Selective NMR Characterization of a Stable E Conformer of Quercetin in an Antioxidant Active Fraction of Ugni molinae Turcz

The identification of bioactive compounds in Ugni molinae Turcz. extracts has gained interest due to the well‐known biological properties of the plant. Previous research has identified two main types of bioactive compounds: polyphenols and pentacyclic triterpenes. In this study, an ethyl acetate extract from the leaves of the 14‐4 genotype of U. molinae was separated by column chromatography (CC), and one fraction exhibited an antioxidant capacity 11 times higher than the crude extract, evaluated through DPPH assay. The presence of the pentacyclic triterpene asiatic acid (AA) was identified through NMR experiments. The AA concentration, estimated by standard addition NMR experiments was 2.18 mg/mL, which corresponds to 14.5% of the active fraction (AF). Two polyphenolic compounds were confirmed by HPLC‐MS and NMR experiments: gallic acid and a stable conformer of quercetin. The structure was elucidated by coordinated analysis of HPLC‐MS, along with several NMR experiments. These included 1 H NMR, 13 C NMR, HSQC, HMBC, SELCOSY, SELTOCSY, variable temperature, and standard addition experiments. Variable‐temperature NMR experiments provide evidence of no isomer exchange over a broad temperature range. Consequently, the isomer previously described as a conformer was reclassified as E conformer. These findings expand current knowledge of the chemical composition of U. molinae extracts and polyphenols.