The epidermal growth factor receptor (EGFR) is the key mediator in signal transduction pathways that regulate numerous cellular functions, including cell proliferation, survival, adhesion, migration, and differentiation [1, 2]. Activation of EGFR has been observed in the majority of solid tumor patients, including lung, breast, colorectal, ovarian, and bladder cancers [3]. Due to the vital role of EGFR in tumor growth, disruption of its signaling pathway by inhibiting receptor tyrosine kinases with small molecules has become a compelling approach to development of targeted cancer therapy [4–6]. Therefore, several kinase inhibitors targeting EGFR are acknowledged as promising therapeutic drugs for treatment of a wide variety of human cancers [7–9]. Recently many structurally diverse compounds have been synthesized as EGFR inhibitors characterized by efficient anti-proliferative activity, including Erlotinib [10], Icotinib [11], EGFR-IN-12 [12], and WZ3146 [13] (Fig. 1).

Fig. 1.
figure 1

Representative EGFR kinase inhibitors.

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The known inhibitors present a typical pharmacophore feature where the central nitrogen heterocyclic rings, such as quinazoline and pyrimidine, constitute an important scaffold for EGFR inhibitors [14–16]. 3,9-Diazatetraasterane derivatives compose a class of cage compounds characterized by the regular structure, high energy and large lipophilic properties that ensure their potential biological activity (Fig. 2) [17–19]. It is considered that 3,9-diazatetraasteranes can influence upon tumor cells. In an attempt to discover new scaffolds, a series of 3,9-diazatetraasteranes 1 was synthesized as EGFR kinase inhibitors, and molecular docking was performed for demonstrating 3,9-diazatetraasteranes 1 as beneficial agents for EGFR inhibition. Biological evaluation against A431 and HepG2 cell lines was carried out using Erlotinib as a reference.

Fig. 2.
figure 2

Structure of 3,9-diazatetraasteranes 1.

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RESULTS AND DISCUSSION

Molecular docking. The molecular docking study was performed by AutoDock 4.2 (Scripps Research Institute, La Jolla, CA, USA) [20]. The EGFR kinase domain crystal structure (PDB ID: 1M17) was selected as the receptor and Erlotinib was used as the reference [21]. All 3,9-diazatetraasteranes were docked into the binding site, and the binding affinities data are presented in Table 1.

Table 1. Molecular docking results for 3,9-diazatetraasteranes 1
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As shown in Table 1, 3,9-diazatetraasteranes 1 exhibited some binding capacity with EGFR, with ΔG ranging from –8.76 to –4.57 kcal/mol and Ki ranging from 0.38 to 448.77 μM. The binding capacity was similar to that of Erlotinib (ΔG = –7.11 kcal/mol, Ki = 6.09 μM). Among the products 1c (ΔG = –8.76 kcal/mol, Ki = 0.38 μM) exhibited the highest binding capacity with EGFR due to presence of the methoxy group on benzene ring. Nevertheless, introduction of multiple methoxy groups on benzene ring reduced binding of 1e (ΔG = –6.53 kcal/mol, Ki = 16.22 μM), 1f (ΔG = –6.62 kcal/mol, Ki = 14.06 μM) and 1g (ΔG = –6.76 kcal/mol, Ki = 11.14 μM). Introduction of the hydroxyl group in 1b (ΔG = –7.31 kcal/mol, Ki = 4.35 μM) also resulted in higher binding capacity than that of the other compounds. Influence of substituents at nitrogen atom has been speculated to affect the binding capacity. When the substituent at nitrogen atom was the benzyl or aryl group, the binding capacity decreased significantly [1h–1n (R2 = Bn) and 1o–1u (R2 = Ph, 4-Me-Ph or 4-OMe-Ph)].

For investigating the binding site and mode of action, compound 1c was chosen as the representative structure and docked into the active site of EGFR. Compound 1c was nicely bound to the ATP binding pocket of EGFR by the H-bond interactions. The higher binding capacity could be attributed to H-bonds with ASP831, CYS773 and LYS721. The N atom of the cage scaffold formed two hydrogen bonds with hinge residue ASP831, and carbonyl of the ester group formed H-bond with the hinge residue CYS773. The O atom of another ester group formed H-bond with the backbone NH of LYS721 (Fig. 3).

