NL194919C - Process for oxidizing carbohydrates. - Google Patents

Description

1 194919

Process for oxidizing carbohydrates

The invention relates to a process for oxidizing carbohydrates containing a primary hydroxyl group with a hypohalogenite in the presence of a catalytic amount of a di-tertiary-alkyl-nitroxyl to the corresponding carboxylic acid.

Such a method is known from the article by Davis and Flitsch (Tetrahedron Lett. (1993) 1181-1184). According to that known method, methyl or n-octyl glucosides or fructose acetonide, in other words protected monosaccharides, are oxidized with TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) in a two-phase system consisting of dichloromethane and an aqueous phase with therein sodium hydrogencarbonate, potassium bromide and tetrabutylammonium bromide to which sodium hypochlorite is added. Thereby yields of 55-67% are obtained. The protected monosaccharides are not sensitive to the side reactions of carbohydrates with hypochlorite such as chain breakdown and the solvent system used lends itself poorly to other carbohydrates, in particular polysaccharides.

A method for the oxidation of carbohydrates in which an excess of sodium nitrite is used is known from the work of T.J. Painter et al. (Carbohydrate Res. 55, 95-103 (1977), ibid. 140, 61-68 (1985)).

The oxidation of carbohydrates, such as starch and cellulose, is important because with it the properties of the carbohydrates can be changed in a desired direction. For example, oxidized carbohydrates are useful as thickeners, gelling agents, binders, swelling agents, stabilizers and complexing agents (phosphate substitutes). Most processes for the oxidation of polymeric carbohydrates are to a greater or lesser extent associated with an undesired depolymerization (hydrolysis). Furthermore, such an oxidation is not always specific: for example, starch can be oxidized both at the primary hydroxyl group at the 6-position, which leads to a carboxy starch with an intact carbon skeleton, and at the secondary hydroxyl groups at the 2,3-positions, which leads to a breakdown of the carbon-carbon chain 25 in the glucose units ("dicarboxy starch").

Oxidized carbohydrates with an intact carbon skeleton, i.e., carbohydrates oxidized to the primary hydroxyl function, generally referred to as polyuronic acids, are often advantageous for certain applications, such as complexing agent or stabilizer.

According to the above-described known method according to T.J. Painter et al., Cellulose or amylose is oxidized with sodium nitrite in phosphoric acid. Thereby a product is obtained which, in the case of the oxidation of cellulose, has a content of 87.5% of glucuronic acid and, in the case of oxidation of amylose, a yield of 66 and 86% of a content of glucuronic acid of 67 -75% and 52-56% respectively.

Disadvantages of this known method are the high oxidant consumption and the long reaction time (24 hours or more). In addition, the reaction mixture must meet certain viscosity and foam requirements. Furthermore, a higher effective yield is desired.

A method has now been found which does not have the disadvantages mentioned and has a higher specificity and selectivity (for oxidation of primary alcohol functions) than the known oxidations

To this end, the process mentioned in the preamble is characterized in that the carbohydrate is oxidized in a basic aqueous medium in a single-phase system. So one does not work in a two-phase system as applied by Davis and Flitsch (see above).

Thanks to the method according to the invention, an oxidized carbohydrate can be obtained in a considerably shorter time (a few minutes), when consuming no more than an approximately stoichiometric amount of oxidizing agent, which, with regard to oxidation degree, selectivity of the oxidation and avoidance of depolymerization, is superior to the products of the known processes.

The di-tertiary-alkyl-nitroxyl may, for example, be di-tert-butyl-nitroxyl, but is in particular a cyclic compound which satisfies the following formula: O * 55 In this formula, A represents a chain of preferably two or three atoms, in particular carbon atoms (methylene groups) or a combination of one or two carbon atoms with an oxygen or nitrogen atom, which may optionally be substituted with one or more groups, such as an alkyl group, an alkoxy group or an oxo group. Most preferably, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is used in the process of the invention. The di-tert-alkyl-nitroxyl can optionally be prepared in the reaction medium, for example by oxidizing the corresponding ditert-alkyl-amine with hydrogen peroxide and tungstate.

