Process for the preparation of alpha-hydroxy esters by grignard coupling and thiolation

Abstract

The present disclosure provides processes for preparing alpha-hydroxy esters by adding a vinyl Grignard reagent to an oxalate and thiolizing the resulting double bond. Also provided are alpha-hydroxy esters and synthetic intermediates prepared according to the processes disclosed herein, as well as compositions comprising the alpha-hydroxy esters.

Description

[0001] Related applications

[0002] This application claims priority to International Application No. PCT / CN2019 / 120393, filed on November 22, 2019, which is incorporated herein by reference in its entirety for any purpose. Invention Field

[0003] This disclosure provides a process for preparing α-hydroxy esters by adding a vinyl Grignard reagent to an oxalate ester and then thiolated the resulting double bond. It also provides α-hydroxy esters and synthetic intermediates prepared according to the process disclosed herein, compositions comprising said α-hydroxy esters, and methods of using said compositions. Background Technology

[0004] α-hydroxy ester analogs of natural amino acids can be used as dietary supplements and in studies of enzymatic processes and protein function. The synthesis of these esters typically employs strong acids (such as H₂SO₄) or... Acid-catalyzed Fischer esterification of the corresponding acid and alcohol in the presence of cation exchange resins, acid-mediated hydrolysis of the corresponding nitriles in the presence of strong acids, or enzyme-mediated processes are all possible. However, acid-catalyzed pathways lead to degradation of starting materials and products, as well as product contamination by dimer and oligomer components. Such methods typically provide low yields and require complex purification techniques to separate the target compound from polymer byproducts. Enzymatic pathways require expensive and sensitive reagents and specific reaction conditions.

[0005] A particularly important α-hydroxy ester is isopropyl 2-hydroxy-4-(methylthio)butyrate (HMBi). HMBi is the isopropyl ester of 2-hydroxy-4-(methylthio)butyrate (HMBA), a hydroxy analog of methionine. HMBi is used to help supplement methionine in ruminants, including cows. Sufficient methionine levels in dairy cows help maintain the required levels of milk protein synthesis, and consequently, milk production. However, severe methionine deficiency in animal feed has become a major limiting factor in dairy cow diets. HMBi is a chemical derivative of methionine that diffuses easily and rapidly across the rumen wall, thus avoiding degradation by rumen microorganisms. Once HMBi passes through the rumen wall, it is metabolized in the liver and can be used for milk protein synthesis in dairy cows.

[0006] Additional processes are required to synthesize α-hydroxy esters (such as HMBi) using inexpensive, non-toxic reagents and mild reaction conditions, and to provide product esters in high yield and purity. Summary of the Invention

[0007] In one aspect, this disclosure relates to a method for preparing compounds of formula (I):

[0008]

[0009] in

[0010] R 1 It is C 1-4 Alkyl; and

[0011] R 2 It is C 1-8 Alkyl or C 4-7 cycloalkyl; and

[0012] R 3 and R 4 Each is independently selected from H, methyl, and ethyl;

[0013] The method includes using a compound of formula (IV):

[0014]

[0015] Coupling with a vinyl Grignard reagent of formula (A) in which X is Br or Cl:

[0016]

[0017] Compounds of formula (III) are formed:

[0018]

[0019] And to convert compounds of formula (III) into compounds of formula (I).

[0020] In another aspect, this disclosure relates to a method for preparing a compound of formula (I), which includes reducing a compound of formula (II) with a reducing agent:

[0021]

[0022] Compounds of formula (I) are formed.

[0023] In some respects, compounds of formula (I) are compounds of formula (IA):

[0024]

[0025] In another aspect, this disclosure relates to a method for preparing compounds of formula (IA):

[0026]

[0027] The method includes:

[0028] Oxalic acid was esterified with isopropanol to form diisopropyl oxalate;

[0029] Diisopropyl oxalate is coupled with vinyl magnesium bromide to form a compound of formula (III-A):

[0030]

[0031] The compound of formula (III-A) is thiolated with CH3SH to form a compound of formula (II-A):

[0032]

[0033] And compounds of the reduced form (II-A) to form compounds of the form (IA).

[0034] In another aspect, this disclosure relates to compounds of formula (I) or formula (IA) prepared according to any of the methods described herein.

[0035] On the other hand, this disclosure relates to isopropyl 2-oxobut-3-enoic acid.

[0036] In another aspect, this disclosure relates to animal feed compositions comprising compounds of formula (I) as described herein (IA). In some aspects, the animal feed is cow feed, such as dairy cow feed.

[0037] In another aspect, this disclosure relates to a method of supplying dairy cows with bioavailable methionine, comprising administering to the dairy cows a compound or animal feed composition of formula (I) or formula (IA) as described herein. In another aspect, this disclosure relates to a method of supplying dairy cows with at least about 50% bioavailable methionine, comprising administering to the dairy cows a compound or animal feed composition of formula (I) or formula (IA) as described herein. In another aspect, this disclosure relates to a method of improving milk obtained from dairy cows, comprising supplying dairy cows with a compound or animal feed composition of formula (I) or formula (IA) as described herein.

[0038] In another aspect, this disclosure relates to a method for improving the condition of cows, which includes supplying cows with compounds or animal feed compositions of formula (I) or formula (IA) as described herein. Attached Figure Description

[0039] Figure 1A It is diisopropyl oxalate. 13 C NMR spectra, as described in Example 1.

[0040] Figure 1B It is diisopropyl oxalate. 1 H NMR spectra, as described in Example 1.

[0041] Figure 2A It is isopropyl 2-oxo-4-methylthiobutyrate. 13C NMR spectra, as described in Example 3.

[0042] Figure 2B It is isopropyl 2-oxo-4-methylthiobutyrate. 1 H NMR spectra, as described in Example 3.

[0043] Figure 3A It is isopropyl 2-hydroxy-4-methylthiobutyrate (HMBi) 13 C NMR spectra, as described in Example 5.

[0044] Figure 3B It is isopropyl 2-hydroxy-4-methylthiobutyrate (HMBi) 1 H NMR spectra, as described in Example 5. Detailed Implementation

[0045] Unless otherwise stated, the terminology used in this disclosure has its simple and common meaning as understood by one of skill in the art. The following terms, as used in the specification and claims, are defined for the purposes of this disclosure and have the following meanings.

[0046] As used herein, the terms “isopropyl 2-hydroxy-4-(methylthio)butyrate,” “HMBi,” and “isopropyl 2-hydroxy-4-(methylthio)butyrate” refer to esters having the following structure (where R… 1 It is methyl and R 2 It is a compound of formula (I) of isopropyl, shown below as formula (IA).

