Heteropoly acid catalyzed synthesis of 3,4-dihydropyrimidin-2(1H)-ones

Catalysis Communications 8 (2007) 279–284 www.elsevier.com/locate/catcom

Heteropoly acid catalyzed synthesis of 3,4-dihydropyrimidin-2(1H)-ones Sanjeev P. Maradur, Gavisiddappa S. Gokavi

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Kinetics and Catalysis Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416 004, Maharashtra, India Received 14 December 2005; received in revised form 31 May 2006; accepted 31 May 2006 Available online 16 June 2006

Abstract Simple and improved conditions have been found to carry out the Biginelli reaction for the synthesis of 3,4-dihydropyrimidin-2(1H)one derivatives. This synthesis was performed using 11-molybdo-1-vanadophosphoric acid (H4PMo11VO40) in ethanol solution. Compared with the classical Biginelli reaction conditions, this new method has the advantage of excellent yields (85–95%). Ó 2006 Elsevier B.V. All rights reserved.

1. Introduction Tandem reactions, sometimes also called domino, sequential, cascade, consecutive, iterative, zipper, and one-pot (one-flask) multi component reactions (MCRs) reactions, link several transformations together in a single synthetic step. Typically, an initial reaction produces an intermediate that undergoes further transformations with strategically positioned reactive centers in the same molecule, with other compounds in the reaction mixture, or with additional reagents introduced after the initial transformation takes place [1]. Over the past decades, tandem reactions have gained wide acceptance because they increase synthetic efficiency by decreasing the number of laboratory operations required and the quantities of chemicals and solvents used. Furthermore, they frequently permit efficient access to unique chemical structures and occasionally result in greater reaction selectivity. Tandem reactions can provide products with the diversity needed for the discovery of new lead compounds or lead optimization employing combinatorial chemistry techniques [2–6]. The search and discovery of such reactions on one hand [7], and the full exploitation of already known

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Corresponding author. Tel.: +91 2315690571; fax: +91 231269333. E-mail addresses: [email protected] (S.P. Maradur), gsgokavi@ hotmail.com (G.S. Gokavi). 1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.05.048

one on the other hand, is therefore of considerable current interest. One such reaction, which belongs in the latter category, is the venerable Biginelli dihydropyrimidine synthesis. Aryl-3,4-dihydropyrimidinones have recently received great attention because of their wide range of therapeutic and pharmacological properties, such as antiviral, antitumor, antibacterial, and antiinflammatory behaviour [8]. Furthermore, these compounds have emerged as the integral backbones of several calcium channel blockers, antihypertensives, a1a-adrenergic antagonists, and neuropeptide Y (NPY) antagonists [9]. Moreover, several alkaloids containing the dihydropyrimidine core structure have been isolated from marine sources and also exhibit interesting biological properties. Most notably among these are the batzelladine alkaloids, which are found to be potent HIV gp-120-CD4 inhibitors [10]. Thus, the synthesis of this heterocyclic nucleus is of much current importance. The so-called Biginelli reaction often suffers, however, from low yields of products, particularly in case of substituted aromatic and aliphatic aldehydes [11]. This problem has led to the development of multi-step synthetic strategies that produce relatively higher yields, but lack the simplicity of the original one-pot-Biginelli protocol [12]. Thus, the Biginelli reaction has received renewed interest from researchers discovering milder and more-efficient procedures that are applicable to a wide range of substituents in all three components and proceed in better yields. As a

