Introduction

Chromenes have been widely employed as important intermediates in the synthesis of many natural products and medical agents. They are widely distributed in nature displaying diverse range of biological activities (Cassidy et al., 1992). These are found to be interesting sharing common structural features for a family of potassium channel activating drugs (Ashwood et al., 1986). They also serve as the framework of tannins (Rochfort et al., 1996), which are becoming important because of their health-promoting effects found in red wine, vegetables, fruits and teas.

4-Chlorochromene derivatives are extensively used as versatile building blocks for the synthesis of many oxygen heterocyclic systems with potential biological activity (Rimbault et al., 1985).

Chromenes and fused chromenes have raised the interest of researchers because of their potential for different biological activities including antibacterial (El-Agrody et al., 2000, Zamocka et al., 1992), antifungal (Ohira and Yatagai, 1993) antitumor (Mohr et al., 1975), and antiviral (Martinez and Marco, 1997) activities. Coumarin derivatives have been reported to exhibit anti-inflammatory (Jung et al., 2008), antimicrobial (Mulwad and Shirodkhar, 2002), antioxidant (Kusanur et al., 2004), anticancer (Musiliyu et al., 2011) and chemoprophylactic (Nofal et al., 2005) activities. The pyranocoumarins soulattrolide, inophyllum G-1, cordatolide A and oblongulide are found to exhibit potential anti-HIV activity (Patil et al., 1993). Herein we hypothesized that hybrid compounds containing both coumarin and chromene moieties, called pyranocoumarins may exhibit better biological activity.

In recent years, microwave-assisted organic synthesis (MAOS) has gained popularity as an environmental benign technology. Microwave-assisted synthesis leads to significant reduced reaction times, enhanced conversions and known to be environment friendly. Earlier, we have reported 4-chlorochromene derivatives possessing potential antimicrobial activity (Ashok et al., 2012). Encouraged by the pharmacological activities of chlorochromenes, chromenes and fused chromenes, we wish to report some new 4-Chloro-8-methyl-2-phenyl-1,5-dioxa-2H-phenanthren-6-ones by simple and convenient microwave irradiation method. Here, we described the synthesis of some new pyranocoumarin derivatives (3a–o) from 8-acetyl-7-hydroxy-4-methyl-2H-chromen-2-one 1 and their antimicrobial activity (Fig. 1).

Fig. 1
figure 1

Representative examples of pyranocoumarin derivatives, exhibiting anti-HIV activity

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Results and discussion

Chemistry

The synthetic route to compounds 3a–o was shown in Scheme 1. Compounds 2a–o were synthesized according to the literatures (Khan and Sharma, 1993). The condensation of 8-acetyl-7-hydroxy-4-methyl coumarin 1 with aromatic or hetero aromatic aldehydes in the presence of piperdine under microwave irradiation gave substituted (E)-1-(7-hydroxy-4-methyl-8-coumarinyl)-3-phenyl-2-propen-1-ones 2a–o in excellent yields. Subsequently, these chalcones 2a–o on reaction with Vilsmeier–Haack reagent (DMF/POCl3) yielded substituted 4-chloro-8-methyl-2-phenyl-1,5-dioxa-2H-phenanthren-6-ones 3a–o. Initially, we performed the reaction at room temperature but there was no product formed. Optimum results were obtained when the temperature was maintained at 90–100 °C by using 6 equivalents of POCl3.

Scheme 1
scheme 1

Synthesis of substituted 4-chloro-8-methyl-2-phenyl-1,5-dioxa-2H-phenanthren-6-ones

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In case of microwave irradiation method, optimum results obtained by irradiating at 160 W for 4–5 min. The crude product was purified using column chromatography to afford pure product.

The yields obtained with microwave irradiation were better than conventional heating method (Table 1). Microwave irradiations are known to be useful for variety of organic reactions due to short reaction time, cleaner reactions, easier work-up and good yield.

Table 1 Physical data of synthesized compounds 3a–o
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Spectroscopic characterisation

Compound 3a was characterized as 4-chloro-8-methyl-2-phenyl-1,5-dioxa-2H-phenanthren-6-one by using spectral data.

IR characterisation

In the IR spectrum, 3a showed a peak due to the C–O–C group at 1,082 cm−1. The peak at 1,732 cm−1 is due to the C–O stretching of carbonyl group.

1H NMR characterisation

In 1H NMR spectrum, the methyl protons attached to C8 appeared as a doublet at δ 2.41 ppm. The protons, H2, H3, H7 and H9 appeared as doublets at δ 5.87 ppm, δ 6.15 ppm, δ 6.21 ppm and δ 6.89 ppm, respectively. The multiplet at δ 7.39–7.50 ppm with proton integration 6 is due to aromatic protons and H10.