[0047]

[0048] As used herein, the terms “2-hydroxy-4-(methylthio)butyrate,” “2-hydroxy-4-(methylthio)butyric acid,” and “HMBA” refer to compounds having the following structures.

[0049]

[0050] The compounds described herein may exist in racemic form, as a single enantiomer, or as a mixture of enantiomers. Thus, for example, HMBi refers to racemic HMBi (or "DL-HMBi"), or D-HMBi or L-HMBi, or a mixture thereof.

[0051] The compounds described herein may also exist in salt form. The chemical formulas shown herein should be understood to include the structures shown and their salt forms. For example, when a compound contains a carboxylic acid, the formula also covers the salt form of its conjugate base (carboxylate), such as sodium, potassium, magnesium, or calcium salts. When a compound contains an indole or imidazole group, the formula also covers the salt of its conjugate acid, such as HCl salts.

[0052] "alkyl" means a linear saturated monovalent hydrocarbon group with 1 to 8 carbon atoms (e.g., 1 to 6 carbon atoms, 1 to 4 carbon atoms, or 1 to 3 carbon atoms), or a branched saturated monovalent hydrocarbon group with 3 to 8 carbon atoms (e.g., 3 to 6 carbon atoms, 3 to 4 carbon atoms, or 3 carbon atoms), such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl (including all isomers), etc.

[0053] "Cycloalkyl" refers to a cyclic saturated monovalent hydrocarbon group with 3 to 10 carbon atoms, such as cyclopropyl, cyclobutyl, cyclopentyl, or cyclohexyl.

[0054] "Optional" or "optionally" means that the event or situation described below may but not necessarily occur, and the description includes examples of the event or situation occurring as well as examples of the event or situation not occurring. For example, an alkyl group "optionally substituted with -OH" means that -OH may but not necessarily be present, and the description includes cases where the alkyl group is substituted with -OH and cases where the alkyl group is not substituted with -OH.

[0055] The term "reaction solvent" refers to an organic liquid used to carry dissolved reactants. In some embodiments, one of the reaction reagents is used as both a reagent and a reaction solvent. In other embodiments, the reagent is diluted in a different reaction solvent.

[0056] The term "acid catalyst" refers to an acid added to a reaction in a substoichiometric amount to catalyze the reaction. Acid catalysts can be Brønsted acids (such as acids with a pKa less than 7, such as HCl, H₂SO₄, KHSO₄, acetic acid, etc.) or Lewis acids (such as boric acid). In some embodiments, the acid is generated in situ, for example, by reacting acetyl chloride or TMSCl with water or alcohol.

[0057] The term "concentration" refers to the amount of solute in a solvent. In this document, concentration may be described as weight % or molar concentration (M) or equivalent concentration (N).

[0058] The term “heptane” or “n-heptane” refers to pure n-heptane or n-heptane in a mixture with other C7 isomers (e.g., at least 90% n-heptane and at least 95% total C7 isomers).

[0059] The term "reflux temperature" or "reflux" refers to the temperature at which the reaction solvent boils; typically, a condenser is used to cool solvent vapors and condense them back into the reaction vessel. The precise temperature at which a given solvent reaches reflux can vary due to environmental factors.

[0060] The term "about" refers to a numerical value, including, for example, integers, fractions, and percentages, whether explicitly indicated or not. The term "about" generally refers to a range of numerical values ​​(e.g., ±5-10% of the listed range) that a person skilled in the art would consider equivalent to the listed values ​​(e.g., having the same function or result). When terms such as "at least" and "about" precede a list of numerical values ​​or ranges, the term modifies all values ​​or ranges provided in the list. In some cases, the term "about" may include rounded numerical values.

[0061] The terms “extraction,” “extraction,” or “extracting” refer to the process of distributing material between an organic phase and an aqueous phase. In some respects, extraction is performed on a reaction mixture or a concentrated residue of the reaction mixture. The “extract” is the organic phase separated from the aqueous phase. As used herein, extraction does not cover purification methods performed on crude reaction products, such as simple distillation, vacuum distillation, azeotropic distillation, fractional distillation, continuous distillation, rapid chromatography, HPLC, or recrystallization.

[0062] As used herein, “purification” or “purifying” refers to the method of separating reaction products after a reaction has been completed. Purification methods include simple distillation, vacuum distillation, azeotropic distillation, fractional distillation, continuous distillation, rapid chromatography, HPLC, or recrystallization.

[0063] The term “substantially,” such as “substantially in monomeric form,” refers to the purity of the compound of formula (I) relative to its dimer and / or oligomeric analogues.

[0064] As used herein, the term "dimer" or "dimeric compound" refers to a compound in which two molecules of a given monomer structure, or one molecule of two different monomer structures, are condensed into a single molecule. As used herein, the term "oligomer" or "oligomeric compound" refers to a compound in which more than two molecules of a given monomer structure, or more than two molecules of at least two different monomer structures, are condensed into a single polymeric structure. HMBi can form homooligomers or heterooligomers (containing at least one HMBi monomer unit).

[0065] The term "purity" or expression of a percentage of a compound (e.g., x%HMBi) refers to the purity of a compound in a sample, as determined by weight, by GC analysis, and / or by HPLC analysis. In some respects, purity by weight is determined by GC or HPLC analysis using UV detection.

[0066] The term "purity by weight" refers to the purity of a compound in a sample relative to the purity of other components in the sample, where the ratio of the mass of the compound to the mass of the sample is expressed as a percentage.

[0067] The term "purity" in gas chromatography (GC) or HPLC refers to the purity (expressed as a percentage) calculated from the peak area of ​​the compound of interest relative to the sum of the peak areas in the chromatogram. In some respects, purity is determined by HPLC using UV detection.

[0068] In some respects, purity is the purity required by the marketing regulations for regulated products. In the case of, for example, HMBi, the compound contains 0.5% or less water (e.g., as determined by Karl Fischer analysis). (See Commission Implementing Regulation (EU) No. 469 / 2013 of 22 May 2013.)

[0069] The terms “crude product”, “crude derivative”, and “crude compound” refer to a sample of a compound obtained from the reaction mixture after concentrating the reaction mixture and / or extracting the reaction mixture into an organic solvent and concentrating the organic extract.

[0070] The term "animal feed composition" refers to a product suitable for use in animal nutrition. In some respects, an animal feed composition is animal feed (e.g., food or drinking water containing supplements), and in other respects, an animal feed composition is a feed additive. Feed additives are suitable for mixing with animal feed or with drinking water.

