Huperzine A (1, Hup A), a lycodine-type Lycopodium alkaloid isolated from Thai clubmosses Huperzia squarrosa (G. Forst.) Trevis., H. carinata (Desv. ex. Poir.) Trevis., H. phlegmaria (L.), and Phlegmariurus nummulariifolius (Blume) Chambers (Lycopodiaceae), exerts inhibitory activity on acetylcholinesterase, a known target for Alzheimer’s disease therapy. This study investigated the structure–activity relationship of C(2)-functionalized and O- or N-methyl-substituted huperzine A derivatives. In silico-guided screening was performed to search for potential active compounds. Molecular docking analysis suggested that substitution at the C(2) position of Hup A with small functional groups could enhance binding affinity with AChE. Consequently, 12 C(2)-functionalized and four O- or N-methyl-substituted compounds were semi-synthesized and evaluated for their eeAChE and eqBChE inhibitory activities. The result showed that 2-methoxyhuperzine A (10) displayed moderate to high eeAChE inhibitory potency (IC₅₀ = 0.16 μM) with the best selectivity over eqBChE (selectivity index = 3633). Notably, this work showed a case of which computational analysis could be utilized as a tool to rationally screen and design promising drug molecules, getting rid of impotent molecules before going more deeply on labor-intensive and time-consuming drug discovery and development processes.

Two new Lycopodium alkaloids, squarrosine A (1) and pyrrolhuperzine A (2), were isolated from the Thai and Philippine plant Huperzia squarrosa. (R)-2-Piperidineacetic acid (5) was a known alkaloid, but has now been isolated for the first time from a natural source. Their structures were elucidated using extensive spectroscopic analyses and, for pyrrolhuperzine A (2), confirmation by chemical transformation. The new compounds exhibited moderate acetylcholinesterase inhibitory activities. ^* Dedicated to Professor Dr. Dr. h. c. mult. Kurt Hostettmann in recognition of his outstanding contribution to natural product research.