Pentafluorophenylammonium triflate (PFPAT) catalyzed facile construction of substituted chromeno[2,3-d]pyrimidinone derivatives and their antimicrobial activity

Abstract

A new, simple thermally efficient and solvent-free condensation of 2-amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylate derivatives with coumarin-3-carboxylic acid employing pentafluorophenylammonium triflate (PFPAT) as an inexpensive organocatalyst for the synthesis of a series of ethyl 4,5-dihydro 7-methyl-2-(2-oxo-2H-chromen-3-yl)-4-oxo-5-aryl-3H-chromeno[2,3-d]pyrimidine-6-carboxylate derivatives is described. This method has the advantages of high yields, a cleaner reaction, simple methodology, short reaction times, easy workup, and greener conditions. All the compounds were evaluated for their in vitro antimicrobial activity against different bacterial and fungal strains.

Introduction

Coumarins (2-oxo-2H-chromene) are the family of lactones containing benzopyran skeletal framework. Coumarin derivatives have been established as well-known naturally occurring oxygen-heterocyclic compounds isolated from various plants which occupy a special role in nature [1]. The plant extracts containing coumarin-related heterocycles are employed as herbal remedies in traditional systems of medicine. They belong to the flavonoid class of plant secondary metabolite. Coumarin derivatives constitute an important class of heterocyclic compounds that have attracted significant attention in recent years [2,3]. They have attracted intense interest because of their diverse pharmacological properties. Cancer, a diverse group of diseases characterized by uncontrolled growth of abnormal cells, is a major worldwide problem. It is a fatal disease standing next to the cardiovascular disease in terms of morbidity and mortality. A series of coumarin–chalcone hybrids have been synthesized and evaluated for their in vitro cytotoxicity against a panel of four human cancer cell lines and normal fibroblasts (NIH3T3) [4]. Tuberculosis (TB) is a common and often deadly infectious disease caused by various strains of mycobacterium, usually Mycobacterium tuberculosis. Tuberculosis has been considered to be a disease of poverty for many years with quite rare occurrence in the developed countries. A new series of 4-(3-coumarinyl)-3-benzyl-4-thiazolin-2-one benzylidenehydrazones were synthesized, and they were evaluated for anti-tuberculosis activity against M. tuberculosis H37Rv in BACTEC 12B medium using the BACTEC 460 radiometric system [5]. Coumarin derivatives also used as anti-HIV [6], antioxidant [7], dyslipidemic [8], anti-inflammatory agents [9], and antimicrobial agents [10].

In view of the pharmaceutical importance of heterocyclic compounds containing coumarin moiety, many methods have been developed. Coumarin derivatives are synthesized using different catalysts like nano-crystalline ZnO [11], heteropoly acids [12] and tetrabutylammonium bromide [13]. Recently, chromeno pyrimidinone derivatives [14] and quinoxaline derivatives containing the coumarin moiety [15] are reported. Various chromeno pyrimidinones are prepared under solvent-free condition at 120 °C in the presence of 10 mol% of ionic liquid [14].

Although these methods are quite satisfactory, many of them employ considerable amounts of hazardous organic solvents, which are not environmentally friendly, for carrying out the reactions and/or for extraction and purification (column chromatography). Furthermore, these methods are not suitable in terms of the recent trends in process chemistry, because of the use of metallic catalysts. Therefore, a method using a nonmetallic catalyst is desirable. Organo-catalysts have gained interesting attraction in recent years due to economic and environmental considerations. These catalysts are generally inexpensive and easily available. They can conveniently be handled and removed from the reaction mixture, thus making the experimental procedure simple and eco-friendly. The leading contenders for environmentally acceptable processes are supported reagents.

PFPAT as an efficient organo-catalyst was applied in various transformations. From the literatures, it was found that PFPAT is a useful catalyst for multi-component reactions (MCRs) [16–22], since it is low toxic catalyst, air and water compatible and has remarkable ability to suppress side reactions in acid-sensitive substrates.

Recently, Funatomi et al. reported the application of pentafluorophenylammonium triflate (C₆F₅NH₃OTf; PFPAT) as a novel heterogeneous catalyst in organic transformation such as esterification of carboxylic acids with alcohols [16], C-acylations of enol silyl ethers or ketene silyl (thio)acetals with acid chlorides [17] and Mukaiyama aldol and Mannich reactions using ketene silyl acetals with ketones and oxime ethers [18]. Further, PFPAT also used as the catalyst for acylation of alcohols, phenols, and amines [19], one-pot condensation of β-naphthol, aldehydes and cyclic 1,3-dicarbonyl compounds [20], synthesis of xanthenes derivatives [21], and Biginelli reaction [22]. However, to the best of our knowledge, there are no examples on the use of PFPAT as catalyst for the synthesis of ethyl-4,5-dihydro 7-methyl-2-(2-oxo-2H-chromen-3-yl)-4-oxo-5-aryl-3H-chromeno[2,3-d]pyrimidine-6-carboxylate derivatives.

Recently, we have reported the synthesis of biologically active heterocyclic molecules, such as 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives [23], polyhydroquinoline derivatives [24], 2-amino-6-(2-oxo-2Hchromen-3-yl)-4-arylnicotinonitrile derivatives [25], and 2-arylbenzothiazole derivatives [26] by multi-component reactions. In continuation of our research on the development of environmentally friendly procedures, we now describe the synthesis of ethyl-4,5-dihydro 7-methyl-2-(2-oxo-2H-chromen-3-yl)-4-oxo-5-aryl-3H-chromeno[2,3-d]pyrimidine-6-carboxylates using PFPAT as an efficient novel organocatalyst. These compounds were synthesized using 2-amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylates (Scheme 1).

