Flavonoid C-glycosides from star fruit and their antioxidant activity

• Six C-glycosides and an O-glucoside of flavonoid were isolated from star fruit.
• Their structures were identified by spectroscopic and chemical methods.
• Four were new and three others were reported from star fruit for the first time.
• All the compounds showed potent ABTS radical cation scavenging activity.

Star fruit (the fruit of Averrhoa carambola L., Oxalidaceae) is popular in tropical regions. Chemical study of the fruit yielded four new dihydrochalcone C-glycosides, carambolasides A (1), B (2), C (3), and D (4), in addition to hovertichoside C (5), isovitexin 2″-O-α-l-rhamnopyranoside (6), and carambolaflavone (7). Their structures were identified by spectroscopic and chemical methods, including electronic circular dichroism (ECD) calculation. Compounds 5–7 were reported from the fruit for the first time. Compounds 1–7 were evaluated for antioxidant activity using three assays. All of them exhibited 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) radical cation scavenging activity with IC50 values ranging from 7.6 to 3.9 µM, which were more potent than l-ascorbic acid (IC50 = 11.6 µM). However, they were inactive in scavenging 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and in ferric reducing antioxidant power (FRAP).