Nanomaterial applications have expanded beyond materials science into the biomedical,chemical and electronics fields because of their high surface area volume ratios,conductivity, magnetic susceptibility and catalytic activity ^{[1, 2]}.

Recently,magnetic nanoparticles (MNPs) have attracted growing interest owing to their unique properties and potential applications in various fields. As heterogeneous catalyst they have gained popularity in organic synthesis due to simple work-up procedures,environmentally benign nature,reusability,low cost, and ease of isolation ^{[2, 3]}. Magnetic nanoparticles have also gained recognition as potential environmentally benign replacements of the conventional Lewis acid and base catalysts in various organic synthetic processes ^[4]. MNPs as solid acid catalysts have served as important functional materials in industrial processes. In particular,for reactions in which water is involved either as a reactant or product,only several solid acids show acceptable performance. The development of new solid acids is expected to have a major impact on industrial applications as well as for basic research. This problem could be overcome by designing different Brønsted acids (SO₃H,HClO₄,HBF₄) on γ-Fe₂O₃@SiO₂ ^{[5, 6, 7]} and functionalized hydroxyapatite-encapsulated-γ-Fe₂O₃ magnetic nanoparticles ^{[8, 9, 10, 11]}.

On the other hand,nitrogen heterocycles occupy an important position in natural product and medicinal chemistry. Fused heterocyclic systems,incorporating a pyrimidine ring in their structures,play important roles in various biological and pharmaceutical processes ^{[12, 13, 14, 15, 16, 17, 18, 19, 20]}. In particular,the pyrido[2.3-d]pyrimidine ring system is embedded in a number of biologically active compounds with bactericidal ^[21],antipyretic ^[22],antitumor ^[23],medicinal ^[24] and antihistaminic ^[25] activities.

Numerous methods for the synthesis of pyrido[2.3-d]pyrimidine derivatives using 2,6-diaminopyrimidine-4(3H)-one ^{[26, 27]}, 6-aminouracil and 6-aminothiouracil have been reported in the past several years,which involve the reaction between b-unsaturated carbonyl compounds (aldehyde,ketone and ester) and 1,3-diketones ^[28] with 6-aminouracil or 6-aminothiouracil, and arylidenemalononitrile derivatives with 6-aminouracil ^[29] or 6-aminothiouracil ^{[30, 31]}. However some of these methods exhibit disadvantages such as long reaction times,high loading of catalyst,non-recyclability of the catalyst,high reaction temperature and waste generation. In order to advance the development of environmentally friendly procedures and sustainable methods for the synthesis of biologically important compounds,we report here a novel method for the three-component one-pot synthesis of pyrido[2.3-d]pyrimidine using [g-Fe₂O₃@-HAp-SO₃H] as a recyclable nanocatalyst. 2. Experimental

Melting points were measured on an Electrothermal 9100 apparatus. IR spectra were determined on a Shimadzo IR-470 spectrometer. ¹H NMR and ¹³C NMR spectra were recorded on a 400 MHz Bruker DRX-400 in DMSO-d6using TMS as an internal standard. Elemental analyses were performed on a Carlo-Erba EA1110CNNO-S analyzer and agreed (within 0.30) with the calculated values. XRD was carried out on a on a Philips X-Pert MPD diffractometer using Co tube. Scanning electron microphotographs (SEM) were obtained on a PHILIPS XL30 electron microscope. All the chemicals were purchased from Merck and used without further purification. All solvents used were dried and distilled according to standard procedures. [γ-Fe₂O₃@Hap-SO₃H] was synthesized according to the procedures reported in Ref. ^[2]. Then to a mixture of 6-amino-2-(methylthio or ethylthio)pyrimidin-4(3H)-one (1 mmol),Meldrum’s acid (1 mmol) and aryl aldehydes (1 mmol) was added [γ-Fe₂O₃@Hap-SO₃H] (10 mg,0.9 mol%) and the reaction mixture was stirred mechanically at 60 ℃. After the completion of the reaction,which was monitored by TLC analysis,the reaction mixture was diluted with hot ethanol and the catalyst was easily separated from the reaction mixture by an external magnet. The product obtained was collected by filtration,washed with ethanol and recrystallized from appropriate solvent to furnish the desired pure product (4a-n). Some data of selected compounds are listed below.

