4,6-Dihydroxysalicylic Acid-Catalyzed Oxidative Condensation of Benzylic Amines and Aromatic Ketones for the Preparation of 2,4,6-Trisubstituted Pyridines and Its Application to Metal-Free Synthesis of G-Quadruplex Binding Ligands.

4,6-Dihydroxysalicylic acid was activated under air to catalyze the one-pot oxidative condensation reaction of benzylamines with acetophenones in the presence of BF3·Et2O, affording 2,4,6-trisubstituted pyridines in yields of 59-91%. During this metal-free oxidative condensation reaction, the benzylamines not only provided the aryl moiety at the 4-position of the pyridines but also acted as the nitrogen donor. This method can be applied to the metal-free synthesis of G-quadruplex binding ligands by the sequential addition of 4-chlorobutyryl chloride and pyrrolidine to the reaction system of the 2,4,6-trisubstituted pyridine synthesis.

Introduction

Pyridines are one of the most important N-containing heterocycles with applications in many fields[1] such as organic synthesis,[2] materials science,[3] and pharmaceutical science.[4] Currently, n class="Chemical">pyridine-based natural compounds continue to be discovered and studied for their properties and biosynthesis.[5] Within the pyridine family, 2,4,6-trisubstituted pyridines have garnered great attention because their excellent thermal stability[6] makes them excellent ligands for various metals.[7] For example, cyclometalated Au(III) complexes featuring a tridentate C^N^C scaffold have proven to be effective anticancer drugs (Figure , [Au(Ph-C^N^C)Cl]).[8] Moreover, 2,4,6-triarylpyridines are a class of G-quadruplex binding ligands that stabilize G-quadruplex DNA (G4-DNA) and provide an efficient approach to cancer treatment (Figure , G-quadruplex binding ligands).[9]

Figure 1

Structure of [Au(Ph-C^N^C)Cl] and G-quadruplex binding ligands.

The traditional synthetic strategies toward the construction of 2,4,6-trisubstituted pyridines require three components, one of which is an additional n class="Chemical">nitrogen source.[10] Recently, simple two-component condensation reactions for the synthesis of 2,4,6-trisubstituted pyridines have been developed.[11] However, most of their starting materials, such as oximes[12] and chalcones,[13] are not commercially available. Benzylamines are considered efficient substrates for the synthesis of 2,4,6-trisubstituted pyridines and play a dual role of providing an aryl functionality at the 4-position of the pyridines as well as being a nitrogen source. The use of transition metal catalysts is one of the powerful methods to synthesize 2,4,6-triarylpyridines via the oxidative condensation reaction of benzylamines with acetophenones.[14] Moreover, metal-free versions of the reaction were recently reported, as shown in Scheme . Although these limited methods provide complementary pathways toward the synthesis of 2,4,6-triarylpyridines, corrosive trifluoromethanesulfonic acid (eq 1),[15] iodine at relatively high temperature (140 °C) (eq 2),[16] or complex photoredox catalyst eosin Y with a noncatalytic amount of BF3·Et2O (50 mol %) upon additional photoirradiation (eq 3)[17] were needed. Because of these disadvantages, development of a one-pot synthesis of drug candidates via the application of 2,4,6-triarylpyridines becomes difficult. In addition, their limited substrate scopes, especially difficulties with amino groups, make it impossible to apply these reactions to the synthesis of G-quadruplex binding ligands. Therefore, the development of metal-free, catalytic methods for 2,4,6-triarylpyridines synthesis is strongly desired.

Scheme 1

Synthesis of 2,4,6-Trisubstituted Pyridines

We recently developed a metal-free method to oxidize n class="Chemical">benzylamines to imines using salicylic acid derivatives as organocatalysts, and successfully applied this efficient method to synthesize N-containing heterocycles and blue dyes.[18] Herein, we report an efficient protocol to synthesize 2,4,6-trisubstituted pyridines by the 4,6-dihydroxysalicylic acid-catalyzed oxidative condensation of benzylamines and acetophenones in the air, and the application of this method to the synthesis of G-quadruplex binding ligands in two steps (Scheme , eq 4). Note that most of the previously reported procedures to synthesize G-quadruplex binding ligands require three or four steps; all of these procedures start from the three-component reaction of aldehydes, 4-aminoacetophenones, and nitrogen donors to obtain 2,4,6-triarylpyridines, followed by stepwise reaction of the isolated 2,4,6-triarylpyridines with 4-chlorobutyryl chloride and pyrrolidine.[9] To our delight, our present work provides the first example for the synthesis of G-quadruplex binding ligands starting from benzylamines and 4-aminoacetophenones. The resulting mixtures containing 2,4,6-triarylpyridines bearing amino groups were then allowed to react directly with 4-chlorobutyryl chlorides without an intermediate purification step. After a simple reaction workup, the residues were next reacted with pyrrolidines to successfully afford G-quadruplex binding ligands. Therefore, this metal-free method for the synthesis of 2,4,6-trisubstituted pyridines and G-quadruplex binding ligands is facile, efficient, and time-saving.

Results and Discussion

To begin the study, 4,6-dihydroxysalicylic acid (2.5 mol %)[18] was employed as an organocatalyst, and 4-methoxyacetophenone (2a, 1.0 mmol) was chosen as the model substrate for condensation with n class="Chemical">benzylamine (1a, 0.5 mmol) under 0.1 MPa of O2 atmosphere (via an O2 balloon) at 100 °C for 24 h. A survey of different solvents (0.5 mL) suggested that dimethyl sulfoxide (DMSO) was the optimal solvent to afford 2,4,6-trisubstituted pyridine 4a in 45% yield (Table , entries 1–7). Increasing the amount of 1a up to 1.0 mmol resulted in the formation of 4a in 56% yield under an O2 atmosphere (entry 8). Removing the O2 balloon and conducting the reaction in air slightly improved the yield of 4a (entry 9). Therefore, we continued to optimize the reaction in air. A decrease in the amount of DMSO (0.1 mL) was favorable for the formation of 4a (entry 10). BF3·Et2O was found to be an efficient additive for the reaction, and the loading of BF3·Et2O could be decreased to 10 mol % (entries 11–17). In the presence of BF3·Et2O (5 mol %), 1.0 mmol of 2a could be coupled with 1.5 mmol of benzylamine to form 4a in 77% yield (entry 13). Further increasing the amount of BF3·Et2O to 10 mol % promoted the formation of 4a (79%, entry 14). A higher loading of 4,6-dihydroxysalicylic acid (5 mol %) gave 4a in 81% yield, whereas an even higher loading (10 mol %) was not needed (entries 15–16). Using 5 mol % of 4,6-dihydroxysalicylic acid and increasing the amount of BF3·Et2O (20 mol %) hindered the reaction (entry 17). A large excess of benzylamine (3.0 mmol) gave a similar yield of 4a (entry 18) to the reaction with less benzylamine (1.5 mmol, entry 15). The neat reaction was inefficient for the formation of 4a (entry 19). The optimal conditions for the reaction were 4-methoxyacetophenone (2a, 1.0 mmol) and benzylamine (1a, 1.5 mmol) in the presence of 4,6-dihydroxysalicylic acid (5 mol %) as the organocatalyst and BF3·Et2O (10 mol %) as the additive at 100 °C for a short reaction time (18 h) in the air to form 4a in 83% yield (entry 20). A shorter reaction time (12 h), lower temperature (80 °C), or higher temperature (120 °C) gave disappointing results (entries 21–23).

