Process for the preparation of vinyl ketocarboxylic acid esters

Abstract

A method for producing vinyl esters of carboxylic acids, in particular vinyl esters of ketocarboxylic acids, which can be α-ketocarboxylic acids or β-ketocarboxylic acids. Said vinyl esters of carboxylic acids can have hydrogen in their vinyl groups, preferably deuterium in their vinyl groups. The vinyl esters can be hydrogenated with parahydrogen, transferring the spin of the parahydrogen to the carbonyl carbon atom of the carboxyl group (which is 13 C), which can then be converted to hyperpolarized aryl by hydrolysis of the ester group. 13 Carboxylic acids having C at the carbonyl carbon atom of the carboxyl group, particularly ketocarboxylic acids, can be prepared.

Description

[Technical field]

[0001] The present invention relates to a process for preparing vinyl esters of carboxylic acids, in particular vinyl esters of ketocarboxylic acids, which can be α-ketocarboxylic acids or β-ketocarboxylic acids. The vinyl esters of carboxylic acids can have hydrogen in their vinyl group, preferably deuterium in their vinyl group. The vinyl esters can be hydrogenated with parahydrogen, transferring the spin of the parahydrogen to the carbonyl carbon atom of the carboxyl group (the carbonyl carbon atom being 13 C), which can then be converted to hyperpolarized aryl by hydrolysis of the ester group. 13 Carboxylic acids, particularly ketocarboxylic acids, can be prepared which have a C in the carbonyl carbon atom of the carboxyl group. 13 The transfer of spin order brought about by hydrogenation of the vinyl group with parahydrogen to achieve hyperpolarization of the C carbonyl atom can be accelerated by magnetic field cycling or by appropriate NMR pulse sequences.

[0002] The process for producing vinyl esters of carboxylic acids, especially vinyl esters of ketocarboxylic acids, is high yielding and 13 of C-carbonyl-labeled ketocarboxylic acids, and as the carbonyl atom hyperpolarized 13 The present invention has the advantage that it allows the preparation of deuterated vinyl esters of ketocarboxylic acids having C.

[0003] The product of the process is 1- 13 C-hyperpolarized carboxylic acids, preferably 1- 13 C-hyperpolarized ketocarboxylic acids, which are suitable for use, for example, as magnetic labeling molecules in magnetic resonance imaging (MRI) diagnostics. Preferred ketocarboxylic acid esters are 13 Hyperpolarization at the carbonyl carbon atom of the carboxyl group for use in C-MRI 13 C is a pyruvic acid vinyl ester (also called vinylpyruvate). [Background technology]

[0004] EP3063119B1 (Patent Document 1) provides an unsaturated ester of a carboxylate, which is hydrogenated with parahydrogen to produce a parahydrogenated ester, followed by 1- 13 The spin order transition to the C carbonyl carbon atom results in 1- 13 A compound containing a C-hyperpolarized carboxylate is produced, and the ester is converted by removal of the hydrogenating group to give a 1- 13 It has been described that the parahydrogenation of 1-C-hyperpolarized carboxylic acids can be achieved by the addition of aryl esters and propargyl esters. In particular, allyl esters and propargyl esters have been proposed as substrates for parahydrogenation because of their ease of synthesis from the corresponding alcohols and carboxylic acids. 13 Vinyl esters (with spin-ordered H atoms) are more efficient substrates for C-hyperpolarization 13 C-carbonyl) is more difficult to prepare. No route for the synthesis of vinylpyruvate that could give acceptable yields has been described.

[0005] The unsaturated ester is held in an organic solvent and reacted therein with parahydrogen, then a magnetic field is applied to cycle the organic phase and mix it with the aqueous phase, hydrolysis of the ester and the formation of water-soluble 1- 13 Promotes enrichment of C-hyperpolarized carboxylic acids.

[0006] Chukanov et al., ACS Omega 2018, 3, 6673-6682 (Non-Patent Document 1) reported the synthesis of Pd acetate from vinyl acetate and pyruvic acid. II reported a 6% yield for the synthesis of vinylpyruvate using 1,2-dichlorophenylhydrazine and KOH at 25° C. for 3 days, which was considered inefficient and too low for spin transfer from parahydrogen.

[0007] Salnikov et al., ChemPhysChem 2021, 22, 1389-1396 (Non-Patent Document 2) describe a synthetic procedure with a slightly improved yield (8%) related to Chukanov 2018, which was also not sufficient to perform hyperpolarization experiments.

[0008] J. van den Broeke, E. de Wolf, B.-J. Deelman, G. van Koten, Adv. Synth. Catal. 2003, 345, 625-635 (Non-Patent Document 3) describe rhodium-diphosphine catalysts bearing alkanes or cycloalkanes or alkane bridges as substituents on the phosphine ligands.

[0009] Steemers, L., L. Wijsman, JH van Maarseveen, Advanced Synthesis & Catalysis 2018) 360, 4241-4245 (Non-Patent Document 4) describes the preparation of vinyl esters of carboxylic acids.

[0010] W.-X. Lv., Q. Li, J.-L. Li, Z. Li, E Lin, D.-H. Tan, Y.-H. Cai, W.-X. Fan, .H. Wang, Angew. Chem. Int. Ed. 2018, 57, 16544-16548 (Non-Patent Document 5) describes the preparation of vinylboronic acid MIDA esters (MIDA = methyliminodiacetic acid).

