JPH11255747A - Dihydropyridinecarboxylic acid derivative and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To industrially obtain the subject compound in high yield, useful as a biologically active substance, an intermediate for medicaments, agrochemicals, or the like, by subjecting a specific pyridinedicarboxylic acid derivative to an electrode reduction in the presence of a solvent and a supporting electrolyte. SOLUTION: This dihydropyridinedicarboxylic acid derivative of formula II R<1> and R<2> are each H or an alkyl; (R<3> )n is (n) substituents of R<3> ; [C5 H5- NN] is a dihydropyridine ring, (n) is 0-2} is obtained by subjecting a pyridinedicarboxylic acid derivative (preferably, a pyridine-2,3-dicarboxylic acid ester, a pyridine-2,4-dicarboxylic acid ester, or the like) of formula I [C5 H3- NN] is pyridine ring} to an electrode reduction in the presence of a solvent and a supporting electrolyte (preferably, methanol, p-toluenesulfonic acid tetraethylammonium salt, or the like). As the electrode reduction, a constant current process is preferable. As an anode material, a noble metal, such as silver, platinum or the like, and as a cathode material, a carbon material, various metals are used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dihydropyridine dicarboxylic acid derivative useful as a physiologically active substance and as an intermediate for pharmaceuticals and agricultural chemicals, and a novel method for producing the dihydropyridine dicarboxylic acid derivative by an electrode reduction reaction.

[0002]

2. Description of the Related Art In recent years, synthetic organic chemistry utilizing an electrode reaction has attracted attention as an energy-saving, resource-saving, and pollution-free process.

[0003] References relating to the organic electrode reaction include, for example, "Organic Electrosynthesis" (published by Kodansha Scientific in 1981, Shigeru Torii), "Chemistry and Education" (1997, Vol. 45, No. 3, pp. 124-133); R. Rifi
and F. H. Covitz, "Introduc
Tion to Organic Electroch
emiry ", AJ Fly," Synthet
ic Organic Electrochemist
ry ", 2nd Ed., John Wiley, Ne
w York (1989), Organic Elec
Trochemistry, An Introduct
ion and a Guide ", 3rd Ed.,
by H. Lund and M.S. M. Baizer,
Marcel Dekker (1991) and the like.

As a patent document, there is a description of p by an electrode oxidation reaction.
-Acetoxybenzaldehyde or a method for producing an acetal thereof (JP-A-56-87683); 2,6-di-tert-butyl-4-methylphenol indirect electrolytic oxidation reaction (JP-A-62-13585); A method of synthesizing an indoaniline-based dye or an indophenol-based dye by coupling p-nitrosoaniline, nitrosophenol or a derivative thereof in the presence of a coupler and a base by an electrode reaction (JP-A-6-263993);
Production method of methyl-substituted paraanisidines by electrolytic reduction reaction (Japanese Patent Publication No. 57-15573); Production method of 1,2-dihydrophthalic acids by electrode reduction reaction
288039) and the like. In addition, Chemical Special 86 (issued by Doujin Kagaku), Electroorganic
Chemistry, pp. 157-174, entitled "Electroorganic Chemi from a Patent Perspective."
"Possibility of story" is published, and Topp
ics Current Chemistry, 148,
Springerverlag Electroch
emiry, D.E. Defner. Contains a large number of patent information.

[0005] On the other hand, the search for physiologically pharmacologically active substances has become an important issue for diversifying viruses and fungi from the viewpoint of pharmacy, pesticide and pathology. Various basic skeletons of such compounds are known. For example, amino acid derivatives, aminopyridine derivatives of vitamin B6, and evenninomicin derivatives, which are antibiotics, are one of physiologically pharmacologically active substances.

Further, nifedipine having the following structure (Niphdipine)
Some of the dihydropyridine dicarboxylic acid derivatives, including edipines, are well known as coronary vasodilators.

[0007]

Embedded image 1,4-Dihydropyridine dicarboxylic acid ester which is an intermediate of such a compound can be obtained by reducing pyridine-3,5-dicarboxylic acid ester with, for example, lithium aluminum hydride.

Generally, dihydropyridines are produced by reducing a pyridine derivative using a reducing agent such as aluminum amalgam, lithium aluminum hydride, acetic anhydride and zinc powder.

