JPH0729966B2 - Optically active compound - Google Patents

Description

TECHNICAL FIELD The present invention relates to a novel optically active compound. More specifically, it relates to an optically active compound useful as a component of a liquid crystal composition.

[Prior Art] A ferroelectric liquid crystal material is not only a combination of liquid crystal compounds exhibiting a chiral smectic phase, but also a liquid crystal compound exhibiting a smectic C phase or a smectic H phase or a liquid crystal compound which is a matrix of a liquid crystal composition and is optically active. It is known that it can be obtained by adding a compound. [For example, Ann.Phys., 3,237
(1978)].

[Problems to be Solved by the Invention] However, many of the optically active compounds used for this purpose have an optically active 2-octanol or an optically active 2-butanol as a raw material and have a small intramolecular dipole moment. Since alcohol is used, when it is added to a liquid crystal compound or a liquid crystal composition as a base, the response is slow and practically difficult.

[Means for Solving the Problems] The inventors of the present invention have earnestly studied for the purpose of obtaining an optically active compound that gives a high response speed when added to a liquid crystal compound or a liquid crystal composition as a matrix, and as a result, The invention was reached.

That is, the present invention has the general formula [In the formula, R and R'are an alkyl group having 3 to 20 carbon atoms which may have an asymmetric carbon atom, and X is a single bond (direct bond), -O-, -S-, -COO. −, −OCO− or −OCOO
And A ₁ and A ₂ may be substituted with a substituent selected from the group consisting of a fluorine atom, a chlorine atom, a nitro group and a cyano group. Or And Y is —C≡C— or —CH ₂ CH ₂ —, n is an integer of 0 to 5, and C ^* represents an asymmetric carbon atom].

In the general formula (1), the alkyl group having 3 to 20 carbon atoms which may have an asymmetric carbon atom for R is a linear alkyl group (n-propyl group, n-butyl group, n-pentyl group,
n-hexyl group, n-heptyl group, n-octyl group, n
-Nonyl group, n-decyl group, n-undecyl group, n-dodecyl group, n-tridecyl group, n-tetradecyl group, n
-Pentadecyl group, n-hexadecyl group, n-heptadecyl group, n-octadecyl group, etc., branched alkyl group (isopropyl group, 1-methylpropyl group, 1-methylbutyl group, 1-methylpentyl group, 1-methylhexyl group) , 1-methylheptyl group, 1-methyloctyl group, 2
-Methylpropyl group, 2-methylbutyl group, 2-methylpentyl group, 2-methylhexyl group, 2-methylheptyl group, 2-methyloctyl group, 2-methylnonyl group, 2
-Methyldecyl group, 2-methylundecyl group, 2-methyldodecyl group, 2-methyltridecyl group, 2-methyltetradecyl group, 3-methylbutyl group, 3-methylpentyl group, 3-methylhexyl group, 3- Methylheptyl group,
3-methyloctyl group, 4-methylpentyl group, 4-methylhexyl group, 4-methylheptyl group, 4-methyloctyl group, 5-methylhexyl group, 5-methylheptyl group, 5-methyloctyl group, 5- Methylnonyl group, 5-
Methyldecyl group, 6-methylheptyl group, 6-methyloctyl group, 6-methylnonyl group, 6-methyldecyl group,
7-methyloctyl group, 7-methylnonyl group, 7-methyldecyl group, 8-methylnonyl group, 8-methyldecyl group, 9-methyldecyl group, 9-methylundecyl group, etc.] Optically active alkyl group (optical activity 1 -Methylpropyl group, optically active 1-methylbutyl group, optically active 1
-Methylpentyl group, optically active 1-methylhexyl group,
Optically active 1-methylheptyl group, optically active 1-methyloctyl group, optically active 2-methylbutyl group, optically active 2-
Methylpentyl group, optically active 2-methylhexyl group, optically active 2-methylheptyl group, optically active 2-methyloctyl group, optically active 2-methylnonyl group, optically active 2-methyldecyl group, optically active 2-methylundecyl group , Optically active 2-methyldodecyl group, optically active 2-methyltridecyl group, optically active 2-methyltetradecyl group, optically active 3-methylpentyl group, optically active 3-methylhexyl group, optically active 3-methylheptyl group , Optically active 3-methyloctyl group, optically active 4-methylhexyl group, optically active 4-methylheptyl group, optically active 4-methyloctyl group, optically active 5-methylheptyl group, optically active 5-methyloctyl group, optical Active 5-methylnonyl group, optically active 5-methyldecyl group, optically active 6-methyloctyl group,
Optically active 6-methylnonyl group, optically active 6-methyldecyl group, optically active 7-methylnonyl group, optically active 7-methyldecyl group, optically active 8-methyldecyl group, optically active 9
-Methylundecyl group, etc.)] and the like. Of these, an alkyl group having 4 to 16 carbon atoms is preferable, and in view of minimizing the reduction of the liquid crystal temperature range when added to the liquid crystal compound or the liquid crystal composition as the base, the carbon number is particularly preferable. 6 to 14 alkyl groups.

