Dual Role of Glyoxal in Metal-Free Dicarbonylation Reaction: Synthesis of Symmetrical and Unsymmetrical Dicarbonyl Imidazoheterocycles.

A practical and efficient method has been developed for the dicarbonylation of imidazoheterocycles using glyoxals as dicarbonyl precursors under metal-free conditions in acetic acid. A series of symmetrical and unsymmetrical dicarbonyl imidazoheterocycles was synthesized in good yields. Aryl and alkyl glyoxals also demonstrated excellent reactivity under similar reaction conditions and delivered corresponding dicarbonyl imidazoheterocycles in high yields. It is believed that the glyoxal plays a dual role both as a dicarbonyl source and as an oxidant in this transformation. A probable mechanistic pathway has been proposed based on control experiments and electrospray ionization high-resolution mass spectrometry analysis.

Introduction

Dicarbonyl compounds are valuable structural motifs present in a variety of natural products and bioactive compounds that possess a broad spectrum of pharmacological activities.[1] For example, 1,2-bis-carbonyl compounds have been demonstrated to be a potent, selective inhibitor of the human carboxylesterases enzyme which is responsible for the detoxification of xenobiotics.[1a,1e,2] Dicarbonyl compounds are also used as powerful building blocks for the synthesis of N-heterocyclic compounds such as imidazoles,[3] quinoxalines,[4] and triazines[5] as well as ligands[6] such as N-heterocyclic carbenes and diamines. 1,2-Dicarbonyls also possess exceptionally useful synthetic potential as pronucleophiles[7] and can be exploited in cascade or sequential reactions for the synthesis of important hetero or carbocycles. Thus, the development of newer synthetic methods for the dicarbonylation has always been a fascinating topic in organic chemistry.[8] In recent years, much attention has been paid towards oxidative coupling reaction under metal-free conditions, which overcome the drawbacks associated with metal catalyzed reactions. For example, C-3 dicarbonylation of indoles have been achieved using arylglyoxal via the aminocatalytic approach,[9] acetophenones in I2/dimethyl sulfoxide (DMSO)/pyrrolidine,[10] aryl acetaldehydes in I2/DMSO employing an assembled two-step continuous flow system.[11] Recently, Pan and coworkers established a regioselective N-1 and C-2 dicarbonylation of 3-substituted indoles with arylglyoxal hydrates.[12] N-1 indolyl diketones were obtained in the presence of copper acetate, while C-2 indolyl diketones were obtained under metal-free reaction conditions.

On the other hand, the imidazo[1,2-a]pyridine motif is a key structural unit found in numerous biologically active molecules, particularly in many commercially available drugs such as alpidem, zolpidem, necopidem, zolmidine, and saripidem.[13] In the last decade, substantial efforts have been devoted for the synthesis and functionalization of the imidazoheterocycles.[14] A few successful and elegant methods have been developed for the synthesis of unsymmetrical dicarbonylated imidazo[1,2-a]pyridines in the recent past using aryl acetaldehydes,[15] aryl ketones,[16] N,N-disubstituted acetamide, or acetone,[17] tertiary α-amino phenylethanone,[18] and arylglyoxals[19] as the dicarbonyl source (Scheme ). All these methods require the use of metal catalysts (Cu and Fe), ligands, and an equimolar amount of external halogenated oxidants and bases. Although some investigation towards metal-free dicarbonylation of indoles[9−12] has been conducted, metal-free dicarbonylation of imidazoheterocycles are rare.[20] In most of the developed protocols for dicarbonylation, the emphasis has been on the synthesis of unsymmetrical dicarbonylated compounds and synthesis of symmetrical dicarbonylated imidazoheterocycles has remained unexplored. During the preparation of our manuscript, Guo and coworkers reported a catalyst-free method for the synthesis of dicarbonylation of imidazoheterocycles with glyoxal hydrates.[21] In this method also only arylglyoxals were utilized to synthesize unsymmetrical dicarbonylated imidazoheterocycles. Thus, the development of metal-free dicarbonylation to obtain biologically active symmetrical and unsymmetrical dicarbonyl imidazoheterocyles under mild reaction conditions is still challenging and highly desirable.

Scheme 1

Synthesis of Symmetrical and Unsymmetrical Dicarbonyl Imidazoheterocyles

In continuation of our sustained efforts towards synthesis and functionalization of imidazoheterocycles,[22] herein, we report a metal-free approach for the dicarbonylation of imidazoheterocycles using glyoxals as dicarbonyl precursors in acetic acid. It is believed that the glyoxal plays a dual role both as a dicarbonyl source and as an oxidant in this transformation. A probable mechanistic pathway has been proposed based on control experiments and electrospray ionization high-resolution mass spectrometry (ESI-HRMS) analysis.

Results and Discussion

We started our studies by choosing 2-phenylimidazo[1,2-a]pyridine (1a) and glyoxal (40 wt % aq solution; 2) as the model substrates. The initial reaction was performed with 1a (0.51 mmol) and 2 (2.55 mmol) using AcOH (5 mL—160 equiv) as the solvent at 100 °C. To our delight, the respective 1,2-bis(2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3a) was formed in a 69% yield in 8 h (Table , entry 1). Encouraged from this result under catalyst-free conditions, we decided to optimize the reaction conditions for the model reaction by varying different reaction parameters such as the amount of acetic acid in the reaction mixture, amount of glyoxal, temperature, reaction medium, and so forth. The results for the optimization reactions are shown in Table . The results indicated that the best yield of 3a was obtained using acetic acid (3 mL), glyoxal (118 μL) at 100 °C (Table , entry 2). It was observed that on decreasing the concentration of glyoxal, the yield of 3a decreased from 70 to 38% (Table , entries 5–7). Similarly, the yield of 3a decreased to 47% on decreasing the reaction temperature to 50 °C and the desired product was not present at 25 °C (Table , entry 8–10). The use of PivOH instead of AcOH resulted in the formation of 3a in 52% yield (Table , entry 11). The use of aqueous medium such as water/AcOH, water/TsOH/sodium lauryl sulphate (SLS), and water/SLS resulted in a diminished yield of 3a (Table , entry 12–14). The product 3a was not formed in the absence of acetic acid when the reaction was performed in organic solvents such as N,N-dimethylformamide (DMF), xylene, and DMSO under standard reaction conditions (Table , entry 15–17).

Table 1

Optimization of the Reaction Conditionsa

entry	reaction medium (y mL)	amount of 2 (x μL)	temp (°C)	yield (%)b
1	CH3COOH (5 mL)	118	100	69
2	CH3COOH(3 mL)	118	100	70
3	CH3COOH (1.5 mL)	118	100	58
4	CH3COOH (0.75 mL)	118	100	51
5	CH3COOH (3 mL)	94	100	68
6	CH3COOH (3 mL)	70	100	45
7	CH3COOH (3 mL)	47	100	38
8	CH3COOH (3 mL)	118	75	60
9	CH3COOH (3 mL)	118	50	47
10	CH3COOH (3 mL)	118	25	NR
11	PivOH (3 mL)	118	100	52
12	CH3COOH (300 μL) + H2O (2 mL)	118	100	30
13	H2O (2 mL) + SLS (50 mol %) + TsOH (10 mol %)	118	100	20
14	H2O (2 mL) + SLS (50 mol %)	118	100	10
15	DMF (3 mL)	118	100	NR
16	xylene (3 mL)	118	100	NR
17	DMSO (3 mL)	118	100	NR

Reaction conditions: 1a (0.100 g; 0.51 mmol), 2 (x μL, 40 wt % aq solution), reaction medium (y mL), and reflux for 8 h.

Isolated yields. Abbreviations used in table: PivOH = pivalic acid; SLS = sodium lauryl sulphate; TsOH = p-toluene sulfonic acid; DMF = N,N-dimethylformamide. DMSO = dimethyl sulfoxide.

