Method for Separating Geometric Isomers

Abstract

The present invention provides a method for separating a compound represented by formula (1) or (2) (wherein P1 is a hydrogen atom or a hydroxyl-protecting group, R1 is a linear or branched, C1-6 alkyl group optionally substituted by a phenyl group, A is an alkenylene group, and R2 is a hydroxyl group, a C1-3 alkoxy group, a mono(C1-3 alkyl)amino group, or a di(C1-3 alkyl)amino group) from a geometrical isomer therefor, the geometrical isomer being one with respect to the double bond contained in A, the method comprising treating a mixture including the compound and the geometrical isomer by chromatography using, as a stationary phase, silica gel modified with an acidic functional group.

Description

Method for separating geometric isomers

[0001] The present invention relates to a method for separating geometric isomers.

[0002] Prostaglandins (PGs) are a general term for a group of endogenous physiologically active substances synthesized in vivo from arachidonic acid through metabolism by cyclooxygenase. There are many types of prostaglandins, including prostaglandin H ２ , prostaglandin D ２ , prostaglandin E １ , prostaglandin E ２ , prostaglandin F ２α , prostaglandin I ２ Prostaglandins are known to be involved in a variety of physiological functions via their specific G protein-coupled receptors.

[0003] Prostaglandins have a distinctive chemical structure, comprising a cyclopentane ring with four asymmetric carbon atoms and two aliphatic side chains. For this reason, they have long been a target of synthetic research and a seed for drug discovery, and various prostaglandin derivatives have been developed to date. (3aS,4R,5S,6aR)-(+)-hexahydro-5-hydroxy-4-(hydroxymethyl)-2H-cyclopenta[b]furan-2-one, which is used as a common intermediate, is also known as "Corey lactone." The chemical structures of Corey lactone and representative commercially available prostaglandin derivatives are shown below.

[0004] Patent No. 5209542 International Publication No. 2011 / 095990 International Publication No. 2012 / 011128 International Publication No. 2015 / 136317

[0005] Prostaglandins have a functionalized cyclopentane ring at the center of their chemical structure, with long aliphatic side chains on two adjacent carbon atoms, one of which bears a carboxyl group or carboxylic acid ester. Prostaglandins are generally produced by the following method, starting with a common synthetic intermediate and then adjusting the oxidation step of the substituent in a subsequent process. Many prostaglandins have a cis-type double bond in the aliphatic side chain, and how to remove this geometric isomer during chemical synthesis presents a challenge. Compounds with a cis-type double bond are also called Z-isomers, and compounds with a trans-type double bond are also called E-isomers.

[0006] Therefore, an object of the present invention is to provide a method for separating compounds having a structure similar to that of prostaglandins from their geometric isomers.

[0007] The present invention provides the following [1] to [5]: [1] A method for separating a compound represented by formula (1) or (2) from its geometric isomer, wherein the geometric isomer is a geometric isomer at a double bond contained in A, and the method comprises treating a mixture containing the compound and its geometric isomer by a chromatography method using acidic functional group-modified silica gel as a stationary phase. [In the formula, P １ and P ２ are each independently a hydrogen atom or a protecting group for a hydroxyl group, and R １ is a straight-chain or branched C alkyl group optionally substituted with a phenyl group; １－６ A is an alkyl group; ３－１０ is an alkenylene group, and R ２ is a hydroxyl group, C １－３ Alkoxy group, mono(C １－３ alkyl)amino group or di(C １－３ alkyl)amino group, is a single bond or a double bond.] [2] A is [3] The method according to [1], １ but, [4] The method according to [1] or [2], wherein P １[5] The method according to any one of [1] to [4], wherein the acidic functional group-modified silica gel is silica gel modified with a carboxy group or a sulfo group.

[0008] According to the present invention, a method for separating a compound having a structure similar to that of a prostaglandin from its geometric isomers can be provided.

[0009] The details of the present invention are described in detail below.

[0010] One embodiment of the present invention is a method for separating a compound represented by formula (1) or (2) from its geometric isomer, wherein the geometric isomer is a geometric isomer at a double bond contained in A, and the method comprises treating a mixture containing the compound and its geometric isomer by a chromatography method using acidic functional group-modified silica gel as a stationary phase. [In the formula, P １ and P ２ are each independently a hydrogen atom or a protecting group for a hydroxyl group, and R １ is a straight-chain or branched C alkyl group optionally substituted with a phenyl group; １－６ A is an alkyl group; ３－１０ is an alkenylene group, and R ２ is a hydroxyl group, C １－３ Alkoxy group, mono(C １－３ alkyl)amino group or di(C １－３ alkyl)amino group, is a single bond or a double bond.