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result, several improved procedures have been reported recently using Lewis acids, as well as protic acids, as promoters [13,14]. Asymmetric versions of the Biginelli reaction have also been reported using Garner aldehyde or a sugar derived aldehyde, b-keto ester and urea to produce optically active 3,4-dihydropyrimidinones [15]. Recently, environmentally benign approaches have been developed using solvent free conditions, utilization of heteropoly acids [16]. Heteropoly acids due to their unique physicochemical properties are widely used as homogeneous and heterogeneous acid and oxidation catalysts. They are also of great interest as model systems for studying fundamental problems of catalysis [17–19]. For catalysis, Keggin-type V HPAs (H8x Xx MVI H8xþn Xx MVI where 12 O40 , 12n Vn O40 , IV IV V V VI V X = Si , Ge , P , As ; M = Mo , W ) are of importance. The considerable number of studies performed during past 20–25 years allowed to formulate the selection principles of effective catalysts in the series of Keggin-structure HPAs. Their significantly higher Brønsted acidity, compared with the acidity of traditional mineral acid catalysts, is of great importance for catalysis [17–20]. Many new catalytic processes for basic and fine organic syntheses based on their employment have been developed. In the future, the number of such processes will undoubtedly increase because HPA-based catalysts have higher activity than known traditional catalysts. Using HPA-based catalysts, it is frequently possible to obtain higher selectivity and successfully solve ecological problems.

2. Experimental section 2.1. General remarks Melting points are uncorrected. 1H spectra were recorded in CDCl3/[d6]DMSO using TMS as the internal standard. The catalyst, 11-molybdo-1-vanadophosphoric acid, H4PMo11VO40 (HPVMo11) was systematically characterized by various analytical and spectroscopic techniques such as inductively coupled plasma-atomic emission spectroscopy (ICP-AES) with Labtam Plasma Lab 8440 equipment, thermogravimetric analysis (TGDTA on a Dupont 9900/2100) TG/DTA system under static air at a heating rate of 5 °C min1 and Fourier transform-infrared spectroscopy (FT-IR, Nicolet Impact-400) using the KBr pellet technique. 2.2. Catalyst preparation The catalyst was prepared according to the procedure reported in the literature [21]. A stoichiometric mixture of 0.98 g (0.01 mol) of phosphoric acid, 0.91 g (0.005 mol) of vanadium pentoxide and 14.4 g (0.11 mol) of molybdenum trioxide was suspended in 150 ml of distilled water. The mixture was stirred for 3 h at 80 °C. After cooling down to room temperature and removal of insoluble molybdates and vanadates, the heteropoly acid solution was evaporated and dried at 85 °C for 10 h yielding orange crystals of HPVMo11. ICP analysis indicated that the composition of the sample was P, 1.51; Mo, 51.89; V, 2.67, that

Table 1 11-Molybdo-1-vanadophosphoric acid (HPVMo11) catalysed synthesis of Biginelli 3,4-dihydropyriminones Entry

R0

R00

X

Time (h)

Product

Yield (%)a,b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Ph 2-NO2AC6H4 3-NO2AC6H4 4-ClAC6H4 3-MeOAC6H4 4-HOAC6H4 C6H5CH@CH Furfural Ph 2-NO2AC6H4 3-NO2AC6H4 4-ClAC6H4 3-MeOAC6H4 4-HOAC6H4 Ph 2-NO2AC6H4 3-NO2AC6H4 4-ClAC6H4 3-MeOAC6H4 2-HOAC6H4 Ph Ph Ph

OEt OEt OEt OEt OEt OEt OEt OEt OMe OMe OMe OMe OMe OMe Me Me Me Me Me Me OEt OMe Me

O O O O O O O O O O O O O O O O O O O O S S S

6 6.5 6 6 6 6.5 6 6 6 8 6 4.5 5 8 6.5 7.5 6 6 5.5 8 5 6 6

4a 4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 4l 4m 4n 4o 4p 4q 4r 4s 4t 4u 4v 4w

94 85 90 95 86 90 91 86 93 81 83 91 95 89 93 83 91 90 96 89 93 90 93

a b

All the products were characterized by 1H NMR spectroscopy. Yield refers to the isolated pure products after recrystallization.

