HETEROCYCLES

Short Paper | Regular issue | Vol. 85, No. 2, 2012, pp. 441-448
Received, 3rd December, 2011, Accepted, 26th December, 2011, Published online, 10th January, 2012.
DOI: 10.3987/COM-11-12406

■ An Efficient One-Pot Synthesis of 1-Amino-3-cyano-4-aryl-10-ethoxycarbonylazuleno[2,1-b]pyrans

Dao-Lin Wang,* Shan-Shan Feng, Qing-Tao Cui, and Jia-Yi Yu

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

Abstract

An efficient and easy method for the one-pot three-component synthesis of 2-amino-3-cyano-4-aryl-10-ethoxycarbonylazuleno[2,1-b]pyran derivatives by the three-componenet condensation of ethyl 2-hydroxyazulene-1-carboxylate, aldehydes, and malononitrile in the presence of diazabicyclo[2.2.2]octane (DABCO) has been described.

It is well known that pyrans are important core units in a number of natural products1 and photochromic materials.2 Compounds with pyran ring system have many pharmacological properties and play important roles in biochemical process. Moreover, 4H-pyrans are useful intermediates for the synthesis of various compounds.3,4 Therefore, preparation of this heterocyclic nucleus has gained great importance in organic synthesis.
The azulene constitute was the best known of polycyclic nonbenenoid aromatic compounds and have long fascinated chemists with their beautiful colors and unusual electronic properties.5 Azulenes have attracted interest in medicine as antiulcer drugs6 anticancer agents,7 and as antioxidant therapeutics for neurode-generative conditions.8 A variety of heteroarylazulenes have so far been obtained on the viewpoints of chemical properties and physiological activities by several methods.9 Recently, our group has developed several synthesis method s of heteroarylazulenes.10 In this work, we report one-pot synthesis of 2-amino-3-cyano-4-aryl-10-ethoxycarbonylazuleno[2,1-b]pyrans by condensation of ethyl 2-hydroxyazulene-1-carboxylate (1), aldehydes (2a), and malononitrile using inexpensive and convenient available diazabicyclo[2.2.2]octane (DABCO) as catalyst (Scheme 1).

In this three-component reaction, we found that catalysts had significant effects on the reaction time and yields (Table 1). The results indicated that this reaction could not take place in the presence of proton acids and classical Lewis acids as catalysts in EtOH under reflux conditions for 120 min, such as TsOH, NH2SO3H, CF3CO2H, ZnCl2, and MgCl2 (Table 1, entries 1-5). Then, we carried out the reaction using bases as the catalysts under the same reaction conditions, such as Et3N, DBU, piperidine and DABCO (Table 1, entries 6-11). To our delight, the desired condensation product 4a was obtained in moderate to excellent yields (45-92%), which meant that this three-component condensation reaction of ethyl 2-hydroxyazulene-1-carboxylate (1), benzaldehyde (2a), and malononitrile (3) could proceed smoothly catalyzed by bases. Furthermore, we found that the yields of 4a were improved as the amount of DABCO increased from 10 to 20%, and the yields plateaued when the amount of DABCO was further increased to 30% (Table 1, entry 11). Therefore, 20 mol% of DABCO was considered to be the most suitable.

Furthermore, the reaction was optimized by screening the solvent such as CH2Cl2, CH2ClCH2Cl, THF, MeCN, AcOH, EtOH, toluene and DMF. The reaction using DMF and EtOH as the solvents gave the corresponding product 4a in high yields and in short reaction time. From the economical and environmental point of view, EtOH was chosen as the reaction medium for all further reactions. Therefore, the best reaction conditions were obtained by using 20 mol % of ammonium acetate as the catalyst in EtOH at reflux temperature.
A series of aromatic and heterocyclic aldehydes were selected to undergo the condensation in the presence of DABCO. As shown in Table 2, aromatic aldehydes (2) carrying either electron-donating or electron-withdrawing substituents reacted efficiently giving excellent yields (Table 2, entries 1-12). Hence, the effect of the nature of the substituents on the aromatic ring showed no obvious effect on this conversion. Furthermore, heterocyclic aldehydes could react smoothly to give the corresponding products 4 in good yields (Table 2, entries 13, 14).

We propose a mechanism of the DABCO-catalyzed condensation as shown in Scheme 2. The condensa- tion of ethyl 2-hydroxyazulene-1-carboxylate 1, aldehyde 2, malononitrile 3 may occur by a mechanism of Knoevenagel condensation, Michael addition, intramolecular cyclization, and tautomerization. Initially, intermediate 5 is formed by Knoevenagel condensation of aldehyde 2 and malononitrile 3 by the action of DABCO.