Scheme 1.

2-Amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylate derivatives.

Experimental

Apparatus and analysis

Chemicals were purchased from Merck, Fluka, and Aldrich Chemical Companies. All yields refer to isolated products unless otherwise stated. ¹H NMR (500 MHz) and ¹³C NMR (125 MHz) spectra were obtained using Bruker DRX-500 Avance at ambient temperature, using TMS as internal standard. FT-IR spectra were obtained as KBr disks on Shimadzu spectrometer. Mass spectra were determined on a Varion – Saturn 2000 GC/MS instrument. Elemental analysis was measured by means of Perkin Elmer 2400 CHN elemental analyzer flowchart.

Preparation of the catalyst (PFPAT)

To a solution of 2,3,4,5,6-pentafluoroaniline (25 mmol) in toluene (25 mL), CF₃SO₃H (25 mmol) was added at 0–5 °C. The reaction mixture was stirred at the same temperature for 30 min. After this time, the solvent was evaporated in vacuo, and the crude product was collected and washed with hexane to give the pure catalyst in 92% yield [16].

General procedure to synthesis of 2-amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylate derivatives using ZrOCl₂·8H₂O (5 mol%) as catalyst

A mixture of ethyl acetoacetate (1 mmol), aldehydes (1 mmol), malononitrile (1 mmol), and catalyst ZrOCl₂·8H₂O (5 mol%) in 5 mL of EtOH/H₂O[50/50(v/v)] were refluxed for appropriated time. After the TLC indicates the disappearance of starting materials, the reaction was cooled to room temperature, ethanol (20 mL) was added, and the insoluble material was filtered to separate the catalyst. The filtrate was concentrated under vacuum, and the crude residue was purified by recrystallization. 2-Amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylate was obtained as crystals. The recovered catalyst can be washed consequently with diluted acid solution, water, and then acetone. After drying, it can be reused without noticeable loss of reactivity. The products were identified by IR, ¹H NMR, ¹³C NMR, mass, elemental analysis, and melting points.

Spectral data for the selected synthesized compounds

2-Amino-3-cyano-6-methyl-4-(4-N,N-dimethylaminophenyl)-4H-pyran-5-ethylcarboxylate (4d)

(KBr, cm⁻¹): 3413, 3342, 3214, 2217, 1662, 1638, 1484, 1203, 785; ¹H NMR (500 MHz, CDCl₃) δ: 1.20 (t, J = 7.2 Hz, 3H, CH₃CH₂), 2.66 (s, 6H, N(CH₃)₂), 2.28 (s, 3H, CH₃), 4.11 (q, J = 7.2 Hz, 2H, CH₃CH₂), 4.94 (s, 1H, CH), 5.17 (s, 2H, NH₂), 7.11 (d, J = 7.2 Hz, 2H, ArH), 7.34 (d, J = 7.2 Hz, 2H ArH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 15.1, 19.2, 39.8, 57.4, 59.8, 105.8, 120.3, 125.2, 128.3, 129.1, 131.1, 144.8, 147.1, 166.5 ppm; MS (ESI): m/z 328 (M+H)⁺. Anal. Calcd. for C₁₈H₂₁N₃O₃ (%): C, 66.05; H, 6.42; N, 12.84. Found: C, 66.00; H, 6.41; N, 12.85.

2-Amino-3-cyano-6-methyl-4-(4-fluorophenyl)-4H-pyran-5-ethylcarboxylate (4f)

IR (KBr, cm⁻¹): 3428, 3329, 3205, 2216, 1667, 1636, 1483, 1219, 793. ¹H NMR (500 MHz, CDCl₃) δ: 1.13 (t, J = 7.0 Hz, 3H, CH₃CH₂), 2.26 (s, 3H, CH₃), 4.06 (q, J = 7.0 Hz, 2H, CH₃CH₂), 4.90 (s, 1H, CH), 5.21 (s, 2H, NH₂), 7.10 (d, J = 7.4 Hz, 2H, ArH), 7.32 (d, J = 7.4 Hz, 2H ArH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 14.5, 19.6, 39.4, 58.0, 60.4, 105.3, 120.3, 125.0, 129.1, 131.1, 144.7, 146.7, 167.5 ppm; MS (ESI): m/z 303 (M+H)⁺. Anal. Calcd. for C₁₆H₁₅FN₂O₃ (%): C, 63.57; H, 4.96; N, 9.27. Found: C, 63.50; H, 4.95; N, 9.28.

2-Amino-3-cyano-6-methyl-4-(4-methoxyphenyl)-4H-pyran-5-ethylcarboxylate (4g)

IR (KBr, cm⁻¹): 3429, 3337, 3219, 2220, 1675, 1644, 1488, 1219, 779. ¹H NMR (500 MHz, CDCl₃) δ: 1.16 (t, J = 7.4 Hz, 3H, CH₃CH₂), 2.24 (s, 3H, CH₃), 3.62 (s, 3H, OCH₃), 4.17 (q, J = 7.2 Hz, 2H, CH₃CH₂), 4.87 (s, 1H, CH), 5.15 (s, 2H, NH₂), 7.07 (d, J = 7.2 Hz, 2H, ArH), 7.34 (d, J = 7.2 Hz, 2H ArH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 14.9, 19.8, 40.6, 58.6, 60.6, 106.3, 119.9, 125.7, 128.4, 129.2, 131.2, 144.8, 147.3, 167.6 ppm; MS (ESI): m/z 315 (M+H)⁺. Anal. Calcd. for C₁₇H₁₈N₂O₄ (%): C, 64.97; H, 5.73; N, 8.92. Found: C, 64.90; H, 5.70; N, 8.91.

