Preparation of dihydrolipoic acid

Abstract

A method of preparing ( R)-dihydrolipoic acid (RDLA) is disclosed. In various embodiments described herein, the method of producing RDLA minimizes yield loss due to the oxidation of RDLA to lipoic acid. The method of producing RDLA minimizes the evolution of hydrogen gas during the reduction of lipoic acid to RDLA. The method includes selectively reducing the disulfide bond in the cyclic moiety of alpha-lipoic acid (shown below) without reducing the carboxylic acid moiety.

Description

[0001] PREPARATION OF DIHYDROLIPOIC ACID CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims benefit of U.S. Provisional Application No. 63 / 567,004 filed March 19, 2024, which is hereby incorporated herein by reference in its entirety.

[0003] FIELD OF THE INVENTION

[0004] The disclosed invention is generally in the field of preparation of hidhydrolipoic acid.

[0005] BACKGROUND OF THE INVENTION

[0006] The enantiomeric compound (R)-dihydrolipoic acid (RDLA) is a small molecule with antioxidant activity and is currently being developed for use as a dietary supplement. As such, large quantities of the compound are required. For various reasons, however, the synthesis of (R)-dihydrolipoic acid on a large scale (e.g., over 1 kilogram) has been problematic.

[0007] The preparation of RDLA on a large scale is problematic because the reduction reaction is re vers able, which means dihydrolipoic acid can oxidize back to lipoic acid in the presence of oxygen and cause a decrease in purity. Furthermore, the reducing agent can result in the evolution of hydrogen gas during the reaction. The oxidation of RDLA back to lipoic acid and the evolution of hydrogen gas can also occur after the reaction, during a workup procedure. Hydrogen gas is highly flammable, and the production of hydrogen gas can be extremely dangerous during large scale manufacturing.

[0008] Improved methods of making RDLA are needed.

[0009] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.

[0010] BRIEF SUMMARY OF THE INVENTION

[0011] In various embodiments, a method of preparing (R)-dihydrolipoic acid (RDLA) is disclosed. In various embodiments described herein, the method of producing RDLA minimizes yield loss due to the oxidation of RDLA to lipoic acid. In various embodiments described herein, the method of producing RDLA minimizes the evolution of hydrogen gas during the reduction of lipoic acid to RDLA.

[0012] In some forms the method includes selectively reducing the disulfide bond in the cyclic moiety of alpha-lipoic acid (shown below) without reducing the carboxylic acid moiety. The IUPAC name for a-lipoic acid is (R)-5-(l,2-dithiolan-3-yl)pentanoic acid.

[0013] In some embodiments, the method comprises the preparation of a first solution comprising alpha-lipoic acid in a first reaction vessel. In some embodiments, the first solution comprises a base or basic reagent. In some embodiments, the base is sodium hydroxide (NaOH). In such embodiments, the molarity of the sodium hydroxide is in a range of 0.1M to 5.0M.

[0014] In some embodiments, the first reaction vessel containing the first solution is maintained at a temperature in a range of 23-65°C, or 35-60°C, or 40-50°C.

[0015] In some embodiments, the method comprises the preparation of a second solution comprising a reducing agent in a second reaction vessel. In some embodiments, the reducing agent is sodium borohydride (NaBt ).

[0016] In some embodiments, the second reaction vessel containing the second solution is maintained at a temperature in a range of 23-65°C, or 35-60°C, or 40-50°C.

[0017] In some embodiments, the method comprises the addition of the second solution to the first solution (e.g., from the second reaction vessel to the first reaction vessel). In some embodiments, the method comprises the addition of the second solution to the first solution (e.g., from the second reaction vessel to the first reaction vessel).

[0018] In some embodiments, the addition of the second solution comprising the reducing agent to the first solution comprising the alpha-lipoic acid causes an exothermic reaction. In such embodiments, the temperature of the first reaction vessel containing the first solution increases after the addition of the second solution. In some embodiments, the temperature of the first reaction vessel is monitored during the addition step and the first reaction vessel is maintained at a temperature in the range of 45-50°C.

[0019] In some embodiments, the second solution is added to the first solution in portions to maintain the first reaction vessel at a desired temperature.

[0020] In some embodiments, the method comprises a reaction period after the addition of the second solution to the first solution.

[0021] In some embodiments, the status of the reaction is monitored using one or more analytical techniques.

[0022] In some embodiments, the method comprises a workup procedure after the reaction has been completed or stopped. In some embodiments, the method and workup procedure comprises the preparation of a third solution comprising an acid in a third reaction vessel.

[0023] In some embodiments, the method and workup procedure comprises a step of transferring the reaction mixture from the first reaction vessel to the acid solution in the third reaction vessel. In some embodiments, the addition of the reaction mixture from the first reaction vessel to the acid solution in the third reaction vessel causes an exothermic reaction.

[0024] In some embodiments, the method and workup procedure comprises a step isolating or extracting the RDLA from the solution in the third reaction vessel. In some embodiments, the method comprises a fourth reaction vessel comprising one or more organic solvents.

[0025] In some embodiments, after the resting period, the method and workup procedure comprises a step of separating the aqueous layer from the organic layer in the fourth reaction vessel (workup station).