13C NMR characterisation

In 13C NMR spectrum, peak corresponding to methyl carbon appeared at δ 18.9 ppm. The peak at δ 77.9 ppm corresponding to C2 confirms the formation of 4-chloro-8-methyl-2-phenyl-1,5-dioxa-2H-phenanthren-6-one.

Mass spectral data

In the MS of 3a, the base peak appeared at 325 corresponding to (M + H)+.

Microbiology

Antibacterial activity

The synthesized novel compounds (3a–o) were screened for antibacterial activity against different types of bacterial strains viz. Gram-positive bacterial strains Bacillus subtillis and Staphylococcus aeureus; Gram-negative bacterial strains Klebsiella pneumoniae and Escherichia coli at a concentration of 10 and 20 µg/mL.

Some of the synthesized compounds have shown potent activity and some compounds have shown moderate activity compared to standard drug Gatifloxacin at a concentration of 10 and 20 µg/mL.

The compound 3e (R=H, Ar=3,4-dimethoxyphenyl), 3f (R=Cl, Ar=3,4,-dimethoxyphenyl) and 3o (R=H, Ar=2-thienyl) shown potent activity with zone of inhibition (Table 2) against Staphylococcus aeureus, Bacillus subtilis, Escherichia coli and Klebsiella pneumoniae respectively compared to the standard drug at a concentration of 10 and 20 µg/mL. The compounds 3d (R=Cl, Ar=4-methoxyphenyl), 3k (R=H, Ar=2-chlorophenyl) and 3l (R=Cl, Ar=2-chlorophenyl) have shown similar activity profile compared to the standard drug at a concentration of 10 and 20 µg/mL. All other compounds have shown moderate activity compared to standard.

Table 2 Antibacterial activity of compounds 3a–o at different concentrations 20 and 20 µg/mL
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Antifungal activity

The antifungal activity of synthesized compounds 3a–o were tested against three pathogenic fungi viz. Fusarium oxysporum, Aspergillus nigerzeae, and Aspergilus flavus, by the poison plate technique at a concentration of 100 µg/mL (Song et al., 2004). Some synthesized compounds shown moderate to high antifungal activity compared to standard drug Clotrimazole at a concentration of 100 µg/mL. Compound 3e (R=H, Ar=3,4-dimethoxyphenyl), 3o (R=H, Ar=2-thienyl) have shown better activity than standard drug against Aspergillus nigerzeae, Aspergilus flavus and Fusarium oxysporum. The compound 3f (R=Cl, Ar=3,4-dimethoxyphenyl) and 3j (R=Cl, Ar=4-methylphenyl) have shown similar activity compared to standard drug against tested fungi where as the remaining compounds shown moderate activity against pathogenic fungi, compared to standard (Figs. 2, 3).

Fig. 2
figure 2

Antibacterial activity against gram-positive bacterial strains

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Fig. 3
figure 3

Antibacterial activity against gram-negative bacterial strains

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Experimental

Materials

Melting points were determined in open capillaries and are uncorrected. Purity of the compounds was checked by TLC on silica gel 60 F254 (Merck). Microwave reactions were carried out in a multi SYNTH series microwavw system (Milestone).

SYNTH series microwave system (Milestone)

1H NMR and 13C NMR spectra were recorded on Bruker Avance II 400 spectrometer using TMS as an internal standard. IR spectra were recorded in KBr on a Shimadzu FTIR 8400S spectrophotometer. Mass spectra were recorded on a GCMS-QP 1000 mass spectrometer (Fig. 4).

Fig. 4
figure 4

Antifungal activity

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Biological assay

Antibacterial activity

The synthesized novel compounds 3a–o were screened for their Antibacterial activity against different types of bacterial strains, they are Gram-negative bacterial strains of Klebsiella pneumoniae and Escherichia coli, Gram-positive bacterial strains of Bacillus subtilis and Staphylococcus aeureus at a concentration of 10 and 20 µg/mL .

The cultures were diluted with 5 % saline autoclaved and the final volume was made with concentration approximately 105–106 CFU/mL. The synthesized compounds were diluted in acetone for antibacterial biological assays. For agar disc diffusion method (Benson et al., 1990), the liquid form of test compound was soaked on to the disc and then allowed to air dry, such that the disc gets completely saturated with test compound. The saturated chemical discs were introduced onto the upper layer of the medium evenly flooded with the bacteria.

