Stereochemistry of flavonoidal alkaloids from Dracocephalum rupestre

Phytochemical studies on the aerial parts of Dracocephalum rupestre led to the isolation of four groups of flavonoidal alkaloids, dracocephins A–D. They were elucidated as conjugates of flavanone with pyrrolidin-2-one on the basis of extensive spectroscopic analysis. The two stereogenic centers rendered each group of the dracocephins as two pairs of enantiomers simultaneously. All of the sixteen isomers were separated successfully by chiral HPLC and their stereochemical features were determined by their CD data and single-crystal X-ray diffraction analysis of one stereoisomer. The additive relation of the chiroptical contributions resulting from the two stereogenic centers was generalized. The CD contribution of the chiral carbon in the pyrrolidin-2-one ring was proposed by subtraction of their respective contributions.