[0011] Steemers et al. (Adv. Synth. Catal. 2018, 360, 4241-4245) describe a method for reacting deuterated vinylboronic acid MIDA esters with aryl carboxylic acids. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] EP3063119B1 [Non-patent literature]

[0013] [Non-Patent Document 1] Chukanov et al., ACS Omega 2018, 3, 6673-6682 [Non-Patent Document 2] Salnikov et al., ChemPhysChem 2021, 22, 1389-1396 [Non-Patent Document 3] J. van den Broeke, E. de Wolf, B.-J. Deelman, G. van Koten, Adv. Synth. Catal. 2003, 345, 625-635 [Non-Patent Document 4] Steemers, L., L. Wijsman, JH van Maarseveen, Advanced Synthesis & Catalysis 2018) 360, 4241-4245 [Non-Patent Document 5] W.-X. Lv., Q. Li, J.-L. Li, Z. Li, E Lin, D.-H. Tan, Y.-H. Cai, W.-X. Fan, .H. Wang, Angew. Chem. Int. Ed. 2018, 57, 16544-16548 [Non-Patent Document 6] Steemers et al. (Adv. Synth. Catal. 2018, 360, 4241-4245) Summary of the Invention [Problem to be solved by the invention]

[0014] The object of the present invention is to provide a method for the preparation of hyperpolarized 1-(2-phenylpropanediol) derivatives, after spin transfer and ester hydrolysis, for use as diagnostic compounds, in particular in magnetic resonance imaging. 13The object of the present invention is to provide an efficient method for producing ketocarboxylic acid vinyl esters and their para-hydrogenation products, which result in C carbonyl carbon compounds. Preferably, the method of the present invention provides a method for producing hyperpolarized 1- 13 A further object is to prepare compounds having a 1-C carbonyl carbon atom. 13 The objective of this study is to provide a ketocarboxylic acid vinyl ester that enables more efficient spin order transfer to the C carbonyl carbon. [Means for solving the problem]

[0015] Description of the Invention The present invention achieves the above object by the features of the claims and in particular provides a process for the preparation of ketocarboxylic acid vinyl esters and, as a by-product, trialkylsilyl halides, and thus chlorides, fluorides or bromides, by reacting carboxylic acid halides, preferably chlorides, fluorides or bromides, of ketocarboxylic acids with trialkylsilyl enol ethers. The ketocarboxylic acid vinyl esters obtained can then be para-hydrogenated by hydrogenation with parahydrogen, resulting in para-hydrogenation of the vinyl group. The 1-alkylsilyl halide of the ketocarboxylic acid can then be converted from the para-hydrogenated vinyl ester to a trialkylsilyl halide of the ketocarboxylic acid. 13 The polarization transfer to the C carbonyl carbon can be accelerated by magnetic field cycling or by an appropriate NMR pulse sequence. Subsequent hydrolysis of the ester bond results in the hyperpolarized 1- 13 The ketocarboxylic acid, and therefore the carboxylic acid chloride, can be produced by the reaction of 1- 13 C carbonyl carbon. Preferably, the trialkylsilyl enol ether is a trialkylsilyl vinyl ether.

[0016] The ketocarboxylic acid can be an alpha (α)-ketocarboxylic acid or a beta (β)-carboxylic acid.

[0017] In the reaction schemes below, the halogen atoms are represented by the preferred chlorine. Carboxylic acid chlorides can be prepared by reacting a ketocarboxylic acid to the carboxylic acid chloride according to one of the following reaction schemes:

[0018] [ka] The ketocarboxylic acid chloride is generated from the corresponding ketocarboxylic acid by reacting it with a chlorinating agent, for example, oxalyl chloride as shown here. As an alternative to oxalyl chloride, thionyl chloride or phosphorus-III-chloride or phosphorus-V-chloride can be used. The preferred agents are oxalic chloride and DMF as catalyst, or 1-chloro-N,N,2-trimethylpropenylamine (tetramethyl-α-chlorenamine, TMCE) or any other suitable chlorinating agent. Although the ketocarboxylic acid chloride can be isolated in certain cases, due to its instability, the ketocarboxylic acid chloride is preferably further reacted in solution immediately after its generation.

[0019] In a subsequent step, an α- or β-ketocarboxylic acid halide, such as an α- or β-ketocarboxylic acid chloride, is reacted with a trialkylsilyl enol ether to give the corresponding vinyl ester.

[0020] [ka] X represents a halogen atom, preferably Cl, Br or F. The upper part shows a reaction scheme relating to an α-ketocarboxylic acid halide, and the lower part shows a reaction scheme relating to a β-ketocarboxylic acid halide.

[0021] In the trialkylsilyl enol ether, R 9 , R 10 , R 11 are each independently H or alkyl, e.g., C1-C 12 R can be alkyl. 1 , R 2 , R3 are each independently H or alkyl, e.g., C1-C 12 R can be alkyl, preferably trialkylsilyl enol ether linked to a fully deuterated vinyl group. 1 , R 2 , R 3 Each of is D.

[0022] Advantageously, vinyl esters of ketocarboxylic acids are deuterated in the vinyl group (R 1 , R 2 , R 3 All of the above are D), and the carbonyl atom of the ketocarboxylic acid chloride is 13 It is enriched in C.

[0023] In a preferred embodiment, the ketocarboxylic acid, and thus the acid chloride of the ketocarboxylic acid that is reacted with the trialkylsilyl enol ether, is fully deuterated and has deuterium instead of hydrogen atoms. A preferred deuterated ketocarboxylic acid is pyruvate, in which the terminal methyl groups are fully deuterated. Thus, in the preferred vinyl esters of ketocarboxylic acids, the vinyl groups are also fully deuterated. A preferred vinyl ester of ketocarboxylic acid that can be obtained by the process of the present invention has the following structure:

[0024] [ka] (wherein R1, R2 and R3 are each deuterium, and R is a linear or branched C1-C cyclic aryl group that is preferably fully deuterated. 12 Alkyl, such as CD3, -CD2-CD3 or -(CD2) n -CD3 (n=1, 2, 3 or 4), or R is a linear or branched C1-C aryl group having a carboxyl or carbonyl ester, preferably fully deuterated at the terminus. 12 Alkyl, e.g. -(CD2) n -COOD, -(CD2) n-COO (CD=CD2), and the carboxyl carbon of ketocarboxylic acid is 13 C).

