Method for producing alkylfurancarboxylic acid ester

Abstract

Provided is a new method for producing an alkylfurancarboxylic acid ester, the method comprising conducting an oxidation / reduction reaction between an alkylfuranaldehyde and a compound represented by A-OH (wherein A is a C1-C10 organic group) in the presence of a catalyst and a base, wherein the catalyst is an N-heterocyclic carbene having a pKa of 33.0 or less, the pKa being calculated from a free energy of a stable structure in methanol of the catalyst molecule determined by an SMD method using Gaussian 16.

Description

Method for producing alkylfuran carboxylic acid esters

[0001] The present invention aims to provide a method for producing an alkylfuran carboxylic acid ester, and more particularly, to a method for producing an alkylfuran carboxylic acid ester using an alkylfuran aldehyde as a starting material.

[0002] Methods for producing alkyl furan carboxylic acid esters have been investigated. For example, Non-Patent Document 1 describes the production of 5-methyl-2-furoate from 5-methyl-2-furaldehyde.

[0003] Stereoselective Synthesis of 1,2-Cyclopropanecarboxylated Furanoids: Applications towards the Preparation of Marine Natural Products and Unnatural Amino Acids, University of Regensburg, Graduation Thesis, 2006

[0004] Although the method described in Non-Patent Document 1 is known, a new method for producing alkyl furan carboxylic acid esters is still needed. The present invention aims to solve this problem and to provide a new method for producing alkyl furan carboxylic acid esters.

[0005] In light of the above-mentioned problems, the present inventors conducted research and found that the nucleophilic reaction to alkylfuranaldehyde can be sufficiently promoted by using an N-heterocyclic carbene having a predetermined pKa or less as a catalyst, thereby completing the present invention. Specifically, the above-mentioned problems have been solved by the following means. <1> A method for producing an alkylfurancarboxylic acid ester, comprising: carrying out an oxidation-reduction reaction between an alkylfuranaldehyde and a compound represented by A-OH (where A is an organic group having 1 to 10 carbon atoms) in the presence of a catalyst and a base, wherein the catalyst is an N-heterocyclic carbene having a pKa of 33.0 or less, calculated from the free energy obtained by determining a stable structure of the catalyst molecule in methanol by the SMD method using Gaussian 16. <2> A method for producing an alkylfurancarboxylic acid ester according to <1>, further comprising carrying out the oxidation-reduction reaction in the presence of an oxidizing agent. <3> A method for producing an alkylfurancarboxylic acid ester according to <2>, wherein the oxidizing agent is a quinone. <4> A method for producing an alkylfurancarboxylic acid ester according to <2>, wherein the oxidizing agent includes at least one oxidizing agent represented by formula (O). (In formula (O), R １ ~R ８ R each independently represents a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkenyl group having 2 to 12 carbon atoms, an alkynyl group having 2 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, an acyl group having 2 to 12 carbon atoms, or a hydroxy group. １ ~R ８ is an alkyl group, an alkenyl group, an alkynyl group, an alkoxy group, an aryl group, or an acyl group, １ ~R ８ Each group in the formula may have a substituent bonded to a carbon atom in the formula, such as an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, a hydroxy group, a halogen atom, a cyano group, or a nitro group. m is 1 or 0. R １ and R ２ , R ３ and R ４ , R ５ and R ６ , R ７ and R ８may be bonded to or fused with each other to form a ring.) <5> The method for producing an alkylfurancarboxylic acid ester according to <4>, wherein the oxidizing agent represented by formula (O) includes at least one oxidizing agent represented by formula (O-3). (In formula (O-3), R ３０１ ~R ３０８ are each independently R in formula (O). １ represents a group having the same meaning as the above.) <6> The method for producing an alkylfuran carboxylic acid ester according to any one of <1> to <5>, wherein the pKa of the catalyst is 16.0 or higher. <7> The method for producing an alkylfuran carboxylic acid ester according to any one of <1> to <6>, wherein the pKa of the catalyst is 30.0 or lower. <8> The method for producing an alkylfuran carboxylic acid ester according to any one of <1> to <7>, wherein the catalyst comprises at least one catalyst selected from the group consisting of triazolium, imidazolinium, imidazolium, and thiazolium. <9> The method for producing an alkylfuran carboxylic acid ester according to any one of <1> to <8>, wherein the catalyst comprises at least one catalyst represented by the following formula (C): (In formula (C), R Ｃ１ and R Ｃ２ each independently represents a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, a heteroaryl group having 4 to 16 carbon atoms, a halogenated alkyl group having 1 to 12 carbon atoms, a halogenated aryl group having 6 to 18 carbon atoms, an alkylaryl group having 7 to 20 carbon atoms, or an arylalkyl group having 7 to 20 carbon atoms. Z is -S- or -NR ｃ３ X represents a methine group (=CR Ｃ５ -), a nitrogen atom (=N-), or a methylene group (-CR Ｃ６ ２ When X is a methine group or a nitrogen atom, the dashed line in the formula represents a double bond, and when X is a methylene group, the dashed line in the formula represents a single bond. Ｃ３ , R Ｃ５ and R Ｃ６are each independently a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkenyl group having 2 to 12 carbon atoms, an alkynyl group having 2 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an acyl group having 2 to 12 carbon atoms, a hydroxy group, a carboxy group, or a halogen atom. － represents a counter anion. Ｃ２ and R Ｃ３ , R Ｃ２ and R Ｃ５ , R Ｃ２ and R Ｃ６ may be bonded to or fused to each other to form a ring.) <10> The method for producing an alkylfurancarboxylic acid ester according to <9>, wherein the catalyst represented by formula (C) includes at least one of the catalysts represented by formulas (C2) to (C6): (In formula (C2), R １１ is R Ｃ１ R represents a group having the same meaning as ２１ is R Ｃ２ R represents a group having the same meaning as ３１ is R Ｃ３ R represents a group having the same meaning as ５１ is R Ｃ５ represents a group having the same meaning as － represents a counter anion. ２１ and R ３１ , R ２１ and R ５１ , R １１ and R ５１ may be bonded to or fused with each other to form a ring. (In formula (C3), R １２ is R Ｃ１ R represents a group having the same meaning as ２２ is R Ｃ２ R represents a group having the same meaning as ３２ is R Ｃ３ R represents a group having the same meaning as ６２ is R Ｃ６ represents a group having the same meaning as － represents a counter anion. ２２ and R ３２ , R ２２ and R ６２ , R １２ and R ６２ may be bonded to or fused with each other to form a ring. (In formula (C4), R １３ is R Ｃ１ R represents a group having the same meaning as ２３ is R Ｃ２ R represents a group having the same meaning as ３３ is R Ｃ３ represents a group having the same meaning as － represents a counter anion. ２３ and R ３３ may be bonded to or fused with each other to form a ring. (In formula (C5), R １４ is R Ｃ１ R represents a group having the same meaning as ７４ , R ８４ , R ９４ are each independently R Ｃ２ represents a group having the same meaning as － represents a counter anion.) (In formula (C6), R １５ is R Ｃ１ R represents a group having the same meaning as ２５ is R Ｃ２ R represents a group having the same meaning as ５５ is R Ｃ５ represents a group having the same meaning as － represents a counter anion. １５ and R ５５ , R ５５ and R ２５ may be bonded to or fused to each other to form a ring.) <11> The method for producing an alkylfuran carboxylic acid ester according to any one of <1> to <10>, wherein the alkylfuran aldehyde is a compound represented by formula (F1): (In formula (F1), R is an alkyl group having 1 to 10 carbon atoms.) <12> The method for producing an alkyl furan carboxylic acid ester according to any one of <1> to <11>, wherein the alkyl furan carboxylic acid ester is a compound represented by formula (F2). (In formula (F2), R is an alkyl group having 1 to 10 carbon atoms, and A is an organic group having 1 to 10 carbon atoms.) <13> The method for producing an alkyl furan carboxylic acid ester according to any one of <1> to <12>, wherein in the compound represented by A-OH, A is an alkyl group having 1 to 5 carbon atoms.

