Construction of Biologically Active Five- and Six-Membered Fused Ring Pyrimidine Derivatives from 1,3-Diarylthiobarbituric Acids (DTBA)

Abstract

Several derivatives of fused pyrimidines were synthesized in maximum yields by using the respective condensation products, namely, 5-ethoxymethylene-1,3-diaryl-2-thiobarbituric acids and 5-phenyl-methylene-1,3-diaryl-2-thiobarbituric acids, which can be obtained from 1,3-diarylthiobarbituric acids (DTBA). These condensation products possessing three electrophilic centres could undergo cyclocondensation with various binucleophiles to give various fused heterocycles of pyrimidine derivatives, such as pyrazolo[3,4-d]pyrimidine-6-thiones, 5,7-diaryl-4-oxo-isoxazolo[5,4-d]pyrimidine-6-thiones, 5-oxo-pyrimido[4,5-d]pyrimidine-7-thiones, 2-thioxo-pyrano[2,3-d]pyrimidine-4-ones, pyrido[2,3-d]pyrimidines, quinazoline-4-oxo-2-thiones.

1. Introduction

The development of physiologically highly potent fused pyrimidines is a challenging task for synthetic organic chemists [1]. It is well known that pyrimidines either in isolated or in fused state are associated with a number of biological activities [2, 3, 4, 5, 6]. Moreover the pyrimidine nucleus containing thiouriedo linkage (-NH-C(S)-NH-) is pharmaceutically important as the development of medicine mainly arose from the heterocyclic compounds containing nitrogen and sulphur atoms [7, 8, 9]. Due to a wide range of biological activities exhibited by pyrimidine derivatives, these compounds occupy a unique place in the field of biological and medicinal chemistry. In view of such wide applications, several derivatives of fused pyrimidines were synthesized in maximum yields by using 1,3-diarylthiobarbituric acids, 1 (DTBA) which can be prepared in one pot reaction by treating 1,3-diarylthioureas with malonic acid in presence of acetyl chloride [10]. They are generally stable at room temperature and can be stored indefinitely without apparent decomposition. Having an active methylene group, they can furnish several condensation products for easy cyclization to give various fused heterocyclic compounds of interest. When DTBA was reacted with ethyl orthoformate and benzaldehyde to the condensation products, namely, 5-ethoxymethylene-1,3-diaryl-2-thiobarbituric acids (2) and 5-phenyl-methylene-1,3-diaryl-2-thiobarbituric acids (3), respectively were formed (Figure 1) [11]. As 2 and 3 possess three electrophilic centres, they could undergo cyclocondensation with various binucleophiles to give various fused heterocycles of pyrimidine derivatives [12].

Figure 1.

Thiobarbituric acid derivatives.

2. Pyrazolo-pyrimidines

The methods reported for the synthesis of pyrazolo-pyrimidines involved a number of steps and yields were poor [13, 14, 15, 16, 17, 18]. A convenient route for the synthesis of pyrazolo-pyrimidines was described [11, 19]. The condensation products, 2 and 3 have been utilized as three carbon fragments for the synthesis of 4-oxo-pyrazolo[3,4-d]pyriimidines, 4 and 6, respectively. When 2 was treated with hydrazine hydrate in the presence of ethanol and acetic acid, the corresponding pyrazolo[3,4-d]pyrimidin-6-thione (4) was yielded in 65–82% overall yields (Figure 2). Similarly, when 2b was treated with phenyl hydrazine in ethanol and acetic acid, 2-phenyl-5,7-bis(2′-methylphenyl)-4-oxo-pyrazolo[3,4-d]pyrimidin-6-thione (5b) was produced in 55% yield. However, when 3 was refluxed with hydrazine hydrate in ethanol and acetic acid, the corresponding (3H)-3-phenyl-5,7-diaryl-4-oxo-pyrazolo[3,4-d]pyrimidin-6-thiones (6) were obtained in 60–75% overall yields (Figure 2). It was observed that 6 could be oxidized to give the respective 3-oxo-pyrazolo-[3,4-d]pyrimidin-6-thiones (7) [11, 12, 19].

Figure 2.

Pyrazolo-pyrimidines.