[0025] In embodiments in which the vinyl group of the ketocarboxylic acid is partially deuterated, preferably fully deuterated, the spin order introduced into the vinyl group by parahydrogenation is the same as that of the carbonyl group. 13 Deuteration of the R group in α-keto carboxylic acid vinyl esters is more effective at reducing the amount of deuteration in the hyperpolarized 13 C resulting in a longer half-life (T1).

[0026] Suitable catalysts for the reaction of acid chlorides with trialkylsilyl enol ethers are transition metal salts or transition metal complexes. Among the catalysts tested, PdCl2 is preferred since it gave the best yield.

[0027] [Table 1]

[0028] Systematic variation of the solvent gave the following yields, showing that all solvents gave yields above 50%, with benzonitrile and dichloromethane being preferred, and THF and acetonitrile being the most preferred.

[0029] [Table 2]

[0030] Preferably, the vinyl group of the trialkylsilyl enol ether is fully deuterated. In general, a fully deuterated trialkylsilyl nolether of a vinyl group can be prepared by reacting fully deuterated tetrahydrofuran (THF) (also called deutero-oxolane-d8) with butyllithium (BuLi) to obtain the lithium salt of the deuterated enol ether of acetaldehyde, and further reacting the resulting product with a trialkylhalosilane, such as a trialkylchlorosilane, for example, according to the following reaction scheme:

[0031] [ka] Here, X is a halogen atom, preferably chlorine.

[0032] The alkyl group R of the trialkylchlorosilane, and hence of the trialkylsilyl enol ether of the deuterated vinyl 9 , R 10 , R 11 are each the same or independently, e.g., C1 to C 12 For example, R9, R10 and R11 can each independently be branched alkyl, such as isopropyl or tert-butyl, or phenyl, for example, R9, R10 are each methyl and R11 is isopropyl or tert-butyl; or R9, R10 are each methyl and R11 is phenyl.

[0033] In vinylpyruvate, when the vinyl group bears hydrogen, after parahydrogenation and application of an appropriate NMR pulse sequence, about 15% of the spin order is in the 1- 13 C carbonyl carbon as hyperpolarization, whereas the same procedure applied to the deuterated vinyl group resulted in 1- 13 It was found to result in the transfer of approximately 96% of the hyperpolarization to the C carbonyl carbon.

[0034] [Table 3]

[0035] Ketocarboxylic acids, either undeuterated or preferably fully deuterated or deuterated at their vinyl groups, are stable and can be stored for long periods of time. The following steps must be carried out in a magnetic field, preferentially inside an MRI scanner. Each step must be carried out as quickly as possible, since spin order and hyperpolarization decrease exponentially with relaxation.

[0036] In a method for producing a ketocarboxylic acid having a hyperpolarized carbonyl atom, a reaction mixture obtained by reacting a carboxylic acid halide of a ketocarboxylic acid with a trialkylsilyl enol ether, preferably a vinyl ester of a ketocarboxylic acid ester isolated from said reaction mixture, and optionally, when said reaction mixture is stored for a certain period of time, the isolated vinyl ester of the ketocarboxylic acid ester is contacted with parahydrogen to parahydrogenate the vinyl group of the ketocarboxylic acid ester, followed by the transfer of the spin order from the parahydrogen atom to the ketocarboxylic acid group. 13 The C carbonyl carbon is hyperpolarized and the completed reaction mixture resulting from the parahydrogenation is subjected to hydrolysis of the ester bond to give the hyperpolarized 1- 13 A ketocarboxylic acid having a C carbonyl carbon is produced, and the resulting completed reaction mixture is then subjected to subsequent separation of the ketocarboxylic acid from the reaction mixture. The separated ketocarboxylic acid is thus suitable for use in diagnosis, for example as a contrast agent for use in MRI diagnosis. (1) Specifically, prior to in vivo application, vinyl esters of ketocarboxylic acids are dissolved in water or organic solvents (e.g., chloroform, pentane) and treated with parahydrogen under pressure in the presence of a suitable hydrogenation catalyst, which transfers the inherent spin order of parahydrogen to the vinyl group (currently an ethyl group). (2) Then, by applying magnetic field cycling or an appropriate NMR pulse sequence, the spin order is shifted from the ethyl group to the adjacent carbonyl group. 13This transfer of hyperpolarization is much more efficient if the vinyl group is deuterated. Further deuteration of the alkyl group in the ketocarboxylic acid increases the half-life (T1) of the hyperpolarized state, which is of the order of only 0.5-2 minutes. (3) Hydrolysis of the ethyl ester with a strong base, e.g., aqueous NaOH, gives the free hyperpolarized ketocarboxylate, e.g., pyruvate, and ethanol. In a two-phase system, the latter two compounds are in the aqueous phase, while the catalyst and unreacted starting compounds are in the organic (e.g., chloroform or pentane) phase. (4) The strongly basic aqueous solution is brought to pH 7.4 by adding a buffer. Before the hyperpolarized compound is injected in vivo, residual traces of hydrogenation catalysts must be removed, since these transition metal catalysts are toxic. A simple method to remove the catalyst is filtration on an ion exchange column. Residual traces of organic solvents must also be removed, for example by passing the solution through a column of activated charcoal. This purification step can also be applied at an earlier stage.

[0037] The purified aqueous solution of the hyperpolarized ketocarboxylic acid is now ready for injection and MRI measurements.

[0038] This embodiment is a hyperpolarized 1- 13 The present invention has the advantages of being able to produce a ketocarboxylic acid having a C carbonyl carbon in a short period of time while providing the ketocarboxylic acid in high material yield and hyperpolarization yield, as well as in high purity.

[0039] For parahydrogenation, a hydrogenation catalyst is added to the reaction mixture.