[0006] The present invention makes it possible to provide a novel method for producing alkylfurancarboxylic acid esters.

[0007] Hereinafter, a detailed description of an embodiment of the present invention (hereinafter simply referred to as "the present embodiment") will be given. Note that the following present embodiment is an example for explaining the present invention, and the present invention is not limited to this embodiment. In this specification, the term "to" is used to mean that the numerical values ​​before and after it are included as lower and upper limits. In this specification, various physical property values ​​and characteristic values ​​are those at 23°C unless otherwise specified. In the description of groups (atomic groups) in this specification, a notation that does not indicate substituted or unsubstituted includes both groups (atomic groups) that have no substituents and groups (atomic groups) that have substituents. For example, "alkyl group" includes not only alkyl groups that have no substituents (unsubstituted alkyl groups) but also alkyl groups that have substituents (substituted alkyl groups). In this specification, when a notation that does not indicate substituted or unsubstituted is used, unsubstituted is preferred. If the measurement method, etc., of the specifications shown in this specification differ depending on the year, they will be based on the specifications as of January 1, 2021, unless otherwise specified. pKa means acid dissociation constant.

[0008] The method for producing an alkylfuran carboxylic acid ester according to the present embodiment includes carrying out an oxidation-reduction reaction between an alkylfuran aldehyde and a compound represented by A-OH (where A is an organic group having 1 to 10 carbon atoms) in the presence of a catalyst and a base, wherein the catalyst is an N-heterocyclic carbene having a pKa of 33.0 or less, calculated from the free energy obtained by determining the stable structure of the catalyst molecule in methanol using a solvation model density (SMD) model with Gaussian 16. This configuration provides a novel method for producing an alkylfuran carboxylic acid ester. Furthermore, the present embodiment effectively promotes an intermolecular oxidation-reduction reaction between the alkylfuran aldehyde and the compound represented by A-OH (where A is an organic group having 1 to 10 carbon atoms), thereby producing an alkylfuran carboxylic acid ester in high yield. Since alkylfuran aldehyde can promote an intermolecular oxidation-reduction reaction, there is an advantage in that no salt is formed as a by-product. Furthermore, the present embodiment allows the production of an alkylcarboxylic acid ester in a single reaction step.

[0009] In the production method of this embodiment, the alkyl furan aldehyde is a substrate for the oxidation-reduction reaction, and is usually added to the reaction system. However, the alkyl furan aldehyde may also be an intermediate for producing an alkyl furan carboxylic acid ester.

[0010] The alkyl furan aldehyde is a furan having an alkyl group and an aldehyde group as substituents, and may have other substituents. Examples of other substituents include an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, a hydroxy group, a halogen atom, a cyano group, or a nitro group, but it is preferable that the alkyl furan aldehyde has no substituents other than the alkyl group and the aldehyde group. The number of carbon atoms in the alkyl group having 1 to 10 carbon atoms in the alkyl furan aldehyde is preferably 8 or less, more preferably 6 or less, even more preferably 5 or less, still more preferably 4 or less, even more preferably 3 or less, and may be 2 or less, or even 1.

[0011] The molecular weight of the alkylfuran aldehyde is preferably 111 to 500, and more preferably 111 to 300.

[0012] The alkylfuran aldehyde is preferably a compound represented by formula (F1). (In formula (F1), R is an alkyl group having 1 to 10 carbon atoms.)

[0013] The number of carbon atoms in the alkyl group for R is preferably 8 or less, more preferably 6 or less, even more preferably 5 or less, still more preferably 4 or less, still more preferably 3 or less, and may be 2 or less, or may be 1. Specific examples of R include a methyl group, an ethyl group, an isopropyl group, an n-propyl group, an isobutyl group, an n-butyl group, and a tert-butyl group, with a methyl group, an ethyl group, and an isopropyl group being preferred, and a methyl group being more preferred.

[0014] In this embodiment, only one type of alkylfuran aldehyde may be used, or two or more types may be used.

[0015] The target product of the production method of this embodiment is an alkyl furan carboxylic acid ester. Typically, the alkyl furan carboxylic acid ester produced (product mixture) is removed from the reaction system, and if necessary, the catalyst is separated and impurities (by-products) are removed. However, the alkyl furan carboxylic acid ester may also be an intermediate for producing another compound. That is, further reactions may be carried out in the same reaction system.