2-Amino-3-cyano-6-methyl-4-(4-nitrophenyl)-4H-pyran-5-ethylcarboxylate (4h)

IR (KBr, cm⁻¹): 3430, 3338, 3209, 2202, 1668, 1644, 1489, 1203, 774. ¹H NMR (500 MHz, CDCl₃) δ: 1.19 (t, J = 7.4 Hz, 3H, CH₃CH₂), 2.31 (s, 3H, CH₃), 4.14 (q, J = 7.3 Hz, 2H, CH₃CH₂), 4.92 (s, 1H, CH), 5.07 (s, 2H, NH₂), 7.15 (d, J = 7.4 Hz, 2H, ArH), 7.44 (d, J = 7.4 Hz, 2H ArH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 15.2, 20.2, 39.3, 58.3, 59.7, 105.7, 119.3, 125.6, 128.1, 129.0, 131.0, 144.1, 147.4, 167.0 ppm; MS (ESI): m/z 330 (M+H)⁺. Anal. Calcd. for C₁₆H₁₅N₃O₅ (%): C, 58.35; H, 4.56; N, 12.76. Found: C, 58.30; H, 4.53; N, 12.75.

General procedure for the synthesis of ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-phenyl-3H-pyrano[2,3-d]pyrimidine-6-carboxylate by PFPAT

A mixture of 2-amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylate 4a–j (1 mmol), coumarin-3-carboxylic acid (1 mmol) and PFPAT (5 mol%) were heated at 80 °C for about 5.5–7.0 h (Scheme 2). After completion of the reaction as indicated by TLC, 2 mL of water was added and stirred at room temperature for 20 min. The precipitated product was filtered, washed with water, dried and purified over column chromatography using silica gel (230–400 mesh) with n-hexane and ethyl acetate (8:2) as eluent. The aqueous layer containing catalyst was recovered, washed with acetone, dried and reused for subsequent reactions without loss in its activity and product yield.

Scheme 2.

Ethyl 4,5-dihydro 7-methyl-2-(2-oxo-2H-chromen-3-yl)-4-oxo-5-aryl-3H-chromeno[2,3-d]pyrimidine-6-carboxylate derivatives.

Recycling and reusing of the catalyst

The catalyst is soluble in water and could therefore be recycled as the filtrate. The catalyst was recovered by evaporation of the water, washed with hexane, dried at 50 °C under vacuum for 1 h, and reused in another reaction without appreciable reduction in the catalytic activity.

Spectral data for the synthesized compounds (6a–j)

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-phenyl-3H-pyrano[2,3-d] pyrimidine-6-carboxylate (6a)

IR (KBr, cm⁻¹): 3311, 1714, 1677, 1638, 1600, 1208; ¹H NMR (500 MHz, CDCl₃) δ: 1.18 (t, J = 7.4 Hz, 3H, CH₃CH₂), 2.22 (s, 3H, CH₃), 4.12 (q, J = 7.2 Hz, 2H, CH₃CH₂), 4.53 (s, 1H, CH), 7.01–7.41 (m, 5H, ArH), 7.75–7.92 (m, 4H, ArH), 8.66 (s, 1H, Coumarin H), 9.07 (s, 1H, NH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 16.4, 20.1, 26.4, 36.0, 37.4, 100.7, 113.5, 116.1, 118.0, 118.7, 121.3, 124.5, 126.4, 127.0, 129.3, 130.8, 134.0, 136.8, 153.0, 154.2, 157.0, 163.7, 170.4 ppm; MS(ESI): m/z 456 (M+H)⁺; Anal. Calcd. for C₂₆H₂₀N₂O₆: C, 68.42; H, 4.38; N, 6.14%. Found: C, 68.31; H, 4.33; N, 6.14%.

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-(3-hydroxyphenyl)-3H-pyrano[2,3-d pyrimidine-6-carboxylate (6b)

IR (KBr, cm⁻¹): 3362, 3308, 1712, 1675, 1640, 1609, 1212; ¹H NMR (500 MHz, CDCl₃) δ: 1.10 (t, J = 7.2 Hz, 3H, CH₃CH₂), 2.18 (s, 3H, CH₃), 4.22 (q, J = 7.2 Hz, 2H, CH₃CH₂), 4.58 (s, 1H, CH), 7.09–7.49 (m, 4H, ArH), 7.71–7.90 (m, 4H, ArH), 8.70 (s, 1H, Coumarin H), 9.01 (s, 1H, NH), 9.66 (s, 1H, OH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 15.9, 20.2, 26.3, 36.4, 37.3, 100.6, 114.0, 116.4, 117.7, 118.8, 121.0, 124.3, 126.2, 127.2, 129.4, 130.4, 134.5, 136.8, 153.2, 154.5, 156.9, 163.6, 170.3 ppm; MS(ESI): m/z 473 (M+H)⁺; Anal. Calcd. for C₂₆H₂₀N₂O₇: C, 66.10; H, 4.24; N, 5.93%. Found: C, 66.01; H, 4.20; N, 5.90%.

