The synthesis of analogues and bioisosteres of the acetylcholinesterase inhibitor donepezil in an attempt to identify novel anti-Alzheimer’s agents

Abstract

As part of an in-house project, the CSIR-housed BioFocus library composed of approximately 20’000 thousand compounds were screened for activity against AChE by van der Westhuizen (PhD candidate) using a combination of in silico and in vitro techniques. Three scaffolds were isolated from this study which showed activity against acetylcholinesterase (AChE) and were chosen as chemical inspiration for this project. Twenty-six virtual scaffolds were designed based on these three compounds and analysed in an in silico screening process to determine which compounds would have comparable or improved activity towards AChE. The affinity these compounds exhibited towards the enzyme AChE was quantified as a docking score. These scores were generated by assessing the stability of the compound-enzyme complex, which could be interpreted similarly to an energy state, and exhibits a more favourable association as a lower value. The compounds were also analysed in terms of physical and pharmacokinetic parameters to assess how likely the compounds were to be successful pharmaceuticals. All compounds were predicted to be blood-brain barrier penetrants and exhibited reasonable affinity towards AChE. Three compounds, N,1-dibenzylpiperidine-4-carboxamide, N-benzhydryl-1-benzylpiperidine-4-carboxamide and 1-benzyl-N-(1,2-diphenylethyl)piperidine-4-carboxamide displayed docking scores of -12.89, -9.84 and -14.09 respectively, and were predicted to be potential AChE inhibitors, in addition they were also predicted to be pharmacologically attractive. The synthesis of compounds N,1-dibenzylpiperidine-4-carboxamide and N-benzhydryl-1-benzylpiperidine-4-carboxamide proved to be successful and these compounds could be isolated with good purity in 7% and 16% yield respectively. The synthesis of 1-benzyl-N-(1,2-diphenylethyl)piperidine-4-carboxamide provided a plethora of challenges and could not be successfully isolated. N-Benzhydryl-1-benzylpiperidine-4-carboxamide was selected as the compound with the best combination of activity, pharmacokinetic parameters and synthetic accessibility, and which had room for improvement under structure activity relationship studies. An in silico structure activity relationship study was undertaken looking at the substitution of the N-benzhydryl ring system with aliphatic, aniline or substituted benzyl ring systems. It was shown that benzyl ring systems in general were more attractive showing better docking scores with more attractive pharmacokinetic profiles. An automated library synthesis of these and related compounds was envisaged using flow chemistry. The N,1-dibenzylpiperidine-4-carboxamide scaffold was successfully translated to flow and the synthesis of an additional three analogues were successfully demonstrated. Unfortunately, due to the global COVID-19 shutdown the flow process could not be fully optimized and entire library envisaged could not be synthesized. Ultimately, 1-benzyl-N-(4-methoxybenzyl)piperidine-4-car-boxamide, 1-benzyl-N-(4-(trifluoro-methyl)benzyl)piperidine-4-carboxamide, 1-benzyl-N-(4-fluoro-benzyl)piperidine-4-carboxamide and 1-benzyl-N-(4-methylbenzyl)piperidine-4-carboxamide were successfully synthesized using the automated method in low yields but good purity.