The discs were dipped in different chemical samples, were placed over the evenly spread bacterial nutrient media, and incubated at 37 °C for 24–48 h for better inhibition of bacteria. The zones of inhibition were measured after 24–48 h. All the experiments were carried out in triplicates and the results were expressed as zone of Inhibition in mm. The zones of inhibition of synthesized compounds 3a–o were compared with the zone of inhibition of standard antibiotic concentrations of gatifloxacin (10, 20 µg/mL). The Antibacterial activity was evaluated and the results are presented in Table 2.

Antifungal activity

The antifungal activity of synthesized compounds 3a–o was tested against three pathogenic fungi, namely Fusarium oxysporum, Aspergillus niger, and Aspergilus flavus, by the poison plate technique at a concentration of 100 µg/mL. Three kinds of fungi were incubated in PDA at 25 ± 1 °C for 5 days to get new mycelium for antifungal assay, then a mycelia as discs of approximately 0.45 cm diameter cut from the culture medium were picked up with a sterilised inoculation needle and inoculated in the center of PDA plate. Test compounds were dissolved in acetone (10 mL) then added to the Potato Dextrose Agar medium (PDA, 90 mL). The final concentration of compounds in the medium was adjusted to 100 µg/mL. The inoculated plates were incubated at 25 ± 1 °C for 5 days. Acetone was diluted with sterilised distilled water and used as control, while clotrimazole (100 µg/mL) was used as standard control for each treatment three replicates of experiments were carried out. The radial growth of the fungal colonies was measured on the 6th day. The Antifungal activity was evaluated and the results are presented in Table 3.

Table 3 Antifungal activity of the compounds 3a–o
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General procedures

Synthesis of compounds 10-chloro-4-methyl-8-aryl-2H,8H-pyrano[2,3-f] Chromen-2-ones(3a–3o) under conventional conditions

DMF (5 mL) was taken into round bottomed flask and it was cooled to 0–5 °C. POCl3 (0.006 mol) was added drop wise to it under stirring. It was stirred at 0–5 °C for 15 min and then substituted (E)-1-(7-Hydroxy-4-methyl-8-coumarinyl)-3-phenyl-2-propen-1-ones 2a–o (0.001 mol) solution in 3 mL of DMF was added to it at 0–5 °C. It was maintained at 0–5 °C for 30 min. The reaction mixture was heated in water bath for 6–8 h. After the completion of the reaction (monitored by TLC, EtOAc:Hexane, 1:3 v/v), the reaction mixture was poured into ice–water and neutralised with 10 % NaOH solution, and it was extracted with Chloroform (2 × 20 mL); the combined organic layer was washed with 10 mL water and was dried over anhydrous magnesium sulphate. The solvent was evaporated and the residue was purified by using silica gel column chromatography to afford pure product 3a–o.

Synthesis of compounds 4-chloro-8-methyl-2-phenyl-1,5-dioxa-2H-phenanthren-6-ones (3a–3o) under microwave irradiation

DMF (5 mL) was taken into round bottomed flask and it was cooled to 0–5 °C. POCl3 (0.006 mol) was added drop wise to it under stirring. It was stirred at 0-5 °C for 15 min and then substituted (E)-1-(7-hydroxy-4-methyl-8-coumarinyl)-3-phenyl-2-propen-1-ones 2a–o (0.001 mol) solution in 3 mL of DMF was added to it at 0–5 °C. It was maintained at 0–5 °C for 30 min. The reaction mixture was placed in the microwave oven and subjected to microwave irradiation at 160 W for 4–5 min. The progress of reaction was monitored by TLC (EtOAc: Hexane, 1:4 v/v). After the completion of the reaction, it was poured into ice-water and neutralised with 10 % NaOH solution and extracted with chloroform (2 × 20 mL) and the combined organic layer was washed with 10 mL water, and it was dried over anhydrous magnesium sulphate. The solvent was evaporated and the residue was purified by using silica gel column chromatography to afford pure product 3a–o.

4-Chloro-8-methyl-2-phenyl-1,5-dioxa-2H-phenanthren-6-one (3a)

Colourless solid, yield = 80 %, mp 158–160 °C; IR (KBr) (cm−1): 1082 (C–O–C), 1732 (C=O); 1H NMR (400 MHz, CDCl3) δ: 2.41 (d, 3H, =C–CH3, J = 1.0 Hz), 5.87 (d, 1H, H2, J = 4.5 Hz), 6.15 (d, 1H, H3, J = 4.5 Hz), 6.21 (d, 1H, H7, J = 1.0 Hz), 6.89 (d, 1H, H9, J = 8.7 Hz) 7.39–7.50 (m, 6H, H10, Ar–H); 13C NMR (100 MHz, CDCl3) δ: 18.9 (CH3), 77.9 (C-2), 111.1 (C-4a), 113.4 (C-8a), 115.2 (C-7), 118.4 (C-10), 125.1 (C-9), 125.5 (C-3), 126.2 (C, Ar–C), 127.0 (C, Ar–C), 128.7 (2C, Ar–C), 129.1 (2C, Ar–C), 137.3 (C-4), 148.6 (C-5a), 151.4 (C-8) 152.6 (C-10a), 159.3 (C-6). MS (m/z): 325 (M + H)+ (100 %). Anal. Calcd. for C19H13ClO3: C, 70.27; H, 4.03. Found: C, 70.24; H, 3.99.

