We used solvent extractions, SiO2 column chromatographies, and HPLC to isolate from the bark of Thanakha (Hesperethusa
crenulata L.) an active crystalline compound for absorbing UV-A radiation (320 to 380 nm). Analyses of lowand
high-resolution FAB-MS revealed a compound, named marmesin, with a formula of C14H1404 and a molecular
mass of 246. To determine its chemical structure, we conducted 300 MMz NMR analyses using various probes, 1H,
13C, and DEPT 13C. Our NMR data showed a structure of 2,3-dihydro-2(1 -hydroxy.1 -methylethyl)-furanocoumarin. This
active compound contains UV-absorbing chromophores, an aromatic ring, a double bond at C3-C4, and a carbonyl at
C2. Its ~m~ is 335 rim, indicating that marmesin could be commercially useful as a natural UV-A-filtering product.