New Neuraminidase Inhibitory Alkaloids from the Mangrove Soil-Derived Fungus Arthrinium sp. SCSIO 41305

New alkaloid, (E)-2-(hydroxyimino)-4-methylpentanamide (1) and a new cyclopentanob pyridine, 4-hydroxy-7-methyl-6, 7-dihydro-5H-cyclopentacpyridin-5-one (2), together with ten known compounds (3 – 12) were isolated from the mangrove soil-derived fungus Arthrinium sp. SCSIO 41305. Extensive spectroscopic analysis and X-Ray crystallographic analysis were used to elucidate the structure of (E)-2-(hydroxyimino)-4-methylpentanamide (1), including its absolute configuration. All the isolated compounds (1–12) were evaluated for their antimicrobial and enzyme inhibitory activities against acetylcholinesterase (AChE), neuraminidase (NAs), and phosphatidylinositol 3-kinase (PI3K). Among them, compounds 1 and 3 exhibited strong neuraminidase inhibitory activity with IC₅₀ values of 12.04, 1.92 μmol L⁻¹ (IC₅₀ 20 μmol L⁻¹ for oseltamivir acid), while compounds 5, 6, 8, and 10 showed moderate neuraminidase inhibitory activity, and compounds 6 – 10 displayed weak enzyme inhibitory activities against PI3K.