In the general formula (1), X is preferably a single bond (direct bond) or -O-.

A is 1 to 4 selected from the group consisting of a fluorine atom, a chlorine atom, a nitro group and a cyano group.
~ May be substituted with 2 substituents Is mentioned.

A may be optionally substituted by 1 to 4 fluorine atoms Is.

n is an integer of 0 to 5, and when the value of n increases, the response speed tends to be slow when added to the liquid crystal compound or the liquid crystal composition as a base, and preferably 0 to 3 is taken into consideration. It is an integer, particularly preferably an integer of 0 to 1.

In the general formula (1), the alkyl group having 3 to 20 carbon atoms which may have an asymmetric carbon atom represented by R ′ is the same linear or branched alkyl group as R in the general formula (1), or the general formula An optically active alkyl group similar to R in (1) can be mentioned. Of these, an alkyl group having 3 to 16 carbon atoms is preferable, and in view of minimizing the reduction of the liquid crystal temperature range when added to the liquid crystal compound or liquid crystal composition as the base, the carbon number is particularly preferable. 4 to 12 alkyl groups.

Specific examples of the optically active compound represented by the general formula (1) include compounds having groups and numbers as shown in Table-1 to Table-5.

(I) When Y is -C≡C- and n is 0 (II) When Y is -C≡C- and n is 1. (III) Y is -CH ₂ CH ₂ - and, when n is 0 (IV) A ₁ And when Y is -C≡C- (V) A ₁ And Y is -CH ₂ CH _2- Each symbol in Table 1 represents the following structure.

PEN; nC ₅ H ₁₁ − HEX; nC ₆ H ₁₃ − HEP; nC ₇ H ₁₅ − OCT; nC ₈ H ₁₇ − NON; nC ₉ H ₁₉ − DEC; nC ₁₀ H ₂₁ − UND; nC ₁₁ H ₂₃ − DOD ; nC ₁₂ H ₂₅ − ^* : ^Indicates optical activity.

The compound included in the general formula (1) can be synthesized, for example, through the following steps. (In the formulas below, R, X, A, R'and n are the same as those in the general formula (1).) (I) Y is -C≡C-, n is 0 and A ₂ is (II) Y is -C≡C-, n is 0 and A ₂ is (III) When Y is -C≡C- and n is 1. When - (IV) Y is -CH ₂ CH ₂ That is, Y is -C≡C-, n is 0 and A ₂ is In this case, the compound of general formula (4) can be obtained by heating the compound of general formula (2) and the compound of general formula (3) in anhydrous dimethylformamide, anhydrous dimethylsulfoxide or anhydrous tetrahydrofuran. The compound of general formula (5) can be obtained by hydrogenating the compound of general formula (4) with Pd / C. After diazotizing the compound of the general formula (5), the compound of the general formula (6) can be obtained by reacting with a halide of an alkali metal. The compound of the general formula (6) is reacted with a compound of the general formula (7) in triethylamine in an inert gas atmosphere using a zero-valent or divalent palladium catalyst to give the compound of the general formula ( The compound of 8) can be obtained.

Y is -C≡C-, n is 0 and A ₂ is In this case, m-fluoroaniline can be halogenated to obtain the compound of general formula (10), and then the amino group can be oxidized to obtain the compound of general formula (11). The compound of general formula (12) can be obtained by heating the compound of general formula (11) and the compound of general formula (3) in anhydrous dimethylformamide, anhydrous dimethylsulfoxide or anhydrous tetrahydrofuran. The compound of general formula (13) can be obtained by hydrogenating the compound of general formula (12) with Pd / C. After diazotizing the compound of the general formula (13),
The compound of the general formula (14) can be obtained by reacting with a halide of an alkali metal. The compound of the general formula (14) and the compound of the general formula (7) in triethylamine,
The compound of the general formula (15), which is the compound of the present invention, can be obtained by reacting with a 0-valent or 2-valent palladium catalyst in an inert gas atmosphere.

