The endocannabinoid system comprises a complex network of lipid signaling mediators in which different proteins participate in the modulation of numerous physiological and pathophysiological processes ^[1][2][1,2]. Cannabinoid receptor type II (CB2R) belongs to the family of heptameric receptors coupled to G proteins (GPCRs). This receptor was identified and cloned from HL60 cells [3] and was initially considered as ‘peripheral cannabinoid receptor’ due to its wide distribution in peripheral cells and tissues, particularly in those of the immune system ^[4][5][4,5]. However, later studies showed its expression also within the Central Nervous System (CNS) especially under states of inflammation ^[4][6][4,6]. Various studies have shown that the activation of CB2R can block activation of microglia cells but has little effect on the normal functioning of neurons within the CNS ^[7][8][7,8]. Several reports indicate that the activation of CB2R is analgesic and CB2R agonists have been shown to suppress responses in animal models of both acute and neuropathic pain ^[5][9][10][5,9,10]. Additionally, cannabinoids have well-established anti-inflammatory properties and recently, effects in the gut-lung-skin barrier epithelia have been reported showing promising results in in vitro and in vivo animal studies [11]. Furthermore, the endocannabinoid system is intimately related to neurological function and neurodegenerative diseases with animal models studies showing beneficial effects for the treatment of brain injuries and multiple sclerosis [12]. Therefore, CB2R agonists represent potential alternatives for the treatment of pain and inflammation both in the peripheral and CNS [13].

The discovery of the CB2R directed research efforts towards the understanding of its role and action. Several reports on the structural requirements for ligand binding to the receptor led to the discovery of many different families of cannabinoid ligands including classical cannabinoids structurally related to THC, eicosanoids analogous to endocannabinoids and synthetic cannabinoids, most of the latter being heterocycles, aminoalkylindoles (represented by WIN55212-2), arylpyrazoles, quinolones and pyridone carboxamide derivatives.

Heterocyclic compounds represent an important source of pharmacologically active molecules and more than 85% of all biologically active compounds contain heterocyclic scaffolds [14]. They are frequently used to alter physicochemical properties of molecules such as lipophilicity, polarity and hydrogen bonding capacity which can improve the pharmacodynamic and pharmacokinetic profile [15]. The pyridone heterocycle is a 6-membered aromatic ring with a carbonyl group and a nitrogen heteroatom which has found great use in drug discovery strategies [16]. Relevant characteristics associated to this structure have been described by Y. Zhan and A. Pike, such as its ability to act as both a hydrogen bond acceptor and donor; act as a bioisostere of amides, phenyls and other nitrogen and oxygen-containing heterocycles, and the capacity to modulate the lipophilicity, solubility, and metabolic stability [16].

Previous reports have explored the 2-pyridone scaffold in the cannabinoid system particularly in the CB2R with promising results (Figure 1) ^{[17][18][19][20][21][22]}[17,18,19,20,21,22]. Kusakabe et al., reported a 2-pyridone-based compound displaying high CB2R affinity and selectivity. They proposed that the pyridone scaffold could provide optimal lipophilicity for the design of CB2R ligands and predicted possible hydrophobic interactions with W194 and F117 [19].

2. Chemistry

All compounds were synthesized as shown in Scheme 1. Firstly, three N-aryl-4,6-dimethyl-2-oxo-1,2-dihydropyridine-3-carboxylic acids were synthesized from compound 3 using different substituted anilines (step c, Scheme 1) to obtain the corresponding substituted amides 4a, 4b and 4c. These amides were cyclized using acetylacetone and piperidine as a catalyst to yield compounds 5a, 5b and 5c. The pyridone derivatives were then hydrolyzed using potassium hydroxide in ethanol 80% under reflux heating obtaining the carboxylic acids derivatives 6a, 6b and 6c. It is noteworthy to mention that hydrolysis of the nitrile derivatives proved to be harder than expected and both acid and basic conditions were studied. As described in Scheme 2, different side products were obtained depending on the reaction conditions. Only decarboxylated product 9a was obtained under acidic medium while under basic medium, the obtained product depended upon the reaction temperature. Heating the reaction below 100 °C stopped the reaction at the amide intermediate 10a whereas heating the reaction above 100 °C completely hydrolyzed the precursors to product 6a (95% relative yield). The carboxylic acid derivatives 6a, 6b and 6c were finally reacted with different cycloalkyl amines (cyclohexylamine, cycloheptylamine and 1-adamantylamine), according to Scheme 3. The products 7 and 8 were obtained using the same synthetic procedure whereby the respective amines were coupled to the carboxylic acid in the presence of BOP as coupling reagent. The relative yield for compounds 7 varied between 50% and 70% and for compounds 8 varied between 70 and 80%.