Iodine-catalyzed electrophilic substitution of indoles: Synthesis of (un)symmetrical diindolylmethanes with a quaternary carbon center.

A novel, versatile approach for the synthesis of unsymmetrical 3,3'-diindolylmethanes (DIMs) with a quaternary carbon center has been developed via iodine-catalyzed coupling of trifluoromethyl(indolyl)phenylmethanols with indoles. In contrast to previously reported methods, the new procedure is characterized by chemoselectivity, mild conditions, high yields, and scalability to obtain gram amounts for biological studies. Selected compounds were found to display affinity for cannabinoid receptors, which are promising drug targets for the treatment of inflammatory and neurodegenerative diseases.

Introduction

Diindolylmethanes (DIMs) represent an important class of indole alkaloids, that are constituents of pharmaceuticals [1-7] and agrochemicals [8-9]. DIM derivatives possess a variety of biological activities (Figure 1) [10]. Unsubstituted DIM (I), for example, exhibits antimicrobial [5], anticancer [11-13], and anti-inflammatory effects (Figure 1) [14]. There is preclinical evidence for activity against several types of cancer [15], and DIM has been clinically evaluated for the treatment of prostate cancer [16] and showed promise for the treatment of cervical dysplasia [17]. The related trisindoline (II) was reported to possess antibiotic activity [18], while DIM derivatives III and IV also showed anticancer activities (Figure 1). Owing to their exciting biological activities, DIM derivatives have recently received increasing attention from synthetic organic chemists, biologists, and pharmacologists.

Figure 1

Diindolylmethanes and reported biological activities.

In general, DIMs can be synthesized via electrophilic substitution of indoles by aldehydes or ketones in the presence of conventional Lewis or Brønsted acids as catalysts [19]. This strategy is straightforward, but it always only provides symmetrical DIMs. The synthesis of unsymmetrical DIM derivatives, however, remains challenging, and merely sporadic examples are reported in literature [20-21].

Methods for the introduction of fluorinated groups into organic molecules are of high interest due to fluorine’s unique physical and chemical properties, such as its small size, high electronegativity, and high C–F bond dissociation energy [22-24]. Organofluoro compounds developed as drug molecules often display increased metabolic stability and bioavailability compared to non-fluorinated analogs [25]. Considering the ever-growing demand for organofluorine compounds, the development of new methodologies that allow the incorporation of fluorine atoms into bioactive molecules is highly desired and will also be addressed herein.

Recently, the use of indol-3-ylmethanols as electrophiles has emerged as a powerful strategy for constructing synthetically valuable indol-3-yl-containing molecules. In particular, the reaction of indol-3-ylmethanols with indoles has become a useful route for the preparation of tertiary unsymmetrical 3,3'-DIMs [26-35]. However, synthetic methods for efficient synthesis of unsymmetrical 3,3'-DIMs with a quaternary carbon center, including trifluoromethyl-substituted 3,3'-DIMs, are still rare.

Sasaki et al. reported the reaction of trifluoromethyl(indolyl)phenylmethanols with indoles in the presence of trifluoroacetic acid (TFA) and CHCl3 (Figure 2) [36]. Very recently, Ling et al. reported the same reaction in the presence of Ga(OTf)3 in acetonitrile (Figure 2) [37]. Although these methods are certainly useful, they have several undeniable drawbacks, including the use of heavy-metal catalysts and the necessity of employing indoles bearing bulky substituents at their 2-position (Ling et al.), or the need for chlorinated solvents (Sasaki et al.), as well as difficulty to scale up the reactions to a multigram scale, as well as a generally rather limited substrate scope. Therefore, finding a robust method with a broad substrate scope and functional group tolerance is highly desirable.

Figure 2

Synthetic strategies toward trifluoromethylated unsymmetrical quaternary DIMs.

As part of our continuous efforts to prepare biologically active DIM derivatives [38], we herein report an innovative approach to synthesize unsymmetrical 3,3'-diindolylmethanes (DIMs) with a fluoromethyl-containing quaternary carbon center via an iodine-catalyzed coupling reaction of trifluoromethyl(indolyl)phenylmethanol with indole derivatives. This method has also been extended to the synthesis of pentafluoro-ethylated and heptafluoro-propylated DIMs in excellent yields. Selected compounds were evaluated in radioligand binding studies for their affinities towards cannabinoid CB1 and CB2 receptors.

