Synthesis of heterocyclic compounds has emerged as a powerful technique for generating new molecules useful for drug discovery.1 Coumarin derivatives have been reported to exhibit anti-inflammatory,2 antimicrobial,3 antioxidant,4 anticancer,5 and antiviral6 activities. Naturally occurring coumarins are of great interest due to their diverse pharmacological properties, and this attracts attention of many medicinal chemists to further derivatization of the heterocycle backbone and screening the resulting compounds for their biological activity. Chromenes and fused chromenes are biologically important compounds with antiHIV,7 antifungal,8 antitumor,9 and antiviral10 activities. It is reported in the literature that when one biodynamic heterocyclic system is coupled with another heterocyclic system, biological activity increases.11 Thus, pyrane ring condensed with coumarin ring system gives polycyclic molecules, called pyranochromenes or pyranocoumarins which may exhibit better biological activity.

Many of the natural compounds containing a pyranochromene moiety are known for various biological activities12 (Fig. 1). Among them xanthyletin, a linear pyranocoumarin, posseses antitubercular activity.13 Seselin, an angular pyranocoumarin isolated from citrus roots, has shown activity against skin cancer.14 A large variety of natural products have been described as antiHIV agents, and pyranocoumarin derivatives are also among them. The pyranocoumarins (+)-calanolide A,15 inophyllum G-1,16 and pseudocordatolide C17 have their potential application for treating HIV infection. The present work describes synthesis of some new arylpyranocoumarins and their antimicrobial activity.

Fig. 1
figure 1

Structures of some naturally occurring pyranochromenes.

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Aryl–aryl and aryl–vinyl bond formation is very important in organic synthesis and has a wide range of applications of industrial interest, including the synthesis of pharmaceuticals, herbicides, polymers, and other materials.18 , 19 The coupling of aryl and vinyl halides with organoboronic acids is one of the most important metalcatalyzed cross-coupling reactions, known as the Suzuki or Suzuki–Miyaura coupling. This palladium-catalyzed reaction is certainly one of the most attractive methods for preparing biaryl and arylated compounds because of its functional-group tolerance, the use of stable and non-toxic organoboron reagents, and the possibility of using aqueous solvents as reaction medium.20

The application of microwave irradiation (MW) can provide enhanced reaction rate and improved product yield in the formation of a variety of carbon–heteroatom bonds. During recent years, microwaves have been extensively used for carrying out chemical reactions and have become a useful non-conventional energy source for performing organic synthesis.21 Our research group has been making considerable efforts towards designing and carrying out innovative synthetic protocols in organic synthesis adopting a more environmentally friendly approach.22 – 24

Earlier we have reported some 4-chlorochromone derivatives which showed potential biological activity.25 In view of the potential bioactivity of pyranocoumarins, we have carried out the synthesis of some new 10-aryl-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3-f]chromene-9-carbaldehydes 4a–j using the Suzuki coupling under microwave irradiation. The synthesized compounds were screened for their antibacterial and antifungal activity.

The synthetic route to compounds 4a–j is shown in Scheme 1. The reaction of flavanone 1 with Vilsmeier reagent (DMF/POCl3) by microwave irradiation for 5 min gave 10-chloro-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano-[2,3-f]chromene-9-carbaldehyde (2)26 with 80% yield. Subsequently compound 2 underwent the Suzuki coupling with arylboronic acids 3a–j, yielding 10-aryl-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3-f]chromene-9-carbaldehydes 4a–j. Na2CO3 was chosen as the base and Pd(PPh3)4 was used as the catalyst.27 Initially, we optimized the Suzuki coupling conditions by running the synthesis of compound 4a at different catalyst loadings (Table 1). The use of 10 mol % Pd(PPh3)4 was found to be the optimal condition. The proposed structure of compound 4a was confirmed by spectral data as 4-methyl-2-oxo-8,10-diphenyl-2,8-dihydropyrano[2,3-f]chromene-9-carbaldehyde. The base peak in the mass spectrum of compound 4a at m/z 395 corresponding to [M+H]+ confirms the Suzuki coupled product. We observed that the use of MW irradiation instead of conventional heating brought further improvement of the reaction yield along with a significant decrease of the reaction time (Tables 1, 2).

Scheme 1
scheme 1

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Table 1 Yields of compound 4a at different catalyst loads and heating methods
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Table 2 Yields of the synthesized compounds 4a–j
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The synthesized novel compounds 4a–j (Table 3) were screened for their antibacterial activity against different types of bacterial Gram-positive and Gram-negative strains at a concentration of 100 μg/ml.

