Catalyst-free synthesis of benzofuran-fused pyrido[4,3-d]pyrimidines from 2-(2-hydroxyaryl)acetonitrile and 4,6-dichloropyrimidine-5-carbaldehyde through domino condensation reactions

Abstract

A rapid, one-pot, catalyst-free approach to novel benzofuran-fused pyrido[4,3-d]pyrimidines with good antitumor activities via a cascade S_NAr/cyclization/condensation reaction through 2-(2-hydroxyphenyl)acetonitriles and 4,6-dichloropyrimidine-5-carbaldehyde was developed.

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Introduction

The pursuit of large diversified compound libraries in drug discovery and other organic synthesis continues to stimulate the design and development of new concepts and innovative synthetic strategies. The use of cascade reactions is one of the most efficient strategies for rapid generation of complex molecules from simple starting materials and multiple-step reactions are occurred in on pot.¹ What is more, it is easier to operate with only one single reaction solvent, workup procedure, and purification step than traditional synthetic methods with several individual steps.² In addition, it is well accepted that cascade reactions were associated with economies of time, labour, cost, and waste generation, which is advantageous from the viewpoint of green chemistry and sustainability.³ Therefore, cascade reactions are ideal synthetic tools to address the demand of constructing diverse heterocycles with high efficiency and environmentally benign reaction conditions.

Heterocyclic rings are of remarkable biological and chemical significance in many fields. Benzofurans or fused benzofurans are ubiquitous in compounds displaying interesting biological properties such as antifungal agents⁴ and inhibitors of tubulin polymerization,⁵ hepatitis C virus RNA-dependent RNA polymerase⁶ and mycobacterium protein tyrosine phosphatase B (mPTPB).⁷ Moreover, fused-polycyclic compounds also occur in many natural products and biologically active molecules with potent bioactivities (Fig. 1).⁸ Thus a simple and facile synthetic method for generation of these heterocyclic molecules is highly desirable.

Fig. 1 Fused-polycyclic compounds in natural and medicinal compounds.

Over the last few decades, both (2-hydroxyphenyl)acetonitrile⁹ and 4,6-dichloropyrimidine-5-carboxaldehyde¹⁰ have been widely applied in drug discovery owing to their potential for the synthesis of various bioactive compounds as precursors, especially (2-hydroxyphenyl)acetonitrile and related compounds have been used to construct bioactive 2-aminobenzofurans.^9a,11

Recently, our group and others have reported a similar method for preparation of dibenzo[b,f]oxepins and their analogues via a one-pot cascade reaction through 2-(2-hydroxyphenyl)acetonitriles and 2-bromobenzaldehydes (Scheme 1).^9f,i Herein, we treated 4,6-dichloropyrimidine-5-carboxaldehyde with 2-(2-hydroxyphenyl)acetonitrile under similar conditions. Much to our surprise, 2-aminobenzofuran-containing tetracyclic product B, determined by X-ray crystallography (Fig. 2), was obtained but not dibenzo[b,f]oxepin product A, even though only one equivalent 2-(2-hydroxyphenyl)acetonitrile was used. The reaction might go through a cascade S_NAr/cyclization/condensation reaction pathway (scheme 1, path B) rather than a sequential aldol condensation and intramolecular ether formation reaction pathway (scheme 1, path A).

Scheme 1 Established methods for synthesis of novel fused-polycyclic heterocycles.

Fig. 2 ORTEP plot of 3a.

Therefore, a highly efficient and eco-friendly synthetic method to construct various benzofuran-containing tetracyclic compounds was developed from inexpensive, readily available starting materials via a cascade S_NAr/cyclization/condensation reaction and most of these fused-tetracyclic compounds showed antiproliferative activity in vitro. Among the compounds tested, compound 3k and the control etoposide (VP-16) were almost equally potent in SRB assay (Table 3).