4,10-Dichloro-8-methyl-2-phenyl-1,5-dioxa-2H-phenanthren-6-one (3b)

Colourless solid, yield = 83 %, mp 154–156 °C; IR (KBr) (cm−1): 1084 (C–O–C), 1734 (C=O); 1H NMR (400 MHz, CDCl3) δ: 2.36 (d, 3H, =C–CH3, J = 1.0 Hz), 5.98 (d, 1H, H2, J = 4.7 Hz), 6.21 (d, 1H, H7, J = 1.0 Hz), 6.26 (d, 1H, H3, J = 4.7 Hz), 7.34–7.52 (m, 6H, H9, Ar–H); 13C NMR (100 MHz, CDCl3) δ: 18.9 (CH3), 77.9 (C-2), 111.1 (C-4a), 113.4 (C-8a), 115.2 (C-7), 118.4 (C-10), 125.1 (C-9), 125.5(C-3), 126.2 (C, Ar–C), 127.0 (C, Ar–C), 128.7 (2C, Ar–C), 129.1 (2C, Ar–C), 137.3 (C-4), 148.6 (C-5a), 151.4(C-8) 152.6 (C-10a), 159.3 (C-6). MS (m/z): 358 (M+) (100 %). Anal. Calcd. for C19H12Cl2O3: C, 63.53; H, 3.37. Found: C, 63.50; H, 3.34.

4-Chloro-2-(4-methoxyphenyl)-8-methyl-1,5-dioxa-2H-phenanthren-6-one (3c)

Colourless solid, yield = 83 %, mp 176–178 °C; IR (KBr) (cm−1): 1088 (C–O–C), 1725 (C=O), 1H NMR (400 MHz, CDCl3) δ: 2.38 (d, 3H, =C-CH3, J = 1.2 Hz), 3.80 (s, 3H,-OCH3) 5.79 (d, 1H, H2, J = 4.2 Hz), 6.11 (d, 1H, H3, J = 4.2 Hz), 6.18 (d, 1H, H7, J = 1.2 Hz), 6.83 (d, 1H, H10, J = 8.4 Hz), 6.89 (d, 2H, Ar–H, J = 8.1 Hz), 7.37 (d, 2H, Ar–H, J = 8.1 Hz), 7.44 (d, 1H, H9, J = 8.4 Hz); 13C NMR (100 MHz, CDCl3) δ: 19.0 (CH3), 55.3 (OCH3), 76.4 (C-2), 109.6 (C-4a), 112.3 (C-8a), 113.3 (C-7), 114.1 (C-10), 114.9 (2C–Ar), 124.6 (C-9), 125.5 (C-3), 126.4 (C–Ar), 129.0 (C-4), 129.8 (2C-Ar), 150.2 (C–Ar), 152.3 (C-5a), 157.3 (C-8), 160.0 (C-10a), 160.2 (C-6). MS (m/z): 355 (M + H)+ (100 %). Anal. Calcd. for C20H15ClO4: C, 67.71; H, 4.26. Found: C, 67.67; H, 4.21.

4,10-Dichloro-2-(4-methoxyphenyl)-8-methyl-1,5-dioxa-2H-phenanthren-6-one (3d)

Colourless solid, yield = 83 %, mp 170–172 °C; IR (KBr) (cm−1): 1088 (C–O–C), 1730 (C=O), 1H NMR (400 MHz, CDCl3) δ: 2.36 (d, 3H, =C-CH3, J = 1.2 Hz), 3.81 (s, 3H, –OCH3) 5.99 (d, 1H, H2, J = 4.2 Hz), 6.21 (d, 1H, H7, J = 1.2 Hz), 6.26 (d, 1H, H3, J = 4.2 Hz), 7.34–7.48 (m, 4H, Ar–H), 7.52 (s, 1H, H9); 13C NMR (100 MHz, CDCl3) δ: 18.9 (CH3), 55.8 (OCH3), 77.9 (C-2), 111.1 (C-4a), 113.4 (C-8a), 115.2 (C-7), 118.4 (C-10), 124.1 (C-9), 125.3 (C-3), 126.2 (C–Ar), 127.0 (C-4), 128.7 (2C–Ar), 129.1(2C–Ar), 137.3 (C-5a), 148.6 (C–Ar), 151.4 (C-8), 152.6 (C-10a), 159.6 (C-6). MS (m/z): 413 (M + Na)+ (100 %). Anal. Calcd. for C20H14Cl2O4: C, 61.72; H, 3.63. Found: C, 61.69; H, 3.60.