[0071] The term "carrier" refers to a suitable carrier for use in animal feed additives. Suitable carriers include water (for liquid or solid feed additives) or silica (for solid feed additives). In some aspects, the carrier is silica (silica). In some aspects, the feed additive comprises the compound and silica in a ratio of 3:2.

[0072] In some respects, animal feed includes pelleted, protein-rich feeds supplemented with 2.5% or 1% HMBi (e.g., based on peanut, rapeseed meal, and / or soybean meal). In some respects, animal feed includes approximately 45% and approximately 50% of grains (corn, barley, wheat, and / or wheat by-products) supplemented with 0.5% or 3.0% HMBi. In some respects, animal feed includes molasses-containing mashed feeds or pelleted feeds, each supplemented with 2.5% or 1% HMBi.

[0073] The term "application" refers to the provision of a supplement to a target animal. Application can be oral, such as by ingesting food or drinking water containing the compound, or by injection or other methods of administration.

[0074] As used herein, “improved milk” means that the quality and / or quantity of milk produced by a treated cow or a group of treated cows is improved compared to milk produced by an untreated counterpart. Improvements in milk include, for example, increased protein content (e.g., increased α, β, and / or κ proteins), increased fat content, and / or increased volume of milk produced.

[0075] As used herein, “improved condition of cows” means that the health indicators of treated cows or a group of treated cows are improved compared to the health indicators of their untreated counterparts. Improved condition of cows can refer to, for example, an increase in certain characteristics relative to untreated animals; for example, weight gain.

[0076] As used herein, improvements in fertility include, for example, shortening the interval between calving and reproduction and / or increasing the percentage of fertilization during insemination.

[0077] As used in this article, “improvement in liver function” includes, for example, a reduction in metabolic problems, an improvement in very low-density lipoprotein levels, a reduction in blood ketosis, and / or a reduction in the incidence of hepatic steatosis.

[0078] As used in this article, "increased energy" means, for example, that the fermentation process in the rumen is stimulated, leading to an increase in digestible organic matter, thereby providing more energy for the animal.

[0079] Synthesis process

[0080] This disclosure relates to a method for preparing a compound of formula (I) or formula (IA) and / or an intermediate by one or more of the following reactions: a) using R 2 -OH esterification of oxaloyl chloride or oxalic acid to form oxalate diester; b) coupling the oxalate diester with an alkenyl Grignard reagent to form an alkenyl-substituted α-keto ester (2-oxobutyrate); c) thiolation of the alkenyl-substituted α-keto ester to form 4-alkylthio-2-oxobutyrate; and d) reduction of 4-alkylthio-2-oxobutyrate to form a compound of formula (I) or (IA). Oxalic acid may be used, for example, in the form of oxalic acid or oxalate dihydrate.

[0081] In some embodiments, this disclosure relates to methods for preparing compounds of formula (I):

[0082]

[0083] in

[0084] R 1 It is C 1-4 Alkyl; and

[0085] R 2 It is C1-8 Alkyl or C 4-7 cycloalkyl; and

[0086] R 3 and R 4 Each is independently selected from H, methyl, and ethyl;

[0087] The method includes using a compound of formula (IV):

[0088]

[0089] Coupling with a vinyl Grignard reagent of formula (A) in which X is Br or Cl:

[0090]

[0091] Compounds of formula (III) are formed:

[0092]

[0093] And to convert compounds of formula (III) into compounds of formula (I).

[0094] In some embodiments, this disclosure relates to a method for preparing a compound of formula (III), which includes coupling a compound of formula (IV) with a vinyl Grignard reagent of formula (A).

[0095] In some implementation schemes, R 1 It is a methyl group.

[0096] In some implementations, each R 2 Selected from methyl, ethyl, and isopropyl. In some embodiments, each R 2 It is isopropyl.

[0097] In some implementation schemes, R 3 and R 4 Each is represented by H.

[0098] In some embodiments, the compound of formula (I) is the same as the compound of formula (IA):

[0099]

[0100] In some embodiments, the compound of formula (III) is a compound of formula (III-A):

[0101]

[0102] In some embodiments, the vinyl Grignard reagent of formula (A) is vinyl-MgCl. In some embodiments, X is Cl. In some embodiments, Grignard coupling is performed in the presence of a salt additive such as LiCl or ZnCl2. In some embodiments, the salt additive is LiCl.

[0103] In some embodiments, coupling includes mixing a compound of formula (I) with a vinyl Grignard reagent of formula (A) in amounts ranging from about 0.8 to about 2.0, or from about 1.0 to about 1.75, or from about 1.0 to about 1.5, or from about 1.2 to about 1.75, or from about 1.4 to about 1.6, or about 1.5 molar equivalents.

[0104] In some embodiments, coupling is performed within a temperature range of about -80°C to about 10°C, or from about -80°C to about -70°C, or from about -50°C to about 10°C, or from about -40°C to about 5°C, or from about -50°C to about -20°C, or from about -30°C to about -20°C, or at a temperature of about -78°C, or about -20°C, or about 0°C. In some embodiments, coupling comprises mixing a compound of formula (I) in MTBE with about 1.5 molar equivalents of a vinyl Grignard reagent of formula (A) at a temperature of about -50°C to about -20°C, or from about -30°C to about -20°C. In some embodiments, the vinyl Grignard reagent is added slowly and / or in portions to the compound of formula (IV).

[0105] In some embodiments, the coupling is performed in an aprotic solvent. In some embodiments, the aprotic solvent is an ether such as MTBE, THF, or Et₂O, optionally mixed with a nonpolar solvent such as heptane or hexane. In some embodiments, the aprotic solvent is MTBE or THF, optionally mixed with heptane. In some embodiments, the coupling reaction concentration is from about 0.25 M to about 1.3 M (moles of the compound of formula (IV) per liter of reaction solvent), or from about 0.4 M to about 1.1 M, or from about 0.4 M to about 0.5 M, or from about 0.9 M to about 1.0 M, or about 0.5 M, or about 1 M.

[0106] In some embodiments, coupling produces a mixture of compounds of formula (III) and formula (III-Z):

[0107]

[0108] The ratio of (III):(III-Z) in the mixture is at least 5:1, or at least 6:1, or at least 7:1, or at least 8:1, or at least 9:1, or at least 10:1, or at least 15:1, or at least 20:1.

[0109] In some embodiments, converting a compound of formula (III) into a compound of formula (I) includes:

[0110] The compound of formula (III) is used with M in it. + Thiolization is the process of using a thiolation reagent of formula (B) or formula (C) with a metal cation:

[0111] R 1 -SH(B)R 1 -S - M + (C),

[0112] Compounds of formula (II) are formed:

[0113]

[0114] And compounds of formula (II) to form compounds of formula (I).