[0009]

However, the reaction with a reducing agent containing a metal in the conventional method, particularly in terms of industrial production, ensures the safety in the handling of the reducing agent and the waste treatment after the reaction. In addition, in terms of reaction yield, selectivity, etc., it has problems in environmental conservation and economic efficiency.

On the other hand, an organic electrode reaction as a new and powerful means of organic synthesis often has an advantage in terms of reaction yield, selectivity, and the like, and the reaction form is various.

The present invention has been made in view of the above-described problems in the prior art, and has as its object to use a reducing agent from the viewpoints of energy saving, resource saving, and non-pollution. A method for obtaining various dihydropyridine dicarboxylic acid derivatives in a high yield industrially and inexpensively by reducing specific double bonds with high selectivity while suppressing generation of by-products by utilizing an electrode reduction reaction, And a novel dihydropyridine dicarboxylic acid derivative.

[0012]

A process for producing a dihydropyridine dicarboxylic acid derivative according to the present invention comprises a pyridine dicarboxylic acid derivative represented by the following formula (I) in the presence of a solvent and a supporting electrolyte.

[0013]

Embedded image [In the formula (I), R ¹ and R ² each independently represent hydrogen (H) or an alkyl group, (R ³ ) _n represents n substituents R ³ , and [C ₅ H _{3 -n} N] represents a pyridine ring,
n represents 0, 1 or 2. Is subjected to an electrode reduction reaction to obtain a dihydropyridine dicarboxylic acid derivative represented by the following formula (II):

[0014]

Embedded image [In the formula (II), R ¹ , R ² and R ³ and n have the same meanings as described above, and [C ₅ H _5-n N] represents a dihydropyridine ring. ] Is manufactured.

This process is expressed as follows.

[0016]

Embedded image The dihydropyridine dicarboxylic acid derivative of the present invention is represented by the above formula (II), and can be produced by the production method of the present invention. Examples of the dihydropyridine dicarboxylic acid derivative represented by the formula (II) include the following dihydropyridine dicarboxylic acids and esters thereof. That is, dihydropyridine-2,3-dicarboxylic acid or its ester, dihydropyridine-2,4-
Dicarboxylic acid or its ester, dihydropyridine-
2,5-dicarboxylic acid or its ester, dihydropyridine-2,6-dicarboxylic acid or its ester, dihydropyridine-3,4-dicarboxylic acid or its ester,
Dihydropyridine-3,5-dicarboxylic acid or an ester thereof, and dihydropyridine-2,6-dimethyl-
3,5-dicarboxylic acid or an ester thereof.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Examples of the pyridine dicarboxylic acid derivative (I) as a reduced starting (or raw material) substance used in the present invention include pyridine-2,3-dicarboxylic acid (quinolinic acid) or an ester thereof, Pyridine-2,4-
Dicarboxylic acid (lutidic acid) or an ester thereof, pyridine-2,5-dicarboxylic acid (isocincomeronic acid) or an ester thereof, pyridine-2,6-dicarboxylic acid (dipicolinic acid) or an ester thereof, pyridine-3,4- Dicarboxylic acid (cinchomeronic acid) or an ester thereof, pyridine-3,5-dicarboxylic acid (diconicotinic acid) or an ester thereof, and pyridine-2,6-dimethyl-3,5-dicarboxylic acid or an ester thereof. Are also available at low cost.

Examples of R ¹ and R ² in the carboxylic acid ester group include a methyl group, an ethyl group, a propyl group,
Examples thereof include a linear or branched alkyl group such as an isopropyl group and a butyl group (for example, one having 4 or less carbon atoms). Examples of R ³ include a methyl group, an ethyl group, a methoxy group, an ethoxy group, a cyano group and the like.

The solvent used in the present invention includes (1) a substance to be reduced (pyridine dicarboxylic acid derivative)
Affinity, (2) dissolving power of supporting electrolyte, (3) polarity (high dielectric constant), (4) electrochemical stability, (5) cost, (6) possibility as a reaction solvent and handling, etc. It is selected in consideration of.

