Sorbose solution

Sterile aqueous D-sorbitol solutions are fermented with y cetobacter subo >gichns in the presence of large amounts of air to complete the microbiological oxidation. The L-sorbose is isolated by crystallisation, filtration, and drying. Various methods for the fermentation of D-sorbitol have been reviewed (60). A.cetobacter suboyydans is the organism of choice as it gives L-sorbose in >90% yield (61). Large-scale fermentations can be carried out in either batch or continuous modes. In either case, stefihty is important to prevent contamination, with subsequent loss of product. 

An alternative approach to increase the oxidation rate is the use of alkaline solutions, because bases enhance the reactivity of L-sorbose and weaken the adsorption strength of 2-KLG. Unfortunately, the rate enhancement at higher pH is accompanied by a drop in selectivity due to the poor stability of 2-KLG in alkaline solutions. To circumvent this problem, we have modified the platinum catalysts by adsorbed tertiary amines and carried out the oxidation in neutral aqueous solution [57], This allowed to enhance the rate without increasing the pH of the bulk liquid, which leads to detrimental product decomposition. Small quantities of amines (molar ratio of amine sorbose = 1 1700, and amine Pts = 0.1) are sufficient for modification. Using amines of pKa a 10 for modification, resulted in a considerable rate enhancement (up to a factor of 4.6) with only a moderate loss of selectivity to 2-KLG. The rate enhancement caused by the adsorbed amines is mainly determined by their basicity (pKa). In contrast, the selectivity of the oxidation was found to depend strongly on the structure of the amine. 

Nickel oxide anodes are another example for a relatively simple oxide electrocatalyst used rather widely in the oxidation of organic substances (alcohols, amines, etc.) in alkaline solutions at relatively low anodic potentials (about +0.6 V RHE). These processes, which occur at an oxidized nickel surface, are rather highly selective. As an example, we mention the industrial oxidation of diacetone-L-sorbose to the corresponding acid in vitamin C synthesis. This reaction occurs at nickel oxide electrodes with chemical yields close to 100%. 

The heterogeneously catalyzed Mn02-mediated oxidation of diacetone-L sorbose to diacetone-2keto-L sorbic acid, the latter being a precursor to vitamin C, at nickel anodes and based on the chemical oxidation of the substrate by NiOOH is of technical relevance. The limiting current density in 1 M KOH solution is under operation conditions only 10 A/cm2 leading to relatively poor space-time yields. Robertson and Ibl showed that acceptable space-time yields can by obtained by using thin layer cells of Swiss roll type (193, 194), which leads to an efficient compression of the cell width to fractions of a millimeter. 

The protonated amino group, however, has no nucleophilic activity, and does not form a hemiacetal an amino sugar can, therefore, be obtained, as a salt, in an otherwise unfavorable form. For example, 6-amino-6-deoxy-L-sorbose can be isolated as the hydrochloride of a furanose form (22) (presumably a). In alkaline solution, immediate ring-expansion to the pyranose form (23) occurs127 this reaction can be reversed under strongly acidic conditions. 4-Amino-4,6-dideoxy-D-glucose hydrochloride forms a 35 65 mixture of the a- and / -pyranose forms in solution.128... 

These spectra have detected only one species of L-sorbose in aqueous solution. However, after prolonged storage in methyl sulfoxide 5-10% of a second component is generated. Although it has not as yet been properly identified, there is no counterpart in spectra of the heptulose, and most likely it is the /f-pyranose—i.e., the existence of furanose forms of 1 and 2 should have about equal probability, whereas the / -pyranose form of 2 in a chair conformation should be much less stable than that of 1. 

There are several reasons for reservations about applying the computer extrapolation of crystal structure data for carbohydrates. One is that much of the crystal structure data refer to unsubstituted sugars which are only soluble in hydroxylic or polar solvents where the conformational analysis may be complicated by hydrolysis, isomerism (muta-rotation) (12), or stereospecific solvent interactions which require a more sophisticated model. However, assuming that such chemical changes do not occur or can be suppressed, there still remain questions to be answered before the conformation observed in the crystal can be accepted as a close enough approximation to that of one or more of the rotomers which may predominate in the solution state. (a-L-Sorbose gives an example of the coexistence of two primary alcohol rotameric... 

The reaction of L-sorbose with acetone, a step in the synthesis of L-ascorbic acid (vitamin C), takes place in acetone solution [Eq. (41)] (361). 

An alternative procedure for the protection of L-sorbose (25), followed by oxidation at C-l and cyclization of the product to L-ascorbic acid, was developed by Hinkley and Hoinowski.257 L-Sorbose (25) was converted into methyl a-L-sorbopyranoside (37) by treatment with methanol and hydrogen chloride.258 Glycoside 37 was then oxidized with air in the presence of a suspension of platinum oxide in aqueous sodium hydrogencarbonate solution at 60°, to afford methyl ot-L-xylo-2-hexulopyranosidonic acid (38), which, when heated in hydrochloric acid, was converted into L-ascorbic acid (1), presumably by way of L-xy/o-2-hexulosonic acid (see Scheme 5). Acid 38 has also been prepared by oxidation of 37 with nitrogen tetraoxide.259,280 Yields were not reported for this reaction sequence, and it appears to offer no potential... 

By treatment of D-gulose (XVII) or D-idose (XVIII) with warm aqueous barium hydroxide solution Van Ekenstein and Blanksma were able to isolate D-sorbose (II) with a melting point of 165° and [< ]d + 42.9° in water. The aldoses were prepared from D-gulonic and D-idonic lactones obtained by the cyanohydrin synthesis from D-xylose. 

It is of interest at this point to mention that the optical rotation of L-sorbose has been studied extensively. Lobry de Bruyn and Van Ekenstein21 reported that no mutarotation was observable however, Pigman and Isbell56 later discovered that L-sorbose does possess a small complex mutarotation. For example, at 20°C., L-sorbose (c, 11.3 l, 4) gave an initial observed rotation (°S) of —57.124, which changed to — 57.498 in 2.69 minutes and attained the final value of —57.768 after two hours. Similar results were observed at 0°C. but at a much slower rate. These authors were of the opinion that the smallness of this mutarotation was due to the fact that the equilibrium solution of L-sorbose is... 

Along with their researches on alkaline isomerization, Lobry de Bruyn and Van Ekenstein21 prepared crystalline D,L-sorbose by evaporating an aqueous solution of the d- and L-isomers. The crystals were later shown to be a racemate compound66 from solubility data. 

A photochemical71 synthesis of an optically inactive ketohexose has been reported by the action of sunlight on an aqueous solution of formaldehyde and oxalic acid, for fifteen months, in a sealed tube. The melting point of the phenylosazone (164°), was believed to indicate the formation of D,L-sorbose, but this evidence is of doubtful value at the present time.11... 

In 1895 Dull,9 who was studying inulin and its products of hydrolysis, found that when either fructose or sorbose was treated with an aqueous solution of oxalic acid under pressure, a substance was obtained which had the formula CeHeOa and resembled furfural in its properties. This substance was further investigated by Kiermayer4 who found that fructose and sucrose were the best sources when they were heated with 0.3% aqueous oxalic acid at 120°. It was however only the fructose portion of the sucrose molecule which was transformed since the glucose moiety was recovered unchanged. Kiermayer prepared several derivatives of CeH Os and from its reactions concluded that its structure was probably /3-hydroxy-S-methylfurfural (III). Van Ekenstein and... 

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