4-Chloro-8-methyl-2-(3,4-dimethoxyphenyl)-1,5-dioxa-2H-phenanthren-6-one (3e)

Colourless solid, yield = 80 %, mp 160–162 °C; IR (KBr) (cm−1): 1086 (C–O–C), 1725 (C=O), 1H NMR (400 MHz, CDCl3) δ: 2.38 (d, 3H, =C–CH3, J = 1.0 Hz), 3.87 (s, 3H, –OCH3), 3.89 (s, 3H, –OCH3), 5.78 (d, 1H, H2, J = 4.6 Hz), 6.11 (d, 1H, H3, J = 4.6 Hz), 6.19 (d, 1H, H7, J = 1.0 Hz), 6.84 (d, 1H, H10, J = 8.7 Hz), 6.86 (d, 1H, Ar–H, J = 8.1 Hz), 7.00 (d, 1H, Ar–H, J = 8.1), 7.26 (s, 1H, Ar–H), 7.47 (d, 1H, H9, J = 8.7 Hz); 13C NMR (100 MHz, CDCl3) δ: 19.02 (CH3), 55.9 (OCH3), 56.0 (OCH3), 77.9 (C-2), 109.6 (C-4a), 110.4 (C-8a), 110.8 (C-7), 112.3 (C-10), 113.2 (C–Ar), 114.9 (C–Ar), 120.2 (C–Ar), 124.6 (C-9), 125.5 (C-3), 126.5 (C–Ar), 130.1 (C-4), 149.1 (C–Ar), 149.6(C–Ar), 150.2 (C-5a), 152.5 (C-8), 157.2 (C-10a), 160.0 (C-6); MS (m/z): 407 (M + Na)+. Anal. Calcd. for C21H17ClO5: C, 65.55; H, 4.45. Found: C, 65.50; H, 4.41.

4,10-Dichloro-8-methyl-2-(3,4-dimethoxyphenyl)-1,5-dioxa-2H-phenanthren-6-one (3f)

Colourless solid, yield = 85 %, mp 158–160 °C; IR (KBr) (cm−1): 1088 (C–O–C), 1728 (C=O), 1H NMR (400 MHz, CDCl3) δ: 2.38 (d, 3H, =C-CH3, J = 1.0 Hz), 3.82 (s, 3H, –OCH3), 3.87 (s, 3H, –OCH3), 5.75 (d, 1H, H2, J = 4.7 Hz), 6.13 (d, 1H, H3, J = 4.7 Hz), 6.17 (d, 1H, H7, J = 1.0 Hz), 7.01(d, 1H, Ar–H, J = 8.1 Hz), 7.04 (d, 1H, Ar–H, J = 8.1 Hz), 7.26 (s, 1H, Ar–H), 7.48 (s, 1H, H9); 13C NMR (100 MHz, CDCl3) δ: 18.8 (CH3), 53.7 (OCH3), 56.3 (OCH3), 76.9 (C-2), 108.9 (C-4a), 109.4 (C-8a), 110.6 (C-7), 112.8 (C-10), 113.4 (C–Ar), 115.9 (C–Ar), 121.2 (C–Ar), 123.6 (C-9), 125.7 (C-3), 126.8 (C–Ar), 130.7 (C-4), 148.1 (C–Ar), 149.8 (C–Ar), 151.2 (C–5a), 152.7 (C-8), 156.2 (C-10a), 160.6 (C-6); MS (m/z): 441 (M + Na)+. Anal. Calcd. for C, 60.16; H, 3.85. C21H16Cl2O5. Found: C, 60.15; H, 3.81.