Preferred solvents satisfying such conditions include methanol (MeOH) and ethanol (EtO).
H) and other alcohol solvents; dimethylformamide (D
Amide solvents such as MF) and pyrrolidones; ether solvents such as tetrahydrofuran (THF), dioxane, trioxane and glyme; sulfolane; a mixed solution of two or more of these; a mixed aqueous solution of one or more of these; Is mentioned.

As a supporting electrolyte for imparting ionic conductivity to the solvent, quaternary alkyl ammonium salts (for example, tetraethylammonium p-toluenesulfonate; quaternary alkyl ammonium halide; OTs of quaternary alkyl ammonium cation) Salt, OMs salt, BF
₄ salt, ClO ₄ salt, and organic salts OAc salt) and the like. When the dielectric constant of the solvent is high, in addition to the organic salts, inorganic salts such as KF, LiClO ₄ , and LiCl; mineral acids such as sulfuric acid, hydrochloric acid, phosphoric acid, and perchloric acid; Bases such as sodium oxide and sodium carbonate can also be used.

Various combinations of the solvent and the supporting electrolyte are possible, but a solvent / supporting electrolyte system which can obtain a product with good selectivity and current efficiency is preferable. Such solvent / supporting electrolyte systems include, for example,
MeOH (methanol) / Et ₄ NOTs (p- toluenesulfonic acid tetraethylammonium salt); DMF
(N, N-dimethylformamide) -EtOH (ethanol) / Et ₄ NOTs; acetic acid or acetic acid aqueous solution / p-toluenesulfonic acid tetraethylammonium salt; methanol or methanol aqueous solution / sulfuric acid; pyrrolidone aqueous solution /
Et ₄ NOTs; glyme aqueous solution / R ₄ NClO ₄ (R is an alkyl group), Bu ₄ NBr; LiCl / CH ₃ NH ₂ ,
_{C 2 H 5 NH 2, H} 2 NCH 2 CH 2 NH 2, lower aliphatic amines, such as aminopyridine; LiCl / HMPA (hexamethylphosphoric amide) -EtOH (mixture); and the like.

The electrode reduction reaction in the present invention consists of the addition of a solvated electron generated reductively at the cathode or an anionic species generated from a substance to be reduced to a double bond.
The solvated electrons are usually generated by alkali metals such as sodium and lithium in liquid ammonia, but can be generated much more easily by using an electrode reduction method.

In the present invention, the solvated electrons are generated only in the solvent / supporting electrolyte reaction system at room temperature and under normal pressure, so that the solvated electrons are more reactive than the alkali metal / liquid ammonia system. The conditions are milder, the operation is much easier and the safety is very high. In addition, solvated electrons have high reactivity, and hydrogenation to an inert double bond of an aromatic ring easily occurs.

As the method of the electrode reaction, any of the constant current method and the constant potential method can be used, but the constant current method is preferable in view of the simplicity of the apparatus and the ease of calculating the reaction time.

Examples of the material of the anode include noble metals such as silver and platinum, various carbon materials, titanium coated with a metal oxide such as titanium oxide and rubidium oxide, lead / lead oxide, magnetite and ferrite. On the other hand, a carbon material and various metals can be used as the cathode.

Although the production method of the present invention can be carried out by a non-diaphragm method, it is preferably carried out by a diaphragm method because the probability of preventing reoxidation of the generated dihydropyridine acid is improved. Examples of the material of the diaphragm include a glass filter, a calcined material, a filter made of a porous resin, and various ion exchange membranes.

The manufacturing method of the present invention can be carried out, for example, as follows. i) A cell used for an electrode reaction is prepared using a carbon electrode as an anode and a platinum plate as a cathode. ii) A diaphragm is used for the anode, into which a solvent and a supporting electrolyte (for example, a 5% w / w Et ₄ NOTs methanol solution) are put. iii) On the cathode side outside the diaphragm, a substrate to be reduced ( Pyridine dicarboxylic acid ester) and solvent and supporting electrolyte (5
% W / w Et ₄ NOTs in methanol solution) and, if necessary, a base (for example, ammonium chloride)), and iv) energize under the reaction conditions to separate the product after completion of the electrode reaction.