2-Ethylthio-5,6-dihydro-5-(4-chlorophenyl)pyrido[2.3-d]pyrimidine-4,7(3H,8H)-dione (4a): White powder; IR (KBr,cm^-1 ): v 3346,3180 (N-H),1647 (CONH); ¹H NMR (400 MHz,CDCl₃): δ 11.99 (br.s,1H,NH),10.33 (br.s,1H,NH),7.25 (d,2H,J = 8.0 Hz, Ar-H),7.03 (d,2H,J = 8.0 Hz,Ar-H),5.44 (s,1H,CH),3.10 (m,3H, CH₂),2.56 (m,1H,diastereotopic CH),1.30 (t,3H,J = 7.2 Hz,CH₃), ¹³C NMR (100 MHz,CDCl₃): δ 163.0,158.3 (C≡0,amide),138.9, 134.8,134.0,129.8,129.0,127.9,109.0,39.3,33.6,24.3,15.2; Anal. Calcd. for C₁₅H₁₄ClN₃O₂S (335.8): C,53.65; H,4.20; N,12.51; Found: C,53.45; H,4.11; N,12.57.

2-Ethylthio-5,6-dihydro-5-(3-hydroxyphenyl)pyrido[2.3-d]-pyrimidine-4,7(3H,8H)-dione (4b): White powder; IR (KBr,cm^-1 ): v 3462 (O-H),3377,3196 (N-H),1647 (CONH); ¹H NMR (400 MHz, CDCl₃): δ 11.93 (br.s,1H,NH),10.52 (s,1H,NH),6.98 (t,1H J = 8.0 Hz,Ar-H),6.49 (m,3H,Ar-H),5.40 (s,1H,CH),3.11 (m,3H, CH₂),2.52 (m,1H,diastereotopic CH),1.30 (t,3H,J = 7.2 Hz,CH₃), ¹³C NMR (100 MHz,CDCl₃): δ 163.0,158.3 (C≡0,amide),138.9, 134.8,134.0,129.8,129.0,127.9,109.0,39.3,33.6,24.3,15.2; Anal. Calcd. for C₁₅H₁₅N₃O₃S (317.3): C,56.77; H,4.76; N,13.24; Found: C,56.65; H,4.67; N,13.11

2-Ethylthio-5,6-dihydro-5-(2,4-dichlorophenyl)pyrido[2.3-d]pyrimidine-4,7(3H,8H)-dione (4c): White powder; IR (KBr, cm^-1 ): v 3383,3194 (N-H),1635 (CONH); ¹H NMR (400 MHz, CDCl₃): δ 12.69 (br.s,1H,NH),10.70 (s,1H,NH),7.59 (d,1H, J = 8.8 Hz,Ar-H),7.47 (d,1H,J = 2.0 Hz,Ar-H),7.34 (dd,1H,J = 8.2, 2.0 Hz,Ar-H),4.49 (d,1H,J = 8.0 Hz,CH),3.17 (m,2H,CH),2.52 (m,1H,diastereotopic CH),1.34 (t,3H,J = 7.4 Hz,CH₃), ¹³C NMR (100 MHz,CDCl₃): δ 163.0 (C=O,amide),129.1,39.3,31.0,24.3, 15.1; Anal. Calcd. for C₁₅H₁₃Cl₂N₃O₃S (370.2): C,48.66; H,3.54; N, 11.35; Found: C,48.70; H,3.44; N,11.21.

2-Ethylthio-5,6-dihydro-5-((E)-5-(2-(4-chlorophenyl)diazenyl)-2-hydroxyphenyl)pyrido[2.3-d]pyrimidine-4,7(3H,8H)-dione (4j): Yellow powder; IR (KBr,cm^-1 ): v 3440 (OH),3200 (N-H),1690 (CONH),1490 (N55N),1280 (C-O); ¹H NMR (400 MHz,CDCl₃):δ 12.70 (br.s,1H,NH),10.84 (br.s,1H,OH),10.61 (br.s,1H,NH),7.80 (d,2H, J = 8.8 Hz,Ar-H),7.70 (dd,1H,J = 8.4,2.4 Hz,Ar-H),7.60 (d,2H, J = 8.8 Hz,Ar-H),7.29 (s,1H,Ar-H),7.05 (dd,1H,J = 8.4,2.4 Hz,Ar-H), 4.46 (d,1H,J = 8.0 Hz,CH),4.06 (m,2H,CH₂),3.05 (dd,. 1H,J = 8.0, 16 Hz,diastereotopic CH),2.55 (m,1H,diastereotopic CH),1.29 (m, 3H,CH₃); ¹³C NMR (100 MHz,CDCl₃): δ 170.4,158.9 (C≡0,amide), 150.6,144.7,134.7,131.1,129.5,129.4,128.7,123.8,123.7,121.5, 116.3,97.1,36.5,30.7,28.2,12.7; Anal. Calcd. for C₂₁H₁₈ClN₅O₃S (455.9): C,55.32; H,3.98; N,15.36; found: C,55.21; H,3.85; N,15.17