[0115] In some embodiments, this disclosure relates to a method for preparing a compound of formula (II), the method comprising thiolation of a compound of formula (III) with a thiolation agent of formula (B) or formula (C).

[0116] In some embodiments, thiolation is performed with a reagent of formula (B) in the presence of an additive. In some embodiments, the additive is an amine base such as triethylamine, diethylamine, pentylamine, or hexylamine; a phosphine such as dimethylphenylphosphine (DMPP) or tris(2-carboxyethyl)phosphine (TCEP); a basic salt such as NaHCO3 or Na2CO3; a Lewis acid such as scandium(III) trifluoromethanesulfonate or anhydrous cerium(III) chloride; or an N-heterocyclic carbene (NHC) complex (e.g., Au-NHC complex). In some embodiments, the additive is triethylamine.

[0117] In some embodiments, the method further comprises generating a thiolating agent of formula (B) from a thiolating agent of formula (C). In some embodiments, the generation is carried out in the presence of an acid catalyst. In some embodiments, the acid catalyst is acetic acid, p-toluenesulfonic acid, or H₂SO₄. In some embodiments, the thiolation is carried out at a temperature ranging from about -40°C to about 10°C, or from about -35°C to about 5°C, or from about -30°C to about -20°C, or at about 0°C.

[0118] In some embodiments, the thiolizing agent is of formula (C), and the thiolization is performed at a temperature ranging from about -80°C to about 35°C, or from about 15°C to about 35°C.

[0119] In some implementation schemes, M + Yes + or K +.

[0120] In some embodiments, coupling includes extracting a compound of formula (III) into an organic solvent to form an extract of formula (III), and thiolation includes adding a thiolation reagent to the extract of formula (III). In this way, the thiolation reaction is performed without purifying the intermediate of formula (III) from the coupling reaction prior to the thiolation reaction. In some embodiments, the procedure is as follows.

[0121]

[0122] In some embodiments, the reduction of the compound of formula (II) is performed in the presence of a reducing agent selected from NaBH4, LiBH4, and Al(O-iPr)3 / iPrOH. In some embodiments, the reducing agent is NaBH4. In some embodiments, thiolation comprises extracting the compound of formula (II) into an organic solvent to form an extract of formula (II), and reduction comprises adding a reducing agent to the extract of formula (II). In this way, reduction is performed without purifying the compound of formula (II) prior to reduction. In some embodiments, coupling comprises extracting the compound of formula (III) into an organic solvent to form an extract of formula (III), thiolation comprises adding a thiolation agent to the extract of formula (III), and extracting the compound of formula (II) into an organic solvent to form an extract of formula (II), and reduction comprises adding a reducing agent to the extract of formula (II). In this way, coupling, thiolation, and reduction are performed without purifying the intermediates of formula (II) and formula (III), as illustrated in the following embodiments.

[0123]

[0124] In some implementations, the reduction is performed using NaBH4 or LiBH4 under the following conditions:

[0125] (a) in an alcohol solvent such as methanol, ethanol or isopropanol; and / or

[0126] (b) Use a reducing agent in amounts of about 0.25 to about 1.0 molar equivalents; and / or

[0127] (c) At temperatures ranging from about -10°C to about 30°C or at about 0°C.

[0128] In some implementations, the reduction is performed using Al(O-iPr)3 / iPrOH at a temperature ranging from about 50°C to about 90°C or at about 80°C.

[0129] In some implementations, the method further includes using R 2-OH esterification of oxaloyl chloride to form a compound of formula (IV). In some embodiments, the esterification is performed in the presence of at least one amine base such as N,N-dimethylpyridine, pyridine, or triethylamine. In some embodiments, the esterification is performed at a temperature ranging from about -5°C to about 30°C.

[0130] In some embodiments, the method further includes using R in the presence of an acid catalyst and an optional desiccant such as an azeotropic water removal agent, a molecular sieve, or a combination thereof. 2 -OH groups esterify oxalic acid to form a compound of formula (IV). In some embodiments, the acid catalyst is selected from p-TsOH; H2SO4; and other similar catalysts. -15、 The catalyst may be a macroporous sulfonic acid resin of M32; aluminosilicate; phosphoric acid; boric acid; acetyl chloride; and acids with a pKa less than 3. In some embodiments, the acid catalyst is p-TsOH or H2SO4. In some embodiments, the acid catalyst is about 0.01 to about 0.1 molar equivalents, or about 0.025 to about 0.05 molar equivalents of p-TsOH, or about 1 to about 3 molar equivalents, or about 2 molar equivalents of H2SO4. In some embodiments, esterification is performed at the reflux temperature of the reaction solvent. In some embodiments, esterification is performed in a reaction solvent selected from toluene, CHCl3, and isopropanol.

[0131] In some embodiments, this disclosure relates to methods for preparing compounds of formula (I):

[0132]

[0133] in

[0134] R 1 It is C 1-4 Alkyl; and

[0135] R 2 It is C 1-8 Alkyl or C 4-7 cycloalkyl; and

[0136] R 3 and R 4 Each is independently selected from H, methyl, and ethyl;

[0137] The method includes reducing the compound of formula (II) with a reducing agent:

[0138]

[0139] To form a compound of formula (I). In some embodiments, the compound of formula (I) is a compound of formula (IA). In some embodiments, the compound of formula (II) is a compound of formula (II-A):

[0140]

[0141] In some embodiments, the reducing compound (II) is carried out in the presence of a reducing agent selected from NaBH4, LiBH4, and Al(O-iPr)3 / iPrOH. In some embodiments, the reducing agent is NaBH4.

[0142] In some implementations, the reduction is performed using NaBH4 or LiBH4 under the following conditions:

[0143] (a) in an alcohol solvent such as methanol, ethanol or isopropanol; and / or

[0144] (b) Use a reducing agent in amounts of about 0.25 to about 1.0 molar equivalents; and / or

[0145] (c) At temperatures ranging from about -10°C to about 30°C or at about 0°C.

[0146] In some implementations, the reduction is performed using Al(O-iPr)3 / iPrOH at a temperature ranging from about 50°C to about 90°C or at about 80°C.