In the presence DABCO, Michael addition of ethyl 2-hydroxy azulene-1-carboxylate 1 on 5 leads to form intermediate 6, followed by intramolecular cyclization and tautomerization, affords the correspondding products 4 (Scheme 2). We think that DABCO serves as a base as well as a proton shuttle to promote this reaction.

In conclusion, the present method discloses a new and simple modification of the condensation of ethyl 2-hydroxyazulene-1-carboxylate 1, aldehyde 2, malononitrile 3 to the synthesis of 2-amino-3-cyano-4-aryl-10-ethylcarbonyl azuleno[2,1-b]pyran derivatives 4 by using DABCO as a catalyst in EtOH. The operational simplicity, mild reaction conditions, short reaction time, and little environmental impact are notable features of this procedure.

EXPERIMENTAL
All melting points were determined on a Yanako MP-3 apparatus and are uncorrected. 1HNMR spectra were recorded on a Bruker spectrometer (400 MHz). IR spectra were measured on Shimadzu IR-740 spectrophotometer. Elemental analyses were performed on EA 2400Ⅱ elemental analyzer (Perkin-Elmer).
Preparation of 2-amino-3-cyano-4-aryl-10-ethoxycarbonyl azuleno[2,1-b]pyran derivatives.
General procedure: A mixture of ethyl 2-hydroxyazulene-1-carboxylate11 (1, 1.0 mmol), aldehyde (2, 1.1 mmol), malononitrile (3, 1.1 mmol), and DABCO (0.3 mmol) in EtOH (15 mL) was heated to reflux under stirring for the given time (Table 2). After completion (by TLC), the reaction mixture was cooled to room temperature, then water (10 mL) was added to the mixture and stirred for 5 min. The solid was filtered and recrystallized to afford the corresponding products. The physical and spectra data of the compounds 4a-n are as follows:
2-Amino-3-cyano-4-phenyl-10-ethoxycarbonylazuleno[2,1-b]pyran (4a): Deep red prism (from EtOH). mp 198-200 ºC; IR (KBr, cm-1): ν 3280 (NH2), 2216 (CN), 1628 (C=O). 1H-NMR (CDCl3): δ 1.26 (3H, t, J = 7.2 Hz, OCH2CH3), 4.59 (2H, q, J = 7.2 Hz, CO2CH2CH3), 4.94 (2H, s, NH2), 5.28 (1H, s, CH), 7.30-7.33 (2H, m), 7.37-7.43 (4H, m), 7.57 (1H, dd, J = 9.6, 10.0 Hz), 7.69 (1H, dd, J = 9.6, 9.6 Hz), 7.99 (1H, d, J = 9.6 Hz), 9.64 (1H, d, J = 10.0 Hz). Anal. Calcd for C23H18N2O3: C 74.58, H 4.90, N 7.56. Found: C 74.67, H 5.13, N 7.68.
2-Amino-3-cyano-4-(4-methylphenyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3b): Deep red prism (from EtOH). mp 192-194 ºC; IR (KBr, cm-1): ν 3312 (NH2), 2209 (CN), 1631 (C=O). 1H-NMR (CDCl3): δ 1.25 (3H, t, J = 7.2 Hz, OCH2CH3), 2.28 (3H, s, CH3), 4.47 (2H, q, J = 7.2 Hz, CO2CH2CH3), 4.75 (2H, s, NH2), 5.29 (1H, s, CH), 7.08 (4H, m), 7.30 (1H, dd, J = 9.6, 10.2 Hz), 7.56 (1H, dd, J = 9.6, 9.6 Hz), 7.66 (1H, dd, J = 9.6, 9.6 Hz), 7.90 (1H, d, J = 10.0 Hz), 9.53 (1H, d, J = 10.0 Hz). Anal. Calcd for C24H20N2O3: C 74.98, H 5.24, N 7.29. Found: C 74.74, H 5.37, N 7.31.