The oxidation of alcohols with hypochlorite in the presence of nitroxyl compounds such as TEMPO is known per se. However, according to the known methods, only simple alcohols are oxidized and a two-phase system is used (see, for example, P. L. Anelli et al., J. Org. Chem. 54, 2970-2972 (1989)).

A catalytic amount of nitroxyl is understood to mean an amount which - after conversion of the nitroxyl radical to the corresponding nitrosonium ion - is less than an amount required for oxidation of all primary hydroxyl groups to carboxyl groups, in particular less than 10% of the amount required for that complete oxidation required amount, according to the following gross reaction equation:

SacCH 2 OH + R 2 N + O + OH "-> SacCHO + R 2 NOH + H 2 O

Here, Sac represents the remainder of a monosaccharide unit and R represents a tertiary alkyl group, wherein the two groups R can be connected to each other. The oxidation with nitroxyl to aldehyde is followed by oxidation of the aldehyde to carboxylic acid with another oxidizing agent such as hypohalogenite (OX '; X - Cl, Br or J), according to the equation:

SacCHO + OX '+ OH' - »SacCOO '+ X + H2 O

A larger amount of nitroxyl compound than 10% by weight is not harmful, but is not attractive because of the higher costs. The catalytic amount of nitroxyl is preferably 0.005-5% by weight, more preferably 0.1-2.5% by weight, and in particular 0.25-1.5% by weight, relative to the carbohydrate. Expressed in mole%, the catalytic amount of nitroxyl relative to the carbohydrate monomer is preferably 0.1-2.5 mole%.

The oxidation can be carried out in water with a hypohalogenite as oxidizing agent, preferably in the form of a salt thereof, such as lithium, sodium, potassium or calcium hypochlorite. Of the oxidizing agent, preferably 0.8-2 equivalents are used, more preferably 0.9-1.5 equivalents and in particular 1-1.2 equivalents. In molar ratio, the amounts of oxidizing agent to be used are 1.6-4.1.8-3 and 2-2.4 mol per mol of monosaccharide unit, respectively.

It is assumed that the hypohalogenite oxidizes the hydroxylamine formed during the oxidation of the hydroxyl group of the polysaccharide back to the corresponding nitroxyl radical or the corresponding nitrosonium ion R2 N + = 0, which can then again oxidize a hydroxyl group, as shown in the reaction equations below, in which the hypohalogenite is represented as OX '.

R2N + 0 + R2NOH + OH '- »2 R2NO + H2O 2 RqNQ · + OX' + H, Q -» 2 RPN + Q + X '+ 2 OH' + 35 R2NOH + OX '-> R2N + 0 + X - + OH '(gross)

As the hypohalogenite, hypochlorite, for example sodium hypochlorite, can be used. Advantageously, use can also be made of hypobromite, which is then preferably obtained in the reaction medium from hypochlorite and bromide. In addition to an equivalent amount or a slight excess of hypochlorite, a deficiency of bromide, or even a catalytic amount of bromide, is sufficient. Preferably 40 0.2-1 equivalent (0.4-2 mol / mol monosaccharide unit) of bromide is used.

The method according to the invention is useful for oxidizing carbohydrates of very different nature and origin (vegetable, animal, microbial, synthetic). Both monomeric carbohydrates (monosaccharides, sugar alcohols) and dimeric, oligomeric and polymeric carbohydrates can be oxidized if they have a primary hydroxyl group. Examples of polymeric carbohydrates 45 are β-glucans, in particular cellulose and fractions, derivatives and hydrolysis products thereof, α-glucans, in particular starch and fractions, derivatives and hydrolysis products - such as amylose and amylodextrin - and cyclic equivalents thereof such as cyclodextrin , further other polysaccharides, such as inulin, and natural or artificial gums, such as xanthan, guar, johannis bread flour, algine, gum arabic, dragant, carrageenan, agar, ghatti, chitin, etc. In particular, the method is suitable for the oxidation of oligosaccharides and polysaccharides, such as starch, cellulose or inulin, or a fraction, a hydrolysis product or a derivative thereof.

