## 1 Introduction  
Dementia is a serious disease that predominantly affects aging patients, especially the elderly population. Alzheimer's disease (AD) is one of the most common causes of dementia and is related to the loss of cognitive functions. Although the mechanism of AD pathogenesis is not fully understood, AD is principally caused by low acetylcholine (ACh) levels, oxidative stress, and \(\beta\)-amyloid plaque deposits [1]. Numerous studies have reported that cognitive functions are related to the cholinesterase (ChE) system, by breaking down the neurotransmitter ACh, and, therefore, ChE inhibition is effective for the treatment of AD [2]. ChE inhibitors suppress the degradation of this chemical messenger in the brain, which is important for memory and learning [3]. The cholinergic hypothesis suggests that the role played by ChE inhibitors in AD is related to neurotransmitter transfer [4]. AD is associated with decreases in neurotransmitter, particularly ACh, and significant increases in acetylcholinesterase (AChE) and butyrylcholinesteraseerase (BChE) levels [5]. In addition, monoamine oxidase (MAO) is responsible for the oxidative deamination of endogenous amines such as serotonin, epinephrine, and other neurotransmitters [6]. The amount of MAO increases with aging and neuronal damage, suggesting that MAO inhibition could be effective in treating AD. Importantly, oxidative stress, associated with free radicals and tissue damage, creates an obstacle in the repair of cognitive functions [7]. On the other hand, \(\beta\)-secretase (\(\beta\)-site amyloid precursor protein cleaving enzyme 1, BACE-1) is a protease that catalyzes the production and deposition of amyloid-\(\beta\) peptide, and it has been considered a promising target for the treatment of AD [8]. Drugs approved by the FDA for the treatment of AD are AChE inhibitors (donepezil, rivastigmine, and galantamine) and \(N\)-methyl-D-aspartate receptor antagonists (meman-time). However, these drugs are not able to stop the progression of AD and only alleviate the symptoms; therefore, further studies are needed in the field of medicinal chemistry to develop agents, which are more effective and show less side effects than the drugs currently used [1]. We examined the structures and possible interactions with AChE and planned to provide the amine group required for enzyme interactions with hydrazide-hydrazone moiety [1]. Therefore, we designed the compounds by taking advantage of the possible activities of parabens, hydrazide hydrazone moieties. Parabens are widely used as preservatives in the food, cosmetic, and pharmaceutical industries. They have low toxicity and effective antimicrobial activity [9], and the most frequently used are methyl-/ethyl-/propyl-paraben [10]. Hydrazides are remarkably effective in the field of pharmaceutical chemistry because they engage in many types of reactions to form different derivatives possessing diverse pharmacological profiles, including antiviral [11], antibacterial [12], antimalarial [13], and antitumor [14] activities. These activities highlight the importance of these compounds and derivatives in drug design. Hydrazones carrying an azomethine structure constitute an important class of compounds and are target molecules for various biological activities [15]. The hydrazide-hydrazone structure is known to have antimycobacterial, antibacterial, antifungal, and carbonic anhydrase inhibition activities [16; 17; 18; 19; 20]. With their wide range of activities, these compounds have been recognized as important building blocks in medicinal chemistry studies. In this study, hydrazide derivatives of ethyl paraben were synthesized in reactions with various benzaldehydes, resulting in 19 tosylated acyl hydrazide derivatives (**3a-t**), and their potential biological activities were evaluated by analyzing their inhibitory activities against MAOs, AChE, BChE, and BACE-1 enzymes, as well as their antioxidant activities.