Field of the Invention

The present invention pertains to a new process for preparing 3-hydroxy-3-methylbutyric acid (HMB) and salts of HMB.

Background of the Invention

3-Hydroxy-3-methylbutyric acid (HMB) is known as a metabolite of the essential amino acid L- leucine. Due to several positive effects, mainly related to either strengthen, or recovering muscle tissue, HMB is used as a nutritional supplement for humans, in particular in strength- and endurance sports (see: J. Int. Soc. Sports Nutr. 2013, 10, 6). In addition, HMB is used in pharmaceutical applications, such as in the treatment of muscular dystrophy.

The positive effects of HMB, however, are not limited to applications in humans. The use of HMB as feed additive or feed supplement has been described for a number of economically relevant livestock, such as poultry and pigs, see e.g. US 5,087,472 or US 2010/0179112 A1 .

The dosage form mainly applied for HMB is the calcium salt, which may be obtained from the free acid. Compared to the free acid, the calcium salt comes with some advantages, such as stability, no odor, and non-corrosive properties.

In the established manufacturing process, the key synthetic reaction step is a Baeyer-Villiger oxidation of diacetone alcohol, whereby the HMB, or its salts, obtained. Sodium hypochloride (NaOCI, described in EP 2 744 489 A1) is used as a typical oxidant in this reaction, thereby yielding the sodium salt of HMB. Neutralization with hydrochloric acid yields HMB as a free acid.

Other reported oxidants include sodium hypobromide (described in CN1417190) or freshly prepared peracetic acid (described in CN103694107). All these reactions come with signficant limitations: a production process based on NaOCI or NaOBr not only produces stoichiometric quantities of salt but also large quantities of undesirable hazardous substances such as chloroform or bromoform. In addition, the yield of the oxidation reaction is rather low. A process based on peracetic acid requires a flammable organic solvent and is as well described by a rather low yield based on both diacetone alcohol and peracetic acid precursors hydrogen peroxide and acetic acid. In an alternative process, HMB can be synthesized directly from te/Y-butanol and carbon monoxide using “Fenton chemistry” (see D. D. Coffmann, R. Cramer, W. e. Mochel, J. Am. Chem. Soc. 1952, 80, 2882-2887). The term “Fenton chemistry” means to form hydroxyl (OH) radicals from hydrogen peroxide with the use of stoichiometric quantities of iron (II) sulphate. In the presence of tert- butanol and carbon monoxide, these hydroxyl radicals can undergo a radical reaction to give the desired product.

It is precisely the necessary use of iron (II) sulphate that makes this approach uneconomical for two reasons: a) iron (II) sulphate is a comparatively expensive raw material and b) salts are formed stoichiometrically in the course of the reaction that must be disposed of.

Accordingly, it was the objective of the present invention to provide a simple and inexpensive manufacturing process for HMB in which less hazardous byproducts are formed and in which the formation of byproducts is minimized, respectively.

Summary of the Invention

The inventors have found a new synthesis route to HMB that meets the requirements of the above objective. In particular, the inventors have unexpectedly found that 3-hydroxy-3-methylbutyric acid (HMB), and salts thereof, can be obtained from te/Y-butanol and carbon monoxide in an aqueous reaction mixture simply be applying electrolytic conditions. In other words, compared to the above- mentioned “Fenton”-route, both the function of the iron (II) sulphate and the function of hydrogen peroxide can be completely replaced by electrochemistry in aqueous solution.

Electrochemistry

Accordingly, the present invention pertains to a method for preparing 3-hydroxy-3-methylbutyric acid (HMB) or salts thereof from te/Y-butanol and carbon monoxide, wherein te/Y-butanol is reacted in an aqueous mixture with carbon monoxide under electrolytic conditions. Detailed Description of the Invention

The term “electrolytic conditions” generally refers to applying an electric potential and current to the respective reaction mixture.

For the method according to the present invention, the electric potential to be applied to the aqueous reaction mixture may be between 1.5 V and 25 V, in particular between 15 V and 23 V and preferably between 18 V and 22 V.

The electrodes may be operated at a current density starting from 1 A/m ² up to 10.000 A/m ² , in particular between 50 A/m ² and 5.000 A/m ² and preferably between 100 A/m ² and 3500 A/m ² .

A wide variety of electrode materials are known in the art. For the method according to the present invention, electrodes allowing the formation of hydroxyl radicals are particularly suitable. For example, the electric potential may be applied to the aqueous reaction mixture using a diamond anode and/or a steel cathode. In contrast to the established platinum anodes, diamond anodes have a larger potential window and suppress oxygen (gaseous O2) formation to the advantage of the formation of hydroxyl radicals in aqueous systems.