Table 1

Optimization of Reaction Conditions for Synthesis of 2,4,6-Trisubstituted Pyridinesa

entry	x	y	solvent (mL)	additive (mmol)	atmosphere	yieldb (%)
1	0.5	0.025	CH3OH (0.5)	none	O2 (0.1 MPa)	5c
2	0.5	0.025	CHCl3 (0.5)	none	O2 (0.1 MPa)	8c
3	0.5	0.025	CH3CN (0.5)	none	O2 (0.1 MPa)	14d
4	0.5	0.025	EtOAc (0.5)	none	O2 (0.1 MPa)	16d
5	0.5	0.025	toluene (0.5)	none	O2 (0.1 MPa)	27
6	0.5	0.025	DMF (0.5)	none	O2 (0.1 MPa)	14
7	0.5	0.025	DMSO (0.5)	none	O2 (0.1 MPa)	45
8	1.0	0.025	DMSO (0.5)	none	O2 (0.1 MPa)	56
9	1.0	0.025	DMSO (0.5)	none	air	60
10	1.0	0.025	DMSO (0.1)	none	air	64
11	1.0	0.025	DMSO (0.1)	BF3·Et2O (0.025)	air	69
12	1.0	0.025	DMSO (0.1)	BF3·Et2O (0.05)	air	74
13	1.5	0.025	DMSO (0.1)	BF3·Et2O (0.05)	air	77
14	1.5	0.025	DMSO (0.1)	BF3·Et2O (0.1)	air	79
15	1.5	0.05	DMSO (0.1)	BF3·Et2O (0.1)	air	81
16	1.5	0.1	DMSO (0.1)	BF3·Et2O (0.1)	air	81
17	1.5	0.05	DMSO (0.1)	BF3·Et2O (0.2)	air	71
18	3.0	0.05	DMSO (0.1)	BF3·Et2O (0.1)	air	82
19	1.5	0.05	neat	BF3·Et2O (0.1)	air	70
20	1.5	0.05	DMSO (0.1)	BF3·Et2O (0.1)	air	83 (81)e
21	1.5	0.05	DMSO (0.1)	BF3·Et2O (0.1)	air	70f
22	1.5	0.05	DMSO (0.1)	BF3·Et2O (0.1)	air	43d,e
23	1.5	0.05	DMSO (0.1)	BF3·Et2O (0.1)	air	67e,g

Conditions: 1a, 2a, 3, additive, and solvent were stirred at 100 °C for 24 h.

Determined by 1H NMR using an internal standard of 1,3,5-trioxane (isolated yield).

Reaction temperature was 60 °C.

Reaction temperature was 80 °C.

Reaction time was 18 h.

Reaction time was 12 h.

Reaction temperature was 120 °C.

With the optimized reaction conditions in hand (Table , entry 20), the scope of the substrates was next investigated. A series of benzylamines were examined to couple with 4-methoxyacetophenone (Table ). More specifically, when benzylamines with a methyl substituent at the ortho and para positions were employed as substrates, the corresponding 2,4,6-triarylpyridines 4b–4c were obtained in high yields. A methoxy substituent at the ortho, meta, and para positions of benzylamines underwent the condensation reaction with 2a to give the desired products 4d–4f. Benzylamines bearing other substituents, such as 4-tert-butyl, 3-chloro, and 4-chloro, were also examined under the standard reaction conditions, and 2,4,6-triarylpyridines 4g–4i were formed in good to high yields. A strong electron-withdrawing group, 4-trifluoromethyl, resulted in a somewhat sluggish reaction for the formation of the product 4j. In addition, several activated primary amines such as 1-naphthylmethylamine (1k), furfurylamine (1l), 2-pyridinemethylamine (1m), and 2-thiophenemethylamine (1n) were examined. 1-Naphthylmethylamine and 2-thiophenemethylamine could be successfully transformed into the corresponding 2,4,6-triarylpyridines 4k and 4n. Product 4k in particular was obtained in excellent yield (91%). The reactions of furfurylamine and 2-pyridinemethylamine did not proceed well, probably because of the low conversion of amines to imines (4l–4m). When 2-pyridinemethylamine was employed, the competitive α-alkylation reaction of ketones resulted in a 32% yield of 6. The gram-scale synthesis of 2,4,6-trisubstituted pyridines was also successful under the standard reaction conditions; 1.38 g (3.75 mmol, 75% isolated yield) of 4a was obtained from 10 mmol of 2a (1.5 g) after increasing the reaction time to 3 days.

Table 2

Synthesis of 2,4,6-Trisubstituted Pyridines Using Various Benzylaminesa

Yield of isolated product is based on 2a.

Gram scale: 1a (15 mmol), 2a (10 mmol), 3 (0.5 mmol), BF3·Et2O (1.5 mmol), and DMSO (1.0 mL) were stirred together for 3 days.

The substrate scope was further investigated using a variety of acetophenone derivatives to couple with n class="Chemical">benzylamine under the optimized reaction conditions (Table ). First, acetophenone derivatives bearing methyl substituents at the ortho, meta, and para positions were examined and produced the corresponding 2,4,6-triarylpyridines 5b–5d in moderate to good yields. It is believed that steric hindrance of the ortho-methyl group might be responsible for the reduced yield of 5b (compare 5b vs 5c and 5d). This situation also occurred in the reaction with ortho-methoxyacetophenone; 5e was formed in a low yield, whereas the outcomes of the meta- and para-methoxy substituents were better (compare 5e vs 5f and Table , 4a). The acetophenone derivative with a para-tert-butyl substituent gave the corresponding product 5g in good yield. In contrast, acetophenone derivatives bearing electron-withdrawing groups such as ortho-chloro, para-chloro, para-bromo, para-iodo, and para-trifluoromethyl gave moderate yields of the products 5h–5l. Activated ketones were also examined under the standard reaction conditions. The reaction of 2′-acetonaphthone afforded the corresponding 2,4,6-triarylpyridine 5m in good yield. When 2-acetylfuran was employed, pyrrole derivative 7, instead of 5n, was formed via the Clauson-Kaas reaction of 2-acetylfuran and benzylamine. 2-Acetylthiophene tolerated the condensation reaction to afford 5o in moderate yield, whereas the reaction of 2-acetylpyridine gave the product 5p in a low yield. An aliphatic ketone was also examined under the optimized reaction conditions and gave an acceptable outcome (5q).

Table 3

Synthesis of 2,4,6-Trisubstituted Pyridines Using Various Acetophenonesa

Yield of the isolated product is based on 2.

