Sodium dithionite oxidation

Carbomethoxyamino-2,l,4-benzothiadiazines, such as 192, can be made by the action of methyl isothiocyanatomethanoate on 2-nitroaniline, followed by reductive ring-closure with sodium dithionite. Oxidation with 3-chloroperbenzoic acid gives the corresponding 2-oxide 193 (71-... 

In the further scans, this first wave is not observed because in the aforementioned potential region, the electrode surface is no longer a pure platinum one but is a rearranged platinum hydroxide surface39. The results described in section 6.2 showed that the limiting-current plateau of the second oxidation wave (first scan) is controlled by transport of dithionite. This indicates that electron transfer from dithionite to PtOH and/or PtO is a much faster process than transport of dithionite towards the electrode. This is confirmed by the fact that in the further scans an identical limiting-current is obtained. The third oxidation wave in the first scan (second wave for the other scans) is attributed to the oxidation of sulphite described earlier. It is formed as a reaction product of the sodium dithionite oxidation and also of the homogeneous decomposition of sodium dithionite. Also in this case, a hysteresis effect is observed for the sulphite forward/back-ward sweep oxidation wave. 

In the preceding sections, it was mentioned several times that the limiting-current of the first wave of the sodium dithionite oxidation is suitable for electroanalytical purposes because of the transport-controlled nature of this limiting-current. Indeed, it was proven earlier that this limiting-current can be correlated with the Levich equation (1.15), showing a linear relationship between limiting-current and sodium dithionite concentration. 

Calibration plot between peak current of the sodium dithionite oxidation reaction and its concentration at a bare gold electrode (curve 1) and at a CoTSPc-modified (curve 2) and CoTSPor-modified (curve 3) gold electrode. T=298.0K. 

Another method employed is the treatment of aqueous solutions of aminophenols with activated carbon (81,82). During this procedure, sodium sulfite, sodium dithionite, or disodium ethylenediaminotetraacetate (82) is added to increase the quaUty and stabiUty of the products and to chelate heavy-metal ions that would catalyze oxidation. Addition of sodium dithionite, hydrazine (82), or sodium hydrosulfite (83) also is recommended during precipitation or crystallization of aminophenols. 

Sodium dithionite is considered only moderately toxic. The solution is reported to have an LD q (rat, oral) of about 5 g/kg. As with sulfites, fairly large doses of sodium dithionite can probably be tolerated because oxidation to sulfate occurs. However, irritation of the stomach by the Hberated sulfurous acid is expected. As a food additive, sodium dithionite is generally recognized as safe (GRAS) (367). 

Traditionally, these dyes are appHed from a dyebath containing sodium sulfide. However, development in dyeing techniques and manufacture has led to the use of sodium sulfhydrate, sodium polysulfide, sodium dithionite, thiourea dioxide, and glucose as reducing agents. In the reduced state, the dyes have affinity for cellulose (qv) and are subsequendy exhausted on the substrate with common salt or sodium sulfate and fixed by oxidation. 

Later, a completely different and more convenient synthesis of riboflavin and analogues was developed (34). It consists of the nitrosative cyclization of 6-(A/-D-ribityl-3,4-xyhdino)uracil (18), obtained from the condensation of A/-D-ribityl-3,4-xyhdine (11) and 6-chlorouracil (19), with excess sodium nitrite in acetic acid, or the cyclization of (18) with potassium nitrate in acetic in the presence of sulfuric acid, to give riboflavin-5-oxide (20) in high yield. Reduction with sodium dithionite gives (1). In another synthesis, 5-nitro-6-(A/-D-ribityl-3,4-xyhdino) uracil (21), prepared in situ from the condensation of 6-chloro-5-nitrouracil (22) with A/-D-ribityl-3,4-xyhdine (11), was hydrogenated over palladium on charcoal in acetic acid. The filtrate included 5-amino-6-(A/-D-ribityl-3,4-xyhdino)uracil (23) and was maintained at room temperature to precipitate (1) by autoxidation (35). These two pathways are suitable for the preparation of riboflavin analogues possessing several substituents (Fig. 4). 

Most vat dyes are based on the quinone stmcture and are solubilized by reduction with alkaline reducing agents such as sodium dithionite. Conversion back to the insoluble pigment is achieved by oxidation. The dyes are appHed by either exhaust or continuous dyeing techniques. In both cases the process is comprised of five stages preparation of the dispersion, reduction, dye exhaustion, oxidation, and soaping. 

Another approach uses the reaction of 6-chloro-5-nitropyrimidines with a-phenyl-substituted amidines followed by base-catalyzed cyclization to pteridine 5-oxides, which can be reduced further by sodium dithionite to the heteroaromatic analogues (equation 97) (79JOC1700). Acylation of 6-amino-5-nitropyrimidines with cyanoacetyl chloride yields 6-(2-cyanoacetamino)-5-nitropyrimidines (276), which can be cyclized by base to 5-hydroxypteridine-6,7-diones (27S) or 6-cyano-7-oxo-7,8-dihydropteridine 5-oxides (277), precursors of pteridine-6,7-diones (278 equation 98) (75CC819). 