Results and discussion

At the outset of our study, the reaction of 2-(2-hydroxyphenyl)acetonitrile (1a) and 4,6-dichloropyrimidine-5-carbaldehyde (2) was used to optimize the reaction conditions (Table 1). Firstly, we wondered whether the catalyst was necessary in the reaction, and finally we were pleased to find that the catalyst-free reaction afforded the product (3a) in an even higher yield compared to the yield with catalyst (Table 1, entries 1 and 2). Then, different bases were tested. Among the base screened, Cs₂CO₃ offered the best yield (Table 1, entries 2–5). Next, the solvent THF and toluene were examined and found to provide moderate yields (Table 1, entries 6 and 7). The amount of base was modulated. By decreasing the amount of Cs₂CO₃ (Table 1, entries 8 and 9), a slightly lower yield was obtained. Finally, when the reaction temperature was raised to 120 °C, the reaction was completed within 1 h yet with a decreased yield (Table 1, entry 10). The yield of product reduced to 73% when the reaction temperature was lowered to 80 °C (Table 1, entry 11). In addition, the reaction time was assessed (Table 1, entries 12 and 13) and we found the reaction couldn't perform completely in 0.5 h. Neither did longer reaction times afford higher yields. Hence, the optimal reaction involved 1a (0.4 mmol), 2 (0.2 mmol), DMF (3 mL) and Cs₂CO₃ (3.0 equiv.) at 100 °C for 1 h under a nitrogen atmosphere.

Table 1 Optimization of the reaction conditions^a

Entry	Base (eq.)	CuI (eq.)	Solvent	Time (h)	Temp. (°C)	Yield^b (%)
a Reaction conditions: 1a (53 mg, 0.4 mmol), 2 (35 mg, 0.2 mmol), solvent (3 mL) under N2. b Isolated yield of 3a.
1	Cs2CO3(3)	0.05	DMF	1	100	74
2	Cs2CO3(3)	0	DMF	1	100	84
3	K2CO3(3)	0	DMF	1	100	50
4	EtONa(3)	0	DMF	1	100	40
5	DBU(3)	0	DMF	1	100	63
6	Cs2CO3(3)	0	toluene	1	100	43
7	Cs2CO3(3)	0	THF	1	65	46
8	Cs2CO3(2)	0	DMF	1	100	73
9	Cs2CO3(1.2)	0	DMF	1	100	63
10	Cs2CO3(3)	0	DMF	1	120	74
11	Cs2CO3(3)	0	DMF	1	80	73
12	Cs2CO3(3)	0	DMF	0.5	100	66
13	Cs2CO3(3)	0	DMF	2	100	79

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The scope and limitations of the reaction were investigated under the optimized reaction conditions. As illustrated in Table 2, both electron-donating and electron-withdrawing groups substituted 2-(2-hydroxyphenyl)acetonitrile could be transformed into the desired products. The yields ranged from 51% to 85%. The positions of substituent had no significant influence on this transformation. Substrates possessing weak electron-withdrawing groups such as chloride and bromine were successfully employed in the reaction (3d, 3i–j), while the fluoride substituent gave a slightly lower yield (3c, 3h). Functional groups at the 3-position included –OCH₃ and –F gave similar results (3b, 3c). Both alkyl and aromatic groups on the 2-(2-hydroxyphenyl)acetonitriles were suitable for substrates (3k–l, 3o–p).

Table 2 Synthesis of 3a–3p under optimized conditions^a

Entry	Substrate	R	Product	Yield^b (%)
a Reaction conditions: 1a–1p (0.4 mmol), 2 (35 mg, 0.2 mmol), Cs2CO3 (195 mg, 0.6 mmol), DMF (3 mL) at 100 °C for 1 h under N2. b Isolated yield of 3.
1	1a	H	3a	84
2	1b	3-OCH3	3b	68
3	1c	3-F	3c	64
4	1d	4-Br	3d	73
5	1e	4-N(CH3)2	3e	58
6	1f	4-OCH3	3f	51
7	1g	4-CF3	3g	57
8	1h	5-F	3h	53
9	1i	5-Cl	3i	85
10	1j	5-Br	3j	72
11	1k	5-CH3	3k	55
12	1l	5-CH(CH3)2	3l	78
13	1m	5-NO2	3m	69
14	1n	5-CN	3n	51
15	1o	5-phenyl	3o	74
16	1p	—	3p	79

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Next, the scope of compound 2 was expanded to other halogen substituted aromatic aldehydes. No desired product was obtained when 2,6-dibromobenzaldehyde/3,5-dichloroisonicotinaldehyde was employed even under copper-assisted conditions. 2,6-Dibromobenzaldehyde gave condensation products and only a trace product could be detected by LC-MS when 3,5-dibromoisonicotinaldehyde was used in this reaction.

