Two New C-benzylated Dihydrochalcone Derivatives from the Leaves of Melodorum siamensis

Two new C-benzylated dihydrochalcone derivatives, 4,2′,4′-trihydroxy-6′-methoxy-3′(2′′-hydroxybenzyl)dihydrochalcone ( 1 ) and 2′,4′-dihydroxy-4,6′-dimethoxy-3′(2′′-hydroxybenzyl)dihydrochalcone ( 2 ), along with six known flavonoid derivatives ( 3 - 8 ), a known dihydrochalcone dimer ( 9 ), three known aromatic esters ( 10 – 12 ), and one known aromatic amide ( 13 ), were isolated from the leaves of Melodorum siamensis . The structures of the compounds were elucidated by spectroscopic analysis, mainly 1D and 2D NMR techniques ( 1 H, 13 C, COSY, HMQC, and HMBC), as well as by comparison with literature data. The isolated compounds with a sufficient amount for biological assays were evaluated for their antimalarial, antimycobactirial, and cytotoxic activities. Compounds 1, 2 , and 13 exhibited strong cytotoxicity against human tumor cell lines KB and NCI-H187, with IC 50 values in the range of 0.66–7.16 µg/mL.