Acetylcholinesterase inhibitory phloroglucinols from tropic Rhodomyrtus tomentosa

Four previously undescribed phloroglucinols, including three pairs of enantiomers, (±)-rhodotomentodimer F, (±)-rhodotomentodimer G, and (±)-rhodotomentomonomer E, and one phloroglucinol-sesquiterpene meroterpenoid, rhodotomentodione E, together with one previously reported congener, (±)-rhodomyrtosone A, were obtained from the leaves of Rhodomyrtus tomentosa. The structures including absolute configurations of previously undescribed isolates were elucidated by extensive spectroscopic analysis (HRESIMS and NMR), ECD calculations, and single-crystal X-ray diffraction. (±)-Rhodotomentodimer F is a rare phloroglucinol derivative conjugated by a β-triketone moiety and an unprecedented resorcinol unit via the formation of a rare bis-furan ring system, whereas (±)-rhodotomentomonomer E shares a rearranged pentacyclic scaffold. Pharmacologically, (±)-rhodotomentomonomer E showed the strongest human acetylcholinesterase (hAChE) inhibitory effect with an IC₅₀ value of 1.04 ± 0.05 μM. Molecular formula studies revealed that hydrogen bonds formed between hAChE residues Glu202, Ser203, Ala204, Gly121, Gly122, Tyr337, and His447 and (±)-rhodotomentomonomer E played crucial roles in its observed activity. These findings indicated that the leaves of Rhodomyrtus tomentosa can supply a rich source of hAChE inhibitors. These inhibitors might potentially be utilized in the therapeutic strategy for Alzheimer's disease, offering promising candidates for further research and development.