When Y is -C≡C- and n is 1, the compound represented by the general formula (16) is reduced by using lithium aluminum hydride in anhydrous ether or anhydrous tetrahydrofuran.
After forming the compound of 7), the hydroxyl group of the compound of general formula (17) can be halogenated with a halogenating agent (for example, thionyl chloride) to obtain the compound of general formula (18). A compound of the general formula (3) and a compound of the general formula (19) in anhydrous dimethylformamide or anhydrous dimethylsulfoxide,
General formula (19) by reacting at room temperature or under heating
Can be obtained. The compound of the general formula (19) is reacted with the compound of the general formula (7) in triethylamine in an inert gas atmosphere using a zero-valent or divalent palladium catalyst to give the compound of the general formula ( The compound of 20) can be obtained.

When Y is —CH ₂ CH ₂ —, the general formula (8) or the general formula (1
The compound of general formula (21) can be obtained by hydrogenating the compound of 5) with Pd / C.

The compound contained in the general formula (3), which is a raw material of the above compound, can be synthesized, for example, through the following steps. (R'in the following formula is the same as in the general formula (1).) When R'does not contain an oxygen atom, the compound of general formula (24) obtained by reacting trifluoroacetic acid ethyl ester with a Grignard reagent at -20 ° C or lower is reduced with sodium borohydride or lithium aluminum hydride. Thus, a racemic compound of the general formula (25) can be obtained.
Asymmetric hydrolysis of a racemic ester of general formula (26) obtained by reacting a compound of general formula (25) with an acid chloride (eg acetyl chloride) in an aqueous solution using lipase,
After isolation, a metal salt of the optically active alcohol of the general formula (29), which is a raw material of the compound of the present invention, can be obtained by reacting with an alkali metal hydride (for example, sodium hydride). Further, by chemically hydrolyzing the unreacted ester recovered at the time of asymmetric hydrolysis, an optically active alcohol having a configuration opposite to that of the above optically active alcohol can be obtained. It can be used as a raw material for compounds.

Further, the optically active site represented by R or R'in the general formula (1) is derived from the corresponding optically active alcohol or optically active carboxylic acid, and is one of these optically active alcohols and / or optically active carboxylic acids. Is a commercially available product; derived by the asymmetric reduction of the corresponding ketone with an enzyme, a microorganism, or an asymmetric metal catalyst; or is mainly derived from an optically active amino acid, an optically active oxyacid, and an optically active oxyacid ester existing as natural products, etc. It can be obtained by the method of.

Liquid crystal is generally used as a liquid crystal composition composed of two or more kinds of components, and the optically active compound of the present invention can also be used as a component of the liquid crystal composition. The liquid crystal composition includes a smectic liquid crystal, for example, a smectic liquid crystal having no optically active site [2-p-alkyloxyphenyl-5-
Alkylpyrimidine, 2-p-alkanoyloxyphenyl-5-alkylpyrimidine, 2-p-alkyloxycarbonylphenyl-5-alkylpyrimidine, 2-
p-Alkylphenyl-5-p-alkyloxyphenylpyrimidine, 2-p-alkyloxy-m-fluorophenyl-5-alkylpyrimidine, 2-p-alkyloxyphenyl-5- (trans-4-alkylcyclohexyl) pyrimidine , 2-p-alkyloxyphenyl-5-alkylpyridine, 2-p-alkyloxy-m
-Fluorophenyl-5-alkylpyridine, 2-p-
(P′-alkylphenyl) phenyl-5-alkylpyrimidine, 2-p-alkylphenyl-5-p-alkylphenylpyrimidine, p-alkyloxyphenyl-
5-alkylpicolinate, 2-p-alkyloxyphenyl-5-alkyloxypyrazine, 2-p-alkylphenyl-5-alkylpyrimidine, 2-p-alkyloxyphenyl-5-alkyloxypyrimidine, 2
-P-alkylphenyl-5-alkyloxypyrimidine, 4-alkyloxy-4'-biphenylcarboxylic acid-p '-(alkyloxycarbonyl) phenyl ester, 4-alkyloxy-4'-biphenylcarboxylic acid-alkyl ester, etc. ] And / or ferroelectric liquid crystal [optically active 4-alkyloxy-4′-biphenylcarboxylic acid-p ′-(2-methylbutyloxycarbonyl)]
Phenyl ester, optically active 4-n-alkyloxy-
4'-biphenylcarboxylic acid-2-methylbutyl ester, optically active p-alkyloxybenzylidene-p'-
Amino-2-chloropropyl cinnamate, optically active p
-Alkyloxybenzylidene-p'-amino-2-methylbutyl cinnamate, etc.] and / or conventional chiral smectic liquid crystals [optically active 4- (p-alkyloxybiphenyl-p'-oxycarbonyl) -4'-
(2-Methylbutyloxycarbonyl) cyclohexane, optically active pn-alkyloxybenzylidene-
p ′-(2-methylbutyloxycarbonyl) aniline and the like] may be contained. Further, it may contain a chiral compound which does not exhibit liquid crystallinity and / or a dichroic dye such as an anthraquinone dye and an azo dye.