[0026] In some embodiments, the RDLA oil obtained from the workup procedure is sparged with an inert gas, such as nitrogen, to remove any residual solvent and oxygen.

[0027] In some embodiments, the method and workup procedure comprises a filtering step. In such embodiments, the RDLA oil isolated from the organic layer is filtered to remove any insoluble solids.

[0028] In some embodiments, the method comprises a purification step of the RDLA oil.

[0029] Additional advantages of the disclosed method and compositions will be set forth in part in the description which follows, and in part will be understood from the description, or can be learned by practice of the disclosed method and compositions. The advantages of the disclosed method and compositions will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.

[0030] DETAILED DESCRIPTION OF THE INVENTION

[0031] The disclosed method and compositions can be understood more readily by reference to the following detailed description of particular embodiments and the Example included therein and to the Figures and their previous and following description.

[0032] Reference will now be made in detail to the exemplary embodiment(s), examples of which is / are illustrated in the examples. Before describing the exemplary embodiments, it is noted the embodiments reside primarily in combinations of method steps related to the preparation of dihydrolipoic acid or a specific enantiomer thereof. Accordingly, the method steps have been represented where appropriate, showing only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

[0033] It is to be understood that the disclosed method and compositions are not limited to specific synthetic methods, specific analytical techniques, or to particular reagents unless otherwise specified, and, as such, can vary. For example, references to “various embodiments,” “some embodiments,” “other embodiments,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include that feature, structure, or characteristic. Such phrases are not necessarily referring to the same embodiment. When a particular feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether explicitly described or not.

[0034] It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

[0035] A. DEFINITIONS

[0036] A recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. As will be understood by one skilled in the art, ranges disclosed herein encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art, language such as “up to,” “at least,” “greater than,” “less than,” and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member. Thus, for example, a composition having 1-3 components refers to compositions having 1, 2, or 3 components. Similarly, a composition having 1-5 components refers to compositions having 1, 2, 3, 4, or 5 components, and so forth.

[0037] As used herein and in the appended claims, singular articles such as “a” and “an” and “the” and similar referents in the context of describing the elements (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context.

[0038] As used herein, the use of examples, or exemplary language (e.g., “such as”), is intended to illuminate the embodiments and does not pose a limitation on the scope of the claims unless otherwise stated. No language in the specification should be construed as indicating any nonclaimed element as essential.

[0039] As used herein, the terms “about” and “substantially” will be understood by persons of ordinary skill in the art and will vary to some extent depending upon the context in which it is used. If there are uses of the term which are not clear to persons of ordinary skill in the art, given the context in which it is used, “about” and “substantially” will mean up to plus or minus 10% of the particular term.

[0040] B. Dihydrolipoic Acid

[0041] The chemcial name of dihydrolipoic acid is 6,8-bis(sulfanyl)octanoic acid. The structure of dihydrolipoic acid (shown below) includes an eight carbon chain substituted at the Cl position with a carboxylic acid and at each of the C6 and C8 positions with a thiol. The R-enantiomerof dihydrolipoic acid (RDLA) has a stereocenter at the C-6 position.

[0042] C. METHODS

[0043] In various embodiments described herein, the method of producing RDLA minimizes yield loss due to the oxidation of RDLA to lipoic acid. In various embodiments described herein, the method of producing RDLA minimizes the evolution of hydrogen gas during the reduction of lipoic acid to RDLA.

[0044] In some embodiments, the method comprises the preparation of a first solution comprising alpha-lipoic acid in a first reaction vessel. In some embodiments, the first solution comprises a base or basic reagent. In some embodiments, the base is sodium hydroxide (NaOH). In such embodiments, the molarity of the sodium hydroxide is in a range of 0.1M to 5.0M, including any subranges (e.g., 0.3M to 0.7M, 0.4M to 0.6M), the endpoints (e.g., 0.1M, 0.6M), and any intermediate points (e.g., 0.4M, 0.5M, 0.6M) contained therein. In some embodiments, the base is an aqueous solution having water as the solvent.

[0045] In some embodiments, the first reaction vessel containing the first solution is maintained at a temperature in a range of 23-65°C, or 35-60°C, or 40-50°C. In some embodiments, the first reaction vessel containing the first solution is maintained at a temperature of about 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C, or 50°C.

[0046] In some embodiments, the method comprises the preparation of a second solution comprising a reducing agent in a second reaction vessel. In some embodiments, the reducing agent is sodium borohydride (NaBF ). In some embodiments, the sodium borohydride is an aqueous solution. In some embodiments, the concentration of the aqueous solution of sodium borohydride is in a range of 5% to 20%, including any subranges (e.g., 8% to 18%), the endpoints (e.g., 5%, 18%), or any intermediate points (e.g., 9%, 10%, 11%, 12%, 13%, 14%, 15%) contained therein.

[0047] In some embodiments, the second reaction vessel containing the second solution is maintained at a temperature in a range of 23-65°C, or 35-60°C, or 40-50°C. In some embodiments, the second reaction vessel containing the second solution is maintained at a temperature of about 23°C, 24°C, 25°C, 26°C, 27°C, 28°C, 29°C, or 30°C.