After having optimized reaction conditions in hand (Table , entry 2), the utility of the present approach was systematically investigated by reacting different imidazo[1,2-a]pyridines (1a–r) with glyoxal (2) (Table ). Imidazo[1,2-a]pyridines having electron donating groups (EDGs) viz. p-Me, p-OMe on the C-2 phenyl ring (1b and 1c) reacted smoothly with 2 and afforded respective symmetrical dicarbonylated products 3b and 3c in 73 and 75% yields, respectively. Similarly, imidazo[1,2-a]pyridines with electron withdrawing groups (EWGs) such as p-F, p-Cl, p-Br, and p-CF3 on the C-2 phenyl ring also reacted smoothly and gave the corresponding dimeric products 3d–g in good yields (60–67%). Although, the substituent on the C-2 phenyl ring did not affect the reaction outcome significantly, relatively better yields were observed in the case of imidazo[1,2-a]pyridines with EDGs on the C-2 phenyl ring. Imidazo[1,2-a]pyridine with C-2 thiophene-yl (1h) also yielded the corresponding product 3h in 77% yield.

Table 2

Synthesis of Symmetrical Dicarbonyl Imidazoheterocyclesa,b

Reaction conditions: 1 (0.51 mmol), 2 (2.55 mmol; 40 wt % aq solution), and AcOH (3 mL) at 100 °C for 8 h, under air.

Isolated yields.

In the case of 2-(tert-butyl)imidazo[1,2-a]pyridine (1i), the respective dimeric product (3i) was not obtained under these conditions which may be attributed to the steric factor of the tert-butyl group. The reaction of 2-phenylimidazo[1,2-a]pyridine with different substituents on the pyridine ring (1j–l) with 2 afforded the desired dimeric product in 3j–l in 61–70% yield. Interestingly, the reaction of 6-methyl-2-(4-nitrophenyl)imidazo[1,2-a]pyridine (1m) with 2 also gave the desired product 3m in 58% yield. Finally, 2-arylimidazo[1,2-a]pyrimidines (1n–p) and 6-arylimidazo[2,1-b]thiazoles (1q–r) were allowed to react with 2 under standard reaction conditions. To our delight, the corresponding dimeric products (3n–p and 3q–r) were obtained in good yields (51–69%). These experiments demonstrated that the scope of the reaction was general for differently substituted imidazoheterocycles.

With the success for the synthesis of symmetrical dicarbonyl imidazoheterocyles under metal-free conditions, we next attempted the synthesis of unsymmetrical dicarbonyl imidazoheterocyles using equimolar ratios of two different imidazo[1,2-a]pyridines with glyoxal (2) under optimized reaction conditions. The cross-coupling reaction of various substituents present on the C-2 aryl ring (p-OMe, p-Cl and p-NO2) and pyridine ring (6-Br and 7-Me) of imidazo[1,2-a]pyridine were amenable to the reaction conditions and afforded the respective unsymmetrical dicarbonyl products (3s, 3t, and 3u) in good yields (34–45%) as compared to the corresponding symmetrical products (Scheme ).

Scheme 2

Synthesis of Unsymmetrical Dicarbonylated Imidazo[1,2-a]pyridine (3s–3u)

In order to check the versatility of the present approach, we tested different arylglyoxals as the carbonyl source with imidazoheterocycles under similar reaction conditions to access unsymmetrical dicarbonyl imidazoheterocycles (Table ). The reaction of 1a with phenylglyoxal (4a) led to the formation of the desired product that is 1-phenyl-2-(2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (5aa) in 82% yield. Next, the reaction of imidazo[1,2-a]pyridines having EDG viz. p-Me and EWGs such as p-Cl, p-Br, p-CF3, and m-NO2 on the C-2 phenyl ring were reacted with phenylglyoxal (4a). The desired products that is 1-phenyl-2-(2-arylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-diones (5ba–5ta) were obtained in good to high yields (70–90%). Similarly, imidazo[1,2-a]pyridines with substituents on the pyridine ring as well as on the C-2 phenyl ring reacted smoothly with 4a to afford the corresponding dicarbonyl compounds (5ja, 5ma, 5ua, and 5va) in good to excellent yields (Table ). 2-(4-Methoxyphenyl)imidazo[1,2-a]pyrimidine (1o) and 6-(4-methylphenyl)imidazo[2,1-b]thiazole (1r) also reacted with 4a to give the corresponding dicarbonyl products 5oa and 5ra in 80 and 77% yields, respectively. Finally, the substrate scope in terms of arylglyoxals was elaborated by reacting 1a with arylglyoxals (4b–f) bearing different functional groups on the aryl ring. The desired 1-aryl-2-(2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-diones (5ab–af) were obtained in good to excellent yields (77–92%). No significant electronic effect of substituents was observed on the yield of dicarbonyl products. Heterocyclic glyoxal that is 2-oxo-2-(thiophen-2-yl)acetaldehyde (4g) also reacted efficiently with 1a and afforded the desired product 5ag in 93% yield. To our delight, the reaction of 2-cyclohexyl-2-oxoacetaldehyde (4h) with 1a under similar reaction conditions afforded the desired product 5ah in 91% yield. However, the reaction of benzaldehyde with 1a under identical reaction conditions failed to deliver the desired phenyl(2-phenylimidazo[1,2-a]pyridin-3-yl)methanone (5ai) product.

Table 3

Synthesis of Unsymmetrical Dicarbonyl Imidazoheterocyclesa,b

Reaction conditions: 1 (0.51 mmol), 2 (0.76 mmol), and AcOH (3 mL), 100 °C, 3 h, under air.

Isolated yields.

It is worth mentioning here that the reaction of 1-methyl-2-phenyl-1H-indole (6a) and 1-methyl-1H-indole (6b) with glyoxal (2) under standard reaction conditions did not give the corresponding symmetrical dicarbonyl indole derivatives and instead resulted in the formation of the corresponding 2,2-bis(indol-3-yl)acetaldehydes (7a–b) in 63 and 65% yields, respectively (Scheme ).

Scheme 3

Reaction of Indoles with Glyoxal

To check the practical utility of the present approach, the gram-scale reaction was performed using 1a (1.0 g; 5.1 mmol) with 2 (25.5 mmol, 40 wt % aq solution) and 4a (1.03 g, 7.6 mmol) under the standard reaction conditions. To our delight, the desired products 3a and 5aa were formed in 62 and 72% yields, respectively (Scheme ).

Scheme 4

Gram Scale Synthesis of 3a and 5aa

Some control experiments were carried out to gain some understanding of the reaction mechanism of this metal-free dicarbonylation (Scheme ). Initially, radical trapping experiments were performed by using different radical scavengers [such as 2,2,6,6-tetramethylpiperidine-1-oxyl (3 equiv), 1,1-diphenylethylene (3 equiv), and 2,6-di-tertbutyl-4-methylphenol (3 equiv)] to shed light on the kind of mechanism involved for this transformation (Scheme , eq i). In all cases, no significant reduction in the yield of 3a was observed, thus the possibility of a radical-mediated reaction was ruled out. Next, the reaction of 1a with 2 was performed under an inert atmosphere using identical reaction conditions to justify the role of air as an oxidant for this reaction (Scheme , eq ii). However, to our surprise, we isolated the desired product 3a in 62% yield along with the isolation of another compound 8 in 18% yield (Scheme , eq iii). We established the molecular structure of 8 as 2-oxo-2-(2-phenylimidazo[1,2-a]pyridin-3-yl)ethyl acetate. When 8 was allowed to react with 1a under standard reaction conditions without using glyoxal, 3a was formed in less than 10% after 8 h. This indicated that 8 is not the intermediate of this transformation but it is a side product. The formation of traces of 3a from 8 may be due to the aerobic oxidation of de-acetylated intermediate 12 (produced from 8) followed by nucleophilic addition of 1a. We also checked the reaction of 1a with 4a under an inert atmosphere (Scheme , eq iv). The desired product 5aa was obtained in 65% yield indicating that air is not a prominent oxidant in this transformation. Key intermediates were traced by performing ESI-HRMS analysis of the model reaction (Scheme , eq v). Based on formation of 8 and other related side products in the reaction mixture as well as significant yield of dicarbonyl products under a nitrogen atmosphere made us believe that glyoxal also acts as the oxidant in this transformation. The benzoin derivative obtained by nucleophilic addition of imidazoheterocycle to glyoxal is oxidized to the dicarbonyl derivative (benzil) by glyoxal through hydrogen transfer. Arylglyoxals has been reported to oxidize thiols to the corresponding disulfides,[23] however, to the best of our knowledge this is the first finding where glyoxal has been found to oxidize benzoins. At this stage, we cannot rule out the possibility of simultaneous aerobic oxidation of the benzoins to the corresponding dicarbonyl derivatives.