A liquid crystal composition exhibiting ferroelectricity causes an optical switching phenomenon when a voltage is applied, and it is possible to manufacture a display element having a fast response by utilizing this (for example, JP-A-56-107216 and JP-A-59-118744). , NA Clark (NAClar
k), STLagerwall;
Applied Physics Letter
Lttetter) 36 , 899 (1980), etc.].

The liquid crystal composition of the present invention has a cell interval of 0 to 5 to 10 μ
It can be used as an optical switching element (display element) by vacuum-sealing in a liquid crystal cell of m, preferably 0.5 to 3 μm and installing a polarizer on both sides.

The above liquid crystal cell was provided with a transparent electrode, and the surface was subjected to orientation treatment.
It can be manufactured by laminating two glass substrates with a spacer interposed therebetween.

Examples of the spacer include alumina beads, glass fiber, and polyimide film. As the alignment treatment method, a normal alignment treatment, for example, a polyimide film, a rubbing treatment, or a SiO oblique vapor deposition can be applied.

[Examples] Hereinafter, the present invention will be further described with reference to Examples, but the present invention is not limited thereto.

Example 1 Production of optically active pn-decyloxy-p ′-(1-trifluoromethylheptyloxy) tran (Compound No. 4 in Table 1) (I) Optically active p- (1-trifluoromethylheptyloxy) ) Production of nitrobenzene Into 200 ml of dry dimethylformamide containing 2.0 g of sodium hydride (purity 60%), 1-form of the optically active S-isomer at room temperature is added.
Add 9.2 g of trifluoromethylheptanol dropwise at a speed that does not cause intense hydrogen generation, and
-Prepared the sodium salt of trifluoromethylheptanol. After stirring for 1 hour at room temperature after the completion of dropping,
12.4 g of p-iodonitrobenzene was added and the mixture was refluxed for 2 hours. After cooling, the mixture was poured into ice water and extracted with hexane. The hexane layer was washed with water, washed with 1N hydrochloric acid and washed with water, and then separated and purified with a silica gel column to obtain 7.7 g of oily optically active p- (1-trifluoromethylheptyloxy) nitrobenzene.

(II) Production of Optically Active p- (1-Trifluoromethylpeptyloxy) aniline Optically Active p- (1-Trifluoromethylheptyloxy) Nitrobenzene 7.7 g in 100 ml of ethanol containing 5% Pd-C as a catalyst (5% Palladium-carbon) (0.5 g) was added, and the mixture was stirred at room temperature under a hydrogen atmosphere at normal pressure for reduction. After confirming that the absorption of hydrogen disappeared, the catalyst was removed by filtration and ethanol was removed under reduced pressure to obtain 6.7 g of oily optically active p- (1-trifluoromethylheptyloxy) aniline.