[0040] Preferably, the catalyst is a homogeneous hydrogenation catalyst, preferentially of the general formula [Rh(diphosphine)(diene)] +Rhodium complexes of, for example, [1,4-bis-(diphenylphosphino)-butane]-(1,5-cyclooctadiene)-rhodium(I)-tetrafluoroborate (79255-71-3) or (bicyclo[2.2.1]hepta-2,5-diene)-[1,4-bis-(diphenylphosphino)-butane]-rhodium(I)-tetrafluoroborate (82499-43-2). These hydrogenation catalysts are soluble in chloroform. It was found that when parahydrogenated vinyl esters are hydrolyzed by adding an aqueous alkaline solution to the organic chloroform phase, not only does the desired acid portion of the ester migrate to the aqueous phase, but also a certain amount of hydrogenation catalyst and chloroform migrate to the aqueous phase high enough with respect to toxic effects in the recipient. For hydrogenation in water, the hydrogenation catalyst can be a water-soluble rhodium complex in which the diphosphine ligand is 1,4-bis-[(phenyl-3-propanesulfonate)-phosphine]-butane disodium salt.

[0041] In one embodiment, the present invention generally relates to a process for para-hydrogenating a keto carboxylic acid vinyl ester, preferably a fully deuterated keto carboxylic acid vinyl ester, with a para-hydrogenation catalyst that is soluble in a solvent consisting of an alkane, e.g. a C3-C10 alkane, preferably a C4-C8 alkane (which in each case can be an n-alkane or a cycloalkane or a mixture of at least two of these), preferably said catalyst is essentially soluble only in said solvent but insoluble in alkaline aqueous solution.

[0042] Generally, the parahydrogenation catalyst is a Rh complex having one diphosphine ligand or two monophosphine ligands, a diene ligand, such as 1,5-cyclooctadiene or norbornadiene. + Preferably, the solubility of the catalyst in a solvent of low polarity is increased by using an alkane or cycloalkane or alkane bridge as a substituent on the phosphine ligand, and additionally a large, low-coordinating anion, such as a tetraarylborane, as the anion.

[0043] Preferably, the parahydrogenation is in the presence of a parahydrogenation catalyst having formula I

[0044] [ka] Preferably, the catalyst is present in a solvent consisting of an alkane, for example a C3-C10 alkane, preferably a C4-C8 alkane, which in each case may be an n-alkane or a cycloalkane, or a mixture of at least two of these, that is liquid at the reaction temperature. A preferred alkane is pentane, for example n-pentane and / or isopentane, more preferably only n-pentane. The parahydrogenation catalyst of formula I has the advantage of being soluble in alkanes, in particular pentane, but essentially insoluble in the aqueous phase, for example in the alkaline aqueous phase containing the hyperpolarized alpha keto acid, which is preferably pyruvic acid or a salt thereof, preferably partially or fully deuterated, which is the hydrolysis product of the parahydrogenated vinyl ester of a ketocarboxylic acid. The catalyst of formula I has the advantage of not transferring from the solvent alkane phase to the adjacent aqueous phase, which effectively minimizes the toxicity of the aqueous phase. The organic phase, consisting of a C3-C10 alkane, preferably a C4-C8 alkane, preferably pentane, n-pentane and / or isopentane, has the advantage that it does not essentially transfer to the adjacent aqueous phase and has low toxicity in the recipient human patient. Thus, for the rapid production of hyperpolarized ketocarboxylic acids or salts thereof in the aqueous phase, the method comprises para-hydrogenating a vinyl ester of the ketocarboxylic acid in the presence of a catalyst of formula I in a solvent, followed by the addition of the carboxylic acid moiety. 13 It is preferred to hyperpolarize C and hydrolyze the resulting ester by adding an alkaline aqueous phase to the solvent, which results in the transfer of the hyperpolarized ketocarboxylate anion into the aqueous phase essentially without catalyst or solvent transfer into the aqueous phase.

[0045] As an alternative to a solvent consisting of an alkane, the para-hydrogenation of the ketocarboxylic acid vinyl ester can be carried out in supercritical carbon dioxide in the presence of a catalyst, preferably a catalyst of formula I, for example under a carbon dioxide partial pressure of at least 30 bar. When para-hydrogenating the ketocarboxylic acid vinyl ester in supercritical carbon dioxide, it is preferred to release carbon dioxide after para-hydrogenation, followed by addition of an aqueous alkaline solution to hydrolyze the ester bond from the hyperpolarized ketocarboxylic acid. There, optionally, the para-hydrogenated ketocarboxylic acid vinyl ester after release of carbon dioxide can be contacted with a solvent, for example an alkane, for example an alkane as described above, in particular pentane, to dissolve the catalyst in pentane, and then add an aqueous alkaline solution.

[0046] In embodiments where the solvent comprises supercritical carbon dioxide, parahydrogenation is preferably followed by release of carbon dioxide, then addition of an alkane solvent, followed by addition of an aqueous alkaline solution, thereby providing the hyperpolarised ketocarboxylate anion in aqueous solution whilst substantially retaining the catalyst in the alkane solvent.

[0047] It has been found that the method of the present invention gives a yield of at least 60%, for example up to 85%, preferably up to 95% for α-keto carboxylic acid. This high yield is also obtained after separating the keto carboxylic acid from the reaction mixture. It is currently believed that a high yield and therefore a high purity of the keto carboxylic acid is produced by the above steps of the reaction, in particular by reacting the carboxylic acid chloride of the keto carboxylic acid with a trialkylsilyl enol ether, and thus minimizing the occurrence of back reactions or dimerization, for example aldol reactions, since they are currently believed to cause very low yields in the synthesis described in Chukanov et al., ACS Omega 2018. For the same reason, the yield for β-keto carboxylic acid according to our procedure is somewhat lower than the yield for α-keto carboxylic acid.

