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 cyclopentano[ b ] pyridine, 4-hydroxy-7-methyl-6,7-dihydro-5 H -cyclopenta[c]pyridin-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 IC 50 values of 12.04, 1.92 µmolL −1 (IC 50 20 µmolL −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.