S.P. Maradur, G.S. Gokavi / Catalysis Communications 8 (2007) 279–284

is, the found atomic ratio of P/Mo/V was: 1.00/11.11/1.07. If the sample were pure H4PMo11VO40, the composition should be P, 1.54; Mo, 52.38; V, 2.53. 2.3. General procedure A mixture of aldehyde (5 mmol), ethyl acetoacetate (5 mmol), urea (10 mmol), and HPVMo11 (0.01 mmol) in ethanol (10 mL) was stirred at 80 °C for the appropriate time (Table 1). On completion of the reaction, as indicated by TLC, the resulting mixture was poured into water and product was separated and recrystallized from methanol to yield pure dihydropyrimidinone. The products were identified by 1H NMR, and physical data with those reported in the literature [13,14,22]. 2.3.1. Ethyl-6-methyl-2-oxo-4-phenyl-3,4dihydropyrimidine-5-carboxylate (4a) MP: 203–205 °C. 1H NMR (300 MHz, CDCl3/ [d6]DMSO) d 1.15 (t, 3H, ACH2CH3), 2.32 (s, 3H, CH3), 4.06 (q, 2H, ACH2CH3), 5.35 (s, 1H, CH), 6.82 (s, 1H, NH), 7.36 (s, 5H, ArAH), 8.78 (s, 1H, NH). 2.3.2. Ethyl-4-(2-nitrophenyl)-6-methyl-2-oxo-3,4dihydropyrimidine-5-carboxylate (4b) MP: 224–225 °C. 1H NMR (300 MHz, CDCl3/ [d6]DMSO) d 1.37 (t, 3H, ACH2CH3), 2.48 (s, 3H, CH3), 4.35 (q, 2H, ACH2CH3), 5.77 (s, 1H, CH), 5.90 (s, 1H, NH), 7.47–8.24 (m, 5H, ArAH & NH). 2.3.3. Ethyl-4-(3-methoxyphenyl)-6-methyl-2-oxo-3,4dihydropyrimidine-5-carboxylate (4e) MP: 213–215 °C. 1H NMR (300 MHz, CDCl3/ [d6]DMSO) d 1.20 (t, 3H, ACH2CH3), 2.33 (s, 3H, CH3), 3.80 (s, 3H, OCH3), 4.08 (q, 2H, ACH2CH3), 5.39 (s, 1H, CH), 5.34 (s, 1H, NH), 6.79–7.26 (m, 5H, ArAH & NH). 2.3.4. Methyl-6-methyl-2-oxo-4-phenyl-3,4dihydropyrimidine-5-carboxylate (4i) MP: 210–213 °C. 1H NMR (300 MHz, CDCl3/ [d6]DMSO) d 2.39 (s, 3H, CH3), 3.62 (s, 3H, CO2CH3), 5.39 (s, 1H, CH), 5.82 (s, 1H, NH), 7.24–7.34 (m, 5H, ArAH), 8.20 (s, 1H, NH). 2.3.5. Methyl-4-(3-methoxyphenyl)-6-methyl-2-oxo-3,4dihydropyrimidine-5-carboxylate (4m) MP: 192–195 °C. 1H NMR (300 MHz, CDCl3/ [d6]DMSO) d 1.19 (t, 3H, ACH2CH3), 2.33 (s, 3H, CH3), 3.78 (s, 3H, OCH3), 4.07 (q, 2H, ACH2CH3), 5.32 (s, 1H, CH), 6.72–7.436 (m, 5H, ArAH & NH), 8.67 (s, 1H, NH). 2.3.6. Acetyl-6-methyl-4-phenyl-2-oxo -3,4dihydropyrimidine (4o) MP: 233–235 °C. 1H NMR (300 MHz, CDCl3/ [d6]DMSO) d 2.10 (s, 3H, COCH3), 2.35 (s, 3H, CH3), 5.36 (s, 1H, CH), 7.22–7.65 (m, 6H, ArAH & NH), 8.97 (s, 1H, NH).

