A phytochemical study of Hopea hainanensis has led to the isolation of three new polyphenols and one known compound. The most important of these compounds are hopeahainols A (2) and B (3), which contain an unprecedented carbon skeleton. The structures were elucidated by analysis of the spectroscopic data including single-crystal X-ray spectroscopy and computational methods. Hopeahainol A was an acetylcholinesterase inhibitor with an IC50 value of 4.33 μM, which is comparable to that of huperzine A, a presently prescribed drug for the treatment of Alzheimer, while other similar structures were inactive. This observation was complemented by a 3D interaction model of the inhibitor with active sites.