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-(3-nitrophenyl)-3H-pyrano[2,3-d]pyrimidine-6-carboxylate (6c)

IR (KBr, cm⁻¹): 3296, 1720, 1680, 1644, 1611, 1216; ¹H NMR (500 MHz, CDCl₃) δ: 1.09 (t, J = 7.0 Hz, 3H, CH₃CH₂), 2.26 (s, 3H, CH₃), 4.26 (q, J = 7.0 Hz, 2H, CH₃CH₂), 4.44 (s, 1H, CH), 7.03–7.33 (m, 4H, ArH), 7.68–7.88 (m, 4H, ArH), 8.80 (s, 1H, Coumarin H), 9.05 (s, 1H, NH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 16.2, 20.0, 26.7, 36.1, 37.1, 100.2, 113.7, 115.7, 117.6, 119.0, 121.4, 124.4, 126.7, 127.5, 128.6, 129.4, 130.6, 134.6, 136.8, 153.3, 154.5, 156.8, 162.9, 170.1 ppm; MS(ESI): m/z 502 (M+H)⁺; Anal. Calcd. for C₂₆H₁₉N₃O₈: C, 62.27; H, 3.79; N, 8.38%. Found: C, 62.22; H, 3.74; N, 8.35%.

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-(N,N-dimethylaminophenyl)-3H-pyrano[2,3-d]pyrimidine-6-carboxylate (6d)

IR (KBr, cm⁻¹): 3304, 1704, 1688, 1633, 1604, 1200; ¹H NMR (500 MHz, CDCl₃) δ: 1.12 (t, J = 7.2 Hz, 3H, CH₃CH₂), 2.27 (s, 3H, CH₃), 2.74 (s, 6H, N(CH₃)₂), 4.19 (q, J = 7.4 Hz, 2H, CH₃CH₂), 4.39 (s, 1H, CH), 7.08–7.17 (m, 2H, ArH), 7.34–7.48 (m, 2H, ArH), 7.74–7.82 (m, 4H, ArH), 8.77 (s, 1H, Coumarin H), 9.24 (s, 1H, NH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 15.5, 20.3, 26.5, 36.5, 37.3, 46.5, 100.4, 113.9, 116.0, 118.1, 118.8, 122.0, 124.6, 126.3, 127.7, 129.5, 130.1, 134.3, 136.5, 153.0, 154.3, 156.7, 163.0, 170.2 ppm; MS(ESI): m/z 500 (M+H)⁺; Anal. Calcd. for C₂₈H₂₅N₃O₆: C, 67.33; H, 5.01; N, 8.42%. Found: C, 67.35; H, 5.00; N, 8.37%.

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-(4-chlorophenyl)-3H-pyrano[2,3-d]pyrimidine-6-carboxylate (6e)

IR (KBr, cm⁻¹): 3294, 1716, 1677, 1640, 1609, 1206; ¹H NMR (500 MHz, CDCl₃) δ: 1.16 (t, J = 7.1 Hz, 3H, CH₃CH₂), 2.19 (s, 3H, CH₃), 4.14 (q, J = 7.2 Hz, 2H, CH₃CH₂), 4.53 (s, 1H, CH), 7.11–7.24 (m, 2H, ArH), 7.42–7.52 (m, 2H, ArH), 7.76–7.96 (m, 4H, ArH), 8.75 (s, 1H, Coumarin H), 9.12 (s, 1H, NH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 16.6, 20.7, 26.7, 35.9, 36.8, 101.0, 114.2, 116.2, 117.5, 119.1, 121.2, 124.8, 126.0, 127.5, 129.0, 130.1, 134.7, 136.9, 153.6, 154.2, 156.9, 162.7, 170.4 ppm; MS(ESI): m/z 491 (M+H)⁺; Anal. Calcd. for C₂₆H₁₉ClN₂O₆: C, 63.61; H, 3.87; N, 5.71%. Found: C, 63.58; H, 3.86; N, 5.73%.

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-(4-fluorophenyl)-3H-pyrano[2,3-d]pyrimidine-6-carboxylate (6f)

IR (KBr, cm⁻¹): 3314, 1722, 1682, 1646, 1616, 1214; ¹H NMR (500 MHz, CDCl₃) δ: 1.19 (t, J = 7.2 Hz, 3H, CH₃CH₂), 2.20 (s, 3H, CH₃), 4.17 (q, J = 7.2 Hz, 2H, CH₃CH₂), 4.55 (s, 1H, CH), 7.07–7.16 (m, 2H, ArH), 7.46–7.57 (m, 2H, ArH), 7.66–7.74 (m, 4H, ArH), 8.88 (s, 1H, Coumarin H), 9.10 (s, 1H, NH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 15.9, 20.0, 26.4, 35.8, 36.7, 101.2, 113.9, 116.4, 117.7, 118.6, 121.5, 124.0, 125.9, 127.8, 129.4, 130.1, 133.9, 136.5, 153.4, 154.6, 157.2, 163.5, 170.3 ppm; MS(ESI): m/z 475 (M+H)⁺; Anal. Calcd. for C₂₆H₁₉FN₂O₆: C, 65.82; H, 4.01; N, 5.91%. Found: C, 65.80; H, 4.00; N, 5.90%.