A mechanistic proposal for the synthesis is depicted in Scheme 2. The starting material 1a and 2 underwent a nucleophilic aromatic substitution (S_NAr) reaction to obtain intermediate A, followed by sequential cyclization to form the intermediate B, which could undergo isomerization to afford C. The mechanism of cyclization was similar to the literature.^4a Finally, the intermediate C underwent dehydrated aromatization to form the product 3a.

Scheme 2 Proposed reaction mechanism.

Subsequently, some of these products have been evaluated for their antitumor activity against human hepatic carcinoma cell line BEL-7402, colon cancer cell line HT29 and stomach cancer cell line SGC-7901. The results were summarized in Table 3. It suggested that compounds 3g–k showed certain antitumor activities, especially the compound 3k had almost equally potency with VP-16.

Table 3 Inhibitory activity (IC₅₀) of some compounds in vitro^a

Compd	IC50^b^,^c [μM]
	BEL-7402	HT-29	SGC-7901
a The experiments were based on using the SRB assay. b Exposure time: 72 h. c The average IC50 values were determined by at least three independent tests.
3a	95.50	78.73	>100
3e	>100	85.17	>100
3f	4.36	7.38	>100
3g	50.23	24.00	74.10
3h	12.59	13.67	11.04
3i	13.24	17.59	11.73
3j	8.20	16.57	7.45
3k	5.36	4.63	4.45
3l	>100	51.87	>100
3n	>100	>100	>100
VP-16	2.99	4.84	10.03

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Conclusions

In summary, we have completed the catalyst-free synthesis of novel benzofuran-fused pyrido[4,3-d]pyrimidines from two simple substrates via one-pot domino reaction under mild conditions. The flexibility of usable substrates indicated that various substituted 2-(2-hydroxyphenyl)acetonitriles were suitable for this reaction. To our satisfaction, we could obtain novel benzofuran-fused pyrido[4,3-d]pyrimidines with good antitumor activity and the synthetic method involved the formation of at least ten new bonds and three new rings through a one pot, eco-friendly cascade reaction.

Experimental

General information

Analytical thin layer chromatography (TLC) was HSGF254 (0.15–0.2 mm thickness, Yantai Huiyou Company, China). Column chromatography was carried out on silica gel (200–300 mesh). Proton and carbon magnetic resonance spectra (¹H NMR and ¹³C NMR) were recorded on Varian Mercury-300/400 and Varian Mercury-400/500 spectrometers. Tetramethylsilane (TMS) was used as internal standard. Chemical shifts (δ = ) are reported in parts per million (ppm). Data are reported as follows: chemical shift, multiplicity (brs = broad singlet, d = doublet, dd = doublet of doublet, dt = doublet of triplet, td = triplet of doublet, m = multiple, s = singlet and t = triplet), coupling constants (Hz). IR spectra were recorded on a FTIR instrument. HRMS spectra were recorded on a Finnigan/MAT-95 spectrometer. Melting points were measured by Büchi 510 melting point apparatus and were uncorrected.

General procedure of biology

The sulforhodamine B (SRB) assay: cells were seeded into 96-well plates on day 0 and exposed to 2-fold serial drug dilutions on day 1. On day 4, the cells were fixed by adding 10% pre-cooled trichloroacetic acid. After 1 h at 4 °C, the plates were washed with distilled water, dried, and then stained with SRB (Sigma, MO) in 1% acetic acid. SRB in the cells was dissolved in 10 mM Tris–HCl and was measured at 515 nm using spectra-MAX190 (Molecular Devices, CA). The cell proliferation inhibition rate was calculated as: proliferation inhibition (%) = [1 − (A515_treated/A515_control)] × 100%.

