Prenylated flavanones from Monotes engleri:: On-line structure elucidation by LC/UV/NMR

The CH2Cl2 extract of Monotes engleri GILG. (Dipterocarpaceae) showed antifungal activity against the yeast Candida albicans in our bioautographic TLC assays. After a first fractionation of the crude extract, the bioactivity was located in one of the fractions. To perform an efficient targeted isolation of the active compounds. LC/UV/MS and LC/UV/NMR analyses of the crude extract and the active fraction were performed. LC/UV.LC/MS, and LC/NMR data (1D and 2D) allowed the identification of 1 as (2S)-2,3-dihydro-5.7-dihydroxy-2-{3-hydroxy-4-[(3-methylbut-2-enyl)oxy]phenyl}-4H-1-benzopyran-4-one, a new prenylated flavanone, named monotesone A. Subsequent isolation of 1 has permitted the determination of its absolute configuration on the basis of CD measurements. Three other prenylated flavanones 2-4 were isolated from the same extract. Compound 3 was identified as 2-(3,5-dihydroxyphenyl)-2,3-dihydro-5,7-dihydroxy-6,8-bis(3-methylbut-2-enyl)-4H-t-benzopyran-4-one, another new natural product, named monotesone B. The structures of 2 and 4 were established as selinone and lonchocarpol A, respectively. The antifungal activity against Candida albicans was determined for all compounds.