Synthesis, Biological Evaluation, Molecular Docking and Molecular Dynamics of Substituted Thieno[2,3-d]pyrimidine Derivatives as Potential Anti-Alzheimer Agents.

Thienopyrimidine derivatives are emerging as potent scaffolds for cholinesterase inhibition in Alzheimer's disease therapy. In this work, a novel series of substituted thieno[2,3-d]pyrimidines was synthesized via Gewald's reaction, followed by cyclization and functionalization through nucleophilic substitution and hydrazone formation. Structural confirmation was achieved using spectroscopic techniques, and biological evaluation was performed against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), with donepezil and rivastigmine as reference drugs. Compound 4 emerged as the most potent and selective AChE inhibitor (IC50 = 0.58 µM), while compound 7 also showed strong AChE inhibition (IC50 = 0.63 µM). Notably, compound 9 exhibited superior BChE inhibition (IC50 = 3.05 µM) compared to donepezil (IC50 = 8.41 µM). Dual inhibitory activity was observed for compounds 5, 6, and 11, highlighting their multitarget potential. Molecular dynamics simulations (200 ns) and MM/GBSA binding free energy calculations provided mechanistic insights. Compound 4 showed the most favorable binding energy (ΔGbind = -59.16 kcal/mol), driven by hydrogen bonds with Tyr121 and Glu199 and π-π stacking with Trp83. Residue-level decomposition identified Tyr121, Trp83, Glu199, and Tyr338 as critical contributors to binding stability. Structure-activity relationship analysis confirmed that nitrogen-containing substituents and cyclic amino moieties enhance potency, whereas bulky aromatic groups reduce activity. These findings establish thieno[2,3-d]pyrimidine derivatives as promising candidates for the development of next-generation anti-Alzheimer agents.