A facile one-pot multi-component synthesis of novel adamantine substituted imidazo[1,2-a]pyridine derivatives: identification and structure–activity relationship study of their anti-HIV-1 activity

Abstract

In this study, a series of adamantine substituted imidazo[1,2-a]pyridine derivatives were designed and synthesized through a one-pot multi-component Groebke–Blackburn–Bienaymé reaction. Their anti-HIV activities were evaluated using an HIV-1_IIIB/TZM-bl indicator cell culture system, in which compounds 19d, 19e and 19m were found to be the most potent inhibitors with EC₅₀ values of 0.048, 0.061 and 0.077 μM, respectively. Furthermore, the modeling results provided valuable insight into how compound 19d gave good efficacy against HIV-1 cells.

---

Before the discovery of azidothymidine (AZT) in 1987, acquired immunodeficiency syndrome (AIDS) was considered to be an incurable and potentially fatal disease. AZT, known as a nucleoside reverse transcriptase inhibitor (NRTI), was the first breakthrough in AIDS therapy.¹ At that time, it opened the doors to new hope for the treatment of HIV-1 infection and converted an incurable disease to a curable one. Since then, several types of anti-HIV-1 drugs have been developed. For example, saquinavir 1 and nevirapine (NVP) 2 were used as a protease inhibitor and non-nucleoside reverse transcriptase inhibitor (NNRTI), respectively.² This symbolized significant progress in HIV-1 treatment. Thereafter, delavirdine (DLV) 3, efavirenz (EFV) 4, etravirine 5 and rilpivirine 6, as the new generation of NNRTIs, have already been approved for use in HIV-1 treatment (Scheme 1).^3–5

Scheme 1 Current anti-HIV-1 drugs.

Nowadays, HIV-1 infection can be controlled by the use of multiple antiretroviral drugs. There are several classes of antiretroviral drugs available for the treatment of HIV-1 infection. Among them, highly active antiretroviral therapy (HAART) is one of the best treatments targeting the virus at different stages of its life-cycle.⁶ However, long-term use of HAART by patients triggers the emergence of drug-resistant and drug-related adverse effects.^7,8 Thus, the identification of novel anti-HIV-1 agents with antiviral activity against drug-resistant strains of HIV-1 is critically urgent.

As mentioned above, NNRTIs play an important role in the HARRT. To overcome the problem of drug resistance, several NNRTIs have been recently developed such as rilpivirine,⁹ fosdevirine (GSK2248761) 7,^10,11 lersivirine (UK-453061) 8,¹² RDEA806 9¹³ and are currently in clinical trial (Scheme 2). In 2010, pyridine based NNRTIs was reported having strong inhibitory effect on polymerase activity.¹⁴

Scheme 2 NNRTIs in development.

Recently, Gomha and co-workers reported pyrazolo[4,3-d]isoxazole derivatives were screened for their antiviral activities against two viral strains of HIV-1 (RF and IIIB) with EC₅₀ value up to 0.25 nM.¹⁵ In 2012, Wang and co-workers developed 6-aryluracils which exhibited highly potent anti-HIV-1 activity against both on HIV-1SF₃₃ and HIV-1A₁₇ replication in MT4 cell.¹⁶ In 2015, Frey and co-workers developed analogues of the catechol diether series having picomolar activity against HIV strains with wild-type RT Y181C and K103N/Y181C variants.¹⁷

N-Fused heterocycles and adamantane are components of the important classes of molecules that were found in a variety of natural products and biologically active compounds. In particular, imidazo[1,2-a]pyridines derivatives, which can be efficiently synthesized through one-pot procedure from 2-aminopyridines and aldehydes,^18–25 have drawn much attention due to their significant pharmaceutical properties such as anticancer,^26–29 antivirals,^30–33 antimicrobials,^34–37 anti-Parkinson,³⁸ antimutagenics,³⁹ antihypoxia,⁴⁰ anticonvulsants,⁴¹ antisecretory,^42,43 and as antiinflamatories etc.⁴⁴ In 2011, Dahan-Farkas and co-workers developed a range of 6-substituted imidazo[1,2-a]pyridines as anti-cancer agents against the colon cancer cell lines HT-29 and Caco-2 with moderate IC₅₀ values, e.g. 10 in Scheme 3.⁴⁵ At the same year, Bode and co-workers reported a library of compounds with imidazo[1,2-a]pyridines skeleton such as 11 which was prepared using the Groebke reaction with moderate anti-HIV-1 activity of 14.47 μM (Scheme 3).⁴⁶

Scheme 3 Imidazo[1,2-a]pyridine based anti-HIV compounds.

On the other hand, there are number of adamantly bearing molecules reported in the literature which have long been known for their anti-HIV-1 activity.⁴⁷ Kolocouris reported the anti-HIV-1 activity of compound 12 in cell culture.^48,49 Other adamantine derivatives, such as 2-(1-adamantyl) piperidines 13 and 14, were developed by Stamatiou and co-workers exhibited modest anti-HIV activities (Scheme 4).^50,51 Burstein and co-workers evaluated newly developed antiviral compounds consisting of an adamantine moiety and a water-soluble polyanionic matrix which could inhibit HIV-1 infection in lymphoblastoid cells, HeLa CD4 + β-galactosidase (MAGI) cells with IC₅₀ value up to 93 μM.⁵² Balzarini group reported that the thiazolidin-4-one derivative 15 bearing a adamantyl substituent exhibited modest anti-HIV-1 activity (EC₅₀ = 0.67 μM).⁵³

Scheme 4 Amantadine based Anti-HIV compounds.

During the past few years,^54–59 we have focused on the development of novel non-nucleoside reverse transcriptase inhibitors, a large amount of small molecule NNRTIs, such as indole-based α-amino acids,⁶⁰ indole-based trifluoropropanoates⁶¹ and halolactones⁶¹ were developed. In order to continue pursuing in this field, we envisioned that incorporation of imidazo[1,2-a]pyridines⁶² and adamantine^63,64 moieties in one molecule could enhance their anti-HIV activity. Herein, the imidazo[1,2-a]pyridines derivatives bearing adamantane tail were designed and synthesized, and their anti-HIV activities were evaluated using a HIV-1_IIIB/TZM-bl indicator cell culture system.

Results and discussion

Chemistry

The new series of imidazo[1,2-α]pyridine derivatives 19 and 21 were designed as shown below in order to find-out the effect of different substituents on the anti-HIV-1 activity. The compounds imidazo[1,2-a]pyridines 19 were prepared according to previously reported method⁶⁵ and the procedure was shown in Scheme 5. The 2-aminopyridine, isonitriles and aldehydes were reacted in one-pot to form the desired products imidazo[1,2-a]pyridines derivatives in good yields.

Scheme 5 Synthesis of imidazopyridines.

Structure–activity relationship analysis

The compounds were assayed for their anti-HIV-1 activity by using a HIV-1_IIIB/TZM-bl indicator cell culture system. All of the newly synthesized compounds were tested, and the results are summarized in Tables 1 and 2. Most of the compounds showed moderate to good inhibitory activities against HIV-1_IIIB in a HIV-1_IIIB/TZM-bl indicator cell culture system. Out of all these compounds 19d, 19e and 19m were found to be the most potent inhibitors with EC₅₀ values of 0.048, 0.061 and 0.077 μM, respectively. It is worth outlining that the substituents R¹, R² and R³ have significant effect on the anti-HIV-1 activity. Compound 19g exhibited better anti-HIV-1 activity than 19b and 19c, in which the methoxyl group substituted on the phenyl ring, giving a moderate EC₅₀ of 3.52 μM in TZM-bl cells. However, when the substitution occurred at para position of the phenyl ring with halogen atoms, the potency increased accordingly. For example, compound 19t gave good EC₅₀ value 0.30 μM. It was also observed that the hydroxyl group at para position of phenyl ring had better influence on anti-HIV activity than meta position in the cases of 19f and 19y with EC₅₀ 0.163 and 0.447 μM, respectively. Further study revealed that with the insertion of ethoxyl and nitro groups at ortho position of phenyl ring induced more inhibition than their analogues (19d and 19e) with EC₅₀ values 0.048 and 0.061 μM. On the other hand, chloro group substituted at meta position (19x) had better effect on the anti-HIV activity as compared to the other groups such as hydroxyl, fluoro and bromo (19h, 19g and 19v). It was also observed that the insertion of electron withdrawing such as chloro, bromo, nitrile and trifluoromethyl groups at R¹ diminished the activity of compounds as compared with the parent analogues. It was found that electron withdrawing group diminished the activity of compounds 19j–19s as compared to the corresponding parent analogues 19d and 19e. A dramatic decrease was observed in the activity from 0.048 μM to 3.705 μM in the case of 19r having trifluoromethyl group at R¹ position. When the bromo (19l) and nitrile (19n) groups were substituted at the R¹ position, the EC₅₀ value shifted to 1.382 and 1.633 μM, respectively; whereas the chloro and ethyl substituted derivatives showed moderate activity. Furthermore, when the cytotoxicity (CC₅₀ values) was evaluated, most of the compounds showed moderate cytotoxicity, especially, compounds 19d and 19e showed SI (selectivity index) values of 426.6 and 661.3, respectively.

Table 1 Anti-HIV-1 activity of imidazo[1,2-a]pyridine derivatives bearing an adamantine tail^a