Fig. 3. Cation-exchange chromatography of protein standards. Column poly(aspartic acid) Vydac (10 pm), 20 x 0.46 cm. Sample 25 pi containing 12.5 pg of ovalbumin and 25 pg each of the other proteins in the weak buffer. Flow rate 1 ml/min. Weak buffer 0.05 mol/1 potassium phosphate, pH 6.0. Strong buffer same +0.6 mol/1 sodium chloride Elution 80-min linear gradient, 0-100% strong buffer. Peaks a = ovalbumin, b = bacitracin, c = myoglobin, d = chymotrypsinogen A, e = cytochrom C (reduced), / = ribonuclease A, g = cytochrome C (oxidised), h = lysozyme. The cytochrome C peaks were identified by oxidation with potassium ferricyanide and reduction with sodium dithionite [47]...

By contrast, alkylamination of naphthazarin (7) in the presence of sodium dithionite followed by oxidation gives l,4-bis(alkylamino)-5,8-naphthoquinone (31).18,19 However, Kikuchi and co-workers20 obtained isomeric l,5-bis(alkylamino)-4,8-naphthoquinone (32) from the reaction of leuco naphthazarin (33) with alkylamine They also isolated 5-alkylamino-leuco-naphthazarin (34) as an intermediate, which is further aminated at the 1-position to give 32. Bloom and Dudek21 have studied the structure of leuco aminonaphthoquinones and their tautomeric equilibria in solution. They concluded that the reaction of leuco naphthazarin (33) or the leuco compound (35) derived from l,5-diamino-4,8-naphthoquinone (36) with methylamine gives mixtures of l,4-bis(methylamino)-31 (R = Me) and 1,5-bis(methylamino)naphthoquinones 32 (R = Me) after oxidation of leuco aminonaphthoquinones (Scheme 10). Some of the structures of leuco aminonaphthoquinones are shown in Scheme ll.20... 

The technetium(III) complexes are synthesized by the direct reduction of pertechnetate with an appropriate reductant in the presence of the desired ligand. However, when sodium dithionite is used as a reductant, the oxidation state of the synthesized complex varies from III to V, depending significantly on the nature of the coexisting ligand. 

For satisfactory whiteness on wool, it is essential for the fibre to be well scoured and bleached, either oxidatively with hydrogen peroxide or by reduction using stabilised sodium dithionite. Brightener is usually applied together with the dithionite bleach. To achieve the highest possible whiteness, the wool should first be scoured to remove natural waxes and other contaminants, then bleached with peroxide and finally treated with FBA during a second bleach with dithionite. 

The stripping of cellulosic materials dyed with reactive dyes is carried out by alkaline reduction followed by hypochlorite oxidation, preceded by a boiling treatment with EDTA if metal-containing dyes have been used. For example, a treatment with 5 gA sodium carbonate or sodium hydroxide and 5g/l sodium dithionite at the boil is followed by a treatment in 0.5-1 °Tw hypochlorite, an antichlor and thorough rinsing. 

By far the most important reducing system for the batchwise application of vat dyes is sodium dithionite (Na2S204) in a solution of sodium hydroxide. Obviously the theoretical concentrations required will depend on the number of keto groups in the dye molecule and on its relative molecular mass and concentration, but the reaction can be represented as in Scheme 12.20 for an anthraquinonoid dye with two keto groups. The effect of air oxidation on alkaline... 

The preparation of 3,5-bis(/3-D-glycopyranosyl)-l,2,4-thiadiazoles has been accomplished via the oxidation of the corresponding acylated C-(/3-D-glycopyranosyl)thioformamides with potassium and sodium dithionite. The synthesis is completed by a Zemplen deacylation. This is an interesting extension to a type A synthesis which has previously only been suitable for arylthioamides <2001T5429>. 

The key intermediate 124 was prepared starting with tryptophyl bromide alkylation of 3-acetylpyridine, to give 128 in 95% yield (Fig. 37) [87]. Reduction of 128 with sodium dithionite under buffered (sodium bicarbonate) conditions lead to dihydropyridine 129, which could be cyclized to 130 upon treatment with methanolic HC1. Alternatively, 128 could be converted directly to 130 by sodium dithionite if the sodium bicarbonate was omitted. Oxidation with palladium on carbon produced pyridinium salt 131, which could then be reduced to 124 (as a mixture of isomers) upon reaction with sodium boro-hydride. Alternatively, direct reduction of 128 with sodium borohydride gave a mixture of compounds, from which cyclized derivative 132 could be isolated in 30% yield after column chromatography [88]. Reduction of 132 with lithium tri-f-butoxyaluminum hydride then gave 124 (once again as a mixture of isomers) in 90% yield. 

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