To gain insight into the reaction mechanism of this pyridine synthesis, several control experiments were conducted as shown in Scheme (eqs 5–10). As expected given our previous work,[18] n class="Chemical">benzylamine (1a) could be oxidized to imine 8 under the standard reaction conditions, and 10% of aldehyde 9 was also generated, most probably because of water from the air (Scheme , eq 5). Imine 8 was employed as substrate to couple with 2a and afforded 4a in 38% yield, whereas amine 10, which could also be generated from benzylamine, failed to give the pyridine product (eqs 6 and 7). The use of imine 8 in combination with benzylamine (1a, 0.5 mmol) as substrates for reaction with 2a provided 4a in 73% yield (eq 8). These results suggested that an imine might be the intermediate for this pyridine synthesis, and that benzylamines play the dual role of providing the aryl moiety at the 4-position of the 2,4,6-trisubstituted pyridines as well as acting as the nitrogen donor. When the reaction of benzylamine (1a, 1.0 mmol), ketone (2a, 1.0 mmol), and 4-tolualdehyde (11, 1.0 mmol) was conducted under the standard reaction conditions, 4c bearing a para-methyl-substituted pyridine was obtained in 49% yield, whereas only a trace of 4a was formed (eq 9). This result might indicate that an aldehyde is the key intermediate for this pyridine synthesis. Then, the reaction of 1a, 2a, and 11 was also performed in argon atmosphere, and as a result, 49% of imine 12 was formed as the main product (eq 10). Compounds 13a and 14, which were speculated as the intermediates for this reaction, were also detected. Moreover, 7% of 2,4,6-trisubstituted pyridine 4c was formed probably via the oxidation of 1,4-dihydropyridine 17 (vide infra) by a trace amount of oxygen in reagents. These results suggest that the nitrogen source for the formation of 2,4,6-trisubstituted pyridine is benzylamine.

Scheme 2

Control Experiments

On the basis of these control experiments, a mechanism for the synthesis of 2,4,6-trisubstituted pyridines is proposed, as shown in Scheme . In the presence of n class="Chemical">4,6-dihydroxysalicylic acid, benzylamine (1a) can be oxidized to imine 8.[18] Then, the hydrolysis of the imine may generate aldehyde 9 and 1a, which is a reversible reaction. Promoted by BF3·Et2O, the keto–enol tautomerism of ketone 2a gives enol 15, which subsequently undergoes a condensation reaction with aldehyde 9 to afford the intermediate 13b. Besides, the condensation of 2a and 1a forms the intermediate 14, which transforms to 16 in the presence of BF3·Et2O. A further addition reaction occurs between 16 and 13b, leading to the formation of 1,4-dihydropyridine 17, which undergoes the oxidative aromatization to form the pyridine product 4a.

Scheme 3

Possible Pathway for the Synthesis of 2,4,6-Trisubstituted Pyridines

2,4,6-Triarylpyridines are a class of G-quadruplex binding ligands that stabilize G4-DNA, which is very important for the treatment of n class="Disease">cancer.[9] Herein, we applied this metal-free synthetic method of pyridines to synthesize a series of G-quadruplex binding ligands by adding 4-chlorobutyryl chloride and pyrrolidine (Table ). First, benzylamine (1a) was employed as the substrate, and the oxidative condensation reaction of 1a with 4-aminoacetophenone (2r) was conducted. Then, 4-chlorobutyryl chloride was added directly into the reaction mixture and the mixture was stirred overnight (>16 h) at 60 °C. The resulting dark mixture was then basified with a saturated NaHCO3 solution to obtain a dark oil, which was easily separated from the mixture by pouring out the water. The dark oil was then reacted with pyrrolidine overnight (>16 h) at room temperature to successfully afford G-quadruplex binding ligand 18a in 65% yield. Thereafter, a series of benzylamine derivatives were examined to synthesize G-quadruplex binding ligands. Benzylamines with electron-donating groups such as methyl, methoxy, and tert-butyl at the para positions afforded the corresponding ligands 18b–18d in moderate yields. Benzylamines bearing electron-withdrawing groups such as para-chloro and para-trifluoromethyl also gave the G-quadruplex binding ligands 18e–18f in satisfactory yields. 1-Naphthylmethylamine was also examined, and the product 18g was successfully obtained. This method provides the first example for the synthesis of G-quadruplex binding ligands starting from benzylamines and 4-aminoactophenones. Compared with reported methods,[9] which all require three or four steps, this procedure is efficient and time-saving, only requiring two steps.

Table 4

Synthesis of G-Quadruplex Binding Ligands Using Various Benzylaminesa

Yield of the isolated product is based on 2r.

Conclusions

We have developed a metal-free and efficient method to synthesize n class="Chemical">2,4,6-trisubstituted pyridines from benzylamines and acetophenones in the presence of 4,6-dihydroxysalicylic acid and boron trifluoride-diethyl etherate in air. During this pyridine synthesis, benzylamines play a dual role of providing the aryl moiety at the 4-position of the pyridines and acting as the nitrogen donor. 4,6-Dihydroxysalicylic acid acts as an organocatalyst for the oxidation of benzylamines to imines, which then undergo a hydrolysis reaction to generate aldehydes. Facilitated by boron trifluoride-diethyl etherate, the condensation reaction of aldehydes with acetophenones and benzylamines occurs, and the adducts are readily oxidized to afford 2,4,6-trisubstituted pyridines. This facile method can be applied to the synthesis of G-quadruplex binding ligands by using 4-aminoacetophenone as the substrate and subsequently adding 4-chlorobutyryl chloride and pyrrolidine to obtain the corresponding G-quadruplex binding ligands in moderate yields. In the drug discovery process, the synthetic methods that require metal catalysts may cause significant problems because of the metal residues in the products. To avoid metal contamination of the final products, development of metal-free synthetic methods is strongly desired. Further investigations of metal-free oxidations are currently underway.

Experimental Section

General Remarks

Unless otherwise stated, all starting materials and additives were purchased from commercial sources and used without further purification. All solvents were distilled and degassed with nitrogen before use. n class="Chemical">1H NMR spectra were recorded on a JEOL JNM-ECS400 (400 MHz) FT NMR system or JEOL JNM-ECX400 (400 MHz) FT NMR system in CDCl3 with (CH3)4Si as an internal standard. 13C NMR spectra were recorded on a JEOL JNM-ECX400 (100 MHz) FT NMR or JEOL JNM-ECS400 (100 MHz) FT NMR in CDCl3. IR spectra are reported in wave numbers (cm–1). Electron ionization (EI) mass spectra were obtained by employing double focusing mass spectrometers.

General Procedure for the Synthesis of 2,4,6-Trisubstituted Pyridines (4)

The desired benzylamine derivative 1 (1.5 mmol), 4-methoxyacetophenone (2a, 150.18 mg, 1.0 mmol), 4,6-dihydroxysalicylic acid (3, 8.5 mg, 0.05 mmol), and DMSO (0.1 mL) were added to a round-bottom flask, and then BF3·Et2O (12.5 μL, 0.1 mmol) was added to the mixture. The reaction mixture was stirred at 100 °C for 18 h under air, then cooled to room temperature. The residue was purified by silica gel chromatography (eluent: hexane/ethyl acetate) to give 4.

2,6-Bis(4-methoxyphenyl)-4-phenylpyridine () [CAS: 50553-98-5].[17] Yellow solid, 149.2 mg, 81% (isolated yield), mp 131–132 °C (lit. mp 129–131 °C); 1H NMR (400 MHz, CDCl3): δ 8.16 (d, J = 8.7 Hz, 4H), 7.78 (s, 2H), 7.74 (d, J = 6.9 Hz, 2H), 7.44–7.55 (m, 3H), 7.04 (d, J = 8.7 Hz, 4H), 3.89 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.6, 157.1, 150.1, 139.5, 132.5, 129.2, 129.0, 128.5, 127.3, 115.9, 114.2, 55.5.