Fig. 3.
figure 3

(a) Binding site of 1c with EGFR. (b) Binding mode of 1c with EGFR.

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Chemistry. Based on the earlier report [22], 3,9-diazatetraasteranes 1 were synthesized by [2+2] photocycloaddition reaction of 1,4-dihydropyridines 2 (Scheme 1).

Scheme 1.
scheme 1

Synthesis of 3,9-diazatetraasteranes 1.

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The starting compounds, 1,4-dihydropyridines 2, were synthesized by the reaction of aromatic aldehydes with the ester and appropriate anilines in acetic acid [23]. Structures of 3,9-diazatetraasteranes 1 are characterized by 1H and 13C NMR, and HRMS. The crystal structure of 1a (CCDC: 1484747) was confirmed by single crystal X-ray diffraction (Fig. 4).

Fig. 4.
figure 4

ORTEP diagram of the crystal structure of 1a.

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Biological evaluation. Biological activity of 3,9-diazatetraasteranes 1 was tested against A431 and HepG2 cell lines by the CCK assay using Erlotinib as a control [24, 25] (Table 2). Both cell lines were usually used as the test cells in the studies of EGFR kinase inhibitors.

Table 2. IC50 values of 3,9-diazatetraasteranes 1 against cancer cell lines
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According to the data accumulated in Table 2, most of target compounds exhibited efficient inhibitory activity against both cell lines, and these indicated that the cage scaffold of 3,9-diazatetraasteranes could maintain and even enhance the antiproliferative activity. Among the products, 1c exhibited the most potent antiproliferative activity against the A431 and HepG2 cell lines compared to positive controls, which may be attributed to introduction of the methoxy groups into benzene ring. Also, 1b and 1d demonstrated high inhibitory activity against both cell lines. Among the compounds with different substituents at nitrogen atom, substitution of the benzyl group (1h–1n) or the aryl group (1o–1u) resulted in a significant reduction in cellular potencies on A431 and HepG2 cell lines in comparison with compounds (1a–1g). Overall, these results were consistent with the molecular docking data.

EXPERIMENTAL

All chemicals were purchased from commercial sources and used without further purification. All solvents were reagent grade and, when necessary, were purified and dried using the standard methods. Irradiation for the photocycloaddition reaction was conducted using a 400 W ACE medium-pressure mercury vapor lamp. The reactions were monitored by TLC on 0.25 mm silica gel plates (60GF-254). Melting points were determined using an X-5 apparatus (open capillaries, uncorrected values). NMR spectra were measured on a Bruker ARX-400 MHz spectrometer using TMS as an internal standard and CDCl3 as a solvent, unless indicated otherwise. High-resolution mass spectra (HRMS) were measured on an Agilent G3250AA LC/MSD TOF mass spectrometer with an ESI source.

Synthesis of 1,4-dihydropyridines 2. A mixture of an aromatic aldehyde (0.05 mol) with ethyl propiolate (0.10 mol), amines (0.05 mol) and 5.0 mL acetic acid was heated in a steam bath for 25 min then crystallized from methanol/water (vol : vol = 4 : 1) and recrystallized from acetone/hexane (vol : vol = 1 : 1) to give the pure product.

General procedure for synthesis of 3,9-diazatetraasteranes 1a–1u. The desired 1,4-dihydropyridine 2 (5 mmol) was dissolved in 100 mL solution of THF/methanol (vol : vol = 1 : 1), which then was poured into a photochemical reactor. The solution was irradiated by a 400 W mercury lamp under the atmosphere of nitrogen. The process was complete within 5–6 h. Then the mixture was concentrated under reduced pressure, the precipitate was filtered off and recrystallized from dichloromethane/methanol (vol : vol = 4 : 1) to give the corresponding pure 3,9-diazatetraasterane.

Tetraethyl 6,12-diphenyl-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1a).

structure 1

Yield 41%, white solid, mp 208.5–211.2°C. 1H NMR spectrum, δ, ppm: 0.99 t (12H, CH3), 3.92–4.00 m (8H, CH2), 3.96 s (2H, CH), 4.35 s (4H, CH), 7.15–7.51 m (10H, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 43.7, 48.7, 55.0, 60.8, 126.8, 127.8, 130.8, 137.3, 173.1. HRMS (ESI): m/z: calcd 603.2701 for C34H39N2O8 [M + H]+, found 603.2704.