[0147] In some embodiments, the method further includes wherein R 3 and R 4 Compounds of formula (III) selected independently of H, methyl, and ethyl:

[0148]

[0149] Use M among them + Thiolization is the process of using a thiolation reagent of formula (B) or formula (C) with a metal cation:

[0150] R 1 -SH(B)R 1 -S - M + (C),

[0151] To form a compound of formula (II). In some embodiments, the compound of formula (II) is a compound of formula (II-A), and the compound of formula (III) is a compound of formula (III-A):

[0152]

[0153] In some embodiments, this disclosure relates to methods for preparing compounds of formula (IA):

[0154]

[0155] The method includes:

[0156] Oxalic acid was esterified with isopropanol to form diisopropyl oxalate;

[0157] Diisopropyl oxalate is coupled with vinyl magnesium bromide to form a compound of formula (III-A):

[0158]

[0159] The compound of formula (III-A) is thiolated with CH3SH to form a compound of formula (II-A):

[0160]

[0161] And compounds of the reduced form (II-A) to form compounds of the form (IA).

[0162] In some embodiments, the methods described herein provide compounds of formula (I) or formula (IA) with a purity of at least about 95% by GC, HPLC, and / or by weight. In some embodiments, the methods provide crude compounds of formula (I) or formula (IA) with a purity of at least about 95%, or at least about 96%, or at least about 97%, or at least about 98% by weight, GC, and / or HPLC, wherein the crude compounds of formula (I) or formula (IA) have not been purified or have only been purified by fractionation. In some embodiments, the method provides crude compounds of formula (I) or formula (IA) that are substantially in monomeric form, or that contain less than about 5% by weight, or less than about 3% by weight, of dimer and / or oligomer compounds, wherein the crude compounds of formula (I) or formula (IA) have not been purified or have only been purified by fractionation.

[0163] compound products

[0164] In some embodiments, the reaction provides a crude compound of formula (I) or formula (IA) having a purity of at least about 80%, or at least about 90%, or at least about 95%, or at least about 96%, or at least about 97%, or at least about 98% by weight (and / or by GC or HPLC), wherein the crude compound of formula (I) or formula (IA) has not been purified or has only been purified by fractionation. In some embodiments, the reaction provides a crude compound of formula (I) or formula (IA) that is substantially in monomeric form, or that contains less than 5% by weight or less than 3% by weight of dimer and / or oligomer compounds, wherein the crude compound of formula (I) or formula (IA) has not been purified or has only been purified by fractionation.

[0165] In some embodiments, this disclosure relates to compounds of formula (I) or formula (IA) prepared according to the methods described herein. In some embodiments, this disclosure relates to compounds of formula (I) or formula (IA) wherein the purity of said compound, by weight (and / or by GC or HPLC), is at least about 80%, or at least about 90%, at least about 95%, or at least about 96%, or at least about 97%, or at least about 98%, and said compound has not been purified or has only been purified by fractionation. In some embodiments, said compound is substantially in monomeric form or mixed with less than about 5% by weight or less than about 3% by weight of dimer and / or oligomer compounds.

[0166] In some implementations, the HMBi (Formula (IA)) product has one or more of the following performance specifications: (a) the HMBi monomer content and chemical purity by weight or by HPLC analysis is at least about 95%; (b) the water content by Karl Fischer analysis is less than about 0.5%; and (c) the pH value is less than about 6.0 (measured at a concentration of 1% in water).

[0167] This document also discloses compounds of formula (I) or formula (IA) prepared according to any of the methods described herein. In some embodiments, the compound is of formula (I) or formula (IA), wherein the compound has a purity of at least about 95%, or at least about 96%, or at least about 97%, or at least about 98% by weight (and / or by GC or HPLC), and the compound is a crude compound that has not been purified and / or has only been purified by fractionation. In some embodiments, the compound is of formula (I), wherein R... 1 It is -CH2CH2-S-CH3, and R 2It is isopropyl, or the compound is a compound of formula (IA). In some embodiments, the compound is substantially in monomeric form, or mixed with less than about 5% by weight or less than about 3% by weight of a dimer and / or oligomer compound.

[0168] Animal feed compositions and uses

[0169] In some aspects, this disclosure relates to animal feed compositions comprising compounds of formula (I) or formula (IA) as described herein. In some embodiments, the animal feed composition is suitable for administration to ruminants such as cattle, cows, sheep, antelopes, deer, giraffes, bovids (e.g., bison, buffalo, or yaks), goats, and / or gazelles. In some embodiments, the animal feed composition is a cow feed composition (such as a dairy cow feed composition) or an additive for cow feed (such as dairy cow feed). In some embodiments, the animal feed composition is a dairy cow feed composition.

[0170] In some embodiments, the animal feed composition is animal feed or an animal feed additive. In some embodiments, the animal feed additive is in liquid or solid form, wherein the liquid form comprises the compound and optionally a liquid carrier, while the solid form comprises the compound mixed with a solid carrier, optionally wherein the solid carrier is silica (silica), and optionally wherein the ratio of the compound to the solid carrier is from about 5:1 to about 1:5 or 3:2. In some embodiments, the feed composition is a liquid feed additive or a solid feed additive. In some embodiments, the animal feed composition is a drinking water additive. In some embodiments, the liquid feed additive or drinking water additive has a pH value ranging from about 4.0 to about 7.5.

[0171] In some embodiments of the animal feed composition, R 1 It is -CH2CH2-S-CH3, and R 2 It is isopropyl. In some embodiments, the compound is a compound of formula (IA).

[0172] In some embodiments, this disclosure relates to a method of supplying dairy cows with bioavailable methionine, comprising administering to the dairy cows a compound or animal feed composition described herein. In some embodiments, administration comprises feeding the dairy cows a feed composition containing said compound. In some embodiments, this disclosure relates to a method of supplying dairy cows with at least about 50% bioavailable methionine, comprising administering to the dairy cows a compound or animal feed composition as described herein. In some embodiments, this disclosure relates to a method of improving milk obtained from dairy cows, comprising supplying the dairy cows with a compound or animal feed composition as described herein. In some embodiments, the improvement of milk comprises an increase in the protein content of the milk. In some embodiments, the improvement of milk comprises an increase in the fat content of the milk. In some embodiments, this disclosure relates to a method of improving the condition of cows, comprising supplying the cows with a compound or animal feed composition as described herein. In some embodiments, the improvement of the condition of cows comprises an improvement in fertility. In some embodiments, the improvement of the condition of cows comprises an improvement in liver function. In some embodiments, the improvement of the condition of cows comprises an increase in energy.

[0173] In some respects, any of the reactions described herein can be performed using a continuous flow apparatus.

[0174] Example

[0175] Equipment. All millimolecular-scale experiments were conducted using 100 mL or 250 mL three-necked round-bottom flasks equipped with magnetic stir bars, dropping funnels, and thermometers. The reaction flasks were equipped with condensers and thermometers to monitor the reaction temperature. For reactions run under reflux, the reaction mixture was heated using a silicone oil bath. For experiments conducted at temperatures below room temperature, liquid nitrogen baths or salt / ice mixture baths were used. All kilogram-scale experiments were conducted using 5 L jacketed reactors. Concentration and / or purification of intermediates and crude products were performed using laboratory-scale vacuum distillation apparatus, rotary evaporators, or column chromatography, or as otherwise described in the examples below.