Entry	Compd	R^1	R^2	R^3	EC50^b (μM)	CC50^c (μM)	SI^d
a All data are mean values ± standard deviation for at least three independent experiments.b EC50: effective concentration (μM) for 50% inhibition of HIV-1 (HIV-1IIIB strain) as evaluated with the luciferase activity in TZM-bl cells.c CC50: cytotoxic concentration (μM) to induce 50% death of noninfected cells, as evaluated with the MTT method in TZM-bl cells.d SI: selectivity index calculated as CC50/EC50 ratio.e ND: not determined.f Not calculated because the EC50 was too high.
1	19a	H		4-OCH3	32.450 ± 3.530	54.35 ± 15.26	1.7
2	19b	H		3,4-(OCH3)2	5.990 ± 1.670	29.73 ± 8.92	5.0
3	19c	H		H	3.520 ± 1.580	94.62 ± 21.25	26.9
4	19d	H		2-NO2-4-OH	0.048 ± 0.017	20.52 ± 7.17	426.6
5	19e	H		2-OEt-4-OH	0.061 ± 0.009	40.14 ± 12.88	661.3
6	19f	H		4-OH	0.163 ± 0.063	22.25 ± 10.01	136.5
7	19g	H		3-F-4-OH	0.130 ± 0.045	22.28 ± 10.87	171.4
8	19h	H		3-OH-4-OH	1.34 ± 0.57	76.26 ± 20.21	56.9
9	19i	H		3,4-OC2H2O–	0.316 ± 0.110	10.22 ± 4.73	32.3
10	19j	Cl		2-OEt-4-OH	0.303 ± 0.082	10.52 ± 5.26	34.7
11	19k	Cl		4-OH	0.271 ± 0.098	22.13 ± 8.39	81.7
12	19l	Br		2-OEt-4-OH	1.382 ± 0.500	34.85 ± 11.82	25.2
13	19m	CF3		2-OEt-4-OH	0.077 ± 0.029	117.83 ± 7.64	252.5
14	19n	CN		2-OEt-4-OH	1.633 ± 0.590	20.32 ± 6.77	12.4
15	19o	Br		4-OH	0.289 ± 0.132	10.27 ± 5.25	35.5
16	19p	Et		2-NO2-4-OH	0.321 ± 0.154	15.74 ± 7.17	49.0
17	19q	Br		2-NO2-4-OH	0.331 ± 0.095	8.69 ± 3.31	26.3
18	19r	CF3		2-NO2-4-OH	3.705 ± 1.050	15.25 ± 6.77	14.5
19	19s	CN		2-NO2-4-OH	0.253 ± 0.098	20.04 ± 6.06	79.2
20	19t	H		4-Br	0.300 ± 0.113	21.80 ± 7.34	72.7
21	19u	H		3-OEt-4-OH	1.330 ± 0.066	13.89 ± 6.69	210.3
22	19v	H		3-Br-4-OH	0.303 ± 0.150	36.52 ± 11.87	120.5
23	19w	H		2-F-4-OH	1.330 ± 0.627	24.40 ± 9.81	18.4
24	19x	H		3-Cl-4-OH	0.077 ± 0.048	17.30 ± 6.10	225.0
25	19y	H		3-Me-4-OH	0.447 ± 0.195	25.73 ± 10.18	57.6
26	19z	H		2-NO2-4-OH	>100	ND^e	—^f
27	19z1	H		2-NO2-4-OH	>100	ND	—
28	2				0.042 ± 0.014	144.3 ± 42.6	3435.72

---

Table 2 Anti-HIV-1 activity of imidazo[1,2-a]pyridine derivatives bearing an indole moiety and an adamantine tail^a

Entry	Compd	R^4	R^5	EC50^b (μM)	CC50^c (μM)	SI^d
a All data are mean values ± standard deviation for at least three independent experiments.b EC50: effective concentration (μM) for 50% inhibition of HIV-1 (HIV-1IIIB strain) as evaluated with the luciferase activity in TZM-bl cells.c CC50: cytotoxic concentration (μM) to induce 50% death of noninfected cells, as evaluated with the MTT method in TZM-bl cells.d SI: selectivity index calculated as CC50/EC50 ratio.
1	21a	H	5-F	12.125 ± 3.300	141.74 ± 23.25	11.7
2	21b	H	5-OCH3	4.410 ± 1.970	155.33 ± 30.54	35.2
3	21c	H	H	1.590 ± 0.729	62.74 ± 18.52	39.5
4	21d	H	5-Me	0.384 ± 0.058	21.18 ± 9.33	55.2
5	21e	H	7-Me	0.452 ± 0.179	15.63 ± 8.57	34.6
6	21f	H	5-CN	0.353 ± 0.137	10.30 ± 4.17	29.2
7	21g	1-Me	H	0.268 ± 0.068	17.15 ± 9.33	64.0
8	21h	H	5-Cl	0.688 ± 0.213	7.69 ± 3.84	11.3
9	21i	H	6-Me	0.601 ± 0.166	5.80 ± 2.01	9.7
10	21j			0.534 ± 0.164	16.73 ± 7.32	31.3
11	2			0.042 ± 0.014	144.3 ± 42.6	3435.72

---

The structural–activity relationship analysis of alternate substituents present on the phenyl ring and imidazo[1,2-a]pyridine bearing adamantine is presented in Fig. 1. The results demonstrate that substituted phenyl ring at the 4-position has better HIV-1 inhibitory activity, for example compound 19t with Br at the 4-position of the phenyl ring gives lower EC₅₀ value than compound 19c. Furthermore, the OH group which can form a hydrogen bonding interaction with residue Glu 238 is the best substituent for the anti-HIV-1 activity, such as compound 19f, this can be explained by the molecular modeling studies in Fig. 2. For other substituents at the 2- or 3-position of the phenyl ring, we found that compounds 19d and 19e with 2-NO₂-4-OH and 2-OEt-4-OH substituents at the phenyl ring respectively, gave the best anti-HIV-1 activities as compared with the reference NVP 2, in which nitro group formed two hydrogen bondings with residue Lys 101. Decreased HIV-1 inhibitory activity occurred in the presence of 3-OEt-4-OH at the phenyl ring, while 3-Cl-4-OH substituted phenyl ring gave decreased but considerable activity as compared with compounds 19d and 19e. Finally, we researched on the groups at the imidazo[1,2-a]pyridine ring (19j–s) and the results showed that substituted imidazo[1,2-a]pyridine ring generally gave lower anti-HIV-1 activities, except compound 19m showed comparable activity to those of the best compounds 19d and 19e of the series. It should also be noted that the compounds 19z and 19z1 bearing the same imidazo[1,2-a]pyridine structure, but lacking the adamantyl moiety, was devoid of antiviral activity (entries 26–27).

Fig. 1 Graphical representation of the effect of substituents present on the imidazo[1,2-a]pyridine and phenyl rings on the anti-HIV-1 activity.

Fig. 2 (A) Molecular orientation of 19d and interaction between the imidazo[1,2-a]pyridine derivative of 19d and RT by AutoDock Vina docking result with good score; (B) the surface type between 19d with RT.

Meanwhile, indole is also one of the most used bioactive components for drug discovery, including anti-HIV-1 agent.⁶⁵ Therefore, indole based imidazo[1,2-a]pyridine (21) derivatives were also synthesized and their anti-HIV activities were evaluated, the results were summarized in Table 2. Surprisingly, when the phenyl ring was replaced with the indole ring, decreased potency was usually observed and the substitution plays a major role on the anti-HIV-1 activity. Compounds 21g, in which the indole ring substituted by methyl group at N-1 position, showed the most potent activity (entry 7).

Among them, 21f showed moderate activity with IC₅₀ value up to 0.353 μM, in contrast, 21a, 21b and 21h, having fluoro, methoxyl, and chloro group at the same position gave lower activity.

Moreover, in order to ascertain whether the compounds were also inhibitory in primary T-lymphocyte cells using virus strains that belong to different clades of HIV-1,⁶⁶ compounds 19d and 19e were included in this study (Table 3). These compounds showed good inhibitory activity against virus members that belonged to clade A, clade B, clade C and group O. The clade viruses (92UG029 and BCF02) were X-tropic, while the clade viruses (92US657 and 93IN101) were R-tropic. These data indicated that this class of compounds possess a broad spectrum of anti-HIV-1 activity.

Table 3 Inhibitory activity of compounds 19d and 19e against different primary HIV-1 strains in peripheral blood mononuclear cells

EC50^a (μM)
Entry	Compd	Clade A (92UG029)	Clade B (92US657)	Clade C (93IN101)	Group O (BCF02)
a EC50: 50% effective concentration or compound concentration required to inhibit HIV-1 p24 production in virus-infected PBMC. Data are the mean of two independent experiments.
1	19d	0.057	0.065	0.092	0.084
2	19e	0.074	0.086	0.124	0.136

---

Molecular modeling studies

In order to further understanding the structure–binding relationships of imidazo[1,2-a]pyridine derivatives in the non-nucleoside binding site (NNBS) of RT, we took a molecular modeling approach. In the modeling study, we chose the crystal structure of WT RT NNBS with (2-(((2-(3,4-dihydroquinolin-1(2H)-yl)-2-oxoethyl)(methyl)amino)-methyl)quinazolin-4(1H)-one) (PDB: 4KFB)^67–69 and compound 19d as the receptor and ligand, respectively. AutoDock Vina⁷⁰ was chosen to study the binding modes of compound 19d and the result showed excellent binding score (−12.41 kcal mol⁻¹) in the NNBS of RT where three hydrogen bonding interactions were formed between the nitro group and residue Lys 101, OH group and residue Glu 238 which can explain that compounds with NO₂ group gave better activities. In the proposed structure of protein-19d complex, adamantine tail was buried deep in the site formed by residues Leu 100-Lys 102-Lys 103-Val 106-Phe 227-Leu 234-Pro 236-Tyr 318. The imidazo[1,2-a]pyridine ring forms several π–π interactions with residues Tyr 181, Tyr 188, and Trp 229 in the hydrophobic pocket, which is mainly formed by the side chains of Tyr181, Tyr 188, Phe 227, Trp 229, and Leu 234.

Conclusion

The biological properties of the imidazo[1,2-a]pyridine class of compounds have drawn much attention in recent years, however their effect on HIV-1_IIIB/TZM-bl indicator cell culture system has not yet been established. In the present study, a number of imidazo[1,2-a]pyridin-3-amines were synthesised in a one-pot procedure from appropriate aldehydes, isocyanides and 2-aminopyridines using the Groebke reaction and their anti-HIV activities were evaluated. The SAR results indicate that the NO₂ and hydroxyl group at ortho and para position of phenyl ring, played important role in anti-HIV-1 activity. Further study on mechanism of action of these compounds is currently under way.

Experimental section

Materials and methods

Unless otherwise noted, reagents and materials were obtained from commercial suppliers and were used without further purification. Reactions were monitored by thin layer chromatography (TLC) and column chromatography purification was performed using 230–400 mesh silica gel. NMR spectra were measured on Bruker DRX and DMX spectrometers at 400 MHz for ¹H spectra and at 100 MHz for ¹³C spectra and calibrated from the residual solvent signal. All final products were characterized by ¹H NMR, ¹³C NMR and MS analyses.

Live subject statement

All experiments were performed in compliance with the relevant laws and institutional guidelines, and the institutional committee (the study was approved by the Ethics Committee of Zhongnan Hospital of Wuhan University. All experiments were performed in accordance with ethical standards laid down in the Declaration of Helsinki). The informed consent was obtained for any experimentation with human subjects.

Representative procedure for the synthesis of N-(1-alkyl)-2-substituted aryl-imidazo[1,2-a]substitutedpyridin-3-amine 19

To the solution of substituted 2-aminopyridine (0.25 mmol) in anhydrous acetonitrile, were added aldehyde (0.28 mmol), a catalytic amount of 4 N HCl/dioxane (10 μL) and isonitrile (0.24 mmol). The reaction mixture was refluxed for 2–6 hours and the reaction was monitored by thin layer chromatography (TLC). After the reaction mixture was cooled to room temperature, solvent was removed and the residue was purified through column chromatography.