Tetraethyl 6,12-bis(4-hydroxyphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1b).

structure 2

Yield 49%, white solid, mp 218.8–220.7°C. 1H NMR spectrum (DMSO-d6), δ, ppm: 0.92 t (12H, CH3), 3.72 s (2H, CH), 3.82–3.90 m (8H, CH2), 3.98 s (4H, CH), 4.44 s (2H, NH), 6.52 d (4H, J = 8.6 Hz, Ar-H), 7.22 d (4H, J = 8.6 Hz, Ar-H), 9.12 s (2H, OH). 13C NMR spectrum (DMSO-d6), δ, ppm: 14.3, 43.0, 49.0, 54.9, 60.4, 114.6, 128.8, 132.3, 156.1, 173.7. HRMS (ESI): m/z: calcd 635.2599 for C34H39N2O10 [M + H]+, found 635.2597.

Tetraethyl 6,12-bis(4-methoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1c).

structure 3

Yield 52%, white solid, mp 225.7–226.9°C. 1H NMR spectrum, δ, ppm: 1.02 t (12H, CH3), 3.72 s (6H, OCH3), 3.95–4.04 m (8H, CH2), 4.24 s (2H, CH), 4.26 s (4H, CH), 6.58 d (4H, J = 8.6 Hz, Ar-H), 7.35 d (4H, J = 8.6 Hz, Ar-H). 13C NMR spectrum, δ, ppm: 13.9, 43.1, 49.2, 55.2, 60.3, 114.6, 128.7, 132.3, 156.4, 173.2. HRMS (ESI): m/z: calcd 663.2912 for C36H43N2O10 [M + H]+, found 663.2916.

Tetraethyl 6,12-bis(2,4-dimethoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1d).

structure 4

Yield 54%, white solid, mp 231.4–233.7°C. 1H NMR spectrum, δ, ppm: 1.00 t (12H, CH3), 3.74 s (6H, OCH3), 3.79 s (6H, OCH3), 3.89–3.94 m (8H, CH2), 4.32 s (4H, CH), 4.48 s (2H, CH), 4.44 s (2H, NH), 6.32 d (2H, J = 8.7 Hz, Ar-H), 6.41 d (2H, J = 8.7 Hz, Ar-H), 7.86 s (2H, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 33.9,48.8, 55.1, 55.3, 55.4, 60.5, 97.8, 103.9,118.8, 133.8, 158.5, 159.4, 173.6. HRMS (ESI): m/z: calcd 723.3124 for C38H47N2O12 [M + H]+, found 723.3128.

Tetraethyl 6,12-bis(2,3,4-trimethoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1e).

structure 5

Yield 20%, white solid, mp 213.5–214.9°C. 1H NMR spectrum, δ, ppm: 1.02 t (12H, CH3), 3.52 s (2H, NH), 3.78 s (6H, OCH3), 3.79 s (6H, OCH3), 3.90 s (6H, OCH3), 3.88–3.99 m (8H, CH2), 4.28 s (4H, CH), 4.33 s (2H, CH), 6.57 d (2H, J = 9.0 Hz, Ar-H), 7.71 d (2H, J = 9.0 Hz, Ar-H). 13C NMR spectrum, δ, ppm: 13.9, 34.3, 48.8, 55.3, 56.0, 56.3, 56.4, 60.6, 96.6, 117.5, 142.7, 148.0, 152.0, 173.5. HRMS (ESI): m/z: calcd 783.3335 for C40H51N2O14 [M + H]+, found 783.3338.

Tetraethyl 6,12-bis(2,4,5-trimethoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1f).

structure 6

Yield 43%, white solid, mp 217.2–220.4°C. 1H NMR spectrum, δ, ppm: 0.99 t (12H, CH3), 3.22 s (2H, NH), 3.78 s (6H, OCH3), 3.79 s (6H, OCH3), 3.84 s (6H, OCH3), 3.87–3.96 m (8H, CH2), 4.28 s (4H, CH), 4.51 s (2H, CH), 6.39 s (2H, Ar-H), 7.93 s (2H, Ar-H). 13C NMR spectrum, δ, ppm: 13.9, 33.1, 48.9, 55.6, 55.7, 60.4, 60.7, 60.8, 106.6, 127.3, 141.2, 152.4, 152.5, 173.6. HRMS (ESI): m/z: calcd 783.3335 for C40H51N2O14 [M + H]+, found 783.3331.