281

2.3.7. 5-Acetyl-4-(2-nitrophenyl)-6-methyl-2-oxo-3,4dihydropyrimidine (4p) MP: 239–242 °C. 1H NMR (300 MHz, CDCl3/ [d6]DMSO) d 2.23 (s, 3H, COCH3), 2.39 (s, 3H, CH3), 5.54 (s, 1H, CH), 5.38 (br, 1H, NH), 7.30–8.20 (m, 4H, ArAH), 8.89 (s, 1H, NH). 2.3.8. 5-Acetyl-4-(3-methoxyphenyl)-6-methyl-2-oxo-3,4dihydropyrimidine (4s) MP: 229–233 °C. 1H NMR (300 MHz, CDCl3/ [d6]DMSO) d 2.11 (s, 3H, COCH3), 2.35 (s, 3H, CH3), 3.78 (s, 3H, OCH3), 5.35 (s, 1H, CH), 6.78–7.53 (m, 5H, ArAH & NH), 8.71 (s, 1H, NH). 3. Results and discussion 3.1. Catalyst characterization 3.1.1. Thermal analysis The catalyst was initially characterized by thermal analysis method. The degree of hydration of the catalyst (HPVMo11), was determined from TGA analysis and corresponded to the formulae H4PMo11VO40 Æ 15H2O. The TGA of the catalyst (HPVMo11) (Fig. 1) shows a weight loss due to the loss of coordinated water molecules between 60 and 200 °C. From the percentage weight loss the number of water molecules were calculated to be 15 where as Tang and Zhang reported 13 water molecules for the same catalyst [23]. It has already been reported that the degree of hydration in these heteropoly compounds depends on various factors such as relative humidity, degree of drying, solution acidity, temperature, etc. [24–27]. 3.1.2. Infrared spectra For comparison the infrared spectra of phosphomolybdic acid is also recorded. The infrared spectra of phosphomolybdic acid and 11-molybdo-1-vanadophosphoric acid exhibit bands in the range 3200–3400 cm1 due to m(OAH) and m(HAOAH), respectively, for water of crystallization and constitutional water present in the heteropoly acids (Fig. 2) [28–30]. Apart from these bands the unsubstituted heteropoly acid shows four major peaks at 1065, 962, 923 and 788 due to m(PAOa), m(Mo@Od), m(MoAObAMo) and m(MoAOcAMo), respectively, where the subscripts a, b, c and d indicate different types of oxygen atoms in the Keggin unit. These bands are shifted to lower frequency region in the vanadium containing heteropoly acid (HPVMo11) due to the weakening of PAO and MoAO bonds and also may be due to the increase in the number of protons from 3 to 4. 3.2. Catalytic activity In this report, we disclose an efficient and high-yielding protocol for the synthesis of 3,4-dihydropyrimidinones involving the three-component, one-pot condensation of an aldehyde, b-dicarbonyl compounds and urea using het-

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Fig. 1. TGA curve of 11-molybdo-1-vanadophosphoric acid (HPVMo11).

Fig. 2. FT-IR spectra of: (A) phosphomolybdic acid (H3PMo12O40) and (B) 11-molybdo-1-vanadophosphoric acid (HPVMo11).

eropoly acid as novel and environmentally benign, acid catalysts. It has been reported that heteropoly compounds are active catalysts for oxidation of ethane [31], n-butane [32], n-pentane [33] and isobutyricacid [34–38]. In these reactions, the incorporation of vanadium atoms into molybdophosphoric acid has been reported to enhance catalytic performances. Therefore, the states and the role of vanadium in the working state of the heteropoly acid catalysts are of great interest. Thus, treatment of benzaldehyde, ethyl acetoacetate and urea in the presence of the 11molybdo-1-vanadophosphoric acid (H4PMo11VO40) in ethanol at reflux resulted in the formation of 4-phenyl3,4-dihydropyrimidinone in 92% yield (Scheme 1).