Scheme 5

Control Experiments

On the basis of preliminary mechanistic studies and relevant literature reports,[12,21,23] a plausible mechanism is proposed for the formation of 3a (Scheme ). We believe initially acetic acid mediated nucleophilic addition of 1a to 2 leads to the formation of α-hydroxy aldehyde (benzoin) 9 through intermediate A. Formation of 9 was confirmed by ESI-HRMS analysis of the reaction mixture (Scheme , eq v). Intermediate 9 then underwent glyoxal mediated oxidation or aerial oxidation to the corresponding keto aldehyde intermediate 13. The formation of product 3a under a nitrogen atmosphere and decline yield of 3a with reducing the amount of glyoxal indicates the involvement of glyoxal as the oxidant in this reaction. This hypothesis was further supported by the formation of intermediate 11 observed in the ESI-HRMS analysis of the reaction mixture (Scheme , eq v). The intermediate 13 underwent nucleophilic addition with another molecule of 1a followed by deprotonation to produce the intermediate 10 which upon oxidation provided the desired product 3a.

Scheme 6

Probable Mechanistic Pathway for Dicarbonylation Reaction

Conclusions

In conclusion, a metal-free method has been developed for the synthesis of symmetrical and unsymmetrical dicarbonyl imidazoheterocycles using glyoxals as the carbonyl source. The present method demonstrated practical and green reaction conditions to afford unexplored symmetrical dicarbonyl imidazoheterocycles in good yields. More importantly, unsymmetrical dicarbonyl imidazoheterocycles were also obtained in high yields as compared to the previously reported methodologies under metal-free conditions. The method is amenable for a scale-up reaction which has been demonstrated through the gram scale reaction. Mechanistic investigation revealed that glyoxal has a dual role in this transformation both as a dicarbonyl source as well as an oxidant.

Experimental Section

General Information

Melting points were determined in open capillary tubes on an EZ-Melt automated melting point apparatus and are uncorrected. All the compounds were unambiguously characterized by 1H, 13C, and IR and further confirmed through ESI-HRMS analysis. The reactions were monitored by using thin layer chromatography (TLC) on 0.2 mm silica gel F254 plates (Merck). 1H and 13C NMR spectra were recorded on a Bruker Avance 400 and 100 MHz instrument, respectively in CDCl3 as a solvent using tetramethylsilane (TMS) as an internal standard. Peak multiplicities of 1H NMR signals were designated as s (singlet), br s (broad singlet), d (doublet), dd (doublet of doublet), t (triplet), q (quartet), m (multiplet), and so forth. Chemical shifts (δ) and coupling constants (J) are reported in parts per million (ppm) relative to the residual signal of TMS in deuterated solvents and hertz, respectively. ESI-HRMS were recorded using Agilent Technologies 6545 Q-TOF LC/MS. IR spectra were recorded using a Fourier transform infrared (FT-IR) spectrophotometer, and the values are reported in cm–1. Column chromatography was performed over silica gel (60–120 and 100–200 mesh) using EtOAc–n-hexane as an eluent. All chemicals were obtained from the commercial suppliers and used without further purification. All the imidazoheterocycles were prepared by our reported methods.

General Experimental Procedure for the Synthesis of Symmetrical Dicarbonyl Imidazoheterocycles (3a–3r)

A clean oven dried 25 mL round bottom flask was charged with 1 (0.51 mmol) and 2 (2.55 mmol; 40 wt % aq solution) in acetic acid (3 mL). The reaction mixture was stirred at 100 °C for 8 h. The reaction progress was monitored by TLC. After completion, the reaction mass was allowed to cool at ambient temperature. Acetic acid was neutralized by saturated solution of NaHCO3 (30 mL) and extracted with EtOAc (3 × 15 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated under reduced pressure. The residue was purified in silica gel (60–120 mesh) column chromatography using EtOAc–n-hexane as the eluent to afford the corresponding products (3a–3r) in moderate to good yields (51–77%).

1,2-Bis(2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3a)

A yellow solid was obtained; yield: 79 mg, 70%; mp: 241 °C; FT-IR (neat): 3047, 1790, 1728, 1612, 1388 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.25 (d, J = 6.8 Hz, 2H), 7.60–7.49 (m, 4H), 7.21 (d, J = 7.6 Hz, 4H), 6.98–6.94 (m, 6H), 6.77 (t, J = 7.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 182.3, 157.0, 147.5, 132.7, 130.6, 129.0, 128.3, 128.2, 127.4, 120.7, 117.0, 115.0; ESI-HRMS: calcd for C28H19N4O2+ [M + H]+, 443.1503; found, 443.1523.

1,2-Bis(2-(p-tolyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3b)

An orange solid was obtained; yield: 87.5 mg, 73%; mp: >300 °C; FT-IR (neat): 3040, 1736, 1604, 1465, 1381 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.25 (d, J = 6.8 Hz, 2H), 7.61 (d, J = 9.2 Hz, 2H), 7.49 (t, J = 8.0 Hz, 2H), 7.08 (d, J = 7.6 Hz, 4H), 6.98 (td, J = 7.2, 1.6 Hz, 2H), 6.77 (d, J = 7.6 Hz, 4H), 1.98 (s, 6H); 13C NMR (100 MHz, CDCl3): δ 182.4, 156.8, 147.4, 138.7, 130.3, 130.2, 129.0, 128.3, 128.0, 120.8, 117.0, 114.2, 21.2; ESI-HRMS: calcd for C30H23N4O2+ [M + H]+, 471.1816; found, 471.1802.

1,2-Bis(2-(4-methoxyphenyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3c)

A pale yellow solid was obtained; yield: 96 mg, 75%; mp: 260 °C; FT-IR (neat): 3102, 1735, 1605, 1535, 1467 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.27 (dt, J = 6.8, 2.0 Hz, 2H), 7.60 (dt, J = 8.8, 1.2 Hz, 2H), 7.52–7.48 (m, 2H), 7.12 (d, J = 8.4 Hz, 4H), 6.95 (td, J = 6.8, 1.2 Hz, 2H), 6.47 (d, J = 8.8 Hz, 4H), 3.58 (s, 6H); 13C NMR (100 MHz, CDCl3): δ 182.5, 159.7, 156.6, 147.5, 130.3, 129.4, 128.9, 125.4, 120.8, 116.8, 114.3, 112.9, 54.8; ESI-HRMS: calcd for C30H23N4O4+ [M + H]+, 503.1714; found, 503.1694.