[0011] In the method according to this embodiment, the combination of two or more geometric isomers to be separated is a compound represented by formula (1) or (2), which is in a relationship of cis isomer and trans isomer with respect to the double bond contained in A. １ When a double bond is also present in the side chain (having the formula (I)), a total of four geometric isomers are possible.

[0012] P １ and P ２are each independently a hydrogen atom or a hydroxyl-protecting group. A hydroxyl-protecting group is a substituent used in organic synthesis reactions for the purpose of protecting the hydroxyl group from reacting with a reactant. The hydroxyl-protecting group is not particularly limited, and examples thereof include acetal-based protecting groups such as a methoxymethyl group, an ethoxyethyl group, a benzyloxymethyl group, and a tetrahydropyranyl group; ether-based protecting groups such as a benzyl group, a p-methoxybenzyl group, and a p-nitrobenzyl group; acyl-based protecting groups such as an acetyl group, a pivaloyl group, a benzoyl group, and a p-methoxybenzoyl group; and silyl-based protecting groups such as a trimethylsilyl group, a triethylsilyl group, a tert-butyldimethyl group, a triphenylsilyl group, and a phenyldimethylsilyl group.

[0013] R １ is a straight-chain or branched C alkyl group optionally substituted with a phenyl group; １－６ It is an alkyl group. １－６ The alkyl group is an alkyl group having 1 to 6 carbon atoms, and specific examples thereof include a methyl group, an ethyl group, a 1-propyl group, a 2-propyl group, a 1-butyl group, a 2-butyl group, a tert-butyl group, a 1-pentyl group, a 2-pentyl group, a 3-pentyl group, a 1,1-dimethylpropyl group, a 1-hexyl group, a 2-hexyl group, and a 3-hexyl group. １ is the linear or branched C １－６ The alkyl group may be substituted with a phenyl group.

[0014] A is C ３－１０ The double bond in A is an alkenylene group, and specific examples thereof include a propenylene group, a butenylene group, a pentenylene group, a hexenylene group, a heptenylene group, an octenylene group, a nonylene group, and a decenylene group. The position of the double bond in A is not particularly limited. According to the separation method of this embodiment, geometric isomers (cis isomers and trans isomers) at the double bond can be separated.

[0015] R ２ is a hydroxyl group, C １－３ Alkoxy group, mono(C １－３ alkyl)amino group or di(C １－３ C is an amino group. １－３The alkoxy group is a group in which an alkyl group having 1 to 3 carbon atoms is substituted with an oxygen atom, and specifically, is a methoxy group, an ethoxy group, a 1-propoxy group, or a 2-propoxy group. １－３ The di(C alkyl)amino group is a group in which one alkyl group having 1 to 3 carbon atoms is substituted on a nitrogen atom, and specific examples thereof include a monomethylamino group, a monoethylamino group, a mono(1-propyl)amino group, and a mono(2-propyl)amino group. １－３ The di(alkyl)amino group is a group in which two alkyl groups having 1 to 3 carbon atoms are substituted on a nitrogen atom, and specific examples thereof include a dimethylamino group, a diethylamino group, a di(1-propyl)amino group, a di(2-propyl)amino group, and an ethyl(methyl)amino group.

[0016] The separation method according to this embodiment includes treating a mixture of geometric isomers by a chromatography method using acidic functional group-modified silica gel as a packing material (stationary phase).

[0017] The acidic functional group-modified silica gel used as a stationary phase in the chromatography method may be any silica gel modified with an acidic functional group. Examples of acidic functional group-modified silica gel include carboxyl group-modified silica gel and sulfo group-modified silica gel. The chromatography method is preferably normal phase chromatography.

[0018] The shape of the acidic functional group-modified silica gel may be spherical or crushed, preferably spherical. Spherical silica gel has a uniform surface area and can be packed uniformly in a column, which improves the degree of separation when separated by chromatography.