[0016] The alkyl group of the alkyl furan carboxylic acid ester (the alkyl group corresponding to R in formula (F2)) is the same as the alkyl group of the alkyl furan aldehyde (the alkyl group corresponding to R in formula (F1)). The alkyl furan carboxylic acid ester is preferably a compound represented by formula (F2). (In formula (F2), R is an alkyl group having 1 to 10 carbon atoms, and A is an organic group having 1 to 10 carbon atoms.) R has the same meaning as R in formula (F1). A has the same meaning as A in the compound represented by A-OH.

[0017] In the production method of this embodiment, an oxidation-reduction reaction is carried out between an alkylfuranaldehyde and a compound represented by A-OH (wherein A is an organic group having 1 to 10 carbon atoms). That is, the compound represented by A-OH acts as a nucleophile that nucleophiles the formyl group of the alkylfuranaldehyde. The organic group having 1 to 10 carbon atoms, represented by A in A-OH, is preferably a hydrocarbon group having 1 to 10 carbon atoms or a group formed by combining a hydrocarbon group having 1 to 10 carbon atoms with -O- and / or -C(=O)-; more preferably an alkyl group having 1 to 10 carbon atoms or a group formed by combining an alkyl group having 1 to 10 carbon atoms with -O- and / or -C(=O)-; and preferably an alkyl group having 1 to 10 carbon atoms. The number of carbon atoms in A is preferably 8 or less, more preferably 6 or less, even more preferably 5 or less, even more preferably 4 or less, even more preferably 3 or less, and may be 2 or less, or may be 1. The hydrocarbon group represented by A is preferably a linear, branched, or cyclic alkyl group or aryl group, and more preferably a linear alkyl group (primary alcohol). Use of a primary alcohol tends to further improve reactivity and increase yield. The molecular weight of the compound represented by A-OH is preferably 32 or more, and is preferably 500 or less, more preferably 300 or less, even more preferably 200 or less, and even more preferably 100 or less.

[0018] Examples of the compound represented by A-OH include methanol, ethanol, isopropanol, isobutanol, n-propanol, n-butanol, and the compounds shown below, with methanol being preferred.

[0019] The compound represented by A-OH in this embodiment may be the same substance as the solvent, which will be described in detail later. For example, the compound represented by A-OH and the solvent may both be methanol.

[0020] In the oxidation-reduction reaction, the amount of the compound represented by A-OH (nucleophile) relative to 1 mole of substrate is preferably 1 mole or more, more preferably more than 1 mole, even more preferably 1.5 moles or more, and even more preferably 2.5 moles or more. Furthermore, the amount of the compound represented by A-OH (nucleophile) relative to 1 mole of substrate is preferably 20 moles or less, more preferably 10 moles or less, even more preferably 8 moles or less, even more preferably 5 moles or less, and even more preferably 4 moles or less. In the oxidation-reduction reaction of this embodiment, only one type of compound represented by A-OH may be used, or two or more types may be used. When two or more types are used, it is preferable that the total amount be in the above-mentioned range. When the compound represented by A-OH is also used as a solvent, the blending amount thereof is the same as the preferred range of the blending amount of the solvent.

[0021] In the production method of this embodiment, a redox reaction is carried out in the presence of a catalyst and a base. The catalyst is an N-heterocyclic carbene whose pKa (hereinafter simply referred to as "catalyst pKa") calculated from the free energy after determining the stable structure of the catalyst molecule in methanol using a solvation model density (SMD) with Gaussian 16 is 33.0 or less. More specifically, the pKa of the catalyst was calculated by using the software Gaussian 16 and performing a structural optimization calculation of the gaseous molecule under the conditions of B3LYP / 6-31+G(d). Vibrational calculations were performed on the obtained structure, and it was confirmed that it was a stable structure without imaginary vibrations. When determining the stable structure, the conformations before and after acid dissociation were kept identical except for the presence or absence of hydrogen. The free energies at 298.15 K of the molecule in the gaseous state and in the solvent were determined by single-point calculation using M06-2X / 6-311++G(d,p). In this calculation, the SMD solvation model was used to take into account the solvent effect. Methanol was used as the solvent. The proton solubility free energy used was -255.6 kcal / mol. From the free energy of each state calculated using this method, the free energy difference ΔGsoln before and after acid dissociation was calculated, and the pKa was calculated. By using a catalyst with a pKa of 33.0 or less calculated using the above method, the conversion rate of alkylfuran aldehyde can be increased, and the reaction rate (reaction cycle) of the reaction system can be increased.

[0022] The pKa of the catalyst is preferably 32.0 or less, more preferably 31.5 or less, and further preferably 31.0 or less, 30.5 or less, 30.0 or less, 29.5 or less, 29.0 or less, 28.5 or less, 28.0 or less, 27.5 or less, 27.0 or less, 26.5 or less, 26.0 or less, 25.5 or less, and 25.0 or less. By setting the pKa at or below the upper limit, the yield of the obtained alkyl furan carboxylic acid ester is further improved. Furthermore, the pKa of the catalyst is preferably 15.0 or more, more preferably 16.0 or more, and further preferably 17.0 or more, 17.5 or more, 18.0 or more, 18.5 or more, 19.0 or more, 19.5 or more, 20.0 or more, 20.5 or more, 21.0 or more, 21.5 or more, 22.0 or more, 22.5 or more, and 23.0 or more. In the oxidation-reduction reaction of this embodiment, only one of the catalysts may be used, or two or more of them may be used. When two or more of them are used, each catalyst must satisfy the above-mentioned preferred range.

[0023] The catalyst having a pKa of 33.0 or less promotes the reaction by the action of a base. The catalyst having a pKa of 33.0 or less preferably contains at least one selected from the group consisting of triazolium, imidazolinium, imidazolium, and thiazolium, more preferably contains at least one selected from the group consisting of triazolium, imidazolinium, and imidazolium, even more preferably contains at least one selected from the group consisting of triazolium and imidazolinium, and still more preferably contains at least one triazolium.