1,2-Bis(2-(4-fluorophenyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3d)

A pale yellow solid was obtained; yield: 79 mg, 65%; mp: 299 °C; FT-IR (neat): 3140, 1735, 1612, 1527 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.26 (dt, J = 6.8, 1.2 Hz, 2H), 7.64 (d, J = 8.8 Hz, 2H), 7.60–7.56 (m, 2H), 7.18–7.15 (m, 4H), 7.06 (td, J = 6.8, 1.6 Hz, 2H), 6.66 (t, J = 8.4 Hz, 4H); 13C NMR (100 MHz, CDCl3): δ 182.1, 162.7 (d, JC–F = 250.3 Hz), 155.7, 147.4, 131.1, 129.9 (d, JC–F = 8.5 Hz), 128.9 (d, JC–F = 3.2 Hz), 128.8, 120.7, 117.1, 115.2, 114.7 (d, JC–F = 21.8 Hz); ESI-HRMS: calcd for C28H17F2N4O2+ [M + H]+, 479.1314; found, 479.1287.

1,2-Bis(2-(4-chlorophenyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3e)

A dark brown solid was obtained; yield: 82 mg, 63%; mp: 326 °C; FT-IR (neat): 3062, 2924, 1789, 1728, 1604 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.25 (d, J = 6.8 Hz, 2H), 7.67–7.58 (m, 4H), 7.13–7.11 (m, 6H), 6.95 (d, J = 7.6 Hz, 4H); 13C NMR (100 MHz, CDCl3): δ 182.0, 155.5, 147.4, 135.2, 131.4, 131.3, 129.2, 128.6, 127.9, 120.8, 117.1, 115.5; ESI-HRMS: calcd for C28H17Cl2N4O2+ [M + H]+, 511.0723; found, 511.0716.

1,2-Bis(2-(4-bromophenyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3f)

A yellow solid was obtained; yield: 92 mg, 60%; mp: >300 °C; FT-IR (neat): 2916, 1728, 1612, 1396 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.27–9.22 (d, J = 6.8, 2H), 7.67–7.58 (m, 4H), 7.16 (td, J = 6.7, 1.8 Hz, 2H), 7.12–7.02 (m, 8H); 13C NMR (100 MHz, CDCl3): δ 182.0, 155.5, 147.3, 131.7, 131.68, 130.8, 129.4, 128.6, 123.6, 120.7, 117.1, 115.7; ESI-HRMS: calcd for C28H17Br2N4O2+ [M + H]+, 598.9713; found, 598.9696.

1,2-Bis(2-(4-(trifluoromethyl)phenyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3g)

A bright yellow solid was obtained; yield: 99 mg, 67%; mp: 319–320 °C; FT-IR (neat): 3048, 1728, 1627, 1319 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.23 (d, J = 6.8 Hz, 2H), 7.64–7.55 (m, 4H), 7.37–7.31 (m, 4H), 7.26 (d, J = 8.0 Hz, 4H), 7.05 (td, J = 6.4, 1.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 181.8, 155.3, 147.3, 136.0, 131.5, 130.8 (q, JC–F = 33 Hz), 128.5, 128.3, 124.5 (q, JC–F = 3.3 Hz), 123.4 (q, JC–F = 271 Hz), 120.7, 117.1, 116.0; ESI-HRMS: calcd for C30H17F6N4O2+ [M + H]+, 579.1250; found, 579.1251.

1,2-Bis(2-(thiophen-2-yl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3h)

An orange solid was obtained; yield: 89 mg, 77%; mp: 255 °C; FT-IR (neat): 3071, 1713, 1605, 1551 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.48 (d, J = 7.2 Hz, 2H), 7.65 (d, J = 8.8 Hz, 2H), 7.59–7.54 (m, 2H), 7.08 (td, J = 6.8, 1.2 Hz, 2H), 7.01 (dd, J = 3.6, 0.8 Hz, 2H), 6.83 (dd, J = 5.4, 1.2 Hz, 2H), 6.57 (dd, J = 5.0, 3.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 182.2, 150.0, 147.8, 134.3, 130.9, 129.4, 129.0, 127.4, 126.4, 120.4, 117.2, 115.2; ESI-HRMS: calcd for C24H15N4O2S2+ [M + H]+, 455.0631; found, 455.0619.

1,2-Bis(7-methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3j)

A pale yellow solid was obtained; yield: 73 mg, 61%; mp: 281–283 °C; FT-IR (neat): 3055, 1643, 1604, 1589 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.10 (d, J = 7.2 Hz, 2H), 7.33 (s, 2H), 7.21 (d, J = 7.2 Hz, 4H), 6.96 (t, J = 7.6 Hz, 4H), 6.80–6.75 (m, 4H), 2.49 (s, 6H); 13C NMR (100 MHz, CDCl3): δ 182.0, 157.2, 147.9, 142.4, 132.9, 128.2, 127.3, 120.5, 117.1, 115.7, 21.7. (ESI): calcd for C30H23N4O2+ [M + H]+, 471.1816; found, 471.1815; ESI-HRMS: calcd for C30H23N4O2+ [M + H]+, 471.1816; found, 471.1804.

1,2-Bis(6-bromo-2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3k)

A pale yellow solid was obtained; yield: 95 mg, 62%; mp: 311–312 °C; FT-IR (neat): 3055, 1735, 1612, 1481 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.27–9.26 (m, 2H), 7.59 (dd, J = 9.6, 2.0 Hz, 2H), 7.47 (d, J = 9.2 Hz, 2H), 7.17–7.15 (m, 4H), 7.03 (t, J = 6.8 Hz, 4H), 6.97–6.93 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 182.0, 156.7, 145.8, 133.8, 132.2, 128.9, 128.5, 128.3, 127.6, 120.86, 117.5, 109.7; ESI-HRMS: calcd for C28H17Br2N4O2+ [M + H]+, 598.9713; found, 598.9692.

1,2-Bis(6-iodo-2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3l)

A yellow solid was obtained; yield: 124 mg, 70%; mp: 307 °C; FT-IR (neat): 3125, 3055, 1728, 1612, 1466 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.347–9.345 (m, 2H), 7.70 (dd, J = 9.2, 2.0 Hz, 2H), 7.36 (d, J = 9.6 Hz, 2H), 7.16–7.14 (m, 4H), 7.03 (t, J = 7.3 Hz, 4H), 6.99–6.95 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 182.0, 156.4, 145.9, 138.5, 133.6, 132.2, 128.5, 128.3, 127.6, 120.6, 117.9, 78.2; ESI-HRMS: calcd for C28H17I2N4O2+ [M + H]+, 694.9435; found, 694.9412.

1,2-Bis(6-methyl-2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3m)

A yellow solid was obtained; yield: 83 mg, 58%; mp: 336–337 °C; FT-IR (neat): 3094, 1736, 1613, 1512 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.04 (s, 2H), 7.87 (d, J = 8.8 Hz, 4H), 7.54 (d, J = 9.2 Hz, 2H), 7.39–7.36 (m, 6H), 2.42 (s, 6H); 13C NMR (100 MHz, CDCl3): δ 181.4, 154.0, 146.8, 146.2, 138.6, 134.7, 128.8, 127.0, 126.5, 122.5, 121.0, 116.8, 18.2; ESI-HRMS: calcd for C30H21N6O6+ [M + H]+, 561.1517; found, 561.1496.

1,2-Bis(2-phenylimidazo[1,2-a]pyrimidin-3-yl)ethane-1,2-dione (3n)

A yellow solid was obtained; yield: 70 mg, 62%; mp: 287–288 °C; FT-IR (neat): 3117, 1767, 1605, 1373 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.41 (dd, J = 6.4, 2.0 Hz, 2H), 8.76 (dd, J = 4.4, 2.0 Hz, 2H), 7.29–7.27 (m, 4H), 7.05–7.01 (m, 6H), 6.87 (t, J = 7.2 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 182.2, 158.3, 154.5, 149.9, 136.3, 132.1, 129.0, 128.7, 127.6, 119.0, 110.9; ESI-HRMS: calcd for C28H17N6O2+ [M + H]+, 445.1408; found, 445.1404.