Tetraethyl 6,12-bis(3,4,5-trimethoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1g).

structure 7

Yield 55%, white solid, mp 218.5–221.1°C. 1H NMR spectrum, δ, ppm: 1.03 t (12H, CH3), 3.79 s (6H, OCH3), 3.80 s (12H, OCH3), 3.83 s (2H, CH), 3.97–4.03 m (8H, CH2), 4.31 s (4H, CH), 6.85 s (4H, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 33.3, 46.9, 51.6, 56.0, 56.5, 57.2,60.3, 97.0, 116.5, 143.2, 149.3, 154.4, 173.8. HRMS (ESI): m/z: calcd 783.3335 for C40H51N2O14 [M + H]+, found 783.3337.

Tetraethyl 3,9-dibenzyl-6,12-diphenyl-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1h).

structure 8

Yield 38%, white solid, mp 233.7–235.2°C. 1H NMR spectrum, δ, ppm: 0.98 t (12H, CH3), 3.94–4.02 m (8H, CH2), 4.28 s (4H, CH), 4.30 s (2H, CH), 4.49 s (4H, CH2), 7.07–7.33 m (20H, Ar-H). 13C NMR spectrum, δ, ppm: 13.9, 42.6, 50.9, 59.0, 60.8, 60.9, 126.7, 127.4, 127.5, 128.2, 129.5, 130.6, 137.4, 137.9, 173.6. HRMS (ESI): m/z: calcd 783.3640 for C48H51N2O8 [M + H]+, found 783.3645.

Tetraethyl 3,9-dibenzyl-6,12-bis(4-hydroxyphenyl)-3,9-diazahexacyclo[6.4.0.02,7.04,11.05,10]dodecane-1,5,7,11-tetracarboxylate (1i).

structure 9

Yield 53%, white solid, mp 240.4–243.5°C. 1H NMR spectrum (DMSO-d6), δ, ppm: 0.98 t (12H, CH3), 3.82–3.98 m (8H, CH2), 4.10 s (4H, CH), 4.15 s (2H, CH), 4.41 s (4H, CH2), 6.49 d (4H, J = 8.5 Hz, Ar-H), 6.98 d (4H, J = 8.5 Hz, Ar-H), 7.28–7.35 m (10H, Ar-H), 9.19 s (2H, OH). 13C NMR spectrum (DMSO-d6) δ, ppm: 13.6, 41.6, 51.0, 58.7, 60.2, 60.9, 126.0, 127.4, 127.5, 128.1, 129.5, 131.2, 137.3, 154.8, 172.9. HRMS (ESI): m/z: calcd 815.3538 for C48H51N2O10 [M + H]+, found 815.3543.

Tetraethyl 3,9-dibenzyl-6,12-bis(4-methoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1j).

structure 10

Yield 43%, white solid, mp 171.2–173.5°C. 1H NMR spectrum, δ, ppm: 1.01 t (12H, CH3), 3.75 s (6H, OCH3), 3.89–3.99 m (8H, CH2), 4.24 s (2H, CH), 4.21 s (4H, CH), 4.48 s (4H, CH2), 6.59 d (4H, J = 8.8 Hz, Ar-H), 7.13 d (4H, J = 8.8 Hz, Ar-H), 6.88–7.34 m (10H, Ar-H). 13C NMR spectrum, δ, ppm: 13.6, 41.6, 51.2, 55.0, 58.7, 60.1, 60.5, 125.6, 127.4, 127.8, 128.1, 129.4, 131.1, 137.3, 154.4, 172.1. HRMS (ESI): m/z: calcd 843.3851 for C50H55N2O10 [M + H]+, found 843.3855.

