Metal-free synthesis of functionalized tacrine derivatives and their evaluation for acetyl/butyrylcholinesterase and α-glucosidase inhibition‡

A mild and greener protocol was developed for C–C (C(sp3)−H functionalization) and C–N bond formation to synthesize functionalized tacrine derivatives using a biodegradable and reusable deep eutectic solvent [(DES) formed from N,N′-dimethyl urea and L-(+)-tartaric acid in a 3 : 1 ratio at 80 °C]. The condensation of 9-chloro-1,2,3,4-tetrahydroacridines with a variety of aromatic aldehydes gave unsaturated compounds via C(sp3)−H functionalization (at the C-4 position) with good yields. The substituted N-aryl tacrine derivatives were obtained from the condensed products of 9-chloro-1,2,3,4-tetrahydroacridine with substituted anilines via the nucleophilic substitution reaction (SN2 type) in the DES with good yields. This is the first example of C4-functionalized tacrine derivatives, highlighting the dual capacity of the DES to serve as both a catalyst and a solvent for facilitating C–N bond formation on acridine. The generated compounds were evaluated for acetyl/butyrylcholinesterase (AChE/BChE) and α-glucosidase inhibitory activity. It was found that the majority of the compounds reported here were significantly more potent inhibitors than the standard inhibitor tacrine (AChE IC50 = 203.51 nM; BChE IC50 = 204.01 nM). Among the compounds screened, 8m was found to be more potent with IC50 = 125.06 nM and 119.68 nM towards AChE and BChE inhibition respectively. The α-glucosidase inhibitory activity of the compounds was tested using acarbose as a standard drug (IC50 = 23 100 nM) and compound 8j was found to be active with IC50 = 19 400 nM.