General procedure for the synthesis of compounds 3a–3p

2-(2-Hydroxyphenyl)acetonitrile 1a (53 mg, 0.4 mmol), 4,6-dichloropyrimidine-5-carbaldehyde 2 (35 mg, 0.2 mmol), Cs₂CO₃ (195 mg, 0.6 mmol) and anhydrous DMF (3 mL) were added to an oven-dried 25 mL two-necked reaction flask. Then the system was degassed and filled with nitrogen. The reaction mixture was stirred and heated at 100 °C for 1 h. After completion of the reaction, the resulting solution was cooled to room temperature, the mixture was filtrated and the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel using petroleum ether/ethyl acetate (5 : 1) as the eluent to provide the desired products 3a. (3a) contains the supplementary crystallographic data for this paper.

3-(Benzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3a)

Yellow solid (84%), m.p. 276–278 °C; ¹H NMR (400 MHz, DMSO-d₆) δ = 9.48 (s, 1H), 9.29 (s, 1H), 8.51 (d, J = 7.7 Hz, 1H), 8.46 (brs, 2H), 7.89 (d, J = 8.2 Hz, 1H), 7.64 (td, J = 7.8, 1.4 Hz, 1H), 7.60–7.54 (m, 2H), 7.43 (d, J = 8.0 Hz, 1H), 7.20 (td, J = 7.5, 1.1 Hz, 1H), 7.12 (td, J = 7.8, 1.1 Hz, 1H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.43, 163.94, 163.34, 158.38, 153.92, 151.30, 149.97, 149.84, 128.55, 128.37, 124.93, 124.12, 123.88, 122.57, 122.10, 118.09, 114.63, 112.53, 110.37, 108.63, 89.12; IR (KBr) 3386, 3069, 2952, 2922, 1658, 1602, 1562, 1520, 1496, 1452, 1377, 1315, 1196, 1053, 742 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₁H₁₂O₂N₄: 352.0960, found: 352.0954.

7-Methoxy-3-(8-methoxybenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3b)

Yellow solid (68%), m.p. 288–290 °C; ¹H NMR (300 MHz, DMSO-d₆) δ = 9.46 (s, 1H), 9.27 (s, 1H), 8.35 (brs, 2H), 8.06 (d, J = 7.6 Hz, 1H), 7.47 (t, J = 8.0 Hz, 1H), 7.27 (d, J = 8.2 Hz, 1H), 7.17–7.07 (m, 2H), 6.81 (d, J = 7.2 Hz, 1H), 4.03 (s, 3H), 3.92 (s, 3H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.21, 163.69, 163.35, 158.32, 151.37, 149.99, 145.55, 144.34, 142.82, 138.12, 130.01, 125.66, 124.80, 123.90, 115.59, 114.58, 110.87, 110.83, 108.79, 105.93, 89.48, 56.53, 56.31; IR (KBr) 3431, 3317, 2964, 1624, 1597, 1562, 1537, 1520, 1493, 1377, 1180, 1057, 766, 727 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₃H₁₆O₄N₄: 412.1172, found: 412.1167.

7-Fluoro-3-(8-fluorobenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3c)

Yellow solid (64%), m.p. 278–280 °C; ¹H NMR (400 MHz, DMSO-d₆) δ = 9.49 (s, 1H), 9.35 (s, 1H), 8.51 (brs, 2H), 8.31 (dd, J = 6.9, 1.9 Hz, 1H), 7.62–7.54 (m, 2H), 7.37 (dd, J = 7.8, 0.6 Hz, 1H), 7.21–7.14 (m, 1H), 7.05–6.99 (m, 1H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.46, 164.05, 163.24, 158.71, 151.40, 150.99, 147.53 (d, ¹J_CF = 248.43 Hz), 146.74 (d, ¹J_CF = 244.41 Hz), 140.49 (d, ²J_CF = 10.26 Hz), 135.95 (d, ²J_CF = 10.56 Hz), 132.39 (d, ⁴J_CF = 2.28 Hz), 126.04 (d, ³J_CF = 6.64 Hz), 125.84 (d, ⁴J_CF = 2.22 Hz), 125.12 (d, ³J_CF = 6.64 Hz), 119.85, 114.98, 114.77 (d, ²J_CF = 14.18 Hz), 114.12 (d, ⁴J_CF = 3.00 Hz), 108.80 (d, ²J_CF = 16.00 Hz), 108.60 (d, ⁴J_CF = 2.28 Hz), 89.18; IR (KBr) 3444, 3327, 1633, 1597, 1564, 1520, 1493, 1437, 1381, 1315, 1257, 1194, 1051, 771 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₁H₁₀O₂N₄F₂: 388.0772, found: 388.0744.