4-Chloro-2-(4-fluorophenyl)-8-methyl-1,5-dioxa-2H-phenanthren-6-one (3g)

Colourless solid, yield = 84 %, mp 175–177 °C; IR (KBr) (cm−1): 1085 (C–O–C), 1734 (C=O), 1H NMR (400 MHz, CDCl3) δ: 2.38 (d, 3H, =C–CH3, J = 1.0 Hz), 5.81 (d, 1H, H2, J = 4.7 Hz), 6.11 (d, 1H, H7, J = 1.0 Hz), 6.21 (d, 1H, H3, J = 4.7 Hz), 6.84 (d, 1H, H10, J = 8.7 Hz), 6.92 (d, 2H, Ar–H, J = 8.0 Hz), 7.35 (d, 2H, Ar–H, J = 8.0 Hz), 7.45 (d, 1H, H9, J = 8.7 Hz); 13C NMR (100 MHz, CDCl3) δ: 18.9 (CH3), 76.2 (C-2), 109.3(C-4a), 112.3 (C-8a), 113.2 (2C-Ar), 115.6 (C-8a), 118.5 (C-10), 123.2 (C-3), 125.6 (C-9), 127.8 (2C–Ar), 129.5 (C–Ar), 136.8 (C-4), 148.4 (C-5a), 150.8 (C-8) 152.6 (C-10a), 160.2 (C-6), 161.2 (C–Ar). MS (m/z): 365 (M + Na)+ (100 %). Anal. Calcd. for C19H12ClFO3: C, 66.58; H, 3.53. Found: C, 66.53; H, 3.49.

4,10-Dichloro-2-(4-fluorophenyl)-8-methyl-1,5-dioxa-2H-phenanthren-6-one (3h)

Colourless solid, yield = 80 %, mp 172–174 °C; IR (KBr) (cm−1): 1088 (C–O-C), 1736 (C=O), 1H NMR (400 MHz, CDCl3) δ: 2.36 (d, 3H, = C–CH3, J = 1.0 Hz), 5.99 (d, 1H, H2, J = 5.0 Hz), 6.21 (d, 1H, H7, J = 1.0 Hz), 6.26 (d, 1H, H3, J = 5.0 Hz), 7.37 (d, 2H, Ar–H, J = 8.0 Hz), 7.47 (d, 2H, Ar–H, J = 8.0 Hz), 7.52 (s, 1H, H9); 13C NMR (100 MHz, CDCl3) δ: 18.9 (CH3), 76.9 (C-2), 110.0 (C-7), 111.5 (C-4a), 113.6 (C-8a), 115.1 (2C-Ar), 124.1 (C-9), 125.3(C-10), 126.2 (C-3), 128.7 (C–Ar), 129.1 (2C–Ar), 137.3 (C-4), 148.6 (C-5a), 151.4 (C-8), 152.6 (C-10a), 159.6 (C-6), 160.3 (C–Ar). MS (m/z): 377(M + H)+ (100 %). Anal. Calcd. for C19H11Cl2FO3: C, 60.50; H, 2.94. Found: C, 60.46; H, 2.90.

4-Chloro-8-methyl-2-(p-tolyl)-1,5-dioxa-2H-phenanthren-6-one (3i)

Colourless solid, yield = 80 %, mp 142–144 °C; IR (KBr) (cm−1): 1086 (C–O–C), 1730 (C = O); 1H NMR (400 MHz, CDCl3) δ: 2.34 (s, 3H,-CH3), 2.37 (d, 3H, =C–CH3, J = 1.0 Hz), 5.81 (d, 1H, H2, J = 4.5 Hz), 6.12 (d, 1H, H3, J = 4.5 Hz), 6.18 (d, 1H, H7, J = 1.0 Hz), 6.84 (d, 1H, H10, J = 8.7 Hz), 7.17 (d, 2H, Ar–H, J = 8.0 Hz), 7.33 (d, 2H, Ar–H, J = 8.0 Hz), 7.45 (d, 1H, H9 J = 8.7 Hz); 13C NMR (100 MHz, CDCl3) δ: 19.0 (CH3), 21.2 (CH3–Ar), 77.8 (C-2), 112.3 (C-4a), 113.2 (C-8a), 115.2 (C-7), 124.6 (C-10), 125.4 (C-9), 126.2 (C-3), 126.4 (C–Ar), 127.3 (C–Ar), 129.1 (C–Ar), 129.5 (2C–Ar), 129.8 (2C–Ar), 134.9 (C-4), 139.0 (C-5a), 152.9 (C-8), 157.3 (C-10a), 160.0 (C-6). MS (m/z): 361 (M + Na)+. Anal. Calcd. for C20H15ClO3: C, 70.90; H, 4.46. Found: C, 70.86; H, 4.42.