2,6-Bis(4-methoxyphenyl)-4-(o-tolyl)pyridine () yellow solid, 156.6 mg, 82% (isolated yield), mp 121–122 °C; 1H NMR (400 MHz, CDCl3): δ 8.13 (d, J = 8.7 Hz, 4H), 7.54 (s, 2H), 7.31–7.35 (m, 4H), 7.02 (d, J = 9.2 Hz, 4H), 3.88 (s, 6H), 2.35 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.6, 156.4, 151.3, 140.2, 135.3, 132.4, 130.8, 129.4, 128.5, 128.4, 126.2, 118.1, 114.2, 55.5, 20.5; IR (KBr, ν/cm–1): 2933, 2841, 1596, 1539, 1514, 1422, 1394, 1233, 1173, 1030, 834, 825, 773, 764; HRMS (EI) m/z: calcd for C26H23NO2 [M]+, 381.1729; found, 381.1725.

2,6-Bis(4-methoxyphenyl)-4-(p-tolyl)pyridine () [CAS: 681443-63-0].[19] Yellow solid, 154.8 mg, 81% (isolated yield), mp 133–134 °C (lit. mp 128–129 °C); 1H NMR (400 MHz, CDCl3): δ 8.16 (d, J = 8.7 Hz, 4H), 7.76 (s, 2H), 7.64 (d, J = 8.2 Hz, 2H), 7.33 (d, J = 8.2 Hz, 2H), 7.03 (d, J = 8.7 Hz, 4H), 3.89 (s, 6H), 2.44 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.6, 157.1, 150.0, 139.0, 136.5, 132.6, 129.9, 128.5, 127.1, 115.7, 114.2, 55.5, 21.4.

4-(2-Methoxyphenyl)-2,6-bis(4-methoxyphenyl)pyridine () white solid, 153.9 mg, 77% (isolated yield), mp 122–123 °C; 1H NMR (400 MHz, CDCl3): δ 8.13 (d, J = 8.7 Hz, 4H), 7.74 (s, 2H), 7.39–7.45 (m, 2H), 7.00–7.11 (m, 6H), 3.88 (s, 6H), 3.87 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.4, 156.7, 156.4, 147.8, 132.8, 130.6, 130.0, 128.9, 128.5, 121.2, 118.5, 114.1, 111.6, 55.8, 55.5; IR (KBr, ν/cm–1): 2948, 2845, 2835, 1607, 1576, 1544, 1545, 1494, 1456, 1422, 1394, 1280, 1242, 1173, 1028, 831, 759, 592; HRMS (EI) m/z: calcd for C26H23NO3 [M]+, 397.1678; found, 397.1673.

4-(3-Methoxyphenyl)-2,6-bis(4-methoxyphenyl)pyridine () white solid, 161.5 mg, 81% (isolated yield), mp 133–135 °C; 1H NMR (400 MHz, CDCl3): δ 8.16 (d, J = 8.6 Hz, 4H), 7.76 (s, 2H), 7.44 (t, J = 7.9 Hz, 1H), 7.32 (d, J = 7.7 Hz, 1H), 7.25–7.26 (m, 1H), 6.99–7.04 (m, 5H), 3.91 (s, 3H), 3.89 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.6, 160.3, 157.1, 150.0, 141.0, 132.5, 130.2, 128.5, 119.7, 115.9, 114.2, 113.1, 55.6, 55.5; IR (KBr, ν/cm–1): 2940, 2845, 1598, 1576, 1545, 1515, 1486, 1422, 1393, 1287, 1252, 1233, 1205, 1169, 1053, 1021, 871, 834, 785, 699; HRMS (EI) m/z: calcd for C26H23NO3 [M]+, 397.1678; found, 397.1678.

2,4,6-Tris(4-methoxyphenyl)pyridine () [CAS: 33567-23-6].[17] White solid, 159.6 mg, 80% (isolated yield), mp 131–133 °C (lit. mp 133–134 °C); 1H NMR (400 MHz, CDCl3): δ 8.15 (d, J = 8.7 Hz, 4H), 7.73 (s, 2H), 7.69 (d, J = 8.7 Hz, 2H), 7.01–7.06 (m, 6H), 3.88 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.6, 160.5, 157.1, 149.6, 132.6, 131.7, 128.5, 128.4, 115.4, 114.6, 114.2, 55.6, 55.5.

4-(4-(Tert-butyl)phenyl)-2,6-bis(4-methoxyphenyl)pyridine () yellow solid, 167.8 mg, 79% (isolated yield), mp 123–124 °C; 1H NMR (400 MHz, CDCl3): δ 8.15 (d, J = 8.7 Hz, 4H), 7.77 (s, 2H), 7.68 (d, J = 8.2 Hz, 2H), 7.54 (d, J = 8.2 Hz, 2H), 7.03 (d, J = 8.7 Hz, 4H), 3.89 (s, 6H), 1.39 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.6, 157.0, 152.3, 150.0, 136.6, 132.6, 128.5, 127.0, 126.2, 115.7, 114.2, 55.5, 34.9, 31.5; IR (KBr, ν/cm–1): 2970, 2348, 1607, 1539, 1514, 1461, 1427, 1307, 1240, 1174, 1034, 826, 592; HRMS (EI) m/z: calcd for C29H29NO2 [M]+, 423.2198; found, 423.2200.

4-(3-Chlorophenyl)-2,6-bis(4-methoxyphenyl)pyridine () [CAS: 2097002-94-1].[10d] Yellow solid, 151.2 mg, 75% (isolated yield), mp 120–121 °C (lit. mp 122–125 °C); 1H NMR (400 MHz, CDCl3): δ 8.15 (d, J = 8.6 Hz, 4H), 7.69–7.71 (m, 3H), 7.56–7.60 (m, 1H), 7.42–7.44 (m, 2H), 7.03 (d, J = 9.1 Hz, 4H), 3.88 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.7, 157.3, 148.7, 141.3, 135.2, 132.2, 130.4, 128.9, 128.5, 127.4, 125.5, 115.6, 114.2, 55.5.

4-(4-Chlorophenyl)-2,6-bis(4-methoxyphenyl)pyridine () [CAS: 857500-67-5].[19] Yellow solid, 160.6 mg, 80% (isolated yield), mp 123–124 °C (lit. mp 114–116 °C); 1H NMR (400 MHz, CDCl3): δ 8.14 (d, J = 8.7 Hz, 4H), 7.71 (s, 2H), 7.65 (d, J = 8.7 Hz, 2H), 7.48 (d, J = 8.2 Hz, 2H), 7.03 (d, J = 8.7 Hz, 4H), 3.88 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.7, 157.3, 148.9, 137.9, 135.1, 132.3, 129.4, 128.6, 128.5, 115.5, 114.2, 55.5.