In the electrode reaction in the production method of the present invention,
The reaction scale can be easily expanded. Taking a reaction system on a 5 mol scale of dimethyl pyridinedicarboxylate as a reaction substrate as an example, 5 to 20 L (liter) of a solvent and 0.3 to 7.0 of a supporting electrolyte are provided on the anode side.
mol, 20 to 80 L of solvent and 1 to 20 mol of supporting electrolyte were used on the cathode side outside the diaphragm, and -40 to 5
The reaction can be carried out by energizing at a temperature of 0 ° C. and 5 to 40 A for 10 to 250 hours.

As shown in the examples described below, the starting pyridinedicarboxylic acid derivative represented by the above formula (I) is selectively reduced by an electrode reduction reaction, and is obtained in high yield in the above formula (I).
The objective dihydropyridine dicarboxylic acid derivative represented by I) (hereinafter, sometimes simply referred to as dihydro form) is provided.

The dihydro form (dihydropyridine derivative) as a product includes 1,2-dihydro form, 1,4-dihydro form, 2,3-dihydro form, 2,5-dihydro form,
There are five types of 3,4-dihydro compounds, but only a part of them has been known so far, and the position of the double bond has not always been clear. In the present invention, the molecular structure of these dihydropyridine derivatives was confirmed by various spectroscopic means (NMR, IR, MASS) including the position of the double bond.

[0032]

According to the production method of the present invention, a specific double bond can be reduced with high selectivity without using a reducing agent by using an electrode reduction reaction while suppressing the generation of by-products. Various dihydropyridine dicarboxylic acid derivatives can be obtained at low cost and high yield industrially using easily available pyridine carboxylic acids as a starting material.

Further, according to the present invention, a novel dihydropyridine dicarboxylic acid derivative can be provided.

[0034]

EXAMPLES The present invention will now be described specifically with reference to examples, but the present invention is not limited to these examples.

Example 1 (1,2-dihydropyridine-
Synthesis of dimethyl 2,3-dicarboxylate) A carbon electrode (a carbon rod with a surface area of 1.5 cm ² ) as an anode,
A cell used for an electrode reaction was prepared using a platinum plate (12 cm ² ) as a cathode.

A ceramic porcelain diaphragm is used for the anode, and 5% w / w Et ₄ NOTs (p-toluenesulfonic acid tetraethylammonium salt) methanol solution 8
dimethyl-2,3-dicarboxylate 0.97 g (5 mmol), 5% w
32 ml of / wEt ₄ NOTs methanol solution and 0.25 g of ammonium chloride were added.

While maintaining the temperature at 5-10 ° C.,
After applying a current of 7.0 F / mol at 2 A, the catholyte was poured into 200 ml of saturated aqueous sodium hydrogen carbonate, and 150 ml of ethyl acetate was added.
Extracted three times. The organic layer was washed with 50 ml of water and saturated saline 5
The mixture was washed once with 0 ml, dried by adding magnesium sulfate, and filtered, and then the solvent was distilled off to obtain an oily target compound, dimethyl 1,2-dihydropyridine-2,3-dicarboxylate (1,2-dicarboxylate). 0.82 g of a dihydro compound) was obtained at a yield of 83.0%.

The ¹ H-NMR spectrum, ¹³ C-NMR spectrum, IR spectrum and GC-MS spectrum of this compound are shown in FIG. 1, FIG. 2, FIG. 3 and GC-MS, respectively. GC-MS spectrum showed a molecular ion peak at m / e = 197. Note that m / e = 1 in FIG.
The low molecular ion peak of 97 and the high molecular ion peaks of 138 and 78 indicate that one or two -CO
This is because the OMe group comes off.

Example 2 (1,4-dihydropyridine-
Synthesis of dimethyl 3,4-dicarboxylate) The same conditions as in Example 1 except that dimethyl 3,4-pyridinedicarboxylate (5 mmol) was used instead of dimethyl pyridine-2,3-dicarboxylate on the cathode side in Example 1. , And treated in the same manner as in Example 1 to give the target compound 1,4-dihydropyridine-3 as a white solid.
Dimethyl 4-dicarboxylate (1,4-dihydro form)
78 g were obtained with a yield of 79.0%.

¹ H-NMR, ¹³ C-NM of this compound
Table 1 shows the results of R, IR and GC-MS analysis.

