Flavonolignans

An interesting combination of flavonoid and lig-nan structures is found in a group of compounds called flavonolignans. They arise by oxidative coupling processes between a flavonoid and a phenylpropanoid, usually coniferyl alcohol. Thus, the dihydroflavonol taxifolin through one-electron oxidation may provide a free radical, which may combine with the free radical generated from coniferyl alcohol (Figure 4.44). This would lead to an adduct, which could cyclize by attack of the phenol nucleophile on to the quinone methide system provided by coniferyl alcohol. The product would be silybin, found in Silybum marianum (Compositae/Asteraceae) as a mixture of two trans diastereoisomers, reflecting a lack of stereospeci-ficity for the original radical coupling. In addition,

D-Glc

D-Glc

OH O

OH O

OH O quercetin

; rutinose rutinose

= rhamnosyl(a1^6)glucose i hesperetin hesperidin ^OH

OH O quercetin

; rutinose rutin

OH O

OH O

L-Rha neohesperidose '

= rhamnosyl(a1^2)glucose i hesperetin neohesperidin

Rha-Glc-O

OH O

neohesperidose \ naringenin

OH O

naringin

Figure 4.42

Rha-Glc-O

OH O

Rha-Glc-O

OH O

Figure 4.42

Rha-Glc-O

H2 / catalyst

OH O

Rha-Glc-O

H2 / catalyst

Rha-Glc-O

OH O R1 = OH, R2 = H, naringin dihydrochalcone R1 = OMe, R2 = OH, neohesperidin dihydrochalcone

OH O

OH O R1 = OH, R2 = H, naringin dihydrochalcone R1 = OMe, R2 = OH, neohesperidin dihydrochalcone

Figure 4.43

the regioisomer isosilybin (Figure 4.45), again a mixture of trans diastereoisomers, is also found in Silybum. Silychristin (Figure 4.45) demonstrates a further structural variant which can be seen to originate from a mesomer of the taxifolin-derived free radical, in which the unpaired electron is localized on the carbon ortho to the original 4-hydroxyl function. The more complex structure in silydianin is accounted for by the mechanism shown in Figure 4.46, in which the initial coupling product cyclizes further by intramolecular attack of an enolate nucleophile on to the quinoneme-thide. Hemiketal formation finishes the process. The flavonolignans from Silybum* (milk thistle) have valuable antihepatotoxic properties, and can provide protection against liver-damaging agents. Coumarinolignans, which are products arising by a similar oxidative coupling mechanism which combines a coumarin with a cinnamyl alcohol, may be found in other plants. The benzodioxane ring as seen in silybin and isosilybin is a characteristic feature of many such compounds.

Reactie Naringenin Neohesperidose

one-electron oxidation

OH O taxifolin radical coupling one-electron oxidation

OH O taxifolin w

OH O

OH O

radical coupling

OH O

OH O

silybin (diastereoisomeric pair)

OH O

nucleophilic attack of OH OMe on to quinonemethide (two | stereochemistries possible)

"oh

silybin (diastereoisomeric pair)

^OMe OH

coniferyl alcohol

Figure 4.44

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