6-Bromo-3-(9-bromobenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3d)

Yellow solid (73%), m.p. 278–280 °C; ¹H NMR (400 MHz, DMSO-d₆) δ = 9.44 (s, 1H), 9.31 (s, 1H), 8.49 (brs, 2H), 8.40 (d, J = 8.3 Hz, 1H), 8.24 (d, J = 1.3 Hz, 1H), 7.74 (dd, J = 8.3, 1.5 Hz, 1H), 7.71 (d, J = 1.6 Hz, 1H), 7.50 (d, J = 8.3 Hz, 1H), 7.35 (dd, J = 8.3, 1.6 Hz, 1H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.49, 164.02, 163.08, 158.53, 154.20, 151.16, 150.52, 150.19, 128.23, 126.94, 125.12, 121.85, 120.72, 119.29, 115.94, 114.86, 113.49, 113.35, 108.27, 88.79; IR (KBr) 3429, 3070, 1633, 1600, 1560, 1523, 1489, 1373, 1186, 1047, 889, 808 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₁H₁₀O₂N₄Br₂: 507.9171, found: 507.9153.

3-(9-(Dimethylamino)benzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)-N6,N6-dimethylbenzofuran-2,6-diamine (3e)

Brown solid (58%), m.p. > 300 °C; ¹H NMR (400 MHz, DMSO-d₆) δ = 9.30 (s, 1H), 9.11 (s, 1H), 8.49 (brs, 2H), 8.21 (d, J = 8.7 Hz, 1H), 7.41 (d, J = 8.6 Hz, 1H), 7.06 (d, J = 2.1 Hz, 1H), 6.94 (dd, J = 8.9, 2.2 Hz, 1H), 6.83 (d, J = 2.1 Hz, 1H), 6.65 (dd, J = 8.7, 2.1 Hz, 1H), 3.05 (s, 6H), 2.91 (s, 6H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.22, 163.32, 162.54, 157.41, 156.47, 151.70, 151.38, 149.86, 147.79, 145.73, 123.96, 118.80, 117.18, 114.48, 111.21, 110.60, 109.75, 109.48, 95.69, 94.49, 89.29, 41.28, 40.80; IR (KBr) 3323, 3234, 3062, 2887, 2806, 1630, 1614, 1599, 1556, 1506, 1437, 1385,1319, 1227, 1105, 989, 808 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₅H₂₂O₂N₆: 438.1804, found: 438.1791.

6-Methoxy-3-(9-methoxybenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3f)

Brown solid (51%), m.p. 234–236 °C; ¹H NMR (400 MHz, DMSO-d₆) δ = 9.37 (s, 1H), 9.18 (s, 1H), 8.50 (brs, 1H), 8.34 (d, J = 8.3 Hz, 1H), 7.51–7.43 (m, 2H), 7.18–7.07 (m, 2H), 6.82 (d, J = 8.1 Hz, 1H), 3.90 (s, 3H), 3.80 (s, 3H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.62, 163.86, 162.75, 160.41, 157.63, 156.16, 155.45, 150.85, 147.67, 124.23, 121.01, 118.60, 115.35, 114.37, 113.38, 113.29, 110.80, 108.85, 97.36, 97.15, 89.27, 56.33, 56.15; IR (KBr) 3431, 2939, 2188, 1620, 1604, 1560, 1437, 1377, 1311, 1142, 1032, 823 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₃H₁₆O₄N₄: 412.1172, found: 412.1154.

