HETEROCYCLES

Short Paper | Regular issue | Vol. 92, No. 1, 2016, pp. 133-140
Received, 12th November, 2015, Accepted, 26th November, 2015, Published online, 11th December, 2015.
DOI: 10.3987/COM-15-13364

■ An Efficient One-Pot Synthesis of Azuleno[2,1-b]pyridazines

Dao-Lin Wang,* Jian Ma, Xiao-Ce Shi, and Jian-Ying Wu

Liaoning Key Laboratory of Synthesis and Application of Functional Compound, College of Chemistry & Chemical Engineering, Bohai University, Jinzhou 121001, China

Abstract

A simple and efficient procedure was developed for the synthesis of 1-aryl-3-cyano-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazines (3) in good yields by condensation of ethyl 1-cyanoacetyl-2-methoxyazulene-3-carboxylate (1) with aryldiazonium salt (2) in pyridine via a domino reaction. This reaction provides a new procedure for synthesis of heterocyle-fused azulenes.

The pyridazine nucleus is an interesting heterocyclic ring that plays the role of a pharmacophore in several classes of derivatives that show a variety of pharmacological properties.1 During the last decade, considerable attention has been devoted to various pyridazinones because of their synthetic versatility, their well-balanced physicochemical properties, and the wide range of pharmacological activities such as antihypertensive,2 anti-HIV,3 antibacterial,4 aldose reductase inhibitor,5 and hepatoprotective agents.6
On the other hand, azulenes have attracted interest in medicine as antiulcer drugs,7 anticancer agents,8 and as antioxidant therapeutics for neurodegenerative conditions.9 A variety of heterocyle-fused azulenes have attracted the interest owing to its unusual chemical properties.10 Thus, preparation and reactivities of a number of heterocyle-fused azulenes have already been revealed by many research groups.11 Recently, our research group has reported the synthesis of new heteroarylazulene derivatives.12 As part of a continuing effort in our laboratory toward the development of azulene chemistry, we became interested in exploring the reactivity and synthetic applications of cyanoacetylated azulene13 to 1-aryl-3-cyano-4-oxo-10-ethoxy- carbonylazuleno[2,1-b]pyridazines (3) by condensation of ethyl 1-cyanoacetyl-2-methoxy-azulene-3- carboxylate (1) with aryldiazonium salt (2) via a domino reaction under mild reaction conditions (Scheme 1).

In this study, we first optimized the reaction of ethyl 1-cyanoacetyl-2-methoxyazulene-3-carboxylate (1) with phenyldiazonium salt (2a) as model substrates. In our preliminary experiments 1.0 mmol of 1 was treated with 1.0 mmol of 2a in pyridine at room temperature. The reaction was complete in 6 h. After work-up of the reaction mixture, 3-cyano-1-phenyl-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3a) was obtained in 82% as orange needles. Its structure was determined from the spectral data as well as elemental analysis (C22H15N3O3). In the IR spectrum, two carbonyl signals at 1708 and 1625 cm-1 and CN signals at 2231 cm-1 are observed. The 1H NMR spectrum shows signals at δ 7.59 (dd, J = 9.2, 9.2 Hz, 1H), 7.87 (dd, J = 9.6, 9.6 Hz, 1H), 7.95 (dd, J = 9.6, 9.6 Hz, 1H), 8.94 (d, J = 10.4 Hz, 1H), and 10.08 (d, J = 9.2 Hz, 1H) for seven-membered protons, together with ethoxycarbonyl protons at δ 0.96 (t, J = 7.2 Hz, 3H, OCH2CH3), 3.57 (q, J = 7.2 Hz, 2H, CO2CH2CH3), and phenyl at δ 7.48-7.53 (m, 5H). In the 13C NMR spectrum, two carbonyl signals at 167.1 and 171.5 are observed.
In an initial endeavor, we carried out the reaction of 1 and phenyldiazonium salt (2a) using the different solvents (Table 1). We screened different solvents such as ethanol, methanol, acetonitrile, tetrahydrofuran, N,N-dimethylformamide, water and pyridine at room temperature. As shown in the Table 1, the best yield was obtained when pyridine was used as a solvent.

Using the optimized conditions, we next explored the scope and generality of the process. Our preparative results are summarized in Table 2. A variety of azuleno[2,1-b]pyridazines 3 containing neutral, electron- rich, and electron-poor aryl substituents could be isolated in good yields.

The proposed mechanism of the process is summarized in Scheme 2. The sequence involves an initial conjugate addition of 1 with aryldiazonium salt (2) in pyridine to producing the 2-arylhydrazolononitrile B. This then undergoes SNAr cyclizations of the 2-methoxy of azulenyl group by attack of NH group via loss of methanol leads to yield the tricyclic product (3).