[0024] More specifically, the catalyst having a pKa of 33.0 or less preferably contains at least one catalyst represented by the following formula (C): (In formula (C), R Ｃ１ and R Ｃ２each independently represents a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, a heteroaryl group having 4 to 16 carbon atoms, a halogenated alkyl group having 1 to 12 carbon atoms, a halogenated aryl group having 6 to 18 carbon atoms, an alkylaryl group having 7 to 20 carbon atoms, or an arylalkyl group having 7 to 20 carbon atoms. Z is -S- or -NR ｃ３ X represents a methine group (=CR Ｃ５ -), a nitrogen atom (=N-), or a methylene group (-CR Ｃ６ ２ When X is a methine group or a nitrogen atom, the dashed line in the formula represents a double bond, and when X is a methylene group, the dashed line in the formula represents a single bond. Ｃ３ , R Ｃ５ and R Ｃ６ are each independently a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkenyl group having 2 to 12 carbon atoms, an alkynyl group having 2 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an acyl group having 2 to 12 carbon atoms, a hydroxy group, a carboxy group, or a halogen atom. － represents a counter anion. Ｃ２ and R Ｃ３ , R Ｃ２ and R Ｃ５ , R Ｃ２ and R Ｃ６ , R Ｃ１ and R Ｃ５ , R Ｃ１ and R Ｃ６ may be bonded to or condensed with each other to form a ring.) The methylene group (—CR Ｃ６ ２ -) Two R's included Ｃ６ may be the same or different. Hereinafter, when two or more groups with the same symbol are present in one compound, the two or more groups with the same symbol may be the same or different.

[0025] In this embodiment, the alkyl group is intended to include not only linear and branched alkyl groups but also cycloalkyl groups, and the alkylaryl group and arylalkyl group are intended to include groups in which a cyclic alkyl and an aryl (e.g., a benzene ring) are fused together.

[0026] R Ｃ１ and R Ｃ２ each independently represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 12 carbon atoms, a halogenated alkyl group having 1 to 6 carbon atoms, a halogenated aryl group having 6 to 12 carbon atoms, an alkylaryl group having 7 to 13 carbon atoms, or an arylalkyl group having 7 to 13 carbon atoms. The halogen atom contained in the halogenated alkyl group and the halogenated aryl group is preferably a fluorine atom or a chlorine atom.

[0027] R Ｃ３ , R Ｃ５ and R Ｃ６ are each independently preferably a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkenyl group having 2 to 6 carbon atoms, an alkynyl group having 2 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, an acyl group having 2 to 7 carbon atoms, a hydroxy group, a carboxy group, or a halogen atom, more preferably a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms, even more preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, and still more preferably a hydrogen atom.

[0028] There is no particular limitation as long as it is a monovalent anion, but Y － is preferably an anion containing a halogen, F － , Cl － ,Br － , I － , B.F. ４ － is preferred.

[0029] When the catalyst represented by the above formula (C) has a methine group in X, it is preferably represented by the following formula (C2). (In formula (C2), R １１ is R Ｃ１ R represents a group having the same meaning as ２１ is R Ｃ２ R represents a group having the same meaning as ３１ is R Ｃ３ R represents a group having the same meaning as ５１ is R Ｃ５ represents a group having the same meaning as －represents a counter anion. ２１ and R ３１ , R ２１ and R ５１ , R １１ and R ５１ may be bonded to or fused with each other to form a ring.

[0030] R １１ and R ３１ are each independently preferably an alkyl group having 1 to 12 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms. ２１ , R ５１ is preferably a hydrogen atom.

[0031] When the catalyst represented by the above formula (C) has a methylene group in X, it is preferably represented by the following formula (C3). (In formula (C3), R １２ is R Ｃ１ R represents a group having the same meaning as ２２ is R Ｃ２ R represents a group having the same meaning as ３２ is R Ｃ３ R represents a group having the same meaning as ６２ is R Ｃ６ represents a group having the same meaning as － represents a counter anion. ２２ and R ３２ , R ２２ and R ６２ , R １２ and R ６２ may be bonded to or fused with each other to form a ring.

[0032] R １２ and R ３２ are each independently preferably an alkyl group having 1 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, or an arylalkyl group having 7 to 20 carbon atoms. ２２ , R ６２ is preferably a hydrogen atom.

[0033] When the catalyst represented by the above formula (C) has a nitrogen atom in X, it is preferably represented by the following formula (C4). (In formula (C4), R １３ is R Ｃ１ R represents a group having the same meaning as ２３ is RＣ２ R represents a group having the same meaning as ３３ is R Ｃ３ represents a group having the same meaning as － represents a counter anion. ２３ and R ３３ may be bonded to or fused with each other to form a ring.

[0034] R １３ and R ３３ are each independently preferably an alkyl group having 1 to 12 carbon atoms, more preferably an alkyl group having 1 to 5 carbon atoms, even more preferably an alkyl group having 1 to 3 carbon atoms, and most preferably a methyl group. ２３ is preferably a hydrogen atom.

[0035] The catalyst represented by the formula (C4) is R ２３ and R ３３ When a ring is formed by the above, it is preferably represented by the following formula (C5). (In formula (C5), R １４ is R Ｃ１ R represents a group having the same meaning as ７４ , R ８４ , R ９４ are each independently R Ｃ２ represents a group having the same meaning as － represents a counter anion.)

[0036] R １４ is preferably a halogenated aryl group having 6 to 18 carbon atoms or an alkylaryl group having 7 to 20 carbon atoms. ７４ , R ８４ , R ９４ is preferably a hydrogen atom.

[0037] When the catalyst represented by the above formula (C) has a sulfur atom in Z, it is preferably represented by the following formula (C4). (In formula (C6), R １５ is R Ｃ１ R represents a group having the same meaning as ２５ is R Ｃ２ R represents a group having the same meaning as ５５ is R Ｃ５ represents a group having the same meaning as － represents a counter anion. １５ and R ５５ , R５５ and R ２５ may be bonded to or fused with each other to form a ring.

[0038] In this embodiment, a catalyst represented by formula (C4) is particularly preferred.

[0039] Examples of the catalyst include the following compounds. It goes without saying that the present embodiment is not limited to these.

