Oxalic acid esterification, 72 652 Oxaloacetic acid, in citric acid cycle, 6 633 Oxalosuccinic acid, in citric acid cycle, 6 633... 

There are a total of three ascorbic acid processes either in place or in late stages of development with firm expectations of commercialization (i) the traditional chemical Reichstein-Griissner synthesis (ii) the two-step fermentation process to 2-ketogulonic acid with subsequent chemical esterification/lactonization to ascorbic acid and (iii) the one-step fermentation to 2-ketogulonic acid with the same last chemical step. Figure 20.8 and Table 20.3 provide an overview of the three processes. 

Peptide Modification lodination was carried out on a stainless steel probe target by adding 0.1 % aq. I2 (1 pi) to the dried peptide (ca. 1 pmol). The reaction was stopped after 1 minute by addition of ascorbic acid and the MALDI matrix, a-cyano cinnamic acid in excess. Esterification with ethanol was carried out using the method of Hunt et al. (15), where an acetylchloride and ethanol solution (1 6, v v) was added (5 pi) to the peptide dried in a microcentrifuge tube (ca. 1 pmol). After incubation for 15 minutes at room temperature a 2 mM p-mercaptoethanol (in ethanol) solution was added (1 pi) and the mixture was dried. The matrix, a-cyano-4-hydroxycinnamic acid (2 pi), was added to the micro tube and after 5 minutes 1 pi of this matrix was removed and applied to a target. 

A second synthesis in which a Ci fragment was coupled with a C5 fragment was reported in which acid chloride (8) was converted to the acyl nitrile (9) by use of silver cyanide (23) (Scheme 6). Hydrolysis and esterification produced ethyl 3,4,5,6-tetra-0-acetyl-DL-xyio-2-hexulo-sonate. The conditions required for the conversion of this material to DL-ascorbic acid will be discussed later. No yields were reported for this reaction sequence. This synthesis has not been used for the preparation of analogues or radiolabeled derivatives of L-ascorbic acid as has the osone-cyanide synthesis first reported (9-12). In contrast to the osone-cyanide synthesis (Scheme 5) in which a 3-ketogulonic acid derivative is produced, the acid chloride-silver cyanide synthesis (Scheme 6) results in the formation of a 2-ketogulonic acid derivative (2a) as an intermediate in the ascorbic acid synthesis. 

Cousins et al. (38) have reported the formation of ascorbic acid 6-0-sulfate, 27, from the esterification of ascorbic acid with sulfur trioxide in sulfuric acid. The 6-sulfate derivatives have been proposed for use as anti-cholesteremics and as inhibitors of nitrosamine formation. 

Acid-catalysed esterification of ascorbic acid, e.g. acetylation, produces initially the O-6-acylated derivative and, under more stringent conditions, a 5,6-diester. Fhe crystalline 5,6-diacetate is well known. More vigorous conditions still are needed to obtain the 2,3,4,6-tetra-acetate. 

The antioxidant activity of ascorbic acid requires the 2-and 3-hydroxyl groups to be unsubstituted. We observed previously that a-tocopherol behaved differently from its carboxylic acid analog, Trolox, in bulk corn oil triacylglycerols compared to the corresponding oil-in-water emulsions. The same behavior is observed when ascorbic acid is compared to ascorbyl palmitate. Although esterification of the 5- or 6-hydroxyl increases the lipid solubility of ascorbic acid, the resulting esters are less active as antioxidants in bulk oil... 

Moreno-Perez S, FUice M, Guisan JM, Fernandez-Lorente G. Synthesis of ascorbyl oleate by trans-esterification of olive oU with ascorbic acid in polar organic media catalyzed by immobilized Upases. Chem Phys Lipids 2013 174 48-54. 

The monopalmitate of ascorbic acid (109) has been prepared by a method (110) that has been applied only to a few other carbohydrates. The method consists of reacting ascorbic acid and fatty acid in 95% sulfuric acid at room temperature. Since the esterification is reported to take place for primary alcohol groups, the method may have value in the preparation of other pure monoesters. The monoesters of ascorbic acid offer promise as antioxidants for edible fats and oils (111). 

Fig. 15.1 The Reichstein process and microbial fermentation processes for L-ascorbic acid production. Three pathways for vitamin C production are shown The Reichstein process includes six chemical steps (Ca-Cf) and one microbial step (Ml). Ca catalytic hydrogenation, Cb protection of hydroxyl groups with acetone, Cc oxidation, Cd deprotection of acetone, Ce esterification with methanol, Cyiactonization. M2 microbial process includes three chemical steps (Ca, Ce, Cf) and three microbial steps shown as M2 in the figure. M3 microbial process includes one chemical step (Ca) and three microbial steps (shown as M3)...

Ferrous ion-induced Hpid peroxidation of rat liver mitochondria was accelerated by phosphate (Yamamoto et al. 1974). Preincubation of rat liver microsomes with iron (Fe)/ascorbate (50 pM/ 200 pM), known to induce peroxidation, resulted in a significant inhibition of (i) the rate-limiting enzyme in cholesterol biosynthesis, HMG-CoA reductase (46 %, P <0.01, (ii) the crucial enzyme control-Hng the conversion of cholesterol in bile acids, cholesterol 7a-hydroxylase (48%, P <0.001), and (iii) the central enzyme for cholesterol esterification, acyl-CoAxholesterol acyltransferase (ACAT, 80%, P <0.0001) (Brunet etal. 2000). The disturbances of these key enzymes coincided with a high rate of malondialdehyde production (350%, P <0.007) and the loss of polyunsaturated fatty adds (36.19 1.06% vs. 44.24 0.41% in controls, P <0.0008). While a-tocopherol simultaneously neutrahsed lipid peroxidation, preserved microsomal fatty acid status, and restored ACAT activity, it was not effective in preventing Fe/ascorbate-induced inactivation of both HMG-CoA reductase (44%, P <0.01) and cholesterol 7a-hydroxylase (71%, P< 0.0001). 

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