JP2737304B2 - Chiral ferrocene derivatives - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a chiral ferrocene derivative, and more particularly to a chiral ferrocene derivative useful as a catalyst for asymmetric synthesis.

[Conventional technology]

In an asymmetric synthesis method, which is one of the methods for producing an optically active substance, a catalyst for asymmetric synthesis is often used. Well-known catalysts include compounds derived from natural products such as ephedrine and prolinol derivatives.

However, these compounds derived from natural products often have substrate specificity, and there are some substrates that exhibit high enantioselectivity and others that do not. Therefore, there are reactions that are not applicable or inefficient.

Therefore, for the purpose of reducing such substrate specificity and improving characteristics such as reaction efficiency, attempts have been made to improve catalysts for asymmetric synthesis derived from natural products. However, it is often not easy to change the substituent on the asymmetric carbon, and in many cases, an asymmetric synthesis catalyst having desired characteristics cannot be obtained.

[Problems to be solved by the invention]

Therefore, an object of the present invention is to provide a novel catalyst for asymmetric synthesis that replaces the compound derived from a natural product.

It is a further object of the present invention to provide a catalyst for asymmetric synthesis in which a substituent on an asymmetric carbon can be changed relatively easily.

[Means for solving the problem]

The present invention relates to a chiral ferrocene derivative represented by the following general formula [I].

(In the formula, R ¹ represents a lower alkyl group having 1 to 6 carbon atoms, R ²
And R ³ are the same or different and represent a lower alkyl group having 1 to 6 carbon atoms, a phenyl group or a benzyl group, or R ² and R ³ are
Each form a heterocyclic ring having 4 to 6 carbon atoms with the nitrogen atom to which they are bonded, R ⁴ and R ⁵ are the same or different, and hydrogen, carbon 2
Represents a lower alkyl group or a phenyl group, or R ⁴ and R ⁵
Forms a cycloalkyl group having 5 to 7 carbon atoms or a 10-hydroanthracenyl group with the carbon atom to which each is bonded. However, R ⁴ and R ⁵ are not simultaneously hydrogen, and R ¹ ,
When R ² and R ³ are methyl groups, R ⁴ and R ⁵ are not phenyl groups at the same time. Hereinafter, the present invention will be described in detail.

The lower alkyl group having 1 to 6 carbon atoms of R ¹ , R ² and R ^{3 in} the general formula [I] is, for example, methyl, ethyl, n-propyl,
iso-propyl, n-butyl, tert-butyl, n-pentyl, n-hexyl and the like. In particular, R ¹
Is preferably methyl, and R ² and R ³ are methyl,
Ethyl and iso-propyl are preferred. Examples of the heterocyclic ring formed by R ² and R ³ together with the nitrogen atom to which they are bonded include pyrrolidyl, piperidyl, and the like, with piperidyl being particularly preferred.

Examples of the lower alkyl group having 2 to 6 carbon atoms represented by R ⁴ and R ⁵ include ethyl, n-propyl, iso-propyl, n-butyl, tert-butyl, n-pentyl, n-hexyl and the like. Can be. Particularly, iso-propyl and tert-butyl are preferred. Examples of the cycloalkyl group formed by R ⁴ and R ⁵ together with the carbon atom to be bonded include cyclopentyl, cyclohexyl, and cycloheptyl, and cyclohexyl is particularly preferable.

Table 1 shows R ^{1 to} R ⁵ of specific examples of the compound of the present invention.

The method for producing the compound of the present invention will be described below.The compound of the present invention is lithiated by reacting ferrocene iodide represented by the general formula (II) with n-butyllithium, and then converting the lithiated compound to the general formula (III) By reacting with a ketone (or aldehyde) represented by

As the organolithium compound used for the lithiation,
In addition to n-butyllithium, sec-butyllithium, t-
Butyllithium, methyllithium and phenyllithium can be exemplified. The amount of the organolithium compound to be used is suitably 0.1 to 2.0 equivalents, preferably 0.7 to 1.5 equivalents, based on ferrocene iodide [II]. These organolithium compounds are preferably used as a 5 to 30% solution of hexane or ether. Further, the lithiation reaction is carried out in a solvent (for example, ethers such as ethyl ether and tetrahydrofuran; pentane, hexane,
In the presence of hydrocarbons such as heptane; aromatic hydrocarbons such as benzene, toluene, xylene and dichlorobenzene; or one or a mixture of two or more of these solvents) under the following conditions. is there.