2,6-Bis(4-methoxyphenyl)-4-(4-(trifluoromethyl)phenyl)pyridine () white solid, 129.7 mg, 59% (isolated yield), mp 123–124 °C; 1H NMR (400 MHz, CDCl3): δ 8.15 (d, J = 8.7 Hz, 4H), 7.81 (d, J = 8.2 Hz, 2H), 7.77 (d, J = 8.2 Hz, 2H), 7.73 (s, 2H), 7.03 (d, J = 8.7 Hz, 4H), 3.88 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.8, 157.3, 148.7, 143.1, 132.1, 130.9 (q, J = 32.5 Hz), 128.5, 127.7, 126.1 (q, J = 3.8 Hz), 124.2 (q, J = 272.8 Hz), 115.7, 114.2, 55.5; IR (KBr, ν/cm–1): 2934, 2829, 2363, 1607, 1545, 1514, 1429, 1391, 1328, 1246, 1175, 1113, 1071, 1020, 826, 579, 517; HRMS (EI) m/z: calcd for C26H20F3NO2 [M]+, 435.1446; found, 43.1445.

2,6-Bis(4-methoxyphenyl)-4-(naphthalen-1-yl)pyridine () yellow solid, 189.5 mg, 91% (isolated yield), mp 119–120 °C; 1H NMR (400 MHz, CDCl3): δ 8.16 (d, J = 8.7 Hz, 4H), 7.92–7.96 (m, 3H), 7.71 (s, 2H), 7.44–7.59 (m, 4H), 7.02 (d, J = 9.2 Hz, 4H), 3.87 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.7, 156.6, 150.2, 138.5, 133.9, 132.3, 131.2, 128.8, 128.6, 128.5, 126.8, 126.7, 126.3, 125.7, 125.5, 118.8, 114.2, 55.5; IR (KBr, ν/cm–1): 2940, 2837, 2370, 1607, 1542, 1513, 1394, 1241, 1173, 1029, 834, 802, 780, 578; HRMS (EI) m/z: calcd for C29H23NO2 [M]+, 417.1729; found, 417.1730.

4-(Furan-2-yl)-2,6-bis(4-methoxyphenyl)pyridine () light gray solid, 32.3 mg, 18% (isolated yield), mp 149–150 °C; 1H NMR (400 MHz, CDCl3): δ 8.14 (d, J = 8.6 Hz, 4H), 7.81 (s, 2H), 7.57 (d, J = 1.8 Hz, 1H), 7.02 (d, J = 8.6 Hz, 4H), 6.94 (d, J = 3.2 Hz, 1H), 6.56 (q, J = 1.7 Hz, 1H), 3.88 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.7, 157.1, 152.3, 143.6, 139.0, 132.4, 128.4, 114.1, 112.2, 111.7, 108.3, 55.5; IR (KBr, ν/cm–1): 2845, 1609, 1539, 1516, 1489, 1456, 1240, 1173, 1018, 832, 759, 579; HRMS (EI) m/z: calcd for C23H19NO3 [M]+, 357.1365; found, 357.1364.

2′,6′-Bis(4-methoxyphenyl)-2,4′-bipyridine () yellow solid, 40.8 mg, 22% (isolated yield), mp 132–133 °C; 1H NMR (400 MHz, CDCl3): δ 8.79 (d, J = 4.5 Hz, 1H), 8.21 (d, J = 9.1 Hz, 4H), 8.18 (s, 2H), 7.82–7.91 (m, 2H), 7.34–7.37 (m, 1H), 7.04 (d, J = 8.6 Hz, 4H), 3.89 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.7, 157.3, 155.8, 150.2, 148.1, 137.2, 132.4, 128.6, 123.8, 121.2, 115.1, 114.1, 55.5; IR (KBr, ν/cm–1): 2363, 1609, 1514, 1471, 1394, 1291, 1249, 1175, 1030, 829, 785; HRMS (EI) m/z: calcd for C24H20N2O2 [M]+, 368.1525; found, 368.1525.

2,6-Bis(4-methoxyphenyl)-4-(thiophen-2-yl)pyridine () [CAS: 170634-00-1]. Yellow solid, 143.1 mg, 77% (isolated yield), mp 172–173 °C; 1H NMR (400 MHz, CDCl3): δ 8.13 (d, J = 9.1 Hz, 4H), 7.75 (s, 2H), 7.59 (dd, J = 3.6, 0.9 Hz, 1H), 7.42 (dd, J = 5.2, 1.1 Hz, 1H), 7.16 (dd, J = 5.0, 3.6 Hz, 1H), 7.03 (d, J = 8.6 Hz, 4H), 3.88 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.7, 157.3, 142.9, 142.4, 132.3, 128.5, 128.5, 126.8, 125.2, 114.2, 114.1, 55.5; IR (KBr, ν/cm–1): 2837, 1600, 1545, 1511, 1423, 1405, 1239, 1172, 1035, 835, 822, 710, 580; HRMS (EI) m/z: calcd for C23H19NO2S [M]+, 373.1136; found, 373.1130.

1-(4-Methoxyphenyl)-3-(pyridin-2-yl)propan-1-one () [CAS: 343596-75-8].[14a] Yellow solid, 78.6 mg, 32% (isolated yield), mp 50–51 °C; 1H NMR (400 MHz, CDCl3): δ 8.52 (d, J = 4.5 Hz, 1H), 7.98 (d, J = 9.1 Hz, 2H), 7.59 (td, J = 7.7, 1.8 Hz, 1H), 7.25 (d, J = 7.7 Hz, 1H), 7.10 (dd, J = 7.2, 5.0 Hz, 1H), 6.92 (d, J = 8.6 Hz, 2H), 3.86 (s, 3H), 3.45 (t, J = 7.2 Hz, 2H), 3.22 (t, J = 7.5 Hz, 2H); 13C{1H} NMR (100 MHz, CDCl3): δ 198.0, 163.6, 161.1, 149.4, 136.5, 130.5, 130.1, 123.5, 121.3, 113.8, 55.6, 37.7, 32.4.

General Procedure for the Synthesis of 2,4,6-Trisubstituted Pyridines (5)

Benzylamine (1a, 160.74 mg, 1.5 mmol), the desired acetophenone derivative 2 (1.0 mmol), 4,6-dihydroxysalicylic acid (3, 8.5 mg, 0.05 mmol), and DMSO (0.1 mL) were added to a round-bottom flask, and then BF3·Et2O (12.5 μL, 0.1 mmol) was added to the mixture. The reaction mixture was stirred at 100 °C for 18 h under air, then cooled to room temperature. The residue was purified by silica gel chromatography (eluent: hexane/ethyl acetate) to give 5.

2,4,6-Triphenylpyridine () [CAS: 580-35-8].[17] white solid, 114.3 mg, 74% (isolated yield), mp 137–138 °C (lit. mp 136–137 °C); 1H NMR (400 MHz, CDCl3): δ 8.21 (d, J = 8.2 Hz, 4H), 7.90 (s, 2H), 7.76 (d, J = 7.7 Hz, 2H), 7.43–7.56 (m, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 157.7, 150.3, 139.7, 139.2, 129.3, 129.2, 129.1, 128.9, 127.3, 127.3, 117.3.

4-Phenyl-2,6-di-o-tolylpyridine () [CAS: 816446-55-6].[10f] White solid, 103.5 mg, 62% (isolated yield), mp 134–135 °C (lit. mp 129–130 °C); 1H NMR (400 MHz, CDCl3): δ 7.72 (d, J = 6.8 Hz, 2H), 7.59 (s, 2H), 7.42–7.51 (m, 5H), 7.26–7.30 (m, 6H), 2.48 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.3, 148.9, 140.9, 138.6, 136.1, 130.8, 130.0, 129.3, 129.2, 128.4, 127.3, 126.0, 120.3, 20.8.