It is well known that the fused heterocycles possessing the pyrazolo[3,4-d]pyrimidine nucleus serve as a class of compounds which exhibited a remarkable variety of biological activities [20, 21, 22, 23]. The pyrazolo[3,4-d]pyrimidine compounds have been found to show similar characteristics to purines, since they are isomeric structural purine analogues, which have resulted in several potent antagonists in biological systems [24, 25].

3. Isoxazolo-pyrimidines

Most of the methods for the preparation of the isoxazolo[5,4-d]pyrimidine system are initiated from the construction of an isoxazole ring first and followed by pyrimidine ring closure [26, 27]. DTBAs were also used as precursors for the synthesis of various isoxazolo[5,4-d]pyrimidines [28]. When 2 and 3 were treated with hydroxylamine hydrochloride in dehydrated alcohol and acetic acid gave 5,7-diaryl-4-oxo-isoxazolo[5,4-d]pyrimidin-6-thiones (8) in 65–85% overall yields and 3,5,7-triaryl-2,3-dihydro-4-oxo-isoxazolo[5,4-d]pyrimidin-6-thones (9) in 60–70% overall yields, respectively (Figure 3) [28].

Figure 3.

Isoxazolo-pyrimidines.

It was reported that the isoxazolo-pyrimidine derivatives exhibited antifungal, antibacterial and many other important biological properties [29, 30, 31]. The biological activities in terms of the antifungal and antibacterial properties of the compounds (8 and 9) were determined by the standard disc diffusion method. The bacteria (Escherichia coli and Bacillus subtilis) and fungi (Penicillium species, Aspergillus sp., Fusarium sp. and Trichoderma sp) were grown in nutrient agar and potato dextrose agar (PDA) plates, respectively. Compound 9c was found to have the highest activity against coli form organisms whereas other compounds were moderated and have less activity. The studied compounds were found to be inactive against some species of fungi, except 8b was found to have antibiotic property in penicillium species.

4. Furo-pyrimidines

It was reported [32] that furo[2,3-d]pyrimidines 10 were prepared by condensing 1,3-diarylthiobarbituric acids (1) with benzoin in presence of p-toluene sulphonic acid. In another reaction, derivatives of furo[2,3-d]pyrimidines 11 were prepared by direct condensation of the corresponding 1,3-diarythiobarbituric acids, 1 with chloroacetone in presence of triethylamine (Figure 4).

Figure 4.

Furo-pyrimidines.

5. Thieno-pyrimidines

The synthesis of thieno[2,3-d] pyrimidines 13 which are found to be associated with varied biological and pharmacological activities, on condensation of 12 with mesityl thioglycolate was reported (Figure 5) [33].

Figure 5.

Thieno-pyrimidines.

6. Pyrimido-pyrimidines

In view of the various physiological properties, the pyrimido[4,5-d]pyrimidines were generally prepared from derivatives of enamines [34, 35]. Various substituted 2-amino-6,8-diaryl-5-oxo-pyrimido[4,5-d]pyrimidin-7-thiones (14) in 62–75% overall yields can be synthesized by reacting the respective compounds 2 with guanidine nitrate in the presence of sodium methoxide and methanol (Figure 6) [36]. Similarly, the condensation products 3 were reacted with guanidine nitrate in sodium methoxide and methanol affording the corresponding substituted 3,4-dihydro-5-oxo-pyrimido[4,5-d] pyrimidine-7-thiones, 15 in good yields (Figure 6) [36].

Figure 6.

Pyrimido-pyrimidines.

The derivatives of pyrimido-pyrimidines are quite effective in the inhibition of cancer cell sickness [37], exhibition of diuretic activities and anti-inflammatory activities [38]. The antifungal activity for the compounds 14 and 15 was screened against Aspergillus niger at different concentrations by agar growth paper disc method using Czapeck’s nutrient medium and DMF was used as control. The average percentage inhibition after 96 hr. was determined and the results were compared with those obtained using commercial fungicide Carbendazim. Most of the compounds were found to be fairly active at the concentrations of 1000 ppm comparable with standard fungicide. It is noteworthy that introduction of pyrimido moiety in the pyrimidine nucleus enhances the fungitoxicity to some extent and furthermore, chloro-substituted heterocycles (14c and 15c) were more active than methyl-substituted compounds (14b and 15b).