1,2-Bis(2-(4-methoxyphenyl)imidazo[1,2-a]pyrimidin-3-yl)ethane-1,2-dione (3o)

A brown solid was obtained; yield: 89 mg, 69%; mp: 271–273 °C; FT-IR (neat): 2986, 1767, 1605, 1528 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.45 (dd, J = 6.8, 2.0 Hz, 2H), 8.74 (dd, J = 4.0, 2.0 Hz, 2H), 7.20 (d, J = 8.4 Hz, 4H), 7.02 (dd, J = 6.8, 4.4 Hz, 2H), 6.53 (d, J = 8.7 Hz, 4H), 3.64 (s, 6H); 13C NMR (100 MHz, CDCl3): δ 182.4, 160.3, 157.9, 154.1, 150.0, 136.1, 130.0, 124.7, 119.0, 113.2, 110.5, 55.0; ESI-HRMS: calcd for C28H21N6O4+ [M + H]+, 505.1619; found, 505.1599.

1,2-Bis(2-(4-bromophenyl)imidazo[1,2-a]pyrimidin-3-yl)ethane-1,2-dione (3p)

A brownish yellow solid was obtained; yield: 78 mg, 51%; mp: 353–354 °C; FT-IR (neat): 3110, 1755, 1612, 1480 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.42 (dd, J = 6.8, 2.0 Hz, 2H), 8.84 (dd, J = 4.3, 2.1 Hz, 2H), 7.23 (dd, J = 6.8, 4.4 Hz, 2H), 7.18–7.11 (m, 8H); 13C NMR (100 MHz, CDCl3): δ 182.0, 156.7, 155.5, 149.8, 135.7, 131.0, 129.9, 124.4, 118.9, 111.7; ESI-HRMS: calcd for C26H15Br2N6O2+ [M + H]+, 600.9618; found, 600.9595.

1,2-Bis(6-phenylimidazo[2,1-b]thiazol-5-yl)ethane-1,2-dione (3q)

A pale brown solid was obtained; yield: 74 mg, 64%; mp: 263–265 °C; FT-IR (neat): 3117, 1728, 1612, 1466 cm–1; 1H NMR (400 MHz, CDCl3): δ 8.03 (d, J = 4.4 Hz, 2H), 7.24–7.22 (m, 4H), 7.12–7.02 (m, 4H), 7.03–6.98 (m, 2H), 6.89 (d, J = 4.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 180.9, 157.3, 155.3, 132.6, 128.5, 128.3, 127.5, 123.4, 121.8, 113.9; ESI-HRMS: calcd for C24H15N4O2S2+ [M + H]+, 455.0631; found, 455.0620.

1,2-Bis(6-(p-tolyl)imidazo[2,1-b]thiazol-5-yl)ethane-1,2-dione (3r)

A dark grey solid was obtained; yield: 80 mg, 65%; mp: 243 °C; FT-IR (neat): 3093, 1735, 1605, 1465 cm–1; 1H NMR (400 MHz, CDCl3): δ 8.05 (d, J = 4.4 Hz, 2H), 7.11 (d, J = 8.0 Hz, 4H), 6.93–6.90 (m, 6H), 2.16 (s, 6H); 13C NMR (100 MHz, CDCl3): δ 181.0, 157.3, 155.3, 138.9, 130.0, 128.3, 128.0, 123.4, 121.9, 113.2, 21.3; ESI-HRMS: calcd for C28H19N4O2S2+ [M + H]+, 483.0944; found, 483.0924.

General Experimental Procedure for the Synthesis of Unsymmetrical Dicarbonyl Imidazoheterocycles with Glyoxal (3s–3u)

A clean oven dried 25 mL round bottom flask was charged with equimolar ratios of two different imidazoheterocycles (0.51 mmol) and 2 (2.55 mmol; 40 wt % aq solution) in acetic acid (3 mL). The reaction mixture was stirred at 100 °C for 8 h. The reaction progress was monitored by TLC. After completion, the reaction mass was allowed to cool at ambient temperature. Acetic acid was neutralized by saturated solution of NaHCO3 (30 mL) and extracted with EtOAc (3 × 15 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated under reduced pressure. The residue was purified in silica gel (100–200 mesh) column chromatography using EtOAc–n-hexane as the eluent to afford the corresponding products (3s–3u) in 34–45% yields.

1-(2-(4-Chlorophenyl)imidazo[1,2-a]pyridin-3-yl)-2-(2-(4-methoxyphenyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3s)

A yellow solid was obtained; yield: 116 mg, 45%; mp: 293–295 °C; FT-IR (neat): 3125, 1736, 1605, 1535 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.26 (t, J = 7.2 Hz, 2H), 7.63–7.51 (m, 4H), 7.13–7.06 (m, 5H), 7.00–6.93 (m, 3H), 6.48 (d, J = 8.4 Hz, 2H), 3.59 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 182.5, 182.0, 159.9, 156.9, 155.1, 147.5, 147.4, 134.9, 131.3, 131.3, 130.5, 129.5, 129.2, 128.9, 128.6, 127.8, 125.3, 120.9, 120.7, 117.0, 116.9, 115.1, 114.6, 113.0, 54.9; ESI-HRMS: calcd for C29H20ClN4O3+ [M + H]+, 507.1218; found, 507.1207.

1-(6-Bromo-2-phenylimidazo[1,2-a]pyridin-3-yl)-2-(7-methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3t)

A white solid was obtained; yield: 92.6 mg, 34%; mp: 301–303 °C; FT-IR (neat): 3143, 3055, 1620, 1608, 1485 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.33 (d, J = 0.8 Hz, 1H), 9.07 (d, J = 7.2 Hz, 1H), 7.58 (dd, J = 9.2, 2.0 Hz, 1H), 7.47 (d, J = 9.2 Hz, 1H), 7.35 (s, 1H), 7.19 (td, J = 8.4, 1.3 Hz, 4H), 7.03–6.97 (m, 4H), 6.93–6.89 (m, 1H), 6.83–6.79 (m, 2H), 2.51 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 182.9, 181.2, 157.3, 156.7, 147.9, 145.7, 142.7, 133.6, 132.9, 132.3, 129.0, 128.6, 128.2, 128.2, 128.1, 128.0, 127.5, 127.4, 121.0, 120.5, 117.4, 117.2, 115.8, 109.5, 21.7. ESI-HRMS: calcd for C29H20BrN4O2+ [M + H]+, 535.0764; found, 535.0763.

1-(7-Methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)-2-(2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (3u)

A white solid was obtained; yield: 112.4 mg, 44%; mp: 232–233 °C, FT-IR (neat): 2924, 2850, 1616, 1597, 1512, 1485 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.24 (d, J = 6.8 Hz, 1H), 9.06 (d, J = 7.2 Hz, 1H), 7.83 (d, J = 8.4 Hz, 2H), 7.64–7.54 (m, 2H), 7.36–7.34 (m, 3H), 7.22–7.20 (m, 2H), 7.04–6.95 (m, 3H), 6.82–6.78 (m, 2H), 2.45 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 182.0, 181.4, 157.7, 153.7, 147.9, 147.3, 146.9, 143.9, 138.8, 132.4, 131.0, 129.0, 128.8, 128.7, 128.2, 127.8, 127.5, 127.0, 122.5, 121.1, 120.5, 117.6, 117.2, 116.1, 115.4, 21.5. ESI-HRMS: calcd for C29H20N5O4+ [M + H]+, 502.1510; found, 502.1503.

