Developments of pyridodipyrimidine heterocycles and their biological activities

Abstract

The entitled review aimed to assemble and highlight the synthetic approaches and biological aspects of heterocycles with pyridodipyrimidine motifs. The recent synthetic approaches were categorized according to the accomplishments of the approaches under catalyst or catalyst-free conditions. The topic involved the synthesis of substituted tricyclic systems and spirocyclic systems. The present study offered an overview of the recent literature in addition to a scope of the preceding literature. The proposed mechanisms of the varied target products were discussed. Pyridodipyrimidine displayed potential and privileged cytotoxic, antioxidant, and antimicrobial performances. The competitions, challenges, and prospects are also deliberated.

Introduction

Pyridodipyrimidine heterocycles are compounds of fused tricyclic systems with four or five nitrogen atoms, for example, pyrido [2,3-d:6,5-d']dipyrimidines. These isomers are analogs of tetra- or penta-aza- anthracene or phenanthrene and can exist in linear or angular forms (Fig. 1). The number of conceivable linear isomers is seven, while the possible angular isomers form probable eighteen isomeric analogs. Formerly, different approaches were applied for the synthesis of pyridodipyrimidine compounds, for instance, the reactions of 2-formyl-1 H-pyrrole with N,N-diaryl barbituric acids, and liquor ammonia [1], reactions of 1,3-dimethyl-6-aminouracil with aryl aldehydes under catalytic conditions of p-TSA in [bmim] Br media [2], solvent-free reactions of 6-aminouracil with diverse aldehydes catalyzed by SBA-15-SO₃H [3], acetic acid-catalyzed reactions of aryl aldehydes with 6-amino-N-methyluracil under microwave irradiated conditions [4], p-TSA catalyzed reactions of isatins with 2,6-diaminopyrimidin-4(3H)-one [5]. Other procedures involved the usage of sulfonic acid maintained-hydroxyapatite-encapsulated-γ-Fe₂O₃ [6], and multistep syntheses [7]. Pyridodipyrimidines are a class of nitrogen-bearing heterocycles that established considerable interest owing to their uses in numerous fields [8–13]. The compounds with pyridopyrimidine skeleton demonstrated privileged biological aspects on a large scale such as DHFR inhibitors [14], EGFr inhibitors [14–16], antitumor [17], antiviral, anti-inflammatory, and antimicrobial effects [18–23]. This review aims to highlight an example of polycyclic compounds containing the pyrimidine and pyridine cores of distinct biological significance. Through our previous research on the study of the importance of reactive synthons in the preparation of many heterocycles [24, 25] and many reviews that discussed the chemistry of related fused pyrimidine [26–33] and pyridopyrimidine compounds [34–36], we discuss here the recent advancements on the chemistry of pyridodipyrimidine compounds. The subject of the research is concerned with studying the biological impacts and the distinct preparation approaches for this class of compounds in recent years.

Fig. 1.

The linear and angular isomeric sample structures of pyridodipyrimidines

Brief history of pyridodipyrimidine class of compounds

Heterocycles with pyridodipyrimidine cores are fascinating compounds owing to their resilient oxidizing aptitude [37]. Al-Hassan et al. [38] synthesized 10-D-ribitylpyridodipyrimidine-tetraone 1 in relatively low yield (21%) by reaction of 6-D-ribitylaminopyrimidine-dione with triethyl orthoformate in refluxing ethanol (Eq. 1). As reported by Yoneda et al. [39], dihydropyridodipyrimidine-triones 2a–h were efficiently prepared by reactions of the corresponding aminopyrimidine-diones with triethyl orthoformate or trimethyl orthoformate in DMF under heating conditions (Eq. 2). Suresh and Mohan [1] reported the synthesis of pyrido-dipyrimidines 3a–f by reactions of 2-formyl-1H-pyrrole with 1,3-diaryl barbituric acids and liquor ammonia in ethanol after a short time under heating conditions. The plausible mechanism postulated for this reaction involved the generation of arylidine intermediates through Knoevenagel condensation of the aldehyde with1,3-diarylbarbituric acid. Micheal's addition of 1,3-diaryl barbituric acid to the generated arylidine intermediates followed by the interaction with liquor ammonia produced the desired products (Eq. 3). Shaker et al. [40] reported the one-pot multicomponent synthesis of pyridodipyrimidines 4a–e through reactions of 6-amino-uracil with aryl aldehydes and thiobarbituric acid under microwave irradiated (800 Watt), and solvent-free conditions (Eq. 4). Compound 5 was synthesized by treatment of 6-amino-thiouracil with 1,3-diphenyl-4-formyl-1H-pyrazole in methanol/Conc. HCl and utilized as a precursor for the synthesis of pentacyclic systems 6 by reactions with oxo-N-phenylhydrazonoyl halides [41] (Eq. 5; Scheme 1).

Scheme 1.

Synthetic routes of the miscellaneous former pyrido-dipyrimidines

On the other hand, compounds 7a,b were prepared by reactions of 6-amino-1-(benzyl)/(2-chlorobenzyl)pyrimidine-2,4(1H,3H)-diones with formaldehyde in EtOH/AcOH followed by reflux in DMF/TEA (Eq. 6) [42]. Youssif et al. [43] defined the synthesis of compounds 8a–c through reactions of aryl aldehydes with 6-amino-2-thiouracil in refluxing acetic acid (Eq. 7). Azev et al. [44] reported the synthesis of compound 9 (Eq. 8) as a mixture with pyridopyrimidines 10a, b by reactions of alkyl aldehydes with 1,3-dimethyl-6-amino-uracil in formic acid. Treatment of disubstituted amino-uracile derivatives with Vilsmeier reagent produced 12a, b in very low yields (6–7%) (Eq. 9) [45]. Khalafi‐Nezhad et al. [46] reported the synthesis of 13a–m was achieved through four-component one-pot reactions of aryl aldehydes with aryl amines, barbituric/thiobarbituric acids and 2-thioxo-2,3-dihydropyrimidin-4(1H)-one through heating in ethanol containing H₃PW₁₂O₄₀ catalyst (Eq. 10; Scheme 2).

Scheme 2.

Synthesis of the diverse formerly prepared pyrido-dipyrimidines

In a multi-step synthetic procedure, El-Gazzar et al. [47] employed the excessive reactivity of enaminonitriles 14a–c to construct the pyrido-dipyrimidine systems 15–18 through reactions with formic acid, acetic acid, urea, and thiourea (Scheme 3). Furthermore, Toobaei et al. [48] prepared pyrido-dipyrimidines substituted with sugar chains 19 through reactions of barbituric/thiobarbituric acids with sugar and 4,4'-oxydianiline in ethanol catalyzed by p-TSA under gently heating conditions (Eq. 11). Bazgir et al. [49] greenly synthesized pyrido-dipyrimidines 20 (12 examples) through one-pot three-component reactions of 6-amino-uracil/thiouracils with aryl aldehydes, and barbituric/thiobarbituric acids in the water contained p-TSA (Eq. 12). Bhat et al. [50] defined the utility of DBU as a proficient catalyst for the one-pot three-component synthesis of pyridodipyrimidines 21 in 72–89% yields through reactions of aldehydes, thiobarbituric acid, and 6-amino-uracil in ethanol at reflux temperature (Eq. 13). Under microwave-irradiated conditions, Dabiri et al. [2] developed the synthesis of pyridodipyrimidine-tetrones 22 by reactions of aryl aldehydes with 1,3-dimethyl-6-amino-uracil. The proceeding of the reactions in acetic acid produced 20–30% yields from the products 22, while the use of [bmim]Br/ p-TSA under heating conditions gave desired products with 50–54% yields along with the formation of acyclic products 23 (Eq. 14; Scheme 4).

Scheme 3.

Synthesis of pyridodipyrimidines from reactive enaminonitriles

Scheme 4.