Table 3 Antibacterial activity of compounds 2, 4a–j
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From the screening studies it is evident that the replacement of chlorine atom by an aryl group enhanced the activity of compounds against all tested bacteria. Compounds 4a,c,f–j showed high activity against Grampositive Staphylococcus aureus, and compounds 4a,b,g showed better activity against Gram-positive Bacillus subtilis, compound 4g being more active than the standard drug amoxicillin against both strains.

The activity of compounds 4b,f,g was high against Gram-negative Escherichia coli, and compounds 4a–h had high activity against Gram-negative Pseudomonas aeruginosa compared to the standard drug. It was observed that compound 4g exhibited broad spectrum of antibacterial activity against all the tested strains.

The antifungal activity of the synthesized compounds 4a–j (Table 4) was tested against three pathogenic fungi at a concentration of 100 μg/ml. Replacement of chlorine atom by aryl group enhanced the activity of compounds against Aspergillus niger, Penicillium italicum. In case of Fusarium oxysporum, there was a minor enhancement of activity in comparison to compound 2. Compounds 4a–h showed high activity against Aspergillus niger, compounds 4a,b,e,g showed high activity against Penicillium italicum, and compounds 4a,b,c,d,g had high activity against Fusarium oxysporum compared to standard drug mycostatin at a concentration of 100 μg/ml. It was observed that compounds 4a,b,g showed broad spectrum of antifungal activity against all tested fungal species.

Table 4 Antifungal activity of compounds 2, 4a–j
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An efficient microwave synthesis of pyranocoumarin derivatives have been carried out successfully under mild reaction conditions. All the final compounds were investigated for their in vitro antimicrobial activity. Substitution of chlorine atom at pyranocoumarin ring by an aryl group enhanced the antibacterial activity. The p-acetylphenyl derivative exhibited broad spectrum of antibacterial activity against all the tested bacterial and fungal strains, and two other compounds showed a promising antifungal activity against tested strains compared to the standard drug mycostatin. The microwave irradiation process proved to be a simple environmentally friendly technique with high yields and short reaction times.

Experimental

IR spectra were recorded in KBr on a Shimadzu FTIR 8400S spectrophotometer. 1H and 13C NMR spectra were recorded on a Bruker Avance II 400 spectrometer (400 and 100 MHz, respectively) in CDCl3 using TMS as internal standard. Mass spectra were recorded on a Shimadzu GCMS-QP 1000 mass spectrometer. The elemental analysis was carried out on a Vario-11 CHN analyzer. Melting points were determined in open glass capillaries on a Stuart SMP30 apparatus and are uncorrected. Purity of the compounds was checked by TLC on silica gel 60 F254 (Merck). All the microwave irradiation experiments were performed in a CEM Discover microwave system and reaction temperatures were monitored by an equipped IR sensor.

Synthesis of 10-aryl-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehydes 4a–j. Conventional heating (General method). 10-Chloro-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3-f]chromene-9-carbaldehyde (2) (353 mg, 1.0 mmol), Pd(PPh3)4 (115 mg, 0.10 mmol), arylboronic acid (1.2 mmol), and Na2CO3 (318 mg, 3.0 mmol) were sequentially added into a roundbottomed flask. The mixture was dissolved in DMF (5 ml) and degassed with nitrogen over 15 min. The reaction mixture was stirred at 80°C for 6–8 h (Table 2) under nitrogen atmosphere. After the reaction was completed, the mixture was diluted with EtOAc and washed with H2O and brine solution. The organic layer was washed with H2O (10 ml) and dried over anhydrous MgSO4. The solvent was evaporated, and the residue was purified by using silica gel column chromatography (hexane–AcOEt, 3:1).

Synthesis of 10-aryl-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehydes 4a–j. Microwave heating (General method). A degassed mixture of 10-chloro-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3-f]-chromene-9-carbaldehyde (2) (353 mg, 1.0 mmol), Pd(PPh3)4 (115 mg, 0.10 mmol), arylboronic acid (1.2 mmol), Na2CO3 (318 mg, 3.0 mmol), and DMF (3 ml) was introduced into a microwave reaction vessel equipped with a magnetic stirrer. The vessel was sealed and then placed into the microwave cavity. Initial microwave irradiation of 180 W was used, the temperature being ramped from room temperature to the desired 80°C temperature. The reaction mixture was heated at this temperature under continuous stirring for the appropriate time (Table 2). The reaction mixture was then cooled to room temperature, diluted with EtOAc (20 ml), and washed with H2O and brine solution. The organic layer was washed with water (10 ml) and dried over anhydrous MgSO4. The solvent was evaporated, and the residue was purified by using silica gel column chromatography (hexane–AcOEt, 3:1).

