The present invention relates to a process for preparing conveniently an ascorbic acid-2-monophosphate salt, particularly an L-ascorbic acid-2-monophosphate (hereinafter sometimes abbreviated to xe2x80x9cAsMPxe2x80x9d) salt in a high yield, by hydrolyzing an ascorbic acid-2-polyphosphate or a salt thereof, particularly an L-ascorbic acid-2-polyphosphate (hereinafter sometimes abbreviated to xe2x80x9cAsPPxe2x80x9d) or a salt thereof, as the starting material in the presence of magnesium ion.
L-Ascorbic acid (vitamin C) is known to have various physiological and pharmacological actions, and particularly by virtue of its effect of preventing melamine pigmentation, L-ascorbic acid has been used in whitening cosmetics. However, L-ascorbic acid is unstable to oxygen or heat, therefore, L-ascorbic acid derivatives stable against oxygen or heat have been heretofore formed by converting the hydroxyl group at the 2-position into a phosphate.
A salt, particularly a magnesium salt of L-ascorbic acid-2-monophosphate (hereinafter the xe2x80x9cmagnesium L-ascorbic acid-2-monophosphatexe2x80x9d is sometimes abbreviated to xe2x80x9cAPMxe2x80x9d) is being used as a stabilized vitamin C derivative. APM exhibits excellent stability in cosmetic materials and scarcely decomposes. Furthermore, APM is easily absorbed through the skin and by the action of phosphatase present inside the human body, L-ascorbic acid is liberated to bring about various physiological actions such as prevention of melamine pigmentation.
High-purity AsMP salts are useful as a stabilized derivative of L-ascorbic acid and can be used in cosmetics, medical products, food additives, feed and other various industrial fields.
For monophosphorylating L-ascorbic acid, three methods described below are known. A first method comprises monophosphorylating L-ascorbic acid using phosphorus oxychloride as the phosphorylating agent and this method is described, for example, in JP-B-45-30328 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d), JP-B-52-18191, JP-B-59-4438 and JP-A-2-27969 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and the like. The second method comprises transposing the phosphoric acid group of a phosphoric acid donor (e.g., adenosine triphosphate, pyrophosphoric acid and the like) to L-ascorbic acid using an enzyme and this method is described, for example, in JP-A-2-283283. The third method comprises reacting a soluble salt of metaphosphoric acid used as a phosphorylating agent with L-ascorbic acid and drying the aqueous solution to obtain AsMP salt and this method is described, for example, in JP-A-5-155893.
The first method above is most frequently used and according to Carbohydrate. Res., 67, 127-138 (1978), L-ascorbic acid-2-monophosphate is obtained as a main product in the form of tricyclohexylamine salt in a yield of 86%. The by-products are L-ascorbic acid-3-monophosphate, L-ascorbic acid-2-diphosphate (hereinafter sometimes abbreviated to xe2x80x9cAsDPxe2x80x9d) and 2,2xe2x80x2-bis(L-ascorbic acid)phosphate. For the purification thereof, a complicated step such as ion exchange chromatography is necessary. L-ascorbic acid has four hydroxyl groups active in the reaction with phosphorus oxychloride and phosphorus oxychloride has three active sites, therefore, many by-products are produced. Furthermore, chloride ion generated from phosphorus oxychloride after the reaction is 3 molar times the phosphorus oxychloride used and this requires purification by electrodialysis and the like.
The second method is advantageous from the viewpoint that generation of by-products is prevented but still has the defect that the productivity is low. According to JP-A-2-283283, AsMP is produced using an enzyme fixed to an ion exchange resin and using diphosphoric acid (pyrophosphoric acid) as a phosphoric acid donor. However, the yield of AsMP of 34% is very low. Also, the amount of AsMP produced is as low as 1.7% in terms of the concentration in the reaction solution and large scale reaction equipment is necessary. Furthermore, in order to obtain high-purity AsMP salt, purification through many steps such as removal of a large amount of water and recovery of unreacted L-ascorbic acid is required.
The third method is a method of drying L-ascorbic acid-2-triphosphate (hereinafter sometimes abbreviated to xe2x80x9cAsTPxe2x80x9d) in an aqueous solution and thereby stepwise hydrolyzing the phosphate bonded to produce AsMP. According to JP-A-5-155893, AsMP is produced by preparing an aqueous solution of AsTP from a soluble salt of metaphosphoric acid and L-ascorbic acid, adjusting the pH thereof to about 5.5 to 6.5 and performing drying at a temperature of from 120 to 180xc2x0 C. As known from the pH of aqueous solution, this stepwise hydrolysis reaction of phosphate is acid hydrolysis reaction. Therefore, although it is set forth therein that sodium and calcium are advantageous as the cation present in the aqueous solution, the alkali property of the cation has no relation to the acid hydrolysis, since the pH of the aqueous solution is acidic.