[0040] In the oxidation-reduction reaction, the amount of the catalyst having a pKa of 33.0 or less relative to 1 mole of substrate is preferably 0.0001 moles or more, more preferably 0.001 moles or more, even more preferably 0.01 moles or more, even more preferably 0.04 moles or more, even more preferably 0.05 moles or more, even more preferably 0.1 moles or more, and particularly preferably 0.2 moles or more. By setting the amount to be equal to or greater than the above lower limit, the oxidation-reduction reaction tends to proceed more effectively. The amount of the catalyst relative to 1 mole of substrate is preferably 5.0 moles or less, more preferably 3.0 moles or less, even more preferably 1.0 moles or less, even more preferably 0.5 moles or less, and even more preferably 0.4 moles or less. In the oxidation-reduction reaction of this embodiment, only one catalyst having a pKa of 33.0 or less may be used, or two or more catalysts may be used. When two or more catalysts are used, it is preferable that the total amount be within the above range.

[0041] In the production method of this embodiment, it is preferable to further carry out the oxidation-reduction reaction in the presence of an oxidizing agent. By using an oxidizing agent, the progression of side reactions can be more effectively suppressed. The type of oxidizing agent is not particularly limited, and may be an organic compound or an inorganic compound. The amount of oxidizing agent can be determined appropriately depending on the type of oxidizing agent. For example, the amount of oxidizing agent per mole of substrate is preferably 0.01 moles or more and preferably 20.0 moles or less. In the oxidation-reduction reaction of this embodiment, only one type of oxidizing agent may be used, or two or more types may be used. When two or more types are used, it is preferable that the total amount is within the above range.

[0042] In this embodiment, when the oxidizing agent is an organic compound, examples thereof include hypervalent organic iodine compounds such as quinone, nitrobenzene, and 2-iodobenzoic acid, nitroxy radical compounds such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), and organic peroxides such as m-chlorobenzoic acid, with quinone being preferred. Here, quinone refers to a compound in which two hydrogen atoms bonded to the benzene ring of an aromatic hydrocarbon are each replaced with an oxygen atom. Additionally, oxidizing agents described in Oxidizing Agents for Organic Synthesis, Third Edition, published by Fujifilm Wako Pure Chemical Industries, Ltd., can also be used, the contents of which are incorporated herein by reference.

[0043] The molecular weight of the quinone used as the oxidizing agent is preferably 108 or more, and is preferably 1000 or less, more preferably 800 or less, and may be 600 or less.

[0044] The oxidizing agent preferably contains at least one oxidizing agent represented by formula (O). (In formula (O), R １ ~R ８ R each independently represents a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkenyl group having 2 to 12 carbon atoms, an alkynyl group having 2 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, an acyl group having 2 to 12 carbon atoms, or a hydroxy group. １ ~R ８is an alkyl group, an alkenyl group, an alkynyl group, an alkoxy group, an aryl group, or an acyl group, １ ~R ８ Each group in the formula may have a substituent bonded to a carbon atom in the formula, such as an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, a hydroxy group, a halogen atom, a cyano group, or a nitro group. m is 1 or 0. R １ and R ２ , R ３ and R ４ , R ５ and R ６ , R ７ and R ８ may be bonded to or fused with each other to form a ring.

[0045] R １ ~R ８ are preferably each independently a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, or an alkoxy group having 1 to 12 carbon atoms, more preferably each independently a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or an alkoxy group having 1 to 8 carbon atoms, and even more preferably each independently a hydrogen atom or an alkyl group having 1 to 6 carbon atoms.

[0046] In formula (O), m is 0 and R １ and R ２ , R ３ and R ４ An example in which the groups are fused together to form a 6-membered ring is shown in the following formula (O-1). (In formula (O-1), R １０１ ~R １０８ are each independently R in formula (O). １ represents a group having the same meaning as

[0047] R １０１ ~R １０８ are each independently the above R １ and is preferably a hydrogen atom or an alkyl group having 1 to 6 carbon atoms.

[0048] In formula (O), m is 1, and R １ and R ２ , R ３ and R ４ , R ５ and R６ , R ７ and R ８ An example in which the groups are fused together to form a 6-membered ring is shown in the following formula (O-2). (In formula (O-2), R ２０１ ~R ２０４ are each independently R in formula (O). １ represents a group having the same meaning as

[0049] R ２０１ ~R ２０４ are each independently the above R １ n1, n2, n3, and n4 each independently represent an integer of 1 to 4.

[0050] In formula (O), m is 1, and R １ and R ２ , R ３ and R ４ , R ５ and R ６ , R ７ and R ８ are not condensed with each other to form a ring, the following formula (O-3) can be mentioned. (In formula (O-3), R ３０１ ~R ３０８ are each independently R in formula (O). １ represents a group having the same meaning as

[0051] R ３０１ ~R ３０８ are each independently the above R １ and is preferably a hydrogen atom or an alkyl group having 1 to 12 carbon atoms, more preferably a hydrogen atom or an alkyl group having 1 to 8 carbon atoms.

[0052] In formula (O), m is 0 and R １ and R ２ , R ３ and R ４ are not condensed with each other to form a ring, the following formula (O-4) can be mentioned. (In formula (O-4), R ４０１ ~R ４０４ are each independently R in formula (O). １ R represents a group having the same meaning as ４０１ ~R ４０４are each independently the above R １ R is preferably a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, or an alkoxy group having 1 to 12 carbon atoms, more preferably a hydrogen atom or an alkyl group having 1 to 12 carbon atoms, and even more preferably a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. ４０１ ~R ４０４ When is an alkyl group or an alkoxy group, the alkyl group or the alkoxy group may have a halogen atom as a substituent bonded to a carbon atom in the formula.

[0053] In this embodiment, formula (O-1), formula (O-3) and formula (O-4) are preferred, formula (O-3) and (O-4) are more preferred, and formula (O-3) is even more preferred.

[0054] Examples of the oxidizing agent include the following compounds. It goes without saying that the present embodiment is not limited to these. Note that tBu is a tert-butyl group.

[0055] In the oxidation-reduction reaction, the amount of quinone relative to 1 mole of substrate is preferably 0.01 mol or more, more preferably 0.05 mol or more, even more preferably 0.1 mol or more, even more preferably 0.5 mol or more, and even more preferably 0.8 mol or more. By adjusting the amount to be equal to or greater than the lower limit, the yield of the alkyl furan carboxylic acid ester tends to be further improved. Furthermore, the amount of oxidizing agent relative to 1 mole of the substrate may be 3.0 mol or less, 2.5 mol or less, 2.0 mol or less, 1.5 mol or less, or 1.2 mol or less. In the oxidation-reduction reaction of this embodiment, only one quinone may be used, or two or more quinones may be used. When two or more quinones are used, it is preferable that the total amount be within the above range.