4-Methyl-2-oxo-8,10-diphenyl-2,8-dihydropyrano[2,3- f ]-chromene-9-carbaldehyde (4a). Yield 87%, pale-yellow solid, mp 160–162°C. IR spectrum, ν, cm−1: 1078 (C–O–C), 1749 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.31 (3H, d, J = 1.0, CH3); 5.97 (1H, d, J = 1.0, H-3); 6.54 (1H, s, 8-CH); 6.82–6.93 (2H, m, H-2',6'); 6.99 (1H, d, J = 8.8, H-6); 7.21–7.41 (8H, m, H-3',4',5' H Ph); 7.53 (1H, d, J = 8.8, H-5); 9.68 (1H, s, CHO). 13C NMR spectrum, δ, ppm: 18.9 (CH3); 73.2; 111.8; 112.3; 114.2 (C-3); 114.9; 121.6; 126.9; 128.4; 128.5; 128.6; 128.9; 129.2; 129.7; 134.2; 137.6; 148.7; 152.0; 152.3; 157.9 (C-6a); 158.7 (C=O); 190.5 (CHO). Mass spectrum, m/z (I rel, %): 395 [M+H]+ (100). Found, %: C 79.21; H 4.57. C26H18O4. Calculated, %: C 79.17; H 4.60.

10-(4-Methoxyphenyl)-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehyde (4b). Yield 85%, pale-yellow solid, mp 154–156°C. IR spectrum, ν, cm−1: 1084 (C–O–C), 1745 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.31 (3H, d, J = 1.0, CH3); 3.88 (3H, s, OCH3); 5.97 (1H, d, J = 1.0, H-3); 6.52 (1H, s, 8-CH); 6.98–7.00 (3H, m, H-6, 2H Ar); 7.24–7.39 (7H, m, 2H Ar, H Ph); 7.53 (1H, d, J = 8.8, H-5); 9.70 (1H, s, CHO). 13C NMR spectrum, δ, ppm: 18.9 (CH3); 55.4; 73.2 (C-8); 112.3; 113.3; 114.1 (2C); 114.5; 114.9; 126.2; 126.9 (2C); 128.4 (2C); 128.5 (2C); 129.5; 134.3; 137.6; 142.1; 148.5; 151.9; 152.3; 158.0; 158.8; 160.6 (C=O); 190.7 (CHO). Mass spectrum, m/z (I rel, %): 425 [M+H]+ (100). Found, %: C 76.44; H 4.78. C27H20O5. Calculated, %: C 76.40; H 4.75.

10-(4-Fluorophenyl)-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehyde (4c). Yield 85%, pale-yellow solid, mp 175–177°C. IR spectrum, ν, cm−1: 1080 (C–O–C), 1742 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.31 (3H, d, J = 1.0, CH3); 5.97 (1H, d, J = 1.00, H-3); 6.54 (1H, s, 8-CH); 6.98–7.00 (3H, m, H-6, 2H Ar), 7.27–7.29 (3H, m, H-3',4',5'); 7.36–7.38 (2H, m, H-2',6'); 7.53–7.55 (3H, m, H-5, H Ar); 9.66 (1H, s, CHO). 13C NMR spectrum, δ, ppm: 19.3 (CH3); 73.7 (C-8); 112.7; 114.0; 115.2; 115.5; 120.2; 121.3; 121.6; 122.0; 122.3; 122.9; 124.5; 126.9; 128.6; 129.0; 129.1; 129.6; 134.4; 136.1; 150.4; 152.4; 158.4; 159.3; 160.2 (C=O); 161.2; 190.8 (CHO). Mass spectrum, m/z (I rel, %): 413 [M+H]+ (100). Found, %: C 75.74; H 4.16. C26H17FO4. Calculated, %: C 75.72; H 4.15.