N-(1-Adamantyl)-2-(4-methoxyphenyl)imidazo[1,2-a]pyridin-3-amine 19a. 46% yield; mp 227–229 °C. IR (KBr, cm⁻¹): 3450 (N–H), 3302, 3080, (C–H, Ar), 2991, 2902 2845 (CH₂, CH), 1556 (C–C, Ar), 1444 (C=N), 1298 (C–N), 1193 (C–OCH₃). ¹H NMR (400 MHz, CDCl₃) δ 8.24 (d, J = 6.9 Hz, 1H, Ar-H), 7.92–7.85 (m, 2H, Ar-H), 7.51 (d, J = 9.0 Hz, 1H, Ar-H), 7.09 (ddd, J = 8.9, 6.6, 1.2 Hz, 1H, Ar-H), 7.00–6.93 (m, 2H, Ar-H), 6.73 (td, J = 6.8, 0.9 Hz, 1H, Ar-H), 3.85 (s, 3H, OCH₃), 3.02 (s, 1H, NH), 1.93–1.44 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, CDCl₃) δ 158.05, 141.91, 139.40, 129.30, 127.88, 121.83, 117.02, 113.63, 111.05, 56.24, 55.24, 43.85, 36.16, 29.66. HRMS (ESI) calcd for C₂₄H₂₈N₃O [M + H]⁺ 374.2232, found 374.2206.

N-(1-Adamantyl)-2-(2,4-dimethoxyphenyl)imidazo[1,2-a]pyridin-3-amine 19b. 39% yield; mp 270–272 °C. IR (KBr, cm⁻¹): 3433 (N–H), 3199, 3062 (C–H, Ar), 2989, 2899 2848 (CH₂, CH), 1598 (C–C, Ar), 1444 (C=N), 1284 (C–N), 1190 (C–OCH₃). ¹H NMR (400 MHz, CDCl₃) δ 8.56 (d, J = 6.8 Hz, 1H, Ar-H), 7.78 (d, J = 8.9 Hz, 1H, Ar-H), 7.72–7.65 (m, 2H, Ar-H), 7.45–7.36 (m, 1H, Ar-H), 7.06 (td, J = 6.9, 0.9 Hz, 1H, Ar-H), 6.70 (d, J = 8.5 Hz, 1H, Ar-H), 5.25 (s, 1H, NH), 4.10 (s, 3H, OCH₃), 3.86 (s, 3H, OCH₃), 1.90–1.26 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, CDCl₃) δ 149.51, 148.34, 136.24, 130.20, 123.25, 121.05, 118.71, 115.27, 111.98, 110.84, 110.42, 56.88, 55.65, 43.95, 35.95, 29.56. HRMS (ESI) calcd for C₂₅H₃₀N₃O₂ [M + H]⁺ 404.2338, found 404.2311.

N-(1-Adamantyl)-2-phenyl-imidazo[1,2-a]pyridin-3-amine 19c. 30% yield; mp 231–233 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3267, 3043 (C–H, Ar), 2912, 2848 (CH₂, CH), 1558 (C–C, Ar), 1444 (C=N), 1217 (C–N), ¹H NMR (400 MHz, CDCl₃) δ 8.27 (d, J = 6.9 Hz, 1H, Ar-H), 7.98–7.90 (m, 2H, Ar-H), 7.53 (d, J = 9.0 Hz, 1H, Ar-H), 7.43 (t, J = 7.6 Hz, 2H, Ar-H), 7.35–7.24 (m, 1H, Ar-H), 7.12 (ddd, J = 8.9, 6.6, 1.2 Hz, 1H, Ar-H), 6.76 (td, J = 6.8, 1.0 Hz, 1H, Ar-H), 3.10 (s, 1H, NH), 1.92–1.43 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, CDCl₃) δ 141.90, 139.29, 135.09, 128.15, 127.36, 117.15, 111.32, 56.68, 43.81, 36.14, 29.65. HRMS (ESI) calcd for C₂₃H₂₆N₃ [M + H]⁺ 344.2127, found 344.2102.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]-3-nitro-phenol 19d. 43% yield; mp 287–289 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3354, 3073 (C–H, Ar), 2899, 2846 (CH₂, CH), 1585 (C–C, Ar), 1433 (C=N), 1230 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.73 (s, 1H, Ar-H), 8.41 (d, J = 6.9 Hz, 1H, Ar-H), 7.84 (d, J = 8.9 Hz, 1H, Ar-H), 7.44 (d, J = 9.0 Hz, 1H, Ar-H), 7.32–7.05 (m, 2H, Ar-H), 6.97–6.74 (m, 2H, Ar-H), 4.39 (s, 1H, NH), 1.84–1.36 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 160.67, 141.41, 140.97, 132.60, 124.13, 123.69, 118.25, 114.36, 111.24, 56.21, 40.06, 35.74, 28.95. HRMS (ESI) calcd for C₂₃H₂₅N₄O₃ [M + H]⁺ 405.1927, found 405.1900.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]-3-ethoxy-phenol 19e. 30% yield; mp 281–283 °C. IR (KBr, cm⁻¹): 3442 (N–H), 3332 (O–H), 3257, 3161, 3091 (C–H, Ar), 2980, 2900, 2848 (CH₂, CH), 1606 (C–C, Ar), 1444 (C=N), 1247 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 9.16 (s, 1H, –OH), 8.49 (d, J = 6.9 Hz, 1H, Ar-H), 7.78 (d, J = 1.8 Hz, 1H, Ar-H), 7.62 (dd, J = 8.3, 1.9 Hz, 1H, Ar-H), 7.51 (d, J = 8.9 Hz, 1H, Ar-H), 7.34–7.23 (m, 1H, Ar-H), 6.96 (t, J = 6.5 Hz, 1H, Ar-H), 6.85 (d, J = 8.21 Hz, 1H, Ar-H), 4.71 (s, 1H, NH), 4.11 (q, J = 7.0 Hz, 2H, –CH₂–), 1.88–1.36 (m, 18H, adamantyl-H, –CH₃). ¹³C NMR (100 MHz, DMSO-d₆) δ 146.51, 146.15, 139.79, 125.46, 124.38, 121.96, 120.78, 115.24, 113.14, 111.92, 66.83, 55.80, 43.20, 35.78, 29.02, 14.87. HRMS (ESI) calcd for C₂₅H₃₀N₃O₂ [M + H]⁺ 404.2338, found 404.2309.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]phenol 19f. 25% yield; mp 292–294 °C, IR (KBr, cm⁻¹): 3444 (N–H), 3271, 3176, 3091 (C–H, Ar), 2904, 2848, (CH₂, CH), 1614 (C–C, Ar), 1436 (C=N), 1282 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 9.83 (s, 1H, –OH), 8.62 (d, J = 6.7 Hz, 1H, Ar-H), 7.99 (d, J = 8.5 Hz, 2H, Ar-H), 7.62 (d, J = 8.7 Hz, 1H, Ar-H), 7.48 (t, J = 7.7 Hz, 1H, Ar-H), 7.13 (t, J = 6.7 Hz, 1H, Ar-H), 6.87 (d, J = 8.5 Hz, 2H, Ar-H), 4.83 (s, 1H, NH), 1.88–1.41 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 157.37, 139.10, 129.22, 124.74, 122.99, 122.15, 115.05, 114.34, 112.87, 55.89, 40.05, 35.72, 29.00. HRMS (ESI) calcd for C₂₃H₂₆N₃O [M + H]⁺ 360.2076, found 360.2048.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]-2-fluoro-phenol 19g. 37% yield; mp 296–298 °C. IR (KBr, cm⁻¹): 3431 (N–H), 3334, 3084 (C–H, Ar), 2910, 2848 (CH₂, CH), 1597 (C–C, Ar), 1496 (C=N), 1292 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.13 (s, 1H, –OH), 8.55 (dd, J = 29.0, 6.8 Hz, 1H, Ar-H), 7.98 (dd, J = 13.1, 1.8 Hz, 1H, Ar-H), 7.82 (d, J = 8.3 Hz, 1H, Ar-H), 7.53 (d, J = 8.9 Hz, 1H, Ar-H), 7.37–7.28 (m, 1H, Ar-H), 7.09–6.93 (m, 2H, Ar-H), 4.77 (s, 1H, NH, Ar-H), 1.89–1.41 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 151.76, 149.38, 144.35, 139.90, 135.20, 124.54, 124.11, 122.43, 115.23, 112.13, 109.50, 55.91, 43.15, 35.74, 29.01. HRMS (ESI) calcd for C₂₃H₂₅FN₃O [M + H]⁺ 378.1982, found 378.1950.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]benzene-1,2-diol 19h. 87% yield; mp 334–335 °C. IR (KBr, cm⁻¹): 3421.72 (N–H), 3271, 3089 (C–H, Ar), 2904, 2848 (CH₂, CH), 1614 (C–C, Ar), 1446 (C=N), 1278 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 9.50 (s, 1H, –OH), 9.21 (s, 1H, –OH), 8.73 (d, J = 6.7 Hz, 1H, Ar-H), 7.77–7.60 (m, 2H, Ar-H), 7.55 (d, J = 2.1 Hz, 1H, Ar-H), 7.43 (dd, J = 8.2, 2.1 Hz, 1H, Ar-H), 7.30 (t, J = 6.3 Hz, 1H, Ar-H), 6.87 (d, J = 8.3 Hz, 1H, Ar-H), 4.92 (s, 1H, NH), 1.89–1.42 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) 146.46, 145.21, 138.64, 122.69, 119.67, 118.54, 115.60, 109.69, 109.38, 56.16, 42.86, 35.65, 29.00. HRMS (ESI) calcd for C₂₃H₂₆N₃O₂ [M + H]⁺ 376.2025, found 376.2001.

N-(1-Adamantyl)-2-(2,3-dihydro-1,4-benzodioxin-6-yl)imidazo-[1,2-a]pyridin-3-amine 19i. 57% yield; mp 267–269 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3296 (C–H, Ar), 2974, 2914, 2846 (CH₂, CH), 1585 (C–C, Ar), 1502 (C=N), 1278 (C–N), 1126 (O–CH₂–O). ¹H NMR (400 MHz, CDCl₃) δ 8.25 (d, J = 6.9 Hz, 1H, Ar-H), 7.50 (t, J = 5.4 Hz, 2H, Ar-H), 7.43 (dd, J = 8.4, 2.0 Hz, 1H, Ar-H), 7.28 (s, 1H, Ar-H), 7.10 (ddd, J = 8.9, 6.6, 1.2 Hz, 1H, Ar-H), 6.91 (d, J = 8.4 Hz, 1H, Ar-H), 6.74 (td, J = 6.8, 1.0, 1H, Ar-H), 4.30 (s, 4H, –CH₂), 3.03 (s, 1H, NH), 1.94–1.46 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, CDCl₃) δ 143.33, 143.01, 141.76, 138.97, 128.69, 121.46, 117.01, 111.15, 64.50, 65.37, 56.62, 43.81, 36.16, 29.67. HRMS (ESI) calcd for C₂₅H₂₈N₃O₂ [M + H]⁺ 424.20, found 402.2149.

