The potential of nitro group containing compounds as bioactive substances has been widely reported over the years, with nitroimidazole compounds being of most importance [1]. The structural activity relationship has also been reported whereby the position of the nitro group within the imidazole ring has a large influence on the activity [2]. In general, 2-nitroimidazole derivatives (e.g. benznidazole) have trypanocidal and trichomonacidal activities; 4-nitroimidazoles only have immunosuppressive action; and 5-nitroimidazoles (e.g. metronidazole) are responsible for the greatest therapeutic success of this class, having compounds with antiparasitic and antimicrobial action amongst other biological actions [3], [4].

Neglected tropical diseases (NTDs) involve a group of diseases present in 149 countries, which are directly related to social and economic fragility. There are several diseases that are classified by the World Health Organisation [5] in this group which are caused by viruses, bacteria or parasites. These NTDs are accepted to currently affect about one billion people, mainly in the tropical zones of the globe where the most vulnerable populations of developing countries are concentrated [5]. Chagas disease which is caused by the parasite Trypanosoma cruzi, (T. cruzi) is most prominent in Latin America, where approximately 6 million people are infected by the parasite and at least 50,000 deaths annually are reported [6]. Infection with this parasite in humans can cause significant health problems including fatal cardiomyopathy and damage to the gastrointestinal tract [7], [8], [9], [10], [11]. There is currently no vaccine available, therefore drug treatment with nitroheterocycles such as benznidazole, nifurtimox and fexinidazole are the only options [12], [13], [14], [15], [16]. However, due to side effects and prolonged administration, they are not considered to be a practical or particularly effective treatment system for Chagas disease [6].

There are many research groups (including ours) that are studying compounds for the treatment of T. cruzi., and some interesting papers include: Mello et al. [17] who reported 4-nitroimidazole analogues of benznidazole (Fig. 1A), Sánchez-Pavón et al. [18] who reported bromine containing nitroimidazoles (Fig. 1B), Boechat et al. [19] studying substituted 2-nitroimidazole (Fig. 1C) and Von Trompowsky et al. [20] who reported analogues of megazol (Fig. 1D).

Research and development of new drug candidates with sufficient pharmacological efficacy is required to combat Chagas disease. As previously mentioned, the nitro group plays an important role in the activity of metronidazole – it is thought that a reversible radical nitro anion is formed which causes damage to biological moieties, hence making the molecule effective [21], [22]. For this reason, we have postulated and investigated whether the nitration of eugenol / nitroimidazole hybrids could increase the activity of the hybrids in the same manner. In addition to eugenol, we have also studied dihydroeugenol and guaiacol moieties as they are structurally similar.

We evaluated the possibility of functionalising natural phenols with a nitro group through regioselective nitration (both ortho and meta to the phenolic hydroxyl), using methodologies already in the domain of our research group [6], [23], [24], [25], [26]. Eugenol (2-methoxy-4-(2-propenyl)phenol) is present in the essential oil of cloves, nutmeg and cinnamon; and the literature reports several pharmacological properties such as antibacterial, antifungal, antiparasitic, cytotoxic and antioxidant properties [27], [28], [29], [30], [31]. The structural analogue dihydroeugenol has been studied in order to further elucidate the structure activity relationship. Guaiacol (derived from the guaco leaf) with analgesic, antioxidant and antimicrobial properties [32], presents a great structural similarity to eugenol and dihydroeugenol, and has also been studied.

The project plan involves the condensation of a nitroimidazole unit with different natural phenols to obtain a new structural pattern – with a single molecule containing the two units [33]. There are studies that demonstrate that this type of hybridisation strategy generates a therapeutic gain, as the new hybrid compound often has greater affinity and effectiveness compared to the individual subunits [34]. Two different coupling methodologies have been explored in order to analyse the influence of the connector on bioactivity:

“Click” chemistry using a triazole connector – produces the AC series of compounds (Fig. 2). “click” chemistry is a concept introduced in 2001 by K. Barry Sharpless to describe thermodynamically favourable reactions capable of connecting two molecules by forming a triazole ring in a simple way and with high yields [35]. Using the triazole compound as a connector has a number of benefits; they have recognised pharmacological activity and good chemical stability to remain inert during reactions [35], [36], [37], [38], [39].

Classic direct etherification – produces the AD series of compounds (Fig. 2). For comparison, this direct coupling methodology ensures no presence of a connector between the subunits, which allows a good comparison to analyse the influence the triazole connector has on activity.

Our research group has previously reported the synthesis of compounds AC1 - AC4 which informed the direction of this research [40]. Those previously reported compounds are reproduced in this work as they allow a full picture of the structural-activity relationship. The in silico studies of all compounds are reported here for the first time. The general hybrid structures proposed in this study (Fig. 2) are expected to be new prototypes for studies against infectious agents such as anaerobic / microaerophilic microorganisms, parasites and protozoa, many of which cause diseases which are considered to be NTDs [41].