4-Phenyl-2,6-di-m-tolylpyridine () [CAS: 2040475-81-6].[10f] White solid, 125.9 mg, 75% (isolated yield), mp 157–158 °C (lit. mp 132–135 °C); 1H NMR (400 MHz, CDCl3): δ 8.01 (s, 2H), 7.97 (d, J = 8.2 Hz, 2H), 7.86 (s, 2H), 7.75 (d, J = 6.8 Hz, 2H), 7.45–7.55 (m, 3H), 7.40 (t, J = 7.5 Hz, 2H), 7.24–7.27 (m, 2H), 2.48 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 157.9, 150.2, 139.8, 139.3, 138.5, 129.93, 129.2, 129.1, 128.8, 128.0, 127.3, 124.5, 117.3, 21.8.

4-Phenyl-2,6-di-p-tolylpyridine () [CAS: 16112-41-7].[10f] White solid, 130.4 mg, 78% (isolated yield), mp 160–161 °C (lit. mp 158–159 °C); 1H NMR (400 MHz, CDCl3): δ 8.10 (d, J = 8.2 Hz, 4H), 7.83 (s, 2H), 7.73 (d, J = 7.2 Hz, 2H), 7.44–7.53 (m, 3H), 7.31 (d, J = 8.2 Hz, 4H), 2.42 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 157.5, 150.2, 139.4, 139.1, 137.0, 129.5, 129.2, 129.00, 127.3, 127.1, 116.7, 21.5.

2,6-Bis(2-methoxyphenyl)-4-phenylpyridine () [CAS: 1428423-32-8].[10f] Yellow solid, 91.6 mg, 50% (isolated yield), mp 154–155 °C (lit. mp 153–154 °C); 1H NMR (400 MHz, CDCl3): δ 7.99 (s, 2H), 7.95 (dd, J = 7.7, 1.8 Hz, 2H), 7.72 (d, J = 7.2 Hz, 2H), 7.49 (t, J = 7.5 Hz, 2H), 7.41–7.44 (m, 1H), 7.37 (td, J = 7.8, 1.4 Hz, 2H), 7.10 (t, J = 7.5 Hz, 2H), 7.02 (d, J = 8.6 Hz, 2H), 3.88 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 157.2, 156.1, 147.8, 139.6, 131.7, 129.9, 129.8, 129.1, 128.7, 127.5, 121.6, 121.2, 111.6, 55.9.

2,6-Bis(3-methoxyphenyl)-4-phenylpyridine () [CAS: 1333120-04-9].[10f] Yellow solid, 127.2 mg, 69% (isolated yield), mp 65–67 °C (lit. mp 76–78 °C); 1H NMR (400 MHz, CDCl3): δ 7.87 (s, 2H), 7.80 (s, 2H), 7.74 (t, J = 6.6 Hz, 4H), 7.40–7.54 (m, 5H), 7.00 (dd, J = 7.9, 2.0 Hz, 2H), 3.92 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 160.2, 157.3, 150.3, 141.2, 139.1, 129.8, 129.3, 129.1, 127.3, 119.7, 117.6, 114.9, 112.8, 55.5.

2,6-Bis(4-(tert-butyl)phenyl)-4-phenylpyridine () [CAS: 2070893-06-8].[20] White solid, 157.0 mg, 75% (isolated yield), mp 155–157 °C (lit. mp 158–160 °C); 1H NMR (400 MHz, DMSO-d6): δ 8.12 (d, J = 8.2 Hz, 4H), 7.83 (s, 2H), 7.72 (d, J = 7.7 Hz, 2H), 7.42–7.53 (m, 7H), 1.38 (s, 18H); 13C{1H} NMR (100 MHz, DMSO-d6): δ 157.6, 152.2, 150.0, 139.4, 137.1, 129.2, 129.0, 127.3, 127.0, 125.8, 116.7, 34.8, 31.5.

2,6-Bis(2-chlorophenyl)-4-phenylpyridine () [CAS: 2056081-58-2].[12a] Yellow solid, 115.0 mg, 61% (isolated yield), mp 148–149 °C (lit. mp 147–149 °C); 1H NMR (400 MHz, CDCl3): δ 7.88 (s, 2H), 7.73–7.76 (m, 4H), 7.44–7.54 (m, 5H), 7.33–7.41 (m, 4H); 13C{1H} NMR (100 MHz, CDCl3): δ 157.3, 148.5, 139.4, 138.4, 132.4, 132.0, 130.3, 129.8, 129.3, 127.4, 127.2, 121.7.

2,6-Bis(4-chlorophenyl)-4-phenylpyridine () [CAS: 72666-43-4].[12a] Light yellow solid, 121.3 mg, 64% (isolated yield), mp 190–192 °C (lit. mp 186–187 °C); 1H NMR (400 MHz, CDCl3): δ 8.14 (d, J = 8.6 Hz, 4H), 7.87 (s, 2H), 7.72–7.74 (m, 2H), 7.48–7.57 (m, 7H); 13C{1H} NMR (100 MHz, CDCl3): δ 156.5, 150.7, 138.8, 137.9, 135.4, 129.3, 129.1, 128.5, 127.3, 117.3.

2,6-Bis(4-bromophenyl)-4-phenylpyridine () [CAS: 74918-94-8].[12a] Light yellow solid, 165.6 mg, 71% (isolated yield), mp 198–199 °C (lit. mp 195–197 °C); 1H NMR (400 MHz, CDCl3): δ 8.07 (d, J = 8.6 Hz, 4H), 7.87 (s, 2H), 7.72–7.74 (m, 2H), 7.65 (d, J = 8.6 Hz, 4H), 7.49–7.56 (m, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 156.6, 150.8, 138.8, 138.4, 132.0, 129.4, 128.8, 127.3, 123.8, 117.3.

2,6-Bis(4-iodophenyl)-4-phenylpyridine () [CAS: 2103202-38-4]. Yellow solid, 141.1 mg, 50% (isolated yield), mp 176–177 °C; 1H NMR (400 MHz, CDCl3): δ 7.92 (d, J = 8.6 Hz, 4H), 7.83–7.86 (m, 6H), 7.72 (d, J = 6.8 Hz, 2H), 7.47–7.55 (m, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 156.7, 150.7, 138.9, 138.8, 138.0, 129.3, 128.9, 127.3, 117.3, 95.7; IR (KBr, ν/cm–1): 2348, 1601, 1558, 1544, 1506, 1411, 1004, 816, 762, 692; HRMS (EI) m/z: calcd for C23H15I2N [M]+, 558.9294; found, 558.9293.

4-Phenyl-2,6-bis(4-(trifluoromethyl)phenyl)pyridine () [CAS: 2056081-57-1].[12a] Light yellow solid, 110.0 mg, 50% (isolated yield), mp 152–153 °C (lit. mp 152–154 °C); 1H NMR (400 MHz, CDCl3): δ 8.30 (d, J = 8.2 Hz, 4H), 7.97 (s, 2H), 7.74–7.79 (m, 6H), 7.49–7.58 (m, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 156.4, 151.1, 142.7, 138.5, 131.3 (q, J = 32.5 Hz), 129.6, 129.4, 127.6, 127.3, 125.9 (q, J = 3.8 Hz), 124.3 (q, J = 271.8 Hz), 118.4.

