Two pairs of unusual scalemic enantiomers from Isatis indigotica leaves

Two pairs of scalemic enantiomers with a novel carbon skeleton of 2-[1′-(4″-hydroxy-3″,5″-dimethoxyphenyl)ethyl]-2-methoxyindolin-3-one, named isatidifoliumindolinones A-D (1-4), were isolated from an aqueous extract of Isatis indigotica leaves (da qing ye). Their structures including absolute configurations were determined by spectroscopic data analysis combined with comparison of their experimental CD and calculated ECD spectra. Postulated biosynthetic pathways of 1-4 were discussed. The four stereoisomers exhibited stereochemistry-dependent activity against LPS-induced NO production in BV2 cells.