Synthesis of a series of pyridodipyrimidines

Additionally, spirooxindoles were prepared by reactions of isatins with two equivalents of 2,6-diaminopyrimidin-4(3H)-one in ethanol contained p-toluene sulfonic acid [5]. Hirota et al. [51] employed the reactivity of 5-((dimethylamino)methylene)-6-imino-1,3-dimethyldihydropyrimidine-2,4-(1H,3H)-dione in its reactions with 1,3-dimethyl-6-amino-uracil or barbituric/thiobarbituric acid derivatives in DMF for the synthesis of di- and tetra-substituted pyrido-dipyrimidines 24 (Eq. 15). Ram et al. [52] also designed the tricyclic systems 25 by reactions of mono-, or disubstituted-barbituric/thiobarbituric acids with 5-((dimethylamino)methylene)-6-imino-1,3-dimethyl-dihydropyrimidine-2,4(1H,3H)-dione in DMF under heating conditions (Eq. 16). The synthesis of the titled compounds was also accomplished under microwave-irradiation conditions from reactions of 1-methyl-6-amino-uracil with aryl aldehydes in acetic acid [4]. On the other hand, Mohsenimehr et al. [6] applied the utility of encapsulated-γ-Fe₂O₃ nanoparticles for the synthesis of pyrido [2,3-d:6,5-d′]dipyrimidines 26. The best product yields were obtained from the method that involved the use of nano-catalyst than applying the reactions in AcOH/DMF (1:1) at room temperature or using ultrasonic irradiation conditions (Eq. 17; Scheme 5). In another route, pyrido-dipyrimidines were synthesized in 66–95% yields by reactions of 2-methyl-6-(substituted-amino)pyrimidin-4(3H)-one with 2,4,6-trichloropyrimidine-5-carbaldehyde in refluxing acetic acid [53]. Alternatively, a series of tri-substituted-pyrido-dipyrimidine-tetraones were synthesized by reactions of 3-substituted-6-(substituted-amino) pyrimidine-2,4(1H,3H)-diones with 6-chloro-3-substituted-2,4-dioxo-1,2,3,4-tetrahydro-pyrimidine-5-carbaldehydes [37].

Scheme 5.

Synthesis of tricyclic pyridodipyrimidines

Besides, a series of spiro-pyridodipyrimidines were efficiently synthesized by reactions of 1,2-diketones e.g. isatins with 1,3-dimethyl-6-aminouracil [54]. Rostamizadeh et al. [3] greenly employed nano-catalytic environments using SBA-15-SO₃H for the construction of pyrido [2,3-d:6,5-d’]dipyrimidines 27 under solvent-free conditions (Eq. 18). Nagamatsu et al. [55] defined the synthesis of pyrido-dipyrimidine-tetraones 28 via the multi-step synthesis involving the treatment of 6-hydroxy-3-substituted-pyrimidine-2,4-diones with Vilsmeier reagent followed by reactions with 6-alkylamino-3-methyluracil or treatment of 6-alkylamino-3-methyluracil with Vilsmeier reagent followed by reactions with 6-chloro-3-methyluracil in DMF under heating conditions (Eq. 19 & 20; Scheme 6). Besides, the considered tetra-substituted tricyclic heterocyclic systems were efficiently prepared by refluxing of 1,3-disubstituted-pyrimidine-2,4,6(1H,3H,5H)-triones with 6-amino-1,3-dimethyl-2,4-dioxo-1,2,3,4-tetrahydro-pyrimidine-5-carbaldehyde in pyridine [56].

Scheme 6.

Synthesis of pyridodipyrimidine heterocycles

Yamato et al. [57] investigated the synthesis of bis(pyrido-dipyrimidines) 29 through reactions of bis(3-methyluracil-6-ylamino)alkanes with 6-chloro-5-formyl-3-methyluracil or 2,4,6-tri-chloropyrimidine-5-carbaldehyde in acetic acid (Eq. 21). Disubstituted-tricyclic-tetraone/trione systems 30, and 31 were synthesized as reported by Yoneda et al. [58] from the reaction of 2,4,6-tri-chloropyrimidine-5-carbaldehyde with 6-(substituted-amino)-3-methylpyrimidine-2,4(1H,3H)-dione (Eq. 22& 23; Scheme 7). Bis-(pyridodi-pyrimidines) “two pyrido-dipyrimidine motifs are connected with polymethylene chains” have efficiently synthesized and applied as redox catalysts, in which these compounds oxidized alcohols to the corresponding ketones. The bis(pyridodipyrimidines) were synthesized through condensation of bis(6-chloro-5-formyluracil-3-yl)alkanes with 6-alkylaminouracils [59].

Scheme 7.

Synthesis of tricyclic pyridodipyrimidines, and Bis-(pyridodi-pyrimidines)

Synthetic approaches

Multicomponent reactions with catalyst-free conditions

The catalyst-free reactions are protocols of reactions that involved simplicity, economic, eco-friendly, ease of product separation, and less sensitivity to air or moisture. The literature reports highlighted the significance of reactions that were achieved under catalyst-free conditions e.g. supporting C–C bond formation [60]. An approach that involved the synthesis of dihydropyrido-dipyrimidine derivatives under catalyst-free and ambient conditions was reported by Brahmachari et al. [61].

The multicomponent one-pot reactions of barbituric/ thiobarbituric acids or 1,3-dimethylbarbituric acid with aryl/alkyl amines and aryl/alkyl aldehydes gave a series of pyrido-dipyrimidines 32 and bis(pyrido-dipyrimidines 33 (Scheme 8 and 9). The multicomponent reactions proceeded using (2:1:1) of the barbituric acid derivatives, amines, and aldehydes for the synthesis of pyridodipyrimidines, respectively, while (4:2:1) molar ratios were used for the synthesis of bis(dihydropyrido-dipyrimidines. The method was accomplished in water at room temperature. This method provided noticeable features including the ease of product isolation, green procedure, catalyst-free, no extra purification of the products, improved yields, and media reusability.

Scheme 8.

Synthesis of pyridodipyrimidines

Scheme 9.

Synthesis of bis-pyridodipyrimidines

The probable mechanism for the synthesis of pyridodipyrimidines and bis(pyrido-dipyrimidines involved Claisen-Schmidt condensation between one mole of barbituric/thiobarbituric acids with aldehydes to generate the arylidene intermediates A-1. The next step is the interaction of another mole of barbituric acid derivatives with the amines to generate the Schiff bases, which followed tautomerization to form the enamine forms A-4. Michael's addition was the next step between intermediates A-1 and A-4 to generate the intermediates A-7, which upon tautomerization generated intermediates A-8. The 6-oxo-trig progression led to ring closure with the formation of the intermediate A-9. The last step involved the condensation of intermediates A-9 gave the desired products 32 (Scheme 10) [61].

Scheme 10.

The postulated mechanism for the synthesis of pyridodipyrimidines

Milović et al. [62] developed a green process for the preparation of pyridodipyrimidines (34 azaheterocyclic pyridodipyrimidines) 34 as DNA-base hybrids. Thus, one-pot multicomponent reactions of thiobarbituric acid (2 equivalents), vanillin (1 equivalent), and aryl amines (1 equivalent) in water under catalyst-free, and stirring conditions at room temperature furnished the tricyclic products, pyridodipyrimidines 34 in respectable yields (42–98%) (Scheme 11). In the case of reactions of thiobarbituric acid with 3-methoxyaniline, and aldehydes containing two aldehyde groups c'–j', these reactions were accomplished by molar ratios 4:2:1, respectively. The proposed mechanism in this work was projected through the formation of enamine intermediates (enamine path) and arylidene intermediates (Knoevenagel condensation path), this was accompanied by Michael addition of both intermediates to give the respective pyridodipyrimidines. The NMR analysis verified the presence of interfacial hydrogen bonding that referred to the role of water in the activation of the reaction sequence.

Scheme 11.

Multicomponent green synthesis of pyridodipyrimidines

Multicomponent reactions under catalytic conditions

Catalytic nanoparticles

Owing to the developed stability of catalytic nanoparticles, they are intended for various applications instead of bulky materials [63–66]. The increased surface area owing to the reduced nano-size of the particles enables the proficient interaction with the reactants for improved yields after a short time. The insolubility of these catalysts also supports the catalyst reusability due to conceivable isolation from the reaction mixture [67–71].

Mahmoudi et al. [72] defined the preparation of aryl-pyrido-dipyrimidinones/thiones 35 through reactions involving the interaction of aryl aldehydes with barbituric/thiobarbituric acids or their dimethyl derivatives and 6-amino-uracil under ultrasonic irradiation conditions (Scheme 12). The reactions were catalyzed by TEDA-BAIL@MIL-101(Cr) nanocomposite, which delivered ease of preparation, worthy product yields, and ease of catalyst reusability. The procedure was considered also a green procedure for the preparation of the tricyclic systems from promptly accessible materials.