2-Methylthio-5,6-dihydro-5-((E)-5-(2-(4-chlorophenyl)diazenyl)-2-hydroxyphenyl)pyrido[2.3-d]pyrimidine-4,7(3H,8H)-dion (4m): Yellow powder; IR (KBr,cm^-1 ): v 3440 (OH),3200 (N-H), 1690 (CONH),1490 (N55N),1280 (C-O); ¹H NMR (400 MHz,CDCl₃): δ 12.67 (br.s,1H,NH),10.91 (br.s,1H,OH),10.60 (s,1H,NH),7.79 (d,2H,J = 8.8 Hz,Ar-H),7.70 (dd,1H,J = 8.4,2.4 Hz,Ar-H),7.59 (d, 2H,J = 8.8 Hz,Ar-H),7.30 (s,1H,Ar-H),7.04 (d,1H,J = 8.4,Hz,ArH),4.47 (d,1H,J = 8.0 Hz,CH),3.05 (dd,1H,J = 8.0,16 Hz, diastereotopic CH),2.56 (s,3H,CH₃),2.51 (br.s,1H,diastereotopic CH); ¹³C NMR (100 MHz,CDCl₃): δ 170.4,158.9 (C≡0,amide), 150.6,144.8,134.8,131.1,129.3,128.7,123.8,123.7,121.5,116.3, 97.1,36.5,28.2,12.7; Anal. Calcd. for C₂₀H₁₆ClN₅O₃S (441.9): C, 54.36; H,3.65; N,15.85; found: C,54.21; H,3.47; N,15.66.

2-Methylthio-5,6-dihydro-5-((E)-5-(2-(4-nitrophenyl)diazenyl)-2-hydroxyphenyl)pyrido[2.3-d]pyrimidine-4,7(3H,8H)-dione (4n): Orange powder; IR (KBr,cm^-1 ): v 3440 (OH),3200 (N-H), 1690 (CONH),1490 (N55N),1280 (C-O); ¹H NMR (400 MHz, CDCl₃): δ 12.78 (br.s,1H,NH),11.25 (br.s,1H,OH),10.61(s,1H, NH),7.79 (d,2H,J = 8.8 Hz,Ar-H),7.70 (dd,1H,J = 8.4,2.4 Hz,ArH),7.59 (d,2H,J = 8.8 Hz,Ar-H),7.30 (s,1H,Ar-H),7.04 (d,1H, J = 8.4,Hz,Ar-H),4.47 (d,1H,CH),3.06 (m,1H,diastereotopic CH), 2.56-2.67 (m,4H,diastereotopic CH,CH₃); ¹³C NMR (100 MHz, CDCl₃): δ 170.4,167.5 (C≡0,amide),150.3,147.6,131.2,130.2, 124.9,123.0,122.9,118.4,115.2,103.7,36.,28.2,12.7; Anal. Calcd. for C₂₀H₁₆N₆O₅S (452.4): C,53.10; H,3.56; N,18.57; found: C, 52.91; H,3.44; N,18.41. 3. Results and discussion

Following our continued studies in the development of benign methods in the synthesis of biologically important heterocycles ^{[32, 33, 34, 35, 36, 37, 38]},and exploiting the valuable catalytic properties of magnetic nanoparticles ^{[6, 7, 9, 10]} we were interest in the synthesis of novel derivatives of pyrido[2.3-d]pyrimidine using [γ-Fe₂O₃@-HAp-SO₃H] as a nanocatalyst.

Recently,as a result of the intense interest in the fabrication of core-shell of magnetic particles,hydroxyapatite has received considerable attention as one of the most ideal biocompatible materials for encapsulated iron oxide NPs. This class of materials has reliable chemical stability,biocompatibility and versatility in surface modification. HAp-encapsulated-γ-Fe₂O₃ [Fe₂O₃@HAp] and [γ-Fe₂O₃@HAp-SO₃H] nanocrystallites were prepared according to the reported procedures ^[2] (Scheme 1). The prepared nanocrystallites were characterized by XRD,IR and SEM (Fig. 1a-d).

Scheme 1

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Scheme 1.Preparation of [γ-Fe2O3@HAp] and [γ-Fe2O3@HAp-SO3H].

Fig. 1

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Fig. 1. (a) XRD spectra,(b) IR spectrum and (c,d) SEM photographs of the [γ-Fe2O3@HAp-SO3H].

In XRD analysis (Fig. 1a),the resulted patterns are in agreement with those of the tetragonal structure of γ-Fe₂O₃(1999JCPDS Card No. 13-0458). Diffraction peaks at around 18.4°,30.2°,35.7°,43.6°, 56.2° and 63.1° are related to the (1 1 1),(2 2 0),(3 1 1),(4 0 0), (4 4 0) and (5 1 1) patterns of maghemite. In addition,characteristic diffraction peaks of hydroxyapatite based on the standard XRD pattern of HAp (JCPDS Card No. 09-432) are readily recognized from the XRD. In the IR spectrum (Fig. 1b),the hydroxyl band appeared at 3400 cm^-1 ,the S55O group appeared at 1380 cm^-1 and surface phosphate groups in the hydroxyapatite cover,overlapped with S-O stretching peak at 560 and 600 cm^-1 .