[0056] In the present embodiment, when the oxidizing agent is an inorganic compound, examples thereof include permanganates such as potassium permanganate; chromic acids such as potassium dichromate and chromium oxide; nitric acids such as nitric acid and potassium nitrate; halogens such as fluorine, chlorine, bromine, and iodine; peroxides such as aqueous hydrogen peroxide and sodium peroxide; oxides such as copper (II) oxide, lead (IV) oxide, and manganese (IV) oxide; and metal salts such as iron chloride and copper sulfate. Oxides are preferred, and manganese oxide is more preferred.

[0057] In the oxidation-reduction reaction, the amount of the inorganic compound as an oxidizing agent relative to 1 mole of the substrate is preferably 1.00 moles or more, more preferably 3.00 moles or more, and even more preferably 5.00 moles or more, and is preferably 20.0 moles or less, more preferably 15.0 moles or less, and even more preferably 10.0 moles or less. In the oxidation-reduction reaction of this embodiment, only one type of inorganic compound may be used as the oxidizing agent, or two or more types may be used. When two or more types are used, it is preferable that the total amount be within the above range.

[0058] In the production method of this embodiment, the oxidation-reduction reaction is preferably carried out in the presence of a solvent. The solvent is not particularly limited as long as it can dissolve a part or all of the alkylfuran aldehyde and does not interfere with the oxidation-reduction reaction. Examples of the solvent used in the production method of this embodiment include aromatic hydrocarbon solvents, aliphatic hydrocarbon solvents, amide solvents, ether solvents, alcohol solvents, halogenated solvents, and ester solvents. Aromatic hydrocarbon solvents, ether solvents, and alcohol solvents are preferred, and ether solvents and sulfoxide solvents are more preferred.

[0059] Specific examples of aromatic hydrocarbon solvents include benzene and toluene. Specific examples of amide solvents include acetonitrile, N,N-dimethylacetamide, and N,N-dimethylformamide. Specific examples of ether solvents include tetrahydrofuran (hereinafter also referred to as THF) and diethyl ether. Specific examples of alcohol solvents include methanol, ethanol, and isopropanol. Alcohol solvents can also serve as nucleophiles. Specific examples of halogen solvents include dichloromethane, dichloroethane, and chloroform. Specific examples of ester solvents include ethyl acetate. Specific examples of sulfoxide solvents include dimethyl sulfoxide.

[0060] In the oxidation-reduction reaction, the amount of solvent used is not particularly limited, but from the viewpoints of productivity and energy efficiency, it is preferably 0.5 times by mass or more, more preferably 0.8 times by mass or more, and even more preferably 1.0 times by mass or more relative to the substrate (alkylfuran aldehyde). Furthermore, the amount of solvent used may be preferably 200 times by mass or less, more preferably 100 times by mass or less, even more preferably 50 times by mass or less, even more preferably 40 times by mass or less, and even more preferably 30 times by mass or less relative to the substrate. In the oxidation-reduction reaction, only one solvent may be used, or two or more solvents may be used. When two or more solvents are used, it is preferable that the total amount is within the above range.

[0061] In the production method of this embodiment, it is preferable to carry out the oxidation-reduction reaction in the presence of a base. Carrying out the reaction in the presence of a base tends to further improve the yield of the alkylfurancarboxylic acid ester. The base may be an organic or inorganic base. Examples of the base include alkylamines, alkanolamines, polyamines, hydroxylamine, cyclic amines, quaternary ammonium, alkali metal-containing compounds, and alkaline earth metal-containing compounds. Alkylamines, polyamines, cyclic amines, and alkali metal-containing compounds are preferred, and diazabicycloundecene, sodium carbonate, sodium hydroxide, triethylamine, sodium methoxide, and potassium carbonate are more preferred. The oxidation-reduction reaction preferably contains at least one of diazabicycloundecene, sodium carbonate, and potassium carbonate, more preferably contains diazabicycloundecene and / or sodium carbonate, and even more preferably contains diazabicycloundecene.

[0062] In the oxidation-reduction reaction, the amount of base per mole of catalyst is preferably 1 mole or more. By setting the amount to be equal to or greater than the lower limit, the oxidation-reduction reaction tends to proceed more effectively. Furthermore, the amount of base per mole of substrate is preferably 200 moles or less, more preferably 100 moles or less. In the oxidation-reduction reaction of this embodiment, only one type of base may be used, or two or more types may be used. When two or more types are used, the total amount is preferably within the above range.

[0063] The reaction temperature of the oxidation-reduction reaction in the production method of this embodiment is not particularly limited, but is preferably −80° C. or higher, more preferably 0° C. or higher, even more preferably 15° C. or higher, still more preferably 20° C. or higher, and even more preferably 25° C. or higher. The reaction temperature of the oxidation-reduction reaction is preferably 200° C. or lower, more preferably 150° C. or lower, even more preferably 100° C. or lower, still more preferably 50° C. or lower, and even more preferably 40° C. or lower. In the production method of this embodiment, the reaction temperature may be the same except for the initial temperature increase and the final temperature decrease (with a ±5° C. variation allowed as an error), or the reaction may be carried out in two or more stages. In this embodiment, the reaction temperature is preferably the same except for the initial temperature increase and the final temperature decrease (with a ±5° C. variation allowed as an error).

[0064] The reaction time of the oxidation-reduction reaction in the production method of this embodiment is preferably 1 minute or more, may be 30 minutes or more, may be 1 hour or more, or may be 1.5 hours or more, and the reaction time of the oxidation-reduction reaction is preferably 50 hours or less, may be 30 hours or less, or may be 25 hours or less.

[0065] In this embodiment, for example, when producing an alkylfurancarboxylic acid ester, a solvent, a catalyst, a base, and a compound represented by A-OH are added in this order to an alkylfuranaldehyde as a substrate.