The process according to the invention can be used for the preparation of fully carboxylated carbohydrates, in other words of polyuronic acids. However, the process can also be advantageously used to prepare partially carboxylated carbohydrates in which only a 55 part of the primary hydroxyl groups of the carbohydrate have been oxidized. Carbohydrates with a carboxyl content of at least 75%, in particular of at least 90%, are preferably prepared. The invention also relates to polyuronic acids with a uronic acid content of at least 90%.

3, 194919

The process according to the invention is carried out in a basic reaction medium. In particular, a pH of 9-13 is used, preferably a pH of 10.3-11.5.

The reaction temperature can vary from about -5 ° C to about 30 ° C. Preferably, the reaction is conducted at temperatures of 10 ° C or less and more preferably at about 0-5 ° C.

After the reaction, the mixture can be worked up and the oxidized carbohydrate can be isolated by adding a solvent in which the inorganic substances dissolve and the product does not dissolve, for example an alcohol. Further purification can be carried out in a manner known per se. The yields of uronic acid are generally above 90%. The di-tertiary alkyl nitroxyl used as a catalyst can be recovered from the reaction mixture, for example by extraction with an ether.

10

Examples

General

Potato starch (water-soluble, 21.5% amylose, 10% water) and B-cyclodextrin (13.5% water) were from Avebe. Amylodextrin (degree of polymerization 25) was obtained from waxy corn starch with pullulanase (see Dutch patent 165500). Microcrystalline cellulose (Avicel) was from Merck. 2,2,6,6-Tetramethylpiperidine-1-oxyl was of analytical grade (Sigma). Polygalacturonic acid (98%) was from Sigma; D (2] o (99.9%) from Isotec Inc. and ethanol (96%) from Gist-Brocades.

NMR spectra were recorded with a VARIAN UNITY-400 spectrometer (1 H resonance frequency 400 MHz, 13 C resonance frequency 101 MHz). 13 C NMR spectra were recorded "gated decoupled", making a quantitative determination possible. All samples were dissolved in D 2 O. HPLC analysis was performed with a 7.5 x 600 mm Ultrapac TSK G5000PW column coupled to an R1 detector (Spectra Physics, SP 8430). The eluent used was phosphate buffer (0.1 M NaH 2 PO 4. 2H 2 O with 1 M NaOH at pH 7). The absorbance at 520 nm was measured with a Perkin-Elmer Lambda 5 UV / VIS spectrophotometer. Centrifuge was performed in a Sorvall RC-5B device.

The reaction mixtures were worked up by pouring into 96% (70% of the volume of the reaction mixture), causing the product to precipitate. The white precipitate was centrifuged, taken up in ethanol / water (70/30 v / v), centrifuged again, taken up in 96% ethanol and centrifuged again. The resulting product was dried under reduced pressure.

The glucuronic acid content was determined using the uronic acid determination of Blumenkrantz and Abdoe-Hansen, Anal. Blochem. 54, 484 (1973). A calibration line was made from polygalacturonic acid (5, 10, 15, 20 pg). Samples of 20 µg of the reaction product were always measured and the percentage of glucuronic acid was determined on the basis of the calibration curve. The carboxyl content of the product was determined by titration of 0.2 g of product with a 0.10 M NaOH solution, or by adding an excess of 0.1 M calcium acetate solution and back titration of the released acetic acid with 0.10 M NaOH. The relative molecular weight was determined by HPLC. The products were further characterized by NMR.