(III) Production of optically active p- (1-trifluoromethylheptyloxy) iodobenzene By adding 6.7 g of optically active p- (1-trifluoromethylheptyloxy) aniline to 200 ml of water containing 25 g of 36% hydrochloric acid. It was cooled to below 5 ° C. A separately prepared aqueous sodium nitrite solution (1.7 g of sodium nitrite, 10 ml of water) was added dropwise thereto at 5 ° C or lower. After the dropping was completed, the mixture was stirred for an additional hour, and then an aqueous potassium iodide solution (20 g potassium iodide, 2 g water) was added.
0 ml) was added, the temperature was slowly returned to room temperature, and the mixture was stirred until the generation of nitrogen disappeared. After completion of the reaction, extract with hexane,
The hexane layer was washed with water, washed with aqueous sodium hydrogen sulfite, and washed with water to remove hexane, to obtain 7.7 g of oily optically active p- (1-trifluoromethylheptyloxy) iodobenzene.

(IV) Production of optically active pn-decyloxy-p '-(1-trifluoromethylheptyloxy) tran 1.3% pn-decyloxyphenylacetylene and optically active p- (1-trifluoromethylheptyloxy) 1.9 g of iodobenzene was reacted in 50 ml of triethylamine with 40 mg of bistriphenylphosphine palladium dichloride and 10 mg of copper (I) iodide as catalysts under a nitrogen atmosphere at room temperature overnight. After completion of the reaction, extraction was performed with toluene from which triethylamine was removed. By washing the toluene layer with 1N hydrochloric acid water, washing with water, and then distilling off the toluene,
A black oily substance was obtained. This substance was dissolved in hexane, passed through a short column packed with silica gel, and then recrystallized twice with ethanol to obtain optically active pn-decyloxy-p '-(1-tri) of white crystals of the compound of the present invention. 2.0 g of fluoromethylheptyloxy) tran was obtained.
The structure of the compound is NMR (nuclear magnetic resonance spectroscopy), M
Confirmed by S (mass spectrometry), IR (infrared absorption spectrum analysis) and elemental analysis. IR spectrum of the above compound,
H-NMR spectrum and F-NMR spectrum are shown in FIGS. 1, 2 and 3, respectively.

Elemental analysis value: Theoretical value (%) Actual value (%) C: 74.42 C: 74.51 H: 8.33 H: 8.19 0: 6.20 O: 6.29 F: 11.05 F: 11.01 Example 2 Optical activity p-n-decyloxy-p ′-(1-Trifluoromethylheptyloxymethyl) tran (No. in Table 2
72 compounds) (I) Production of optically active p- (1-trifluoromethylheptyloxymethyl) iodobenzene Optically at room temperature into 100 ml of dry dimethylformamide containing 0.4 g of sodium hydride (60% pure). 1 of active S-form
1.84 g of trifluoromethylheptanol was added dropwise at a speed at which hydrogen generation did not become violent to prepare an optically active sodium salt of 1-trifluoromethylheptanol of R-form. After the addition was completed, the mixture was stirred for 1 hour at room temperature, and then 2.50 g of p-chloromethyliodobenzene (this compound was obtained by reducing p-iodobenzoic acid with lithium aluminum hydride and then chlorinating with thionyl chloride. ) Was added and the mixture was stirred overnight. After completion of the reaction, the reaction mixture was poured into ice water and extracted with hexane. The hexane layer was washed with water, washed with 1N HCl water and washed with water, and then hexane was distilled off to obtain 3.8 g of oily optically active p- (1-trifluoromethylheptyloxymethyl) iodobenzene.

(II) Production of optically active pn-decyloxy-p '-(1-trifluoromethylheptyloxymethyl) tolan 1.3 g of pn-decyloxyphenylacetylene and optically active p- (1-trifluoromethylheptyloxy) Methyl) iodobenzene (2.0 g) in 50 ml of triethylamine and bistriphenylphosphine palladium chloride as a catalyst
40 mg and 10 mg of copper (I) iodide were reacted under a nitrogen atmosphere at room temperature overnight. After completion of the reaction, triethylamine was removed and the mixture was extracted with toluene. The toluene layer was washed with 1N hydrochloric acid and washed with water, and then the toluene was distilled off to obtain a black oily substance. This substance was dissolved in hexane, passed through a short column packed with silica gel, and then recrystallized twice with ethanol to give optically active pn-decyloxy-p '-(1 as white crystals of the compound of the present invention.
-Trifluoromethylheptyloxymethyl) tran 2.
I got 20g. The IR spectrum, H-NMR spectrum and F-NMR spectrum of the above compound are shown in FIG. 4 and FIG. 5, respectively.
Shown in Figures and 6.