4-[3-(1-Adamantylamino)-7-chloro-imidazo[1,2-a]pyridin-2-yl]-3-ethoxy-phenol 19j. 42% yield; mp 249–252 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3348, 3251, 3184 (C–H, Ar), 2904, 2848 (CH₂, CH), 1604 (C–C, Ar), 1413 (C=N), 1282 (C–N), 1186 (O–CH₂). ¹H NMR (400 MHz, DMSO-d₆) δ 9.36 (s, 1H, –OH), 7.80–7.72 (m, 2H, Ar-H), 7.61 (d, J = 6.4 Hz, 1H, Ar-H), 7.22 (d, J = 5.7 Hz, 1H, Ar-H), 6.90 (d, J = 8.3 Hz, 1H, Ar-H), 4.97 (s, 1H, NH), 4.12 (q, J = 7.0 Hz, 2H, –CH₂–), 1.89–1.36 (m, 18H, adamantyl-H, –CH₃). ¹³C NMR (100 MHz, DMSO-d₆) δ 147.34, 146.35, 138.59, 126.03, 120.98, 112.78, 109.32, 63.95, 56.10, 40.08, 35.69, 29.02, 14.83. HRMS (ESI) calcd for C₂₅H₂₉ClN₃O₂ [M + H]⁺ 438.1938, found 438.1916.

4-[3-(1-Adamantylamino)-7-chloro-imidazo[1,2-a]pyridin-2-yl]phenol 19k. 39% yield; mp 305–307 °C. IR (KBr, cm⁻¹): 3430 (N–H), 3097, (C–H, Ar), 2900, 2846 (CH₂, CH), 1564 (C–C, Ar), 1508 (C=N), 1269 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 9.62 (s, 1H, –OH), 8.47 (d, J = 7.3 Hz, 1H, Ar-H), 7.99 (d, J = 8.7 Hz, 2H, Ar-H), 6.99 (dd, J = 7.3, 2.0 Hz, 1H, Ar-H), 6.82 (d, J = 8.7 Hz, 2H, Ar-H), 6.82 (d, J = 8.7 Hz, 1H, Ar-H), 4.69 (s, 1H, NH), 1.87–1.40 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 157.00, 139.83, 137.65, 129.77, 129.06, 122.32, 115.89, 114.87, 114.16, 112.62, 55.87, 43.13, 35.75, 29.02. HRMS (ESI) calcd for C₂₃H₂₅ClN₃O [M + H]⁺ 394.1686, found 394.1656.

4-[3-(1-Adamantylamino)-7-bromo-imidazo[1,2-a]pyridin-2-yl]-3-ethoxy-phenol 19l. 27% yield; mp 285–287 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3348, 3248, 3186 (C–H, Ar), 2900, 2848 (CH₂, CH), 1521 (C–C, Ar), 1455 (C=N), 1280 (C–N), 1184 (O–CH₂). ¹H NMR (400 MHz, DMSO-d₆) δ 9.25 (s, 1H, –OH), 8.51 (d, J = 7.3 Hz, 1H, Ar-H), 7.92–7.67 (m, 2H, Ar-H), 7.60 (d, J = 6.6 Hz, 1H, Ar-H), 7.22 (d, J = 7.1 Hz, 1H, Ar-H), 6.88 (d, J = 8.3 Hz, 1H, Ar-H), 4.86 (s, 1H, NH), 4.12 (q, J = 6.9 Hz, 2H, –CH₂–), 1.89–1.36 (m, 18H, adamantyl-H, –CH₃). ¹³C NMR (100 MHz, DMSO-d₆) δ 147.06, 146.27, 139.38, 134.79, 125.64, 123.21, 122.58, 120.94, 116.35, 116.10, 115.37, 113.17, 63.91, 56.02, 40.09, 35.71, 29.02, 14.85. HRMS (ESI) calcd for C₂₅H₂₉BrN₃O₂ [M + H]⁺ 482.1443, found 482.1408.

4-[3-(1-Adamantylamino)-7-(1,1,1trifloromethyl)-imidazo[1,2-a]pyridin-2-yl]-3-ethoxy-phenol 19m. 41% yield; mp 255–257 °C. IR (KBr, cm⁻¹): 3402 (N–H), 3086 (C–H, Ar), 2906, 2852 (CH₂, CH), 1562 (C–C, Ar), 1477 (C=N), 1253 (C–N), 1163 (O–CH₂). ¹H NMR (400 MHz, DMSO-d₆) δ 9.07 (s, 1H, –OH), 8.59 (d, J = 7.2 Hz, 1H, Ar-H), 7.99–7.74 (m, 2H, Ar-H), 7.67 (d, J = 6.5 Hz, 1H, Ar-H), 7.10 (d, J = 5.6 Hz, 1H, Ar-H), 6.85 (d, J = 8.2 Hz, 1H, Ar-H), 4.78 (s, 1H, NH), 4.11 (q, J = 6.9 Hz, 2H, –CH₂–), 1.89–1.37 (m, 18H, adamantyl-H, –CH₃). ¹³C NMR (100 MHz, DMSO-d₆) δ 146.57, 146.12, 141.08, 138.59, 123.58, 123.31, 122.51, 120.92, 113.20, 109.50, 106.21, 63.83, 56.08, 43.28, 35.75, 29.02, 14.87. HRMS (ESI) calcd for C₂₆H₂₉F₃N₃O₂ [M + H]⁺ 472.2212, found 472.2277.

4-[3-(1-Adamantylamino)-7-cyano-imidazo[1,2-a]pyridin-2-yl]-3-ethoxy-phenol 19n. 30% yield; mp 308–310 °C. IR (KBr, cm⁻¹): 3444 (N–H), 3354, 3275, 3163 (C–H, Ar), 2910, 2850 (CH₂, CH), 1544 (C–C, Ar), 1477 (C=N), 1253 (C–N), 1132 (O–CH₂). ¹H NMR (400 MHz, DMSO-d₆) δ 9.58 (s, 1H, –OH), 8.88 (d, J = 7.1 Hz, 1H, Ar-H), 7.79 (d, J = 1.7 Hz, 1H, Ar-H), 7.66 (d, J = 6.6 Hz, 1H, Ar-H), 7.57 (d, J = 6.9 Hz, 1H, Ar-H), 7.50–7.18 (m, 2H, Ar-H), 6.96 (d, J = 8.3 Hz, 1H, Ar-H), 5.27 (s, 1H, NH), 4.15 (q, J = 6.9 Hz, 2H, –CH₂–), 1.89–1.37 (m, 18H, adamantyl-H, –CH₃). ¹³C NMR (100 MHz, DMSO-d₆) δ 147.26, 145.43, 134.77, 125.17, 123.55, 120.31, 117.48, 116.93, 115.86, 114.50, 112.14, 62.90, 55.68, 38.91, 34.42, 27.87, 13.63. HRMS (ESI) calcd for C₂₆H₂₉N₄O₂ [M + H]⁺ 429.2291, found 429.2217.

4-[3-(1-Adamantylamino)-7-bromo-imidazo[1,2-a]pyridin-2-yl]phenol 19o. 31% yield; mp 310–312 °C. IR (KBr, cm⁻¹): 3427 (N–H), 3275, 3151 (C–H, Ar), 2904, 2846 (CH₂, CH), 1564 (C–C, Ar), 1431 (C=N), 1282 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 9.79 (s, 1H, –OH), 8.52 (d, J = 7.3 Hz, 1H, Ar-H), 7.98 (d, J = 8.6 Hz, 2H, Ar-H), 7.86 (s, 1H, Ar-H), 7.25 (d, J = 5.8 Hz, 1H, Ar-H), 6.86 (d, J = 8.6 Hz, 2H, Ar-H), 4.84 (s, 1H, NH), 1.87–1.40 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 157.56, 139.23, 129.23, 125.74, 122.61, 116.34, 115.10, 108.78, 56.04, 40.09, 35.69, 29.01. HRMS (ESI) calcd for C₂₃H₂₅BrN₃O [M + H]⁺ 438.1181, found 438.1148.

4-[3-(1-Adamantylamino)-7-ethyl-imidazo[1,2-a]pyridin-2-yl]-3-nitro-phenol 19p. 67% yield, mp 296–298 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3354, 3080, (C–H, Ar), 2908, 2846 (CH₂, CH), 1587 (C–C, Ar), 1450 (C=N), 1267 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.69 (s, 1H, –OH), 8.30 (d, J = 7.0 Hz, 1H, Ar-H), 7.82 (d, J = 8.9 Hz, 1H, Ar-H), 7.22–7.20 (m, 2H, Ar-H), 6.89–6.76 (m, 2H, Ar-H), 4.27 (s, 1H, NH), 2.64 (q, J = 7.5 Hz, 2H, –CH₂–), 1.84–1.36 (m, 15H, adamantyl-H), 1.25–1.21 (m, 3H, –CH₃). ¹³C NMR (100 MHz, DMSO-d₆) δ 160.60, 141.44, 141.40, 140.31, 132.75, 126.72, 123.19, 118.18, 114.22, 113.61, 112.68, 55.10, 40.09, 35.76, 28.96, 27.57, 14.48. HRMS (ESI) calcd for C₂₅H₂₉N₄O₃ [M + H]⁺ 433.2240, found 433.2207.

4-[3-(1-Adamantylamino)-7-bromo-imidazo[1,2-a]pyridin-2-yl]-3-nitro-phenol 19q. 63% yield; mp 310–312 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3344, 3093 (C–H, Ar), 2904, 2848 (CH₂, CH), 1585 (C–C, Ar), 1450 (C=N), 1271 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.74 (s, 1H, –OH), 8.37 (d, J = 6.9 Hz, 1H, Ar-H), 7.86 (d, J = 8.9, 2H, Ar-H), 7.78 (d, J = 1.4 Hz, 1H, Ar-H), 7.21 (d, J = 2.7 Hz, 1H, Ar-H), 7.07 (dd, J = 7.3, 1.9 Hz, 1H, Ar-H), 6.89 (dd, J = 8.9, 2.7 Hz, 1H, Ar-H), 4.48 (s, 1H, NH), 1.84–1.36 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 160.77, 141.30, 140.96, 136.15, 132.09, 124.31, 118.64, 118.26, 116.85, 114.75, 114.61, 55.31, 40.09, 35.70, 28.96. HRMS (ESI) calcd for C₂₃H₂₄BrN₄O₃ [M + H]⁺ 483.1032, found 483.1001.