[0176] Example 1. Synthesis of diisopropyl oxalate from oxalic acid

[0177]

[0178] In a 5L laboratory reactor, oxalic acid (1 kg, 11.1 mol) was added to isopropanol (1700 mL) with stirring. A clear solution was formed. Subsequently, p-toluenesulfonic acid monohydrate (47.67 g, 2.5 mol%) in 200 mL of toluene was slowly added to the solution. The reaction mixture was heated and stirred under reflux for 24 h. The water generated was continuously removed by azeotropic extraction using a Dean-Stark separator to drive the reaction to completion. The reaction mixture was cooled, neutralized with 500 mL of saturated NaHCO3 aqueous solution, and partitioned between 400 mL (2x) toluene and 1 L (2x) water. The combined organic phases were washed with 1 L of saturated NaCl aqueous solution. The organic phases were separated and the solvent was removed under vacuum. The crude material was purified by heated distillation under high vacuum to give 1740 g (90%) of diisopropyl oxalate as a colorless oil. 13 C NMR(100MHz,CDCl3)δ(ppm)157.96,71.44,21.63( Figure 1A ); 1 H NMR (400MHz, CDCl3) δ5.13 (hept, J=6.3Hz, 2H), 1.33 (d, J=6.2Hz, 12H) ( Figure 1B ).

[0179] Various other suitable reaction conditions were investigated using oxalic acid dihydrate (items 1-5) or oxalic acid (items 6-7) as starting materials, as shown in Table 1. Add to the reaction mixture... Molecular sieves (1-2 g per 5 g of starting material) are used to remove additional water during the reaction. Post-treatment includes diluting the reaction mixture with ethyl acetate, neutralizing to pH 7 with a saturated aqueous NaHCO3 solution, separating the layers, washing the organic extract with a saturated aqueous NaHCO3 solution and a saturated aqueous NaCl solution, and concentrating to provide a crude residue.

[0180] Table 1.

[0181]

[0182]

[0183] Example 2. Synthesis of diisopropyl oxalate from oxalyl chloride

[0184]

[0185] Oxaloyl chloride (1019 g) was slowly added in portions to a 3 L sample of isopropanol at 0 °C in a 5 L glass-lined laboratory reactor while stirring, maintaining the temperature between 0 and 5 °C. After the addition was complete, the reaction mixture was gradually heated to room temperature and stirred for 12 hours. The mixture was concentrated by rotary evaporation and high vacuum to obtain a crude product. The crude product was diluted with dichloromethane (1000 mL) and then with a saturated aqueous solution of NaHCO3 (3 × 500 mL) to provide organic extracts. The first two washings were back-extracted with dichloromethane (1 L each) to obtain two more organic extracts. These three organic extracts were dried over a saturated aqueous solution of NaCl (3 × 500 mL), combined, concentrated, and purified by distillation to obtain diisopropyl oxalate in 86% yield. 1 H NMR (400MHz, CDCl3) δ5.13 (hept, J=6.3Hz, 2H), 1.33 (d, J=6.2Hz, 12H).

[0186] Various other suitable reaction conditions were investigated, as shown in Table 2.

[0187] Table 2.

[0188]

[0189] Example 3.2 Synthesis of isopropyl oxo-4-methylthiobutyrate (small-scale experiment)

[0190]

[0191] Step 1, Grignard reaction. A mixture of diisopropyl oxalate (1.4 g, 8 mmol, 1.0 equivalent), 16 mL of solvent (MTBE, MTBE / heptane mixture, or THF) and 2 equivalents of LiCl (0.68 g, 16 mmol when used) was cooled to the test temperature in a liquid nitrogen bath or salt bath (as shown in Table 3). A vinyl magnesium chloride solution (1.6 M in THF) was slowly added, and the mixture was stirred until the starting material was exhausted (see Table 3). The reactants were quenched by washing with saturated NH4Cl (2 × 100 mL). The product was extracted with EtOAc (2 × 100 mL), dried over Na2SO4, and filtered. The yield of isopropyl 2-oxo-3-butenoate was determined by GC / MS. The extract was used directly in the next step without purification.

[0192] Table 3.

[0193]

[0194] *In this embodiment, diisopropyl oxalate is added to the Grignard reagent.

[0195] ND = Not measured

[0196] Step 2, thiolization reaction.

[0197] Procedure 1: CH3SH gas was generated by treating 20% ​​w / v of CH3SNa in water with an acid catalyst (AcOH (12 mmol) or TsOH (12 mmol)) at -30 to -20 °C, or with H2SO4 (2 equivalents relative to CH3SNa) at 50 °C for 15 to 30 minutes, as shown in Table 4. The resulting CH3SH was bubbled into a stirred MTBE solution containing 0.1 mL of triethylamine at 0 °C (20 mL). The obtained CH3SH in MTBE was added to the crude product in MTBE from step 1, Table 3, entry 14, at 0 °C or -30 to -20 °C, and the reaction mixture was stirred for 15 to 30 minutes, as shown in Table 4. The reactants were quenched with 2 M HCl, extracted with ethyl acetate, dried (Na2SO4), filtered, and concentrated. The crude material was then used in the next reaction step.

[0198] In entries 1-5 of Table 4, acetic acid or p-toluenesulfonic acid was used to generate CH3SH gas. In entries 1-3, the yields are the separation yields after column chromatography. In entries 4-5, the yields are the separation yields after product distillation. In entries 6-9, CH3SH gas was generated by heating 20% ​​CH3SNa and H2SO4 in water at 50°C.

[0199] Table 4.

[0200] entry acid Temperature; reaction time Yield (2 steps) 1 p-Toluenesulfonic acid 0℃; 30min 27％ 2 p-Toluenesulfonic acid -30 to -20°C; 15 min 46％ 3 Acetic acid -30 to -20°C; 15 min 38％ 4 p-Toluenesulfonic acid -30 to -20°C; 15 min 49％ 5 Acetic acid -30 to -20°C; 15 min 44％ 6 <![CDATA[H2SO4]]> -30 to -20°C; 15 min 20％ 7 <![CDATA[H2SO4]]> -30 to -20°C; 15 min 19％ 8 <![CDATA[H2SO4]]> -30 to -20°C; 15 min 54％ 9 <![CDATA[H2SO4]]> -30 to -20°C; 15 min 63％

[0201] Procedure 2: Add 20% w / v CH3SNa aqueous solution (1 equivalent) and H2SO4 (2 equivalents) to the crude product from step 1, Table 3, entry 11, in THF at -78°C. Allow the reaction mixture to warm to room temperature and stir for 16 h. Quench the reaction mixture with 2M HCl, extract with ethyl acetate (2 × 50 mL), dry (Na2SO4), filter, and concentrate. Then, use the crude product for the next reaction step. Separate the product to obtain the final product in 33% yield. 1 H NMR (400MHz, CDCl3) δ5.12(hept,J=6.2Hz,1H),3.13(t,J=7.2Hz,2H),2.76(t,J=7.2Hz,2H),2.11(s,3H),1.33(d,J=6.3Hz,6H).