10-(2-Fluorophenyl)-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehyde (4d). Yield 80%, pale-yellow solid, mp 170–172°C. IR spectrum, ν, cm−1: 1080 (C–O–C), 1742 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.31 (3H, d, J = 1.0, CH3); 6.01 (1H, d, J = 1.0, H-3); 6.67 (1H, s, 8-CH); 6.90 (1H, d, J = 8.8, H-6); 7.25–7.50 (9H, m, H Ar, H Ph); 7.55 (1H, d, J = 8.8, H-5); 9.68 (1H, s, CHO). 13C NMR spectrum, δ, ppm: 19.3 (CH3); 73.7 (C-8); 112.7; 114.0; 115.2; 115.4; 120.2; 121.3; 121.6; 122.0; 122.3; 122.9; 124.0; 124.5; 125.3; 126.0; 126.9; 128.6; 129.0; 129.1; 129.6; 134.4; 136.1; 150.8; 152.4; 158.4; 159.3; 161.2; 160.2 (C=O); 190.7 (CHO). Mass spectrum, m/z (I rel, %): 413 [M+H]+ (100). Found, %: C 75.75; H 4.17. C26H17FO4. Calculated, %: C 75.72; H 4.15.

10-(4-Chlorophenyl)-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehyde (4e). Yield 80%, pale-yellow solid, mp 170–172°C. IR spectrum, ν, cm−1: 1082 (C–O–C), 1742 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.45 (3H, d, J = 1.0, CH3); 6.20 (1H, d, J = 1.0, H-3); 6.78 (1H, s, 8-CH); 6.96 (1H d, J = 8.8, H-6); 7.33–7.75 (9H, m, H Ar, H Ph); 7.86 (1H, d, J = 8.8, H-5); 9.53 (1H, s, CHO). 13C NMR spectrum, δ, ppm: 19.3 (CH3); 73.7 (C-8); 112.7; 114.0; 115.2 (C-3); 115.4; 120.2; 121.6; 122.9; 124.5; 126.9; 128.6; 129.0; 129.1; 129.6; 134.4; 136.8; 150.3; 152.4; 158.4; 159.3; 160.2 (C=O); 161.2; 190.5 (CHO). Mass spectrum, m/z (I rel, %): 429 [M+H]+ (100). Found, %: C 72.85; H 4.03. C26H17ClO4. Calculated, %: C 72.82; H 4.00.

10-(2,4-Dichlorophenyl)-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehyde (4f). Yield 80%, pale-yellow solid, mp 181–183°C. IR spectrum, ν, cm−1: 1082 (C–O–C), 1742 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.36 (3H, d, J = 1.0, CH3); 6.08 (1H, d, J = 1.0, H-3); 6.69 (1H, s, 8-CH); 6.88 (1H, d, J = 8.8, H-6); 7.27–7.49 (8H, m, H Ar, H Ph); 7.61 (1H, d, J = 8.8, H-5); 9.74 (1H, s, CHO). 13C NMR spectrum, δ, ppm: 19.3 (CH3); 73.7 (C-8); 112.7; 114.0; 114.7 (C-3); 115.2; 120.2; 122.3; 122.9; 124.5; 126.9; 128.6; 129.0; 129.1; 129.6; 134.4; 136.1; 150.3; 152.4; 158.4 (C-6a); 159.3; 161.2; 162.4 (C=O); 190.5 (CHO). Mass spectrum, m/z (I rel, %): 463 [M+H]+ (100). Found, %: C 67.44; H 3.51. C26H16Cl2O4. Calculated, %: C 67.40; H 3.48.

10-(4-Acetylphenyl)-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehyde (4g). Yield 84%, pale-yellow solid, mp 156–158°C. IR spectrum, ν, cm−1: 1084 (C–O–C), 1745 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.31 (3H, d, J = 1.0, CH3); 3.68 (3H, s, COCH3); 5.95 (1H, d, J = 1.0, H-3); 6.54 (1H, s, 8-CH); 7.01 (1H, d, J = 8.8, H-6); 7.29–7.69 (10H, m, H-5, H Ar, H Ph); 9.62 (1H, s, CHO). 13C NMR spectrum, δ, ppm: 19.0 (CH3); 24.3; 76.4 (C-8); 112.0; 112.8; 114.3 (C-3); 114.5; 114.9; 121.5; 122.2 (2C); 123.8; 126.6; 126.8; 128.4; 129.0; 137.8; 150.2; 151.6; 152.3; 157.3 (C-6a); 160.0; 160.2 (C=O); 190.6 (CHO). Mass spectrum, m/z (I rel, %): 437 [M+H]+ (100). Found, %: C 77.08; H 4.64. C28H20O5. Calculated, %: C 77.05; H 4.62.