Furthermore, the product AsMP obtained by this production method has extremely low purity and the yield is also low. As seen from the composition of the product obtained by this method described in the Examples, the AsMP purity is 61% and the yield based on AsTP is 66%. In addition, much L-ascorbic acid, AsDP and AsTP are present as by-products. The presence of L-ascorbic acid shows that the acid hydrolysis reaction of AsTP extended to L-ascorbic acid over AsMP.
This means that it is useless to phosphorylate the hydroxyl group at the 2-position of L-ascorbic acid so as to improve lack of stability of L-ascorbic acid to oxygen or heat. Furthermore, the L-ascorbic acid present in the product decomposes due to oxygen or heat causing coloration or produce oxalic acid which is a skin irritant. The product cannot be used in cosmetics and the like as it is. Therefore, the impurities must be purified by a complicated method such as ion exchange chromatography. For these reasons, this production method of AsMP by the drying of AsTP aqueous solution is not an industrial attractive.
As described above, AsMP salt is useful as a stabilized derivative of L-ascorbic acid, nevertheless, the production methods thereof are complicated and disadvantageous in industry. Under these circumstances, a simplified preparation process is demanded therefor.
As a result of extensive studies to eliminate the above-described defects, the present inventors have found that high-purity AsMP salt can be conveniently prepared in a high yield by hydrolyzing, preferably under heating AsPP, or a salt thereof at a pH of 7 or more as a starting material in the presence of magnesium ion. The present invention has been accomplished based on this finding of a new process.
The present invention provides the following embodiments for preparing an ascorbic acid-2-monophosphate salt:
(1) a process for preparing an ascorbic acid-2-monophosphate salt, comprising hydrolyzing an ascorbic acid-2-polyphosphate or a salt thereof in the presence of magnesium ion at a pH of 7 or more;
(2) the process as described in (1) above, comprising hydrolyzing a reaction product of an ascorbic acid and a polyphosphoric acid or a salt thereof in the presence of magnesium ion at a pH of 7 or more;
(3) the process as described in (1) above, comprising eating a mixture comprising:
(A) an ascorbic acid-2-polyphosphate or a salt thereof,
(B) water,
(C) a magnesium compound capable of supplying magnesium ion in water, and
(D) a base necessary for maintaining a pH of 7 or more;
(4) the process as described in (3) above, wherein the
ascorbic acid-2-polyphosphate or a salt thereof (A) is one or more selected from the group consisting of an L-ascorbic acid-2-triphosphate and a salt thereof, an L-ascorbic acid-2-diphosphate and a salt thereof, and a mixture thereof;
(5) the process as described in (3) above, wherein the magnesium compound (C) is one or more selected from the group consisting of magnesium chloride, magnesium sulfate, magnesium acetate, magnesium nitrate, magnesium oxide, magnesium hydroxide, magnesium carbonate and a mixture thereof;
(6) the process as described in (3) above, wherein the base (D) is one or more selected from the group consisting of an alkali metal hydroxide, an alkali metal carbonate, an alkaline earth metal hydroxide, an alkaline earth metal oxide, an alkaline earth metal carbonate, a tertiary amine and a mixture thereof;
(7) the process as described in (3) above, wherein the compound serving both as the magnesium component (C) and the base (D) is one or more selected from the group consisting of magnesium hydroxide, magnesium oxide and magnesium carbonate;
(8) the process as described in (3) above, wherein the heating temperature is from 35xc2x0 C. to the boiling point of the aqueous reaction solution;
(9) the process as described in (3) above, which includes, after the completion of reaction, adjusting the pH by adding an acid to the reaction solution or the diluted or concentrated solution to precipitate a phosphate salt and removing the phosphate salt;
(10) the process as described in (3) above, which includes, after the completion of reaction, adjusting the pH by adding an acid to the reaction solution or the diluted or concentrated solution to precipitate a phosphate salt and dissolving the precipitated phosphate salt and adding a base to the dissolved solution to precipitate the phosphate salt and removing the phosphate salt;
(11) the process as described in (9) above, which includes crystallizing an ascorbic acid-2-monophosphate salt from the aqueous reaction solution or an aqueous solution after the removal of phosphate using a water-soluble organic solvent; and
(12) the process as described in (11) above, which includes drying the wet ascorbic acid-2-monophosphate salt crystallized from the aqueous reaction solution or an aqueous solution after the removal of phosphate using a water-soluble organic solvent to obtain the ascorbic acid-2-monophosphate salt as a powder.