[0048] In another embodiment, the present invention relates to an alternative method for preparing ketocarboxylic acid vinyl esters, preferably with fully deuterated vinyl moieties or fully deuterated ketocarboxylic acid moieties, more preferably fully deuterated. In this embodiment, pyruvic acid vinyl esters with fully deuterated vinyl moieties (vinylpyruvate-D3) are produced by transfer of vinyl moieties from deuterated alkenylvinylboronic acid esters, such as trivinylboroxine / pyridine complexes. The preparation of vinyl esters of carboxylic acids that are not α-ketocarboxylic acids is described in L. Steemers, L. Wijsman, JH van Maarseveen, Advanced Synthesis & Catalysis 2018) 360, 4241-4245.

[0049] Preferentially, vinylpyruvate-D3 can be prepared by direct reaction of deuterated vinylboronic acid MIDA ester (MIDA = methyliminodiacetic acid) with an a-ketocarboxylic acid in the presence of a metal catalyst (preferentially a Cu(II) salt).

[0050] [ka] Commercially available deuterated vinyl bromide (vinyl bromide-D3) is reacted with Mg in ether under standard conditions to prepare the corresponding Grignard reagent, which is reacted with MIDA anhydride (methyliminodiacetic anhydride) to give deuterated vinyl MIDA boronate (similar to the procedure described, for example, in Lv et al. Angew. Chem. Int. Ed. 2018, 57, 16544-16548). The deuterated vinyl MIDA boronate is reacted with pyruvic acid under conditions described in Steemers et al. (Adv. Synth. Catal. 2018, 360, 4241-4245), who applied this method to aryl carboxylic acids. Vinyl pyruvate-D3 was obtained in 12% yield.

[0051] [ka] In general, the described embodiments can be combined with one another, for example, the respective processes for producing ketocarboxylic acid vinyl esters, preferably fully deuterated ketocarboxylic acid vinyl esters, can be combined with parahydrogenation in an alkane solvent or in supercritical carbon dioxide, preferably with a parahydrogenation catalyst of formula I in each case.

[0052] The invention will now be explained in more detail by means of examples. EXAMPLES

[0053] Example 1: Preparation of trimethylsiloxyethene-D3 A nitrogen flushed oven dried 100 mL flask was charged with 27.9 mL (24.8 g, 344 mmol) of THF-D8 (perdeuterated tetrahydrofuran). The flask was cooled to 0° C. and 7.82 mL (86.0 mmol) of 11 M n-butyllithium in n-hexane was added dropwise. After 0.5 h at 0° C. the solution was warmed to room temperature and after another 0.5 h it was heated to 40° C. The solution was stirred at 40° C. for 16 h, after which the solvent was removed under vacuum. The remaining white solid was dissolved in 20 mL of dry diglyme and further residues of THF were removed under vacuum (40° C., 50 mbar). An additional amount of 30 mL of dry diglyme was added and the solution was cooled to 0° C. Then 11.2 mL (88.0 mmol) of dry trimethylsilyl chloride was added slowly resulting in the formation of a substantial amount of white precipitate. After stirring at room temperature for 2 hours, the crude product was separated from the precipitate by flash distillation of the reaction mixture at 25° C. and 50 mbar (reduced to 10 mbar during the process). The crude product, containing some diglyme, was then further distilled by Kugelrohr distillation at atmospheric pressure and 75° C.

[0054] Example 2: Preparation of Dimethylisopropylsiloxyethene-D3 A nitrogen flushed oven dried 100 mL flask was charged with 27.9 mL (24.8 g, 344 mmol) of THF-D8. The flask was cooled to 0° C. and 7.82 mL (86.0 mmol) of 11 M n-butyllithium in n-hexane was added dropwise. After 0.5 h at 0° C., the solution was warmed to room temperature and after another 0.5 h, heated to 40° C. The solution was stirred at 40° C. for 16 h, followed by removal of the solvent under vacuum. The residue was dissolved in 50 mL of dry diethyl ether and cooled to 0° C. Then, 10.85 mL (9.47 g, 69.3 mmol) of dry dimethylisopropylsilyl chloride was slowly added to the solution, resulting in the formation of a substantial amount of white precipitate. After stirring at room temperature for 2 h, the solution was transferred to a separatory funnel and washed once with 50 mL of saturated sodium bicarbonate solution and twice with 50 mL of distilled water. The organic layer was dried over magnesium sulfate and the solvent was removed in vacuo. The crude product obtained (>95%) was used without further purification.

[0055] Example 3: Preparation of vinyl pyruvate: In a preferred embodiment for obtaining vinyl pyruvate, 10 mL of dry dichloromethane was placed in an oven-dried 50 mL two-neck flask flushed with nitrogen. The flask was cooled to 0° C. and 0.961 mL (1.58 g, 12.5 mmol) of oxalyl chloride was added. 0.787 mL (1.00 g, 11.2 mmol) of pyruvic acid was mixed with 5 mL of dry dichloromethane and 13 drops of N,N-dimethylformamide. The mixture was added dropwise to the cooled solution in the two-neck flask. The reaction was stirred at 0° C. for 2 hours and then at room temperature for an additional 2 hours. The solution was carefully degassed under Schlenk conditions. During the process, the volume of the solution was reduced from 15 mL to 10 mL (herein referred to as solution A).

[0056] A nitrogen-flushed oven-dried 50 mL two-neck flask was charged with 80.0 mg (0.450 mmol) of palladium(II) chloride and 10 mL of dry dichloromethane was added. The flask was cooled to 0° C. and 1.84 mL (1.43 g, 12.34 mmol) of trimethyl(vinyloxy)silane was added as the silyl enol ether. The solution was stirred at 0° C. for 0.5 h, then solution A was added dropwise over 0.5 h at 0° C. After complete addition, the solution was warmed to room temperature and stirred for an additional 20 h. The solution was transferred to a single-neck flask and a spatula tip of hydroquinone (approximately 30 mg, 0.27 mmol) was added and the solvent was removed under reduced pressure (600 mbar, 30° C.). The residue was taken up in 3 mL of dichloromethane:n-pentane (1:1) and then purified by flash column chromatography using a gradient of n-pentane, dichloromethane. The product was isolated as a slightly yellow oil in 68% yield.

