New insight into the biosynthesis of flavanolignans in the white-flowered variant of Silybum marianum.

It has been demonstrated that besides the known flavanolignan constituents of the white-flowered variant of Silybum marianum, (-)-silandrin A (3a) and (-)-isosilandrin A (4a); their trans-benzodioxane diastereomers, (-)-silandrin B (3b) and (-)-isosilandrin B (4b), are also produced by the plant. Moreover, the isolation of their cis-benzodioxane diastereomers, (-)-isocisilandrin (5) and cisilandrin (6), confirm that the previously proposed biosynthetic pathway involving a nonselective O-beta coupling is correct.