[0019] The average particle size of the acidic functional group-modified silica gel may be 3 to 500 μm, preferably 5 to 300 μm, and more preferably 30 to 200 μm.

[0020] The column length may be 5 to 200 cm, preferably 10 to 150 cm, and more preferably 15 to 100 cm. If the column length is 15 cm or more, the degree of separation is improved and the range of choice of eluent becomes wider.

[0021] As the eluent (mobile phase), an organic solvent well known to those skilled in the art can be used. Examples of organic solvents include aliphatic hydrocarbon solvents such as pentane, hexane, and heptane; aromatic hydrocarbon solvents such as toluene; halogenated hydrocarbon solvents such as dichloromethane and chloroform; ester solvents such as ethyl acetate and propyl acetate; and alcohol solvents such as methanol, ethanol, and 2-propanol. These solvents can be appropriately selected in consideration of the solubility of the crude product to be separated, and can also be mixed in any ratio in consideration of their mutual compatibility. Examples of mixed solvents include binary mixed solvents such as combinations of hexane and ethanol, hexane and isopropanol, and hexane and ethyl acetate, and ternary mixed solvents such as combinations of hexane, methanol, and isopropanol.

[0022] As used herein, "separating a compound represented by formula (1) or (2) from its geometric isomer" means that, when the content of the desired compound is taken as 100, the content of the corresponding geometric isomer is 2 or less, preferably 1 or less, and more preferably 0.5 or less. Furthermore, the purity of the compound represented by formula (1) or (2) after separation may be 90% or more, preferably 95% or more, and more preferably 97% or more, 98% or more, or 99% or more.

[0023] The present invention will be described in more detail below with reference to examples and comparative examples.

[0024] Abbreviations used in the examples and the like should be understood as meanings well known to those skilled in the art unless otherwise specified. For example, the meanings of some abbreviations are as follows: THP: 2-tetrahydropyranyl Ph: phenyl

[0025] Example 1 Purification of (Z)-7-[(1R,2R,3R,5S)-3,5-dihydroxy-2-[(3R)-3-hydroxy-5-phenylpentyl]cyclopentyl]hept-5-enoic acid ((Z)-IFL-FA)

[0026] Example 1a The crude product (EZ mixture of IFL-FA, 238 mg) was dissolved in dichloromethane and purified by chromatography under the following separation conditions. Fractions in which the E-isomer was not detected under the following analytical conditions were collected to obtain the Z-isomer (yield: 206 mg, 90%).

[0027] <Separation conditions> Eluent: hexane: isopropanol = 10:1 Packing material: CROMATOREX COOH MB100-40 / 75 (trade name, manufactured by Fuji Silysia Corporation, spherical silica gel (average particle size 40 to 75 μm, pore size 10 nm)) Fraction volume: 1 to 2 mL / fraction <Analysis conditions> Column: YMC-Pack SIL (trade name, manufactured by YMC Corporation, inner diameter: 4.6 mm, length: 25 cm) Mobile phase: hexane: ethanol: acetic acid = 91:9:0.05 Flow rate: 1 mL per minute Detection wavelength: 215 nm Injection volume: 20 μL

[0028] The content of each isomer before and after separation is shown in Table 1. The content of each isomer was calculated based on the area under the curve of the chromatogram obtained under the above analytical conditions. (E)-IFL-FA is (E)-7-[(1R,2R,3R,5S)-3,5-dihydroxy-2-[(3R)-3-hydroxy-5-phenylpentyl]cyclopentyl]hept-5-enoic acid.

[0029] Example 1b The crude product (EZ mixture of IFL-FA, 224 mg) was dissolved in dichloromethane and purified by chromatography under the following separation conditions. Fractions in which the E-isomer was not detected under the following analytical conditions were collected to obtain the Z-isomer (yield: 142 mg, 68%).

[0030] <Separation conditions> Eluent: hexane: isopropanol = 10:1 Packing material: CROMATOREX SO3H MB100-40 / 75 (trade name, manufactured by Fuji Silysia Corporation, spherical silica gel (average particle size 40 to 75 μm, pore size 10 nm)) Fraction volume: 1 to 2 mL / fraction <Analysis conditions> Column: YMC-Pack SIL (trade name, manufactured by YMC Corporation, inner diameter: 4.6 mm, length: 25 cm) Mobile phase: hexane: ethanol: acetic acid = 91:9:0.05 Flow rate: 1 mL per minute Detection wavelength: 215 nm Injection volume: 20 μL

[0031] The content of each isomer before and after separation is shown in Table 2. The content of each isomer was calculated based on the area under the curve of the chromatogram obtained under the above analytical conditions. The content of the E-isomer was 0.0%, and the Z-isomer was obtained with high purity.