Many of the earlier methods reported for the syntheses of pyrimido[4,5-d]pyrimidine derivatives require toxic chemicals, long reaction times, afford low to moderate yields and require many steps. A simple one-pot method for the synthesis of thioxo pyrimido[4,5-d]pyrimidinone derivatives by treatment of N,N-diethylthiobarbituric acids, benzaldehydes and thiourea/urea with NaOEt as the catalyst under solvent-free conditions was reported (Figure 1). Several 5-aryl-2,7-dithioxo-pyrimido[4,5-d]pyrimidine-4-ones; 4-aryl-7-thioxo-pyrimido[4,5-d]pyrimidine-2,5-diones were synthesized using an eco-friendly and efficient, multi-component reaction (MCR) under solvent free conditions. The reactions proceed via Biginelli type condensation of aromatic aldehyde, thiobarbituric acid and urea or thiourea in presence of catalytic amount of NaOEt. The synthesized compounds were screened for anti-inflammatory activity. It was observed that the compounds containing fluoro-substituents gave good yields and showed high anti-inflammatory activities. There are no reports available on the formation of Biginelli products using NaOEt as a catalyst under solvent-free conditions (Figure 7).

Figure 7.

Synthesis of thioxo pyrimido[4,5-d]pyrimidinone derivatives.

7. Pyrano-pyrimidines

Pyrano[2,3-d]pyrimidine derivatives could be synthesized via a multicomponent domini Knoevenagel/hetero Diels-Alder reaction of 1,3-dimethyl barbituric acid with an aromatic aldehyde and ethyl vinyl ether or 2,3-dihydrofuran in presence of 1 mol % of indium (III) chloride. The reaction also proceeds in aqueous media without using any catalyst, but the yield is comparatively less (65–75%). Preparation of naturally occurring complex molecules containing a uracil ring possesses significant synthetic challenges. The development of clinically useful anticancer (5-fluorouracil) and antiviral drugs (AZT, DDC, DDI, BVDV) has renewed interest in the synthetic manipulation of uracils.

Ahluwalia et al. [39] reported the synthesis of pyrano[2,3-d] pyrimidines 16 by reacting thiobarbturic acids, 1 with mesityl oxide in the presence of pyridine (Figure 8). The products were identical with the products obtained by the reaction of appropriate thiobarbituric acids with acetone in presence of triethylamine. The reaction of thiobarbituric acids with mesityl oxide in the absence of any base gave the corresponding open chain compounds 17 which on heating with glacial acetic acids and phosphorus pentoxide gave the corresponding cyclic compounds 18 [39].

Figure 8.

Pyrano-pyrimidines.

Although a variety of routes for the synthesis of these compounds have been appeared in the literature, the majority of them involve a number of steps, drastic conditions, long reaction time and low yields [40, 41, 42, 43, 44]. Moreover, very few methods are reported for the synthesis of 2-thioxo-pyrano[2,3-d]pyrimidine-4-ones, as most of the methods reported are of pyrano[2,3-d]pyrimidines. In search of an efficient method and in continuation to our studies on fused pyrimidine derivatives, we report the full details of the work and studies related to the synthesis of 7-amino-1,3-diaryl-5-phenyl-2-thioxo-pyrano[2,3-d]pyrimidin-4(1H)-ones (19) from DTBA by reacting with various arylidenes, such as, phenylmethylenemalononitrile, ethyl phenylmethylene-cyanoacetate and phenylmethylenecyanoacetamide in presence of sodium methoxide and methanol [45]. Thus, the reaction of DTBA with arylidenes and sodium methoxide in presence of methanol by refluxing for 6 hr. afforded the compounds 19 in 70–82% overall yields (Figure 9).

Figure 9.

2-Thioxo-pyrano[2,3-d]pyrimidine-4-ones.

Compounds containing dihydro-5H-pyrano[2,3-d]pyrimidines moiety have interesting biological properties [46]. In addition, compounds having a chalcone unit attached to the pyranopyrimidine ring are efficient herbicides [47]. The antifungal screening of the synthesized compounds (19) were done in vitro at 100 and 300 μg/ml solution against the two types of fungi, namely, Fusarium oxysporum and Helminthosporium oryzae using acetone as solvent. The zones of inhibition were measured in mm. The result showed that most of the compounds exhibit moderate to high activity against both fungi. Incorporation of chlorine increased the activity of high order [48]. The synthesized compounds, 19 were also screened against gram negative bacteria Escherichia coli (MTCC 739) and gram positive bacterium Bacillus subtilis (MTCC 121). The agar cup-plate method was used and nutrient agar was culture medium for the antibacterial activity. The solvent acetone served as a control. The zone of inhibition frame was measured in mm. The result of antibacterial activity showed that compounds 19b and 19e showed the most active compound against B. subtilis only, while 19a and 19c do not show any zone of inhibition against these bacteria [48].