Scheme 12.

Multicomponent synthesis of pyridodipyrimidines under ultrasonic irradiations

On the other side, mesoporous nanocatalyst such as nano-[SiO₂-RNMe₂SO₃H] [Cl] was prepared by Kohzadian et al. [8] and applied for the efficient synthesis of pyridodipyrimidine analogs 36 under solvent-free conditions (Scheme 13). Thus, reactions of thiobarbituric acid (2 equivalents), aryl aldehydes (1 equivalent), and NH₄OAc (1.4 equivalents) in the presence of the nano-catalyst (0.02 g) under heating, stirring, and solvent-free conditions gave the target products with amended yields after a short time. The nanocatalyst was prepared through three steps involving the reaction of tetramethylethane-1,2-diamine with (3-chloropropyl)-trimethoxysilane followed by an interaction with nano-silica in the second step and a reaction with sulfurochloridic acid in the final step. The nano-catalyst was effective owing to the aggregation factor of the nanoparticles and it was recyclables four times without the notable loss of activity.

Scheme 13.

Multicomponent synthesis of pyridodipyrimidines under nano-catalytic, and solvent-free conditions

The nano-catalyst supported the activation of the carbonyl group of aryl aldehydes along with the enolization of thiobarbituric acid for Knoevenagel condensation, which generated the arylidene intermediates B-1. In the second step, the nano-catalyst activated another mole of thiobarbituric acid in its interaction with NH₄OAc for the generation of the amino intermediates B-2. The interaction of both intermediates B-1, and B-2 through the Michael addition step generated the intermediate B-3 followed by condensation giving the desired products, pyridodipyrimidines 36 (Scheme 14) [8].

Scheme 14.

The probable mechanism for the synthesis of pyridodipyrimidines

Otherwise, Mamaghani et al. [73] reported the synthesis of dithioxopyridodipyrimidine-diones 37 through a green protocol under HAp-encapsulated-γ-Fe₂O₃[γ-Fe₂O₃@HAp-SO₃H] catalytic, and ambient conditions (Scheme 15). The procedure involved the reactions of two equivalents of 6-amino-2-thiouracil with a variety of aryl aldehydes in DMF under heating (110 °C) and catalytic conditions. The products were prepared under catalytic conditions using a nano-catalyst or acetic acid, in which the nano-catalyst provided improved yields with a high reaction rate. The procedure permits the recyclability of the catalyst with the ease of product isolation. The mechanism of these reactions was proposed through Knoevenagel condensation of the desired 6-amino-2-thiouracil with the aryl aldehydes activated by the nanocatalyst utilizing the acid characters of the applied catalyst to generate the arylidene intermediate. The generated arylidene intermediate followed the Michael addition cascade process with another mole of 6-amino-2-thiouracil. The final step involved the intramolecular cyclization step to produce the target products through the nucleophilic attack of the exocyclic amino group with the release of the catalyst, and ammonia.

Scheme 15.

Multicomponent synthesis of pyridodipyrimidines

Naeimi et al. [9, 10, 74] reported the synthesis of pyrido-dipyrimidines by one-pot multicomponent reactions following different techniques using nano-CuFe₂O₄ as an efficient catalyst. The first method involved one-pot multicomponent reactions of aryl aldehydes with two equivalents of thiobarbituric acid and ammonium acetate under nano-catalytic, and microwave-assisted conditions gave pyrido-dipyrimidines 38a–k in exceptional yields (90–98%). The procedure is preferred over the other methods owing to the improved product yields that are affected by the support of microwave-assisted conditions (100 W), and the productivity of the applied nano-catalyst. The use of one equivalent of terephthalaldehyde in the reactions with thiobarbituric acid (4 equivalents) and two equivalents of NH₄OAc led to the preparation of symmetrical bis-pyrido-dipyrimidine analog 38j in 95% yield (Scheme 16) [10]. The mechanism sequence of this reaction type accomplished also the acid characters of the catalyst to activate the Knoevenagel condensation and supported Michael's addition steps. The catalyst also increased the electrophilic characteristics of the substrates and provided stabilized intermediates via coordination with the electron lone pairs of the oxygen atom.

Scheme 16.

Schematic and projected mechanistic sequence for the synthesis of pyrido-dipyrimidines

Consequently, pyrido-dipyrimidines 39 were also prepared in the second method [74] in water at room temperature using CuFe₂O₄ magnetic nanoparticles (Scheme 17). It was noted that both 4-(dimethylamino)-benzaldehyde, and 2-hydroxybenzaldehyde did not give the desired tricyclic product by this method. The same group has reported the synthesis of pyrido-dipyrimidines 40 in 90–99% yields under sonochemical green conditions using copper ferric nanoparticles as an efficient and recyclable catalyst (Scheme 18) [9]. In addition, the reaction of 4-(dimethylamino) benzaldehyde, and 2-hydroxy-benzaldehyde failed to produce the anticipated products under these conditions. The first method [10] is the most reliable owing to the product yields. The nano-catalyst was prepared by co-precipitation of copper(II) nitrate trihydrate with hydrated ferric chloride in sodium hydroxide solution. Naeimi et al. [75] previously synthesized pyridodipyrimidines under iron (III)-doped Fe-MCM-41 ionic liquid catalysts.

Scheme 17.

Synthesis of pyrido-dipyrimidines under nano-catalytic conditions

Scheme 18.

Synthesis of pyrido-dipyrimidines under nano-catalytic, and sonochemical conditions

Rostami et al. [76] developed the usage of CoFe₂O₄@SiO₂-PA-CC-guanidine-SA as an adept nanocatalyst for the synthesis of pyridodipyrimidines 41 (Scheme 19). Consequently, one-pot multicomponent reactions of aryl aldehyde, barbituric/thiobarbituric acids, and NH₄OAc produced the expected products in brilliant yields (86–95%). The procedure is efficient to be applied for this purpose owing to the low costs, the ease of nano-catalyst preparation, and improved yields along with the possible recyclability of the nano-catalyst. The nature of substituents on the phenyl ring of the aryl aldehydes affected the reaction times and product yields, in which the groups with electron-withdrawing characters are preferred. The catalytic nanoparticles were prepared from ferric chloride, and cobalt chloride treatment in sodium hydroxide solution followed by treatment with TEOS, PEG, 3-aminopropyl-trimethoxysilane, cyanuric chloride, guanidinium chloride, and sulfurochloridic acid. The catalyst maintained the interaction of barbituric/thiobarbituric acids with NH₄OAc to generate the aminopyrimidinone intermediates and activates the aldehyde carbonyl group for condensation with barbituric/thiobarbituric acids to form arylidene intermediates. The mechanism of this type of reaction involved Knoeveangel condensation, Michael addition, and cyclocondensation steps.

Scheme 19.

Nano-catalyzed multicomponent synthesis of pyridodipyrimidines

Zare et al. [77] appraised the usefulness of nano-2-(dimethylamino)-N-(silica-n-propyl)-N,N-dimethylethanaminium chloride for the synthesis of compounds 42 by heating under solvent-free, heating, and stirring conditions (Scheme 20). Thus, reactions of thiobarbituric acid (2 equivalents) with aryl aldehydes (1 equivalent) and NH₄OAc (1.4 equivalents) supplied the target products in notable yields (84–91%). This technique is a green procedure and was preferred over the other methods that involved the usage of Lewis and acidic catalysts. The nano-catalyst is recyclable, delivered reduced reaction time, and enhanced yields. In addition, the procedure has also supplied a simplicity of nano-catalyst preparation through two steps involving the reaction of (3-chloropropyl)trimethoxysilane with tetramethylethane-1,2-diamine in toluene under reflux conditions followed by the reaction with nano-silica gel in refluxing ethyl acetate after the evaporation of toluene.

Scheme 20.