4,10-Dichloro-8-methyl-2-(p-tolyl)-1,5-dioxa-2H-phenanthren-6-one (3j)

Colourless solid, yield = 84 %, mp 139–141 °C; IR (KBr) (cm−1): 1085 (C–O–C), 1734 (C=O), 1H NMR (400 MHz, CDCl3) δ: 2.35 (s, 3H, –CH3) 2.39 (d, 3H, =C–CH3, J = 1.0 Hz), 6.14 (d, 1H, H2, J = 4.5 Hz), 6.24 (d, 1H, H7, J = 1.0 Hz), 6.32 (d, 1H, H3, J = 4.5 Hz), 7.22 (d, 2H, Ar–H, J = 8.0 Hz), 7.35 (d, 2H, Ar–H, J = 8.0 Hz), 7.54 (s, 1H, H9); 13C NMR (100 MHz, CDCl3) δ: 18.9 (CH3), 75.5 (CH, C-2), 113.6 (C-4a), 115.5 (C-8a), 118.3 (C-7), 124.4 (C-10), 125.7 (C-9), 126.4 (C-3), 127.2 (C–Ar), 128.2 (C–Ar), 128.5 (C–Ar), 130.2 (C–Ar), 130.4 (C-4), 132.5 (C-5a), 134.7 (C-8), 151.3, 152.8 (C-10a), 156.5 (C-6). MS (m/z): 395 (M + Na)+ (100 %). Anal. Calcd. for C20H14Cl2O3: C, 64.36; H, 3.78. Found: C, 64.33; H, 3.72.

4-Chloro-2-(o-chlorophenyl)-8-methyl-1,5-dioxa-2H-phenanthren-6-one (3k)

Colourless solid, yield = 82 %, mp 156–158 °C; IR (KBr) (cm−1): 1074 (C–O–C), 1736 (C=O); 1H NMR (400 MHz, CDCl3) δ: 2.41 (d, 3H, = C–CH3, J = 1.0 Hz), 5.87 (d, 1H, H2, J = 4.5 Hz), 6.15 (d, 1H, H3, J = 4.5 Hz), 6.21 (d, 1H, H7, J = 1.0 Hz), 6.89 (d, 1H, H10, J = 8.7 Hz) 7.39-7.49 (m, 5H, H9, Ar–H); 13C NMR (100 MHz, CDCl3) δ: 19.0 (CH3), 77.9 (C-2), 109.6 (C-4a), 112.4 (C-8a), 113.2 (C-10), 113.4 (C-7), 115.0 (C-8a), 124.6 (C-3), 125.1 (C–Ar), 125.6 (C–Ar), 126.3 (C–Ar), 126.5 (C-9), 127.0 (C–Ar), 127.2 (C-5a), 128.8 (C–Ar), 129.0 (C–Ar), 137.9 (C-4), 152.4 (C-8), 157.3 (C-10a), 160.3 (C-6). MS (m/z): 381 (M + Na)+ (100 %). Anal. Calcd. for C19H12Cl2O3: C, 63.53; H, 3.37. Found: C, 63.50; H, 3.34.

4,10-Dichloro-2-(o-chlorophenyl)-8-methyl-1,5-dioxa-2H-phenanthren-6-one (3l)

Colourless solid, yield = 80 %, mp 154–156 °C; IR (KBr) (cm−1): 1080 (C–O–C), 1738 (C=O); 1H NMR (400 MHz, CDCl3) δ: 2.38 (d, 3H, =C–CH3, J = 1.0 Hz), 6.14 (d, 1H, H2, J = 4.5 Hz), 6.23 (d, 1H, H7, J = 1.0 Hz), 6.32 (d, 1H, H3, J = 4.5 Hz), 7.29–7.54 (m, 4H, ArH), 7.55 (s, 1H, H9); 13C NMR (100 MHz, CDCl3) δ: 18.9 (CH3), 75.5 (C-2), 108.5 (C-4a), 113.6 (C-10), 115.3 (C-7), 118.3 (C-8a), 124.4 (C-3), 125.7 (C–Ar), 126.4 (C–Ar), 127.2 (C-9), 127.5 (C–Ar), 128.0 (C-5a), 128.2 (C–Ar), 128.5 (C–Ar), 130.2 (C-4), 132.5 (C–Ar), 151.3 (C-8), 152.8 (C-10a), 159.2 (C-6). MS (m/z): 415 (M + Na)+ (100 %). Anal. Calcd. for C19H11Cl3O3: C, 57.97; H, 2.82. Found: C, 57.94; H, 2.79.

