Pentafluoropropionic acid (PFPA) efficiently catalyzes the one-pot, three-component reaction of aromatic aldehyde, malononitrile, and dimedone to yield tetrahydrobenzo[b]pyran derivatives in high yields. This method is of great value because of its easy processing, short reaction time, environmentally, and high yields.

1. Introduction

Tetrahydrobenzo[b]pyran derivatives are important classes of heterocyclic compounds. Tetrahydrobenzo[b]pyran compounds are known to possess a variety of biological activities, such as anticoagulant, spasmolytic, diuretic, anticancer, and antianaphylactin properties [15]. Furthermore these compounds can be employed as pigments, and they constitute the structural unit of a series of natural products [6, 7]. In view of different biological and chemical applications of tetrahydrobenzo[b]pyran derivatives, the development of suitable synthetic methodologies for generation has been a topic of great interest in recent times [8]. A number of 2-amino-tetrahydropyran derivatives are useful as photoactive materials [9]. Several methods have been reported for the synthesis of tetrahydrobenzo[b]pyran derivatives from aromatic aldehydes, dimedone, and malononitrile, involving the use of catalysts such as ZnO-beta zeolite [10], Mw-NaBr [11], Na2SeO4 [12], Caro’s acide-SiO2 [13], trisodium citrate [14], PPA-SiO2 [15], KF-basic alumina under ultrasound irradiation [16], TEBA [17], 1-butyl-3-methylimidazolium hydroxide [18], sodium hypochlorite [19], 1,4-diazabicyclo[] octane (DABCO) [20], triethylamine [21], [22], silica gel supported polyamine [23], urea-choline chloride [24], 2,2,2-trifluoroethanol [25], tetramethyl ammonium hydroxide [26], ammonium acetate [27], and tetrabutylammonium bromide [28].

However, most of these procedures have significant drawbacks such as harsh reaction conditions, difficult workup, and expensive reagents. These problems promoted us towards further investigation in search for a new catalyst, which will carry out the synthesis of tetrahydrobenzo[b]pyrans under simpler experimental setup. In continuation of our efforts to develop novel synthetic routes using metal-free catalysts in organic reactions [2931], and due to our interest in the synthesis of heterocyclic compounds [32], herein we wish to report an efficient synthesis of tetrahydrobenzo[b]pyran derivatives by cyclocondensation reaction of aromatic aldehydes, dimedone, and malononitrile using pentafluoropropionic acid as a metal-free catalyst (Scheme 1).


2. Experimental

2.1. General

All chemicals were obtained from Merck or Fluka and were used without further purification. Melting points were recorded on an electrothermal type 9100 melting point apparatus and are uncorrected. Infrared spectra were obtained in KBr disks on shimadzu IR-470 spectrometer. 1H NMR and 13C NMR spectra were recorded on a Bruker, DRX-400 Avance Bruker spectrometer, at 400.13 MHz and 100.22 MHz, respectively, in CDCl3 and chemical shifts are in ppm (δ) relative to internal TMS.

2.2. General Procedure for the Synthesis of Tetrahydrobenzo[b]pyrans (4a–j)

A solution of dimedone 1 (1 mmol), an aromatic aldehyde 2a–j (1 mmol), malononitrile 3 (1.2 mmol), and pentafluoropropionic acid (35 mol%) in H2O (10 mL) and EtOH (10 mL) was stirred at room temperature for the time period as indicated in Table 1. The progress of the reaction was monitored by TLC. After completion of reaction, the solid product was collected by filtration and recrystallized from ethanol to afford pure products 4a–j (Table 2) in high yields. All the products were identified by comparison of spectral data (IR and 1H NMR) and m.p. with those reported.

2.3. Physical and Spectral Data for the Selected Compounds

2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-phenyl-4H-benzopyran (4a). m.p. = 227–229°C; IR (KBr) cm−1 3410, 3330 (NH2), 3050 (C–H), 2233 (CN), 1681 (C=O), 1380 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.32–7.19 (m, 5H), 4.56 (s, 2H, NH2), 4.42 (s, 1H, CH), 2.47 (dd, 2H,  Hz, CH2), 2.24 (dd, 2H,  Hz, CH2), 1.13 (s, 3H, CH3), 1.06 (s, 3H, CH3).

2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(3-nitrophenyl)-5-oxo-4H-benzopyran (4b). m.p. = 207–210°C; IR (KBr) cm−1 3420, 3300 (NH2), 3012 (C–H), 2257 (CN), 1710 (C=O), 1557, 1360 (NO2); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 8.11–7.48 (m, 4H), 4.80 (s, 2H, NH2), 4.53 (s, 1H, CH), 2.50 (dd,  Hz, 2H, CH2), 2.25 (dd,  Hz, 2H, CH2), 1.12 (s, 3H, CH3), 1.08 (s, 3H, CH3).