Form improving the conductivity of the aqueous solution, at least conducting salt may be used in small quantities. Said conducting electrolyte are not consumed during the reaction, but significantly improves the efficiency of hydroxyl formation. The initial amount of the at least one conducting electrolyte present in the aqueous reaction mixture (i.e. the amount of of the at least one conducting electrolyte present in the aqueous reaction mixture at the beginning of the reaction) may between 1 wt.% and 20 wt.%, in particular between 5 wt.% and 15 wt.% and preferably between 10 wt.% and 14 wt%.; based on the amount of tert-butanol present in the aqueous reaction mixture.

The HMB reaction product obtainable via the method according to the present invention itself may serve as a conducting electrolyte. In this case, an initial amount of HMB or HMB salt has to be introduced to the reaction mixture before stating the electrochemical reaction.

In general, the at least one conducting electrolyte may be selected from the group consisting of tetraalkyl ammonium hexaflurophosphates, tetraalkyl ammonium tetrafluoroborates, and tetraalkyl imidazolium tritiates, and 3-hydroxy-3-methylbutyric acid or salts thereof (HMB), or any combination of these conducting electrolytes. Conducting electrolytes selected from the group consisting of tetra butyl ammoniumhexafluorophosphate, tetrabutyl ammoniumtetrafluoroborate, 1- butyl-3-methylimidazolium tritiate, and 3-hydroxy-3-methylbutyric acid or salts thereof (HMB), or any combination of these conducting electrolytes are particularly suitable.

Preferably, the conducting electrolyte is 1-butyl-3-methylimidazolium tritiate. The carbon monoxide may be provided to the aqueous reaction mixture in gaseous form, e.g. by simply bubbling carbon monoxide through the aqueous reaction mixture. Alternatively, the carbon monoxide may be obtained in situ from gaseous carbon dioxide. In the latter case, the carbon dioxide is bubbled through the aqueous reaction mixture and is reduced in situ, i.e. under the electrolytic conditions and via electron transfer, to carbon monoxide.

The temperature in the aqueous reaction mixture may be between 10°C and 80°C, in particular between 12°C and 50°C and preferably between 15°C and 35°C.

In the following, the invention is illustrated by non-limiting examples and exemplifying embodiments.

Examples

The general reaction setup consists of a 1 L- reaction device with a carbon monoxide inlet. The reactor is also connected to a pump that allows to circulate the whole reaction mixture through an electrochemical reactor, and back again into the reaction device.

Experiment 1 :

In a 1 L-reaction device, tert-Butanol (50 g, 0.67 mol, 1.0 equiv.) and the electrolyte 1-Butyl-3- methylimidazolium triflate (7.0 g, 0.024 mol, 3.6 Mol-%) were added to water (500 mL) at room temperature. Carbon monoxide (1 bubble/s) was bubbled through the reaction mixture, while circulating the reaction mixture through the electrochemical reactor (cathode: steel, anode: diamond, electrode distance: 1.5 cm, no separator). The electrodes were operated at 125 A/m ² by applying a potential of 20 V. During the reaction, a voltage drop was observed due to HMB formation, which increases the electrical conductivity. After 2 h at room temperature, HMB was obtained in a yield of 73% (10 wt-% in reaction mixture, 58 g, 0.49 mol) as verified by HPLC analysis.

Experiment 2:

In a 1 L-reaction device, tert-Butanol (50 g, 0.67 mol, 1 .0 equiv.) was added to water (450 mL) at room temperature. No additional electrolyte was used. Carbon monoxide (1 bubble/s) was bubbled through the reaction mixture, while circulating the reaction mixture through the electrochemical reactor (cathode: steel, anode: diamond, electrode distance: 1.5 cm, no separator). No electrical current was observed when applying a potential of 20 V. No HMB formation was observed after 2 h at room temperature.

Experiment 3:

In a 1 L-reaction device, te/Y-Butanol (50 g, 0.67 mol, 1.0 equiv.) and HMB (10 g, 0.085 mol, 13 Mol- %) as electrolyte were added to water (440 mL) at room temperature. Carbon monoxide (1 bubble/s) was bubbled through the reaction mixture, while circulating the reaction mixture through the electrochemical reactor (cathode: steel, anode: diamond, electrode distance: 1.5 cm, no separator). The electrodes were operated at 3000 A/m ² by applying a potential of 20 V. After 2 h at room temperature, an additional amount of HMB was formed as verified by HPLC analysis.