Synthesis of pyridodipyrimidines through a green protocol

Mirhosseini-Eshkevari et al. [78] synthesized compounds 43 under catalyzed conditions. The green protocol was accomplished under solvent-free, and nano-catalytic conditions. In this work, the pyridodipyrimidines were prepared through one-pot multicomponent reactions of 6-amino-uracil derivatives with aryl aldehydes and barbituric/thiobarbituric acids. The product yields are varied (87–98%) based on the substituents’ nature attached to the phenyl ring. The ease of the catalyst preparation, improved product yields, catalyst recyclability, superior reactivity of the catalyst, pure product, and simplicity of isolations are benefits of this approach. The mechanism was planned based on the acid characters of the catalyst, HMTA-BAIL@MIL-101(Cr). The catalyst behaves as Brønsted acid in the activation of the carbonyl group of barbituric/thiobarbituric acids for enolization before the condensation with aryl aldehydes “Knoevenagel condensation”. The catalyst also supported the Michael addition step of 6-aminouracil/6-amino-1,3-dimethyl-uracil with the arylidene intermediate D-2. Cyclocondensation of the intermediates D-3 gave the target products (Scheme 21).

Scheme 21.

Green catalyzed synthesis of pyrido-dipyrimidines

Fattahi et al. [79] developed the utility of Fe₃O₄ nanoparticles as an effective ecological catalyst for the one-pot multicomponent synthesis of pyridodipyrimidines 45a–f. In an alternative route, 1,3-dimethylbarbituric acid reacted with aryl aldehydes to give the respective arylidenes, this step proceeded under catalytic conditions using nano-Fe₃O₄ in water/ethanol. The second step is the reactions of the produced arylidenes with another equivalent of 1,3-dimethylbarbituric acid and NH₄OAc under the same previous conditions to give the tricyclic pyrido-dipyrimidines. The structural nature of the substituents at the phenyl ring of the aryl aldehydes controls the reaction sequence. Accordingly, the products 44 g–i were isolated in high yields in the case of the reactions involved the use of aldehydes attached to electron-donating substituents (e.g. 4-MeC₆H₄; 4-MeOC₆H₄; 4-HOC₆H₄) instead of the formation of the predictable products 45 even by increasing the reaction time. The products 45, in general, were formed through the initial formation of the Knoevenagel intermediates 44, which reacted with another equivalent of 1,3-dimethyl-barbituric acid and ammonium acetate. The efforts to isolate the intermediates 44a–f were unsuccessful under these conditions. The Knoevenagel products 44a–i were only isolated under these conditions in the case of carrying the reactions between 1,3-dimethyl-barbituric acid, and aryl aldehydes in the absence of ammonium acetate. In addition, the reactions of 44a–f with the second equivalent of 1,3-dimethyl-barbituric acid, and ammonium acetate afforded 45a–f in high yields (Scheme 22) [79].

Scheme 22.

Alternative synthesis of dihydropyridodipyrimidines

Acid-catalyzed reactions

Acid catalysts are principally applied for organic reactions, in which many acids such as strong acids or organic acids are a source of protons. Strong mineral acids were applied, for example, in the hydrolysis and transesterification of esters. Frequently, the acids enable the protonation of the carbonyl oxygen to improve the electrophilicity at the carbonyl carbon [80, 81].

Unexpectedly, Farag et al. [82] efficiently prepared pyridodipyrimidines 46a–g (Scheme 23) in a one-step synthesis. Therefore, the reactions of 6-amino-uracil with arylidene-malononitrile or ethyl arylidene-cyanoacetate gave the desired products in respectable to exceptional yields. The mechanism sequence, in this case, was planned as the first step involved Michael's addition of the Zwitter-ionic form of 6-amino-uracil on the arylidene-bearing electron-withdrawing function to generate the intermediate E-1. The intermediate E-2 was formed through the elimination of RCH₂CN from the intermediate E-1. Another molecule of 6-amino-uracil tended Michael addition type on the beforehand formed intermediate E-2 followed by intramolecular cyclization generated intermediate E-3. The heterocyclization step was accompanied by the release of the ammonia molecule.

Scheme 23.

Two-component synthesis of pyridodipyrimidines

Abdelmoniem et al. [83] synthesized bis(pyridodipyrimidine-tetraones) through one-pot multicomponent reactions. Thus, the reactions of bis(aldehydes) with four equivalents of 6-amino-uracil in acetic acid at reflux temperature furnished tetrakis(6-aminopyrimidine-2,4-diones) 47 or bis(pyrido-dipyrimidines) 48 as controlled by the reaction conditions. Consequently, the reactions of bis(aldehydes) with 6-amino-uracil in acetic acid gave the desired tetrakis(6-aminopyrimidine-2,4-diones) 47a-c, while the addition of p-toluene sulfonic acid led to the formation of bis(pyridodipyrimidines) 48a-d. In addition, heating 47a in acetic acid containing p-TSA for 1 h gave the desired product 48a directly in one step. The reactions of 6-aminouracil with the bis(aldehydes) in acetic acid contained p-TSA gave the corresponding bis(pyridodipyrimidinyl)phenoxy)acetamides) 48a–d, respectively, in good yields. An alternative route for the synthesis of compounds of series 48 was estimated by heating compounds of series 47 in acetic acid containing p-TSA (Scheme 24).

Scheme 24.

Multicomponent synthesis of bis-pyridodipyrimidines

The projected mechanism for the synthesis of bis(pyridodipyrimidines) 48 from the reactions of bis-aldehydes with 6-amino-uracil was as shown in Scheme 25. In this sequence, the enamine β-carbon of 6-amino-uracil tended to nucleophilic addition to the carbonyl groups of bis(aldehydes) to generate the intermediates F-1. The consequent elimination of water molecules generated the respective arylidene intermediates F-2. Next, Michael's addition step took place for another two molecules of 6-amino-uracil to the intermediates F-2 formed compounds 47. The pyridine ring cyclization was achieved by the release of ammonia molecules through intramolecular cyclization supported by p-TSA to give the products 48 [83].

Scheme 25.

The projected mechanism for the synthesis of bis-pyridodipyrimidines

Attempts by Hawass et al. [84] to prepare bis(dihydropyridodipyrimidine) analogs through direct reactions of 1H-pyrazole-4-carbaldehyde 49 with 6-amino-uracil/thiouracil failed. The reactions of aldehyde 49 with alkyl dibromides 50 in acetic acid yielded the desired bis-aldehydes 51, which reacted with 4 equivalents of 6-amino-uracil/thiouracil derivatives to give bis(dihydropyridodipyrimidine) analogs 52, and 53 (Scheme 26). The mechanism of these reactions involved the reactions of di-aldehyde derivatives with four equivalents of amino-uracil/thiouracil. The first step is a condensation of the aldehyde group with amino-uracil/ thiouracil to form the Knoevenagel condensation intermediates G-1, which interacted with the second equivalent of amino-uracil/thiouracil through Michael-type addition to generate the intermediates G-2. Intramolecular cyclization of the formed intermediates G-2 by nucleophilic attack of the amino group at the imine-carbon generated intermediates G-3. The release of ammonia produced the products 52. The pyridine ring aromatization was achieved by the oxidation step in the case of the preparation of compound 53.

Scheme 26.

Multi-step synthesis, and the proposed mechanism of bis(dihydropyridodipyrimidine) analogs

A series of bis(methylene))bis(1'H-spiro[indoline-3,5'-pyrido[2,3-d:6,5-d']dipyrimidine] derivatives 55a–c were efficiently synthesized by Abdelmoniem et al. [85] under acid-catalyzed, and heating conditions. In this course, bis(1,2-diketones) 54 reacted in a one-pot multicomponent procedure with four equivalents of 6-amino-uracil in acetic acid containing p-TSA to give the corresponding products 55a–c (Scheme 27). The best yield of these reactions is based on the steric factor of the reacted bis(1,2-diketones), in which 1,4- analog was prepared with the best yield (82%), while 1,2- analog was obtained in the lowest yield (73%). The mechanism of these reactions involved the acid catalyst activation for carbonyl groups of isatins by improving electrophilicity for the interaction with amino-uracil. The enamine β-carbons of 6-amino-uracil (2 equiv.) tended nucleophilic addition to the carbonyl groups of 1,2-diketones to generate intermediate H-1. Succeeding condensation led to the formation of intermediates H-2. Next, Michael’s addition of two molecules of 6-amino-uracil to the previously formed intermediates H-2 generated the intermediates H-3. The final step involved the release of two ammonia molecules with pyridine ring cyclization to give the products 55.

Scheme 27.