4-Chloro-8-methyl-2-(1-naphthyl)-1,5-dioxa-2H-phenanthren-6-one (3m)

Colourless solid, yield = 80 %, mp 136–138 °C; IR (KBr) (cm−1): 1084 (C–O–C), 1736 (C = O); 1H NMR (400 MHz, CDCl3) δ: 2.38 (d, 3H, =C–CH3, J = 1.00 Hz), 5.97 (d, 1H, H2, J = 4.5 Hz), 6.21 (d, 1H, H7, J = 1.0 Hz), 6.35 (d, 1H, H3, J = 4.5 Hz), 7.12 (d, 1H, H10, J = 8.7 Hz), 7.34–8.09 (m, 8H, H9, ArH); 13C NMR (100 MHz, CDCl3) δ: 19.0 (CH3), 76.1 (C-2), 109.7 (C-4a), 111.6 (C-8a), 113.2 (C-7), 115.4 (C-10), 118.6 (C-3), 123.4 (C–Ar), 124.6 (C–Ar), 125.1 (C-9), 125.5 (C–Ar), 126.1 (C–Ar), 126.3 (C–Ar), 127.0 (C–Ar), 128.7 (C–Ar), 129.1 (C–Ar), 130.2 (C- Ar), 130.8 (C- Ar), 137.3 (C-4), 148.6 (C-5a), 151.4 (C-8), 152.6 (C-10a), 159.3 (C-6). MS (m/z): 397 (M + Na)+ (100 %). Anal. Calcd. for C23H15ClO3: C, 73.70; H, 4.03. Found: C, 73.67; H, 4.00.

4,10-Dichloro-8-methyl-2-(1-naphthyl)-1,5-dioxa-2H-phenanthren-6-one (3n)

Colourless solid, yield = 80 %, mp 133–135 °C; IR (KBr) (cm−1): 1086 (C–O–C), 1736 (C=O); 1H NMR (400 MHz, CDCl3) δ: 2.36 (d, 3H, =C–CH3, J = 1.0 Hz), 6.21 (d, 1H, H7, J = 1.0 Hz), 6.30 (d, 1H, H2, J = 4.7 Hz), 6.69 (d, 1H, H3, J = 4.7 Hz), 7.40–8.38 (m, 8H, H9, H10, ArH); 13C NMR (100 MHz, CDCl3) δ: 18.9 (CH3), 75.9 (C-2), 111.2 (C-4a), 113.4 (C-8a), 115.2 (C-7), 118.6 (C-3), 123.8 (C-10), 124.7 (C-9), 126.0 (C–Ar), 126.1 (C–Ar), 126.1 (C–Ar), 126.8 (C–Ar), 128.8 (C–Ar), 130.2 (C–Ar), 130.8 (C–Ar), 131.6 (C–Ar), 134.1 (C-4), 148.7 (C-5a), 151.4 (C-8), 152.9 (C-10a), 159.4 (C-6). MS (m/z): 409 (M + H)+ (100 %). Anal. Calcd. for C23H14Cl2O3: C, 67.50; H, 3.45. Found: C, 67.45; H, 3.41.

4-Chloro-8-methyl-2-(thiophen-2-yl)-1,5-dioxa-2H-phenanthren-6-one (3o)

Colourless solid, yield = 85 %, mp 155–157 °C; IR (KBr) (cm−1): 1082 (C–O–C), 1720 (C=O); 1H NMR (400 MHz, CDCl3) δ: 2.38 (d, 3H, = C–CH3, J = 1.0 Hz), 6.07 (d, 1H, H2, J = 4.5 Hz), 6.19 (d, 1H, H7, J = 1.0 Hz), 6.23 (d, 1H, H3, J = 4.5 Hz), 6.86 (d, 1H, H10, J = 8.7 Hz), 6.95–7.33 (m, 3H, Ar–H), 7.45 (d, 1H, H9, J = 8.7 Hz); 13C NMR (100 MHz, CDCl3) δ: 19.0 (CH3), 72.7 (C-2), 109.6 (C-4a), 112.4 (C-8a), 113.5 (C-7), 115.1 (C-10), 123.5 (C-3), 126.1 (C–Ar), 126.5 (C-9), 126.9 (C- Ar), 127.3 (C–Ar), 127.5 (C–Ar), 140.4 (C-4), 150.2 (C-5a), 152.4 (C-8), 156.6 (C-10a), 159.9 (C-6). MS (m/z): 353 (M + Na)+ (100 %). Anal. Calcd. for C17H11ClO3S: C, 61.73; H, 3.35. Found: C, 61.69; H, 3.31.

Conclusion

We synthesized a new series of compounds 3a–o under conventional and microwave irradiation methods. In microwave irradiation method, reactions were completed in short time with better yields compared to conventional method. All the new compounds have been screened for antimicrobial activity. The compounds 3e, 3f, 3o were more potent and the compounds 3f, 3j were moderately potent for pathogenic bacteria where as the compounds 3e, 3o were more potent and the compounds 3f, 3j were moderately potent for pathogenic fungi compared to the standard drugs with their respective concentrations.