Synthesis and antioxidant activity of 2-methylthio-pyrido[3,2-e][1,2,4] triazolo[1,5-a]pyrimidines

Abstract A series of 2-methylthio-pyrido-triazolopyrimidines (1-17) were prepared by the reaction of dimethyl-N-cyanoimidodithiocarbonate with hydrazinopyridine carboxylic acid as starting reactants. Their chemical structures were affirmed with HREI-MS, IR and NMR analyses. The target compounds (1-17) were evaluated for their antioxidant activity using 1,1-diphenyl-2-picryl hydrazyl (DPPH) radical scavenging, ferric reduction antioxidant power (FRAP) and reducing power capability (RPC). The results revealed that some pyrido-triazolopyrimidines showed good activity as antioxidant agents, in particular, compounds 12 and 15 were found to possess good antioxidant activity. Butylated hydroxyl toluene (BHT) was used as reference drug.


Introduction
Extensive research in the organic-medicinal chemistry field has led to the discovery of different classes of bioactive substances, most of them being nitrogen-containing heterocycles. Within the huge number of nitrogenous heterocyclic products, in particular those containing quinazoline, triazole, triazine, benzoquinazoline and triazoloquinazoline platforms, have been broadly scouted, showing to be highly versatile motif during the identification of new worthy and selective bioactive compounds for medicinal purposes.
One of the most challenging proceedings in the medicinal chemistry range is to explore and elaborate more potent drugs with only slight toxic effects and completely reversible. Thus, it is essential to consider the physiochemical features in the design and development of new bioactive constituents as well as their mechanism of action. Hundreds of thousands of new heterocyclic organic compounds are synthesized per year around the globe; many of them are submitted to biological screening for determination of their activities. These random investigation processes have been misfit, but they have led to discovery of new lead compounds whose structures have been optimized and developed to produce clinical agents.
Triazolo-annelated quinazolines were found to be potent adenosine antagonists, herbicidal and fungicidal agents [1][2][3]. In addition, further chemical transformation of their inherent lactam groups and substituent types at positions 2 and 5 were offered access to a variety of derivatives that have shown antihistamine, antimicrobial, antiviral, antihypertensive, anti-inflammatory, antitumor and antioxidant activities [4][5][6][7][8][9][10][11]. In light of the abovementioned facts, we are interested in the conversion of triazolquinazoline moiety into a pyridotriazolopyrimidine scaffold. This scaffold has already been reported as a potent fungicidal and herbicidal agent [2,3] and incorporated in various substances that displayed interesting antimicrobial activity [12]. The emergence of various diseases resulting from oxidative stress such as diabetes, cancer, stroke, and cardiovascular dysfunctions, requires a permanent revaluation search and adjustment of adequate strategy to design and develop new bioactive substances to protect the human body against such diseases. In continuation of our on-going program research dealing with antioxidants, this research deals with the preparation and characterization of 2-methylthio-pyrido-triazolopyrimidines (1-17); thereafter, evaluating their antioxidant activity using different assays.  13

Preparation of pyrido-triazolopyrimidines 13-16
Over a period of 10 min at RT, K 2 CO 3 (1 mmol) was added to a stirred solution of 12 (0.5 mmol) in DMF (8 mL). Then, alkylhalides (1.5 mmol) was added and kept stirring in the reaction mixture at 80 o C for 20 h. The mixture was poured into ice/water, the solid collected and dried.

Antioxidant assays
In our previous paper [8,13], we have reported in detail the methodology routes for determination of DPPH radical scavenging activity %, FRAP values and reducing power of the target compounds.
Ethical approval: The conducted research is not related to either human or animal use.