Synthesis of bis(methylene))bis(1'H-spiro[indoline-3,5'-pyridodipyrimidines]

El-Kalyoubi et al. [86] defined the acid-catalyzed synthesis of 1'H-spiro[indoline-pyrido-dipyrimidine] analogs 57, 59, and 60. Thus, 1,2-diketones 56, and 58 reacted in a one-pot multicomponent procedure with two equivalents of each of 6-amino-uracil/ 1-methyl-6-amino-uracil/ 6-amino-2-thiouracil/ 1-methyl-6-amino-2-thiouracil compounds to give the respective products 57a–d, 59a–e, and 60a–e, respectively (Scheme 28). The reactions were accomplished in acetic acid at reflux temperature. The presence of an acid medium supported the imine-amino and keto-enol tautomerization of 6-amino-uracil/thiouracil derivatives in reactions with isatins. Also, the acid enabled the condensation of isatins with 6-amino-uracil/thiouracil derivatives to form the arylidene intermediates. The mechanistic sequence of this reaction type involved the release of ammonia molecules in the final step.

Scheme 28.

Acid-catalyzed synthesis of 1'H-spiro [indoline-3,5′-pyridodipyrimidines]

Abdelmoniem et al. [87] synthesized three compounds of pyridodipyrimidines 61–63 in water containing p-TSA as a catalyst (Scheme 29). In this route, multicomponent one-pot reactions of isatin with barbituric acid, and 6-amino-uracil or 1,3-dimethyl-6-amino-uracil or 6-amino-thiouracil gave the corresponding products 61–63 in excellent yields (85–86%). The procedure is a simple technique with reasonable yields, readily available materials, and catalysts under green conditions. The mechanism of this sequence involved the protonation of the carbonyl group of isatin for interaction with 6-amino-uracil/thiouracil compounds followed by cyclization through ammonia molecule release.

Scheme 29.

Synthesis of spiro[indoline-pyridodipyrimidines] supported by an acid catalyst

Ionic liquid and Lewis acid catalysts

The progress and achievements of Lewis acids command a vital area in organic and synthetic chemistry fields. The heterogeneous Lewis acids from metal halide to metal oxide with Lewis acid site, are commonly predominating on the industrial scale [88, 89]. The ionic liquids of the type of Lewis acids perform monotonously, with strong attention on halometallate Lewis acidic anions-especially chloroaluminate(III) ILs [90].

Atashrooz et al. [91] utilized an ionic liquid catalyst, [TMBSED][TFA]² for the synthesis of pyridodipyrimidine derivatives 64 under ambient conditions (Scheme 30). The acid–base characters of the catalyst provided a dual functionality of the catalyst to activate the reacted materials. Therefore, the multicomponent one-pot reactions in this sequence involved the use of thiobarbituric acid with aryl aldehydes and NH₄OAc as substrates.

Scheme 30.

Schematic and planned mechanistic route for the design of pyridodipyrimidines

The reactions proceeded under heating, solvent-free and catalytic conditions to prepare the target compounds in excellent yields (87–95%). The [(TMBSED) (TFA)²] catalyst efficiently provided shortened reaction time and improved yields using 17 mol% at 110 °C. The dual functionality of the catalyst as it consists of an acidic SO₃H group, and basic trifluoroacetate, in which the acid site enables activation of the electrophilicity of the carbonyl group in their interactions with nucleophiles. The acidic characteristics of the catalyst also supported the enolization of the carbonyl group in the tautomerization conversion along with the water molecule release. The common role of the basic site of the catalyst that involved the activations of nucleophiles was shown in the proposed mechanistic route in Scheme 30.

Zare et al. [13] developed the utility of [Et₃N–SO₃H][MeSO₃] as an acidic ionic liquid catalyst for the multicomponent synthesis of pyridodipyrimidines in a one-pot step reaction. Thus, reactions of aryl aldehydes with thiobarbituric acid and NH₄OAc in refluxing ethanol under catalytic conditions gave the corresponding pyridodipyrimidines 65a–k in excellent yields (93–99%) (Scheme 31). The method provided ease of catalyst preparation, readily accessible materials, ease of product isolation, improved product yields, and decreased reaction time. The mechanism of this reaction was supposed by the preliminary activation of the carbonyl group of the aldehydes by a sulfonic acid moiety of the catalyst for Knoevenagel condensation with the tautomerized thiobarbituric acid to generate the desired arylidene intermediates. The next step termed the activation of the amidic carbonyl group of another mole of thiobarbituric acid for the condensation with NH₄OAc to generate 6-amino-2-thioxo-2,3-dihydropyrimidin-4(1H)-one intermediate. Both intermediates that were generated from the two steps interacted through Michael’s addition followed by tautomerization, and cyclocondensation to produce the tricyclic derivatives 65.

Scheme 31.

Synthesis of pyridodipyrimidines under ionic liquid catalytic, and heating conditions

Bhosle et al. [92] advanced the synthesis of pyrido-dipyrimidine-tetraones 66 through multicomponent one-pot reactions of barbituric acid with aldehydes, and NH₄OAc (Scheme 32). The reactions proceeded under catalytic conditions using DIPEAc at room temperature. The advantage of this green method lay in the utility of an efficient catalyst, which provided excellent product yields (87–95%) after a short time along with catalyst reusability.

Scheme 32.

Catalyzed synthesis of pyrido-dipyrimidine-tetraones

Saikia et al. [93] developed the utility of ionic liquid catalysts of the sulfonic acid functionalized imidazolium salts type such as [Dsim]Cl for the efficient synthesis of pyridodipyrimidine-tetrone analogs 67. The protocol involves the reactions of 1,3-dimethyl-6-amino-uracil with aryl aldehydes under microwave-assisted conditions (Scheme 33). The procedure is a benefit for improved product yields, reduced reaction time, and catalyst recyclability. The acid property of the catalyst activated the formation of arylidene intermediated in the Knoeveangel condensation step and enable imino-amino tautomerization. The inferred mechanism was projected as the final steps involved the release of ammonia molecules and oxidation to reach the pyridine ring aromatization.

Scheme 33.

Synthesis of pyrido-dipyrimidines catalyzed by an ionic liquid catalyst

In due course, Moosavi-Zare et al. [94] specified the synthesis of pyrido-dipyrimidine-dithione analogs 68 using N-sulfonic acid pyridinium-4-carboxylic acid chloride as an efficient catalyst. The designated green procedure states one-pot multicomponent reactions of thiobarbituric acid (2 equivalents) with aryl aldehydes and ammonium acetate under solvent-free, and heating conditions to prepare the target compounds (Scheme 34). The method is preferred since the ease of catalyst preparation, selectivity, high product yield, reduced reaction time, and catalyst reusability. As proposed from the mechanistic sequence in Scheme 27, the catalyst tended to stimulate the aldehyde carbonyl groups for condensation with thiobarbituric acid and provided cyclocondensation after the Michael addition step. Some of the compounds of this series were formerly prepared under miscellaneous conditions [9, 77, 95].

Scheme 34.

Schematic plausible mechanistic route for the synthesis of pyrido-dipyrimidine-dithiones

A series of pyrido-dipyrimidine-diones 69a–j (75–89% yields) were synthesized as demonstrated by Sun et al. [96] under ambient conditions. Thus, two equivalents of 2,6-diaminopyrimidin-4(3H)-one reacted with aryl aldehydes by heating in water containing Y(OTf)₃ as an efficient catalyst to yield the tricyclic systems 69 (Scheme 35). The best yield was obtained from the reaction of 4-chloro-benzaldehyde with 2,6-diaminopyrimidin-4(3H)-one, while the lowest yield was recorded in the reaction involving 2-nitrobenzaldehyde. The product yield is affected by the nature of substituents on the phenyl ring and the steric factor.

Scheme 35.