O H2N R'CHO

O NH2

O

R'

N

H4PMo11VO40 R''

H

R'' EtOH,

CH3

O

H3C

N H

X

Scheme 1.

In a similar fashion, a variety of aromatic and heterocyclic aldehydes underwent three-component condensation smoothly to afford a wide range of substituted dihydropyri-

S.P. Maradur, G.S. Gokavi / Catalysis Communications 8 (2007) 279–284

+

H

+ R'

OH

O

O H2N

H

R'

H H

NH2

+ OH2

+

R' N

H

NH2

N NH2

O

O -H2O

H O

R' R''OC

Me

NH

O

H

H R'' R'

+

H

N+

NH2

H

Me O

O

H2N

283

O

R' NH2

N O

Dehydration O

R'

C

NH

R'' Me

N

O

H Scheme 2.

midinones. Many of the pharmacologically relevant substitution patterns on the aromatic ring could be introduced with high efficiency by using this procedure (Table 1). Most importantly, aromatic aldehydes carrying either electrondonating or -withdrawing substituents reacted well under the reaction conditions to give the corresponding dihydropyrimidinones in high-to-quantitative yields with high purity. Acid-sensitive furfural also worked well without the formation of any side products (4h, Table 1). Another important feature of this method is survival of a variety of functional groups, such as olefin, nitro, halide, ether, and ester groups, under the reaction conditions (Table 1). Unlike most of the reported methods, this procedure does not require any additives or activators. Some other methods require the use of toxic reagents in combination with Bronsted acids, such as hydrochloric acid and acetic acid, as additives [13e]. This procedure not only preserves the simplicity of the Biginelli reaction, but also produces excellent yield of the products with high purity. Thiourea has been used with similar success to produce the corresponding thio derivatives of dihydropyrimidinones, which are also of much interest with respect to their biological activities (entries 4u, 4v, and 4w, Table 1) [8]. Decreased reaction times and improved yields are realized as a result of the increased reactivity of the substrates on the surface of heteropoly acid. By using heteropoly acid as catalyst, the yields of the one-pot Biginelli reaction [39] can be increased from 20–60% to 81–96% while the reaction times are shortened from 18 h to 4.5–8.0 h. To optimize the conditions, we carried out the reactions using different quantities of reactants. The best results were obtained using a 0.01:1.0:1.0:2.0 mmol ratio of heteropoly acid, aldehyde, 1,3-dicarbonyl compound, and urea or thiourea. In the

absence of the heteropoly acid, the products were obtained in low yields (15–20%) after long reaction times (15–18 h). Thus, this procedure provides easy access to the preparation of substituted pyrimidinones having a wide range of substitution patterns on all three components. The scope and generality of this process is illustrated with respect to the various 1,3-diketones and aldehydes that are tolerated; the results are presented in Table 1. According to the mechanism suggested by Folkers, Johnson and Kappe, we think the reaction may proceed through imine formation from the aldehyde and urea, which is activated by protonation. Subsequent addition of the carbanion derived from 1,3-diketone or b-keto ester to the imine followed by cyclodehydration afford dihydropyrimidin-2(1H)-one (Scheme 2). During the reaction process, the hydrogen ion, H+, is donated by the heteropoly acid. The hydrogen ion not only help the dehydration but also benefit the enolization of 1,3diketone or b-keto ester to form the enolate intermediate. 4. Conclusion In summary, we have found that heteropoly acid 11molybdo-1-vanadophosphoric acid (HPVMo11) is extremely useful and highly efficient homogeneous acid catalyst for the synthesis of biologically potent aryl 3,4-dihydropyrimidinones by means of MCRs three-component condensations of an aldehyde, 1,3-dicarbonyl compound, and urea or thiourea in a one-pot operation. This method is applicable to a wide range of substrates, including aromatic, aliphatic, a,b-unsaturated, and heterocyclic aldehydes, and provides a variety of biologically relevant dihydropyrimidinones in high-to-quantitative yields in short reaction times.

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