6-(Trifluoromethyl)-3-(9-(trifluoromethyl)benzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3g)

Yellow solid (57%), m.p. > 300 °C; ¹H NMR (300 MHz, DMSO-d₆) δ = 9.46 (s, 1H), 9.36 (s, 1H), 8.63 (d, J = 8.1 Hz, 1H), 8.59 (brs, 2H), 8.35 (s, 1H), 7.88 (d, J = 7.2 Hz, 1H), 7.77 (s, 1H), 7.67 (d, J = 8.2 Hz, 1H), 7.50 (d, J = 7.3 Hz, 1H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 165.52, 164.81, 162.98, 158.75, 153.05, 151.59, 151.41, 149.02, 132.71, 128.20 (d, ²J_CF = 31.66 Hz), 125.90, 125.16 (d,¹J_CF = 270.97 Hz),124.60 (d,¹J_CF = 273.18 Hz), 124.52, 121.90 (d, ²J_CF = 32.53 Hz), 121.65, 121.24, 117.89, 114.78, 110.21 (d, ³J_CF = 4.17 Hz), 107.77, 107.38 (d, ³J_CF = 4.17 Hz), 88.88; IR (KBr) 3419, 3300, 1637, 1624, 1606, 1566, 1512, 1479, 1433, 1325, 1117, 1057, 910 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₃H₁₀O₂N₄F₆: 488.0708, found: 488.0693.

5-Fluoro-3-(10-fluorobenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3h)

Yellow solid (53%), m.p. 274–276 °C; ¹H NMR (500 MHz, DMSO-d₆) δ = 9.42 (s, 1H), 9.29 (s, 1H), 8.45 (brs, 2H), 8.08 (dd, J = 8.0, 2.7 Hz, 1H), 7.93 (dd, J = 9.0, 4.0 Hz, 1H), 7.49 (td, J = 9.2, 2.7 Hz, 2H), 7.41 (dd, J = 8.8, 4.3 Hz, 1H), 7.31 (dd, J = 9.6, 2.6 Hz, 1H), 6.90 (td, J = 9.1, 2.6 Hz, 1H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 165.29, 164.71, 163.02, 160.55 (d, ¹J_CF = 236.43 Hz), 159.38 (d, ¹J_CF = 240.09 Hz), 158.49, 151.18, 150.70, 150.08, 146.00, 130.22 (d, ³J_CF = 11.65 Hz), 123.33 (d, ³J_CF = 11.06 Hz), 115.86 (d, ²J_CF = 25.80 Hz), 114.54, 114.08 (d, ³J_CF = 9.88 Hz), 111.03 (d, ³J_CF = 9.73 Hz), 109.24 (d, ²J_CF = 25.85 Hz), 108.53 (d, ⁴J_CF = 3.32 Hz), 108.28 (d, ²J_CF = 25.52 Hz), 104.51 (d, ²J_CF = 26.83 Hz), 89.39 (d, ⁴J_CF = 2.73 Hz); IR (KBr) 3406, 3213, 3138, 2920, 1712, 1637, 1606, 1558, 1520, 1502, 1446, 1356, 1311, 1188, 1047, 793 cm⁻¹; HRMS (EI): m/z [(M − H)⁺] calcd for C₂₁H₉O₂N₄F₂: 387.0694, found: 387.0684.

5-Chloro-3-(10-chlorobenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3i)

Yellow solid (85%), m.p. 280–282 °C; ¹H NMR (400 MHz, DMSO-d₆) δ = 9.36 (s, 1H), 9.25 (s, 1H), 8.45 (brs, 2H), 8.26 (d, J = 2.2 Hz, 1H), 7.86 (d, J = 8.8 Hz, 1H), 7.61 (dd, J = 8.8, 2.3 Hz, 1H), 7.54 (d, J = 2.1 Hz, 1H), 7.40 (d, J = 8.5 Hz, 1H), 7.08 (dd, J = 8.5, 2.2 Hz, 1H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.92, 164.27, 162.90, 158.50, 152.23, 151.05, 150.75, 148.35, 130.63, 129.07, 128.46, 128.16, 123.82, 122.73, 121.41, 117.43, 114.62, 114.25, 111.54, 107.83, 88.72; IR (KBr), 3485, 3398, 2922, 1647, 1604, 1564, 1522, 1500, 1464, 1365, 1317, 1165, 795 cm⁻¹; HRMS (EI): m/z [(M − H)⁺] calcd for C₂₁H₉O₂N₄Cl₂: 419.0103, found: 419.0085.