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-(4-methoxyphenyl)-3H-pyrano[2,3-d]pyrimidine-6-carboxylate (6g)

IR (KBr, cm⁻¹): 3310, 1711, 1668, 1652, 1603, 1205; ¹H NMR (500 MHz, CDCl₃) δ: 1.08 (t, J = 7.2 Hz, 3H, CH₃CH₂), 2.27 (s, 3H, CH₃), 3.62 (s, 3H, OCH₃), 4.10 (q, J = 7.1 Hz, 2H, CH₃CH₂), 4.35 (s, 1H, CH), 7.12–7.30 (m, 2H, ArH), 7.43–7.56 (m, 2H, ArH), 7.70–7.82 (m, 4H, ArH), 8.65 (s, 1H, Coumarin H), 9.06 (s, 1H, NH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 16.1, 20.1, 26.4, 36.1, 37.4, 100.5, 113.8, 115.8, 117.6, 118.7, 121.2, 124.2, 126.1, 127.3, 129.2, 130.1, 134.4, 136.4, 153.7, 154.8, 157.3, 163.0, 170.2 ppm; MS(ESI): m/z 487 (M+H)⁺; Anal. Calcd. for C₂₇H₂₂N₂O₇: C, 66.67; H, 4.53; N, 5.76%. Found: C, 65.70; H, 4.50; N, 5.75%.

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-(4-nitrophenyl)-3H-pyrano[2,3-d]pyrimidine-6-carboxylate (6h)

IR (KBr, cm⁻¹): 3299, 1709, 1671, 1647, 1600, 1210; ¹H NMR (500 MHz, CDCl₃) δ: 1.13 (t, J = 7.2 Hz, 3H, CH₃CH₂), 2.20 (s, 3H, CH₃), 4.20 (q, J = 7.2 Hz, 2H, CH₃CH₂), 4.30 (s, 1H, CH), 7.00–7.15 (m, 2H, ArH), 7.40–7.52 (m, 2H, ArH), 7.69–7.81 (m, 4H, ArH), 8.58 (s, 1H, Coumarin H), 9.21 (s, 1H, NH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 16.7, 20.6, 26.6, 36.4, 37.6, 100.7, 113.3, 116.1, 118.0, 118.5, 121.4, 124.3, 125.8, 127.0, 129.4, 130.1, 134.0, 136.2, 153.3, 154.3, 156.7, 162.6, 170.6 ppm; MS(ESI): m/z 502 (M+H)⁺; Anal. Calcd. for C₂₆H₁₉N₃O₈: C, 62.27; H, 3.79; N, 8.38%. Found: C, 62.29; H, 3.79; N, 8.36%.

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-(4-bromophenyl)-3H-pyrano[2,3-d]pyrimidine-6-carboxylate (6i)

IR (KBr, cm⁻¹): 3292, 1714, 1675, 1644, 1611, 1208; ¹H NMR (500 MHz, CDCl₃) δ: 1.12 (t, J = 7.1 Hz, 3H, CH₃CH₂), 2.16 (s, 3H, CH₃), 4.16 (q, J = 7.2 Hz, 2H, CH₃CH₂), 4.56 (s, 1H, CH), 7.16–7.26 (m, 2H, ArH), 7.46–7.58 (m, 2H, ArH), 7.72–7.90 (m, 4H, ArH), 8.78 (s, 1H, Coumarin H), 9.09 (s, 1H, NH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 16.5, 20.5, 26.5, 35.7, 36.6, 101.1, 114.4, 116.4, 117.4, 119.4, 121.6, 124.6, 126.0, 127.3, 129.2, 130.3, 134.5, 136.7, 153.7, 154.5, 156.7, 162.9, 170.7 ppm; MS(ESI): m/z 535.9 (M+H)⁺; Anal. Calcd. for C₂₆H₁₉BrN₂O₆: C, 58.32; H, 3.55; N, 5.23%. Found: C, 58.28; H, 3.50; N, 5.21%.

Ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-(4-methylphenyl)-3H-pyrano[2,3-d]pyrimidine-6-carboxylate (6j)

IR (KBr, cm⁻¹): 3313, 1714, 1669, 1655, 1603, 1208; ¹H NMR (500 MHz, CDCl₃) δ: 1.09 (t, J = 7.2 Hz, 3H, CH₃CH₂), 2.22 (s, 3H, CH₃), 2.29 (s, 3H, CH₃), 4.14 (q, J = 7.1 Hz, 2H, CH₃CH₂), 4.38 (s, 1H, CH), 7.18–7.35 (m, 2H, ArH), 7.45–7.58 (m, 2H, ArH), 7.77–7.88 (m, 4H, ArH), 8.69 (s, 1H, Coumarin H), 9.14 (s, 1H, NH) ppm; ¹³C NMR (125 MHz, CDCl₃) δ: 16.3, 20.3, 26.6, 27.3, 36.3, 37.8, 100.7, 113.4, 115.4, 117.9, 118.8, 124.4, 126.2, 127.0, 129.5, 130.4, 134.6, 137.0, 154.0, 155.9, 157.5, 163.3, 170.5 ppm; MS(ESI): m/z 471 (M+H)⁺; Anal. Calcd. for C₂₇H₂₂N₂O₆: C, 68.93; H, 4.68; N, 5.95%. Found: C, 68.88; H, 4.65; N, 5.94%.