Reaction temperature: -30 ゜ to 50 ゜, preferably -10 ゜ to 30 ゜ Reaction time: 0.1 to 5 hours Reaction pressure: from normal pressure to pressurization, preferably under 1 to 3 atmospheres Atmosphere: under nitrogen or argon And ketone (or aldehyde) [III]
Is preferably a ketone (or aldehyde) [II
The reaction is carried out by adding an ether solution of [I] to the reaction system. The amount of the ketone (or aldehyde) [III] used is based on the ferrocene iodide [II] used in the lithiation.
It is appropriate to use 0.1 to 2.0 equivalents, preferably 0.7 to 1.5 equivalents. This reaction is preferably performed under the following conditions.

Reaction temperature: -40 to -70 ° C, preferably -20 to 40 ° C Reaction time: 1 to 3 hours Reaction pressure: from normal pressure to pressurized, preferably 1 to 3 atm Atmosphere: under nitrogen or argon After the reaction is stopped by adding an aqueous phosphoric acid solution to the system, the aqueous layer is made alkaline, followed by ether extraction, and the target product, a compound of the general formula [I], can be fractionated by column chromatography. .

As the ketone (or aldehyde) of the general formula [III] among the starting compounds used in the above reaction, commercially available products can be obtained. Further, the compound of the general formula [II] is synthesized as follows.

A compound represented by the general formula [IV] (J. Am. Chem. Soc., 92
Vol. 5389 (1970)) by reacting with methyl iodide to form a quaternary ammonium salt, and then reacting with a primary or secondary amine derivative or ammonia so that R ² and R ³ are other than methyl. Can be synthesized [G. Gokel et al., Angel
w. Chem., Int. Ed. Engl., 9 , 64 (1970).

Reaction to form a quaternary ammonium salt MeI: 0.5 to 40 equivalents Solvents: ketones such as acetone and methyl ethyl ketone,
Nitriles such as acetonitrile, benzonitrile, etc. Temperature: -30 to 80 ° C, preferably -10 to 40 ° C Time: 0.1 to 5 hours Pressure: From normal pressure to pressurized, preferably 1 to 3 atm Atmosphere: Nitrogen or argon alone Separation: Filtration when the product is crystallized, otherwise, addition of ethyl ether or hexane to precipitate crystals, followed by filtration.

Reaction with amine or ammonia HNR ² R ³ : 1 to 30 equivalents Solvent: nitriles such as acetonitrile and benznitrile, ethers such as ethyl ether and tetrahydrofuran Temperature: 0 to 100 ° C, preferably 10 to 90 C Time: 0.5 to 100 hours Pressure: From normal pressure to pressurized, preferably 1 to 3 atm Atmosphere: Under nitrogen or argon Isolation: Recrystallization or column chromatography Tetrahedron is a ferrocene derivative of the general formula [V] , 2
6 , by applying the method described in 5453 (1970)
A haloferrocene derivative of the general formula [II] can be synthesized according to the following scheme.

Apart from the above method, a haloferrocene derivative represented by the general formula [II] can be produced by lithiation of a ferrocene derivative represented by the general formula [V] with an organolithium compound, followed by reaction with a halogenating agent. .

As the organolithium compound used for the lithiation,
Examples include n-butyl lithium, sec-butyl lithium, t-butyl lithium, methyl lithium and phenyl lithium. The amount of the organic lithium compound used is 0.1 to 2.0 equivalents, preferably 0.1 equivalent, to the ferrocene derivative (V).
It is appropriate to use 7 to 1.5 equivalents. In addition, these organolithium compounds are hexane or ether 5 to 30.
% Solution is preferred. Further, the lithiation reaction may be carried out in a solvent (for example, ethers such as ethyl ether and tetrahydrofuran; hydrocarbons such as pentane, hexane and heptane; aromatic hydrocarbons such as benzene, toluene, xylene and dichlorobenzene); In the presence of one or a mixture of two or more of the above) under the following conditions.

Reaction temperature: -30 ° to -50 ° C, preferably -10 to 30 ° C Reaction time: 0.1 to 5 hours Reaction stress: from normal pressure to pressurized, preferably under 1 to 3 atmospheres Atmosphere: under nitrogen or argon The reaction is performed by a known method (Ugi et al., J. Am. Chem. So
c., Vol. 92, 5389 (1970)).