[0032] Example 2 Purification of (Z)-7-[(1R,2R,3R,5S)-5-hydroxy-2-[(3R)-5-phenyl-3-[(tetrahydro-2H-pyran-2-yl)oxy]pentyl]-3-[(tetrahydro-2H-pyran-2-yl)oxy]cyclopentyl]hept-5-enoic acid ((Z)-IFL-PPF)

[0033] The crude product (EZ mixture of IFL-PPF, 392 mg) was dissolved in a mixture of hexane and ethyl acetate and purified by chromatography under the following separation conditions. Fractions containing 0.5% or less of the E-isomer were collected under the following analytical conditions to obtain the Z-isomer (yield: 99 mg, 68%). The analysis was performed after concentrating approximately 0.1 mL of the fraction, dissolving the residue in 1 mL of 2-propanol, adding a catalytic amount of paratoluenesulfonic acid monohydrate, and reacting for approximately 2 hours at room temperature for deprotection, followed by derivatization to IFL-FA (see Example 1).

[0034] <Separation conditions> Eluent: hexane:ethyl acetate = 3:1 to 3:7 Packing material: CROMATOREXSO3H MB100-40 / 75 (trade name, manufactured by Fuji Silysia Corporation, spherical silica gel (average particle size: 40 to 75 μm, pore size: 10 nm)) Fraction volume: 1 to 2 mL / fraction <Analysis conditions> Column: YMC-Pack SIL (trade name, manufactured by YMC Corporation, inner diameter: 4.6 mm, length: 25 cm) Mobile phase: hexane:ethanol:acetic acid = 91:9:0.05 Flow rate: 1 mL per minute Detection wavelength: 215 nm Injection volume: 20 μL

[0035] The content of each isomer before and after separation is shown in Table 3. The content of each isomer was calculated based on the area under the curve of the chromatogram. (E)-IFL-PPF is (E)-7-[(1R,2R,3R,5S)-5-hydroxy-2-[(3R)-5-phenyl-3-[(tetrahydro-2H-pyran-2-yl)oxy]pentyl]-3-[(tetrahydro-2H-pyran-2-yl)oxy]cyclopentyl]hept-5-enoic acid. The E-isomer contained in the Z-isomer after separation was 0.2% when measured under the above analytical conditions. In addition, triphenylphosphine, which was present in a very large amount in the crude product, was also successfully separated.

[0036] Example 3 Purification of propan-2-yl (Z)-7-[(1R,2R,3R,5S)-3,5-dihydroxy-2-[(3R)-3-hydroxy-5-phenylpentyl]cyclopentyl]hept-5-enoate ((Z)-latanoprost)

[0037] Example 3a The crude product (EZ mixture of latanoprost, 200 mg) was dissolved in a mixture of hexane and ethyl acetate and purified by chromatography under the following separation conditions. Fractions containing 0.5% or less of the E-isomer were collected under the following analytical conditions to obtain the Z-isomer (yield: 94 mg, 47%).

[0038] <Separation conditions> Eluent: hexane:ethyl acetate = 3:1 to 3:7 Packing material: CROMATOREXCOOH MB100-40 / 75 (trade name, manufactured by Fuji Silysia, spherical silica gel (average particle size: 40 to 75 μm, pore size: 10 nm)) Fraction volume: 1 to 2 mL / fraction <Analysis conditions> Column: Spherisorb Silica (trade name, manufactured by Waters, inner diameter: 4.6 mm, length: 25 cm) Mobile phase: hexane:ethanol:acetic acid = 91:9:0.05 Flow rate: 1 mL per minute Detection wavelength: 215 nm Injection volume: 20 μL

[0039] The content of each isomer before and after separation is shown in Table 4. The content of each isomer was calculated based on the area under the curve of the chromatogram. (E)-Latanoprost is propan-2-yl (E)-7-[(1R,2R,3R,5S)-3,5-dihydroxy-2-[(3R)-3-hydroxy-5-phenylpentyl]cyclopentyl]hept-5-enoate. The content of the E-isomer was 0.4% under the above analytical conditions, and the Z-isomer was obtained with high purity.