Results and discussion

The synthetic pathway of the title compounds was achieved via 2-amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylates intermediates (4a–j). Considering the broad spectrum of biological activities of 4H-pyrans, synthetic chemists have developed numerous protocols for their syntheses including two-step as well as one-pot three component synthesis, catalyzed by Baker’s yeast [27], MgO [28], hexadecyldimethylbenzyl ammonium bromide (HDMBAB) [29], phenylboronic acid [30], 2,2,2-trifluoroethanol [31], and silica gel-supported polyphosphoric acid (PPA–SiO₂) [32]. However, these methods often suffer from one or the other kind of drawbacks and most of them give moderate yields even after prolonged reaction time. This has clearly indicated that there is still scope to develop an efficient and eco-sustainable method for the synthesis of 4H-pyrans. The intermediates were obtained by the three component condensation of ethyl acetoacetate, aldehydes with malononitrile using ZrOCl₂·8H₂O as catalyst in aqueous ethanol.

In order to optimize the conditions, we studied the reaction of ethyl acetoacetate, benzaldehyde with malononitrile and ZrOCl₂·8H₂O (5 mol%) as a simple model reaction in various conditions. The reaction was performed in various solvents to identify the best solvent condition. A range of solvents like CH₃CN, CH₃Cl, EtOH, and H₂O were examined at reflux condition (Table 1, Enries 1–4). The reaction without any solvent at reflux was not very successful (Table 1, Entry 5). The model reaction was studied at various mixtures of EtOH/H₂O solvent. The EtOH/H₂O[50/50(v/v)] is proven to be the most suitable solvent for this condensation in terms of yield and reaction time (Table 1, Entry 7). We have studied the amount of ZrOCl₂·8H₂O required for the reaction. It was found that when decreasing the amount of the catalyst from 5 mol% to 3 mol%, the yield decreased from 95% to 77% (Table 1, Entry 9). When increasing the amount of the catalyst from 5 mol% to 10 mol%, there is no change in the yield (Table 1, Entry 10). The use of 5 mol% of ZrOCl₂·8H₂O maintaining the yield at 95%, so this amount is sufficient to promote the reaction. In the presence of more than this amount of the catalyst, neither the yield nor the reaction time was improved (Table 1, Entry 10). Encouraged by this successful three component reaction, synthesis of diverse 2-amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylate derivatives 4a–j was undertaken. The aromatic aldehydes bearing electron-withdrawing and electron donating groups were found to be equally effective to produce 2-amino-4H-pyrans 4a–j in very good yields (Table 2).

Table 1.

Optimization of the reaction conditions on the synthesis of 4a: Effect of solvent and catalyst amount.^a

Entry	Solvent	Amount of catalyst (mol%)	Time (h)	Yield (%)b
1	CH3CN	5	3	41
2	CHCl3	5	3	62
3	H2O	5	3	72
4	EtOH	5	3	68
5	None	5	3	31
6	EtOH/H2O[30/70(v/v)]	5	1.5	78
7	EtOH/H2O[50/50(v/v)]	5	1.5	95
8	EtOH/H2O[70/30(v/v)]	5	1.5	80
9	EtOH/H2O[50/50(v/v)]	3	1.5	77
10	EtOH/H2O[50/50(v/v)]	10	1.5	96

^aReaction conditions: ethyl acetoacetate (1 mmol), benzaldehyde (1 mmol) and malononitrile (1 mmol) at reflux.

^bIsolated yield.

Table 2.

Preparation of various 2-amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylate derivatives.^a

Entry	R1	Product	Time (h)	Yield (%)b	Mp (°C)
					Found	Reported
1	H	4a	1.5	95	193–195	195–196 [28]
2	3-OH	4b	1.5	93	162–164	161–162 [28]
3	3-NO2	4c	1.0	90	182–184	182–183 [28]
4	4-N(CH3)2	4d	2.0	88	180–182	–
5	4-Cl	4e	1.5	87	170–172	172–174 [28]
6	4-F	4f	1.5	91	186–188	–
7	4-OCH3	4g	2.0	87	141–143	142–144 [28]
8	4-NO2	4h	2.5	89	182–184	180–182 [28]
9	4-Br	4i	1.5	90	172–174	–
10	4-CH3	4j	2.0	86	178–180	177–179 [28]

^aReaction conditions: ethyl acetoacetate (1 mmol), aldehyde (1 mmol), and malononitrile (1 mmol) in the presence of ZrOCl₂·8H₂O (5 mol%) in EtOH/H₂O[50/50(v/v)] at reflux.

^bIsolated yield.

After the synthesis of 2-amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylate derivatives, we have synthesized Ethyl 4,5-dihydro 7-methyl-2-(2-oxo-2H-chromen-3-yl)-4-oxo-5-aryl-3H-chromeno[2,3-d]pyrimidine-6-carboxylate derivatives. Initially, the reaction between compound 4a and coumarin-3-carboxylic acid was carried out under neat conditions at 80 °C without and with different acid catalyst (phenylboronic acid, bismuth nitrate, silica perchloric acid, sulfamic acid, PFPAT each 5 mol%) and observed maximum yield with PFPAT (Table 3).

Table 3.