Next, halogenation of the lithiated compound can be performed using, for example, iodine, bromine, chlorine, N-iodosuccinimide, N-bromosuccinimide, or N-chlorosuccinimide as a halogenating agent. These halogenating agents can be added to the reaction system as it is or as a solution dissolved in a solvent.

As the solvent, those exemplified above as the lithiation solvent can be used, and the solvents exemplified as the lithiation solvent can also be used as the halogenation reaction solvent. The amount of the halogenating agent to be used is suitably 0.1 to 2.0 equivalents, preferably 0.7 to 1.5 equivalents, relative to the ferrocene derivative [V] used for the lithiation.

The halogenation reaction is preferably performed under the following conditions.

Reaction temperature: -120 ° to 0 ° C, preferably -100 ° to -30 ° C Reaction time: 0.1 to 10 hours Reaction stress: from normal pressure to pressurized, preferably 1 to 3 atm Atmosphere: Nitrogen or argon under halogenation Can be determined by thin layer chromatography. At the end of the reaction, the reaction is stopped by adding water to the reaction system, followed by ether extraction and fractionation of the desired product by column chromatography.

In the haloferrocene derivatives of the general formula (II), the compounds in which R ² and R ³ are methyl groups are quaternary ammonium chloride by methyl iodide and then reacted with a primary or secondary amine to form primary and secondary amines. By appropriately selecting a secondary amine, R ² and R ³ can be converted to a compound other than a methyl group [I. Ugi et al., J. Org. Chem., 37, 3052 (197
2); T. Hayashi et al., Bull. Chem. Soc. Jpn, 53, 1138 (19
80); G. Gokel et al., Angew. Chem., Int. Ed. Engl, 9 , 64 (19).
70)].

〔Usefulness〕

The optically active ferrocene derivative of the present invention can be an effective ligand for Lewis acidic metals, and can be used in an asymmetric introduction reaction using Lewis acidic metals such as zinc, aluminum, titanium, cerium, and nickel. And a catalyst exhibiting high mirror image surface selectivity.

According to the present invention, a chiral ferrocene derivative having various substituents can be provided, and this derivative can be applied to various asymmetric synthesis reactions. The optically active ferrocene derivatives of the present invention can also be used to provide polymers useful as heterogeneous asymmetric synthesis catalysts and gels for polymer optical resolution (FS Arimoto et al., J. Am. Chem. Soc. , 77, 6295 (195
5); see GF Hayes et al., Polymer, 18, 1286 (1977)).

REFERENCE EXAMPLE 1 Under an argon atmosphere, (+)-(R) -N, N-dimethyl-1-ferrocenylethylamine (2.66 g, 10.3 mmol) was added to a 100 ml glass atmospheric pressure reactor having a stirrer, and 25 ml of ether was added. Was dissolved. After cooling with ice, 12.4 ml (0.94 M, 11.7 mmol) of a solution of secondary butyllithium in cyclohexane was added dropwise. The reaction was performed for 1 hour under ice cooling. Dry ice-
After cooling in an acetone bath, a solution of 3.00 g (11.7 mmol) of iodine dissolved in 25 ml of tetrahydrofuran was added dropwise. After reacting for 1 hour under cooling, water was added to stop the reaction. The aqueous layer was made alkaline and extracted with ether. The organic layer was dried with anhydrous sodium sulfate. After the solvent was distilled off, the residue was separated by alumina column chromatography to give (-)-(R) -N, N-dimethyl-1-.
[(S) -2-iodoferrocenyl] ethylamine is 3.
12 g (production yield 79%) were obtained. Further, it was recrystallized from acetonitrile.

▲ [α] ²² _D ▼ = −8.98 ゜ (C1.0, EtOH) mp 78-9 ° C. 60 MHz ¹ H NMR (δ, CDCl ₃ ); 1.50 (3H, d, J = 8.0 Hz) 2.15 (6H, s) , 3.15 (1H, q, J = 7.5 Hz) 4.13 (7H, s), 4.40 to 4.60 (1H, m) IR (KBr) 3100, 2970, 2940, 2802, 2760, 1100, 1000c
m ^-1 Reference Example 2 (R) -N, N-dimethyl-1- obtained in Reference Example 1 in a glass-made atmospheric pressure reactor having a stirrer under an argon atmosphere.
[(S) -2-iodoferrocenyl] ethylamine 3.83
g (10.0 mmol) was added and dissolved in 20 ml of acetone. At room temperature, 2.86 ml (46 mmol) of methyl iodide was added and reacted for 10 minutes. A precipitate was formed by adding 100 ml of ethyl ether, and 5.25 g (10.0 mmo) of a quaternary ammonium salt was collected by filtration.
l) got.