[0048] In some embodiments, the method comprises the addition of the second solution to the first solution (e.g., from the second reaction vessel to the first reaction vessel). In such embodiments, the addition of the second solution can occur in one or more portions. For example, in some embodiments, the entire second solution is delivered to the first solution. In other embodiments, a first portion of the second solution is delivered to the first solution followed by a period (e.g., 1-120 minutes) in which the delivery is stopped, followed by the delivery of a second portion of the second solution to the first solution.

[0049] In some embodiments, the addition of the second solution comprising the reducing agent to the first solution comprising the alpha-lipoic acid causes an exothermic reaction. In such embodiments, the temperature of the first reaction vessel containing the first solution increases after the addition of the second solution. In some embodiments, the temperature of the first reaction vessel is monitored during the addition step and the first reaction vessel is maintained at a temperature in the range of 45-50°C (e.g., 45°C, 46°C, 47°C, 48°C, 49°C). In some embodiments, the second solution is added to the first solution in portions to maintain the first reaction vessel at a desired temperature.

[0050] In some embodiments, the method comprises a reaction period after the addition of the second solution to the first solution. In such embodiments, the reaction period is in the range of 1 hour to 30 hours, including any subranges (e.g., 8 hrs. to 24 hrs.), the endpoints (e.g., 8 hrs., 24 hrs.), or any intermediate points (e.g., 9 hrs., 10 hrs., 11 hrs., 12 hrs., 13 hrs., 14 hrs., 15 hrs., 16 hrs., 17 hrs.) contained therein.

[0051] In some embodiments, the status of the reaction is monitored using one or more analytical techniques. For example, in some embodiments, an aliquot from the reaction mixture can be obtained and tested using nuclear magnetic resonance (NMR), high-performance liquid chromatography (HPLC), gas chromatography (GC), etc. In such embodiments, the status of the reaction, which refers to the existence of any remaining, unreacted, alpha-lipoic acid can be determined. In such embodiments, if unreacted alpha-lipoic acid remains, the reaction period can be extended until the alpha-lipoic acid is no longer detected.

[0052] In some embodiments, the method comprises a workup procedure after the reaction has been completed or stopped. In such embodiments, the workup procedure comprises one or more steps. In some embodiments, the workup procedure comprises a step of cooling the first reaction vessel to a temperature in a range of -15°C to 20°C (e.g., -10°C, -5°C, -4°C, -3°C, -2°C, -1°C, 0°C, 1°C, 2°C, 3°C, 4°C, 5°C).

[0053] In some embodiments, the method and workup procedure comprises the preparation of a third solution comprising an acid in a third reaction vessel. In such embodiments, the acid is hydrochloric acid or citric acid. In some embodiments, the molarity of the acid is in a range of 0.1M to 5.0M, including any subranges (e.g., 0.3M to 3.7M, 1.4M to 2.6M), the endpoints (e.g., 3.7M, 2.6M), and any intermediate points (e.g., 1.0M, 2.0M, 2.5M, 3.0M) contained therein. In some embodiments, the third solution further comprises a solvent. For example, in some embodiments, the acid is an aqueous solution having water as the solvent.

[0054] In some embodiments, the method and workup procedure comprises a step of transferring the reaction mixture from the first reaction vessel to the acid solution in the third reaction vessel. In some embodiments, the addition of the reaction mixture from the first reaction vessel to the acid solution in the third reaction vessel causes an exothermic reaction. In such embodiments, the temperature of the third reaction vessel containing the acid solution increases after the addition of the reaction mixture from the first reaction vessel. In some embodiments, the temperature of the third reaction vessel is monitored during the addition step and the third reaction vessel is maintained at a temperature in the range of 0°C to 20°C (e.g., 1°C, 2°C, 3°C, 4°C, 5°C, 6°C, 7°C, 8°C, 9°C, 10°C, 11°C, 12°C). In some embodiments, the third reaction vessel is maintained at a temperature of 12°C or less, 11°C or less, 10°C or less, 9°C or less, or 8°C or less. In some embodiments, the rate of transfer of the reaction mixture from the first reaction vessel is controlled to maintain the third reaction vessel at a desired temperature. In some embodiments, the method and workup procedure comprises a step isolating or extracting the RDLA from the solution in the third reaction vessel. In some embodiments, the method comprises a fourth reaction vessel comprising one or more organic solvents. In some embodiments, the organic solvent is dichloromethane. In some embodiments, the fourth reaction vessel is a workup station configured to allow aqueous and organic solutions to separate into layers based on their respective densities. In some embodiments, the workup station comprises a valve at the bottom of the vessel that can be opened and closed for draining the solution. In some embodiments, the solution from the third reaction vessel is transferred to the fourth reaction vessel. In such embodiments, the solution from the third reaction vessel is mixed with the one or more organic solvents in the fourth reaction vessel, and the mixture is allowed to rest for a period of time to allow the mixture to separate into organic and aqueous layers. In some embodiments, the resting period is in the range of 1 hour to 30 hours, including any subranges (e.g., 8 to 24 hrs.), the endpoints (e.g., 8 hrs., 24 hrs.), or any intermediate points (e.g., 9 hrs., 10 hrs., 11 hrs., 12 hrs., 13 hrs., 14 hrs., 15 hrs., 16 hrs., 17 hrs.) contained therein. The RDLA, substantially all of the RDLA (e.g., 90% or more, 91% or more, 92% or more, 93% or more, 94% or more, 95% or more, 96% or more, 97% or more, 98% or more, 99% or more), or a majority of the RDLA (e.g., 51% or more, 70% or more, 80% or more), from the mixture will be dissolved in the organic layer due to solubility. In some cases, a minimum amount of RDLA will remain in the aqueous layer.