2-Amino-3-cyano-4-(4-methoxyphenyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3c): Deep red prism (from EtOH). mp 185-187 ºC; IR (KBr, cm-1): ν 3322 (NH2), 2215 (CN), 1642 (C=O). 1H-NMR (CDCl3): δ 1.25 (3H, t, J = 7.2 Hz, OCH2CH3), 3.85 (3H, s, OCH3), 4.59 (2H, q, J = 7.2 Hz, CO2CH2CH3), 4.88 (2H, s, NH2), 5.25 (1H, s, CH), 6.91 (2H, d, J = 8.2 Hz), 7.24 (2H, d, J = 8.2 Hz), 7.37 (1H, dd, J = 9.6, 10.2 Hz), 7.59 (1H, dd, J = 9.6, 9.6 Hz), 7.67 (1H, dd, J = 9.6, 10.0 Hz), 8.01 (1H, d, J = 10.0 Hz), 9.64 (1H, d, J = 10.0 Hz). Anal. Calcd for C24H20N2O4: C 71.99, H 5.03, N 7.00. Found: C 71.86, H 5.26, N 7.14.
2-Amino-3-cyano-4-(4-chlorophenyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3d): Deep red prism (from EtOH). mp 206-208 ºC; IR (KBr, cm-1): ν 332 1 (NH2), 2219 (CN), 1642 (C=O). 1H-NMR (CDCl3): δ 1.25 (3H, t, J = 7.2 Hz, OCH2CH3), 4.59 (2H, q, J = 7.2 Hz, CO2CH2CH3), 4.96 (2H, s, NH2), 5.26 (1H, s, CH), 7.24 (2H, d, J = 8.2 Hz), 7.35 (2H, d, J = 8.2 Hz), 7.43 (1H, dd, J = 9.6, 10.2 Hz), 7.63 (1H, dd, J = 9.6, 10.0 Hz), 7.78 (1H, dd, J = 9.6, 9.6 Hz), 7.94 (1H, d, J = 10.0 Hz), 9.65 (1H, d, J = 10.0 Hz). Anal. Calcd for C23H17ClN2O3: C 68.23, H 4.23, N 6.92. Found: C 68.36, H 4.41, N 7.14.
2-Amino-3-cyano-4-(4-fulorophenyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3e): Deep red prism (from EtOH). mp 212-214 ºC; IR (KBr, cm-1): ν 3325 (NH2), 2206 (CN), 1648 (C=O). 1H-NMR (CDCl3: δ 1.27 (3H, t, J = 7.2 Hz, OCH2CH3), 4.59 (2H, q, J = 7.2 Hz, CO2CH2CH3), 4.91 (2H, s, NH2), 5.27 (1H, s, CH), 7.26 (2H, d, J = 8.8 Hz), 7.39 (2H, d, J = 8.8 Hz), 7.45 (1H, dd, J = 9.6, 10.2 Hz), 7.52 (1H, dd, J = 9.6, 9.6 Hz), 7.69 (1H, dd, J = 9.6, 10.0 Hz), 8.12 (1H, d, J = 10.0 Hz), 9.66 (1H, d, J = 10.0 Hz). Anal. Calcd for C23H17FN2O3: C 71.13, H 4.41, N 7.21. Found: C 71.26, H 4.58, N 7.39.
2-Amino-3-cyano-4-(4-hydroxyphenyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3f): Deep red prism (from EtOH). mp 221-222 ºC; IR (KBr, cm-1): ν 3334 (NH2), 3225 (OH), 2208 (CN), 1640 (C=O). 1H-NMR (DMSO): δ 1.40 (3H, t, J = 7.2 Hz, OCH2CH3), 4.35 (2H, q, J = 7.2 Hz, CO2CH2CH3), 5.10 (1H, s, CH), 6.62 (2H, d, J = 8.4 Hz), 6.95 (2H, s, NH2), 6.96 (2H, d, J = 8.4 Hz), 7.44 (1H, dd, J = 9.6, 10.0 Hz), 7.64 (1H, dd, J = 10.0, 10.0 Hz), 7.77 (1H, dd, J = 9.6, 9.6 Hz), 8.03 (1H, d, J = 9.6 Hz), 9.25 (1H, s, OH), 9.38 (1H, d, J = 10.0 Hz). Anal. Calcd for C23H18N2O4: C 71.49, H 4.70, N 7.25. Found: C 71.63, H 4.84, N 7.36.