SEM analysis (Fig. 1c and d) clearly showed the particles of the catalyst are uniformly nanosized (33-47 nm).

At the outset of this study,the requisite starting material 1 (Scheme 2) was prepared by the condensation of thiourea with ethyl cyanoacetate in sodium ethoxide and alkylated by alkyl halide according to the known procedures ^[39]. Diazenylarylaldehydes (Fig. 2) were conveniently prepared according to the literature report ^[40].

Scheme 2

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Scheme 2.Synthesis of pyrido[2.3-d]pyrimidine derivatives (4a-n)

Fig. 2

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Fig. 2. Structures of 2-hydroxy-5-(aryldiazenyl)benzaldehydes.

To optimize the desired reaction conditions,one-pot threecomponent reaction of 6-amino-2-(methylthio or ethylthio)-pyrimidin-4(3H)-one 1 (1 mmol),Meldrum’s acid 2 (1 mmol) and 4-nitrobenzaldehydes 3a (1 mmol) was used as a model system. The reaction mixture was heated at 80 ℃ with [γ-Fe₂O₃@HAp-SO₃H] (0.01 g) in EtOH,which produced the product 4a in 8 min in 87% yield.

To investigate the effect of various parameters,preparation of 4a as a model reaction was attempted in several solvents such as CH₃CN,CH₂Cl₂,DMF,THF,CH₃OH,EtOH and under solvent-free conditions. The results revealed that the reaction under solventfree conditions produced the product in shortest reaction time and highest yield (94%). To compare the efficiency of [γ-Fe₂O₃@HAp-SO₃H] with various acidic and basic catalysts, preparation of 4a was carried out in refluxing ethanol (Table 1). The result showed that [γ-Fe₂O₃@HAp-SO₃H] under solvent-free conditions gave the best result (entry 12). We also verified the amount of the catalyst in preparation of 4a and the best result was obtained using 0.01 g [γ-Fe₂O₃@HAp-SO₃H] at 60 ℃ under solvent-free condition.

Table 1

Table 1 Comparison of efficiency of various catalysts in one-pot synthesis of 4a in refluxing ethanol at 60 ℃.

Table 1 Comparison of efficiency of various catalysts in one-pot synthesis of 4a in refluxing ethanol at 60 ℃.

Using the optimized conditions,several pyrido[2.3-d]pyrimidine derivatives were synthesized (Scheme 2). The results are summarized in Table 2. The structure of all products was established by spectroscopic methods (IR, ¹H NMR, ¹³C NMR) and elemental analyses.

Table 2

Table 2 One-pot synthesis of pyrido[2.3-d]pyrimidine derivatives (4a-n) in the presence of[γ-Fe2O3@HAp-SO3H] as nanocatalyst at 60 ℃,under solvent-free condition

Table 2 One-pot synthesis of pyrido[2.3-d]pyrimidine derivatives (4a-n) in the presence of[γ-Fe2O3@HAp-SO3H] as nanocatalyst at 60 ℃,under solvent-free condition

We have also examined the reusability of the catalyst. The nanocatalyst was separated from the reaction medium simply by an external magnetic field,washed with ethanol,dried under vacuum and reused for the subsequent reactions. After 10 successive runs the catalytic activity of [γ-Fe₂O₃@HAp-SO₃H] was almost remained unchanged.

The mechanism of this multicomponent reaction involves a Knoevenagel condensation/Michael addition cascade process. To form the reaction product,aryllidene derivatives of Meldrum’s acid are attacked by exocyclic NH₂-group followed by the release of CO₂ and acetone. The use of [γ-Fe₂O₃@HAp-SO₃H] nanocatalyst provides efficient acidic sites and therefore facilitates the reaction (Scheme 3).

Scheme 3

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Scheme 3. A plausible mechanism for the synthesis of pyrido[2.3-d]pyrimidine 4 using [γ-Fe2O3@HAp-SO3H].

In summary,for the first time we showed that [γ-Fe₂O₃@HAp] supported sulfonic acid was an effective heterogeneous catalyst for the one-pot synthesis of pyrido[2.3-d]pyrimidine derivatives from 6-amino-2-(methylthio or ethylthio)-pyrimidin-4(3H)-one, Meldrum’s acid and aryl aldehydes under solvent-free conditions.

The mild reaction conditions,green and cost-effective catalyst, excellent yields,easy work-up procedures,which avoid the use of large volumes of hazardous organic solvents,make it a useful alternative to previously applied procedures. Compared with nonmagnetic nanoparticle catalytic system,the present protocol combines the advantages of solid Brønsted acid and magnetic nanoparticles and offers great potentials for the rapid synthesis of pyrido[2.3-d]pyrimidines.