[0055] In some embodiments, after the resting period, the method and workup procedure comprises a step of separating the aqueous layer from the organic layer in the fourth reaction vessel (workup station). In some embodiments, the workup procedure comprises a step of transferring the organic layer to a container (e.g., round bottomed flask) and then removing the organic solvent. For example, in some embodiments, the solvent is removed using a rotary evaporator device maintained at a temperature in a range of 23-65°C, or 35-60°C, or 40-50°C, including specific values therein, including 33°C, 34°C, 35°C, 36°C, 37°C, 38°C, 39°C, 40°C, 41°C, 42°C, 43°C, 44°C, or 45°C. With the solvent removed, the RDLA remains present as an oil.

[0056] In some embodiments, the RDLA oil obtained from the workup procedure is sparged with an inert gas, such as nitrogen, to remove any residual solvent and oxygen. In some embodiments, the oil is sparged for a period in the range of 1 hour to 120 hours, including any subranges (e.g., 8 hrs. to 96 hrs., 48 hrs. to 80 hrs.), the endpoints (e.g., 8 hrs., 80 hrs.), or any intermediate points (e.g., 24 hrs., 40 hrs., 50 hrs., 60 hrs., 70 hrs., 75 hrs.) contained therein.

[0057] In some embodiments, the method and workup procedure comprises a filtering step. In such embodiments, the RDLA oil isolated from the organic layer is filtered to remove any insoluble solids. In some embodiments, the organic layer is filtered through a fritted funnel, which can be made from glass, porcelain, or ceramic. In some embodiments, the organic layer is filtered through an in-line filter (i.e., a filter positioned between reaction vessels in a large-scale industrial configuration). In some embodiments, the fritted funnel is a course frit with through- holes having a porosity in the range of 40-60 mm and a frit area of about 1520 mm2. In some embodiments, the fritted funnel is a medium frit with through-holes having a porosity in the range of 10-15 mm and a frit area of about 1320 mm2.

[0058] In some embodiments, the method comprises a purification step of the RDLA oil. In some embodiments, the method excludes a purification step and the RDLA oil is used “crude” after isolation. In such embodiments, the omission of the purification step reduces the opportunity for the RDLA oil to be exposed to oxygen and for the RDLA to oxidize back into alpha-lipoic acid.

[0059] In various embodiments, one or more steps of the method, including a single step, a plurality of steps, and all steps of the method, are carried out in an inert atmosphere (e.g., argon or nitrogen gas forced into the reaction vessel or container and any air removed).

[0060] In various embodiments, the reaction vessel for the first solution, the second solution, or the third solution are sparged with an inert gas to remove any oxygen present in the vessel (e.g., in the atmosphere or dissolved in solution). In some embodiments, the reaction vessel is fitted with a gas inlet and a device that delivers the gas directly into the solution. In some embodiments, the first solution, the second solution, or the third solution are sparged with an inert gas for a period of 1 hour to 30 hours, including any subranges (e.g., 8 to 24 hrs.), the endpoints (e.g., 8 hrs., 24 hrs.), or any intermediate points (e.g., 9 hrs., 10 hrs., 11 hrs., 12 hrs., 13 hrs., 14 hrs., 15 hrs., 16 hrs., 17 hrs.) contained therein.

[0061] In various embodiments, the original shipping container for any components of the method, such as the solvents, are sparged with an inert gas to remove any oxygen present in the vessel (e.g., in the atmosphere or dissolved in solution). In some embodiments, the shipping container is fitted with a gas inlet and a device that delivers the gas directly into the solution. In some embodiments, the shipping container is sparged with an inert gas for a period of 1 hour to 30 hours, including any subranges (e.g., 8 to 24 hrs.), the endpoints (e.g., 8 hrs., 24 hrs.), or any intermediate points (e.g., 9 hrs., 10 hrs., 11 hrs., 12 hrs., 13 hrs., 14 hrs., 15 hrs., 16 hrs., 17 hrs.) contained therein.

[0062] In various embodiments, the reaction vessels are fitted with laboratory equipment to facilitate the method. For example, the reaction vessel can include a stirring device, one or more inlets, a gas (e.g., inert gas) feed inlet, a heating apparatus, a cooling apparatus, a temperature recording device, couplings for piping, etc.

[0063] Examples

[0064] The following examples are provided to aid in the understanding of the present disclosure, the true scope of which is set forth in the appended claims. One of skill in the art would appreciate that modifications can be made in the procedures set forth without departing from the spirit of the disclosure.