General Experimental Procedure for the Synthesis of Unsymmetrical Dicarbonyl Imidazoheterocycles with Arylglyoxals (5aa–5ah)

A clean oven dried 25 mL round bottom flask was charged with 1 (0.51 mmol) and arylglyoxal 4 (0.76 mmol) in acetic acid (3 mL). The reaction mixture was stirred at 100 °C for 3 h. The reaction progress was monitored by TLC. After completion, the reaction mass was allowed to cool at ambient temperature. Acetic acid was neutralized by saturated solution of NaHCO3 (30 mL) and extracted with EtOAc (3 × 15 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated under reduced pressure. The residue was purified in silica gel (60–120 mesh) column chromatography using EtOAc–n-hexane as the eluent to afford the corresponding products (5aa–5ah) in moderate to good yields (70–93%).

1-Phenyl-2-(2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (5aa)

A pale yellow solid was obtained; yield: 136 mg, 82%; mp: 121–123 °C (lit.[15] mp 122–124 °C); FT-IR (neat): 2986, 1767, 1682, 1589, 1250 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.86 (d, J = 6.8 Hz, 1H), 7.87 (d, J = 9.2 Hz, 1H), 7.75–7.67 (m, 3H), 7.58 (d, J = 6.8 Hz, 1H), 7.41 (t, J = 7.6 Hz, 2H), 7.32–7.26 (m, 4H), 7.11 (t, J = 7.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 191.4, 184.6, 158.7, 148.4, 134.1, 133.4, 132.9, 130.9, 130.0, 129.6, 129.4, 129.3, 128.6, 127.9, 117.7, 115.8; ESI-HRMS: calcd for C21H15N2O2+ [M + H]+, 327.1128; found, 327.1129.

1-Phenyl-2-(2-(p-tolyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (5ba)

A pale yellow solid was obtained; yield: 147 mg, 85%; mp: 110 °C (lit.[15] mp 109–110 °C); FT-IR (neat): 3024, 1681, 1589, 1489 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.85 (dt, J = 7.2, 0.8 Hz, 1H), 7.85 (dt, J = 9.2, 1.2 Hz, 1H), 7.75 (dd, J = 8.4, 1.6 Hz, 2H), 7.68–7.64 (m, 1H), 7.60–7.56 (m, 1H), 7.41 (t, J = 8.0 Hz, 2H), 7.25–7.19 (m, 3H), 6.91 (d, J = 7.6 Hz, 2H), 2.29 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 191.4, 184.6, 158.9, 148.4, 139.5, 134.0, 133.5, 130.9, 130.0, 129.9, 129.6, 129.3, 128.6, 128.4, 118.9, 117.6, 115.6, 21.3; ESI-HRMS: calcd for C22H17N2O2+ [M + H]+, 341.1285; found, 341.1274.

1-(2-(4-Chlorophenyl)imidazo[1,2-a]pyridin-3-yl)-2-phenylethane-1,2-dione (5ea)

A pale yellow solid was obtained; yield: 129 mg, 70%; mp: 124–126 °C (lit.[21] mp 123–125 °C); FT-IR (neat): 3017, 1767, 1681, 1597, 1404 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.85 (d, J = 6.8 Hz, 1H), 7.87 (d, J = 8.8 Hz, 1H), 7.75–7.68 (m, 3H), 7.62 (t, J = 7.6 Hz, 1H), 7.44 (t, J = 8.0 Hz, 2H), 7.24–7.22 (m, 3H), 7.09 (d, J = 8.0 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 191.5, 184.4, 157.3, 148.4, 135.8, 134.4, 133.2, 131.3, 131.2, 129.5, 129.4, 128.7, 128.2, 127.3, 118.9, 117.7, 116.03; ESI-HRMS: calcd for C21H14ClN2O2+ [M + H]+, 361.0738; found, 361.0730.

1-(2-(4-Bromophenyl)imidazo[1,2-a]pyridin-3-yl)-2-phenylethane-1,2-dione (5fa)

A pale yellow solid was obtained; yield: 155 mg, 75%; mp: 130–133 °C; FT-IR (neat): 3140, 1674, 1597, 1489, 1465 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.85 (d, J = 6.8 Hz, 1H), 7.87 (dt, J = 8.8, 1.2 Hz, 1H), 7.75–7.67 (m, 3H), 7.64–7.59 (m, 1H), 7.44 (t, J = 7.6 Hz, 2H), 7.27–7.23 (m, 3H), 7.16 (d, J = 8.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 191.5, 184.4, 157.2, 148.3, 134.5, 133.2, 131.7, 131.6, 131.2, 131.1, 129.5, 129.3, 128.7, 124.1, 118.9, 117.7, 116.0; ESI-HRMS: calcd for C21H14BrN2O2+ [M + H]+, 405.0233; found, 405.0214.

1-Phenyl-2-(2-(4-(trifluoromethyl)phenyl)imidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (5ga)

A pale yellow solid was obtained; yield: 175 mg, 87%; mp: 120–122 °C (lit.[21] mp 117–119 °C); FT-IR (neat): 3071, 1767, 1667, 1604 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.85 (d, J = 6.8 Hz, 1H), 7.88 (d, J = 9.2 Hz, 1H), 7.73–7.66 (m, 3H), 7.60 (t, J = 7.6 Hz, 1H), 7.41–7.35 (m, 6H), 7.31–7.28 (m, 1H); 13C NMR (100 MHz, CDCl3): δ 191.6, 184.3, 156.8, 148.3, 136.4, 134.5, 133.0, 131.3 (q, JC–F = 32 Hz), 131.29, 129.4, 129.3, 128.7, 124.7 (q, JC–F = 4 Hz), 123.8 (q, JC–F = 270 Hz), 119.1, 117.8, 116.2; ESI-HRMS: calcd for C22H14F3N2O2+ [M + H]+, 395.1002; found, 395.0991.

1-(2-(3-Nitrophenyl)imidazo[1,2-a]pyridin-3-yl)-2-phenylethane-1,2-dione (5ta)

A pale brown solid was obtained; yield: 170 mg, 90%; mp: 197–199 °C (lit.[21] mp 198–200 °C); FT-IR (neat): 3078, 1728, 1674, 1581 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.86 (d, J = 6.8 Hz, 1H), 8.16–8.13 (m, 1H), 8.06 (t, J = 2.0 Hz, 1H), 7.90 (d, J = 8.8 Hz, 1H), 7.76–7.73 (m, 4H), 7.62 (t, J = 7.6 Hz, 1H), 7.45–7.40 (m, 3H), 7.32 (td, J = 6.9, 1.3 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 191.3, 184.0, 155.5, 148.3, 147.2, 135.8, 134.9, 134.8, 132.9, 131.4, 129.5, 129.3, 129.2, 129.0, 125.1, 124.0, 119.1, 117.9, 116.4; ESI-HRMS: calcd for C21H14N3O4+ [M + H]+, 372.0979; found, 372.0966.

1-(7-Methyl-2-phenylimidazo[1,2-a]pyridin-3-yl)-2-phenylethane-1,2-dione (5ja)

An off-white solid was obtained; yield: 161 mg, 93%; mp: 121 °C (lit.[15] mp 142–145 °C); FT-IR (neat): 2994, 1767, 1674, 1643 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.71 (d, J = 6.8 Hz, 1H), 7.73 (d, J = 7.6 Hz, 2H), 7.62 (s, 1H), 7.57 (t, J = 7.6 Hz, 1H), 7.39 (t, J = 7.6 Hz, 2H), 7.30–7.24 (m, 3H), 7.12–7.07 (m, 3H), 2.57 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 191.6, 184.2, 159.0, 148.8, 143.0, 134.0, 133.5, 133.0, 130.0, 129.6, 129.4, 128.5, 128.5, 127.9, 118.7, 118.2, 116.4, 21.8; ESI-HRMS: calcd for C22H17N2O2+ [M + H]+, 341.1285; found, 341.1274.