[0202] Procedure 3, Continuous Flow Reactor: Alternatively, a mixture of isopropyl 2-oxo-3-butenoate and 10 mL of triethylamine is pumped into the reactor at a controlled flow rate. The outlet is further connected to the inlet of a Y-type mixer, another inlet of which is injected (in a controlled flow manner) with MeSH gas. The two components are then mixed and further stirred in a batch reactor while the reaction temperature is maintained at 0°C. Once GC monitoring indicates that the reaction is complete, 1 N HCl is added and the mixture is post-treated as described above.

[0203] Example 4: Synthesis of isopropyl 2-oxo-4-methylthiobutyrate (kilogram-scale synthesis)

[0204] Step 1, Grignard reaction. In a 20 L laboratory reactor, vinyl magnesium chloride (1.7 kg, 10 mol) in anhydrous MTBE (3.4 L) was added dropwise over 1 h with stirring at -30 to -20 °C. Once gas chromatography showed that the vinyl addition was complete, the reaction mixture was quenched at room temperature by adding 1 L of saturated NH4Cl aqueous solution. The organic phase was separated and washed with 500 mL of water, and the aqueous phase was back-extracted with 400 mL of MTBE. The MTBE extracts were combined to give isopropyl 2-oxo-3-butenoate with a conversion greater than 90%, which was used directly in the next step without further purification or distillation.

[0205] Step 2, Thiolization. The MTBE extract from Step 1 was cooled to 0°C in a reactor and treated with triethylamine (10 mL). MeSH gas was generated in situ by reacting a solution of CH3SNa (1.0 equivalent; 20% in water) with H2SO4 (2 equivalent) at 50°C for 30 min. The resulting CH3SH gas was bubbled into a stirred 0°C reaction solution and stirred at 0°C. When GC monitoring indicated that the intermediate had been converted to isopropyl 2-oxo-4-methylthiobutyrate, 1N HCl (780 mL) was added to the reactor to quench the reactants. The organic layer was separated from the aqueous layer, washed with 500 mL of water, and the aqueous phase was extracted with 650 mL (2x) MTBE. The combined organic extracts were concentrated by vacuum evaporation and the product was purified by vacuum distillation to give 1021 g (55%) of isopropyl 2-oxo-4-methylthiobutyrate as a colorless oil. 13 C NMR (100MHz, CDCl3) δ (ppm) 193.21,160.25,70.95,39.33,27.32,21.63,15.74( Figure 2A ); 1H NMR (400MHz, CDCl3) δ5.12(hept,J=6.2Hz,1H),3.13(t,J=7.2Hz,2H),2.76(t,J=7.2Hz,2H),2.11(s,3H),1.33(d,J=6.3Hz,6H)( Figure 2B ).

[0206] Example 5: Synthesis of isopropyl 2-hydroxy-4-methylthiobutyrate (HMBi)

[0207]

[0208] In a 5 L reactor, NaBH4 (99 g, 2.6 mol) was added in portions to a solution of isopropyl 2-oxo-4-methylthiobutyrate (1 kg, 5.25 mol) in methanol (2 L) at 0–5 °C. The resulting reaction mixture was maintained at 0–5 °C and stirred for 1 hour. The reaction mixture was washed with saturated NH4Cl aqueous solution (500 mL). The organic phase was separated, the solvent was removed by vacuum distillation, and the crude product was purified by distillation to give HMBi (859 g, 85% yield, 97% monomeric ester) as a pale yellow oil. 13 C NMR (100MHz, CDCl3) δ (ppm) 174.52, 69.85, 69.34, 33.76, 29.69, 21.87, 21.83, 15.60 ( Figure 3A ); 1 H NMR(400MHz, CDCl3) δ5.08(hept,J=6.3Hz,1H),4.24(dd,J=7.9,3.8Hz,1H),2.97(br,1H),2.67- 2.55(m,2H),2.10-2.01(m,4H),1.93-1.84(m,1H),1.27(d,J=1.9Hz,3H),1.26(d,J=2.2Hz,3H)( Figure 3B ).

[0209] Alternative method for synthesizing 2-hydroxy-4-(methylthio)butyrate isopropyl ester (HMBi) from 2-oxo-4-methylthiobutyrate isopropyl ester (OMBi):

[0210]

[0211] Various reagents and conditions for the preparation of HMBi from isopropyl 2-oxo-4-methylthiobutyrate were screened, including NaBH4, transition metal-catalyzed hydrogenation, and ketone reduction. Several reaction temperatures, times, reagents, and solvents were tested. The conversion and yield of the separated products under each set of conditions were determined, and the results are shown in Table 5.

[0212] Table 5.

[0213] .

Claims

1. A method of preparing a compound of Formula (I): wherein the method comprises coupling a compound of Formula (IV): with a vinyl Grignard reagent of Formula (A) in a ratio of 0.8 to 2.0 (Formula (IV): Formula (A)): wherein the solvent used for the coupling is selected from the group consisting of methyl tert-butyl ether (MTBE), tetrahydrofuran (THF), and diethyl ether (Et20), the reaction temperature range for the coupling is in the range from -80 °C to -20 °C, and X is Br or Cl; to form a compound of Formula (III): and converting the compound of Formula (III) to the compound of Formula (I) comprising: thiolating the compound of Formula (III) with a thiolating reagent of Formula (B) or Formula (C): to form a compound of Formula (II): and reducing the compound of Formula (II) to form the compound of Formula (I), wherein reducing the compound of Formula (II) is performed in the presence of a reducing agent, wherein the reducing agent is 0.25 to 1.0 molar equivalents of NaBH4. (I) 4. The method of claim 1, wherein the compound of Formula (I) is a compound of Formula (I-A): 5 R 1 is C 1-4 alkyl; and R 2 is C 1-8 alkyl or C 4-7 cycloalkyl; and R 3 and R 4 are each independently selected from H, methyl and ethyl; 5. The method of claim 1, wherein the compound of Formula (III) is a compound of (III-A): 6 (IV) 6. The method of claim 1, wherein the vinyl Grignard reagent of Formula (A) is vinyl MgCl. (A)， 7. The method of claim 1, wherein X is Cl.