In the present invention, the ascorbic acid structure may be any of an L-form, a D-form and a racemic compound, however, the present invention is described below by referring to the L-form which is industrially useful.
According to the process of the present invention, an aqueous solution containing AsPP or a salt thereof is hydrolyzed, preferably under heating, at a pH of 7 or more in the presence of magnesium ion and thereby the phosphoric acid ester bond of AsPP is stepwise alkali-hydrolyzed to prepare AsMP.
This preparation process of the present invention comprises allowing magnesium ion to be present together and adjusting the pH to 7 or more, the stepwise hydrolysis from AsPP to AsMP proceeds almost constantly in quantity and the hydrolysis reaction from AsMP to L-ascorbic acid scarcely proceeds.
In the preparation process of the present invention, the ion allowed to be present at the hydrolysis cannot be just any alkaline earth metal ion but must be magnesium ion. The magnesium ion has an action of accelerating the hydrolysis reaction from AsPP to AsMP but not accelerating the hydrolysis from AsMP to L-ascorbic acid. The reasons therefor are not clearly understood but the magnesium ion maintains relatively mild alkalinity even in a strong alkali condition and has low water-solubility and these properties are considered to take a part in the above-described action.
For example, when magnesium oxide was added in 1.7 molar times to an aqueous solution containing AsMP and the solution was heated at 60xc2x0 C. for 6 hours, the amount of residual AsMP was 100 wt %. When the same operation was performed using calcium oxide in place of magnesium oxide, the amount of residual AsMP was 41 wt % and this clearly showed formation of L-ascorbic acid and furthermore formation of oxalic acid by the decomposition of L-ascorbic acid. In the examination where magnesium chloride was added in 1.7 molar times to a solution containing AsMP sodium salt and heated at 60xc2x0 C. for 6 hours, the amount of residual AsMP was also 100 wt %. However, when the same operation was performed using calcium chloride in place of magnesium chloride, the amount of residual AsMP was 63 wt %. The same results were obtained in the examination using barium ion in place of calcium ion.
As such, magnesium ion has an action of not accelerating the hydrolysis of AsMP but calcium ion and the like have no such an action. which can be used in the present invention include polyphosphoric acids such as triphosphoric acid, diphosphoric acid (pyrophosphoric acid) and metaphosphoric acid, and salts thereof. Accordingly, the AsPP usually includes AsTP, AsDP, and salts and mixtures thereof. These may be prepared by the method described in U.S. Pat. 5,149,829 and the like. In the present invention, the dehydration product of an ascorbic acid and a polyphosphoric acid or a salt thereof may be used as it is in the hydrolysis.
According to U.S. Pat. 5,149,829, an L-ascorbic acid and a polyphosphate salt such as sodium trimetaphosphate were reacted at a temperature of from 33 to 35xc2x0 C. while adjusting the pH to 10.5 to 10.7 by NaOH or KOH. After the completion of reaction, the inorganic phosphoric acid compound was removed by precipitation as a calcium salt using calcium chloride at a temperature as low as 10xc2x0 C. Thereafter, the residue was concentrated, crystallized from ethanol and dried to obtain the objective AsPP salt as a mixture of AsMP salt, AsDP salt and AsTP salt. In the method of U.S. Pat. 5,149,829, the inorganic phosphoric acid compound precipitated is removed, however, in the present invention, inorganic phosphoric acids may be mixed in the AsPP salt. In the case where use of AsPP as an acid but not as a salt is intended, the acid may be obtained by dissolving an AsPP salt in water and decationizing it by passing the solution through a strongly acidic cation exchange resin at a low temperature. Also, when use of only pure AsTP or only pure AsDP as a raw material is intended, the raw material may be obtained by purifying the AsPP using chromatography.
In the present invention, the AsPP concentration is, for example, in water, suitably from about 1 to 80 wt %. If the AsPP concentration is low, the reaction slowly proceeds, whereas it is excessively high, the viscosity increases or magnesium salt of AsPP precipitates. Accordingly, the AsPP concentration is preferably from about 5 to 50 wt %, more preferably from about 10 to 30 wt %.