2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(4-methylphenyl)-5-oxo-4H-benzopyran (4d). m.p. = 216–218°C; IR (KBr) cm−1 3421, 3297 (NH2), 2966 (C–H), 2194 (CN), 1674 (C=O), 1369 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.08–7.14 (m, 4H), 4.54 (s, 2H, NH2), 4.38 (s, 1H, CH), 2.46 (dd,  Hz, 2H, CH2), 2.29 (s, 3H, CH3), 2.23 (dd,  Hz, 2H, CH2), 1.12 (s, 3H, CH3), 1.06 (s, 3H, CH3).

2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(3-methoxyphenyl-5-oxo-4H-benzopyran (4f). m.p. = 207–209°C; IR (KBr) cm−1 3397, 3291 (NH2), 2295 (CN), 1678 (C=O), 1384 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.29–6.76 (m, 4H), 4.57 (s, 2H, NH2), 4.40 (s, 1H, CH), 3.81 (m, 3H, CH3), 2.47 (dd,  Hz, 2H, CH2), 2.26 (dd,  Hz, 2H, CH2), 1.14 (s, 3H, CH3), 1.08 (s, 3H, CH3); 13C NMR (100.22 MHz, CDCl3) δ (ppm): 27.7 (CH3), 28.8 (CH3), 32.2 (C), 35.4 (CH2), 40.6 (CH2), 50.6 (CH3), 55.2 (CH), 63.5 (C), 112.3 (CH), 113.5 (CH), 113.9 (C), 118.6 (C), 119.9 (CH), 129.6 (CH), 144.8 (C), 157.4 (C), 159.7 (C), 161.6 (C), 195.8 (C).

2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(4-chlorophenyl)-5-oxo-4H-benzopyran (4h). m.p. = 213–215°C; IR (KBr) cm−1 3421, 3105 (NH2), 2185 (CN), 1686 (C=O), 1356 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.25–7.02 (m, 4H), 4.57 (s, 2H, NH2), 4.41 (s, 1H, CH), 2.47 (s, 2H, CH2), 2.24 (dd,  Hz, 2H, CH2), 1.14 (s, 3H, CH3), 1.05 (s, 3H, CH3).

3. Result and Discussion

In order to optimize the reaction conditions, including solvents and temperature, the reaction was conducted under various conditions and the results are listed in Table 1. In an optimized reaction condition, benzaldehyde (1 mmol), dimedone (1 mmol), and malononitrile (1.2 mmol) in H2O (10 mL) and EtOH (10 mL) were mixed in the presence of pentafluoropropionic acid (35 mol%) as catalyst for 60–80 min. The reaction proceeds very cleanly at room temperature and was free of side products. After completion of the reaction (monitored by TLC), a simple workup affords the products in high yields (Scheme 1). Among the solvents tested, the reaction in H2O, EtOH, CHCl3, and CH3CN using 35 mol% of the catalyst gave a moderate yield of the desired product at room temperature. However in the mixture of EtOH and H2O relatively high yield of the product is obtained at room temperature after 80 min. Without catalyst, in refluxing EtOH, H2O, CHCl3, CH3CN, and mixture of EtOH-H2O or at room temperature in this solvents the reaction times are prolonged and the yields are poor. In the solvent-free conditions, even in the presence of 40 mol% of the catalyst at room temperature, the yields are moderate. The results are summarized in Table 1. For comparison, we also investigated the efficiency of trifluoroacetic acid (TFAA) as catalyst in this model reaction. As shown in Table 1, it can be seen that PFPA proved to be a better catalyst than TFAA in terms of reaction time and yield.

We also evaluated the amount of pentafluoropropionic acid required for this transformation. It was found that the yield of product was affected by the catalyst amount. Increasing the amount of the catalyst up to 35 mol% in the mixture of EtOH and H2O at room temperature increased the yield of the product. Further increase in the catalyst amount did not increase the yield noticeably. In order to show generality and scope of this new protocol, we used various substituted aromatic aldehydes and the results obtained are summarized in Table 2.

In all cases, aromatic aldehydes with substituents carrying either electron-donating or electron-withdrawing groups reacted successfully and gave the expected products in high yields and short reaction times. The type of aldehyde had no significant effect on the reaction. The efficiency of pentafluoropropionic acid as a catalyst for the synthesis of the 2-amino-3-cyano-5,6,7,8-tetrahydro-7, 7-dimethyl-5-oxo-4-phenyl-4H-benzopyran (4a), was compared with that of other catalysts reported in the literature. Some of the results are summarized in Table 3. It is clear from this table that pentafluoropropionic acid is an efficient and environmentally benign catalyst which could be useful in the synthesis of a series of tetrahydrobenzo[b]pyran derivatives.

4. Conclusion

In conclusion, a mild and efficient method is proposed for the one-pot three-component reactions of aromatic aldehydes, dimedone, and malononitrile using pentafluoropropionic acid catalyst for synthesis of tetrahydrobenzo[b]pyran derivatives. Some attractive features of this protocol are high yields, easy workup, and the simplicity of the procedure.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.


This research has been supported by the Islamic Azad University, Tonekabon Branch.