Oxidation-reduction method

Finally in this section on deracemization via cyclic oxidation/reduction methods, there has been some limited work carried out on the deracemization of secondary alcohols. Soda et al. [22] employed lactate oxidase in combination with sodium borohydride to deracemize D/i-lactate (18) via the intermediate pyruvate (19) (Figure 5.12). 

In oxidation-reduction methods bromine is employed as an oxidizing agent in place of iodine, because it is reduced quantitatively be the readily oxidized pharmaceutical organic substances in a reaction which results in either water-insoluble bromine substitution products, for instance ... 

The oxidation-reduction methods with potassium iodate invariably based on the formation of iodine monochloride (ICl) in a medium of strong hydrochloric acid solution. 

In the field, a quick test for the general level of total phenolics in a plant can be the starting point for a more detailed quantitative analysis of the nature and amounts of specific compounds. Various oxidation-reduction methods have been exploited to analyze total phenolics in plant extracts. Many, but not all, phenols form complexes with ferric chloride (FeCy. Ferric chloride gives a color reaction with phenolic compounds. While the phenolate ion is oxidized, the ferric ions are reduced to the ferrous state. 

For acetal formation, besides the methods discussed thus far, further possibilities exist. For instance, oxidation/reduction methods could be envisaged. Thus, as described,the acceptor can be oxidized to the acid. Anomeric 0-acylation is a convenient process (see Section 1.2.4), requiring in the final step reduction of the acyloxy group to the alkoxy group, which could be performed in simple cases with borane. Similarly the donor could oxidized to the lactone stage (and derivatives) and then acceptor addition and reduction would lead to the desired product. However, the efficiency of such methods has to be evaluated on the basis of the existing methodology. 

Chemical destruction by oxidation-reduction methods in aqueous media is well documented [32]. Destruction of small quantities of lead azide with ceric ammonium nitrate is fast but expensive. The electrolytic oxidation of cerous ammonium nitrate can be done without sophisticated apparatus, and it may be pos-... 

The oxidation-reduction method, developed initially by Mukaiyama et al. [133] and related to the previously described organophosphorus methods, has permitted a variety of important solid-phase applications. The mechanism of the activation is complex and involves the oxidation of the triaryl/ alkyl-phosphine to the oxide as well as reduction of the disulfide to the mercapto derivative. However, different active species, such as 81 (Fig. 11), the 2-pyridyl thioester, or even the symmetrical anhydride, have been postulated to form. For the intermediate 81, the peptide bond formation may proceed through a (cyclic transition state. The method has been used for conventional stepwise synthesis [134], acylation of the first protected amino acid to a hydroxymethyl resin, and to achieve segment condensation on a solid support in the opposite direction (N C) [135,136]. Lastly, it has been used for efficient grafting of a polyethylene glycol (molecular weight 2000) derivative to an aminomethyl resin to prepare PEG-PS resins [137]. 

General Procedures for Coupling Reaction Using the Oxidation-Reduction Method [137]... 

An equally interesting and perhaps more practical approach to coupling is the oxidation-reduction method (Mukaiyama et al. 1968)... 

Syntheses of cyclopentadienyl metal compounds by reactions which do not involve the formation of cyclopentadienyl-metal bonds are not within the scope of this review. However, certain cyclopentadienyl metal compounds are difficult to make by the general methods outlined above, and are best made by converting easily prepared compounds without affecting the cyclopentadienyl-metal bonding. The most important reactions of this sort involve changes in the oxidation state of the central metal atom. Oxidation-reduction methods are therefore discussed in detail. In addition, several other general methods for transforming one cyclopentadienyl metal compound to another are listed with little discussion in order to complete the tabular survey of compounds and methods. 

In order to avoid the two-step oxidation-reduction method for conversion of a mixture of 2- and 3-cephem isomers to a 3-cephem, Pfaendler et al. (1976) discovered that a mixture of isomeric benzhydryl ethers... 

Oxidation-reduction methods in which 2, -dichloruphenolindophenol is used. 

Of the various oxidation-reduction methods those based upon the reduction of indophenol have been found to be the most satisfactory. 2,6-Dichlorophenolindophenol is a dye that is blue in alkali and pink in acid. It is reduced by ascorbic acid to the colorless leuco form, as shown by following reaction (1). 

The desired conditions for the reaction are produced by placing sufficient sodium acetate in the indhenol solution to buffer the mixture HPO] and dye to a pH of approximately 3.6. This type of procedure, where applicable, yields the highest precMon and the most specific values of any of the oxidation-reduction methods. 

Lugg (13) discovered that in solutions of pH 1.5 formaldehyde condenses only very slowly vrith ascorbic acid but combines rapidly with cysteine, sulfides, and sulfites. Since condensation of these compounds destroys their reducing property, a principle was established by which ascorbic acid can be determined by an oxidation-reduction method in the presence of the interfering sulfur compounds. 

The oxide reduction method is feasible for Ac, Am, Cm, Bk, Cf, and Es, and is generally the favored process for producing thin-fllm or globular multi-milligram and multigram samples. On the other hand, when only multi-microgram quantities are to be recovered and favorable vapor pressures obtain (Bk and Cf), the Li reduction-vaporization method produces a more attractive bulk sample form. 

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