5-Bromo-3-(10-bromobenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3j)

Yellow solid (72%), m.p. 262–264 °C; ¹H NMR (300 MHz, DMSO-d₆) δ = 9.43 (d, J = 1.7 Hz, 1H), 9.32 (d, J = 1.6 Hz, 1H), 8.53 (s, 1H), 8.41 (s, 2H), 7.88 (dd, J = 7.4, 1.4 Hz, 1H), 7.80–7.76 (m, 1H), 7.71 (d, J = 2.0 Hz, 1H), 7.37 (d, J = 8.6 Hz, 1H), 7.22 (dd, J = 8.5, 2.0 Hz, 1H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.73, 164.12, 162.93, 158.55, 152.62, 151.07, 150.81, 148.74, 131.20, 130.86, 125.71, 124.39, 124.20, 120.28, 116.93, 116.45, 114.68, 112.06, 107.69, 88.58; IR (KBr) 3442, 3136, 2920, 1633, 1606, 1560, 1516, 1495, 1442, 1356, 1311, 1186, 1039, 795, 667 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₁H₁₀O₂N₄Br₂: 507.9171, found: 507.9136.

5-Methyl-3-(10-methylbenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3k)

Yellow solid (55%), m.p. 220–222 °C; ¹H NMR (300 MHz, CDCl₃) δ = 9.69 (s, 1H), 9.27 (s, 1H), 8.42 (brs, 2H), 7.63 (d, J = 8.4 Hz, 1H), 7.42–7.37 (m, 2H), 7.21 (d, J = 8.3 Hz, 1H), 6.98–6.93 (m, 2H), 2.59 (s, 3H), 2.39 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ = 164.37, 163.74, 163.58, 157.26, 152.61, 151.39, 149.31, 148.36, 134.02, 133.65, 128.90, 127.46, 123.82, 123.24, 122.60, 118.87, 114.56, 111.45, 109.82, 109.15, 90.79, 21.69, 21.48; IR (KBr) 3421, 2922, 1852, 2185, 1643, 1601, 1564, 1520, 1496, 1469, 1360. 1317, 1188, 1047, 798 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₃H₁₆O₂N₄: 380.1273, found: 380.1251.

5-Isopropyl-3-(10-isopropylbenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3l)

Yellow solid (78%), m.p. 238–240 °C; ¹H NMR (300 MHz, CDCl₃) δ = 9.72 (s, 1H), 9.30 (s, 1H), 8.49 (d, J = 1.9 Hz, 1H), 7.68 (d, J = 8.5 Hz, 1H), 7.49–7.45 (m, 2H), 7.29–7.25 (m, 1H), 7.04 (dd, J = 8.3, 1.7 Hz, 1H), 6.94 (brs, 2H), 3.25–3.14 (m, 1H), 3.02-2.92 (m, 1H), 1.42 (s, 3H), 1.40 (s, 3H), 1.28 (s, 3H), 1.26 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ = 164.52, 163.76, 163.57, 157.32, 152.80, 151.50, 149.33, 148.53, 145.32, 145.04, 127.45, 126.40, 122.65, 121.46, 120.63, 116.48, 114.63, 111.62, 109.92, 109.39, 91.00, 34.41, 34.32, 24.57; IR (KBr) 3388, 3213, 2958, 2925, 2868, 1633, 1601, 1560, 1495, 1469, 1365, 1315, 1190, 1043, 808 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₇H₂₄O₂N₄: 436.1899, found:436.1884.

5-Nitro-3-(10-nitrobenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3m)

Yellow solid (69%), m.p. > 300 °C; ¹H NMR (300 MHz, DMSO-d₆) δ = 9.49 (s, 1H), 9.39 (s, 1H), 9.08 (d, J = 2.5 Hz, 1H), 8.57 (brs, 2H), 8.46 (dd, J = 9.0, 2.4 Hz, 1H), 8.37 (d, J = 2.5 Hz, 1H), 8.11 (d, J = 9.1 Hz, 1H), 7.97 (dd, J = 8.8, 2.2 Hz, 1H), 7.59 (d, J = 8.9 Hz, 1H); ¹³C NMR (125 MHz, Py-d₅) δ = 164.54, 159.95, 158.53, 154.92, 153.16, 152.95, 152.91, 146.45, 146.41, 131.35, 121.33, 119.41, 116.54, 114.90, 114.34, 111.79, 110.43, 90.92; IR (KBr) 3384, 3095, 1651, 1612, 1564, 1516, 1448, 1342, 1194, 1047, 818 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₁H₁₀O₆N₆: 442.0662, found:442.0600.