2,6-Di(naphthalen-2-yl)-4-phenylpyridine () [CAS: 3557-63-9].[14a] Yellow solid, 157.5 mg, 77% (isolated yield), mp 156–157 °C (lit. mp 156–157 °C); 1H NMR (400 MHz, CDCl3): δ 8.66 (d, J = 0.9 Hz, 2H), 8.39 (dd, J = 8.6, 1.8 Hz, 2H), 8.01 (s, 2H), 7.98 (dd, J = 8.8, 3.4 Hz, 4H), 7.88 (t, J = 4.8 Hz, 2H), 7.77–7.79 (m, 2H), 7.46–7.56 (m, 7H); 13C{1H} NMR (100 MHz, CDCl3): δ 157.6, 150.4, 139.2, 137.1, 133.9, 133.7, 129.3, 129.2, 128.9, 128.5, 127.8, 127.4, 126.6, 126.4, 125.1, 117.6.

4-Phenyl-2,6-di(thiophen-2-yl)pyridine () [CAS: 5562-59-4].[14a] Yellow solid, 93.0 mg, 58% (isolated yield), mp 113–114 °C (lit. mp 108–110 °C); 1H NMR (400 MHz, CDCl3): δ 7.68–7.71 (m, 6H), 7.47–7.54 (m, 3H), 7.42 (d, J = 4.1 Hz, 2H), 7.13–7.15 (m, 2H); 13C{1H} NMR (100 MHz, CDCl3): δ 152.7, 150.3, 145.0, 138.7, 129.2, 128.1, 127.9, 127.2, 125.0, 115.2.

4′-Phenyl-2,2’:6′,2″-terpyridine () [CAS: 58345-97-4].[21] Brown solid, 44.5 mg, 29% (isolated yield), mp 203–204 °C (lit. mp 206–208 °C); 1H NMR (400 MHz, CDCl3): δ 8.72–8.74 (m, 4H), 8.67 (d, J = 8.2 Hz, 2H), 7.85–7.91 (m, 4H), 7.43–7.53 (m, 3H), 7.33–7.36 (m, 2H); 13C{1H} NMR (100 MHz, CDCl3): δ 156.4, 156.1, 150.5, 149.3, 138.6, 137.0, 129.1, 129.0, 127.5, 123.9, 121.5, 119.1.

2,6-Diphenethyl-4-phenylpyridine () [CAS: 1428423-37-3].[14a] Yellow solid, 49.0 mg, 27% (isolated yield), mp 120–121 °C; 1H NMR (400 MHz, CDCl3): δ 7.49 (d, J = 7.7 Hz, 2H), 7.37–7.45 (m, 3H), 7.17–7.30 (m, 10H), 7.09 (s, 2H), 3.08–3.19 (m, 8H); 13C{1H} NMR (100 MHz, CDCl3): δ 161.3, 149.1, 141.8, 138.9, 129.1, 128.9, 128.7, 128.5, 127.2, 126.1, 118.8, 40.3, 36.4.

1-(1-Benzyl-1H-pyrrol-2-yl)ethan-1-one () [CAS: 100713-02-8].[22] Brown oil, 51.9 mg, 26% (isolated yield); 1H NMR (400 MHz, CDCl3): δ 7.21–7.31 (m, 3H), 7.10 (d, J = 6.8 Hz, 2H), 7.00 (q, J = 2.0 Hz, 1H), 6.90 (t, J = 2.0 Hz, 1H), 6.18 (q, J = 2.1 Hz, 1H), 5.58 (s, 2H), 2.41 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 188.5, 138.4, 130.5, 130.5, 128.7, 127.5, 127.2, 120.5, 108.6, 52.7, 27.4; MS (EI) [M]+m/z: 199.

General Procedure for the Synthesis of G-Quadruplex Binding Ligands (18)

The desired benzylamine derivative 1 (1.5 mmol), 4-aminoacetophenone (2r, 135.17 mg, 1.0 mmol), 4,6-dihydroxysalicylic acid (3, 8.5 mg, 0.05 mmol), and DMSO (0.1 mL) were added to a 50 mL round-bottom flask, and then BF3·Et2O (12.5 μL, 0.1 mmol) was added to the mixture. The reaction mixture was stirred at 100 °C for 18 h under air, then cooled to 60 °C. 4-Chlorobutyryl chloride (2.0 mL) was added and the resulting mixture was stirred overnight (>16 h) at 60 °C. The dark reaction mixture was cooled in an ice bath and basified to pH > 7 with a saturated NaHCO3 solution (35 mL). A dark oil appeared and was separated from the mixture by pouring out the water. The dark oil was collected and dissolved in methanol (20 mL), concentrated by vacuum, and then pyrrolidine (1.0 mL) was added to the residue. The resulting dark mixture was stirred overnight (>16 h) at room temperature and cooled in an ice bath. Saturated NaHCO3 solution (35 mL) was then added, and the mixture was stirred for 1 h. A gray solid appeared and was collected by filtration. The filter cake was dissolved in methanol/chloroform (30 mL, 1:4) and concentrated by vacuum. The residue was purified by preparative thin-layer chromatography (eluent: chloroform/methanol/triethylamine = 15/4/1) to give 18.

N,N’-((4-Phenylpyridine-2,6-diyl)bis(4,1-phenylene))bis(4-(pyrrolidin-1-yl)butanamide) () [CAS: 1023617-87-9].[9d] Dark yellow solid, 199.1 mg, 65% (isolated yield), mp 123–125 °C; 1H NMR (400 MHz, CDCl3): δ 9.95 (br, 2H), 8.16 (d, J = 8.7 Hz, 4H), 7.81 (s, 2H), 7.73 (d, J = 7.8 Hz, 2H), 7.68 (d, J = 8.2 Hz, 4H), 7.44–7.54 (m, 3H), 2.67–2.68 (m, 12H), 2.57 (t, J = 6.4 Hz, 4H), 1.90–1.96 (m, 12H); 13C{1H} NMR (100 MHz, CDCl3): δ 171.8, 156.8, 150.1, 139.8, 139.1, 134.9, 129.1, 129.0, 127.7, 127.2, 119.7, 116.2, 55.8, 54.0, 36.7, 23.9, 23.6.

N,N’-((4-(p-Tolyl)pyridine-2,6-diyl)bis(4,1-phenylene))bis(4-(pyrrolidin-1-yl)butanamide) () dark yellow solid, 188.0 mg, 60% (isolated yield), mp 102–104 °C; 1H NMR (400 MHz, CDCl3): δ 9.93 (br, 2H), 8.14 (d, J = 8.7 Hz, 4H), 7.78 (s, 2H), 7.66 (d, J = 8.2 Hz, 4H), 7.62 (d, J = 7.8 Hz, 2H), 7.31 (d, J = 7.8 Hz, 2H), 2.64–2.65 (m, 12H), 2.55 (t, J = 6.4 Hz, 4H), 2.43 (s, 3H), 1.87–1.96 (m, 12H); 13C{1H} NMR (100 MHz, CDCl3): δ 171.9, 156.7, 149.9, 139.7, 139.0, 136.0, 134.9, 129.8, 127.6, 126.9, 119.7, 115.9, 55.8, 54.0, 36.7, 24.0, 23.6, 21.3; IR (KBr, ν/cm–1): 2956, 2800, 1661, 1600, 1516, 1423, 1387, 1246, 1181, 811; HRMS (EI) m/z: calcd for C40H47N5O2 [M]+, 629.3730; found, 629.3729.