[0065] Example 1.1. Preparation of an (R)-alpha lipoic acid solution. A 72L reaction vessel was fitted with an overhead stirring device, a nitrogen inlet, a 2L addition funnel, and appropriate stoppers. The apparatus was purged with nitrogen. A solution was created by dissolving 714g of NaOH in water then diluting it to 34L. The NaOH solution was added to the 72L reaction vessel. The solution was heated to 45 °C. Next, 3.5kg of (R)-alpha lipoic acid was added to the 72L reaction vessel and stirred until fully dissolved.

[0066] Example 1.2. Reduction of (R)-alpha lipoic acid to (R)-dihydrolipoic acid. A 2L addition funnel was charged with 1.82L of a 12% aqueous NaBH4 solution. Approximately half of the 12% NaBH4 solution was added to the 72L reaction vessel in one portion. The temperature of the reaction vessel was allowed to cool down to 48°C. The remaining portion of the 12% NaBH4 solution was added to the 72L reaction vessel, and the reaction mixture was stirred at 45 °C for at least 16 hours.

[0067] Example 1.3. Reaction quench and acidification with aqueous hydrochloric acid. The reaction mixture was chilled to 0°C. A 26L solution of a 2M HC1 was prepared and sparged with nitrogen for at least 16 hours. The 2M HC1 solution was transferred via cannula to the chilled 72L reaction vessel at a rate in which the reaction temperature did not exceed 10 °C. A workup station was setup with overhead stirring, a nitrogen inlet, and appropriate stoppers. The workup station was purged with nitrogen. Equal portions of the contents of the 72L reaction vessel were transferred via cannula to the workup station while stirring.

[0068] Example 1.4. Extraction of (R)-dihydrolipoic acid. Each workup station was charged with 8.75L of nitrogen-sparged dichloromethane (DCM) and stirred for 5 minutes. A biphase was obtained and allowed to separate for at least 16 hours. The DCM layers were combined and then concentrated to an oil via a 20L rotary evaporator at 40°C. The oil was sparged with nitrogen for at least 72 hours.

[0069] Example 1.5. Removal of residual inorganic solids from (R)-dihydrolipoic acid. The crude oil product was filtered through a coarse fritted glass funnel to remove any insoluble solids. The filtration time for 20mL was 0.5 minutes and the flow rate was measured to be 40 mL / min. The flow rate normalized for frit area was 0.026 mL»min-l»mm-2.

[0070] Example 1.6. Recovery and storage of (R)-dihydrolipoic acid-final product. The RDLA (3,445g, 97% yield) was stored in glass or plastic containers packaged under nitrogen.

[0071] Examples 1.1-1.6 provided improvements over alternative processes that were evaluated.

[0072] The following Examples 2.1 to 2.6 were relatively unsuccessful attempts that highlight the unexpected results obtained in Examples 1.1-1.6.

[0073] Example 2.1. Preparation of an (R)-alpha lipoic acid solution and reduction of (R)-alpha lipoic acid to (R)-dihydrolipoic acid. A 500mL 4-neck reaction vessel was fitted with overhead stirring, a thermowell, an addition funnel, a nitrogen inlet, and a rubber septum. The reactor was purged with nitrogen then charged with water (80mL, 5 vol) and (R)-alpha lipoic acid (16.38g, 79 mmol). The thick slurry was stirred at ambient temperature. A solution was prepared by diluting 5mL of 50% wt / wt NaOH solution to 80mL with water. This solution was charged to the addition funnel. The base was added dropwise to the (R)-alpha lipoic acid slurry. Approximately halfway through the addition the reaction mixture became a thick gel. The addition was continued and a clear yellow solution with a pH of 8 was acquired. A syringe was charged with lOmL of 12% NaBHr solution. Upon addition of a few drops of NaBH4 the reagent was observed precipitating out as white spheres. This material dissolved within a few minutes. The pH was then checked and found to be 14. The remaining borohydride solution was added, and the maximum temperature reached was 30°C. The reaction mixture was stirred at 35 °C overnight. The following morning GC-FID analysis showed 26% remaining (R)-alpha lipoic acid. These results indicate that NaBH4 is not stable in aqueous solutions below pH 14. These results further show the sensitivity of the reaction toward temperature.

[0074] Example 2.2.1. Reduction of (R)-alpha lipoic acid to (R)-dihydrolipoic acid. Solid NaBH4 was added to the reaction vessel from Example 1.1 and hydrogen gas immediately evolved after the NaBH4 contacted the aqueous solution. The immediate and significant evolution of hydrogen gas is dangerous and is not acceptable for large scale processes. By contrast, the use of a prepared 12% NaBH4 solution resulted in undetectable amounts of hydrogen gas. Furthermore, the relative amount of NaBH4 was reduced to 0.47 equivalents or less while still driving the reduction reaction to completion.