4-(2-Amino-5-cyanobenzofuran-3-yl)benzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidine-10-carbonitrile (3n)

Yellow solid (51%), m.p. > 300 °C; ¹H NMR (300 MHz, DMSO-d₆) δ = 9.47 (s, 1H), 9.37 (s, 1H), 8.76 (s, 1H), 8.53 (brs, 2H), 8.16–8.06 (m, 2H), 7.96 (s, 1H), 7.63–7.51 (m, 2H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.90, 164.42, 162.82, 158.78, 155.62, 151.90, 151.55, 151.09, 132.11, 130.03, 127.80, 126.30, 123.12, 121.55, 119.92, 119.05, 114.95, 114.25, 111.45, 107.80, 107.59, 106.84, 88.23; IR (KBr) 3365, 3240, 3078, 2229, 1651, 1604, 1562, 1495, 1518, 1460, 1358, 1317, 1196, 1047, 820 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₃H₁₀O₂N₆: 402.0865, found:402.0827.

5-Phenyl-3-(10-phenylbenzofuro[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)benzofuran-2-amine (3o)

Yellow solid (74%), m.p. 188–190 °C; ¹H NMR (300 MHz, DMSO-d₆) δ = 9.53 (s, 1H), 9.31 (s, 1H), 8.65 (s, 1H), 8.48 (brs, 2H), 7.96–7.85 (m, 2H), 7.82 (s, 1H), 7.74 (d, J = 7.4 Hz, 2H), 7.60 (d, J = 7.7 Hz, 2H), 7.54–7.46 (m, 3H), 7.45–7.30 (m, 5H); ¹³C NMR (125 MHz, CDCl₃) δ = 164.51, 163.71, 163.47, 157.35, 153.75, 151.45, 149.58, 149.50, 141.26, 140.90, 137.87, 137.85, 128.83, 128.01, 127.59, 127.42, 127.26, 127.14, 123.13, 122.45, 121.80, 117.21, 114.65, 112.03, 110.37, 109.38, 90.85; IR (KBr) 3431, 3059, 2920, 1727, 1628, 1601, 1560, 1493, 1462, 1358, 1315, 1192, 1047, 762, 698 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₃₃H₂₀O₂N₄: 504.1586, found:504.1585.

1-(Naphtho[1′′,2′′:4′,5′]furo[3′,2′:5,6]pyrido[4,3-d]pyrimidin-4-yl)naphtho[2,1 b]furan-2-amine (3p)

Yellow solid (79%), m.p. 230–232 °C; ¹H NMR (400 MHz, DMSO-d₆) δ = 10.66 (d, J = 3.2 Hz, 1H), 10.36 (d, J = 8.2 Hz, 1H), 10.22 (s, 1H), 10.03 (s, 1H), 8.19–8.07 (m, 2H), 8.01–7.92 (m, 4H), 7.89 (d, J = 8.4 Hz, 1H), 7.65 (t, J = 7.6 Hz, 1H), 7.61–7.51 (m, 2H), 7.43–7.34 (m, 2H); ¹³C NMR (125 MHz, DMSO-d₆) δ = 164.27, 155.01, 152.75, 148.97, 147.71, 144.61, 144.43, 133.40, 131.60, 131.22, 131.18, 129.42, 129.39, 129.19, 129.06, 128.84, 127.81, 127.49, 125.64, 123.68, 123.46, 122.69, 119.57, 117.75, 114.05, 112.78, 112.11, 109.66, 72.59; IR (KBr) 3354, 3060, 2181, 1612, 1576, 1516, 1400, 1362, 1255, 1188, 1032, 814, 750 cm⁻¹; HRMS (EI): m/z [M⁺] calcd for C₂₉H₁₆O₂N₄:452.1273, found:452.1271.

Acknowledgements

This work was supported by the “Interdisciplinary Cooperation Team” Program for Science and Technology Innovation of the Chinese Academy of Sciences, and the National Natural Science Foundation of China (21072205, 81025020) and SIMM1203ZZ-0103.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, characterization data and NMR spectra data. CCDC 945367 and 945368. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra44828b

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