One new flavanone, 4'-hydroxyisorottlerin (2), and two new chalcone derivatives, kamalachalcones C (3) and D (4), were isolated from Mallotus philippensis (kamala tree). The largest compound (4; M r 1098 g/mol) was shown to possess a unique, fused-ring system made of two hydroxy-chalcone units, giving rise to eight fused benzene/pyran units. From the same plant, the following six known compounds were also isolated: kamalachalcone A (5) and B (6), isoallorottlerin (7), isorottlerin (8), 5,7-dihydroxy-8-methyl-6-prenylflavanone (9); 6,6-dimethylpyrano(2'',3'': 7,6)-5-hydroxy-8-methylflavanone (10), and rottlerin (1). The structures of the new compounds were confirmed by in-depth spectral analyses, including 2D-NMR techniques, and the full 13 C-NMR assignments of the known flavanones 1 and 7 ± 10 are published for the first time.
Gnetol (2,3?,5?,6-tetrahydroxy-trans-stilbene), a naturally occurring compound particularly found in the genus Gnetum, had a strong inhibitory eŠect on murine tyrosinase activity. Gnetol (IC 50 , 4.5 mM) was stronger than kojic acid (IC 50 , 139 mM) as a standard inhibitor for murine tyrosinase activity. Moreover, gnetol signiˆcant-ly suppressed melanin biosynthesis in murine B16 melanoma cells.
Five new stilbene dimers were isolated from the lianas of Gnetum parvifolium in addition to known stilbenoids. The structures of the compounds were established on the basis of spectroscopic evidence, including long-range coupling and nuclear Overhauser effect experiments, in NMR spectrum. Among the isolates, 2b-hydroxyampelopsin F showed potent inhibitory activity in the Maillard reaction.
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