[0075] Example 2.2.2. Reduction of (R)-alpha lipoic acid to (R)-dihydrolipoic acid. A 12% NaBH4 solution was added to the reaction vessel from Example 1.1 and the reaction mixture was stirred at room temperature for 72 hours. The resulting RDLA obtained had a purity of only 66.47%. This shows the sensitivity of the reaction toward temperature. Example 2.2.3. Reduction of (R)-alpha lipoic acid to (R)-dihydrolipoic acid. A 12% X;iBI l i solution was added to the reaction vessel from Example 1.1 and the reaction mixture was stirred at 35°C for 16 hours. The resulting RDLA had a purity of only 76.59%. Additionally, 1250mL of hydrogen gas was captured during the process. This shows the sensitivity of the reaction toward temperature.

[0076] Example 2.3. Reaction quench and acidification with aqueous hydrochloric acid. The reaction mixture from Example 1.2 must be stirred at 45°C for at least 16 hours. In alternative examples, the reaction was stirred for less than 16 hours and when the HC1 solution was transferred via cannula to the chilled reaction vessel a significant amount of hydrogen gas was produced. The reaction of the acid with the NaBH4 in solution is dangerous and unacceptable for the large-scale production of RDLA.

[0077] Example 2.4. Extraction of (R)-dihydrolipoic acid. A workup station was charged with chloroform instead of dichloromethane. The procedure resulted in a re-oxidation of the RDLA back to alpha- lipoic acid and a decrease in product yield.

[0078] Example 2.5. Removal of residual inorganic solids from (R)-dihydrolipoic acid. The crude oil product was filtered through a fine fritted glass funnel having a porosity of 4-5.5, um and a frit area of 894 mm2. Compared to Example 1.5, the flow rate was exponentially slower and the filtration time was exponentially longer. The filtration time for 20mL was days long and the flow rate was measured very slow. The flow rate normalized for frit area was not determinable.

[0079] Example 2.6. Recovery and storage of (R)-dihydrolipoic acid-final product. The

[0080] RDLA was not distilled stored in glass or plastic containers packaged under nitrogen. When the RDLA was distilled via vacuum distillation, it was surprisingly found that the RDLA lost purity. During the distillation process, the RDLA reacted intramolecularly to form a cyclic thioester impurity causing a loss in product yield.

[0081] There are several advantages for the process of Examples 1.1 to 1.6 compared to the alternative processes in Examples 2.1 to 2.6. In the context of a large-scale manufacturing process, the decrease in hydrogen gas production from 1250mL to no measurable amount of gas is a significant improvement. Additionally, the elimination of the purification step surprisingly increased the purity and recovery of the RDLA.

[0082] Example 3. Optical rotation. A sample of the RDLA obtained from Example 1.6 was tested for optical rotation. The analysis returned a value of -16.2°, which is consistent with literature values. The results confirm that the process of Examples 1.1 to 1.6 does not racemize the RDLA or result in the production of racemic dihydrolipoic acid. This is an important result because (S)-dihydrolipoic acid does not possess helpful biological properties. Example 4. Microbial testing. The RDLA obtained from Example 1.6 was tested to determine if any microbes were present. The results confirmed that no microbes were present, and the RDLA produced according to the methods described herein are suitable for human use.

[0083] Exemplary embodiments of the methods are described above in detail. The methods are not limited to the specific embodiments described herein, but rather, components of the compositions and / or steps of the method may be utilized independently and separately from other components and / or steps described herein.

[0084] The disclosed method can be further understood in view of the following non-limiting paragraphs.

[0085] 1. A method of preparing (R)-dihydrolipoic acid (RDLA), the method comprising:

[0086] (a) preparing a first solution comprising alpha-lipoic acid in a first reaction vessel;

[0087] (b) preparing of a second solution comprising a reducing agent in a second reaction vessel

[0088] (c) combining the second solution and the first solution, by adding the second solution to the first solution to form a reaction mixture;

[0089] (d) preparing a third solution comprising an acid in a third reaction vessel; and

[0090] (e) transferring the reaction mixture from the first reaction vessel to the acid solution in the third reaction vessel, optionally, wherein one or more steps of the method, including a single step, a plurality of steps, and all steps of the method, are carried out in an inert atmosphere.

[0091] 2. The method of paragraph 1, wherein the first solution comprises a base or basic reagent.

[0092] 3. The method of paragraph 1 or 2, wherein the base is sodium hydroxide (NaOH), optionally, wherein the NaOH has a molarity of the sodium hydroxide is in a range of 0.1M to 5.0M

[0093] 4. The method of any one of paragraphs 1-3, wherein the first reaction vessel containing the first solution is maintained at a temperature in a range of 23-65°C, or 35-60°C, or 40-50°C.

[0094] 5. The method of any one of paragraphs 1-4, wherein the reducing agent is sodium borohydride (NaBHzt), at a concentration in a range of about 5% to about 20%.

[0095] 6. The method of any one of paragraphs 1-5, wherein the second reaction vessel containing the second solution is maintained at a temperature in a range of about 23-65°C, or about 35- 60°C, or about 40-50°C

[0096] 7. The method of any one of paragraphs 1-6, wherein (a) the entire second solution is delivered to the first solution or (b) a first portion of the second solution is delivered to the first solution followed by a period (e.g., 1-120 minutes) in which the delivery is stopped, followed by the delivery of a second portion of the second solution to the first solution.

