Isolation of six isoprenylated biflavonoids from the leaves of Garcinia subelliptica.

The acetone-soluble parts of Garcinia subelliptica leaves were analyzed and six new biflavonoids were isolated, i.e., garciniaflavones A-F (1-6), as well as the five known biflavonoids amentoflavone (7), podocarpusflavone A (8), (+)-morelloflavone (9), (+)-morelloflavone-7"-O-β-glucopyranoside (10), and (+)-4'''-O-methylmorelloflavone (11) and the three triterpenoids oleanan-3-one, β-amyrin, and cycloartenol. The structures of the isolates were established based on spectroscopic analyses, including a detailed NMR spectroscopic investigation. The new biflavonoids are rare mono-isoprenylated derivatives that have a flavone-(3'-8")-flavone core (1-4: amentoflavone type) and a flavanone-(3-8")-flavone core (5, 6: morelloflavone type). The absolute configurations of the morelloflavone-type biflavonoids (5, 6) were confirmed by circular dichroism to be 2R,3S. The biflavonoids with an isoprenyloxy group (1) and a 2-hydroxy-3-methyl-3-butenyl group (2), and the morelloflavone-type biflavonoids with a C(5) unit are the first examples in nature. We found that 7, one of the major biflavonoids, strongly inhibited hypoxia-inducible factor-1 in human embryonic kidney 293 cells under hypoxic conditions.