[0040] Example 3b The crude product (EZ mixture of IFL, 200 mg) was dissolved in a mixture of hexane and ethyl acetate and purified by chromatography under the following separation conditions. Fractions containing 0.5% or less of the E-isomer under the following analytical conditions were collected to obtain the Z-isomer (yield: 64 mg, 32%).

[0041] <Separation conditions> Eluent: hexane:ethyl acetate = 3:1 to 2:3 Packing material: CROMATOREXSO3H MB100-40 / 75 (trade name, manufactured by Fuji Silysia, spherical silica gel (average particle size: 40 to 75 μm, pore size: 10 nm)) Fraction volume: 1 to 2 mL / fraction <Analysis conditions> Column: Spherisorb Silica (trade name, manufactured by Waters, inner diameter: 4.6 mm, length: 25 cm) Mobile phase: hexane:ethanol:acetic acid = 91:9:0.05 Flow rate: 1 mL per minute Detection wavelength: 215 nm Injection volume: 20 μL

[0042] The content of each isomer before and after separation is shown in Table 5. The content of each isomer was calculated based on the area under the curve of the chromatogram. The content of the E-isomer was 0.5%, and the Z-isomer was obtained with high purity.

[0043] Example 4 Purification of (E)-7-((1R,2R,3R)-3-hydroxy-2-((3S,5S,E)-3-hydroxy-5-methylnon-1-en-1-yl)-5-oxocyclopentyl)hept-2-enoic acid ((E)-IEL)

[0044] The crude product (EZ mixture of IEL, 33 mg) was dissolved in dichloromethane and purified by chromatography under the following separation conditions. Fractions in which the E-isomer was not detected under the following analytical conditions were collected to obtain the E-isomer (yield: 13 mg, 56%).

[0045] <Separation conditions> Eluent: hexane:ethanol = 10:1 Packing material: CROMATOREX COOH SMB100-10 (trade name, manufactured by Fuji Silysia Chemical, spherical silica gel (average particle size: 10 μm, pore size: 10 nm)) Fraction volume: 1 to 2 mL / fraction <Analysis conditions> Column: Develosil ODS-5 (trade name, manufactured by Nomura Chemical, inner diameter: 4.6 mm, length: 15 cm) Mobile phase: 0.02 M potassium dihydrogen phosphate buffer: acetonitrile: 2-propanol = 9:5:2 Flow rate: 1 mL per minute Detection wavelength: 215 nm Injection volume: 20 μL

[0046] The content of each isomer before and after separation is shown in Table 6. The content of each compound was calculated based on the area under the curve of the chromatogram. (Z)-IEL is (Z)-7-((1R,2R,3R)-3-hydroxy-2-((3S,5S,E)-3-hydroxy-5-methylnon-1-en-1-yl)-5-oxocyclopentyl)hept-2-enoic acid, and AT-IEL is (E)-7-((1R,2S)-2-((3S,5S,E)-3-hydroxy-5-methylnon-1-en-1-yl)-5-oxocyclopent-3-en-1-yl)hept-2-enoic acid. The content of the E-isomer contained in the Z-isomer after separation was 0.0% under the above analytical conditions. Furthermore, AT-IEL, which was contained in a large amount in the crude product, could also be separated.

[0047] Comparative Example 1 Purification of (Z)-7-[(1R,2R,3R,5S)-5-hydroxy-2-[(3R)-5-phenyl-3-[(tetrahydro-2H-pyran-2-yl)oxy]pentyl]-3-[(tetrahydro-2H-pyran-2-yl)oxy]cyclopentyl]hept-5-enoic acid ((Z)-IFL-PPF)

[0048] The crude product (EZ mixture, 75.8 g) was dissolved in a mixed solvent of hexane and ethyl acetate and purified by chromatography under the following separation conditions to obtain the (Z) isomer (yield: 45.3 g, 96%). (E)-Latanoprost was detected in all collected fractions and could not be separated. The analysis was performed after concentrating approximately 0.1 mL of the fraction, dissolving the residue in 1 mL of 2-propanol, adding a catalytic amount of paratoluenesulfonic acid monohydrate, and reacting for approximately 2 hours at room temperature for deprotection, followed by derivatization to IFL-FA (see Example 1).