[0057] Example 4:1- 13 Preparation of C-vinylpyruvate-D6 13 In a preferred embodiment for obtaining vinylpyruvate-D6, 10 mL of dry dichloromethane was placed in a nitrogen-flushed oven-dried 50 mL two-neck flask. The flask was cooled to 0° C. and 0.961 mL (1.58 g, 12.5 mmol) of oxalyl chloride was added. 0.787 mL (1.00 g, 11.2 mmol) of 13 C pyruvic acid was mixed with 5 mL of dry dichloromethane and 13 drops of N,N-dimethylformamide. The mixture was added dropwise to the cooled solution in a two-neck flask. The reaction was stirred at 0° C. for 2 hours and then at room temperature for an additional 2 hours. The solution was carefully degassed under Schlenk conditions. During the process, the volume of the solution was reduced from 15 mL to 10 mL (herein referred to as solution A).

[0058] A nitrogen-flushed oven-dried 50 mL three-neck flask was charged with 80.0 mg (0.450 mmol) of palladium(II) chloride and 10 mL of dry dichloromethane was added. The flask was cooled to 0° C. and 1.84 mL (1.82 g, 12.38 mmol) of dimethylisopropyl(vinyloxy)silane-D3 was added. The solution was stirred at 0° C. for 0.5 h, then solution A was added using a dropping funnel at 0° C. over 0.5 h. After complete addition, the solution was warmed to room temperature and stirred for an additional 20 h. The solution was transferred to a single-neck flask and a spatula tip of hydroquinone (approximately 30 mg, 0.27 mmol) was added and the solvent was removed under reduced pressure (600 mbar, 30° C.). The residue was taken up in 3 mL of dichloromethane:n-pentane (1:1) and then purified by flash column chromatography using a gradient of n-pentane, dichloromethane. The product was isolated as a slightly yellow oil in 29 percent yield.

[0059] Example 5: Preparation of Divinyl Oxaloacetate (Oxaloacetic Acid Divinyl Ester) To obtain divinyl-oxaloacetate, α-oxaloacetic acid (1.00 g, 7.57 mmol) was placed in a three-necked round-bottom flask under N2 atmosphere and suspended in dichloromethane (75 mmol). The flask was cooled to 0°C and N,N-dimethylformamide (13 drops) was added. Then, oxalyl chloride (1.17 ml, 13.7 mmol) was added dropwise as a solution in dichloromethane (10 ml). The solution was stirred at 0°C for 2 h and at room temperature for another 2 h. The solution was degassed under Schlenk conditions and its volume was reduced to 35 ml to obtain a solution of α-oxaloacetic acid chloride, referred to here as solution A.

[0060] Palladium(II) chloride (48.5 mg, 0.274 mmol) was placed in a round bottom flask under N2 atmosphere and suspended in dichloromethane (10 ml). The suspension was cooled to 0°C and trimethylvinyloxysilane (2.48 ml, 16.7 mmol) was added. The mixture was stirred for 0.5 h and previously prepared solution A was added dropwise over 0.5 h. Without further cooling, the solution was stirred for 20 h. The product was obtained in 17% yield after flash column chromatography using a gradient of n-pentane and dichloromethane. Optionally, the vinyl group of trimethyl(vinyloxy)silane was deuterated.

[0061] Example 6: Preparation of divinyl α-ketoglutarate (2-oxoglutarate divinyl ester) To obtain divinyl-α-ketoglutarate, α-ketoglutaric acid (1.00 g, 6.84 mmol) was placed in a three-necked round-bottom flask under N2 atmosphere and suspended in dichloromethane (75 ml). The flask was cooled to 0°C and N,N-dimethylformamide (13 drops) was added. Then, oxalyl chloride (1.17 ml, 13.7 mmol) was added dropwise as a solution in dichloromethane (10 ml). The solution was stirred at 0°C for 2 h and at room temperature for another 2 h. The solution was degassed under Schlenk conditions and its volume was reduced to 35 ml to obtain a solution of α-ketoglutaroyl chloride, referred to here as solution A.

[0062] Palladium(II) chloride (48.5 mg, 0.274 mmol) was placed in a round bottom flask under N2 atmosphere and suspended in dichloromethane (10 ml). The suspension was cooled to 0°C and trimethylvinyloxysilane (2.48 ml, 16.7 mmol) was added. The mixture was stirred for 0.5 h and previously prepared solution A was added dropwise over 0.5 h. Without further cooling, the solution was stirred for 20 h. The product was obtained in 18% yield after flash column chromatography using a gradient of n-pentane and dichloromethane. Optionally, the vinyl group of trimethyl(vinyloxy)silane was deuterated.

[0063] Example 7: Preparation of vinyl acetoacetate (vinyl-3-oxobutyrate) To obtain vinyl acetoacetate, lithium acetoacetate (1.00 g, 9.26 mmol) was placed in a three-necked round-bottom flask under N2 atmosphere, suspended in dichloromethane (75 ml) and oxalic acid (anhydrous, 0.83 g, 9.26 mmol) and stirred for 5 min. The flask was cooled to 0 °C and DMF (13 drops) was added. Oxalyl dichloride (0.791 ml, 9.26 mmol) was then added dropwise as a solution in dichloromethane (10 ml). The solution was stirred at 0 °C for 2 h and at room temperature for another 2 h. The solution was degassed under Schlenk conditions and its volume reduced to 35 ml to give a solution of acetoacetate chloride, which decomposes rapidly under ambient conditions and cannot be isolated or stored. Palladium(II) chloride (65.1 mg, 0.370 mmol) was placed in a round-bottom flask under N2 atmosphere and suspended in dichloromethane (10 ml). The suspension was cooled to 0° C. and trimethylvinyloxysilane (1.51 ml, 10.2 mmol) was added. The mixture was stirred for 0.5 h and the previously prepared solution of acetoacetic chloride was added dropwise over 0.5 h. Without further cooling, the solution was stirred for 20 h. The product was obtained in 15% yield.