1-(6-Methyl-2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)-2-phenylethane-1,2-dione (5ma)

A yellow solid was obtained; yield: 175 mg, 89%; mp: 166–168 °C; FT-IR (neat): 3063, 2916, 1767, 1674, 1612 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.68 (s, 1H), 7.97 (d, J = 8.4 Hz, 2H), 7.79–7.72 (m, 3H), 7.63–7.57 (m, 2H), 7.48–7.43 (m, 4H), 2.54 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 191.4, 183.8, 155.4, 148.2, 147.3, 139.7, 134.8, 134.3, 133.1, 131.0, 129.6, 128.9, 127.4, 126.9, 122.9, 118.8, 117.0, 18.6; ESI-HRMS: calcd for C22H16N3O4+ [M + H]+, 386.1135; found, 386.1120.

1-(6-Bromo-2-(4-nitrophenyl)imidazo[1,2-a]pyridin-3-yl)-2-phenylethane-1,2-dione (5ua)

A yellow solid was obtained; yield: 202 mg, 88%; mp: 211–212 °C; FT-IR (neat): 3078, 1667, 1612, 1520, 1242 cm–1; 1H NMR (400 MHz, CDCl3): δ 10.03 (s, 1H), 7.99 (d, J = 8.4 Hz, 2H), 7.79–7.74 (m, 4H), 7.66 (t, J = 7.6 Hz, 1H), 7.50–7.44 (m, 4H); 13C NMR (100 MHz, CDCl3): δ 190.8, 184.0, 155.4, 148.4, 146.7, 139.0, 135.0, 134.7, 132.8, 131.0, 129.6, 129.4, 129.0, 123.0, 119.0, 118.4, 111.3; ESI-HRMS: calcd for C21H13BrN3O4+ [M + H]+, 450.0084; found, 450.0064.

1-(6-Bromo-2-(4-chlorophenyl)imidazo[1,2-a]pyridin-3-yl)-2-phenylethane-1,2-dione (5va)

An off-white solid was obtained; yield: 203 mg, 91%; mp: 126–127 °C; FT-IR (neat): 3132, 3055, 1728, 1681, 1612 cm–1; 1H NMR (400 MHz, CDCl3): δ 10.03 (s, 1H), 7.76–7.73 (m, 4H), 7.64 (t, J = 7.6 Hz, 1H), 7.46 (t, J = 7.6 Hz, 2H), 7.23 (d, J = 8.0 Hz, 2H), 7.10 (d, J = 8.4 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 191.1, 184.5, 157.1, 146.7, 136.0, 134.6, 134.5, 133.0, 131.3, 130.9, 129.6, 129.4, 128.8, 128.3, 118.9, 118.2, 110.8; ESI-HRMS: calcd for C21H13BrClN2O2+ [M + H]+, 438.9843; found, 438.9828.

1-(2-(4-Methoxyphenyl)imidazo[1,2-a]pyrimidin-3-yl)-2-phenylethane-1,2-dione (5oa)

A pale brown solid was obtained; yield: 146 mg, 80%; mp: 161–165 °C; FT-IR (neat): 2839, 1735, 1674, 1597, 1450 cm–1; 1H NMR (400 MHz, CDCl3): δ 10.05 (dd, J = 6.8, 2.0 Hz, 1H), 8.89 (dd, J = 4.4, 2.4 Hz, 1H), 7.80–7.78 (m, 2H), 7.62 (t, J = 7.6 Hz, 1H), 7.46–7.42 (m, 2H), 7.33 (d, J = 8.4 Hz, 2H), 7.28–7.26 (m, 1H), 6.65 (d, J = 8.7 Hz, 2H), 3.77 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 190.9, 185.3, 161.1, 159.8, 154.7, 151.0, 136.8, 134.4, 133.2, 131.7, 129.6, 128.7, 124.7, 117.0, 113.6, 111.5, 55.3; ESI-HRMS: calcd for C21H16N3O3+ [M + H]+, 358.1186; found, 358.1171.

1-Phenyl-2-(6-(p-tolyl)imidazo[2,1-b]thiazol-5-yl)ethane-1,2-dione (5ra)

A yellow solid was obtained; yield: 136 mg, 77%; mp: 148–149 °C; FT-IR (neat): 3078, 1790, 1736, 1674 cm–1; 1H NMR (400 MHz, CDCl3): δ 8.56 (d, J = 4.4 Hz, 1H), 7.79–7.77 (m, 2H), 7.61 (t, J = 7.2 Hz, 1H), 7.44 (t, J = 7.6 Hz, 2H), 7.21 (d, J = 7.6 Hz, 2H), 7.14 (d, J = 4.4 Hz, 1H), 6.93 (d, J = 8.0 Hz, 2H), 2.30 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 191.7, 183.1, 158.8, 156.2, 139.6, 134.2, 133.3, 129.9, 129.7, 129.6, 128.7, 128.6, 122.0, 121.9, 114.7, 21.3; ESI-HRMS: calcd for C20H15N2O2S+ [M + H]+, 347.0849; found, 347.0842.

1-(2-Phenylimidazo[1,2-a]pyridin-3-yl)-2-(p-tolyl)ethane-1,2-dione (5ab)

An off-white solid was obtained; yield: 139 mg, 80%; mp: 151–154 °C (lit.[15] mp 152–154 °C); FT-IR (neat): 3009, 1766, 1674, 1589 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.86 (d, J = 6.8 Hz, 1H), 7.87 (d, J = 8.8 Hz, 1H), 7.69–7.64 (m, 3H), 7.33 (d, J = 7.6 Hz, 2H), 7.26–7.20 (m, 4H), 7.12 (t, J = 7.6 Hz, 2H), 2.43 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 191.0, 184.8, 158.6, 148.3, 145.3, 133.0, 131.0, 130.9, 130.0, 129.7, 129.4, 129.3, 127.9, 118.9, 117.6, 115.7, 21.9; ESI-HRMS: calcd for C22H17N2O2+ [M + H]+, 341.1285; found, 341.1273.

1-(4-Methoxyphenyl)-2-(2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (5ac)

An off-white solid was obtained; yield: 140 mg, 77%; mp: 168 °C (lit.[15] mp 165–168 °C); FT-IR (neat): 2994, 1767, 1659, 1597 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.84 (d, J = 6.8 Hz, 1H), 7.85 (d, J = 8.8 Hz, 1H), 7.72 (d, J = 8.8 Hz, 2H), 7.66 (t, J = 8.4 Hz, 1H), 7.33 (d, J = 7.2 Hz, 2H), 7.26–7.21 (m, 2H), 7.13 (t, J = 8.0 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H), 3.89 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 190.1, 185.0, 164.4, 158.5, 148.3, 133.0, 132.0, 130.8, 130.0, 129.4, 129.3, 127.8, 126.6, 119.0, 117.6, 115.7, 113.9, 55.6; ESI-HRMS: calcd for C22H17N2O3+ [M + H]+, 357.1234; found, 357.1223.

1-(4-Chlorophenyl)-2-(2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (5ad)

An off-white solid was obtained; yield: 169 mg, 92%; mp: 124–127 °C (lit.[15] mp 122–125 °C); FT-IR (neat): 2994, 1767, 1674, 1604 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.83 (d, J = 6.8 Hz, 1H), 7.88 (d, J = 8.8 Hz, 1H), 7.71–7.67 (m, 3H), 7.39 (d, J = 8.8 Hz, 2H), 7.31–7.30 (m, 3H), 7.26 (d, J = 6.8 Hz, 1H), 7.15 (t, J = 7.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 190.1, 183.9, 158.8, 148.5, 140.7, 132.9, 131.8, 131.2, 130.8, 130.0, 129.6, 129.3, 129.0, 128.0, 117.7, 116.0; ESI-HRMS: calcd for C21H14ClN2O2+ [M + H]+, 361.0738; found, 361.0728.