Tetraethyl 3,9-dibenzyl-6,12-bis(2,4-dimethoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1k).

structure 11

Yield 51%, white solid, mp 224.2–226.5°C. 1H NMR spectrum, δ, ppm: 0.94 t (12H, CH3), 3.75 s (6H, OCH3), 3.77 s (6H, OCH3), 3.86–3.91 m (8H, CH2), 4.23 s (4H, CH), 4.52 s (4H, CH2), 4.76 s (2H, CH), 6.18 d (2H, J = 8.64 Hz, Ar-H), 7.29 s (2H, Ar-H), 7.23–7.42 m (10H, Ar-H); 7.46 d (2H, J = 8.64 Hz, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 33.2, 50.9, 55.2, 55.5, 59.3, 60.5, 60.9, 97.2, 103.6, 119.4, 127.2, 128.1, 129.6, 133.7, 137.9, 158.5, 159.3, 173.9. HRMS (ESI): m/z: calcd 903.4603 for C52H59N2O12 [M + H]+, found 903.4607.

Tetraethyl 3,9-dibenzyl-6,12-bis(2,3,4-trimethoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1l).

structure 12

Yield 55%, white solid, mp 233.5–235.7°C. 1H NMR spectrum, δ, ppm: 1.01 t (12H, CH3), 3.80 s (6H, OCH3), 3.81 s (6H, OCH3), 3.89–3.98 m (8H, CH2), 3.92 s (6H, OCH3), 4.25 s (4H, CH), 4.53 s (4H, CH2), 4.65 s (2H, Ar-H), 6.34 d (2H, J = 8.92 Hz, Ar-H), 7.20 d (2H, J = 8.92 Hz, Ar-H), 7.28–7.44 m (10H, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 34.3, 51.0, 55.8, 59.3, 60.3, 60.5, 60.7, 61.1, 106.3, 124.2, 127.3, 127.4, 128.1, 129.7, 137.7, 141.1, 152.2, 152.3, 173.7. HRMS (ESI): m/z: calcd 963.4274 for C54H63N2O14 [M + H]+, found 963.4277.

Tetraethyl 3,9-dibenzyl-6,12-bis(2,4,5-trimethoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1m).

structure 13

Yield 43%, white solid, mp 230.7–232.5°C. 1H NMR spectrum, δ, ppm: 0.92 t (12H, CH3), 3.37 s (6H, OCH3), 3.79 s (6H, OCH3), 3.76–3.95 m (8H, CH2), 3.86 s (6H, OCH3), 4.31 s (4H, CH), 4.57 s (4H, CH2), 4.72 s (2H, CH3), 6.39 s (2H, Ar-H), 7.15 s (2H, Ar-H), 7.20–7.36 m (10H, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 42.1, 51.7, 55.9, 59.4, 60.3, 60.5, 61.0, 107.1, 125.2, 127.1, 128.2, 128.5, 133.3, 137.1, 140.2, 152.3, 173.8. HRMS (ESI): m/z: calcd 963.4274 for C54H63N2O14 [M + H]+, found 963.4279.

Tetraethyl 3,9-dibenzyl-6,12-bis(3,4,5-trimethoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1n).

structure 14

Yield 63%, white solid, mp 219.6–223.4°C. 1H NMR spectrum, δ, ppm: 0.99 t (12H, CH3), 3.58 s (12H, OCH3), 3.79 s (6H, OCH3), 4.01–4.07 m (8H, CH2), 4.23 s (2H, CH), 4.33 s (4H, CH), 4.52 s (4H, CH2), 6.46 s (4H, Ar-H), 7.22 m (10H, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 42.4, 51.1, 55.9, 59.7, 60.5, 60.8, 61.0, 107.9, 127.3, 128.3, 128.4, 133.9, 137.0, 137.6, 152.2, 173.8. HRMS (ESI): m/z: calcd 963.4274 for C54H63N2O14 [M + H]+, found 963.4272.

Tetraethyl 3,9-diphenyl-6,12-diphenyl-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1o).

structure 15

Yield 62%, white solid, mp 255.8–256.6°C. 1H NMR spectrum δ, ppm: 0.97 t (12H, CH3), 3.92–4.05 m (8H, CH2), 3.96 s (2H, CH), 5.23 s (4H, CH), 6.94–7.37 m (20H, Ar-H). 13C NMR spectrum, δ, ppm: 13.7, 44.1, 51.8, 57.9, 61.2, 117.5, 120.4, 127.0, 128.0, 129.5, 130.2, 136.6, 149.7, 172.8. HRMS (ESI): m/z: calcd 755.3327 for C46H47N2O8 [M + H]+, found 755.3331.