[0064] Optionally, the vinyl group of trimethyl(vinyloxy)silane was deuterated.

[0065] Parahydrogenation: Substrate precursor 1- 13 C-vinylpyruvate-d3 (or d6) (6.8 mg) and the hydrogenation catalyst [1,4-bis-(diphenylphosphino)-butane]-(1,5-cyclooctadiene)-rhodium(I)-tetrafluoroborate (26 mg [Rh], CAS79255-71-3) were dissolved in 6 mL of chloroform-d.

[0066] An NMR tube (5 mm diameter, medium wall thickness, for high pressure) was filled with 450 μL of the above solution (vinyl pyruvate and Rh catalyst) and connected to the gas line. The gas line was flushed with N2 at 5 bar for about 5 s before being connected to the NMR tube to prevent premature hydrogenation. The NMR tube was inserted into an NMR spectrometer (WB NMR 400 MHz, Avance Neo, 9.4 T, Bruker) with the probe set at 330 K. After a waiting period of 2–3 min to reach temperature stabilization, the pressure was increased by flushing parahydrogen through the sample for 10–15 s at 7 bar. Parahydrogen bubbling was then stopped and the spin-order transfer sequence was performed after 2 s.

[0067] The same procedure can be carried out using other solvents, including alkanes (e.g., pentane, butane, hexane, etc.), acetone, as well as alcohols and chlorine-containing solvents. However, hydrogenation is very inefficient in alkanes because of the low solubility of the catalyst in these solvents.

[0068] Synthesis of n-pentane soluble catalyst: Bis-(1,5-cyclooctadiene)-rhodium(I)-tetrafluoroborate (5 mg, 0.0123 mmol) and tricyclohexylphosphine (6.90 mg, 0.0246 mmol) and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (10.9 mg, 0.0123 mmol) were dissolved in 1 ml of dry DCM. To this clear orange solution, n-pentane was added slowly until a white solid started to precipitate. The solid was filtered off and the solvent was evaporated under reduced pressure to give an orange solid. The orange solid was suspended in 10 ml of n-pentane. The supernatant yellow n-pentane solution was used for the hyperpolarization experiments.

[0069] Parahydrogenation in pentane: 500 μl of the above pentane solution of soluble catalyst was mixed with 10 μl of vinylpyruvate. The sample was then placed in a 5 mm NMR tube fitted with a low pressure hydrogen gas line. The gas line was flushed with N2 at 5 bar for about 5 seconds before being connected to the NMR tube. The NMR tube was inserted into an NMR spectrometer (WB NMR 400 MHz, Avance Neo, 9.4 T, Bruker) with the probe set at 330 K. After a waiting period of 2-3 minutes to reach temperature stabilization, the pressure was increased by flushing parahydrogen through the sample at 7 bar for 10-15 seconds. The parahydrogen bubbling was then stopped and a spin order transfer sequence (PASADENA experiment) was performed after 2 seconds. The pentane system was suppressed with an OPSY (parahydrogen spectroscopy only) sequence.

[0070] Spin-order transition:Spin-order transition sequence:phINEPT+, Kadlecek, Goldman (10.1016 / j.jmr.2012.08.016), SEPP-INEPT (10.1016 / j.pnmrs.2012.03.001), ESOTHERIC ( https: / / doi.org / 10.1002 / open.201800086 ) and their derivatives (including adiabatic and shape RF pulses (10.1021 / jz501754j) or variation of magnetic field (10.1021 / acs.jpcb.5b06222)) to generate 13 C. Experimentally, we found that the polarization of 1- 13 An ESOTHERIC SOT sequence was used which resulted in 100% polarization of C-ethylpyruvate-d6. Other ketocarboxylates result in similar degrees of hyperpolarization with the same procedure.

[0071] Ester hydrolysis: To carry out the hydrolysis, 13C-hyperpolarized ethyl ester (450 μL) is pushed from the parahydrogen reaction chamber into a glass vial previously filled with aqueous NaOH (450 μL, 100 mmol / L). After pushing the hyperpolarized sample into the aqueous solution, the vial is shaken for about 3 s and further left to stand for another 5-10 s to achieve phase separation. The upper aqueous phase is then collected. Traces of residual catalyst are removed by passing the aqueous phase through an ion exchange column and traces of solvent are removed by rapid filtration through activated charcoal. If the ketocarboxylic acid ester is a pyruvate ester, the ester hydrolysis is carried out in acetone-D6. Sodium pyruvate is poorly soluble in acetone and is purified by precipitation, washing and redissolution in water.

[0072] The resulting hyperpolarized aqueous solution is filled into NMR tubes for quality analysis or pre-filled with enzymes or proteins for in vitro studies. If necessary, the pH of the solution is adjusted by adding a more acidic buffer to obtain the desired pH value (approximately 7.4). After quality assurance (pH, purity, temperature), the hyperpolarized 13 C-ketocarboxylic acids can be used for in vivo molecular MRI.

Claims

1. A method for producing a ketocarboxylic acid vinyl ester by reacting a carboxylic acid halide of a ketocarboxylic acid with a trialkylsilyl enol ether.

2. 2. The method of claim 1, wherein the carboxylic acid halide of the ketocarboxylic acid is prepared by reacting the ketocarboxylic acid with a halogenating agent.

3. 3. The method according to claim 1, wherein the halide is chloride, fluoride or bromide.

4. A method for preparing a ketocarboxylic acid vinyl ester by reacting a carboxylic acid with a deuterated alkenyl vinylboronic acid ester in the presence of a metal catalyst.

5. 5. The method of claim 4, wherein the deuterated alkenyl vinylboronic acid ester is a trivinylboroxine / pyridine complex (or methyliminodiacetate boronate).