2-Amino-3-cyano-4-(4-methoxyphenyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3g): Deep red prism (from EtOH). mp 214-216 ºC; IR (KBr, cm-1): ν 3342 (NH2), 2209 (CN), 1642 (C=O). 1H-NMR (CDCl3): δ 1.27 (3H, t, J = 7.2 Hz, OCH2CH3), 3.94 (3H, s, OCH3), 4.50 (2H, q, J = 7.2 Hz, CO2CH2CH3), 4.84 (2H, s, NH2), 5.31 (1H, s, CH), 6.84-6.86 (2H, m), 7.07-7.09 (1H, m), 7.16 (1H, dd, J = 8.0, 8.0 Hz), 7.30 (1H, dd, J = 9.6, 9.6 Hz), 7.52 (1H, dd, J = 9.6, 10.0 Hz), 7.62 (1H, d, J = 9.6, 10.0 Hz), 8.01 (1H, d, J = 10.0 Hz), 9.48 (1H, d, J = 10.0 Hz). Anal. Calcd for C24H20N2O4: C 71.99, H 5.03, N 7.00. Found: C 72.13, H 5.15, N 7.21.
2-Amino-3-cyano-4-(4-hydroxyphenyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3h): Deep red prism (from EtOH). mp 206-207 ºC; IR (KBr, cm-1): ν 3334 (NH2), 3241 (OH), 2218 (CN), 1645 (C=O). 1H-NMR (DMSO): δ 1.36 (3H, t, J = 7.2 Hz, OCH2CH3), 4.40 (2H, q, J = 7.2 Hz, CO2CH2CH3), 5.54 (1H, s, CH), 6.78 (1H, d, J = 8.0 Hz), 6.81 (2H, s, NH2), 6.92 (1H, dd, J = 7.2, 7.2 Hz), 7.01 (1H, d, J = 8.0 Hz), 7.18 (1H, dd, J = 7.2, 7.2 Hz), 7.48 (1H, dd, J = 9.6, 9.6 Hz), 7.51~7.63 (2H, m), 8.63 (1H, d, J = 10.0 Hz), 9.01 (1H, d, J = 9.6 Hz), 10.51 (1H, s, OH). Anal. Calcd for C23H18N2O4: C 71.49, H 4.70, N 7.25. Found: C 71.72, H 4.86, N 7.42.
2-Amino-3-cyano-4-(2,4-dimethoxyphenyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3i): Deep red prism (from EtOH). mp 189-190 ºC; IR (KBr, cm-1): ν 3334 (NH2), 2208 (CN), 1649 (C=O). 1H-NMR (CDCl3, 400MHz) δ (ppm): 1.26 (3H, t, J = 7.2 Hz, OCH2CH3), 3.76 (3H, s, OCH3), 3.91 (3H, s, OCH3), 4.50 (2H, q, J = 7.2 Hz, CO2CH2CH3), 4.71 (2H, s, NH2), 5.63 (1H, s, CH), 6.32 (1H, d, J = 8.4 Hz), 6.50 (1H, s), 6.82 (1H, d, J = 8.4 Hz), 7.32 (1H, dd, J = 9.6, 9.6 Hz), 7.54 (1H, dd, J = 9.6, 10.0 Hz), 7.66 (1H, dd, J = 9.6, 9.6 Hz), 7.98 (1H, d, J = 10.0 Hz), 9.52 (1H, d, J = 10.0 Hz). Anal. Calcd for C25H22N2O5: C 69.76, H 5.15, N 6.51. Found: C 69.84, H 5.28, N 6.71.
2-Amino-3-cyano-4-(3,4-dimethoxyphenyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3j): Deep red prism (from EtOH). mp 209-210 ºC; IR (KBr, cm-1): ν 3334 (NH2), 2208 (CN), 1649 (C=O). 1H-NMR (DMSO): δ 1.40 (3H, t, J = 7.2 Hz, OCH2CH3), 3.66 (6H, s, 2xOCH3), 4.35 (2H, q, J = 7.2 Hz, CO2CH2CH3), 5.17 (1H, s, CH), 6.63 (1H, d, J = 8.0 Hz), 6.80~6.85 (2H, m), 6.98 (2H, s, NH2), 7.46 (1H, dd, J = 9.6, 9.6 Hz), 7.65 (1H, dd, J = 9.6, 10.0 Hz), 7.77 (1H, dd, J = 10.0, 10.0 Hz), 8.08 (1H, d, J = 10.0 Hz), 9.39 (1H, d, J = 10.0 Hz). Anal. Calcd for C25H22N2O5: C 69.76, H 5.15, N 6.51. Found: C 69.87, H 5.23, N 6.63.