8. The method of claim 7, wherein the coupling is performed in the presence of a salt additive.

9. The method of claim 8, wherein the salt additive is LiCl or ZnCl2.

10. The method of claim 1, wherein the coupling comprises mixing the compound of Formula (I) with from 0.8 to 2.0 molar equivalents of the vinyl Grignard reagent of Formula (A). (III); 11. The method of claim 10, wherein the coupling comprises mixing the compound of Formula (I) with from 1.0 to 1.75 molar equivalents of the vinyl Grignard reagent of Formula (A).

12. The method of claim 11, wherein the coupling comprises mixing the compound of Formula (I) with from 1.0 to 1.5 molar equivalents of the vinyl Grignard reagent of Formula (A). R 1 -SH (B) R 1 -S - M + (C), wherein M + is a metal cation, 13. The method of claim 10, wherein the coupling comprises mixing the compound of Formula (I) with from 1.2 to 1.75 molar equivalents of the vinyl Grignard reagent of Formula (A). (I); and (II).

14. The method of claim 13, wherein the coupling comprises mixing the compound of Formula (I) with from 1.4 to 1.6 molar equivalents of the vinyl Grignard reagent of Formula (A).

15. The method of claim 14, wherein the coupling comprises mixing the compound of Formula (I) with 1.5 molar equivalents of the vinyl Grignard reagent of Formula (A).

2. The method of claim 1, wherein each R 2 is selected from the group consisting of methyl, ethyl, and isopropyl.

3. The method of claim 1, wherein R 1 is methyl; and / or wherein each R 2 is isopropyl; and / or wherein R 3 and R 4 are each H.

16. The method of claim 1, wherein the coupling is performed at a temperature ranging from -80 °C to -70 °C. (I-A). ​ (III-A). ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 17. The method of claim 1, wherein the coupling is performed at a temperature ranging from -50 °C to -20 °C.

18. The method of claim 17, wherein the coupling is performed at a temperature ranging from -30 °C to -20 °C.

19. The method of claim 16, wherein the coupling is performed at a temperature of -78 °C.

20. The method of claim 18, wherein the coupling is performed at a temperature of -20 °C.

21. The method of claim 1, wherein the thiolation is performed with the reagent of formula (B) in the presence of an additive.

22. The method of claim 21, wherein the additive is an amine base, a phosphine, a basic salt, a Lewis acid, or an N-heterocyclic carbene (NHC) complex.

23. The method of claim 22, wherein the amine base is triethylamine, diethylamine, pentylamine, or hexylamine.

24. The method of claim 22, wherein the phosphine is dimethylphenylphosphine or tris(2-carboxyethyl)phosphine (TCEP).

25. The method of claim 22, wherein the basic salt is NaHCO3 or Na2CO3.

26. The method of claim 22, wherein the Lewis acid is scandium(III) triflate or anhydrous cerium(III) chloride.

27. The method of claim 22, wherein the N-heterocyclic carbene (NHC) complex is an Au-NHC complex.

28. The method of claim 23, wherein the amine base is triethylamine.

29. The method of claim 21, further comprising generating the thiolated reagent of formula (B) from the thiolated reagent of formula (C).

30. The method of claim 29, wherein the generating is performed in the presence of an acid catalyst.

31. The method of claim 30, wherein the acid catalyst is acetic acid, p-toluenesulfonic acid, or H2SO4.

32. The method of claim 1, wherein M + is Na + or K + .

33. The method of claim 1, wherein the reducing is performed: (a) in an alcoholic solvent; and / or (b) at a temperature ranging from -10 °C to 30 °C.

34. The method of claim 33, wherein the alcoholic solvent is methanol, ethanol, or isopropanol.

35. The method of claim 33, wherein the reducing is performed at 0 °C.

36. The method of claim 35, wherein the alcoholic solvent is methanol, ethanol, or isopropanol.

37. The method of any one of claims 1 to 36, wherein the thiolation comprises extracting the compound of formula (II) into an organic solvent to form an extract of formula (II), and the reducing comprises adding the reducing agent to the extract of formula (II).

38. The method of any one of claims 1 to 36, further comprising esterifying oxalyl chloride with R 2 -OH to form the compound of Formula (IV).

39. The method of any one of claims 1 to 36, wherein the method provides the compound of formula (I) or formula (I-A) in a purity of at least 95% by GC, HPLC, and / or by weight.

40. A method of making a compound of formula (I): wherein (I) ​ R 1 is C 1-4 alkyl; and R 2 is C 1-8 alkyl or C 4-7 cycloalkyl; and R 3 and R 4 are each independently selected from H, methyl and ethyl; The method comprises reducing a compound of formula (II): (I) to form the compound of formula (I).

41. The method of claim 40, wherein R 1 is methyl; and / or wherein each R 2 is isopropyl; and / or wherein R 3 and R 4 are each H.

42. The method of claim 41, wherein the compound of formula (I) is a compound of formula (I-A): (I-A).

43. The method of claim 40, wherein the reducing is performed under conditions of: (a) in an alcoholic solvent; and / or (b) at a temperature ranging from -10 °C to 30 °C.

44. The method of claim 43, wherein the alcoholic solvent is methanol, ethanol, or isopropanol.

45. The method of claim 43, wherein the reducing is performed at 0 °C.

46. The method of claim 45, wherein the alcoholic solvent is methanol, ethanol, or isopropanol.

47. The method of claim 40, further comprising the compound of formula (III): ###0020### wherein R 3 and R 4 are each independently selected from H, methyl, and ethyl. (III), thiolating with a thiolating reagent of formula (B) or formula (C): R 1 -SH (B) R 1 -S - M + (C), wherein M + is a metal cation, to form the compound of formula (II).

48. The method of claim 47, wherein the compound of formula (III) is a compound of formula (III-A): (III-A).

49. A method of making a compound of formula (I-A): ， The method comprises: esterifying oxalic acid with isopropanol to form diisopropyl oxalate; coupling diisopropyl oxalate with vinylmagnesium bromide to form a compound of formula (III-A): (III-A); thiolating the compound of formula (III-A) with CH3SH to form a compound of formula (II-A): (II-A)? and reducing the compound of formula (II-A) to form the compound of formula (I-A).

50. The method of any one of claims 40 to 49, wherein the method provides the compound of formula (I) or formula (I-A) in a purity of at least 95% by GC, HPLC, and / or by weight.

Images