Elemental analysis value: Theoretical value (%) Actual value (%) C: 74.72 C: 74.80 H: 8.49 H: 8.41 0: 6.04 O: 5.81 F: 10.75 F: 10.98 Use example 1 The compound of Example 1 or 2 (the following) Compounds 1 and 2) and a known smectic liquid crystal (compounds A and B below) were mixed in the ratios shown below to obtain a liquid crystal composition and.

Liquid crystal composition Liquid crystal composition Polyvinyl alcohol was applied as an alignment treatment agent, and the composition was injected into a cell having a cell thickness of 2 μm provided with a transparent electrode which was rubbed on the surface and subjected to parallel alignment treatment, to prepare an optical switching element. This element was placed between two orthogonal polarizers, a voltage of ± 10 V was applied, and the response time was calculated from the change in transmitted light intensity.
60 μsec, the composition was about 100 μsec.

[Effects of the Invention] The present invention provides novel optically active compounds, and these optically active compounds have the following remarkable characteristics.

(1) A liquid crystal compound or liquid crystal composition exhibiting a non-chiral smectic C phase or H phase does not exhibit ferroelectricity,
Although the spontaneous polarization value is 0, by adding an optically active compound having a large dipole moment in the molecule of the present invention, it is possible to exhibit a ferroelectric phase having a large spontaneous polarization value, which is fast. Optical response can be obtained.

(2) When added to a liquid crystal compound or liquid crystal composition that already exhibits ferroelectricity, the configuration of the optically active compound that can be added depends on the sign of the spontaneous polarization of the liquid crystal compound or liquid crystal composition, and the configuration is either the R configuration or the S configuration. Although limited to one, the optically active compound of the present invention can be obtained in both R and S forms, and even when added to a liquid crystal compound or liquid crystal composition that already exhibits ferroelectricity, the liquid crystal compound or liquid crystal composition It is possible to remarkably increase the spontaneous polarization value of an object and obtain a fast optical response.

(3) Minimize adverse effects on the phase series and phase transition temperature of the liquid crystal compound or liquid crystal composition that has a linear molecular structure and is a matrix when added to the liquid crystal compound or liquid crystal composition. Is possible.

(4) Unlike conventional compounds that are merely optically active, an alkyl chain containing an asymmetric carbon atom is independently formed on both sides via a skeleton that may have a substituent (for example, a tolan skeleton). The optically active compound is obtained, and physical properties such as helical pitch, spontaneous polarization value, dielectric anisotropy and optical anisotropy when added to a liquid crystal compound or a liquid crystal composition as a matrix are freely controlled and designed. It is possible to

(5) In addition to the use as a compounding component for a ferroelectric smectic liquid crystal composition or a non-chiral smectic liquid crystal composition, a TN type liquid crystal is prepared by adding the optically active compound of the present invention to a nematic liquid crystal composition. It is possible to control reverse domain generation in the cell.

(6) It is easy to control the hydrophobicity necessary for forming the LB film, and a monomolecular laminated film can be obtained.

(7) Good stability against light, heat and moisture.

Because of the above effects, the optically active compound of the present invention is a very useful substance in developing a practical ferroelectric smectic liquid crystal composition.

[Brief description of drawings]

1, 2 and 3 show the IR, H-NMR and F-NMR spectra of the compound obtained in Example 1, respectively, and FIGS. 4, 5 and 6 show the examples. The IR, H-NMR and F-NMR spectra of the compound obtained in 2 are shown.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location C07C 255/55 255/57 323/18 323/19 323/62 // C09K 19/54 B 9279-4H

Claims (1)

[Claims] 1. A general formula [In the formula, R and R'are an alkyl group having 3 to 20 carbon atoms which may have an asymmetric carbon atom, and X is a single bond (direct bond), -O-, -S-, -COO. −, −OCO− or −OCOO
And A ₁ and A ₂ may be substituted with a substituent selected from the group consisting of a fluorine atom, a chlorine atom, a nitro group and a cyano group. Or And Y is —C≡C— or —CH ₂ CH ₂ —, n is an integer of 0 to 5 and C ^* represents an asymmetric carbon atom].