6. The carboxyl carbon of the ketocarboxylic acid is １３ C, or １３ 5. The method according to claim 1 or 4, characterized in that the carbon has a natural or enriched proportion of C.

7. 2. The method according to claim 1, wherein the enol ether moiety of the trialkylsilyl enol ether is fully deuterated.

8. 5. The method according to claim 1 or 4, characterized in that the ketocarboxylic acid is fully deuterated.

9. 5. The method according to claim 1 or 4, wherein the ketocarboxylic acid is pyruvate in which the terminal methyl groups are fully deuterated.

10. 2. The method of claim 1, wherein the enol ether moiety of the trialkylsilyl enol ether is a vinyl group.

11. 11. The method of claim 10, wherein the trialkylsilyl enol ether has a fully deuterated vinyl group and is prepared by reacting fully deuterated tetrahydrofuran with butyllithium (BuLi) and reacting the reaction product with a trialkylhalogensilane.

12. 5. The method according to claim 1, wherein the ketocarboxylic acid is an α-ketocarboxylic acid or a β-ketocarboxylic acid.

13. The vinyl group is para-hydrogenated with para-hydrogen, and then the para-hydrogenated vinyl group is converted to the ketocarboxylic acid. １３ 5. The method of claim 1 or 4, characterized in that the spin order is transferred to the C carboxyl carbon, the ester bond is hydrolyzed, and the resulting hyperpolarized ketocarboxylic acid is separated from the reaction mixture.

14. During parahydrogenation, the ketocarboxylic acid vinyl ester is reacted with at least one C ３ ~C １０ 14. The method of claim 13, wherein parahydrogen is introduced into a solvent comprising or consisting of an alkane in the presence of a hydrogenation catalyst.

15. 15. The method of claim 14, wherein during parahydrogenation, the alkane is placed adjacent to an alkaline aqueous phase in a pressure vessel.

16. 16. The process of claim 15, wherein the aqueous phase contains a ketocarboxylic acid or anion thereof, and the aqueous phase is separated from the alkane and the hydrogenation catalyst by removing the aqueous phase from the alkane solvent phase.

17. The reaction mixture obtained by reacting the carboxylic acid halide of the ketocarboxylic acid with the trialkylsilyl enol ether is contacted with parahydrogen to parahydrogenate the vinyl group of the ketocarboxylic acid ester, and then the carboxylic acid halide of the ketocarboxylic acid group is contacted with parahydrogen to parahydrogenate the vinyl group of the ketocarboxylic acid ester. １３ The spin order is transferred from the para-hydrogen atom to hyperpolarize the C carboxyl carbon, and the resulting reaction mixture is subjected to hydrolysis of the ester bond to form the hyperpolarized 1- １３ 2. The method according to claim 1, characterized in that a ketocarboxylic acid having a C carbonyl carbon is produced, and then the resulting reaction mixture after the completion of the reaction is subsequently subjected to separation of the ketocarboxylic acid.

18. During parahydrogenation, a ketocarboxylic acid vinyl ester is reacted with supercritical CO in a pressure vessel. ２ or the supercritical CO ２ 14. The method of claim 13, wherein the parahydrogen is introduced into a solvent consisting of

19. The parahydrogenation catalyst is a Rh having a diphosphine ligand or two monophosphine ligands and a diene ligand. ＋ 14. The method of claim 13, characterized in that it is composed of ions and anions.

20. supercritical CO ２ The solvent is removed by reducing the pressure in the pressure vessel, followed by separating the residue into an alkaline aqueous phase and, optionally, further C ３ ~C １０ The alkaline aqueous phase is then contacted with an alkane. ３ ~C １０ 19. The method of claim 18, characterized in that it is separated from alkanes.

21. 14. The method of claim 13, wherein the ketocarboxylic acid is separated from the reaction mixture by evaporating the solvent and the trialkylhalogensilane by-product and / or by one chromatographic separation step.

22. 1. A process for preparing deuterated trialkylsilyl enol ethers, particularly for use in the process of claim 1, by reacting fully deuterated tetrahydrofuran (THF) with butyllithium (BuLi) and further reacting the resulting product with a trialkylhalogensilane, such as a trialkylchlorosilane.

23. A method for para-hydrogenating a ketocarboxylic acid vinyl ester, comprising reacting the ketocarboxylic acid vinyl ester in a solvent in the presence of a para-hydrogenation catalyst having formula I, whereby the vinyl moiety is fully deuterated and / or the ketocarboxylic acid moiety is fully deuterated. 【Chemical 1】

24. 24. The method of claim 23, wherein the solvent consists of a C3 to C10 alkane or a mixture of at least two of them.

25. 24. The method of claim 23, wherein the solvent comprises supercritical carbon dioxide and wherein following parahydrogenation, carbon dioxide is released, then an alkane solvent is added, followed by the addition of an alkaline aqueous solution to obtain a hyperpolarized ketocarboxylate anion in said aqueous solution.

26. Vinyl esters of ketocarboxylic acids having the structure: 【Chemistry 2】 wherein R1, R2 and R3 are each deuterium, the carboxyl carbon of the ketocarboxylic acid is 13 C, and R is a linear or branched C １ ~C １２ alkyl, or R is a linear or branched C １ ~C １２ alkyl].

27. The vinyl ester of claim 26, wherein R is a fully deuterated linear or branched C 1 -C 12 alkyl, or R is a fully deuterated linear or branched C 1 -C 12 alkyl having a carboxyl or carbonyl ester.

28. Para-hydrogenation of the vinyl group with para-hydrogen, followed by the formation of a ketocarboxylic acid from the para-hydrogenated vinyl group. １３ 28. Use of a vinyl ester according to claim 26 or 27 for spin order transfer to the C carboxyl carbon, hydrolysis of the ester bond and separation of the resulting hyperpolarized ketocarboxylic acid from the reaction mixture.

Images