[0066] The reaction mixture and the catalyst after the reaction can be separated by a common method such as sedimentation, centrifugation, or filtration. The catalyst is preferably separated under an inert gas atmosphere such as nitrogen or argon, depending on the catalyst used, to prevent ignition. The reaction mixture may be concentrated as needed, and the residue may be used as a raw material or intermediate, or the reaction mixture may be purified by appropriate post-treatment. Specific post-treatment methods include known purification methods such as extraction, distillation, and chromatography. Two or more of these purification methods may be combined.

[0067] In the production method of this embodiment, the higher the conversion rate of the raw material, the better, preferably 50 mol% or more, and more preferably 80 mol% or more. The upper limit is ideally 100 mol%. In the production method of this embodiment, the higher the yield of the alkyl furan carboxylic acid ester, the better, preferably 3% or more, more preferably 10% or more, and even more preferably 30% or more. The upper limit is ideally 100 mol%.

[0068] In the production method of this embodiment, salts are not usually produced as by-products. Therefore, the resulting alkyl furan carboxylic acid esters are preferably used in various applications. In particular, the alkyl furan carboxylic acid esters obtained by the production method of this embodiment are preferably used as various industrial materials or raw materials thereof.

[0069] The present invention will be explained in more detail below with reference to examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. If the measuring instruments used in the examples are difficult to obtain due to discontinuation or the like, measurements can be made using other instruments with equivalent performance.

[0070] Raw Materials <Substrate> MFF: methylfurfural, Fujifilm Wako Pure Chemical Industries, Ltd., 133-11771 iPrFF: isopropylfurfural, Matrix Scientific, Ltd., 020867 <Solvent> THF: tetrahydrofuran, Fujifilm Wako Pure Chemical Industries, Ltd., 206-00483 Toluene: Fujifilm Wako Pure Chemical Industries, Ltd., 204-01866 Methanol: Fujifilm Wako Pure Chemical Industries, Ltd., 137-01823 Dichloromethane: Fujifilm Wako Pure Chemical Industries, Ltd., 135-02441 <Base> DBU: diazabicycloundecene, Tokyo Chemical Industry Co., Ltd., D1270 Sodium carbonate: Fujifilm Wako Pure Chemical Industries, Ltd., 199-01585 <Nucleophile> MeOH (methanol): Fujifilm Wako Pure Chemical Industries, Ltd., 137-01823 nPrOH (n-propyl alcohol): Tokyo Chemical Industry Co., Ltd., P0491

[0071] <Catalyst> CatA, manufactured by Tokyo Chemical Industry Co., Ltd., D4624 CatB: Manufactured by Sigma-Aldrich, 688487-250MG Cat C: Tokyo Chemical Industry Co., Ltd., D3341 Cat D: Tokyo Chemical Industry Co., Ltd., D3711 CatE: Sigma-Aldrich, 708607-1G Cat F: Tokyo Chemical Industry Co., Ltd., D3446 CatG: Tokyo Chemical Industry Co., Ltd., B3158

[0072] <Oxidizing agent> Q1: Fujifilm Wako Pure Chemical Industries, 359-16793 tBu is a tert-butyl group. Q2: Tokyo Chemical Industry Co., Ltd., E0063 Q3: Fujifilm Wako Pure Chemical Industries, 171-00242 Q4: Tokyo Chemical Industry Co., Ltd., D2256 Q8: Tokyo Chemical Industry Co., Ltd., D2152 Q9: Manganese (IV) oxide: Fujifilm Wako Pure Chemical Industries, 138-09675

[0073] Example 1 <Production of methyl furoate> 0.033 g (0.3 mmol) of MFF, 1 g of THF (stabilizer-free), 0.033 g (0.3 eq, 0.09 mmol) of Cat A, 0.014 g (0.3 eq) of DBU, and 0.29 g (3 eq, 0.9 mmol) of methanol were placed in a 20 mL recovery flask in this order. The flask was placed in a dry bath and stirred at 30°C for 2 hours. The reaction solution after the reaction was analyzed using high-performance liquid chromatography (HPLC) and quantified using the absolute calibration curve method. The HPLC analysis conditions were as follows: Apparatus: Shimadzu Corporation Column: GL Sciences ODS-4 (250 mm x 46 mm, film thickness 5 μm) Mobile phase: Water (A) 70%: Acetonitrile (B) 30% → (5 min) → (A) 70%: (B) 30% → (30 min) → (A) 0%: (B) 100% → (10 min) → (A) 0%: (B) 100% Column oven: 40°C Flow rate: 0.5 mL / min Detector: UV (254 nm)

[0074] Examples 2 to 18, Comparative Examples 1 and 2 In Example 1, the changes were made as shown in Table 1 or Table 2, but the rest was carried out in the same manner. The results are shown in Table 1 or Table 2. In the examples in which an oxidizing agent was added, the oxidizing agent was added after the catalyst in the amount shown in Table 1 or Table 2, respectively. The yields in Examples 17 and 18 are values ​​after 3 hours of stirring, and furthermore, the values ​​in parentheses are values ​​after 24 hours of stirring.

[0075]

[0076]

[0077] In the above table, pKa refers to the pKa calculated from the free energy obtained by determining the stable structure of the catalyst molecule in methanol using Gaussian 16 of the catalyst by the SMD method. As is clear from the above results, by setting the pKa of the catalyst to 33.0 or less, it was possible to promote the oxidation-reduction reaction between alkylfuran aldehyde (e.g., MFF) and a compound represented by A-OH (e.g., methanol), thereby producing an alkylfuran carboxylic acid ester (compare Examples 1 to 3 and Comparative Example 1). Furthermore, by adding an oxidizing agent, it was possible to produce an alkylfuran carboxylic acid ester in high yield (compare Examples 4 to 12 and Comparative Example 2).

Claims

1. The method includes carrying out an oxidation-reduction reaction between an alkylfuranaldehyde and a compound represented by A-OH (wherein A is an organic group having 1 to 10 carbon atoms) in the presence of a catalyst and a base, The method for producing an alkylfurancarboxylic acid ester, wherein the catalyst is an N-heterocyclic carbene having a pKa of 33.0 or less, as calculated from the free energy obtained by determining a stable structure of the catalyst molecule in methanol by the SMD method using Gaussian 16.