[0097] 8. The method of any one of paragraphs 1-7, wherein the addition of the second solution comprising the reducing agent to the first solution comprising the alpha- lipoic acid causes an exothermic reaction, optionally, the temperature of the first reaction vessel containing the first solution increases after the addition of the second solution, optionally, wherein, the temperature of the first reaction vessel is monitored during the addition step and the first reaction vessel is maintained at a temperature in the range of about 45-50°C

[0098] 9. The method of any one of paragraphs 1-8, wherein the method comprises a reaction period after the addition of the second solution to the first solution. In such embodiments, the reaction period is in the range of about 1 hour to about 30 hours,

[0099] 10. The method of any one of paragraphs 1-9, wherein the status of the reaction is monitored using one or more analytical techniques, optionally, where in the reaction technique is selected from the group consisting of nuclear magnetic resonance (NMR), high-performance liquid chromatography (HPLC), gas chromatography (GC), wherein any remaining, unreacted, alpha-lipoic acid can be determined, the method further comprising In such embodiments, if unreacted alpha-lipoic acid remains, the reaction period can be extended until the alpha-lipoic acid is no longer detected

[0100] 11. The method of any one of paragraphs 1-10, further comprising a workup procedure after the reaction has been completed or stopped, wherein the workup procedure comprises: (a) one or more steps, of cooling the first reaction vessel to a temperature in a range of about - 15°C to about 20°C and / or (b) transferring the reaction mixture from the first reaction vessel to the acid solution in the third reaction vessel, optionally, wherein the third reaction vessel is maintained at a temperature in the range of about 0°C to about 20°C, and / or (c) isolating or extracting the RDLA from the solution in the third reaction vessel.

[0101] 12. The method of any one of paragraphs 1-11, wherein the acid is hydrochloric acid or citric acid, optionally, wherein the molarity of the acid is in a range of about 0.1M to about 5.0M

[0102] 13. The method of any one of paragraphs 1-12, the method comprises a fourth reaction vessel comprising one or more organic solvents, wherein substantially all of the RDLA from the mixture dissolves in the organic layer due to solubility, optionally, wherein the organic solvent is dichloromethane, the solution from the third reaction vessel is transferred to the fourth reaction vessel, and wherein the fourth reaction vessel is a workup station configured to allow aqueous and organic solutions to separate into layers based on their respective densities.

[0103] 14. The method of any one of paragraphs 1-13, wherein the workup station comprises a valve at the bottom of the vessel that can be opened and closed for draining the solution, wherein, prior to draining the solution, the mixture is allowed to rest for a period of time to allow the mixture to separate into organic and aqueous layers, optionally, wherein the resting period is in the range of about 1 hour to about 30 hours, the method further comprising separating the aqueous layer from the organic layer in the fourth reaction vessel, transferring the organic layer to a container an then removing the organic layer to obtain RDLA present as an oil

[0104] 15. The method of claim any one of paragraphs 1-14, wherein the RDLA oil obtained from the workup procedure is sparged with an inert gas, for a period in the range of about 1 hour to about 120 hours, to remove any residual solvent and oxygen

[0105] 16. The method of claim any one of paragraphs 1-15, comprising filtering the RDLA oil isolated from the organic layer is filtered to remove any insoluble solids, wherein optionally, the method excludes a further purification step.

[0106] 17. The method of any one of paragraphs 1-16, wherein one or more steps of the method, including a single step, a plurality of steps, and all steps of the method, are carried out in an inert, optionally, wherein argon or nitrogen gas is forced into the reaction vessel or container and any air removed

[0107] 18. The method of any one of paragraphs 1-17, wherein the reaction vessel for the first solution, the second solution, or the third solution are sparged with an inert gas to remove any oxygen present in the vessel, optionally, wherein the reaction vessel is fitted with a gas inlet and a device that delivers the gas directly into the solution.

[0108] 19. The method of claim any one of paragraphs 1-18, wherein the first solution, the second solution, the third solution, or the fourth solution are sparged with an inert gas for a period of 1 hour to 30 hours.

[0109] 20. The method of claim any one of paragraphs 1-19, wherein the original shipping container for any components of the method, such as the solvents, are sparged with an inert gas to remove any oxygen present in the vessel, optionally, for a period of about 1 hour to about 30 hours.

[0110] This written description uses examples to disclose the present embodiments, including the best mode, and to enable any person skilled in the art to practice the present embodiments, including making and using any compositions or performing any methods. The patentable scope of the present embodiments is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims, or if they include equivalent elements with insubstantial differences from the literal language of the claims.

[0111] It is understood that the disclosed method and compositions are not limited to the particular methodology, protocols, and reagents described as these can vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention which will be limited only by the appended claims.

[0112] Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, many equivalents to the specific embodiments of the method and compositions described herein. Such equivalents are intended to be encompassed by the following claims.