4-[3-(1-Adamantylamino)-7-(1,1,1-trifloromethyl)-imidazo[1,2-a]pyridin-2-yl]-3-nitro-phenol 19r. 53% yield; mp 296–298 °C. IR (KBr, cm⁻¹): 3444 (N–H), 3348, 3070 (C–H, Ar), 2908, 2850 (CH₂, CH), 1587 (C–C, Ar), 1477 (C=N), 1230 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.81 (s, 1H, –OH), 8.61 (d, J = 7.2 Hz, 1H, Ar-H), 8.05–7.82 (m, 2H, Ar-H), 7.34–7.10 (m, 2H, Ar-H), 6.93 (dd, J = 8.9, 2.6 Hz, 1H, Ar-H), 4.65 (s, 1H, NH), 1.85–1.36 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 160.92, 141.24, 138.79, 138.11, 131.83, 125.59, 125.52, 118.34, 114.88, 106.77, 55.56, 40.09, 35.66, 28.95. HRMS (ESI) calcd for C₂₄H₂₄F₃N₄O₃ [M + H]⁺ 473.1801, found 473.1806.

4-[3-(1-Adamantylamino)-7-cyano-imidazo[1,2-a]pyridin-2-yl]-3-nitro-phenol 19s. 63% yield; mp 310–312 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3342, 3057 (C–H, Ar), 2910, 2850 (CH₂, CH), 1579 (C–C, Ar), 1483 (C=N), 1234 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.28 (s, 1H, –OH), 8.55 (d, J = 7.2 Hz, 1H, Ar-H), 8.24 (s, 1H, Ar-H), 7.90 (d, J = 8.9 Hz, 1H, Ar-H), 7.32–7.13 (m, 2H, Ar-H), 6.93 (dd, J = 8.9, 2.5 Hz, 1H, Ar-H), 5.32 (s, 1H, NH), 1.84–1.36 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 160.99, 141.16, 138.96, 138.82, 131.58, 126.39, 123.45, 118.21, 115.04, 111.48, 104.90, 55.74, 40.08, 35.63, 28.95. HRMS (ESI) calcd for C₂₄H₂₄N₅O₃ [M + H]⁺ 430.1879, found 430.1847.

N-(1-Adamantyl)-2-(4-bromophenyl)imidazo[1,2-a]pyridin-3-amine 19t. 60% yield; mp 244–246 °C. IR (KBr, cm⁻¹): 3446 (N–H), 3265, 3037 (C–H, Ar), 2900, 2848 (CH₂, CH), 1552 (C–C, Ar), 1485 (C=N), 1269 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 8.42 (d, J = 6.9 Hz, 1H, Ar-H), 8.19 (d, J = 8.6 Hz, 2H, Ar-H), 7.80–7.53 (m, 2H, Ar-H), 7.45 (d, J = 9.0 Hz, 1H, Ar-H), 7.18 (ddd, J = 9.0, 6.6, 1.2 Hz, 1H, Ar-H), 6.88 (td, J = 6.8, 1.0 Hz, 1H, Ar-H), 4.69 (s, 1H, NH), 1.88–1.41 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 141.11, 136.71, 134.68, 130.81, 129.54, 124.30, 124.17, 123.17, 119.97, 116.60, 111.12, 109.50, 55.96, 43.24, 35.77, 29.02. HRMS (ESI) calcd for C₂₃H₂₅BrN₃ [M + H]⁺ 422.1232, found 422.1200.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]-2-ethoxy-phenol 19u. 14% yield; mp 273–275 °C. IR (KBr, cm⁻¹): 3446 (N–H), 3340, 3257, 3161, (C–H, Ar), 2980, 2900, 2846 (CH₂, CH), 1598 (C–C, Ar), 1444 (C=N), 1282 (C–N), 1122 (O–CH₂CH₃). ¹H NMR (400 MHz, DMSO-d₆) δ 9.34 (s, 1H, –OH), 8.67 (d, J = 6.8 Hz, 1H, Ar-H), 7.77 (d, J = 1.9 Hz, 1H, Ar-H), 7.67 (d, J = 8.9 Hz, 1H, Ar-H), 7.61 (dd, J = 8.3, 1.9 Hz, 1H, Ar-H), 7.55 (t, J = 7.7 Hz, 1H, Ar-H), 7.18 (t, J = 6.7 Hz, 1H, Ar-H), 6.91 (d, J = 8.3 Hz, 1H, Ar-H), 4.93 (s, 1H, NH), 4.16 (q, J = 6.9 Hz, 2H, –CH₂–), 1.89–1.37 (m, 18H, adamantyl-H, –CH₃). ¹³C NMR (100 MHz, DMSO-d₆) δ 147.43, 146.40, 138.22, 136.68, 136.44, 125.13, 122.37, 121.56, 121.04, 115.48, 114.48, 113.26, 63.97, 56.03, 40.08, 35.68, 29.01, 14.82. HRMS (ESI) calcd for C₂₅H₃₀N₃O₂ [M + H]⁺ 404.2338, found 404.2314.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]-2-bromo-phenol 19v. 52% yield; mp 298–300 °C. IR (KBr, cm⁻¹): 3450 (N–H), 3367, 3311, 3061 (C–H, Ar), 2902, 2846 (CH₂, CH), 1608 (C–C, Ar), 1436 (C=N), 1294 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.54 (s, 1H, –OH), 8.52 (d, J = 6.1, 1H, Ar-H), 8.38 (d, J = 1.5, 1H, Ar-H), 8.03 (dd, J = 8.5, 1.7 Hz, 1H, Ar-H), 7.57–7.49 (m, 1H, Ar-H), 7.40–7.28 (m, 1H, Ar-H), 7.03 (dd, J = 15.0, 7.6, 2H, Ar-H), 4.79 (s, 1H, NH), 1.90–1.42 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 153.52, 139.94, 134.55, 131.85, 127.96, 125.87, 124.40, 122.40, 115.91, 115.25, 112.13, 108.96, 55.89, 43.24, 35.75, 29.02. HRMS (ESI) calcd for C₂₃H₂₅BrN₃O [M + H]⁺ 438.1181, found 438.1149.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]-3-fluoro-phenol 19w. 74% yield; mp 311–313 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3346, 3099, 3043 (C–H, Ar), 2908, 2848 (CH₂, CH), 1589 (C–C, Ar), 1462 (C=N), 1278 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.26 (s, 1H, –OH), 8.55 (d, J = 6.9 Hz, 1H, Ar-H), 7.82–7.41 (m, 2H, Ar-H), 7.46–7.13 (m, 1H, Ar-H), 7.03 (t, J = 6.7 Hz, 1H, Ar-H), 6.83–6.56 (m, 2H, Ar-H), 4.08 (s, 1H, NH), 1.85–1.38 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 161.13, 159.05, 140.01, 132.22, 126.00, 124.37, 123.61, 115.26, 112.42, 111.87, 102.85, 102.61, 55.24, 40.07, 35.66, 28.92. HRMS (ESI) calcd for C₂₃H₂₅FN₃O [M + H]⁺ 378.1982, found 378.1950.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]-2-chloro-phenol 19x. 33% yield; mp 312–314 °C. IR (KBr, cm⁻¹): 3417 (N–H), 3099 (C–H, Ar), 2902, 2846 (CH₂, CH), 1612 (C–C, Ar), 1436 (C=N), 1296 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.47 (s, 1H, –OH), 8.52 (d, J = 6.8 Hz, 1H, Ar-H), 8.21 (d, J = 2.0 Hz, 1H, Ar-H), 7.99 (dd, J = 8.5, 2.0 Hz, 1H, Ar-H), 7.53 (d, J = 8.9 Hz, 1H, Ar-H), 7.43–7.25 (m, 1H, Ar-H), 7.17–6.85 (m, 2H, Ar-H), 4.79 (s, 1H, NH), 1.89–1.41 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 152.66, 139.72, 138.32, 128.93, 127.40, 126.33, 125.25, 124.63, 122.48, 119.38, 116.28, 115.00, 112.40, 55.92, 43.19, 35.74, 29.02. HRMS (ESI) calcd for C₂₃H₂₅ClN₃O [M + H]⁺ 394.1686, found 394.1657.

4-[3-(1-Adamantylamino)imidazo[1,2-a]pyridin-2-yl]-2-methyl-phenol 19y. 23% yield; mp 289–291 °C. IR (KBr, cm⁻¹): 3448 (N–H), 3049 (C–H, Ar), 2904, 2846 (CH₂, CH), 1546 (C–C, Ar), 1444 (C=N), 1276 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 9.34 (s, 1H, –OH), 8.38 (d, J = 6.2 Hz, 1H, Ar-H), 8.06–7.70 (m, 2H, Ar-H), 7.40 (d, J = 8.71 Hz, 1H, Ar-H), 7.21–7.01 (m, 1H, Ar-H), 6.81 (dd, J = 15.6, 7.2 Hz, 2H, Ar-H), 4.51 (s, 1H, NH), 2.19 (s, 3H, –CH₃), 1.89–1.42 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 154.50, 140.73, 138.51, 130.15, 126.18, 123.91, 123.28, 122.98, 121.60, 116.13, 113.98, 110.58, 55.68, 43.33, 35.86, 29.04, 16.09. HRMS (ESI) calcd for C₂₄H₂₈N₃O [M + H]⁺ 374.2232, found 374.2204.

4-[3-(Cyclohexylamino)imidazo[1,2-a]pyridin-2-yl]-3-nitro-phenol 19z. 40% yield; mp 249–251 °C. IR (KBr, cm⁻¹): 3448 (N–H), 3049 (C–H, Ar), 2904, 2846 (CH₂, CH), 1546 (C–C, Ar), 1444 (C=N), 1276 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.78 (s, 1H, –OH), 8.29 (d, J = 6.9 Hz, 1H, Ar-H), 7.86 (d, J = 8.9 Hz, 1H, Ar-H), 7.45 (d, J = 9.1 Hz, 1H, Ar-H), 7.25–7.09 (m, 2H, Ar-H), 6.99–6.78 (m, 2H, Ar-H), 4.59 (d, J = 5.4 Hz, 1H, NH), 2.11–0.85 (m, 11H, cyclohexyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 160.72, 141.20, 140.31, 132.20, 131.74, 126.83, 126.30, 123.57, 123.38, 117.84, 116.92, 114.42, 111.47, 109.50, 55.72, 33.03, 25.33, 24.08. HRMS (ESI) calcd for C₁₉H₂₁N₄O₃ [M + H]⁺ 353.1614, found 353.1611.

3-Nitro-4-[3-(1,1,2,2-tetramethylpropylamino)imidazo[1,2-a]pyridin-2-yl]phenol 19z1. 29% yield; mp 239–241 °C. IR (KBr, cm⁻¹): 3381 (N–H), 3336, 3091 (C–H, Ar), 2927, 2850 (CH₂, CH), 1583 (C–C, Ar), 1444 (C=N), 1261 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.77 (s, 1H, –OH), 8.41 (d, J = 6.9 Hz, 2H, Ar-H), 7.88 (d, J = 8.9 Hz, 1H, Ar-H), 7.44 (d, J = 9.0 Hz, 1H, Ar-H), 7.25–7.05 (m, 2H, Ar-H), 6.99–6.78 (m, 2H, Ar-H), 4.20 (s, 2H, NH), 0.94 (s, 6H, –CH₃), 0.89 (s, 9H, –CH₃). ¹³C NMR (100 MHz, DMSO-d₆) δ 160, 141.22, 140.94, 135.86, 133.05, 126.84, 124.35, 124.18, 123.92, 118.59, 116.79, 114.45, 111.24, 59.35, 55.38, 31.48, 30.98, 28.46. HRMS (ESI) calcd for C₂₀H₂₅N₄O₃ [M + H]⁺ 369.1848, found 369.1845.