Subsequently, the resulting quaternary ammonium salt was dissolved in 130 ml of acetonitrile and then dissolved in 26 ml of diethylamine (2
50 mmol) and stirred at 30 ° C. for 48 hours. Ethyl ether was added and washed with water. The ethyl ether solution was dried over anhydrous sodium sulfate, and the solvent was distilled off. The residue was recrystallized using ethyl alcohol to give (R) -N, N diethyl-1-[(S) -2-iodoferrocenyl] ethylamine 3.70 g (9.0 mmol, production yield 90%). Obtained.

▲ [α] ²⁷ _D ▼ -58.6 ゜ (C 0.596 EtOH) Melting point 49 ° C 90 MHz ¹ H NMR (δ, CDCl ₃ ); 0.98 (6H, t, J = 7.2 Hz) 1.39 (3H, d, J = 6.6 Hz) ) 2.39 (2H, q, J = 6.9 Hz) 2.42 (2H, q, J = 6.9 Hz) 3.90 (1H, q, J = 6.6 Hz) 4.09 (5H, s) 4.15 to 4.21 (2H, m) 4.38 to 4.44 (1H, m) IR (KBr) 3100, 2980, 2820, 1104, 1001, 820 cm ^-1 Reference Examples 3, 4 Same operation as Reference Example 2 except that the amine shown in Table 1 was used instead of diethylamine. Was repeated to obtain a product in Table 1.

Product of Reference Example 3 90 MHz ¹ H NMR (δ, CDCl ₃ ); 1.16 to 1.50 (6 H, m) 1.50 (3 H, d, J = 6.3 Hz) 2.36 (3 H, t, J = 5.1 Hz) 3.70 (1 H , q, J = 7.2 Hz) 4.10 (5H, s) 4.16 to 4.25 (2H,
m) 4.39 to 4.49 (1H, m) IR (KBr) 3100, 2950, 2860, 2805, 2760, 1108, 100
2, 820 cm ^-1 The product of Reference Example 4 90 MHz ¹ H NMR (δ, CDCl ₃ ); 0.96 (6H, d, J = 6.0 Hz) 1.06 (6H, d, J = 6.0 Hz) 1.49 (3H, d, J = 6.3Hz) 2.89 to 3.37 (2H, m) 4.03 (1H, q, J = 6.6Hz) 4.06 (5H,
s) 4.10-4.28 (2H, m) 4.36-4.45 (1H, m) IR (KBr) 3105, 2995, 2898, 1365, 1195, 1109, 100
2,820cm ^-1 Example 1 2.12 g (5.0 mm) of (R) -1-[(S) -2-iodoferrocenyl] -1-piperidinoethane obtained in Reference Example 3 in a glass-made atmospheric pressure reactor having a stirrer under an argon atmosphere.
ol) and dissolved in 12 ml of ethyl ether. After cooling on ice, 3.13 ml of a hexane solution of n-butyllithium (1.60 ml)
M, 5.0 mmol) was added dropwise. After reacting for 5 minutes under ice cooling, 911 mg (5.0 mmol) of benzophenone (15 ml) in ethyl ether was added. The reaction was performed at room temperature for 1 hour. The reaction was stopped by adding an 8% aqueous phosphoric acid solution. After washing the acidic solution with ethyl ether, a concentrated alkaline aqueous solution was added to make the acidic solution strongly alkaline. Extract with ethyl ether,
Dry over anhydrous sodium sulfate. After the solvent was distilled off, the residue was subjected to alumina column chromatography (hexane: AcO
(-)-(R) -1-
1.80 g (3.75 mmol, production yield 75%) of [(S) -2-diphenylhydroxymethylferrocenyl] -1-piperidinoethane were obtained.