EXAMPLE 1

Water-soluble potato starch (dry weight 2 g, 12.3 mmol anhydroglucose unit (AGU)) was dissolved in water (200 ml). TEMPO (1 wt% relative to the polysaccharide (0.02 g, 0.13 mmol)) was then added and dissolved in about 20 minutes. Then 1.5 g (14.6 mmol) of sodium bromide was added and the solution was brought to 0 ° C. A solution of hypochlorite (45 ml, 4% solution, 25.2 mmol) was adjusted to pH 10.8 with 3M HCl and cooled to 0 ° C. The solution was added to the polysaccharide and TEMPO solution in one go. The course of the reaction 45 was monitored on the basis of the lye consumption, which is equivalent to the formation of uronic acid (see figure 1). During the reaction, the temperature rose to a maximum of 5 ° C. After a reaction time of 30 minutes, the mixture was worked up and analyzed as indicated above. The results are shown in Table 1 below. Of the product oxidized at pH 10.8, a 13 C NMR spectrum (101 MHz) was included; the absorbance at 177 ppm is characteristic of the oxidized primary alcohol function at C-6. 50 (see figure 3).

Claims (5)

194919 4 TABLE 1 Yield and uronic acid content of processed products 5 pH yield% consumption ml uronic acid COOH (%) ° 0.5M NaOH® (%) b 9.3 99 24.3 86 9.8 90 25.6 87 10 10.3 97 23, 3 94 10.8 96 24.1 92 92 11.3 89 23.5 95 - Total number of ml of 0.5 M NaOH added for oxidation was stopped. b Determined according to uronic acid assay. Polygalacturonic acid was reference. 15. Calculated with the molecular weight of polyglucionic acid (so M monomer = 176). Example II Example I was repeated, with the difference that the pH was varied. The influence of the pH on the course of the reaction is shown in Figure 2. The results are shown in Table 1. HPLC analysis showed that a significant degradation of the polymer took place at a pH above 11.5. Example III Example I was repeated, with the difference that instead of 0.02 g TEMPO, 0, 0.002, 0.005 and 0.01 g TEMPO, respectively, were used. The course of the reaction is shown in Figure 4. Example IV Example I was repeated, with the difference that instead of 1.5 g of sodium bromide 0, 0.02 and 0.5 g of sodium bromide were used respectively. The course of the reaction is shown in Figure 5. Example V Amylose (dry weight 3 g, 18.5 mmol) was suspended in 200 ml of water. To the suspension were added 0.03 g of TEMPO (0.19 mmol) and 1.5 g of sodium bromide (14.6 mmol). The suspension was brought to 0 ° C and 65 ml of 4% hypochlorite with a pH of 10.6 was added at 0 ° C. The pH was adjusted with a pH stat. kept at 10.6 during the reaction by adding 0.5 M NaOH. The reaction was terminated after 3 hours at 0 ° C by adding an excess of ethanol. 30.5 ml of 0.5 M NaOH was then added. The mixture was worked up as described above. Yield: 92%. Uronic acid content: 75%. Example VI Wheat starch (dry weight 3 g, 18.5 mmol) was suspended in 200 ml of water and oxidized in the manner according to Example V. The reaction was terminated after 2 hours at 0 ° C by adding ethanol. 32 ml of 0.5 M NaOH was then added. After working up in the manner described above, a yield of 94% and a uronic acid content of 81% were found. 45 Conclusions A process for oxidizing carbohydrates containing a primary hydroxyl group with a hypohalogenite in the presence of a catalytic amount of a di-tertiary-alkyl-nitroxyl to the corresponding carboxylic acid, characterized in that the carbohydrate is in a basic aqueous medium oxidizes a single-phase system in 50. Method according to claim 1, characterized in that the carbohydrate is an oligosaccharide or polysaccharide. Method according to claim 2, characterized in that the carbohydrate is starch, cellulose or inulin, or a fraction, a hydrolysis product or a derivative thereof. 5, 194919 Method according to claim 2 or 3, characterized in that a pH of 9-13 is used. Polyuronic acid obtainable by the method according to claim 3, which has a uronic acid content of at least 90%. Hereby 5 sheets of drawing