Chiral resolution and absolute configuration of a pair of rare racemic spirodienone sesquineolignans from Xanthium sibiricum.

A pair of racemic spirodienone neolignan enantiomers, (±)-sibiricumin A, were isolated from the extract of the fruits of Xanthium sibiricum. The resolution of (+)- and (-)-sibiricumin A was achieved by chiral HPLC. The absolute configurations of the racemes were assigned by X-ray and by electronic circular dichroism (ECD). This experiment is the first unambiguous determination of the absolute configuration of spirodienone neolignan.