10-(4-Formylphenyl)-4-methyl-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehyde (4h). Yield 80%, pale-yellow solid, mp 168–170°C. IR spectrum, ν, cm−1: 1084 (C–O–C), 1749 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.45 (3H, d, J = 1.0, CH3); 6.21 (1H, d, J = 1.0, H-3); 6.92 (1H, s, 8-CH); 6.95 (1H, d, J = 8.8, H-6); 7.05 (2H, d, J = 9.0, H Ar); 7.35–7.86 (8H, m, H-5, H Ar, H Ph); 9.96 (1H, s, CHO); 10.24 (1H, s, ArCHO). 13C NMR spectrum, δ, ppm: 18.9 (CH3); 73.2 (C-8); 111.8; 112.3; 114.2 (C-3); 114.9; 121.9; 128.4; 128.5; 128.6;128.9; 129.2; 129.7; 134.2 (C-3); 137.6; 148.7; 152.0; 152.3; 157.9 (C-6a); 158.7; 162.2 (C=O); 188.9; 190.5 (CHO). Mass spectrum, m/z (I rel, %): 423 [M+H]+ (100). Found, %: C 76.79; H 4.31. C27H18O5. Calculated, %: C 76.77; H 4.29.

4-Methyl-10-(naphthalen-1-yl)-2-oxo-8-phenyl-2,8-dihydropyrano[2,3- f ]chromene-9-carbaldehyde (4i). Yield 82%, pale-yellow solid, mp 152–154°C. IR spectrum, ν, cm−1: 1083 (C–O–C), 1746 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.25 (3H, d, J = 1.0, CH3); 5.79 (1H, d, J = 1.0, H-3); 6.62 (1H, s, 8-CH); 7.03 (1H, d, J = 8.8, H-6); 7.37–7.99 (13H, m, H-5, H Ar, H Ph), 9.33 (1H, s, CHO). 13C NMR spectrum, δ, ppm: 18.8 (CH3); 73.5 (C-8); 112.1; 112.4; 113.9 (C-3); 114.9; 124.7; 125.1; 125.5; 126.2; 126.6, 126.9; 128.6; 128.7 (3C); 128.8; 129.2; 131.5; 132.0; 133.5; 133.6; 138.4; 145.3; 146.5; 151.3; 152.1; 157.9 (C=O); 190.5 (CHO). Mass spectrum, m/z (I rel, %): 445 [M+H]+ (100). Found, %: C 81.10; H 4.57. C30H20O4. Calculated, %: C 81.07; H 4.54.

4-Methyl-2-oxo-8-phenyl-10-( p -tolyl)-2,8-dihydropyrano-[2,3- f ]chromene-9-carbaldehyde (4j). Yield 80%, pale-yellow solid, mp 178–180°C. IR spectrum, ν, cm−1: 1078 (C–O–C), 1731 (C=O). 1H NMR spectrum, δ, ppm (J, Hz): 2.43 (3H, s, CH3); 2.44 (3H, d, J = 1.0, 4-CH3); 6.20 (1H, d, J = 1.0, H-3); 6.85 (1H, s, 8-CH); 6.95 (1H, d, J = 8.8, H-6); 7.31–7.86 (10H, m, H-5, H Ar, H Ph); 10.02 (1H, s, CHO). 13C NMR spectrum, δ, ppm: 19.0 (CH3); 55.3 (ArCH3); 76.4 (C-8); 112.3; 113.3; 114.1 (C-3); 114.5; 114.9; 121.5; 122.2; 123.8; 124.0; 126.6; 127.5; 128.4; 129.0; 137.8; 150.2; 151.6; 152.3; 157.3 (C-6a); 159.4 (C=O); 190.2 (CHO). Mass spectrum, m/z (I rel, %): 409 [M+H]+ (100). Found, %: C 79.44; H 4.97. C27H20O4. Calculated, %: C 79.40; H 4.94.

Biological activity tests. All the prepared compounds were screened for their antimicrobial activity against two strains of Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis), two strains of Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa), as well as three strains of fungi (Aspergillus niger, Penicillium italicum, and Fusarium oxysporum). Standard antibiotic drugs amoxicillin for bacteria and mycostatin for fungi were used for comparison. The biological activity of these compounds has been evaluated by filter paper disc method28 on the test compounds and standards dissolved in DMF to obtain concentration 100 μg/ml. The inhibition zones of microbial growth surrounding the filter paper disc (5 mm) were measured in millimeters at the end of an incubation period of 4 days at 47°C for Escherichia coli and at 28°C for other bacteria and fungi. Pure solvent DMF showed no inhibition zone.

Supporting material to this article containing 1H NMR spectra of compounds 2, 4a–e,g–j, 13C NMR spectra of compounds 2, 4a,b,i, and mass spectra of compounds 3, 4a,b is available for the authorized users.