Example 3 (1,4-dihydropyridine-
Synthesis of dimethyl 2,5-dicarboxylate) Instead of pyridine-2,3-dicarboxylate dimethyl on the cathode side in Example 1, dimethyl pyridine-2,5-dicarboxylate (5 mmol) was used, and 5% w on the anode side was used. / WE
Electrode reduction reaction under the same conditions as in Example 1 except that a 5% w / w Et ₄ NOTs solution of a mixed solvent of DMF / methanol = 1: 1 was used instead of the methanol solution of t ₄ NOTs, and a current of 7 F / mol was used. And carried out in the same manner as in Example 1. The obtained product was purified by column chromatography using silica gel as a filler to give 0.12 g of dimethyl 1,4-dihydropyridine-2,5-dicarboxylate (1,4-dihydro form) as a white solid. ,
Obtained in a yield of 12.0%.

¹ H-NMR, ¹³ C-NM of this compound
Table 1 shows the results of R, IR and GC-MS analysis.

Example 4 7F under the same conditions as in Example 1 except that dimethyl pyridine-2,6-dicarboxylate (5 mmol) was used instead of dimethyl pyridine-2,3-dicarboxylate on the cathode side in Example 1. The electrode reduction reaction was performed at a current flow rate of / mol, and the same treatment as in Example 1 was performed. The obtained product was purified by column chromatography using silica gel as a filler to give 0.92 g of the objective compound dimethyl 1,4-dihydropyridine-2,6-dicarboxylate (1,4-dihydro form) as a white solid. In a yield of 92%.

¹ H-NMR, ¹³ C-NM of this compound
Table 1 shows the results of R, IR and GC-MS analysis.

Example 5 Diethyl pyridine-2,3-dicarboxylate (5 mmol) was used instead of dimethyl pyridine-2,3-dicarboxylate on the cathode side in Example 1, and 5% w / w E / E on the anode side.
Electrode reduction reaction under the same conditions as in Example 1 except that a 5% w / w Et ₄ NOTs solution of a mixed solvent of DMF / methanol = 1: 1 was used instead of the methanol solution of t ₄ NOTs, and a current of 7 F / mol was used. And carried out in the same manner as in Example 1. The obtained product was purified by column chromatography using silica gel as a filler to obtain an oily target compound, diethyl 1,2-dihydropyridine-2,3-dicarboxylate (1,2-dihydro form).

[0046]

[Table 1] (1) to (16) in Table 1 are as follows.

(1): shown in FIG.

(2): shown in FIG.

(3): shown in FIG.

(4): As shown in FIG.

(5): (CDCl ₃ , 400 MHz) δ
3.68 (s, 3H), 3.71 (s, 3H), 4.2
7 (d, J = 4.40 Hz, 1H), 4.81-4.8
4 (m, 1H), 6.07-6.10 (m, 1H),
6.57 (broad s, 1H), 7.35 (d, J
= 5.84 Hz, 1H); (6): (CDCl ₃ , 400 MHz) δ39.49,
51.05, 52.13, 97.01, 100.65
125.41, 137.72, 168.16, 174.
22 ppm; (7): (nujol) 3340, 1735, 166
0, 1510, 1440, 1240; (8): m / e = 197 (9): (CDCl ₃ , 400 MHz) δ 3.28
(D, J = 4.40 Hz, 2H), 3.70 (s, 3
H), 3.80 (s, 3H), 5.87-5.90.
(M, 1H), 6.20 (broad s, 1H),
7.19 (d, J = 5.84 Hz, 1H); (10): (CDCl ₃ , 400 MHz) δ23.2
6,51.11,52.43,96.87,113.0
4,128.00,137.40,162.168.2
7 ppm; (11): (nujol) 3375, 1720, 168
0, 1620, 1280, 1105; (12): m / e = 197 (13): (CDCl ₃ , 400 MHz) δ 3.20
(T, J = 3.92 Hz, 2H), 3.80 (s, 3
H), 5.48 (dt, J = 1.48, 3.92 Hz,
2H), 6.11 (broad s, 1H); (14): (CDCl ₃ , 400 MHz) δ24.2.
0,52.04,104.94,130.53,16
2.86 ppm; (15): (nujol) 3320, 1730, 145
5, 1440, 1250; (16): m / e = 197

[Brief description of the drawings]

1 shows a ¹ H-NMR spectrum of a dihydropyridine derivative obtained in Example 1. FIG.