2-Amino-3-cyano-4-(4-hydroxy-3-methoxyphenyl)-10-ethoxycarbonylazuleno[2,1-b]-pyran (3k): Deep red prism (from EtOH). mp 213-215 ºC; IR (KBr, cm-1): ν 3334 (NH2), 3261 (OH), 2208 (CN), 1649 (C=O). 1H-NMR (DMSO): δ 1.40 (3H, t, J = 7.2 Hz, OCH2CH3), 3.67 (3H, s, OCH3), 4.36 (2H, q, J = 7.2 Hz, CO2CH2CH3), 5.11 (1H, s, CH), 6.49 (1H, d, J = 8.0 Hz), 6.64 (1H, d, J = 8.0 Hz), 6.82 (1H, s), 6.96 (2H, s, NH2), 7.44 (1H, dd, J = 9.6, 10.0 Hz), 7.63 (1H, dd, J = 10.0, 10.0 Hz), 7.74 (1H, dd, J = 10.0, 10.0 Hz), 8.07 (1H, d, J = 10.0 Hz), 8.84 (1H, s, OH), 9.37 (1H, d, J = 10.0 Hz). Anal. Calcd for C24H20N2O5: C 69.22, H 4.84, N 6.73. Found: C 69.37, H 4.96, N 6.79.
2-Amino-3-cyano-4-(3-nitrophenyl)-10-ethoxycarbonoxyazuleno[2,1-b]pyran (3l): Deep red prism (from MeOH). mp 209-210 ºC; IR (KBr, cm-1): ν 3339 (NH2), 2212 (CN), 1656 (C=O). 1H-NMR (DMSO): δ 1.41 (3H, t, J = 7.2 Hz, OCH2CH3), 4.36 (2H, q, J = 7.2 Hz, CO2CH2CH3), 5.51 (1H, s, CH), 7.20 (2H, s, NH2), 6.50 (1H, dd, J = 9.6, 9.6 Hz), 7.56 (1H, dd, J = 8.0, 8.2 Hz), 7.64 (1H, d, J = 8.4 Hz), 7.68 (1H, d, J = 9.6 Hz), 7.77 (1H, dd, J = 9.6, 9.6 Hz), 8.04~8.07 (2H, m), 8.09 (1H, d, J = 9.6 Hz), 9.42 (1H, d, J = 10.0 Hz). Anal. Calcd for C23H17N3O5: C 66.50, H 4.12, N 10.12. Found: C 66.64, H 4.33, N 10.26.
2-Amino-3-cyano-4-(2-thienyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3m): Deep red prism (from MeOH). mp 208-210 ºC; IR (KBr, cm-1): ν 3328 (NH2), 2217 (CN), 1657 (C=O). 1H-NMR (DMSO): δ 1.40 (3H, t, J = 7.2 Hz, OCH2CH3), 4.35 (2H, q, J = 7.2 Hz, CO2CH2CH3), 5.63 (1H, s, CH), 6.90 (1H, d, J = 3.2 Hz), 7.12 (2H, s, NH2), 7.13~7.15 (2H, m), 7.28 (1H, d, J = 4.4 Hz), 7.51 (1H, dd, J = 9.6, 9.6 Hz), 7.66 (1H, dd, J = 9.6, 10.0 Hz), 7.81 (1H, dd, J = 9.6, 9.6 Hz), 8.22 (1H, d, J = 9.6 Hz), 9.39 (1H, d, J = 10.0 Hz). Anal. Calcd for C21H16N2O3S: C 67.00, H 4.28, N 7.44. Found: C 67.24, H 4.36, N 7.61.
2-Amino-3-cyano-4-(2-furyl)-10-ethoxycarbonylazuleno[2,1-b]pyran (3n): Deep red prism (from MeOH). mp 187-189 ºC; IR (KBr, cm-1): ν 3341 (NH2), 2213 (CN), 1652 (C=O). 1H-NMR (DMSO): δ 1.40 (3H, t, J = 7.2 Hz, OCH2CH3), 4.39 (2H, q, J = 7.2 Hz, CO2CH2CH3), 4.90 (2H, s, NH2), 5.38 (1H, s, CH), 6.19 (1H, d, J = 2.8 Hz), 6.30 (1H, dd, J = 1.8, 2.6 Hz), 7.30 (1H, d, J = 1.0 Hz), 7.47 (1H, dd, J = 9.6, 9.6 Hz), 7.62 (1H, dd, J = 9.6, 10.0 Hz), 7.76 (1H, dd, J = 9.6, 9.6 Hz), 8.24 (1H, d, J = 9.6 Hz), 9.39 (1H, d, J = 9.6 Hz). Anal. Calcd for C21H16N2O4: C 69.99, H 4.48, N 7.77. Found: C 69.87, H 4.61, N 7.65.

ACKNOWLEDGEMENT
We thank the Innovation Team Foundation of Liaoning Education Department (No. 2008T001).

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