Synthesis of pyrido-dipyrimidine-diones

Naeimi et al. [95] have progressed the multicomponent synthesis of pyrido-dipyrimidines 70a–m under [HNMP]⁺[HSO₄]⁻ catalytic and ultrasonic irradiation conditions (Scheme 36). The advantage of this green protocol involved improved product yields after a short time, low costs, and high product purity. The one-pot four-component reactions of aryl aldehydes, with thiobarbituric acid, and ammonium acetate in water under the optimum conditions gave the respective products with excellent yields (90–98%). The reactions gave the pyridodipyrimidine products in the case of reactions involving 4-(dimethylamino)-benzaldehyde, and 2-hydroxybenzaldehyde but with very low yields (< 5%). The mechanism of this reaction type was assumed as previous research involving the Knoeveangel condensation step to generate the desired intermediates followed by amination of thiobarbituric acid by interaction with ammonium acetate, Michael addition, and cyclization through ammonia molecule release. The mechanism of this work did not signify the role of ammonium acetate in these reactions, as it was pointed out that the cyclization step was accomplished through cyclocondensation instead of the ammonia release. The same reaction sequences were reported by Kidwai et al. [97] using the solid phase of Al₂O₃ as a catalyst under microwave-irradiation conditions, and by Kidwai et al. [97] and Vafaeezadeh et al. [98] using the liquid phase of Al₂O₃ under thermal conditions.

Scheme 36.

Synthesis of pyridodipyrimidines under sonochemical, and catalyzed conditions

Mirhosseini‐Eshkevari et al. [99] applied the usage of Brønsted acidic ionic liquids supported in Zr metal–organic framework (BAIL@UiO-66) as an effective catalyst for the synthesis of spiro[indoline-pyridodipyrimidines] 71a–o. Therefore, reactions of 1,3-dimethyl-6-amino-uracil with isatins, and barbituric/ thiobarbituric acid derivatives under catalytic, and sonochemical conditions gave the target products 71a–o in excellent yields (88–98%) (Scheme 37).

Scheme 37.

The synthetic, and mechanistic sequence of spiro[indoline-pyridodi-pyrimidines]

The procedure enables green protocol, catalyst recyclability, short time, ease of product isolation, and good product yields. The proposed mechanism for this reaction sequence involved the action of the catalyst as Brønsted acid with increased electrophilicity of the carbonyl group of isatin and barbituric acid through the formation of a relatively strong coordination bond. Firstly, protonation of the carbonyl group of isatins enabled the nucleophilic attack by 6-amino-1,3-dimethyl-uracil to generate arylidene intermediate K-3. Next, tautomerization of barbituric acid enabled the nucleophilic attack of the intermediate K-3 through the Michael addition step to generate intermediate K-4. Tautomerization of the intermediate K-4 generated the intermediate K-5, which followed cyclization with the elimination of water molecules to give the anticipated products 71 (Scheme 37).

Heterogeneous organometallic catalysts

Heterogeneous catalysis is a catalyst in which it does not take part in the reaction that it increases. Heterogeneous catalysis involved different phases of the catalyst and reactants such as gas phase reactions supported by the solid catalysts. Surface organometallic chemistry is an area of heterogeneous catalysis that has developed as a result of a qualified analysis of homogeneous and heterogeneous catalysis. Heterogeneous catalysis is mostly preferred in the chemical industry scale, but the progress of the best catalyst was hindered by many factors such as the presence of numerous active sites with low concentrations [100].

On the other hand, Saeidiroshan et al. [101] synthesized a series of pyrido-dipyrimidines 72 using MWCNTs@L-His/Cu(II) catalyst. Subsequently, one-pot three-component reactions of aryl aldehydes with barbituric acid or its 1,3-dimethyl- derivative, and 6-amino-uracil in ethanol under catalytic, and heating conditions gave the desired products 72a–o (Scheme 38). The procedure enabled catalyst reusability without notable loss of its activity. The preparation of the catalyst was achieved through four steps involving the preparation of MWCNTs-OH, MWCNTs-CPTES, MWCNTs@L-Histidine, and MWCNTs@L-His/Cu(II). The yields of the products are affected by the nature of substituents on the phenyl ring of the aryl aldehydes, in which electron-withdrawing groups delivered higher yields over the electron-donating groups. In addition, the rate of the reactions was noticed to be faster in the case of withdrawing groups than in the electron-donating groups. The catalyst role is demonstrated through the action of copper ions as Lewis acid catalyst in the stimulation of the carbonyl group of the aldehyde by increasing electrophilicity and activation of C-H of barbituric acid. The overall steps in this sequence involved Knoeveangel condensation, Michael addition, proton transfer, cyclocondensation, and catalyst release.

Scheme 38.

Procedure for the synthesis of pyridodipyrimidines and the mediated Heterogeneous catalyst

Metal oxide catalysts

The utility of oxide catalysts for improved product yields has attracted research interests in recent years [102–104]. Incessantly, Patil et al. [105] proceeded with the synthesis of pyridodipyrimidines in good yields (80–88%) through multicomponent one-pot reactions involving the usage of P₂O₅ mediated catalysis. As shown in Scheme 39, aryl aldehydes reacted with 6-amino-1,3-dimethyl-uracil, and barbituric/ thiobarbituric acids in ethanol to yield the desired products 73a–k. The researchers tried the synthesis of these compounds under different optimized conditions, for example, the use of other catalysts (acetic acid, catalyst-free, triethylamine, DABCO, and P₂O₅). The best product yield along with shortened reaction time was recorded for the product in a pilot experiment. Also, the maximum product yield after a short reaction time was obtained using 80 mg of the P₂O₅-mediated catalyst. In this reaction sequence, ethanol as a solvent is preferred over acetonitrile, DMSO, DMF, and water for improved product yields.

Scheme 39.

Synthesis of pyridodipyrimidines under P₂O₅ mediated catalytic conditions

Hafez et al. [106] applied the one-pot multicomponent synthesis of pyrido-dipyrimidine-dithione under Al₂O₃ catalytic conditions. Thus, reactions of thiobarbituric acid (2 equivalents) with substituted-1H-pyrazole-4-carbaldehyde and ammonium acetate gave the desired tricyclic product 74 in a good yield (68%) (Scheme 40).

Scheme 40.

Synthesis of pyrido-dipyrimidine-dithione analog

Biological activities of pyridodipyrimidine derivatives

Antimicrobial activity

Antimicrobial activity was described as a collective term for all active principles (agents) that inhibit the growth of bacteria or fungi, hinder the formation of microbial colonies, and might destroy microorganisms. Suresh and Mohan [1] also reported the in vitro assessment of pyridodipyrimidines 3a–f (Fig. 2) as antibacterial agents against Salmonella typhi and Aeromonas hydrophilla species by disc diffusion method. The samples were screened using three different concentrations of each sample (0.5, 1, and 2%). All the samples are active against both bacterial species at 1, and 2% concentrations. Compound 3f (R = 4-Cl) displayed the most potent activity against S. typhi (7 mm) and A. hydrophilla (9 mm) species relative to the results of Streptomycin (10, and 12 mm). In addition, compounds 3a (R = H), 3d (R = 2-OMe), and 3e (R = 4-OMe) displayed the same potency with an inhibition zone at 5 mm against S. typhi species. Mainly, the compounds revealed good potency against the growth of Aeromonas hydrophilla species with inhibition zones ranging from 5 to 9 mm. It was noted that the increase in the sample concentration was accompanied by an increase in toxicity. The variation in the antibacterial potency between the tested compounds is related to the impermeability or diffusion in the ribosomes of the microbial cells.

Fig. 2.

The structures of antibacterial agents, and their results against bacterial species

Compounds 67a–i (Fig. 3) were assessed as antibacterial agents using disc diffusion assay on B. subtilis, S. aureus, K. pneumonia, and E. coli strains. The results verified that all compounds have no activity against B. subtilis, and E. coli species. In contrast, these analogs revealed good antibacterial activities for growth inhibition of S. aureus, and K. pneumonia species (inhibition zone diameters = 10–14 mm). The most potent activities were recorded for compounds 67b (Ar = 4-ClC₆H₄), 67d (Ar = 4-OHC₆H₄), 67e (Ar = 4-NO₂C₆H₄), and 67f (Ar = 4-MeC₆H₄) against S. aureus species with 14 mm of inhibition zone diameters. Analogously, compound 67 g (Ar = 4-NMe₂C₆H₄) is the most potent antibacterial agent against S. aureus species (IZD = 14 mm) [93].

Fig. 3.