1-(4-Nitrophenyl)-2-(2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (5ae)

A pale yellow solid was obtained; yield: 159 mg, 84%; mp: 222–225 °C (lit.[20a] mp 220–224 °C); FT-IR (neat): 3117, 1736, 1681, 1604 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.82 (d, J = 6.8 Hz, 1H), 8.25 (d, J = 8.4 Hz, 2H), 7.90 (d, J = 8.4 Hz, 3H), 7.74 (t, J = 7.6 Hz, 1H), 7.32–7.30 (m, 4H), 7.15 (t, J = 7.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 189.4, 182.8, 159.1, 150.6, 148.7, 137.7, 132.8, 131.6, 130.4, 130.1, 129.8, 129.3, 128.2, 123.7, 117.9, 116.2; ESI-HRMS: calcd for C21H14N3O4+ [M + H]+, 372.0979; found, 372.0967.

1-(2-Phenylimidazo[1,2-a]pyridin-3-yl)-2-(4-(trifluoromethyl)phenyl)ethane-1,2-dione (5af)

A pale grey solid was obtained; yield: 165 mg, 82%; mp: 165 °C; FT-IR (neat): 3062, 1790, 1682, 1604, 1319 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.82 (d, J = 6.8 Hz, 1H), 7.89–7.84 (m, 3H), 7.72–7.66 (m, 3H), 7.32–7.26 (m, 4H), 7.13 (t, J = 7.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 190.1, 183.4, 159.0, 148.6, 136.0, 135.1 (q, JC–F = 32.4 Hz), 132.8, 131.4, 130.0, 129.8, 129.7, 129.3, 128.0, 125.6 (q, JC–F = 3.7 Hz), 123.5 (q, JC–F = 271 Hz), 118.8, 117.8, 116.0. ESI-HRMS: calcd for C22H14F3N2O2+ [M + H]+, 395.1002; found, 395.0984.

1-(2-Phenylimidazo[1,2-a]pyridin-3-yl)-2-(thiophen-2-yl)ethane-1,2-dione (5ag)

A yellow solid was obtained; yield: 157 mg, 93%; mp: 129–130 °C (lit.[20a] mp 128–130 °C); FT-IR (neat): 3062, 1798, 1728, 1658 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.77 (d, J = 6.8 Hz, 1H), 7.86 (d, J = 8.8 Hz, 1H), 7.75 (t, J = 4.8 Hz, 2H), 7.67 (t, J = 7.6 Hz, 1H), 7.41 (d, J = 7.6 Hz, 2H), 7.35 (t, J = 7.6 Hz, 1H), 7.25–7.19 (m, 3H), 7.15 (t, J = 4.4 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 183.6, 182.9, 158.7, 148.4, 140.6, 136.1, 135.6, 133.2, 131.0, 129.8, 129.5, 129.2, 128.4, 128.08, 118.4, 117.7, 115.8; ESI-HRMS: calcd for C19H13N2O2S+ [M + H]+, 333.0692; found, 333.0681.

1-Cyclohexyl-2-(2-phenylimidazo[1,2-a]pyridin-3-yl)ethane-1,2-dione (5ah)

A yellow solid was obtained; yield: 154 mg, 91%; mp: 105–107 °C; FT-IR (neat): 3136, 3032, 1705, 1612, 1493, 1397 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.71 (d, J = 7.2 Hz, 1H), 7.84 (d, J = 8.8 Hz, 1H), 7.64 (t, J = 7.2 Hz, 1H), 7.55–7.53 (m, 2H), 7.47–7.46 (m, 3H), 7.20 (t, J = 6.8 Hz, 1H), 2.56–2.50 (m, 1H), 1.70–1.59 (m, 5H), 1.17–1.05 (m, 5H). 13C NMR (100 MHz, CDCl3): δ 204.4, 185.4, 157.8, 148.1, 133.5, 130.7, 130.3, 129.59, 129.0, 128.4, 118.2, 117.6, 115.6, 46.9, 25.7, 25.5; ESI-HRMS: calcd for C21H21N2O2+ [M + H]+, 333.1596; found, 333.1610.

General Procedure for the Synthesis of 2,2-Bis(indolyl)acetaldehydes (7a–b)

A clean oven dried 25 mL round bottom flask was charged with 6 (0.51 mmol) and glyoxal (2.55 mmol; 40 wt % aq solution) in acetic acid (3 mL). The reaction mixture was heated with stirring at 80 °C for 30 min. The reaction progress was monitored by TLC. After completion, the reaction mass was allowed to cool at ambient temperature. Acetic acid was neutralized by saturated solution of NaHCO3 (30 mL) and extracted with EtOAc (3 × 15 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated under reduced pressure. The residue was purified in silica gel (60–120 mesh) column chromatography using EtOAc–n-hexane as the eluent to afford the corresponding products (7a–b) in good yields (63–65%).

2,2-Bis(1-methyl-2-phenyl-1H-indol-3-yl)acetaldehyde (7a)

A white solid was obtained; yield: 73 mg, 63%; mp: 171–174 °C; FT-IR (neat): 3052, 1720, 1405, 1357, 740 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.94 (d, J = 2.4 Hz, 1H), 7.39–7.29 (m, 10H), 7.23 (t, J = 8.0 Hz, 2H), 7.15 (d, J = 6.4 Hz, 4H), 7.00 (t, J = 7.2 Hz, 2H), 5.21 (d, J = 2.4 Hz, 1H), 3.54 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 199.2, 139.4, 137.1, 131.3, 130.8, 128.3, 128.0, 127.0, 121.6, 120.3, 119.6, 109.4, 107.9, 49.7, 30.8; ESI-HRMS: calcd for C32H27N2O+ [M + H]+, 455.2118; found, 455.2115.

2,2-Bis(1-methyl-1H-indol-3-yl)acetaldehyde (7b)

A yellow solid was obtained; yield: 50 mg, 65%; mp: 107–110 °C; FT-IR (neat): 3055, 1720, 1473, 1327, 1226, 748 cm–1; 1H NMR (400 MHz, CDCl3): δ 10.2 (d, J = 2.0 Hz, 1H), 7.87 (d, J = 8.0 Hz, 2H), 7.63 (d, J = 8.0 Hz, 2H), 7.55 (t, J = 7.2 Hz, 2H), 7.41 (t, J = 7.6 Hz, 2H), 7.28 (s, 2H), 5.64 (d, J = 3.2 Hz, 1H), 4.04 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 197.3, 137.3, 128.1, 127.2, 122.0, 119.5, 119.4, 109.5, 109.3, 48.0, 32.9; ESI-HRMS: calcd for C20H19N2O+ [M + H]+, 303.1492; found, 303.1481.

2-Oxo-2-(2-phenylimidazo[1,2-a]pyridin-3-yl)ethyl Acetate (8)

An off-white solid was obtained; yield: 27 mg, 18%; mp: 122–125 °C; FT-IR (neat): 2931, 1744, 1643, 1404, 1242, 763, 694 cm–1; 1H NMR (400 MHz, CDCl3): δ 9.66 (d, J = 6.8 Hz, 1H), 7.71 (dt, J = 8.8, 1.2 Hz, 1H), 7.55–7.44 (m, 6H), 7.06 (td, J = 7.2, 1.6 Hz, 1H), 4.61 (s, 2H), 2.07 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 183.4, 170.4, 155.4, 147.6, 134.7, 130.0, 129.8, 129.3, 129.0, 128.9, 119.1, 117.5, 115.3, 66.7, 20.5; ESI-HRMS: calcd for C17H15N2O3+ [M + H]+, 295.1077; found, 295.1077.