FIG. 2 shows a ¹³ C-NMR spectrum of the dihydropyridine derivative obtained in Example 1.

FIG. 3 shows an IR spectrum of the dihydropyridine derivative obtained in Example 1.

FIG. 4 shows a GC-MS spectrum of the dihydropyridine derivative obtained in Example 1.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Yama ▲ Saki ▼ Yasuhiro 8-1, Sanarahigashicho, Neyagawa City Inside Orient Chemical Co., Ltd.

Claims (9)

[Claims] 1. A pyridinedicarboxylic acid derivative represented by the following formula (I) in the presence of a solvent and a supporting electrolyte: [In the formula (I), R ¹ and R ² each independently represent hydrogen (H) or an alkyl group, (R ³ ) _n represents n substituents R ³ , and [C ₅ H _{3 -n} N] represents a pyridine ring,
n represents 0, 1 or 2. Is subjected to an electrode reduction reaction to obtain a dihydropyridine dicarboxylic acid derivative represented by the following formula (II): [In the formula (II), R ¹ , R ² and R ³ and n have the same meanings as described above, and [C ₅ H _5-n N] represents a dihydropyridine ring. ], And a method for producing a dihydropyridine dicarboxylic acid derivative. 2. The pyridine dicarboxylic acid derivative is pyridine-2,3-dicarboxylic acid ester, pyridine-2,4-
2. A dicarboxylic acid ester, pyridine-2,5-dicarboxylic acid ester, pyridine-2,6-dicarboxylic acid ester, pyridine-3,4-dicarboxylic acid ester or pyridine-3,5-dicarboxylic acid ester.
The method for producing the dihydropyridine dicarboxylic acid derivative described above. 3. The solvent and the supporting electrolyte are methanol and p-toluenesulfonic acid tetraethylammonium salt, respectively; N, N-dimethylformamide-ethanol and p-toluenesulfonic acid tetraethylammonium salt; acetic acid or an aqueous solution of acetic acid and p-toluene Sulfonic acid tetraethylammonium salt; methanol or aqueous methanol solution and sulfuric acid; pyrrolidone aqueous solution and p-toluenesulfonic acid tetraethylammonium salt; glyme aqueous solution and R ₄ NClO ₄ (R is an alkyl group); glyme aqueous solution and Bu ₄ NBr; LiCl and CH ₃ NH ₂ ;
LiCl and C ₂ H ₅ NH ₂ ; LiCl and H ₂ NCH ₂ C
H ₁ NH ₂ ; or a mixture of LiCl and hexamethylphosphoric amide-ethanol.
Or the method for producing a dihydropyridine dicarboxylic acid derivative according to 2. 4. A dihydropyridine dicarboxylic acid derivative represented by the following formula (II). Embedded image [In the formula (II), R ¹ and R ² each independently represent hydrogen (H) or an alkyl group, (R ³ ) _n represents n substituents R ³ , and [C ₅ H _{5- n} N] represents a dihydropyridine ring, and n represents 0, 1 or 2. ] 5. The dihydropyridine dicarboxylic acid according to claim 4, which is a 1,2-dihydro form, a 1,4-dihydro form, a 2,3-dihydro form, a 2,5-dihydro form, or a 3,4-dihydro form. Derivatives. 6. The 1,2-form according to claim 4, which is represented by the following formula:
Dihydropyridine-2,3-dicarboxylic acid derivatives. Embedded image [Wherein, R ¹ and R ² each independently represent hydrogen (H) or an alkyl group. ] 7. The 1,4- according to claim 4, which is represented by the following formula:
Dihydropyridine-3,4-dicarboxylic acid derivatives. Embedded image [Wherein, R ¹ and R ² each independently represent hydrogen (H) or an alkyl group. ] 8. The 1,4-form according to claim 4, wherein the formula is represented by the following formula:
Dihydropyridine-2,5-dicarboxylic acid derivatives. Embedded image [Wherein, R ¹ and R ² each independently represent hydrogen (H) or an alkyl group. ] 9. The 1,4- according to claim 4, which is represented by the following formula:
Dihydropyridine-2,6-dicarboxylic acid derivatives. Embedded image [Wherein, R ¹ and R ² each independently represent hydrogen (H) or an alkyl group. ]