▲ [α] ²⁵ _D ▼ -203.6 ゜ (C 0.48, EtOH) Melting point 62-69 ° C 90MHz ¹ H NMR (δ, CDCl ₃ ); 0.29-1.70 (6H, m) 1.24 (3H, d, J = 6.3Hz) ) 2.25 (4H, t, J = 5.7Hz) 3.80 (5H, s) 3.89 to 4.01 (1H, m) 4.03 to 4.20 (1H, m) 4.20 to 4.37 (1H, m) 4.40 (1H, q, J = 6.9Hz) 6.98-7.45 (8H, m) 7.50-7.75 (2H, m) 8.72 (1H, s, O
H) IR (KBr) 3460, 3100, 3070, 2950, 2840, 1600, 144
4, 1104, 1002, 820, 762, 753, 703 cm ^-1 Examples 2 to 6 (R) -1-[(S) -2-iodoferrocenyl]-
The same operation as in Example 1 was repeated except that the raw materials shown in Table 2 were used instead of benzophenone as 1-piperidinoethane and ketone, to obtain products shown in Table 2.

Product of Example 2 90 MHz ¹ H NMR (δ, CDCl ₃ ); 0.37 (3H, d, J = 6.6 Hz) 0.85 (3H, d, J = 6.3 Hz) 1.29 (6H, d, J = 6.6 Hz) 1.45 (3H, d, J = 6.6Hz) 1.60-1.97 (1H, m) 2.10 (6H, s) 2.30-2.68 (1H, m) 3.78-3.95 (1H, m) 4.09 (5H, s) 4.11-4.23 (3H, m) 7.69 (1H, s, -OH) IR (KBr) 3420, 3100, 2998, 2898, 2800, 1108, 100
4,828,820 cm ^-1 The product of Example 3 90 MHz ¹ H NMR (δ, CDCl ₃ ); 1.21 (3H, d, J = 6.3 Hz) 1.30 to 2.42 (10H, m) 2.09 (6H, s) 3.90 ~ 4.09 (3H, m) 4.12 (5H, s) 4.29 (1H, q, J = 6.3Hz) 7.18 (1H, s, -OH) IR (KBr) 3450,3100,2950,2800,1105,822cm ^-1 Product of Example 4 90 MHz ¹ H NMR (δ, CDCl ₃ ); 0.63 (6H, t, J = 7.2 Hz) 1.20 (3H, d, J = 6.0 Hz) 1.89 to 2.58 (4H, m) 3.78 (5H , s) 3.89 to 4.03 (1H, m) 4.07 to 4.20 (1H, m) 4.20 to 4.31 (1H, m) 4.65 (1H, q, J = 6.9Hz) 6.98 to 7.48 (8H, m) 7.53 to 7.76 ( 2H, m) 8.70 (1H, s, -O
H) IR (KBr) 3450, 3100, 3060, 2995, 2860, 1598, 110
9,1003,820,755,702Cm ^-1 product 90 MHz ¹ H NMR of Example _{5 (δ, CDCl 3);} 1.32 (3H, d, J = 6.0Hz) 1.35~1.95 (6H, m) 2.64 (4H , t, J = 5.4Hz) 3.20 to 3.39 (1H, m) 3.50 (5H, s) 3.70 to 3.90 (1H, m) 3.90 to 4.02 (1H, m) 4.02 to 4.18 (1H, m) 4.20 to 4.60 ( 2H, m) 7.06 to 7.50 (4H, m) 7.60 to 7.83 (1
H, m) 7.89 to 8.09 (1H, m) IR (KBr) 3460, 3100, 3150, 2950, 2830, 1108, 100
1,820,760,747 cm ^-1 The product of Example 6 90 MHz ¹ H NMR (δ, CDCl ₃ ); 0.69 (6H, d, J = 6.0 Hz) 1.09 (6H, d, J = 6.0 Hz) 1.38 ( 3.85 to 3.22 (2H, m) 3.70 (5H, s) 3.90 to 4.10 (1H, m) 4.10 to 4.23 (1H, m) 4.23 to 4.37 (1H, m) 4.82 (1H , q, J =
6.9Hz) 6.98-7.47 (8H, m) 7.60-7.83 (2H, m) 8.45 (1H, s, -OH) IR (KBr) 3450, 3100, 3070, 2990, 2890, 1600, 110
5, 1000, 819, 750, 700 cm ^-1 Examples 7 to 9 (R) -1-[(S) -2-iodoferrocenyl]-
The same operation as in Example 1 was repeated, except that the raw materials shown in Table 3 were used instead of 1-piperidinoethane, and the aldehydes shown in Table 3 were used instead of benzophenone as the carbonyl compound. The products were two kinds of diastereomers, and were separated by chromatography on alumina or silica gel to obtain each product, and the physical properties of each product were shown. Table 3 shows the results.