[0049] <Separation conditions> Eluent: hexane:ethyl acetate = 3:2 Packing material: BW-300S (trade name, manufactured by Fuji Silysia, crushed silica gel (average particle size: 38 to 75 μm, pore size: 6 nm)) Fraction volume: 20 to 50 mL / fraction <Analysis conditions> Column: YMC-Pack SIL (trade name, manufactured by YMC, inner diameter: 4.6 mm, length: 25 cm) Mobile phase: hexane:ethanol:acetic acid = 91:9:0.05 Flow rate: 1 mL per minute Detection wavelength: 215 nm Injection volume: 20 μL

[0050] The content ratio of each isomer before and after separation is shown in Table 7. The content ratio of each isomer was calculated based on the area under the curve of the chromatogram.

[0051] Comparative Example 2 Purification of propan-2-yl (Z)-7-[(1R,2R,3R,5S)-3,5-dihydroxy-2-[(3R)-3-hydroxy-5-phenylpentyl]cyclopentyl]hept-5-enoate ((Z)-latanoprost)

[0052] The crude product (EZ mixture of latanoprost, 17.3 g) was dissolved in a mixture of hexane and ethyl acetate and purified by chromatography under the following separation conditions to obtain (Z)-latanoprost (yield: 14.6 g, 88%). (E)-latanoprost was detected in all fractions containing (Z)-latanoprost under the following analytical conditions and could not be separated.

[0053] <Separation conditions> Eluent: hexane: ethyl acetate = 3:2 Packing material: Silica gel 60 high purity (trade name, manufactured by Kanto Chemical) Fraction volume: 20 to 50 mL / fraction <Analysis conditions> Column: Spherisorb Silica (trade name, manufactured by Waters, inner diameter: 4.6 mm, length: 25 cm) Mobile phase: hexane: ethanol: acetic acid = 91:9:0.05 Flow rate: 1 mL per minute Detection wavelength: 215 nm Injection volume: 20 μL

[0054] The content ratio of each isomer before and after separation is shown in Table 8. The content ratio of each isomer was calculated based on the area under the curve of the chromatogram.

[0055] Reference Example 1 (Z)-7-((1R,2R,3R)-3-hydroxy-2-((S,E)-3-hydroxyoct-1-en-1-yl)-5-oxocyclopentyl)hept-5-enoic acid (PGE ２ Purification of dinoprostone

[0056] Crudely purified product (PGE ２ and P.G.A. ２ The mixture (35 mg) was dissolved in dichloromethane and purified by chromatography under the following separation conditions. The fractions not containing PGA2 were collected under the following analytical conditions to obtain PGE2 (yield: 20 mg, 100%).

[0057] <Separation conditions> Eluent: hexane:ethanol = 10:1 Packing material: CROMATOREX COOH SMB100-10 (trade name, manufactured by Fuji Silysia, spherical silica gel (average particle size: 10 μm, pore size: 10 nm)) Fraction volume: 1 to 2 mL / fraction

[0058] The content ratio of each isomer before and after separation is shown in Table 9. The content ratio of each isomer was calculated based on the area under the curve of the chromatogram obtained under the following analytical conditions. <Analysis conditions> Column: L-Column 2 ODS (trade name, Chemicals Evaluation and Research Institute, Japan, inner diameter: 4.6 mm, length: 25 cm) Mobile phase: methanol: 0.2% acetic acid aqueous solution (volume ratio) = 58:42 Flow rate: 1 mL per minute Detection wavelength: 210 nm Injection volume: 20 μL

Claims

1. A method for separating a compound represented by formula (1) or (2) from its geometric isomers, wherein the geometric isomers are geometric isomers at the double bond contained in A, and the method comprises treating a mixture containing the compound and its geometric isomers by a chromatography method using an acidic functional group-modified silica gel as a stationary phase. (wherein is a single bond or a double bond.) 2. The method according to claim 1, wherein A is .

3. R １ is the method according to claim 1 or 2 4. P １ The method according to any one of claims 1 to 3, wherein P is a hydrogen atom or a 2-tetrahydropyranyl group.

5. The method according to any one of claims 1 to 4, wherein the acidic functional group-modified silica gel is silica gel modified with a carboxy group or a sulfo group.