N-(1-Adamantyl)-2-(6-fluoro-1H-indol-3-yl)imidazo[1,2-a]pyridine-3-amine 21a. 40% yield, mp 297–299 °C. IR (KBr, cm⁻¹): 3442 (N–H), 3232, 3088 (C–H, Ar), 2906, 2846 (CH₂, CH), 1624 (C–C, Ar), 1463 (C=N), 1230 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 12.06 (s, 1H, NH), 8.93 (d, J = 6.8 Hz, 1H, Ar-H), 8.25 (d, J = 2.0 Hz, 1H, Ar-H), 8.13 (dd, J = 8.8, 5.4 Hz, 1H, Ar-H), 7.95–7.79 (m, 2H, Ar-H), 7.49 (dd, J = 10.0, 4.5 Hz, 1H, Ar-H), 7.32 (dd, J = 9.8, 2.3 Hz, 1H, Ar-H), 7.05 (td, J = 9.4, 2.3 Hz, 1H, Ar-H), 5.27 (s, 1H, NH), 1.82–1.34 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) 156.05, 136.85, 132.97, 132.13, 129.30, 125.87, 124.90, 122.41, 116.12, 113.25, 111.46, 110.50, 110.24, 106.05, 105.80, 102.69, 56.11, 40.06, 35.54, 28.88. HRMS (ESI) calcd for C₂₅H₂₆FN₄ [M + H]⁺ 401.2142, found 401.2114.

N-(1-Adamantyl)-2-(5-methoxy-1H-indol-3-yl)imidazo[1,2-a]pyridin-3-amine 21b. 34% yield; mp 247–249 °C. IR (KBr, cm⁻¹): 3398 (N–H), 3228, 3082 (C–H, Ar), 2906, 2846 (CH₂, CH), 1600 (C–C, Ar), 1483 (C=N), 1261 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 11.89 (d, J = 1.7 Hz, 1H, NH), 8.93 (d, J = 6.8 Hz, 1H, Ar-H), 8.24 (d, J = 2.5 Hz, 1H, Ar-H), 7.98 (d, J = 8.8 Hz, 1H, Ar-H), 7.91–7.82 (m, 1H, Ar-H), 7.69 (d, J = 2.1 Hz, 1H, Ar-H), 7.53–7.37 (m, 2H, Ar-H), 6.86 (dd, J = 8.8, 2.3 Hz, 1H, Ar-H), 5.34 (s, 1H, NH), 3.86 (s, 3H, –OCH₃), 1.82–1.35 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 153.86, 136.79, 131.25, 127.89, 122.08, 116.06, 112.69, 111.41, 102.73, 102.09, 56.03, 55.44, 40.11, 35.58, 28.91. HRMS (ESI) calcd for C₂₆H₂₉N₄O [M + H]⁺ 413.2341, found 413.2314.

N-(1-Adamantyl)-2-(1H-indol-3-yl)imidazo[1,2-a]pyridin-3-amine 21c. 46% yield; mp 266–268 °C. IR (KBr, cm⁻¹): 3427 (N–H), 3223, 3080 (C–H, Ar), 2904, 2846 (CH₂, CH), 1602 (C–C, Ar), 1460 (C=N), 1259 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 12.00 (d, J = 2.0 Hz, 1H, NH), 8.94 (d, J = 6.8 Hz, 1H, Ar-H), 8.28 (d, J = 2.7 Hz, 1H, Ar-H), 8.12 (d, J = 7.8 Hz, 1H, Ar-H), 7.96–7.85 (m, 2H, Ar-H), 7.51 (ddd, J = 9.5, 8.1, 4.7 Hz, 2H, Ar-H), 7.29–7.11 (m, 2H, Ar-H), 5.27 (s, 1H, NH), 1.82–1.34 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 136.78, 136.22, 132.14, 127.55, 122.16, 119.89, 116.21, 112.13, 111.45, 102.14, 56.01, 40.06, 35.55, 28.88. HRMS (ESI) calcd for C₂₅H₂₇N₄ [M + H]⁺ 383.2236, found 383.2209.

N-(1-Adamantyl)-2-(5-methyl-1H-indol-3-yl)imidazo[1,2-a]pyridin-3-amine 21d. 26% yield; mp 250–252 °C. IR (KBr, cm⁻¹): 3421 (N–H), 3230, 3084 (C–H, Ar), 2908, 2848 (CH₂, CH), 1606 (C–C, Ar), 1479 (C=N), 1257 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 11.89 (s, 1H, NH), 8.92 (d, J = 6.3 Hz, 1H, Ar-H), 8.26 (s, 1H, Ar-H), 7.91 (dd, J = 24.9, 11.3 Hz, 3H, Ar-H), 7.58–7.30 (m, 2H, Ar-H), 7.06 (d, J = 8.1 Hz, 1H, Ar-H), 5.28 (s, 1H, NH), 2.45 (s, 3H, –CH₃), 1.83–1.36 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 136.85, 134.56, 131.75, 128.50, 123.67, 120.15, 116.05, 111.75, 111.55, 101.72, 56.03, 40.07, 35.58, 28.90, 21.33. HRMS (ESI) calcd for C₂₆H₂₉N₄ [M + H]⁺ 397.2392, found 397.2366.

N-(1-Adamantyl)-2-(7-methyl-1H-indol-3-yl)imidazo[1,2-a]pyridin-3-amine 21e. 35% yield; mp 270–272 °C. IR (KBr, cm⁻¹): 3427 (N–H), 3228, 3116, 3051 (C–H, Ar), 2900, 2848 (CH₂, CH), 1583 (C–C, Ar), 1494 (C=N), 1267 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 11.18 (s, 1H, NH), 8.35 (dd, J = 39.9, 7.2 Hz, 2H, Ar-H), 8.15 (s, 1H, Ar-H), 7.45 (d, J = 8.8 Hz, 1H, Ar-H), 7.18–7.04 (m, 1H, Ar-H), 6.90 (ddd, J = 35.8, 13.9, 6.9 Hz, 3H, Ar-H), 4.63 (s, 1H, NH), 2.50 (s, 3H, –CH₃), 1.87–1.39 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 140.83, 135.41, 123.66, 121.63, 120.19, 119.05, 115.78, 110.68, 109.75, 55.89, 43.32, 35.86, 29.01, 16.64. HRMS (ESI) calcd for C₂₆H₂₉N₄ [M + H]⁺ 397.2392, found 397.2366.

N-(1-Adamantyl)-2-(5-cyano-1H-indol-3-yl)imidazo[1,2-a]pyridin-3-amine 21f. 30% yield; mp 250–252 °C. IR (KBr, cm⁻¹): 3425 (N–H), 3230 (C–H, Ar), 2906, 2848 (CH₂, CH), 1577 (C–C, Ar), 1462 (C=N), 1290 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 10.00 (s, 1H, NH), 8.86 (d, J = 5.8 Hz, 1H, Ar-H), 8.72 (s, 1H, Ar-H), 8.48 (t, J = 20.1 Hz, 1H, Ar-H), 7.82 (s, 1H, Ar-H), 8.00–7.53 (m, 3H, Ar-H), 7.42 (s, 1H, Ar-H), 5.27 (s, 1H, NH), 1.83–1.36 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 185.41, 140.30, 138.81, 125.66, 123.91, 122.61, 119.91, 117.92, 113.97, 113.26, 104.32, 101.56, 56.09, 40.06, 35.61, 28.92. HRMS (ESI) calcd for C₂₆H₂₆N₅ [M + H]⁺ 408.2188, found 408.2158.

N-(1-Adamantyl)-2-(1-methylindol-3-yl)imidazo[1,2-a]pyridin-3-amine 21g. 26% yield; mp 270–272 °C. IR (KBr, cm⁻¹): 3435 (N–H), 3223, 3049 (C–H, Ar), 2906, 2848 (CH₂, CH), 1558 (C–C, Ar), 1481 (C=N), 1242 (C–N). ¹H NMR (400 MHz, CDCl₃) δ 8.43 (d, J = 7.4 Hz, 2H, Ar-H), 8.17–7.82 (m, 2H, Ar-H), 7.39–7.05 (m, 4H, Ar-H), 6.95 (t, J = 6.8 Hz, 1H, Ar-H), 4.98 (s, 1H, NH), 3.55 (s, 3H, –CH₃), 1.81–1.34 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 136.43, 136.36, 132.18, 130.00, 127.10, 124.68, 124.29, 122.19, 121.93, 120.94, 120.73, 115.09, 111.93, 109.47, 100.51, 56.98, 43.55, 35.89, 32.92, 29.45. HRMS (ESI) calcd for C₂₆H₂₉N₄ [M + H]⁺ 397.2392, found 397.2366.

N-(1-Adamantyl)-2-(5-chloro-1H-indol-3-yl)imidazo[1,2-a]pyridin-3-amine 21h. 70% yield; mp 282–284 °C. IR (KBr, cm⁻¹): 3421 (N–H), 3234, 3082 (C–H, Ar), 2906, 2848 (CH₂, CH), 1558 (C–C, Ar), 1458 (C=N), 1259 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 12.24 (s, 1H, NH), 8.92 (d, J = 6.8 Hz, 1H, Ar-H), 8.48–8.11 (m, 2H, Ar-H), 8.07–7.70 (m, 2H, Ar-H), 7.63–7.38 (m, 2H, Ar-H), 7.24 (dd, J = 8.6, 1.8 Hz, 1H, Ar-H), 5.36 (s, 1H, NH), 1.83–1.35 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 136.95, 134.79, 139.04, 124.46, 120.34, 116.07, 113.67, 111.55, 102.37, 56.16, 40.09, 35.55, 28.90. HRMS (ESI) calcd for C₂₅H₂₆ClN₄ [M + H]⁺ 417.1846, found 417.1814.

N-(1-Adamantyl)-2-(6-methyl-1H-indol-3-yl)imidazo[1,2-a]pyridin-3-amine 21i. 32% yield; mp 259–261 °C. IR (KBr, cm⁻¹): 3427 (N–H), 3159, 3115, 3076 (C–H, Ar), 2900, 2845 (CH₂, CH), 1591 (C–C, Ar), 1498 (C=N), 1273 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 11.06 (d, J = 1.5 Hz, 1H, NH), 8.36 (dd, J = 28.5, 7.5 Hz, 2H, Ar-H), 8.05 (d, J = 2.4 Hz, 1H, Ar-H), 7.45 (d, J = 8.9 Hz, 1H, Ar-H), 7.17 (s, 1H, Ar-H), 7.15–7.06 (m, 1H, Ar-H), 6.94–6.77 (m, 2H, Ar-H), 4.57 (s, 1H, Ar-H), 2.48 (s, 3H, NH), 1.86–1.39 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 140.96, 136.58, 136.30, 129.99, 124.41, 120.60, 115.89, 111.02, 110.48, 109.49, 55.89, 43.29, 35.86, 29.02, 21.39. HRMS (ESI) calcd for C₂₆H₂₉N₄ [M + H]⁺ 397.2392, found 397.2360.

