Sorbose aqueous solution

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. 

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. 

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... 

Fig. 1A-F. The 75.5 MHz H-decoupled NMR spectra in aqueous solution at 40°C of the starting isotopically substituted compounds (A, B) and the synthetic products (C-F) obtained from isomerization reactions catalyzed with molybdic acid. D-(2- C)Hamamelose (C) and D-(2- C)sorbose (E) obtained from D-(2- C)fructose (A) D-(l- C)hamamelose (D) and D-(3- C)sorbose (F) obtained from D-(3- C)fructose (B) (Reprinted from Carbohydrate Research, Vol. 319, Zuzana Hricoviniova-Bflikovd, Milos Hricovini, Maria Petrusovd, Anthony S. Serianni, Ladislav Petrus, Stereospecific molybdic acid-catalyzed isomerization of 2-hexul-oses to branched-chain aldoses. Pages 38-46, Copyright 1999, with permission from Elsevier Science.)...

In this context, it appears to be of relevance to briefly mention some comparable reaction systems for ketose epimerization. Petrus and his group, as well as Petrus, Serianni and co-workers, recently reported the stereospecific molybdic acid catalyzed isomerization of 2-hexuloses to branched-chain aldoses [55-57]. Upon treatment with a catalytic amount of molybdic acid in aqueous solution, the 2-ketohexoses, o-fructose, L-sorbose and o-tagatose, underwent a stereospecific intramolecular rearrangement to give the corresponding 2-C-hydroxyme-thylaldoses, 2-C-hydroxymethyl-D-ribose (o-hamamelose), 2-C-hydroxymethyl-L-lyxose and 2-C-hydroxymethyl-o-xylose, respectively (see Petrus, Petrusova and Hricovfniova, this vol.). 

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 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. 

The Pt-containing (2-4% Pt) nanocomposite beads placed in an aqueous basic solution of L-sorbose catalyze oxidation of the latter with O2 to 2-keto-L-gulonic acid, an intermediate in the production of vitamin C [399]. Within first 100 min at 60—80°C the catalytic activity gradually develops, resulting in a 100% convenion of L-sorbose with the yield of ketogulonic acid up to 98% [400]. With the size of Pt clusters controlled by the size of matrix network meshes, the catalytic properties of the composite material remain stable even after 30—50 reaction cycles... 

---