Results and Discussion

Optimization of the reaction

The reaction conditions were optimized using 2,2,2-trifluoro-1-(5-methoxy-1H-indol-3-yl)-1-phenylethan-1-ol (1a, 5 mmol) and 1H-indole (2a, 5 mmol) as model substrates (Table 1). At first, the reaction was attempted in trifluoroethanol (TFE), and water, respectively, as these solvents had been utilized for the preparation of unsymmetrical DIMs from (1H-indol-3-yl)(phenyl)methanol by Xiao and co-workers [39-40]. However, no product was formed in either solvent even at high temperatures (Table 1, entries 1–3). This is likely due to the steric hindrance of the CF3-substituted quaternary carbon atom in substrate 1a. Therefore, the solvent was changed to H2SO4 (5%) in water (Table 1, entry 4) or glacial acetic acid (entry 5), and the reactions were performed at room temperature. While no reaction occurred in 5% H2SO4, traces of product were observed in acetic acid (entry 5). Therefore, the reaction mixture was gradually heated to 50 °C (Table 1, entry 6), 80 °C (entry 7), and 100 °C (entry 8). To our delight, the formation of the expected product was steadily increased to 32, 47, and 56%, respectively. Nevertheless, it was not possible to further increase the yield of the product using this solvent.

Table 1

Optimization of the reaction conditions for the preparation of 5-methoxy-3-(2,2,2-trifluoro-1-(1H-indol-3-yl)-1-phenylethyl)-1H-indole (3a)a.

Entry	Solvent	Catalyst	Temp. (°C)	Time (h)	Yield (%)b
1	trifluoroethanol	–	rt	24	0c
2	trifluoroethanol	–	80	24	0c
3	H2O	–	100	24	0c
4	5% H2SO4 in H2O	–	rt	24	0c
5	CH3COOH	–	rt	24	traces
6	CH3COOH	–	50	24	32
7	CH3COOH	–	80	24	47
8	CH3COOH	–	100	24	56
9	MeCN	AlCl3(10 mol %)	rt	24	10
10	MeCN	FeCl3(10 mol %)	rt	24	17
11	MeCN	I2 (10 mol %)	rt	24	51
12	MeCN	p-TsOH(10 mol %)	rt	24	5
13	MeCN	InCl3(10 mol %)	rt	12	traces
14	MeCN	FeCl3(10 mol %)	40	12	67
15	MeCN	p-TsOH(10 mol %)	40	24	15
16	MeCN	I2 (10 mol %)	40	5	98
17	MeCN	I2 (10 mol %)	80	5	95
18	MeCN	I2 (5 mol %)	40	12	89

aReactions of 1a (5 mmol) and 2a (5 mmol) were performed in 5 mL of solvent. bIsolated yields after column chromatography. cNo reaction. MeCN, acetonitrile. rt, room temperature.

For subsequent attempts, we investigated the reaction using different Lewis acid catalysts, including AlCl3 (Table 1, entry 9, 10% yield), FeCl3 (entry 10, 17%), I2 (entry 11, 51%), p-TsOH (entry 12, 5%), and InCl3 (entry 13, traces), in acetonitrile at room temperature. Among these, the presence of I2 led to the highest yield of 51% (Table 1, entry 11). This trend was consistent: Upon heating to 40 °C, reactions with FeCl3 (Table 1, entry 14) or p-TsOH (entry 15) yielded 67% and 15% of product, respectively, while 98% of the product was obtained in the presence of I2 (Table 1, entry 16). However, further increase of the reaction temperature to 80 °C did not significantly affect the generation of the product (Table 1, entry 17). Lowering the amount of catalyst from 10 mol % to 5 mol % reduced the product formation (Table 1, entry 18).

Having optimized the reaction conditions (I2, 10 mol %, 40 °C for 5 h in MeCN; entry 16, Table 1), we explored the scope of the reaction. At first, we employed differently substituted indole derivatives (Table 2). A large variety of substituted indoles was well tolerated, and their reactions with 1a provided the desired products in good to excellent yields (61–99%). Reaction of 1a with indoles bearing electron-donating substituents, such as methoxy (2b) or hydroxy (2f), afforded the products in good yields (3b: 67%; 3f: 61%). Coupling of 1a with indoles substituted with electron-withdrawing groups, including cyano (2c), fluoro (2d, 2e, 2h), and bromo (2g), likewise resulted in good to excellent yields of the desired products (3c: 65%, 3d: 99%, 3e: 96%, 3g: 95%, 3h: 91%).

Table 2

Substrate scope of the reaction with differently substitute indole derivatives 2.

Yields of products 3a–o
3a (98%) (81%)[35]	3b (67%)	3c (65%)
3d (99%), (80%)[35]	3e (96%)	3f (61%)
3g (95%)	3h (91%)	3i (77%)
3j (89%)	3k (70%)	3l (72%)
3m (89%)	3n (99%)	3o (87%)

Shifting the position of the methoxy group of 1a from position 5 to 4 (1b) or 6 (1c) also led to the formation of the products in very good yields (3i: 77%, 3j: 89%, 3k: 70%, 3l: 72%). The intermediates 1d with 6-fluoro or 1e without substituent on the indole ring reacted with 5,6-difluoroindole (2h) and formed the desired products in excellent yields of 89% (3m) and 99% (3n). Besides, 2-methylindole (2i) smoothly reacted with 1e, affording product 3o in 87% yield.

