Isolation of Six Isoprenylated Biflavonoids from the Leaves of Garcinia subelliptica

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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 C5 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.