Thereafter, magnesium ion is added to the aqueous solution of AsPP or a salt thereof. In the case where the AsPP is a salt, a water-soluble magnesium compound such as magnesium chloride is preferably used and in the case where the AsPP is an acid, a magnesium compound showing alkalinity such as magnesium oxide is preferably used. Examples of the magnesium ion-supplying compounds include magnesium chloride, magnesium sulfate, magnesium acetate, magnesium nitrate, magnesium oxide, magnesium hydroxide, magnesium carbonate and the like. These compounds may be used individually or as a mixture without any problem.
The amount of magnesium ion added is an equimolar amount or more to the phosphoric acid group of AsPP. As the alkali hydrolysis of AsPP proceeds, phosphoric acid is produced in the aqueous reaction solution and the phosphoric acid produced reacts with magnesium ion present in the aqueous reaction solution to form magnesium hydrogenphosphate or magnesium phosphate. These magnesium salts of phosphoric acid have very low solubility in water and accordingly precipitate out. As a result, magnesium ion in the aqueous solution necessary for the hydrolysis reaction decreases. Accordingly, in order to maintain the magnesium ion in the reaction system even when the hydrolysis reaction of AsPP proceeds, the magnesium ion is required in an equimolar amount or more to the phosphoric acid group of AsPP. In this reaction, the amount of magnesium ion added has no upper limit in particular, however, the presence of excess magnesium ion has no effect on the reaction and this is of no benefit. Accordingly, the amount of magnesium added is preferably from about 1.1 to 2.0 mol, more preferably from 1.3 to 1.8 mol, based on the phosphoric acid group of AsPP.
In the preparation process of the present invention, the pH is suitably set to be 7 or more. If the pH is less than 7, the stepwise hydrolysis reaction in the present invention becomes hydrolysis accelerated by an acid, as a result, the hydrolysis of the objective product AsMP is also accelerated to decompose it into L-ascorbic acid and phosphoric acid. Accordingly, the aqueous reaction solution must be set to a pH of 7 or more using an appropriate alkali. Examples of the alkali used here include alkali metal hydroxides, alkali metal carbonates, alkaline earth metal hydroxides, alkaline earth metal oxides, alkaline earth metal carbonates, tertiary amines, and mixtures of two or more thereof. In the case where the alkaline earth metal compound used is magnesium oxide, magnesium hydroxide or magnesium carbonate, magnesium ion can be supplied to the aqueous reaction solution and thus actions of adjusting the pH and at the same time accelerating the hydrolysis of AsPP can be achieved. In the present invention, substantially no problem arises as long as the pH is 7 or more. However, the reaction slowly proceeds in the alkali region closer to the neutral, whereas reaction in the strong alkali condition requires alkali-resistant equipment. Accordingly, the pH is preferably from 8 to 13, more preferably from 9 to 11.
In addition, heating is preferably performed in the present invention. If the reaction is performed at a low temperature, a very long time is necessary until the completion of reaction and the plant is not utilized efficiently and this is disadvantageous. Accordingly, the reaction is preferably performed at a temperature of from 35xc2x0 C. to the boiling point of aqueous reaction solution, more preferably from 50 to 90xc2x0 C.
The thus-obtained AsMP-containing aqueous solution is diluted or concentrated to an appropriate AsMP concentration and a phosphate salt precipitated (example: phosphoric acid alkali salt) is removed by the solid-liquid separation. At this time, the pH is adjusted with an appropriate acid to effectively precipitate the phosphate salt. Examples of said acid are concentrated or diluted hydrochloric acid. Or, the phosphate salt precipitated may be dissolved by an acid and again precipitated by adding a base.
In the aqueous solution containing high-purity AsMP salt after the removal of the phosphate salt precipitated, AsMP salt is precipitated using a water-soluble organic solvent. Examples of the water soluble organic solvent used here include methanol, ethanol, isopropanol, tetrahydrofuran, dioxane, acetone and the like. Among these, ethanol is preferred in view of the cost and ease of recovery.
The thus-obtained wet AsMP salt is dried by vacuum drying or fluidized bed drying to obtain AsMP salt in a high yield.
In particular, AsMP magnesium salt has excellent stability against heat or light, accordingly, it is used not only for cosmetic powders or lotions but also, for example, for medical products (e.g., oral cavity preparation, eyedrops, products for bath and the like), cosmetics (e.g., lotion, emulsion, cream, pack and the like), food (e.g., bread and the like) and animal feed (e.g., feed for breeding prawn, salmon, yellow tail, eel, carp and the like).