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 transbenzodioxane 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-β coupling is correct.