Chemical constituents from Tribulus terrestris and screening of their antioxidant activity

Abstract Two oligosaccharides ( 1 ,  2 ) and a stereoisomer of di- p -coumaroylquinic acid ( 3 ) were isolated from the aerial parts of Tribulus terrestris along with five known compounds ( 4 – 8 ). The structures of the compounds were established as O -β- d -fructofuranosyl-(2 → 6)-α- d -glucopyranosyl-(1 → 6)-β- d -fructofuranosyl-(2 → 6)-β- d -fructofuranosyl-(2 → 1)-α- d -glucopyranosyl-(6 → 2)-β- d -fructofuranoside ( 1 ), O -α- d -glucopyranosyl-(1 → 4)-α- d -glucopyranosyl-(1 → 4)-α- d -glucopyranosyl-(1 → 2)-β- d -fructofuranoside ( 2 ), 4,5-di- p - cis -coumaroylquinic acid ( 3 ) by different spectroscopic methods including 1D NMR ( 1 H, 13 C and DEPT) and 2D NMR (COSY, TOCSY, HMQC and HMBC) experiments as well as ESI-MS analysis. This is the first report for the complete NMR spectral data of the known 4,5-di- p - trans -coumaroylquinic acid ( 4 ). The antioxidant activity represented as DPPH free radical scavenging activity was investigated revealing that the di- p -coumaroylquinic acid derivatives possess potent antioxidant activity so considered the major constituents contributing to the antioxidant effect of the plant.