N-(1-Adamantyl)-2-(1H-indol-5-yl)imidazo[1,2-a]pyridin-3-amine 21j. 53% yield; mp 263–265 °C. IR (KBr, cm⁻¹): 3427 (N–H), 3259, 3176, 3032 (C–H, Ar), 2902, 2846 (CH₂, CH), 1564 (C–C, Ar), 1477 (C=N), 1263 (C–N). ¹H NMR (400 MHz, DMSO-d₆) δ 11.56 (s, 1H, NH), 8.93 (d, J = 6.6 Hz, 1H, Ar-H), 8.34 (s, 1H, Ar-H), 7.92 (d, J = 7.2 Hz, 3H, Ar-H), 7.53 (dd, J = 32.8, 10.1 Hz, 3H, Ar-H), 6.57 (s, 1H, Ar-H), 5.28 (s, 1H, NH), 1.84–1.38 (m, 15H, adamantyl-H). ¹³C NMR (100 MHz, DMSO-d₆) δ 136.63, 136.22, 130.82, 125.94, 120.44, 118.22, 116.21, 111.76, 111.56, 101.77, 56.20, 40.09, 35.56, 28.95. HRMS (ESI) calcd for C₂₅H₂₇N₄ [M + H]⁺ 383.2236, found 383.2205.

Anti-HIV-1 activity assays

The inhibitory activity of the compounds against the laboratory-adapted HIV-1_IIIB strain was examined using the HIV-1_IIIB/TZM-bl indicator cell culture system as previously described.^60,61 Briefly, 1.5 × 10³ TZM-bl cells were seeded in a volume of 300 μL into each well of a 48-well plate and incubated overnight at 37 °C. The cells were then infected with HIV-1_IIIB at a multiplicity of infection (M.O.I.) of 0.1 and incubated in the presence or absence of tested compounds for two days at 37 °C. After that they were lysed in lysis buffer, and the luciferase substrate was added to the cell lysates as recommended by the manufacturer's protocol (Promega). The luciferase activities were quantified using a luciferase reporter assay system (Promega, Madison, WI). The percentage of the compound inhibitory activity was calculated using the following formula: [1 − (E − N)/(P − N)] × 100%, where N represents the negative control, P represents the positive control and E corresponds to the experimental group. The EC₅₀ values (effective concentration for 50% inhibition) were calculated using the computer program Calcusyn. The inhibitory activity of the experimental compounds in primary HIV-1 replication was determined as previously described. Peripheral blood mononuclear cells (PBMCs) were isolated from the blood of healthy donors by standard density gradient centrifugation using a Histopaque-1077 (Sigma). PBMCs were cultured in RPMI 1640 medium containing 15% FBS, 1% penicillin–streptomycin and IL-2 (20 units per mL). Three-day PHA-stimulated (5 μg mL⁻¹) and polybrene-treated (2 μg mL⁻¹) cells were infected with the corresponding primary HIV-1 strain at a M.O.I. of 0.01 in the presence or absence of the tested compounds. The supernatants were collected on day 4 post-infection and tested for p24 antigen expression by ELISA using the RETRO-TEK HIV-1 p24 antigen ELISA kit (USA). The absorbance of the samples at 450 nm (A450) was read using a Tecan GENios microplate reader (Tecan, Switzerland), and the EC₅₀ values were calculated as described above.

Acknowledgements

We are grateful to the NSFC (81573279, 81373255, 81560574), the Key Scientific Research Projects of Ministry of Education (313040), Hubei Province's Outstanding Medical Academic Leader Program, and Hunan Province Cooperative Innovation Center for Molecular Target New Drug Study for support of this research.