Chemistry
The 3-amino-5-(methylthio) -1, 2, 4-triazole or 5-amino-N-(2,6-dichlorophenyl)-1,2,4-triazole-3-sulfonamide was reacted with 2-chloronicotinic acid to afford the target (1) has been described [2,3]. Herein, the synthesized 2-hydrazinonicotonic acid (A) [14] was reacted with dimethyl-N-cyanoimidodithio carbonte to give the intermediate (B) (Scheme 1). Then, the corresponding 2-methylthio-pyrido-triazolopyrimidin-5one (1) was obtained in a good yield of 74% by treatment of the intermediate B with Conc. HCl. The target 1 was characterized by NMR, IR and MS spectral analyses. The IR spectrum of 1 was characterized by a strong stretching (C=O) peak at 1684 cm -1 . Reaction of 1 with alkyl halides furnished the N-alkylated pyrido-triazolopyrimidines (2)(3)(4)(5)(6)(7)(8)(9)(10)(11) in 69-81% yields (Scheme 1, Table 1). The IR spectra of 2-11 showed the characteristic (C=O) group absorption bands at 1670-1688 cm -1 . Boiling equimolar amounts of 1 and P 2 S 5 in pyridine for 3-4 h has furnished the respective 2-methylthio-pyrido-triazolopyrimidin-5-thione (12) as yellow solid in yield of 90%. The structure of product 12 was proved with IR and 13 C NMR analyses, where C=S group exhibited an intrinsic a weak absorption peak at 1248 cm -1 and a resonance peak at 186.1 ppm, respectively. The expected thioether products (13)(14)(15)(16) were obtained from the reaction of 12 with the appropriate alkyl halides in basic medium. The carbonyl group of 1 was successfully transformed into imidoyl chloride group in 17 by employing POCl 3 as a chlorinating agent. Complete disappearance of the carbonyl peak at 1684 cm -1 in the IR spectrum of 1, confirmed the production of 17. Taking into account the NMR data of structural related compounds [9,15,16], the parent structure 2-methylthio-pyrido-triazolopyrimidine and its derivatives were simply confirmed by 1 H NMR, that showed one AMX spin coupling system and a singlet of a 2-methylthio group at about 2.65 ppm that was recorded at about 14 ppm in all 13 C NMR spectra. This spin coupling system was interpretable for the three protons of pyridoring, i.e. H-8 and H-6 (often broad doublet, each) and H-7 (dd) at about δ 8.9 -7.5 ppm. In all 13 C NMR spectra, the parent structure of all products exhibited nine signals, including the most downfield thionated (C-2) and lactam-carbonyl carbon (C-5) at about 163 and 160 together with the characteristic three aromatic -CH-carbons at about 146, 139 and 113 ppm for C-8, 6 and 7 (1 and 2-11). The C-5 showed a characteristic δ-value at 186.1 ppm for thiocarbonyl function in compound 12, while in the thioalkyl/aralkyl derivatives 13-16 it was assigned as the most downfield carbon at about 168 ppm.

Antioxidant activity
The DPPH, FRAP and RPC assays were applied to assess the in vitro antioxidant activity of the pyrido-triazolopyrimidines (1-17). The obtained results were presented in figures 1-3 and compared with standard synthetic antioxidant. The target pyrido-triazolopyrimidines (1-17) showed various antioxidant activities relative to BHT. Figure: 1 presented the pyrido-triazolopyrimidines (1-17) inhibitory effects at a concentration of 0.5 mg/ml, the same as DPPH radical. The pyrido-triazolopyrimidines 12 and 15 were found to exhibit high DPPH radical scavenging activity among the tested products, however, lower than BHT. showed the lowest activity. From the summarized results, it was noticed that the targets antioxidant is correlated with substituents on the pyridopyrimidine ring. As presented in Table 2, the highest DPPH scavenging activity of 12 and 15 when compared with other pyrido-triazolopyrimidines, may be due to the existence of a thiocarbonyl group in 12 and its transformed thiother in 15, however, compound 15 appeared to be the most active. In accordance with our reported antioxidant results [13], compound 12 could have the same behavior as the reported benzotriazoloquinazolines and expected to undergo sequential proton loss electron transfer (SPLET) mechanism as illustrated in scheme 2. Thus, compound 12 may be showing a lower DE than the others. Each value is expressed as mean ± SD (n = 3). Each value is expressed as mean ± SD (n = 3). Each value is expressed as mean ± SD (n = 3). Antioxidant activity (µmol Trolox /100 g). Generally, in redox reactions of organic compounds, if a C-atom loses a bond to H and gains a bond to a heteroatom (or to another C-atom), it is considered to be an oxidation reaction because the H-element is the least electronegative one. Thus, in the dehydrogenation process the C-atom undergoes an overall loss of electron density and loss of electrons is oxidation. Contrary, in the hydrogenation or reduction of an organic compound a C-atom forms a new bond to H and loses a bond to a heteroatom (or to another C-atom). Accordingly, chemical modification on the parent 1 furnished 2-17 with high diversity in antioxidant activity; in particular, the conversion of C=O in 1 into C=S in 12, has resulted in the increase of antioxidant effects. Similarly, the formation of 13-16 derivatives from 12 positively improved in the activity profiles, which is probably explained due to the presence of an electron-rich sulfur-containing moiety (-S-R).

Conclusions
A total of seventeen new 2-methylthio-pyridotriazolopyrimidines were successfully prepared by the reaction of dimethyl-N-cyanoimidodithiocarbonate and hydrazine-pyridine carboxylic acid as the reactants. In all three assays used, the 5-thione (12) and 5-thiobenzyl (15) products have shown the strongest antioxidant effects.