Preparation of ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-phenyl-3H-pyrano[2,3-d]pyrimidine-6-carboxylate: effect of catalyst.^a

Entry	Catalyst	Amount of catalyst (mol%)	Time (h)	Yield (%)b
1	Phenylboronic acid	5	8	55
2	Bismuth nitrate	5	8	62
3	Silica perchloric acid	5	8	72
4	Sulfamic acid	5	8	68
5	PFPAT	5	6	89
6	None	0	8	Trace
7	PFPAT	10	6	89
8	PFPAT	3	6	84
9	PFPAT	2	6	75

^aReaction conditions: 4a (1 mmol) and coumarin-3-carboxylic acid (1 mmol) at 80 °C.

^bIsolated yield.

The solvents played an important role in the synthesis of chromeno pyrimidine derivatives. Various reaction media were screened (1,4-dioxane, ethanol, acetonitrile, THF, methanol, and t-BuOH) using the model reaction (Table 4, Entries 1–6). It was found that the best results were obtained with 5 mol% of PFPAT under solvent-free condition (Table 4, Entry 7). The reaction was completed in 6 h, and the expected product was obtained in 89% yield.

Table 4.

Preparation of ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-phenyl-3H-pyrano[2,3-d]pyrimidine-6-carboxylate: effect of solvent.^a

Entry	Solvent	Amount of catalyst (mol%)	Time (h)	Yield (%)b
1	1,4-Dioxane	5.0	6.0	66
2	Ethanol	5.0	6.0	82
3	Acetonitrile	5.0	8.0	20
4	THF	5.0	8.0	25
5	Methanol	5.0	6.0	78
6	t-BuOH	5.0	6.0	25
7	None	5.0	6.0	89

^aReaction conditions: 4a (1 mmol) and coumarin-3-carboxylic acid (1 mmol) in the presence of PFPAT (5 mol%); 80 °C.

^bIsolated yields.

At these optimize conditions (solvent-free, 80 °C, 5 mol% of PFPAT), we synthesized various chromeno pyrimidinones 6a–j (Table 5). After completion of the reaction, the catalyst was recovered by evaporating the aqueous layer, washed with acetone, dried and reused for subsequent reactions without significant loss in its activity. The catalyst was recycled for four runs without loss of its activity (Table 5, Entry 1). All the synthesized compounds were confirmed by their analytical and spectroscopic data.

Table 5.

Preparation of various ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-phenyl-3H-pyrano[2,3-d]pyrimidine-6-carboxylate derivatives.^a

Entry	R1	Product	Time (h)	Yield (%)b	Mp (°C)
1	H	6a	6.0	89 (87, 85, 84)c	272–274
2	3-OH	6b	6.0	85	234–236
3	3-NO2	6c	6.0	84	268–270
4	4-N(CH3)2	6d	5.5	82	280–282
5	4-Cl	6e	5.5	87	218–220
6	4-F	6f	5.5	86	286–288
7	4-OCH3	6g	7.0	84	220–222
8	4-NO2	6h	5.0	86	266–268
9	4-Br	6i	5.5	87	244–246
10	4-CH3	6j	6.0	85	234–236

^aReaction conditions: 4a–j (1 mmol) and coumarin-3-carboxylic acid (1 mmol) in the presence of PFPAT (5 mol%) at 80 °C.

^bIsolated yield.

^cReusability of catalyst.

To explain the formation of 6a as a model via the condensation reaction, we have proposed a plausible reaction mechanism, which is illustrated in Scheme 3. Firstly, the protonation of coumarin-3-carboxylic acid by PFPAT as a Brønsted acid was occurred to form a cation intermediate (a). In continue, the formation of (b) resulting from the amidation of (a) with 4a was established. In the next step, the protonation of nitrile group of intermediate (b) following by a cyclo-addition reaction was occurred to form the intermediate (c). In continue the addition reaction of pentafluorophenyl amine (PFPA) followed by ring-opening of the (c) to the intermediate (d) and (e) followed by ring closure of intermediate (e) results in the formation of intermediate (f) that convert to the (6a) as product by the de-protonation reaction. Interestingly, the formation of compound 6a, obtained from the condensation of coumarin-3-carboxylic acid with 4a, confirms the mechanism of the reaction which was rarely described in the literature as Dimroth rearrangement [33,34].

Scheme 3.

Proposed mechanism for the formation of 6a.

Biological evaluations

All the compounds were screened for their antibacterial and antifungal activity. Compounds 6a–j with various substituents in the aromatic ring will be useful in understanding the influence of steric and electronic effects on the biological activity.

Antibacterial activity

The newly synthesized compounds were screened for their in vitro antibacterial activity against Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa), Klebsiella pneumonia (K. pneumonia), and Staphylococcus aureus (S. aureus) bacterial stains by serial plate dilution method. Serial dilutions of the drug in Muller Hinton broth were taken in tubes, and their pH was adjusted to 5.0 using phosphate buffer. A standardized suspension of the test bacterium was inoculated and incubated for 16–18 h at 37 °C. The minimum inhibitory concentration (MIC) was noted by seeing the lowest concentration of the drug at which there was no visible growth.