Next, we studied the substrate scope of the trifluoromethyl(indolyl)phenylmethanols 1 with respect to the substitution of the phenyl ring (Table 3). The derivatives bearing p-tolyl (1f), p-fluorophenyl (1g), or p-bromophenyl (1g) rings reacted with a series of halogenated indoles (2d, 2e, 2f, 2g, 2h, and 2j) providing the unsymmetrical DIMs (3p: 99%, 3q: 98%, 3r: 87%, 3s: 90%, 3t: 82%, 3u: 92%) in excellent yields ranging from 82–99%. It was interesting to see that a compound, in which the phenyl ring of 1 was replaced by the heteroaryl moiety thiophene (1i), reacted efficiently with a series of indole derivatives (2a, 2d, 2h) providing yields of 82–90% (3v–x).

Table 3

Substrate scope of the reaction of 1f–i with trifluoromethyl(indolyl)phenylmethanols 1: modification of the aryl group.

Yields of products 3p–x
3p (99%)	3q (98%)	3r (87%)
3s (90%)	3t (82%)	3u (92%)
3v (89%)	3w (90%)	3x (82%)

We further extended this protocol to the preparation of unsymmetrical pentafluoroethylated and heptafluoropropylated DIM derivatives (Table 4). The (indol-3-yl)phenylmethanol derivative bearing a pentafluoroethyl residue (1j) was efficiently reacted with a series of indole derivatives (2d, 2e, 2h, 2j, and 2k) substituted either both electron-donating or electron-withdrawing groups, and provided the desired products (3y: 96%, 3z: 89%, 3aa: 90%, 3ab: 86%, 3ac: 92%, 3ad: 94%) in excellent yields (86–96%).

Table 4

Substrate scope of the reaction of 1j–l with trifluoromethyl(indolyl)phenylmethanols 1: modification of the trifluoromethyl group.

Yields of products 3y,z,aa–ag
3y (96%)	3z (89%)	3aa (90%)
3ab (86%)	3ac (92%)	3ad (94%)
3ae (65%)	3af (72%)	3ag (70%)

The (indol-3-yl)phenylmethanol derivative bearing a heptafluoropropyl residue (1k) underwent coupling reactions with indole derivatives (2a, 2d, and 2e) and yielded the expected products (3ae: 65%, 3af: 72%, 3ag: 70%) in very good yields (65–72%). It was observed that the yield of heptafluoropropylated DIMs was slightly lower compared to their pentafluoroethylated congeners (compare 3y: 96% vs 3af: 72%, and 3aa: 90% vs 3ag 70%).

Next, the necessity of the fluoroalkyl substituent was investigated to study the scope of the reaction. The non-fluorinated (indol-3-yl)phenylmethanol derivative bearing a methyl residue (1l) and 2c or 2d were reacted applying the optimized conditions. However, the desired products 3ah and 3ai were not produced (Figure 3). These experiments indicated that the presence of a fluoroalkyl substituent was indeed essential for the alkylation of indoles.

Figure 3

Reactions performed to study the scope of the method.

We further investigated the new method’s feasibility for large-scale synthesis (Figure 4). Thus, 2,2,2-trifluoro-1-(5-methoxy-1H-indol-3-yl)-1-phenylethan-1-ol (1a, 1.0 g, 3.10 mmol) and 2,2,3,3,3-pentafluoro-1-(5-methoxy-1H-indol-3-yl)-1-phenylpropan-1-ol (1j, 1.0 g, 2.7 mmol) were reacted with indole (2a, 0.40 g, 3.4 mmol; and 0.347 g, 2.9 mmol, respectively). The reactions proceeded without significant loss in efficiency, affording 1.2 g of 3a (92% yield) and 1.10 g of 3ad (87% yield).

Figure 4

Gram-scale synthesis of unsymmetrical DIMs 3a and 3ad.

The unsubstituted diindolylmethane (I, Figure 1) was previously reported to bind to the cannabinoid receptors, CB1 (Ki 4.3 µM) and CB2 (Ki 1.1 µM) [41]. Both receptors are considered important therapeutic targets, e.g. for neurodegenerative and inflammatory diseases. Selected final products (3a, 3b, 3e, 3g, 3h, 3n, 3ad) were tested for their binding affinities towards human CB1 and CB2 receptors (Table 5).

Table 5

Binding affinities of unsymmetrical fluoromethyl-substituted DIM derivatives for cannabinoid receptors.

