Oxidation of L-sorbose

Direct Oxidation of L-Sorbose to L-xylo-2-HexuIosonic Acid ( 2-Keto-L-gulonic Acid )... 

Clearly, an improved synthesis of L-ascorbic acid would result from the direct oxidation of L-sorbose (25) to L-xy/o-2-hexulosonie acid (28), thus eliminating the protecting-deprotecting steps currently required in the Reichstein-Griissner synthesis (see Scheme 4). Efforts to perform this oxidation may be divided into two categories, namely, chemical and fermentative. The results of each method will be summarized. 

It is apparent from the foregoing discussion that, at the present time, the direct chemical or fennentative oxidation of L-sorbose to h-xylo-2-hexulosonic acid is not efficient enough to compete with the Reich-stein-Griissner protection-oxidation method. 

Recently, Sprinson and Chargaff97 have inferred that periodate oxidation of L-sorbose gives rise to two esters of glycolaldehyde, namely the glyoxylic acid ester (XLII) and probably the glycolic acid ester (XLIII). 

In the recent, especially the patent, literature, a number of experiments are described on the direct oxidation of L-sorbose to 2-/cefo-L-gulonic acid, with chlorates,110 nitrous111 and nitric112 acids and by catalytic oxidation with platinum catalyst.113 Cupric acetate114 in methanol oxidizes L-sorbose to the osone, from which L-ascorbic acid is easily prepared. 

Organic bases such as tetraalkylammonium hydroxides, tertiary amines, and phosphines [46,49,52-54] were employed as additives to improve the activity and selectivity of platinum catalysts in the oxidation of L-sorbose to 2-keto-L-gulonic acid (2-KLG). Rate acceleration was attributed to a beneficial effect of the amine on the hydration of the intermediate aldehyde. The selectivity enhancement obtained with hexamethylenetetramine (HMTA) was attributed to a steric effect involving a complex between HMTA and L-sorbose via hydrogen bonding [52] (see Section 9.3.3.2). 

Figure 7. Oxidation of L-sorbose to 2-keto-L-gulonic acid.

In Chapter 3, Lyudmila M. Bronstein, Valentina G. Matveeva and Esther M. Sulman review metal NP catalysis usingpdymers, in particular, work in Bronstein s group concerning the hydrogenation of drain acetylene alcohols and direct oxidation of L-sorbose. These authors stress the importance of and interest in block copolymers such as pdystyrene-hlock-poly-4-vinylpyridine, PS-b-4VP, and even better poly (ethylene oxide)-block-poly-2-vinylpyridine, PEO-h-P4VP (the latter being used in water). The catalytic efficiency is optimal for the smallest NPs and decrease as the NP size decreases. 

To address these issues, we studied the catalytic behavior of Pt nanoparticles formed in several nanostructured polymeric systems in the selective oxidation of L-sorbose to 2-keto-L-gulonic acid (Table 3.5) [89]. Commercial Pt/7-Al203 (3% Pt, Degussa AG) was used for comparison. The reaction has been conducted in alkaU media (NaHCOs), yet gradual alkaline loading (NaHCOs) provides the highest selectivity, while one-shot results in the highest TOP. 

Fig. 3.14 TEM image of HPS-Pt-2 after the induction period during the direct oxidation of L-sorbose in an aqueous medium. Groups of single Pt nanoparticles are highlighted by circles, whereas substantially enlarged nanoparticles are identified by arrows. Reprinted with permission from Ref. [89]. Copyright (2001) American Chemical Society.

The state of the bi- or multimetallic catalyst is important as segregated single metal sites yield reduced selectivity. These may be formed either by single metal agglomerations on the surface or multilayer adsorption of the secondary component on the initial supported metal catalyst. It has been shown that selectivities and activities may be influenced by the presence of organic modifiers such as amines or phosphines.Mallat et al. described a systematic study for the partial oxidation of L-sorbose with molecular oxygen over Pt/C and Pt/AlgOg catalysts, modified with trace... 

A specially prepared palladium on charcoal catalyst permits the oxidation of L-sorbose to L-xyto-hexulosonic acid in 50 % yield and is of potential value in the industrial production of vitamin C. The inhibitory effect of various... 

In a further example of indium-mediated additions to aldoses, a-(bromo-methyl)acrylic acid added to A-acetylmannosamine in the presence of indium to give branched derivative 27 and its C-4 epimer. Ozonolysis of 27 afforded N-acetylneuraminic acid. In a study of the oxidation of L-sorbose (5% Pt/AlaOa, O2) to 2-keto-L-gulonic acid it was found that the reaction rate and selectivity was improved in the presence of certain tertiary amines. The synthesis of a carbocyclic analogue of iV-acetyl-neuraminic acid is discussed in Chapter 18. 

Here we discuss Pt and Ru compound NPs (no reduction) formed in block copolymers and HPS in catalytic oxidation of L-sorbose and D-glucose (Fig. 2). Table 2 shows catalytic properties of these nanocomposites. 

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