Tetraethyl 3,9-diphenyl-6,12-bis(4-methylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1p).

structure 16

Yield 65%, white solid, mp 288.7–289.9°C. 1H NMR spectrum, δ, ppm: 0.99 t (12H, CH3), 2.18 s (6H, CH3), 3.92 s (2H, CH), 3.94–4.07 m (8H, CH2), 5.22 s (4H, CH), 6.82 d (4H, J = 8.0 Hz, Ar-H), 6.94–7.36 m (10H, Ar-H), 6.96 d (4H, J = 8.0 Hz, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 20.9, 43.6, 51.8, 57.8, 61.1, 117.4, 120.3, 128.7, 129.4, 130.0, 133.5, 136.4, 149.7, 172.9. HRMS (ESI): m/z: calcd 783.3640 for C48H51N2O8 [M + H]+, found 783.3643.

Tetraethyl 3,9-diphenyl-6,12-bis(4-hydroxyphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1q).

structure 17

Yield 63%, white solid, mp >300.0°C. 1H NMR spectrum (DMSO-d6), δ, ppm: 0.94 t (12H, CH3), 3.71 s (2H, CH), 3.88–4.02 m (8H, CH2), 5.07 s (4H, CH), 6.42 d (4H, J = 8.4 Hz, Ar-H), 6.74 d (4H, J = 8.4 Hz, Ar-H), 6.93– 7.41 m (10H, Ar-H). 13C NMR spectrum, δ, ppm: 14.1, 43.0, 51.6, 57.4, 61.4, 115.1, 116.8, 120.8, 126.7, 1230.2, 131.1, 149.5, 156.5, 172.6. HRMS (ESI): m/z: calcd 787.3225 for C46H47N2O10 [M + H]+, found 787.3228.

Tetraethyl 3,9-diphenyl-6,12-bis(4-methoxylphenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1r).

structure 18

Yield 63%, white solid, mp 265.1–266.8°C. 1H NMR spectrum (DMSO-d6), δ, ppm: 1.01 t (12H, CH3), 3.67 s (6H, OCH3), 3.92 s (2H, CH), 3.94–4.05 m (8H, CH2), 5.21 s (4H, CH), 6.58 d (4H, J = 8.8 Hz, Ar-H), 6.95– 7.37 m (10H, Ar-H), 7.01 d (4H, J = 8.8 Hz, Ar-H). 13C NMR spectrum, δ, ppm: 13.9, 43.2, 51.8, 55.0, 57.7, 61.2, 113.3, 117.5, 120.4, 128.6, 129.5, 131.4, 149.7, 158.4, 172.9. HRMS (ESI): m/z: calcd 815.3538 for C48H51N2O10 [M + H]+, found 815.3535.

Tetraethyl 3,9-diphenyl-6,12-bis(4-fluorophenyl)-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1s).

structure 19

Yield 63%, white solid, mp 264.6–265.9°C. 1H NMR spectrum (DMSO-d6), δ, ppm: 0.99 t (12H, CH3), 3.94–4.07 m (8H, CH2), 3.98 s (2H, CH), 5.21 s (4H, CH), 6.75 d (4H, J = 8.8 Hz, Ar-H), 6.97–7.40 m (10H, Ar-H), 7.09 d (4H, J = 8.8 Hz, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 43.3, 51.7, 57.7, 61.3, 114.8/ 115.0, 117.5, 120.8, 129.6, 131.9/ 132.0, 132.3/ 132.4, 149.5, 160.6, 163.1, 172.3; HRMS (ESI): m/z: calcd 791.3138 for C46H45F2N2O8 [M + H]+, found 791.3140.

Tetraethyl 3,9-bis(4-methylphenyl)-6,12-diphenyl-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1t).

structure 20

Yield 43%, white solid, mp 280.5–281.7°C. 1H NMR spectrum, δ, ppm: 0.98 t (12H, CH3), 2.21 s (6H, CH3), 3.92–4.05 m (8H, CH2), 3.94 s (2H, CH), 5.23 s (4H, CH), 6.86 d (4H, J = 8.0 Hz, Ar-H), 6.94–7.37 m (10H, Ar-H), 6.98 d (4H, J = 8.0 Hz, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 20.7, 44.0, 51.8, 57.9, 61.2, 117.5, 120.4, 127.0, 128.0, 129.5, 130.2, 136.6, 149.7, 172.8; HRMS (ESI): m/z: calcd 783.3640 for C48H51N2O8 [M + H]+, found 783.3644.