In conclusion, we have successfully developed facile and efficient method to prepare a series of 1-aryl- 3-cyano-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine derivatives via a domino reaction of ethyl 1- cyanoacetyl-2-methoxyazulene-3-carboxylate with aryldiazonium salt in good yields. Further investigations to elaborate the scope of this methodology and to show the synthetic utility of the heterocycle-fused azulene derivatives obtained are currently in progress in our laboratory.

EXPERIMENTAL
Melting points were determined in open capillaries and are uncorrected. The NMR spectra were recorded with a Bruker Avance 400 spectrometer (400 MHz for 1H and 100 MHz for 13C) in CDCl3 using TMS an internal reference. IR spectra were measured on Shimadzu FTIR-8300 spectrophotometer. C, H and N analyses were performed by a HP-MOD 1106 microanalyzer. The preparation of ethyl 1-cyanoacetyl-2- methoxyazulene-3-carboxylate was according to the literature procedure.13a All other chemicals used in this study were commercially available.
Typical Procedure for the Preparation of 1-Aryl-3-cyano-4-oxo-10-ethoxycarbonylazuleno[2,1-b]- pyridazines.
3-Cyano-1-phenyl-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3a): To a stirred solution of 1 (295 mg, 1.0 mmol) in pyridine (20 mL) was added phenyldiazonium salt (2a) [prepared by addition of NaNO2 solution (69 mg, 1.0 mmol) to a cold solution of aniline (93 mg, 1.0 mmol) containing the appropriate quantity of HC1 at 0 °C with stirring] and the mixture was left at room temperature for 6 h. The solid product formed was collected and recrystallized from EtOH to give 3a (302 mg, 82%); orange needles; mp 240-242 oC; IR (KBr) 2231, 1708, 1625 cm-1; 1H NMR δ 0.96 (t, J = 7.2 Hz, 3H, OCH2CH3), 3.57 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 7.48-7.50 (m, 1H, benzene ring aromatic H), 7.51-7.53 (m, 4H, benzene ring aromatic H), 7.59 (dd, J = 9.2, 9.2 Hz, 1H, azulene ring aromatic H), 7.87 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 7.95 (t, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.94 (d, J = 10.4 Hz, 1H, azulene ring aromatic H), 10.08 (d, J = 9.2 Hz, 1H, azulene ring aromatic H); 13C NMR δ 15.9, 61.3, 97.9, 105.6, 109.9, 111.1, 123.5, 129.2, 130.7, 134.9, 137.7, 137.9, 138.1, 140.7, 141.0, 141.1, 146.9, 150.2, 167.1, 171.5. Anal. Calcd for C22H15N3O3: C, 71.54; H, 4.09; N, 11.38. Found: C, 71.62; H, 4.23; N, 11.56.
3-Cyano-1-(2-methylphenyl)-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3b): orange needles; mp 218-220 oC; IR (KBr) 2235, 1702, 1634 cm-1; 1H NMR δ 1.06 (t, J = 7.2 Hz, 3H, OCH2CH3), 2.37 (s, 3H, CH3), 3.63 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 7.24-7.25 (m, 1H, benzene ring aromatic H), 7.28-7.32 (m, 1H, benzene ring aromatic H), 7.43-7.45 (m, 2H benzene ring aromatic H), 7.80 (dd, J = 9.2, 9.2 Hz, 1H, azulene ring aromatic H), 7.85 (dd, J = 9.2, 9.2 Hz, 1H, azulene ring aromatic H), 7.93 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.94 (d, J = 10.4 Hz, 1H, azulene ring aromatic H), 10.08 (d, J = 9.2 Hz, 1H, azulene ring aromatic H); 13C NMR δ 16.3, 24.7, 61.5, 98.2, 105.6, 109.9, 111.1, 112.6, 113.3, 125.1, 127.0, 129.8, 131.9, 132.8, 133.3, 134.5, 137.3, 137.8, 140.7, 144.3, 146.9, 167.1, 171.5. Anal. Calcd for C23H17N3O3: C, 72.05; H, 4.47; N, 10.96. Found: C, 72.13; H, 4.55; N, 11.15.