The structures of pyridodipyrimidines as antimicrobial agents

The antimicrobial activity was assessed for compounds 70c (Ar = 4-NO₂-C₆H₄), 70f (Ar = 4-Cl-C₆H₄), 70 h (Ar = 2-OH-1-naphthyl), 70i (2-pyridyl), and 70 m (Ar = 4-OMe-C₆H₄) (Fig. 3) using agar diffusion assay against a variety of microbial species. The results identified that all the compounds are inactive against P. aeruginosa, E. coli, P. vulgaris bacterial species, while compounds 70i (10 mm), and 70 h (13.5 ± 0.5 mm) revealed good activities against B. subtilis species. All the compounds have good activities against S. aureus, and S. epidermidis species, specifically, compound 70 h is the most potent against both bacterial species with inhibition zone diameters at 19.5 ± 1.5, and 26.5 ± 1.5 mm, respectively. Furthermore, compounds 70f, 70f, and 70 h revealed potent antifungal activities against C. albicans, A. niger, and A. brasiliensis strains, along with the inactivity of compound 70c against C. albicans, and A. brasiliensis strains. In all cases, compound 70 h is the most potent antimicrobial agent against the various species compared to the results of antibiotics, tetracycline, and nystatin. The MIC tests identified that compounds 70f and 70 h are the most effective at lower concentrations (1.95 μg/mL) against S. epidermidis species [95].

Compounds 75a,b [41] (Fig. 3) were evaluated as antimicrobial agents against S. aureus (ATCC 25,923), B. subtilis (ATCC 6635), S. typhimurium (ATCC 14,028), E. coli (ATCC 25,922), as well as fungal species e.g. C. Albicans (ATCC 10,231) and A. fumigatus. The samples were tested at two concentrations (0.5, and 1 mg/mL), in which compound 75b displayed low activity to inhibit the growth of B. subtilis species (10 mm) relative to Chloramphenicol (35 mm) at the higher concentration. In addition, compound 75b showed low activity against C. albicans (11 mm) in comparison to the result of Cephalothin (35 mm) at a higher concentration. Compound 75a revealed no inhibition for the growth of the bacterial and fungal species.

Antitubercular activity

Antitubercular activity is defined as a drug or influence that works against tuberculosis (a spreadable bacterial infection that frequently affected the lungs). The in vitro antitubercular activity of compounds 73a–k (Fig. 4) was assessed against Mycobacterium tuberculosis H37RV strain. The results established varied ranges of activities were recorded from moderate to good significance. The samples were assessed at different concentrations (0.8–100 µg/mL) involving eight concentrations in the serial dilutions. At high concentrations (50, and 100 µg/mL) the M. tuberculosis H37RV strain recorded susceptibility, while the other concentrations led to inactivity [105].

Fig. 4.

The structures of pyridodipyrimidines as antitubercular agents

Anticancer activity

Anticancer agents are compounds that destroyed or inhibited the growth of cancer cells. The in vitro MTT/MTS cell proliferation assessment is one of the supreme extensively utilized assessments for estimating the earliest anticancer activity of both synthetic compounds, and natural product extracts. The exceedingly dependable, colorimetric-based assay is readily implemented on a wide range of cell lines. The desired pyridodipyrimidines 46a–g (Fig. 5) were in vitro estimated as cytotoxic agents on HePG2, HCT-116, and MCF-7 tumor cells. 5-Fluorouracil, Doxorubicin, and methotrexate were applied as chemotherapeutic standards. The results verified that the compounds revealed auspicious cytotoxic effects on the tested cell lines relative to the results of anticancer standards. The cytotoxic impacts of these compounds against HePG2 cells referred to impacts with IC₅₀ values in the range of 38.48 ± 2.6 to 85.62 ± 4.0 µM. Thus, compounds 46a–g displayed moderate to weak cytotoxic activities against the HePG2 cell line compared to the results of Doxorubicin (IC₅₀ = 4.50 ± 0.2 µM), and 5-Fluorouracil (IC₅₀ = 8.09 ± 0.5 µM). In addition, compound 46f (Ar = 4-NMe₂-C₆H₄) is the most potent analog on HePG2, HCT-116, and MCF-7 tumor cells with IC₅₀ values at 38.48 ± 2.6, 35.91 ± 2.5, and 43.14 ± 2.8 µM, respectively. The compounds are more applicable to hinder the growth of HePG2 cells in most of the cases except in the case of compounds 46b (Ar = 4-NO₂-C₆H₄) (IC₅₀ = 68.59 ± 3.6 µM), 46e (Ar = 4-MeO-C₆H₄) (IC₅₀ = 42.63 ± 2.8 µM), and 46f (IC₅₀ = 35.91 ± 2.5 µM), against HCT-116 cells, along with the potency of compound 46 g (Ar = 4-OH-C₆H₄) (IC₅₀ = 49.87 ± 3.0 µM) against MCF-7 tumor cells. The comparison of the cytotoxic results against the investigated cell lines with the chemotherapeutic standards indicated that the compounds displayed moderate to weak cytotoxic activities based on the scale of comparison that referred to the moderate impacts in the range of IC₅₀ = 21 to 50 µM along with the weak cytotoxic influences for IC₅₀ values higher than 50 µM [82].

Fig. 5.

The structures of pyridodipyrimidines as cytotoxic agents

Additionally, compounds 66a–k (Fig. 5) in this work were in vitro investigated as cytotoxic agents on MCF-7, HeLa, A-549, SK-MEL-2 tumor, non-tumorigenic MCF-10A cell lines, and tyrosinase inhibitors. The results verified that compounds 66a (Ar = C₆H₅; IC₅₀ = 21.88 µM), 66c (Ar = 4-Me-C₆H₄; IC₅₀ = 12.22 µM), 66f (Ar = 4-OH-C₆H₄; IC₅₀ = 22.12 µM) and 66i (Ar = 4-NMe₂-C₆H₄; IC₅₀ = 14.10 µM) displayed potent cytotoxicity against SK-MEL-2 cells. In addition, compounds 66f (IC₅₀ = 16.26–23.31 µM), 66 h (Ar = 2-NO₂-C₆H₄; IC₅₀ = 26.54–38.11 µM), and 66i (IC₅₀ = 12.33–20.12 µM) are potent cytotoxic agents against most of the tested tumor cells. The chemotherapeutic standard, Adriyamicin revealed very strong cytotoxicity with IC₅₀ lower than 10 µM. The comparison of the results of compounds 66a–k to that of the reference standard verified that compounds 66c and 66i showed strong activities against SK-MEL-2 cell lines [92].

Hafez et al. [106] reported that compound 74 (Fig. 5) revealed very strong cytotoxic effects in comparison to the standard doxorubicin against HePG2 (GI₅₀ = 0.01 ± 0.006 µmol/L) (Doxorubicin, GI₅₀ = 0.04 ± 0.009 µmol/L), HCT-116 (GI₅₀ = 0.03 ± 0.004 µmol/L) (Doxorubicin, GI₅₀ = 0.05 ± 0.008 µmol/L), and MCF-7 (GI₅₀ = 0.05 ± 0.007 µmol/L) (Doxorubicin, GI₅₀ = 0.09 ± 0.005 µmol/L) tumor cells. The GI₅₀ is the concentration responsible for 50% of the inhibition of tumor cell growth after continual exposure for two days.

Analgesic and anti-inflammatory activities

Analgesic compounds are a class of compounds that have the selectivity to relieve pain by acting in the CNS and peripheral pain mediators without changing consciousness. Analgesics might be narcotic or non-narcotic. El-Gazzar et al. [47] assessed the analgesic activity of a series of pyridodipyrimidine compounds 15–18 (Fig. 6) performing the tail-flick technique utilizing Wistar albino mice. The results referred to moderate potency after 30 min of reaction accompanied by increased activity over time as the peak levels of the compounds were recorded after two hours. The analgesic impact of the compounds after three hours attended a decline in potency. Compound 15a (59%) appeared the most amended analgesic potency as compared to the results of the other compounds, but this compound was still less active than diclofenac sodium (62%). It was noted that the transformation of the bicyclic pyridopyrimidines 14 to the respective tricyclic systems 15–18 decreased the analgesic acidity.

Fig. 6.