Tetraethyl 3,9-bis(4-methoxylphenyl)-6,12-diphenyl-3,9-diazahexacyclo[6.4.0.0 2,7 .0 4,11 .0 5,10 ]dodecane-1,5,7,11-tetracarboxylate (1u).

structure 21

Yield 68%, white solid, mp 254.2–255.7°C. 1H NMR spectrum, δ, ppm: 0.96 t (12H, CH3), 3.83 s (6H, OCH3), 3.94–4.00 m (8H, CH2), 4.02 s (2H, CH), 5.09 s (4H, CH), 6.91 d (4H, J = 8.8 Hz, Ar-H), 7.07–7.13 m (10H, Ar-H), 7.32 d (4H, J = 8.8 Hz, Ar-H). 13C NMR spectrum, δ, ppm: 13.8, 44.2, 51.8, 55.6, 58.5, 61.1, 114.7, 119.2, 126.9, 128.0, 130.3, 136.9, 143.9, 153.9, 173.0. HRMS (ESI): m/z: calcd 815.3538 for C48H51N2O10 [M + H]+, found 815.3542.

Molecular docking. Molecular docking was performed using AutoDock 4.2 based on the Lamarckian genetic algorithm (LGA) to explore the full range of ligand conformational flexibility with partial flexibility of the receptor. The three-dimensional structures of the ligands were constructed using standard bond lengths and bond angles of the GaussView (Gaussian, Inc., Wallingford, CT, USA). Geometry optimization involved the semi-empirical Austin Model 1 (AM1) method, and the output files were then minimized using the density functional (DFT) method by applying the B3LYP (Becke, Lee, Yang, and Parr) correlation function in the course of the second optimization. The Gasteiger partial charges were assigned using AutoDock Tools. The crystal structure of EGFR complexed with Erlotinib (PDB code: 1M17) was retrieved from the Protein Data Bank. The co-crystallized ligand was extracted from the PDB file, and AutoDock Tools was used to assign polar hydrogens and Gasteiger charges. AutoGrid was used to create affinity grid maps for all atom types. The affinity grids enclosed a volumetric space of 60, 60, and 60 Å in the x, y, and z directions with a grid spacing of 0.375 Å. The docked conformations were generated using the LGA with an initial population size. The rest of the parameters were set to their default values. Docking assessment depended on the size of the data, the estimated free binding energy (ΔG) and the estimated inhibition constant (Ki). The model analysis was performed using the Discovery Studio (Accelrys, Inc., San Diego, CA, U.S.A.).

Biological evaluation. Biological activity of 3,9-diazatetraasteranes 1 was evaluated using the CCK-8 assay on A431 and HepG2 cell lines. The cells (5×103 per well in 96-well plates) were incubated at 37°C overnight in a humidified incubator containing 5% CO2. The target compounds were added to the culture medium at the indicated final concentrations, and the cell cultures were stored for 48 h. Then 10% CCK-8 solution was added into each well, and the cells were further incubated at 37°C for 2 h more. Cell survival was determined by measuring absorbance at 470 nm using a microplate reader, and IC50 value was calculated from a log plot.

CONCLUSIONS

In summary, a novel series of 3,9-diazatetraasterane derivatives has been synthesized as potential EGFR tyrosine kinase inhibitors, and their structures have been characterized by NMR, HRMS and single crystal X-ray diffraction. Molecular docking has been performed with the ligand binding site of EGFR, indicating that 3,9-diazatetraasterane derivatives have exhibited high binding capacity with it. Biological activity has been evaluated against A431 and HepG2 cell lines using CCK-8 assay with Erlotinib as a reference. The compound compound 1c has been determined as the most active product against both cell lines, compared to positive controls. Also, 1b and 1d have demonstrated the higher inhibitory activity against A431 and HepG2. The above results indicate that 3,9-diazatetraasteranes demonstrate high potential for further development as the novel EGFR inhibitors.