Synthesis of new silybin derivatives and evaluation of their antioxidant properties

Silibinin, the major flavonolignan of silymarin, displays a broad spectrum of biological features that are generally ascribed to its antioxidant properties. Silibinin occurs in two diastereoisomeric forms, i.e., silybins A and B, in a ratio of ca. 1 : 1. With a simple and robust purification method, it is now possible to obtain silybins A and B in pure forms, in g-scale amounts, and within a short period of time. Herein, we describe an efficient synthesis strategy to obtain a variety of new and more H2O-soluble derivatives from the single silybins in which the 9″-OH group was converted to a sulfate, N3, phosphodiester, or NH2 group via a solution-phase approach. Thus, eight new compounds have been synthesized, purified by HPLC analysis, and characterized by NMR and MS analyses. To conduct experiments to clarify the many biological properties of pure silibinin diastereoisomers and their derivatives, the synthetic compounds were tested by using the DPPH assay to evaluate their antioxidant activities. The results, even if only for a small number of derivatives, revealed that some of these compounds are much more active than their parent compounds. It is also interesting to consider the synergetic effects.