8. Pyrido-pyrimidines

Very few substituted pyrido[2,3-d]pyrimidines were synthesized from 6-aminouracils [49, 50]. The synthesis of substituted 3-cyano-6,8-diaryl-2,5-dioxo-pyrido[2,3-d]pyrimidin-7-thiones (20) from the corresponding condensation product (2) derived from 1,3-diarythiobarbituric acids were also reported [12]. Thus, the reaction of 2 with cyanoacetamide in presence of sodium isopropoxide and isopropanol afforded the corresponding pyrido[2,3-d]pyrimidines (20) in 62–74% overall yields (Figure 10).

Figure 10.

Pyrido-pyrimidines.

9. Quinazolines

Quinazolines are interesting targets for new method development due to their importance in a broad range of therapeutic areas [51, 52]. Quinazoline derivatives, which possess a wide range of biological activities contain the 4-oxo-2-thioxo-1,2,3,4-tetrahydropyrimidine structural moiety in their heterocyclic rings [53, 54, 55, 56, 57, 58].

The synthetic methods available for the preparation of quinazolines involved the amidation of 2-aminobenzoic acid or its derivatives, i.e. 2-aminobenzonitrile, 2-aminobenzoate, and 2-arylnitrilium salts, followed by oxidative ring closure [59, 60, 61, 62, 63]. Other synthetic pathways include the cyclization of anthranilamides with aldehydes [64], and with ketones or acid chlorides under acidic or basic conditions [65, 66, 67]. These methods involved multistep processes, poor yields, toxic reagents and time-consuming experimental procedures. Moreover, very few methods are reported for the synthesis of 2-thioxoquinazoline-4-ones, as most of the methods reported are of quinazoline-2,4(1H,3H)-diones, from various sources [61, 62, 68, 69, 70, 71, 72, 73, 74, 75].

Recently, Saeed et al. [76] reported the base catalysed intramolecular nucleophilic cyclization of substituted thioureas in the presence of DMF to afford the 2-thioxoquinazoline-4-ones. The solid-phase synthesis for the preparation of2-thioxoquinazoline-4-one had been reported [77, 78, 79, 80]. Treatment of 5-ethoxymethylene-1,3-diaryl-2-thiobarbituric acids (2) with malononitrile in presence of NH₄OAc and acetic acid with ZnCl₂ as catalyst in refluxing condition gave the corresponding 2-thioxoquinazolin-4-ones, 21 in 78–85% overall yields [81] (Figure 11). However, the reaction of 2 with ethyl cyanoacetate in presence of ammonium acetate and acetic acid with ZnCl₂ as a catalyst gave 7-hydroxy-2,3-dihydro-2-thioxo-1,3-diarylquinazolin-4(1H)-ones, 22 in 76–87% overall yields [81].

Figure 11.

Quinazolines.

The synthesized compounds 21 and 22 were screened in vitro for their antimicrobial activities [82, 83]. Cytotoxicity studies were performed for the compounds on human lung cancer cell line A549 using an MTT assay. The A549 cells were grown at 37°C, 5% CO₂ and 100% relative humidity.

10. Benzo[5,6]chromeno[2,3-d]pyrimidines

Novel 2-thioxo-benzo[5,6]chromeno[2,3-d]pyrimidin-4-one derivatives were synthesized in aqueous media using cetylpyridinium chloride (CPC) as micellar catalyst in three-component one-pot reaction involving thiobarbituric acids, aromatic aldehydes and β-napthol [84]. The synthesized compounds were found to show antioxidant and cytotoxic activities (Figure 12).

Figure 12.

Synthesis of 2-thioxo-benzo[5,6]chromeno[2,3-d]pyrimidin-4-one derivatives.

11. Pyrazolopyrano-pyrimidinones

Tricyclic fused pyrazolopyranopyrimidines were synthesized by one-pot, four-component reaction of ethyl acetoacetate, hydrazine hydrate, aromatic aldehydes and barbituric acid in good to excellent yields (88–95%) [85]. Another method for the synthesis of pyrazolopyranopyrimidines was employed using DABCO as catalyst in water [86].

