Introduction

Peptidomimetics are synthetic compounds designed to mimic the structure and function of peptides. In recent decades, this field has been extensively explored as a promising tool for developing new and effective therapeutic agents [1]. Multicomponent reactions, such as the Ugi four-component reaction (Ugi-4CR), offer significant advantages over traditional multistep reactions, primarily by enabling the synthesis of complex organic molecules at lower cost and faster reaction times [2]. Ugi-4CR is a single-pot synthetic method that involves the condensation of an amine, an aldehyde or a ketone, an isocyanide, and a carboxylic acid to produce a wide range of peptidomimetic compounds [3]. Due to their structural diversity and potential biological activity, these products serve as valuable intermediates in pharmaceutical research and drug discovery. Ugi-4CR was employed to synthesize various active pharmaceutical ingredients, including amenamevir [4], lacosamide [5], carfentanil [6], clopidogrel [7] and ivosidenib [8] (Figure 1).

Figure 1.: Examples of marketed drugs prepared using Ugi-4CR chemistry; the amine, aldehyde, isocyanide, and acid components are depicted in blue, green, magenta and red, respectively

Various methods for performing Ugi-4CR have been described in the literature, including conventional solution-phase synthesis, mechanochemical, solid-phase, ultrasound, and microwave-assisted methods [9]. Microwave synthesis can be a greener alternative to conventional synthetic methods in organic chemistry because microwave heating is more energy-efficient, directly heating the reaction mixture and leading to shorter reaction times and reduced need for organic solvents [10]. In recent years, microwave-aided synthesis has emerged as a promising synthetic method with broad applications in peptide [11] and polymer [12] synthesis and nanotechnology [13].

Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) are enzymes that play a critical role in the hydrolysis of various choline-based esters, including the neurotransmitter acetylcholine. AChE is predominantly found in neural tissues, but also in the muscles, heart, and blood, where it is bound to erythrocytes [14]. On the contrary, BChE is widely distributed throughout the body, with the highest levels in plasma and liver, and significant levels also in the brain, heart, lungs, intestines, and kidneys [15-16]. While AChE plays a crucial role in terminating synaptic transmission, the function of BChE is still being unravelled. BChE serves as a crucial backup for AChE by assisting in regulating cholinergic signalling and detoxifying certain drugs and toxins [17]. Dysregulation of AChE activity has been closely associated with neurodegenerative diseases, particularly Alzheimer’s disease. In Alzheimer’s disease, there is a decrease in acetylcholine (ACh) levels and accumulation of amyloid-beta plaques, which contribute to synaptic dysfunction and cognitive decline [18]. To counteract this, cholinesterase inhibitors such as donepezil, rivastigmine, and galantamine are used to enhance cholinergic signalling by preventing the breakdown of acetylcholine [19]. With the progression of Alzheimer’s disease, the ratio of AChE to BChE shifts in favour of BChE, so targeting BChE, as a backup to AChE, offers a promising therapeutic strategy for symptomatic treatment of the disease [20]. While AChE inhibitors have been extensively studied, both progressive (toxic carbamates and organophosphates) and reversible (e.g. tacrine, donepezil), BChE inhibitors have only been systematically investigated over the past few decades. For instance, BChE activity and selectivity were assessed for a series of compounds: thiazole analogues, tetrahydroacridine derivatives with a fluorobenzoic acid moiety, 4-dimethylamine flavonoid derivatives, graveolinine derivatives [21] and bambuterol [22-23]. While most compounds showed low micromolar IC50 values (1-10 μM), several tetrahydroacridine derivatives also exhibited nanomolar potency (below 10 nM). Furthermore, the inhibitory potential of heterocyclic scaffolds containing nitrogen, oxygen, and sulfur was explored, demonstrating that effective inhibitors can be achieved with diverse structural frameworks [24]. Thus, using the Ugi reaction, a diverse library of oxindole-lactam hybrids was recently synthesized, demonstrating promising BChE inhibitory activity in the low micromolar range [25], as well as dehydroabietylamine-derived bistetrazoles [26]. Moderate activity toward cholinesterase was demonstrated with Ugi products based on ortho-tolyl-isocyanide [27], and Ugi cinnamic adduct was discovered in in silico study to be a possible binder of butyrylcholinesterase enzyme [28].

Driven by our interest in developing highly selective BChE inhibitors [29-31] and expanding the knowledge of Ugi product-derived cholinesterase inhibitors, we employed Ugi 4-CR to synthesize peptide-like compounds via conventional and microwave-assisted methods. By varying the stereoelectronic properties of reaction components, a series of compounds with novel molecular scaffolds was prepared and evaluated as potential BChE inhibitors. Inhibition of human AChE (hAChE) and human BChE (hBChE) was evaluated, demonstrating that the molecular scaffold of the prepared compounds provides a robust platform for further optimization of selective hBChE inhibitors.