Structure Elucidation of Flavonolignans

In the early years after the discovery of silybin (1), the structure determination of this compound class was mainly achieved through extensive chemical conversions and degradative reactions (15, 95), which often led to structures that later needed revision. Analysis of comprehensive spectroscopic data along with classical chemical reactions and synthesis approaches has led to the unequivocal determination of the absolute stmctures of flavonolignans (102,135). The complete structure elucidation of flavonolignans is summarized below. 

The flavonoid skeleton present in flavonolignans can be readily identified from their NMR spectra. Silybin (1) and its congeners, practically all of which bear a 3,5,7-trihydroxy-3, 4 -dioxygenated dUiydroflavone moiety, exhibit signals for 

Proton(s) Isosilybin Isosilychristin (75) Silybin A (89) Silychristin Silydianin 

Proton(s) Aegicin (64) Compound Hydnocarpin Salcolin A Sinaiticin (96) 

Like coumarinolignans, the dioxane bridge of flavonolignans undergoes mass spectrometric RDA cleavage to give rise to a substimted flavonoid-quinone ion (H) and a typical phenylpropanoid ion (I), as shown in Fig. 18. 

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