3-Cyano-1-(3-methylphenyl)-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3c): orange needles; mp 216-218 oC; IR (KBr) 2233, 1701, 1628 cm-1; 1H NMR δ 1.06 (t, J = 7.2 Hz, 3H, OCH2CH3), 2.34 (s, 3H, CH3), 3.82 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 7.26-7.28 (m, 1H, benzene ring aromatic H), 7.34-7.36 (m, 1H, benzene ring aromatic H), 7.42-7.45 (m, 2H, benzene ring aromatic H), 7.79 (dd, J = 9.2, 9.2 Hz, 1H, azulene ring aromatic H), 7.91 (dd, J = 9.2, 9.2 Hz, 1H, azulene ring aromatic H), 8.01 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.95 (d, J = 10.4 Hz, 1H, azulene ring aromatic H), 10.07 (d, J = 9.2 Hz, 1H, azulene ring aromatic H); 13C NMR δ 15.3, 25.6, 60.6, 100.2, 112.1, 113.4, 118.6, 125.2, 127.7, 129.8, 131.9, 132.6, 133.4, 135.7, 135.9, 137.3, 137.9, 142.0, 144.4, 145.7, 150.5, 165.5, 172.8. Anal. Calcd for C23H17N3O3: C, 72.05; H, 4.47; N, 10.96. Found: C, 72.19; H, 4.53; N, 11.14.
3-Cyano-1-(4-methylphenyl)-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3d): orange needles; mp 264-266 oC; IR (KBr) 2239, 1705, 1634 cm-1; 1H NMR δ 0.97 (t, J = 7.2 Hz, 3H, OCH2CH3), 2.45 (s, 3H, CH3), 3.56 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 7.35 (d, J = 8.4 Hz, 2H, benzene ring aromatic H), 7.46 (d, J = 8.4 Hz, 2H, benzene ring aromatic H), 7.84 (dd, J = 9.2, 9.2 Hz, 1H, azulene ring aromatic H), 7.93 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.03 (dd, J = 9.2, 9.2 Hz, 1H, azulene ring aromatic H), 9.07 (d, J = 10.4 Hz, 1H, azulene ring aromatic H), 10.07 (d, J = 9.2 Hz, 1H, azulene ring aromatic H); 13C NMR δ 15.9, 19.2, 63.7, 105.6, 109.9, 111.1, 123.1, 123.5, 129.9, 130.4, 134.9, 135.8, 137.3, 137.7, 138.0, 141.2, 141.7, 143.3, 145.8, 146.9, 167.1. Anal. Calcd for C23H17N3O3: C, 72.05; H, 4.47; N, 10.96. Found: C, 72.17; H, 4.61; N, 11.13.
3-Cyano-1-(3,5-dimethylphenyl)-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3e): orange needles; mp 215-217 oC; IR (KBr) 2234, 1704, 1629 cm-1; 1H NMR δ 0.96 (t, J = 7.2 Hz, 3H, OCH2CH3), 2.03 (s, 6H, CH3), 2.44 (s, 3H, CH3), 3.58 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 7.31-7.37 (m, 1H, benzene ring aromatic H), 7.39-7.44 (m, 2H, benzene ring aromatic H), 7.82 (dd, J = 9.2, 9.2 Hz, 1H, azulene ring aromatic H), 7.91 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.01 (dd, J = 9.2, 9.2 Hz, 1H, azulene ring aromatic H), 9.02 (d, J = 9.6 Hz, 1H, azulene ring aromatic H), 10.07 (d, J = 9.0 Hz, 1H, azulene ring aromatic H); 13C NMR δ 15.2, 25.4, 25.6, 60.1, 100.5, 112.6, 113.5, 118.7, 125.2, 127.4, 129.8, 131.9, 132.4, 133.0, 135.3, 137.1, 138.9, 142.2, 144.3, 147.7, 150.8, 168.4. Anal. Calcd for C24H19N3O3: C, 72.53; H, 4.82; N, 10.57. Found: C, 72.64; H, 4.96; N, 10.69.
3-Cyano-1-(2,4,6-trimethylphenyl)-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3f): orange needles; mp 246-248 oC; IR (KBr) 2232, 1704, 1631 cm-1; 1H NMR δ 1.12 (t, J = 7.2 Hz, 3H, OCH2CH3), 1.99 (s, 6H, 2xCH3), 2.37 (s, 3H, CH3), 3.68 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 7.02 (s, 2H, benzene ring aromatic H), 7.77 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 7.90 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 7.99 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.68 (d, J = 10.4 Hz, 1H, azulene ring aromatic H), 10.04 (d, J = 9.6 Hz, 1H, azulene ring aromatic H); 13C NMR δ 14.9, 21.3, 25.2, 25.6, 60.7, 100.2, 112.1, 118.6, 125.4, 127.7, 132.6, 133.4, 135.7, 135.9, 137.3, 137.9, 138.9, 142.0, 144.4, 145.7, 150.5, 166.3, 170.4. Anal. Calcd for C25H21N3O3: C, 72.98; H, 5.14; N, 10.21. Found: C, 73.12; H, 5.29; N, 10.34.