A number of antibacterial disks were placed on the agar for the sole purpose of producing zones of inhibition in the bacterial lawn. Twenty milliliters of agar media was poured into each Petri dish. Excess of suspension was decanted, and plates were dried by placing in an incubator at 37 °C for an hour. Using a punch, wells were made on these seeds agar plates, and minimum inhibitory concentrations of the test compounds in dimethyl sulfoxide (DMSO) were added into each labeled well. A control was also prepared for the plates in the same way using DMSO as a solvent. The Petri dishes were prepared in triplicate and maintained a 37 °C for 3–4 days. Antibacterial activity was determined by measuring the diameter of inhibition zone. Activity of each compound was compared with ciprofloxacin as standard. Zone of inhibition was determined for 6a–j. The results are summarized in Table 6. The MIC values were evaluated at concentration range, 6.25–25 μg/mL. The figures in the table show the MIC values in μg/mL and the corresponding zone of inhibition in mm. From the activity report (Table 6) it was notified that most of the compounds showed moderate activity against all the bacterial strains.

Table 6.

In vitro antibacterial activity of compounds 6a–j.

Compounds	MIC in μg/mL and zone of inhibition in mm
	E. coli	P. aeruginosa	K. pneumonia	S. aureus
6a	12.5(15–18)	12.5(15–18)	12.5(15–18)	12.5(16–18)
6b	6.25(16–19)	6.25(19–21)	6.25(15–18)	6.25(16–18)
6c	12.5(14–17)	12.5(15–18)	12.5(15–18)	12.5(16–18)
6d	12.5(12–15)	12.5(12–15)	12.5(15–18)	12.5(15–18)
6e	6.25(16–18)	6.25(15–18)	6.25(15–18)	6.25(16–18)
6f	6.25(16–18)	6.25(15–18)	6.25(15–18)	6.25(16–18)
6g	25(8–11)	25(9–12)	25(8–11)	25(9–12)
6h	25(8–11)	25(9–12)	25(8–11)	25(9–12)
6i	6.25(18–20)	6.25(16–18)	6.25(16–18)	6.25(16–18)
6j	6.25(18–20)	6.25(15–18)	6.25(16–18)	6.25(18–20)
Ciprofloxacin	6.25(30–35)	6.25(26–32)	6.25(21–25)	6.25(25–28)

Antifungal activity

Newly prepared compounds were also screened for their antifungal activity against Aspergillus flavus (A. flavus), Rhizopus schipperae (R. schipperae), Aspergillus niger (A. niger) and Candida albicans (C. albicans) in DMSO by serial plate dilution method. Sabourauds agar media were prepared by dissolving peptone (1 g). D glucose (4 g) and agar (2 g) in distilled water (100 mL) and adjusting the pH to 5.7. Normal saline was used to make a suspension of sore of fungal strains for lawning. A loopful of particular fungal strain was transferred to 3 mL saline to get a suspension of corresponding species. Twenty milliliters of agar media was poured into each Petri dish. Excess of suspension was decanted, and plates were dried by placing in an incubator at 37 °C for 1 h. Using a punch, wells were made on these seeded agar plates. Minimum inhibitory concentrations of the test compounds in DMSO were added into each labeled well. A control was also prepared for the plates in the same way using solvent DMSO. The Petri dishes were prepared in triplicate and maintained at 37 °C for 3–4 days. Antifungal activity was determined by measuring the diameter of inhibition zone. Activity of each compound was compared with Amphoterecin-B as standard. Zones of inhibition were determined for 6a–j. The results are summarized in Table 7. The MIC values were evaluated at concentration range, 6.25–25 μg/mL. The figures in the table show the MIC values in μg/mL and the corresponding zone of inhibition in mm. All the newly synthesized compounds showed moderate activity against all the fungal strains.

Table 7.

In vitro antifungal activity of compounds 6a–j.

Compounds	MIC in μg/mL and zone of inhibition in mm
	A. flavus	R. schipperae	A. niger	C. albicans
6a	12.5(16–20)	12.5(18–22)	12.5(20–22)	12.5(20–22)
6b	6.25(16–20)	6.25(18–22)	6.25(20–22)	6.25(18–20)
6c	12.5(15–18)	12.5(18–22)	12.5(20–22)	12.5(18–20)
6d	12.5(10–12)	12.5(12–16)	12.5(16–18)	12.5(18–18)
6e	6.25(12–16)	6.25(12–16)	6.25(16–18)	6.25(16–18)
6f	6.25(10–14)	6.25(12–14)	6.25(12–15)	6.25(14–16)
6g	25(10–12)	25(8–11)	25(10–12)	25(10–12)
6h	25(10–12)	25(9–12)	25(10–12)	25(10–12)
6i	6.25(15–16)	6.25(18–22)	6.25(18–22)	6.25(18–20)
6j	6.25(14–18)	6.25(16–14)	6.25(16–18)	6.25(16–18)
Amphoterecin-B	6.25(22–26)	6.25(30–34)	6.25(27–30)	6.25(28–32)

Conclusions

Various derivatives of ethyl 4,5-dihydro-7-methyl-4-oxo-2-(2-oxo-2H-chromen-3-yl)-5-phenyl-3H-pyrano[2,3-d] pyrimidine-6-carboxylate (6a–j) were synthesized from the reaction of 2-amino-3-cyano-6-methyl-4-phenyl-4H-pyran-5-ethylcarboxylates (4a–j) with coumarin-3-carboxylic acid in the presence of PFPAT as reusable and inexpensive Brønsted acidic catalyst under solvent-free condition. All the synthesized compounds were screened for their in vitro antimicrobial activity. The newly synthesized compounds showed moderate activity against all the bacterial and fungal strains.

Conflict of interest

The authors have declared no conflict of interest.