Claims

CLAIMSWe claim:

1. A method of preparing (R)-dihydrolipoic acid (RDLA), the method comprising:(a) preparing a first solution comprising alpha-lipoic acid in a first reaction vessel;(b) preparing of a second solution comprising a reducing agent in a second reaction vessel(c) combining the second solution and the first solution, by adding the second solution to the first solution to form a reaction mixture;(d) preparing a third solution comprising an acid in a third reaction vessel; and(e) transferring the reaction mixture from the first reaction vessel to the acid solution in the third reaction vessel, optionally, wherein one or more steps of the method, including a single step, a plurality of steps, and all steps of the method, are carried out in an inert atmosphere.

2. The method of claim 1 , wherein the first solution comprises a base or basic reagent.

3. The method of claim 2, wherein the base is sodium hydroxide (NaOH), optionally, wherein the NaOH has a molarity of the sodium hydroxide is in a range of 0.1M to 5.0M4. The method of any one of claims 1, wherein the first reaction vessel containing the first solution is maintained at a temperature in a range of 23-65°C, or 35-60°C, or 40-50°C.

5. The method of claim 1, wherein the reducing agent is sodium borohydride (NaBH4), at a concentration in a range of 5% to 20%.

6. The method of claim 1 , wherein the second reaction vessel containing the second solution is maintained at a temperature in a range of 23-65°C, or 35-60°C, or 40-50°C7. The method of claim 1, wherein (a) the entire second solution is delivered to the first solution or (b) a first portion of the second solution is delivered to the first solution followed by a period (e.g., 1-120 minutes) in which the delivery is stopped, followed by the delivery of a second portion of the second solution to the first solution.

8. The method of claim 1, wherein the addition of the second solution comprising the reducing agent to the first solution comprising the alpha-lipoic acid causes an exothermic reaction, optionally, the temperature of the first reaction vessel containing the first solution increases after the addition of the second solution, optionally, wherein, the temperature of thefirst reaction vessel is monitored during the addition step and the first reaction vessel is maintained at a temperature in the range of 45-50°C9. The method of claim 1 , wherein the method comprises a reaction period after the addition of the second solution to the first solution. In such embodiments, the reaction period is in the range of 1 hour to 30 hours,10. The method of claim 9, wherein the status of the reaction is monitored using one or more analytical techniques, optionally, where in the reaction technique is selected from the group consisting of nuclear magnetic resonance (NMR), high-performance liquid chromatography (HPLC), gas chromatography (GC), wherein any remaining, unreacted, alpha-lipoic acid can be determined, the method further comprising In such embodiments, if unreacted alpha- lipoic acid remains, the reaction period can be extended until the alpha-lipoic acid is no longer detected11. The method of claim 1 , further comprising a workup procedure after the reaction has been completed or stopped, wherein the workup procedure comprises: (a) one or more steps, of cooling the first reaction vessel to a temperature in a range of -15°C to 20°C and / or (b) transferring the reaction mixture from the first reaction vessel to the acid solution in the third reaction vessel, optionally, wherein the third reaction vessel is maintained at a temperature in the range of 0°C to 20°C, and / or (c) isolating or extracting the RDLA from the solution in the third reaction vessel.

12. The method of claim 1, wherein the acid is hydrochloric acid or citric acid, optionally, wherein the molarity of the acid is in a range of 0.1 M to 5.0M13. The method of claim 11, the method comprises a fourth reaction vessel comprising one or more organic solvents, wherein substantially all of the RDLA from the mixture dissolves in the organic layer due to solubility, optionally, wherein the organic solvent is dichloromethane, the solution from the third reaction vessel is transferred to the fourth reaction vessel, and wherein the fourth reaction vessel is a workup station configured to allow aqueous and organic solutions to separate into layers based on their respective densities.

14. The method of claim 13, wherein the workup station comprises a valve at the bottom of the vessel that can be opened and closed for draining the solution, wherein, prior to draining the solution, the mixture is allowed to rest for a period of time to allow the mixture to separate into organic and aqueous layers, optionally, wherein the resting period is in the range of 1 hour to 30 hours, the method further comprising separating the aqueous layer fromthe organic layer in the fourth reaction vessel, transferring the organic layer to a container an then removing the organic layer to obtain RDLA present as an oil15. The method of claim 14, wherein the RDLA oil obtained from the workup procedure is sparged with an inert gas, for a period in the range of about 1 hour to about 120 hours, to remove any residual solvent and oxygen16. The method of claim 15, comprising filtering the RDLA oil isolated from the organic layer is filtered to remove any insoluble solids, wherein optionally, the method excludes a further purification step.

17. The method of claim 1, wherein one or more steps of the method, including a single step, a plurality of steps, and all steps of the method, are carried out in an inert, optionally, wherein argon or nitrogen gas is forced into the reaction vessel or container and any air removed18. The method of claim 1, wherein the reaction vessel for the first solution, the second solution, or the third solution are sparged with an inert gas to remove any oxygen present in the vessel, optionally, wherein the reaction vessel is fitted with a gas inlet and a device that delivers the gas directly into the solution.

19. The method of claim 18, wherein the first solution, the second solution, the third solution, or the fourth solution are sparged with an inert gas for a period of 1 hour to 30 hours.

20. The method of claim 1, wherein the original shipping container for any components of the method, such as the solvents, are sparged with an inert gas to remove any oxygen present in the vessel, optionally, for a period of 1 hour to 30 hours.

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