Abstract
The gram-scale isolation of the major flavonolignan diastereoisomers from milk thistle ( Silybum marianum) extract provided an entree into the isolation of two related analogues that are present in extremely minute quantities. The isolation and structure elucidation of these two new compounds, which we have termed isosilybin C and isosilybin D due to their structural similarities to isosilybin A and isosilybin B, respectively, afforded a preliminary analysis of structure-activity relationships toward prostate cancer growth, survival, and apoptotic endpoints.