References

1. K. Wright, Nature, 1986, 323, 283.
2. J. A. Esté and T. Cihlar, Antiviral Res., 2010, 85, 25–33.
3. D. S. T. David, H. B. Ripin, J. Fortunak, S. M. Basha, N. Bivins, C. N. Boddy, S. Byrn, K. K. Catlin, S. R. Houghton, S. T. Jagadeesh, K. A. Kumar, J. Melton, S. Muneer, L. N. Rao, R. V. Rao, P. C. Ray, N. G. Reddy, R. M. Reddy, K. C. Shekar, T. Silverton, D. T. Smith, R. W. Stringham, G. V. Subbaraju, F. Talley and A. Williams, Org. Process Res. Dev., 2010, 14, 1194–1201.
4. W. L. Jorgensen, M. Bollini, V. V. Thakur, R. A. Domaoal, K. A. Spasov and K. S. Anderson, J. Am. Chem. Soc., 2011, 133, 15686–15696.
5. C. Reynolds, C. B. de Koning, S. C. Pelly, W. A. van Otterlo and M. L. Bode, Chem. Soc. Rev., 2012, 41, 4657–4670.
6. G. W. Pratt, A. Fan and C. M. Klapperich, ACS Biomater. Sci. Eng., 2015, 1, 314–319.
7. S. Massari, D. Daelemans, M. L. Barreca, A. Knezevich, S. Sabatini, V. Cecchetti, A. Marcello, C. Pannecouque and O. Tabarrini, J. Med. Chem., 2010, 53, 641–648.
8. R. Costi, M. Métifiot, F. Esposito, G. C. Crucitti, L. Pescatori, A. Messore, L. Scipione, S. Tortorella, L. Zinzula, E. Novellino, Y. Pommier, E. Tramontano, C. Marchand and R. D. Santo, J. Med. Chem., 2013, 56, 8588–8598.
9. H. J. Stellbrink, Eur. J. Med.Res., 2007, 12, 483–495.
10. S. Castellino, M. R. Groseclose, J. Sigafoos, D. Wagner, M. D. Serres, J. W. Polli, E. Romach, J. Myer and B. Hamilton, Chem. Res. Toxicol., 2013, 26, 241–251.
11. F. R. Alexandre, A. Amador, S. Bot, C. Caillet, T. Convard, J. Jakubik, C. Musiu, B. Poddesu, L. Vargiu, M. Liuzzi, A. Roland, M. Seifer, D. Standring, R. Storer and C. B. Dousson, J. Med. Chem., 2011, 54, 392–395.
12. P. Vernazza, C. Wang, A. Pozniak, E. Weil, P. Pulik, D. A. Cooper, R. Kaplan, A. Lazzarin, H. Valdez, J. Goodrich, J. Mori, C. Craig and M. Tawadrous, JAIDS, J. Acquired Immune Defic. Syndr., 2013, 62, 171–179.
13. G. Moyle, M. Boffito, A. Stoehr, A. Rieger, Z. Shen, K. Manhard, B. Sheedy, V. Hingorani, A. Raney, M. Nguyen, T. Nguyen, V. Ong, L. T. Yeh and B. Quart, Antimicrob. Agents Chemother., 2010, 54, 3170–3178.
14. J. J. Kennedy-Smith, N. Arora, J. R. Billedeau, J. Fretland, J. Q. Hang, G. M. Heilek, S. F. Harris, D. Hirschfeld, H. Javanbakht, Y. Li, W. Liang, R. Roetz, M. Smith, G. Su, J. M. Suh, A. G. Villasenor, J. Wu, D. Yasuda and Z. K. Sweeney, Med. Chem. Commun., 2010, 1, 79–83.
15. S. M. Gomha, M. G. Badrey, M. M. Abdallac and R. K. Arafa, MedChemComm, 2014, 5, 1685–1692.
16. X. Wang, J. Zhang, Y. Huang, R. Wang, L. Zhang, K. Qiao, L. Li, C. Liu, Y. Ouyang, W. Xu, Z. Zhang, L. Zhang, Y. Shao, S. Jiang, L. Ma and J. Liu, J. Med. Chem., 2012, 55, 2242–2250.
17. K. M. Frey, D. E. Puleo, K. A. Spasov, M. Bollini, W. L. Jorgensen and K. S. Anderson, J. Med. Chem., 2015, 58, 2737–2745.
18. J. Hu, Y. Li, Y. Wu, W. Liu, Y. Wang and Y. Li, Chem. Lett., 2015, 44, 645–647.
19. (a) A. Maleki, Helv. Chim. Acta, 2014, 97, 587–593; (b) A. Maleki, Z. Alrezvani and S. Maleki, Catal. Commun., 2015, 69, 29–33; (c) M. J. D. Pires, D. L. Poeira and M. M. B. Marques, Eur. J. Org. Chem., 2015, 7197–7234.
20. U. Balijapalli and S. K. Iyer, Dyes Pigm., 2015, 121, 88–98.
21. A. Maleki, RSC Adv., 2014, 4, 64169–64173.
22. K. Pericherla, P. Kaswan, K. Pandey and A. Kumar, Synthesis, 2015, 47, 887–912.
23. Z. Fei, Y. Zhu, M. Liu, F. Jia and A. Wu, Tetrahedron Lett., 2013, 54, 1222–1226.
24. H. Sanaeishoar, R. Nazarpour and F. Mohave, RSC Adv., 2015, 5, 68571–68578.
25. (a) A. Maleki, S. Javanshir and M. Naimabadi, RSC Adv., 2014, 4, 30229–30232; (b) A. K. Bagdi, M. Rahman, S. Santra, A. Majee and A. Hajra, Adv. Synth. Catal., 2013, 355, 1741–1747; (c) S. Santra, A. K. Bagdi, A. Majee and A. Hajra, Adv. Synth. Catal., 2013, 355, 1065–1070; (d) S. Lei, G. Chen, Y. Mai, L. Chen, H. Cai, J. Tan and H. Cao, Adv. Synth. Catal., 2016, 358, 67–73; (e) C. Wang, S. Lei, H. Cao, S. Qiu, J. Liu, H. Deng and C. Yan, J. Org. Chem., 2015, 80, 12725–12732; (f) H. Cao, S. Lei, N. Li, L. Chen, J. Liu, H. Cai, S. Qiu and J. Tan, Chem. Commun., 2015, 51, 1823–1825; (g) H. Cao, X. Liu, L. Zhao, J. Cen, J. Lin, Q. Zhu and M. Fu, Org. Lett., 2014, 16, 146–149; (h) S. Lei, Y. Mai, C. Yan, J. Mao and H. Cao, Org. Lett., 2016, 18, 3582–3585.
26. G. Y. Li, K. H. Jung, H. Lee, M. K. Son, J. Seo, S. W. Hong, Y. Jeong, S. Hong and S. S. Hong, Cancer Lett., 2013, 329, 59–67.
27. R. Ducray, I. Simpson, F. H. Jung, J. W. M. Nissink, P. W. Kenny, M. Fitzek, G. E. Walker, L. T. Ward and K. Hudson, Bioorg. Med. Chem. Lett., 2011, 21, 4698–4701.
28. K. A. Emmitte, B. J. Wilson, E. W. Baum, H. K. Emerson, K. W. Kuntz, K. E. Nailor, J. M. Salovich, S. C. Smith, M. Cheung, R. M. Gerding, K. L. Stevens, D. E. Uehling, R. A. Mook Jr, G. S. Moorthy, S. H. Dickerson, A. M. Hassell, M. A. Leesnitzer, L. M. Shewchuk, A. Groy, J. L. Rowand, K. Anderson, C. L. Atkins, J. Yang, P. Sabbatini and R. Kumar, Bioorg. Med. Chem. Lett., 2009, 19, 1004–1008.
29. C. Jaramillo, J. E. De Diego, C. Hamdouchi, E. Collins, H. Keyser, C. Sánchez-Martínez, M. Del Prado, B. Norman, H. B. Brooks, S. A. Watkins, C. D. Spencer, J. A. Dempsey, B. D. Anderson, R. M. Campbell, T. Legget, B. Patel, R. M. Schultz, J. Espinosa, M. Vieth, F. Zhang and D. E. Timm, Bioorg. Med. Chem. Lett., 2004, 14, 6095–6099.
30. C. Hamdouchi, J. Ezquerra, J. A. Vega, J. J. Vaquero, J. Alvarez-Builla and B. A. Heinz, Bioorg. Med. Chem. Lett., 1999, 9, 1391–1394.
31. M. Lhassani, O. Chavignon, J. M. Chezal, J. C. Teulade, J. P. Chapat, R. Snoeck, G. Andrei, J. Balzarini, E. D. Clercq and A. Gueiffier, Eur. J. Med. Chem., 1999, 34, 271–274.
32. S. Mavel, J. L. Renou, C. Galtier, H. Allouchi, R. Snoeck, G. Andrei, E. D. Clercq, J. Balzarini and A. Gueiffier, Bioorg. Med. Chem., 2002, 10, 941–946.
33. J. B. Veron, H. Allouchi, C. Enguehard-Gueiffier, R. Snoeck, G. Andrei, E. D. Clercq and A. Gueiffier, Bioorg. Med. Chem., 2008, 16, 9536–9545.
34. N. M. Shukla, D. B. Salunke, E. Yoo, C. A. Mutz, R. Balakrishna and S. A. David, Bioorg. Med. Chem., 2012, 20, 5850–5863.
35. T. H. Al-Tel, R. A. Al-Quwasmeh and R. Zaarour, Eur. J. Med. Chem., 2011, 46, 1874–1881.
36. J. T. Starr, R. J. Sciotti, D. L. Hanna, M. D. Huband, L. M. Mullins, H. Cai, J. W. Gage, M. Lockard, M. R. Rauckhorst, R. M. Owen, M. S. Lall, M. Tomilo, H. Chen, S. P. McCurdy and M. R. Barbachyn, Bioorg. Med. Chem. Lett., 2009, 19, 5302–5306.
37. A. Scribner, R. Dennis, S. Lee, G. Ouvry, D. Perrey, M. Fisher, M. Wyvratt, P. Leavitt, P. Liberator, A. Gurnett, C. Brown, J. Mathew, D. Thompson, D. Schmatz and T. Biftu, Eur. J. Med. Chem., 2008, 43, 1123–1151.
38. B. F. McGuiness, A. W. Cole, G. Dong, M. R. Brescia, Y. Shao, I. Henderson, L. L. Rokosz, T. M. Stauffer, N. Mannava, E. F. Kimble, C. Hicks, N. White, P. G. Wines and E. Quadros, Bioorg. Med. Chem. Lett., 2010, 20, 6845–6849.
39. W. M. El-Sayed, W. A. Hussin, Y. S. Al-Faiyz and M. A. Ismail, Eur. J. Pharmacol., 2013, 715, 212–218.
40. M. Frohn, V. Viswanadhan, A. J. Pickrell, J. E. Golden, K. M. Muller, R. W. Bürli, G. Biddlecome, S. C. Yoder, N. Rogers, J. H. Dao, R. Hungate and J. R. Allen, Bioorg. Med. Chem. Lett., 2008, 18, 5023–5026.
41. S. Ulloora, R. Shabaraya, S. Aamir and A. V. Adhikari, Bioorg. Med. Chem. Lett., 2013, 23, 1502–1506.
42. N. Bailey, M. J. Bamford, D. Brissy, J. Brookfield, E. Demont, R. Elliott, N. Garton, I. Farre-Gutierrez, T. Hayhow, G. Hutley, A. Naylor, T. A. Panchal, H. X. Seow, D. Spalding and A. K. Takle, Bioorg. Med. Chem. Lett., 2009, 19, 3602–3606.
43. P. J. Zimmermann, W. Buhr, C. Brehm, A. M. Palmer, M. P. Feth, J. Senn-Bilfinger and W. A. Simon, Bioorg. Med. Chem. Lett., 2007, 17, 5374–5378.
44. U. Grädler, T. Fuchß, W. R. Ulrich, R. Boer, A. Strub, C. Hesslinger, C. Anézo, K. Diederichs and A. Zaliani, Bioorg. Med. Chem. Lett., 2011, 21, 4228–4232.
45. N. Dahan-Farkas, C. Langley, A. L. Rousseau, D. B. Yadav, H. Davids and C. B. de Koning, Eur. J. Med. Chem., 2011, 46, 4573–4583.
46. M. L. Bode, D. Gravestock, S. S. Moleele, C. W. van der Westhuyzen, S. C. Pelly, P. A. Steenkamp, H. C. Hoppe, T. Khan and L. A. Nkabinde, Bioorg. Med. Chem., 2011, 19, 4227–4237.
47. L. Wanka, K. Iqba and P. R. Schreiner, Chem. Rev., 2013, 113, 3516–3604.
48. N. Kolocouris, A. Kolocouris, G. B. Foscolos, G. Fytas, J. Neyts, E. Padalko, J. Balzarini, R. Snoeck, G. Andrei and E. De Clercq, J. Med. Chem., 1996, 39, 3307–3318.
49. G. Fytas, G. Stamatiou, G. B. Foscolos, A. Kolocouris, N. Kolocouris, M. Witvrouw, C. Pannecouque and E. De Clercq, Bioorg. Med. Chem. Lett., 1997, 7, 1887–1890.
50. G. Stamatiou, G. B. Foscolos, G. Fytas, A. Kolocouris, N. Kolocouris, C. Pannecouque, M. Witvrouw, E. Padalko, J. Neyts and E. De Clercq, Bioorg. Med. Chem., 2003, 11, 5485–5492.
51. N. Kolocouris, A. Kolocouris, G. B. Foscolos, G. Fytas, J. Neyts, E. Padalko, J. Balzarini, R. Snoeck, G. Andrei and E. D. Clercq, J. Med. Chem., 1996, 39, 3307–3318.
52. M. E. Burstein, A. V. Serbin, T. V. Khakhulina, I. V. Alymova, L. L. Stotskaya, O. P. Bogdan, E. E. Manukchina, V. V. Jdanov, N. K. Sharova and A. G. Bukrinskaya, Antiviral Res., 1999, 41, 135–144.
53. J. Balzarini, B. Orzeszko-Krzesinska, J. K. Maurin and A. Orzeszko, Eur. J. Med. Chem., 2009, 44, 303–311.
54. X. Han, N. Sun, H. Wu, D. Guo, P. Tien, C. Dong, S. Wu and H. B. Zhou, J. Med. Chem., 2016, 59, 2139–2150.
55. J. Pan, X. Han, N. Sun, H. Wu, D. Lin, P. Tien, H. B. Zhou and S. Wu, RSC Adv., 2015, 5, 55100–55108.
56. X. Han, C. Dong and H. B. Zhou, Adv. Synth. Catal., 2014, 356, 1275–1280.
57. X. Han, H. Wu, W. Wang, C. Dong, P. Tien, S. Wu and H. B. Zhou, Org. Biomol. Chem., 2014, 12, 8308–8317.
58. X. Han, W. Ouyang, B. Liu, W. Wang, P. Tien, S. Wu and H. B. Zhou, Org. Biomol. Chem., 2013, 11, 8463–8475.
59. X. Han, H. Wu, C. Dong, P. Tien, W. Xie, S. Wu and H. B. Zhou, RSC Adv., 2015, 5, 10005–10013.
60. A. A. El-Emam, O. A. Al-Deeb, M. A. Al-Omar and J. Lehmann, Bioorg. Med. Chem., 2004, 12, 5107–5113.
61. J. Balzarini, B. Orzeszko, J. K. Mauri and A. Orzeszko, Eur. J. Med. Chem., 2007, 42, 993–1003.
62. D. B. Salunke, E. Yoo, N. M. Shukla, R. Balakrishna, S. S. Malladi, K. J. Serafin, V. W. Day, X. Wang and S. A. David, J. Med. Chem., 2012, 55, 8137–8151.
63. G. Zhou, D. Wu, B. Snyder, R. G. Ptak, H. Kaur and M. Gochin, J. Med. Chem., 2011, 54, 7220–7231.
64. J. D. Bauman, D. Patel, C. Dharia, M. W. Fromer, S. Ahmed, Y. Frenkel, R. S. Vijayan, J. T. Eck, W. C. Ho, K. Das, A. J. Shatkin and E. Arnold, J. Med. Chem., 2013, 56, 2738–2746.
65. M. L. Bode, A. L. Rousseau, D. Gravestock, S. S. Moleele and C. W. van der Westhuyzen, European Patent WO 2010/032195, 2010.
66. T. An, W. Ouyang, W. Pan, D. Guo, J. Li, L. Li, G. Chen, J. Yang, S. Wu and P. Tien, Antiviral Res., 2012, 94, 276–287.
67. M. L. Bode, D. Gravestock, S. S. Moleele, C. W. van der Westhuyzen, S. Pelly, P. A. Steenkamp, H. C. Hoppe, T. Khan and L. A. Nkabinde, Bioorg. Med. Chem., 2011, 19, 4227–4237.
68. O. Trott and A. J. Olson, J. Comput. Chem., 2010, 31, 455–461.
69. T. An, W. Ouyang, W. Pan, D. Guo, J. Li, L. Li, G. Chen, J. Yang, S. Wu and P. Tien, Antiviral Res., 2012, 94, 276–287.
70. J. J. Martinez-Irujo, M. L. Villahermosa, J. Mercapide, J. F. Cabodevill and E. Santiago, Biochem. J., 1998, 329, 689–698.

---

Footnote

† These two authors contributed equally to this work.

---