3-Cyano-1-(2-methoxyphenyl)-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3g): orange needles; mp 174-176 oC; IR (KBr) 2232, 1704, 1626 cm-1; 1H NMR (CDCl3): δ 1.46 (t, J = 7.2 Hz, 3H, OCH2CH3), 4.30 (s, 3H, OCH3), 4.48 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 6.88-6.91 (m, 1H, benzene ring aromatic H), 7.05 (t, J = 9.2 Hz, 1H, benzene ring aromatic H), 7.18 (t, J = 9.2 Hz, 1H, benzene ring aromatic H), 7.61-7.69 (m, 2H, benzene ring aromatic H + azulene ring aromatic H), 7.67 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 7.75 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.72 (d, J = 10.0 Hz, 1H, azulene ring aromatic H), 9.25 (d, J = 10.0 Hz, 1H, azulene ring aromatic H); 13C NMR δ 15.2, 55.2, 61.5, 99.6, 104.6, 109.9, 111.4, 112.6, 113.5, 125.1, 127.9, 129.8, 131.6, 132.8, 133.5, 134.1, 137.3, 137.6, 140.7, 144.6, 146.2, 166.1, 172.1. Anal. Calcd for C23H17N3O4: C, 69.17; H, 4.29; N, 10.52. Found: C, 69.29; H, 4.46; N, 10.69.
3-Cyano-1-(4-methoxyphenyl)-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3h): orange needles; mp 258-260 oC; IR (KBr) 2233, 1705, 1626 cm-1; 1H NMR δ 0.94 (t, J = 7.2 Hz, 3H, OCH2CH3), 3.54 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 3.88 (s, 3H, OCH3), 7.04 (d, J = 9.2 Hz, 2H, benzene ring aromatic H), 7.49 (d, J = 9.2 Hz, 2H, benzene ring aromatic H), 7.83 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 7.92 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.02 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 9.04 (d, J = 10.4 Hz, 1H, azulene ring aromatic H), 10.06 (d, J = 9.2 Hz, 1H, azulene ring aromatic H); 13C NMR δ 14.9, 55.8, 61.2, 106.4, 113.6, 114.7, 117.8, 125.0, 130.2, 133.1, 133.5, 137.5, 138.0, 140.7, 140.8, 144.0, 145.2, 159.9, 163.9, 165.9, 171.7. Anal. Calcd for C23H17N3O4: C, 69.17; H, 4.29; N, 10.52. Found: C, 69.31; H, 4.43; N, 10.67.
1-(4-Chlorophenyl)-3-cyano-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3i): orange needles; mp 270-272 oC; IR (KBr) 2233, 1704, 1627 cm-1; 1H NMR δ 1.01 (t, J = 7.2 Hz, 3H, OCH2CH3), 3.67 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 8.06-8.09 (m, 4H, benzene ring aromatic H), 8.15 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.20 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.26 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 9.02 (d, J = 10.4 Hz, 1H, azulene ring aromatic H), 9.93 (d, J = 9.2 Hz, 1H, azulene ring aromatic H); 13C NMR δ 15.3, 60.4, 105.7, 113.4, 114.7, 118.6, 125.2, 130.2, 133.2, 133.5, 137.4, 138.3, 140.5, 140.7, 144.0, 145.2, 160.2, 163.7, 166.4, 173.2. Anal. Calcd for C22H14ClN3O3: C, 65.43; H, 3.49; N, 10.41. Found: C, 65.57; H, 3.64; N, 10.56.
3-Cyano-1-(3-nitrophenyl)-4-oxo-10-ethoxycarbonylazuleno[2,1-b]pyridazine (3j): orange needles; mp 291-293 oC; IR (KBr) 2237, 1691, 1722 cm-1; 1H NMR δ 0.96 (t, J = 7.2 Hz, 3H, OCH2CH3), 3.65 (q, J = 7.2 Hz, 2H, CO2CH2CH3), 7.94-7.96 (m, 1H, benzene ring aromatic H), 7.98-8.02 (m, 2H, benzene ring aromatic H ), 8.13 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.22 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.32 (dd, J = 9.6, 9.6 Hz, 1H, azulene ring aromatic H), 8.57 (s, 1H, benzene ring aromatic H), 9.08 (d, J = 10.4 Hz, 1H, azulene ring aromatic H), 9.97 (d, J = 10.0 Hz, 1H, azulene ring aromatic H); 13C NMR δ 15.9, 60.8, 105.2, 112.5, 113.4, 118.8, 125.7, 127.9, 129.7, 131.9, 133.6, 133.8, 135.7, 135.9, 137.3, 140.8, 142.3, 144.5, 145.7, 150.8, 165.7, 172.1. Anal. Calcd for C22H14N4O5: C, 63.77; H, 3.41; N, 13.52. Found: C, 63.89; H, 3.52; N, 13.67.

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