Example 7 Product 1 90 MHz ¹ H NMR (δ, CDCl ₃ ); 1.36 (3H, d, J = 6.6 Hz) 2.22 (6H, s) 3.38-3.58 (1H, m) 3.80-4.40 (3H, m) 4.03 (5H, s) 5.94 (1H, s) 7.12 to 7.68 (5H, m) 7.70 to 8.02 (1H, s, OH). IR (neat) 3102, 3049, 2998, 2849, 2800, 1605, 110
5, 1000, 819, 738, 700 cm ^-1 Product 2 90 MHz ¹ H NMR (δ, CDCl ₃ ); 1.23 (3H, d, J = 6.0 Hz) 2.19 (6H, s) 3.80 (5H, s) 3.90- 4.35 (4H, m) 5.47 (1H, s) 6.00 to 6.70 (1H, -OH) 7.09 to 7.68 (5H, m) IR (neat) 3370, 3100, 3040, 2998, 2798, 1600, 110
4, 1000, 818, 719, 700 cm ^-1 Example 8 Product 1 90 MHz ¹ H NMR (δ, CDCl ₃ ); 0.94 (9H, s) 1.29 (3H, d, J
= 6.6Hz) 2.07 (6H, s) 2.62 (1H, -OH) 3.80 (1H, q, J =
7.5Hz) 3.97-4.17 (4H, m) 4.16 (5H, s) IR (KBr) 3480, 3105, 2999, 2970, 2840, 2800, 110
6, 1102, 818 cm ^-1 Product 2 90 MHz ¹ H NMR (δ, CDCl ₃ ); 1.18 (9H, s) 1.29 (3H, d, J
= 6.0Hz) 2.15 (6H, s) 4.00 (5H, s) 4.05 to 4.18 (3H,
m) 4.19-4.34 (1H, m) 4.52 (1H, s) 7.40-7.80 (1H,-
OH) IR (KBr) 3450, 3102, 3000, 2960, 2850, 2800, 110
8, 1005, 820 cm ^-1 Example 9 Product 1 90 MHz ¹ H NMR (δ, CDCl ₃ ); 0.99 (9H, s) 1.27 (3H, d, J
= 5.4Hz) 1.18 to 1.55 (6H, m) 2.20 to 2.60 (4H, m) 3.79 (1H, q, J = 7.2Hz) 3.97 to 4.37 (4H, m) 4.11 (5H, s) IR (neat) 3100 , 3000, 2950, 2820, 1105, 1000, 820c
m ^-1 product 2 90 MHz ¹ H NMR (δ, CDCl ₃ ); 1.20 (9 H, s) 1.33 (3 H, d, J
= 6.0Hz) 1.29 to 1.56 (6H, m) 2.32 to 2.57 (4H, m) 4.00
(5H, s) 4.03 to 4.20 (3H, m) 4.20 to 4.37 (1H, m) 4.50
(1H, s) IR (neat) 3250, 3105, 3000, 2960, 2840, 1112, 100
9,822cm ^-1

Claims (1)

(57) [Claims] 1. A chiral ferrocene derivative represented by the following general formula [I]. (In the formula, R ¹ represents a lower alkyl group having 1 to 6 carbon atoms, R ²
And R ³ are the same or different and represent a lower alkyl group having 1 to 6 carbon atoms, a phenyl group or a benzyl group, or R ² and R ³ are
Each form a heterocyclic ring having 4 to 6 carbon atoms with the nitrogen atom to which they are bonded, R ⁴ and R ⁵ are the same or different, and hydrogen, carbon 2
Represents a lower alkyl group or a phenyl group, or R ⁴ and R ⁵
Forms a cycloalkyl group having 5 to 7 carbon atoms or a 10-hydroanthracenyl group with the carbon atom to which each is bonded. However, R ⁴ and R ⁵ are not simultaneously hydrogen, and R ¹ ,
When R ² and R ³ are methyl groups, R ⁴ and R ⁵ are not phenyl groups at the same time. )