A new series of triheterocyclic compounds containing pyrazole, pyran, and pyrimidinone rings was synthesized via a one-pot condensation of ethylacetoacetate, hydrazine hydrate, barbituric acid, and aromatic aldehydes in the presence of catalytic amounts of titanium dioxide nanowires [87]. Various functional groups were well tolerated under the optimized reaction conditions. A highly efficient, green protocol, one-pot four-component reaction involving thiobarbituric acid, hydrazine hydrate, ethyl acetoacetate and aromatic aldehydes for the synthesis of 7-thioxo-pyrazolopyrano-pyrimidinone derivatives has been accomplished using SDS (sodium dodecyl sulphate) as a catalyst (Figure 13) [88]. The procedure offers the advantages of green solvent, easy work-up avoiding the chromatographic separation and use of inexpensive, biodegradable, reusable catalyst. These novel 7-thioxo-pyrazolopyrano-pyrimidinone derivatives were screened for antimicrobial, and antioxidant activities. It was found that 3-methyl-4-(2,4-dichlorophenyl)-6,8-diethyl-7-thioxo4,6,7,8-tetra-hydropyrazolo[4′,3′:5,6]pyrano-[2,3-d]pyrimidin-5(1H)-one has shown high antifungal and anti-bacterial activities against the tested fungi and bacteria, which may be due to the presence of chlorine atoms [88]. All the prepared pyrazolopyranopyrimidines were tested as anti-inflammatory agents and some of them revealed moderate to potent anti-inflammatory activity [89].

Figure 13.

Synthesis of 7-thioxo-pyrazolopyrano-pyrimidinone derivatives.

12. Benzo[4,5]thiazolopyrimido[5,4-d]pyrimidines

Medhabati et al. [90] reported the synthesis of 2-thio-5-arylbenzo[4,5]thiazolopyrimido[5,4-d]pyrimidin-4-one derivatives under aqueous medium. Benzo[4,5]thiazolopyrimido[5,4-d]pyrimidines were synthesized by condensation of thiobarbituric acids, 2-aminobenzothiazole and aldehydes using metal-proline as catalyst (Figure 14).

Figure 14.

Synthesis of 2-thio-5-arylbenzo[4,5]thiazolopyrimido[5,4-d]pyrimidin-4-ones.

13. Pyrimido[5,4-e[1,3,4]thiadiazolo[3,2-a]pyrimidines

An efficient one-pot method for the synthesis of 2-methyl-5,7,9-triphenyl-8-thioxo-8,9-dihydro-5H-pyrimido[5,4-e[1,3,4]thiadiazolo[3,2-a] pyrimidin-6(7H)-one and its derivatives was reported. The present protocol is also extendable to a wide variety of substrates. The advantages of this protocol are the use of easily available catalyst, short reaction time, easy work-up, ease of product isolation, and high yield (Figure 15).

Figure 15.

Synthesis of Pyrimido[5,4-e[1,3,4]thiadiazolo[3,2-a]pyrimidines.

14. Conclusion

Many fused pyrimidine derivatives are belonged to the most important heterocyclic systems, and there are numerous synthetic preparative methods for these compounds. However, in some cases, difficult access to key intermediates, or to their precursors, was a serious limitation for the above syntheses. Various fused pyrimidines were synthesized in maximum yields by using the respective condensation products, namely, 5-ethoxymethylene-1,3-diaryl-2-thiobarbituric acids and 5-phenyl-methylene-1,3-diaryl-2-thiobarbituric acids, which can be obtained from 1,3-diarylthiobarbituric acids (DTBA). These condensation products possessing three electrophilic centres could undergo cyclocondensation with various binucleophiles to give various fused heterocycles of pyrimidine derivatives, such as, pyrazolo[3,4-d]pyrimidine-6-thiones, 5,7-diaryl-4-oxo-isoxazolo[5,4-d]pyrimidine-6-thiones, 5-oxo-pyrimido[4,5-d]pyrimidine-7-thiones, 2-thioxo-pyrano[2,3-d]pyrimidine-4-ones, pyrido[2,3-d]pyrimidines, quinazoline-4-oxo-2-thiones, etc.