The structures, and results of pyridodipyrimidines as analgesic, and anti-inflammatory agents

On the other side, there are two basic sorts of anti-inflammatory drugs, specifically, steroidal anti-inflammatory drugs, which decrease inflammation by binding to cortisol receptors, and nonsteroidal anti-inflammatory drugs, which reduce damage by inhibition of cyclooxygenase enzymes. Particularly, the carrageenan-induced paw edema assay was estimated for the assessment of the anti-inflammatory activity of compounds 15–18a–c (Fig. 6) in rats as reported by El-Gazzar et al. [47]. The results certified that the compounds protect the rats from the inflammation caused by carrageenan-induced after 30 min. The activity of the compounds increased over time, reached peak levels after two hours, and declined in activity after three hours. Compounds 15a–c and 18a–c revealed improved activities with potent percentages of protection relative to diclofenac sodium. Therefore, compounds 15c (48%), and 18a (47%) are the most potent compounds in comparison to the tested samples. Based on the SARs analysis, it was noted that the introduction of the amino groups at the C6 position of the tricyclic system improved the anti-inflammatory activity.

Antiviral activity

An antiviral agent is a molecule or substance that fights and inhibited the growth of viruses against viral infections. In this route, El-Kalyoubi et al. [86] in vitro assessed the antiviral activity of a series of spiro[indoline-3,5′-pyrido[2,3-d:6,5-d']dipyrimidine] derivatives 57a–d, 59a–e, and 60a–e (Fig. 7) against SARS-CoV-2 segregated from the Egyptian patients. The impact of various concentrations of the tested samples on the cellular proliferation of the Vero E6 cell line succeeding 24 h of usage was estimated by applying an MTT assay. The results specified that the compounds revealed potent activities ranging from 4.10 to 5.93 µM on SARS-CoV-2 in plaque reduction assay. The results of the tested compounds revealed good potency in comparison to the results of the standard, chloroquine, which displayed 2.24 µM. Accordingly, the results of the plaque reduction assay for the estimation of the % inhibition against SARS-CoV2 specified that the introduction of substituents with X = S, and R = methyl "compound 57d" presented 55% inhibition. Compounds 57a, 59b, 59d, 59e, and 60d displayed the most potent activities on SARS-CoV-2. In particular, compound 57a has R = CH₃ and X = O presented the most potent impact with 84% inhibition against the virus replication than the compounds of this series 57 (compounds with pyrrolidin-1-ylsulfonyl moiety). The introduction of substituents such as CH₂-Ph, –CH₂–C₆H₄–Cl-4 “compounds 57b, and 57c” decreased or diminished the activity with percentages of inhibition at 75% and 0%, respectively. In addition, compounds 59b, and 59e incorporated methyl substituent along with “X = O or S” displayed the most potent activities among the tested samples against replication of SARS-CoV-2 with 99% and 91% for the percentages of inhibition, respectively. Compound 59c displayed 74% inhibition indicating the activity decrease, which is related to the incorporation of the benzyl motif. Compound 59d presented 80% inhibition owing to the presence of R = 4-chlorobenzyl and X = O substituents. Weak activities were recorded for compounds 60a-c, and 60e with percentages of inhibitions ranging from 20 to 70%. Compound 60d is the most potent analog with an inhibition percentage of 82% when compared to the compounds of the same series “60”, but it still has low activity compared to Chloroquine (> 99%). The data refer to the improved activities for the compounds incorporated pyrrolidin-1-yl and piperidin-1-yl moieties along with spiroindoline skeleton than the incorporation of morpholin-1-yl motif.

Fig. 7.

The structures, and results of pyridodipyrimidines as antiviral agents

Antidiabetic activity

α-Glucosidase inhibitors reduce blood glucose by amending the intestinal absorption of carbohydrates. Also, α-amylase inhibitors might act as blockers for carbohydrates by controlling the digestibility, and adsorption in the gastrointestinal diet. Consequently, Toobaei et al. [48] investigated a series of pyridodipyrimidines 19 substituted with sugar chains (Fig. 8) as α-glucosidase and porcine pancreatic α-amylase inhibitors.

Fig. 8.

The structures, and results of pyridodipyrimidines as antidiabetic agent

As shown in Fig. 8, Acarbose was used as a standard drug that verified inhibitory action with the IC₅₀ values of 365.4 and 22.3 μM on yeast and rat enzymes, respectively. The comparison of the results of the inhibitory action of the synthesized compounds 19 against the rat enzyme system revealed that the compounds did not have reasonable ranges of inhibitions. To improve the inhibitory actions of these compounds, the authors in this work suggested future chemical modifications to increase the activity. Additionally, these synthesized compounds 19 revealed competitive inhibitory action, which can be deliberated as a disadvantage owing to the high feed will require a high sample concentration from these compounds to display the equivalent inhibitory action. Compound 19c revealed inhibition with IC₅₀ at 45.20 ± 2.98 µM against yeast α-Gls, while compound 19a is the most effective with IC₅₀ at 88.13 ± 2.01 µM against rat α-Gls, but it was still not effective compared to the standard Acarbose (IC₅₀ = 22.3 ± 0.97 µM). The results also verified lower α-Amy inhibitory actions (lower than 10%) compared to Acarbose (70%) owing to their probable reduced susceptibly for the progress of gastrointestinal discomforts. An important factor that affected the inhibitory action of the compounds is the length of the polyhydroxy chain. Accordingly, the compound with a sugar chain derived from glucose verified reduced inhibitory action than the compound with a sugar chain derived from arabinose. Insignificant enzyme inhibitory activity was noted for the compounds with sugar chains derived from lactose and maltose against α-Gls. The compounds revealed insignificant activity against pancreatic α-Amy in comparison to the results of the standard anti-diabetic drug, Acarbose.

Future prospective

Pyridodipyrimidine compounds include the motif of a pyridine fused to two pyrimidine rings, so researchers have to include the biological and synthetic applications of pyridines, pyrimidines, pyridopyridines and pyridopyrimidines, and pyrimidopyrimidines [107–120] on this type of compound in their future research. The formerly reported methods depend on the activity of different pyrimidine derivatives in their interactions with aldehyde derivatives and/or ammonium acetate to construct a pyridine ring. Therefore, it is possible in the future to adopt different methods for preparing these compounds through reactions of reactants containing a pyridine ring for the construction of a pyrimidine ring. On the other hand, the different reported methods depend on improving the product yields rather than finding alternative approaches for the preparation of pyridodipyrimidines, including obtaining the products in several steps or one step through the reaction of two reactants only. It was also found that researchers' interest in evaluating these compounds biologically is very little, as researchers can in the future explore the different biological activities of these compounds. It is also possible to prepare new compounds from this class of compounds linked to chemically active groups to explore and prepare new and distinct compounds so that they can be exploited in the field of drug design.

Concluding remarks

The current study highlighted the chemistry and biological features of heterocycles integrated pyridodipyrimidine skeleton. The researchers have focused their attention on developing the product yields of these compounds by applying distinctive approaches. It was found that the synthesis of these compounds involved multicomponent one-pot reactions of 6-amino-uracil or 6-amino-2-thiouracil or barbituric/ thiobarbituric acids or their derivatives (2 equivalents) with aryl, and heteroaryl/hetaryl aldehydes, and/ or ammonium acetate. The reactions could be accomplished under catalyst-free or catalytic conditions by applying conventional or microwave-assisted or ultrasonic-irradiation or room-temperature conditions. the competition of the scientists is to find suitable conditions that provided excellent yields, in which the green approaches, low cost, ease of catalyst preparation and usability, product purity, and high reaction rate are preferred. The utility of nanoparticle catalysis delivered high catalyst efficiency to produce improved product yield owing to the high surface area of the nanoparticles of the catalyst that increases its efficiency. The ease of catalyst preparation or availability is a significant factor in these reactions along with the catalyst efficiency. The mechanism of the reactions for the synthesis of pyridodipyrimidines involved, catalyst activation for the carbonyl group of the aldehyde in the Knoeveangel condensation with active methylene, enabling the Michael addition step, and intramolecular cyclization.

Author contributions

MMH: collected the literature survey, participate in drawing the figures, and revision. AYE-K and SHE: prepared the figures and participated in the revision. KME: wrote the manuscript text and participated in the revision. All authors reviewed the manuscript. AMAO: participated in the revision and the corrections requested by the reviewers.

Data availability

The datasets used are the available literature regarding the article topic.

Declarations

Competing interests

The authors declare no competing interests.

Conflict of interest

The authors state no conflict of interest.

Ethical approval

The research does not include dealing with any of the human and/or animal studies and therefore does not require the approval of ethical committees, internal review boards, and established guidelines.