Compound	Structure	Human CB1 receptor	Human CB2 receptor
		Radioligand binding assay
		Ki ± SEM (µM)(vs [3H]CP55,940)	Ki ± SEM (µM)(vs [3H]CP55,940)
I [41]	See Figure 1 for structure	4.3	1.1
3a		4.23 ± 0.03	6.04 ± 0.11
3b		7.64 ± 0.80	4.75 ± 0.34
3e		3.21 ± 0.25	4.47 ± 0.12
3g		4.02 ± 0.22	4.62 ± 0.33
3h		4.78 ± 1.26	9.90 ± 1.45
3n		2.04 ± 0.08	6.14 ± 0.13
3ad		1.82 ± 0.09	11.2 ± 0.5

At the CB1 receptor, compound 3a with a methoxy substituent on one of the two indole rings showed equipotent affinity to lead compound I, while introducing an additional 4-methoxy moiety into the second indole ring reduced binding affinity (3b). Compounds bearing 5-OMe,6'-F (3e), 5-OMe,7'-Br (3g), and 5-OMe,5',6'-diF substitution (3h) exhibited similar binding affinities to lead compound I. 5,6-DiF-DIM derivative 3n (CB1: Ki 2.04 µM) showed a slightly improved binding affinity compared to lead compound I. These results suggest that compounds with small substituents like fluoro on only one indole ring are favorable for CB receptor binding. The pentafluoroethylated DIM derivative 3ad was the best CB1 ligand of the present series with a Ki value of 1.82 µM. The binding curves of 3e and 3ad are depicted in Supporting Information File 1, Figure S1.

Compound 3e showed similar binding affinities at both CB1 and CB2 receptor. Therefore, it was selected to determine and compare its functional activity at both receptor subtypes. Compound 3ad was selected due to its high CB2 receptor affinity and selectivity. It is well known that CB1 receptors exhibit high constitutive activity [41]. Compound 3e reduced the basal activity of CB1 receptors (Supporting Information File 1, Figure S2A) but not that of CB2 receptors indicating that this compound acts as an inverse agonist (EC50 0.786 ± 0.233 µM) at CB1 receptors (Supporting Information File 1, Figure S2B). This effect was less pronounced for 3ad. Non-transfected cells used as controls also did not show any effect after treatment with 3ad (Supporting Information File 1, Figure S2C). DIM was previously shown to be weak inverse agonist at CB1 receptors which is consistent with our current findings for DIM derivatives 3ad and especially 3e [42]. Next, we investigated the antagonistic effect of 3e at CB1 receptors (Supporting Information File 1, Figure S3A). Compound 3e blocked CB1 receptor activation with an IC50 value of 5.68 ± 0.54 µM, while it was weaker in inhibiting CB2 receptor activation. Similarly, 3ad was also able to fully block CB1 receptor activation (IC50 value of 5.22 ± 0.68 µM). Our results indicate that the new DIM derivatives act as potent CB1 receptor antagonists with inverse agonistic activity, i.e., they stabilize the inactive receptor conformation. Further optimization is warranted. This class of compounds also possesses potential for the development of CB2-selective or dual CB1/CB2-receptor antagonists.

Based on previous reports [25-3543-53], a plausible reaction mechanism is proposed for the synthesis of 3a as an example, as depicted in Figure 5. We suggest that the reaction is initiated by iodine-mediated activation of the secondary alcohol in compound 1a (A), followed by elimination of HOI to generate the vinyliminium ion species B (see mesomeric structure C) [52]. This electrophilic intermediate undergoes a C3-selective Friedel−Crafts reaction with 2a to deliver intermediate D, and the catalyst I2 is regenerated by the reaction of HOI and I− (see C to D in box highlighted by dashed line). The intermediate D is stabilized by aromatization yielding product 3a and H2O.

Figure 5

Plausible reaction mechanism for the synthesis of fluoromethylated unsymmetrical DIMs, shown for compound 3a as an example.

Conclusion

The described novel and efficient synthetic protocol provides a convenient access to a wide range of unsymmetrical trifluoromethylated 3,3'-diindolylmethanes via I2-catalyzed Friedel–Crafts alkylation reaction of trifluoromethylated (indol-3-yl)-1-phenylethan-1-ols with substituted indoles. The method was also extended to the synthesis of pentafluoroethylated and heptafluoropropylated-DIMs. It constitutes an important addition to the active field of DIM syntheses facilitating the preparation of unsymmetrical quaternary DIMs without the need for chlorinated solvents, high temperatures, or heavy-metal catalysts. A broad range of substrates is tolerated and the reaction is suitable for large-scale preparation of the target compounds. The outlined methodology allows for the rapid generation of structurally diverse DIM derivatives to study structure–activity relationships, to optimize biological activity and other properties in order to prepare tool compounds and future drugs. Several compounds displayed micromolar binding affinities toward CB1 and CB2 receptors acting primarily as CB1 receptor antagonists/inverse agonists. We are confident that our straightforward new approach will enable us and others to extensively investigate these bioactive molecules and their targets in future studies.

Experimental and analytical data.