N,N’-((4-(4-Methoxyphenyl)pyridine-2,6-diyl)bis(4,1-phenylene))bis(4-(pyrrolidin-1-yl)butanamide) () dark yellow solid, 199.3 mg, 62% (isolated yield), mp 99–101 °C; 1H NMR (400 MHz, CDCl3): δ 9.94 (br, 2H), 8.14 (d, J = 8.2 Hz, 4H), 7.76 (s, 2H), 7.67 (t, J = 8.0 Hz, 6H), 7.03 (d, J = 8.2 Hz, 2H), 3.88 (s, 3H), 2.53–2.64 (m, 16H), 1.87–1.94 (m, 12H); 13C{1H} NMR (100 MHz, CDCl3): δ 172.0, 160.4, 156.6, 149.4, 139.6, 134.9, 131.2, 128.2, 127.6, 119.7, 115.6, 114.5, 55.8, 55.4, 54.0, 36.7, 24.1, 23.6; IR (KBr, ν/cm–1): 2933, 2791, 1668, 1602, 1511, 1429, 1390, 1253, 1178, 1117, 1028, 828; HRMS (EI) m/z: calcd for C40H47N5O3 [M]+, 645.3679; found, 645.3674.

N,N’-((4-(4-(Tert-butyl)phenyl)pyridine-2,6-diyl)bis(4,1-phenylene))bis(4-(pyrrolidin-1-yl)butanamide) () dark yellow solid, 184.0 mg, 55% (isolated yield), mp 113–115 °C; 1H NMR (400 MHz, CDCl3): δ 9.94 (br, 2H), 8.16 (d, J = 8.7 Hz, 4H), 7.81 (s, 2H), 7.67 (t, J = 8.9 Hz, 6H), 7.55 (d, J = 8.2 Hz, 2H), 2.64–2.67 (m, 12H), 2.56 (t, J = 6.4 Hz, 4H), 1.89–1.95 (m, 12H), 1.39 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 171.9, 156.8, 152.3, 150.0, 139.7, 136.2, 135.0, 127.7, 126.9, 126.1, 119.7, 116.2, 55.9, 54.1, 37.1, 34.8, 31.4, 24.0, 23.7; IR (KBr, ν/cm–1): 2955, 1684, 1602, 1516, 1423, 1387, 1253, 1180, 1113, 830; HRMS (EI) m/z: calcd for C43H53N5O2 [M]+, 671.4199; found, 671.4194.

N,N’-((4-(4-Chlorophenyl)pyridine-2,6-diyl)bis(4,1-phenylene))bis(4-(pyrrolidin-1-yl)butanamide) () dark yellow solid, 188.3 mg, 58% (isolated yield), mp 94–96 °C; 1H NMR (400 MHz, CDCl3): δ 9.98 (br, 2H), 8.15 (d, J = 8.7 Hz, 4H), 7.76 (s, 2H), 7.67 (d, J = 7.3 Hz, 6H), 7.49 (d, J = 8.2 Hz, 2H), 2.66–2.67 (m, 12H), 2.57 (t, J = 6.4 Hz, 3H), 1.90–1.94 (m, 12H); 13C{1H} NMR (100 MHz, CDCl3): δ 171.9, 156.9, 148.8, 139.9, 137.5, 135.1, 134.7, 129.3, 128.5, 127.7, 119.7, 115.9, 55.9, 54.1, 36.8, 24.0, 23.7; IR (KBr, ν/cm–1): 2934, 2791, 1601, 1516, 1423, 1386, 1252, 1179, 1093, 1014, 823; HRMS (EI) m/z: calcd for C39H44ClN5O2 [M]+, 649.3184; found, 649.3188.

N,N’-((4-(4-(Trifluoromethyl)phenyl)pyridine-2,6-diyl)bis(4,1-phenylene))bis(4-(pyrrolidin-1-yl)butanamide) () dark yellow solid, 187.3 mg, 55% (isolated yield), mp 105–107 °C; 1H NMR (400 MHz, CDCl3): δ 10.00 (br, 2H), 8.16 (d, J = 8.7 Hz, 4H), 7.77–7.84 (m, 6H), 7.68 (d, J = 8.2 Hz, 4H), 2.55–2.68 (m, 16H), 1.90–1.96 (m, 12H); 13C{1H} NMR (100 MHz, CDCl3): δ 172.0, 157.1, 148.7, 142.7, 140.0, 134.5, 130.9 (q, J = 32.7 Hz), 127.7, 127.6, 126.1 (q, J = 3.8 Hz), 124.1 (q, J = 272.1 Hz), 119.8, 116.1, 55.9, 54.1, 36.9, 24.0, 23.7; IR (KBr, ν/cm–1): 2956, 2800, 2356, 1653, 1601, 1516, 1387, 1324, 1165, 1123, 1067, 1016, 835; HRMS (EI) m/z: calcd for C40H44F3N5O2 [M]+, 683.3447; found, 683.3445.

N,N’-((4-(Naphthalen-1-yl)pyridine-2,6-diyl)bis(4,1-phenylene))bis(4-(pyrrolidin-1-yl)butanamide) () brown solid, 173.3 mg, 52% (isolated yield), mp 115–117 °C; 1H NMR (400 MHz, CDCl3): δ 9.95 (br, 2H), 8.17 (d, J = 8.2 Hz, 4H), 7.94 (d, J = 8.2 Hz, 3H), 7.76 (s, 2H), 7.66 (d, J = 8.7 Hz, 4H), 7.46–7.60 (m, 4H), 2.54–2.65 (m, 16H), 1.88–1.95 (m, 12H); 13C{1H} NMR (100 MHz, CDCl3): δ 171.8, 156.4, 150.2, 139.8, 138.2, 134.8, 133.9, 131.1, 128.8, 128.6, 127.8, 126.8, 126.7, 126.2, 125.5, 119.7, 119.4, 55.9, 54.1, 37.0, 24.0, 23.7; IR (KBr, ν/cm–1): 2963, 2807, 1652, 1600, 1516, 1418, 1247, 1179, 842, 777; HRMS (EI) m/z: calcd for C43H47N5O2 [M]+, 665.3730; found, 665.3731.

General Procedure for the Gram-Scale Synthesis of 2,6-Bis(4-methoxyphenyl)-4-phenylpyridine (4a)

Benzylamine (1a, 1.6 g, 15 mmol), 4-methoxyacetophenone (2a, 1.5 g, 10 mmol), 4,6-dihydroxysalicylic acid (3, 85 mg, 0.5 mmol), and DMSO (1.0 mL) were added to a round-bottom flask, and then BF3·Et2O (187.5 μL, 1.5 mmol) was added to the mixture. The reaction mixture was stirred at 100 °C for 3 days under air, then cooled to room temperature. The residue was purified by silica gel chromatography (eluent: hexane/ethyl acetate) to give 4a (1.38 g).