2. The method for producing an alkylfurancarboxylic acid ester according to claim 1 , further comprising carrying out the oxidation-reduction reaction in the presence of an oxidizing agent.

3. The method for producing alkylfurancarboxylic acid esters according to claim 2, wherein the oxidizing agent is a quinone.

4. The method for producing an alkylfurancarboxylic acid ester according to claim 2, wherein the oxidizing agent comprises at least one oxidizing agent represented by formula (O): 【Chemical 1】 (In formula (O), R １ ~R ８ R each independently represents a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkenyl group having 2 to 12 carbon atoms, an alkynyl group having 2 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, an acyl group having 2 to 12 carbon atoms, or a hydroxy group. １ ~R ８ is an alkyl group, an alkenyl group, an alkynyl group, an alkoxy group, an aryl group, or an acyl group, １ ~R ８ Each group in the formula may have a substituent bonded to a carbon atom in the formula, such as an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, a hydroxy group, a halogen atom, a cyano group, or a nitro group. m is 1 or 0. R １ and R ２ , R ３ and R ４ , R ５ and R ６ , R ７ and R ８ may be bonded to or fused with each other to form a ring.

5. The method for producing an alkylfurancarboxylic acid ester according to claim 4, wherein the oxidizing agent represented by formula (O) includes at least one oxidizing agent represented by formula (O-3): 【Chemistry 2】 (In formula (O-3), R ３０１ ~R ３０８ are each independently R in formula (O). １ represents a group having the same meaning as

6. The method for producing an alkylfurancarboxylic acid ester according to any one of claims 1 to 5, wherein the pKa of the catalyst is 16.0 or more.

7. The method for producing an alkylfurancarboxylic acid ester according to any one of claims 1 to 5, wherein the pKa of the catalyst is 30.0 or less.

8. The method for producing an alkylfurancarboxylic acid ester according to any one of claims 1 to 5, wherein the catalyst comprises at least one selected from the group consisting of triazolium, imidazolinium, imidazolium, and thiazolium.

9. The method for producing an alkylfurancarboxylic acid ester according to any one of claims 1 to 5, wherein the catalyst comprises at least one catalyst represented by the following formula (C): 【Chemistry 3】 (In formula (C), R Ｃ１ and R Ｃ２ each independently represents a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, a heteroaryl group having 4 to 16 carbon atoms, a halogenated alkyl group having 1 to 12 carbon atoms, a halogenated aryl group having 6 to 18 carbon atoms, an alkylaryl group having 7 to 20 carbon atoms, or an arylalkyl group having 7 to 20 carbon atoms. Z is -S- or -NR ｃ３ X represents a methine group (=CR Ｃ５ -), a nitrogen atom (=N-), or a methylene group (-CR Ｃ６ ２ When X is a methine group or a nitrogen atom, the dashed line in the formula represents a double bond, and when X is a methylene group, the dashed line in the formula represents a single bond. Ｃ３ , R Ｃ５ and R Ｃ６ are each independently a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, an alkenyl group having 2 to 12 carbon atoms, an alkynyl group having 2 to 12 carbon atoms, an aryl group having 6 to 18 carbon atoms, an alkoxy group having 1 to 12 carbon atoms, an acyl group having 2 to 12 carbon atoms, a hydroxy group, a carboxy group, or a halogen atom. － represents a counter anion. Ｃ２ and R Ｃ３ , R Ｃ２ and R Ｃ５ , R Ｃ２ and R Ｃ６ may be bonded to or fused with each other to form a ring.

10. The method for producing an alkylfurancarboxylic acid ester according to claim 9, wherein the catalyst represented by formula (C) comprises at least one of the catalysts represented by formulas (C2) to (C6): 【Chemistry 4】 (In formula (C2), R １１ is R Ｃ１ R represents a group having the same meaning as ２１ is R Ｃ２ R represents a group having the same meaning as ３１ is R Ｃ３ R represents a group having the same meaning as ５１ is R Ｃ５ represents a group having the same meaning as － represents a counter anion. ２１ and R ３１ , R ２１ and R ５１ , R １１ and R ５１ may be bonded to or fused with each other to form a ring. 【Chemistry 5】 (In formula (C3), R １２ is R Ｃ１ R represents a group having the same meaning as ２２ is R Ｃ２ R represents a group having the same meaning as ３２ is R Ｃ３ R represents a group having the same meaning as ６２ is R Ｃ６ represents a group having the same meaning as － represents a counter anion. ２２ and R ３２ , R ２２ and R ６２ , R １２ and R ６２ may be bonded to or fused with each other to form a ring. 【Chemistry 6】 (In formula (C4), R １３ is R Ｃ１ R represents a group having the same meaning as ２３ is R Ｃ２ R represents a group having the same meaning as ３３ is R Ｃ３ represents a group having the same meaning as － represents a counter anion. ２３ and R ３３ may be bonded to or fused with each other to form a ring. 【Chemistry 7】 (In formula (C5), R １４ is R Ｃ１ R represents a group having the same meaning as ７４ , R ８４ , R ９４ are each independently R Ｃ２ represents a group having the same meaning as － represents a counter anion.) 【Chemistry 8】 (In formula (C6), R １５ is R Ｃ１ R represents a group having the same meaning as ２５ is R Ｃ２ R represents a group having the same meaning as ５５ is R Ｃ５ represents a group having the same meaning as － represents a counter anion. １５ and R ５５ , R ５５ and R ２５ may be bonded to or fused with each other to form a ring.

11. The method for producing an alkylfurancarboxylic acid ester according to any one of claims 1 to 5, wherein the alkylfuran aldehyde is a compound represented by formula (F1): 【Chemistry 9】 (In formula (F1), R is an alkyl group having 1 to 10 carbon atoms.)

12. The method for producing an alkylfuran carboxylic acid ester according to any one of claims 1 to 5, wherein the alkylfuran carboxylic acid ester is a compound represented by formula (F2): 【Chemistry 10】 (In formula (F2), R is an alkyl group having 1 to 10 carbon atoms, and A is an organic group having 1 to 10 carbon atoms.)

13. The method for producing an alkylfurancarboxylic acid ester according to any one of claims 1 to 5, wherein in the compound represented by A-OH, A is an alkyl group having 1 to 5 carbon atoms.

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