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OH
1/2 H
2
O
1/2 H
2
O
2
COH
3
Peroxidase
O
•
OH
Lignin
OCH
3
OH
OH
Oxidase
1/4 O
2
OCH
3
O
•
1/2 H
2
O
FIGURE 6.9
Polymerization of coniferyl alcohol to a lignin.
6.8 VARIOUS LIGNAN STRUCTURAL UNITS
The lignans form several basic skeletons as shown in Figure 6.11, and represent
another group of phenolic compounds found in plants, such as wheat, flax seeds,
pumpkin seeds, rye, soybeans, broccoli, and some berries. In contrast to lignin, the
lignans are derived via the coupling of only two (or three) substituted C6-C3 mono-
lignols. Again, this reaction is catalyzed by oxidative enzymes. The C6-C3 units are
derived from cinnamyl units, but the basic dimerization reaction to form a lignan
structure is outlined in Figure 6.12 to form the bicyclic lignan pinoresinol. Since the
propyl side chain is usually oxygenated, various secondary cyclizations can occur.
Other examples of lignan structures typically found in plants are podophyllotoxin
and stegnacin (Figure 6.13). When two C6-C3 residues are linked together but not
through the β-carbon atom of the propyl side chain, another class of lignans, known
as the neolignans, is formed and represented by skeleton (3) shown in Figure 6.11.
6.9 LIGNANS AS PHYTOESTROGENS
Certain lignans are classified as phytoestrogens similar to the isoflavone genistein.
Furthermore, when they form part of the human diet, some lignans are metabolized
to form mammalian lignans known as enterodiol and enterolactone by the human
intestinal bacteria. Lignans that undergo this transformation include the lignans,
matairesinol, and hydroxymatairesinol (Figure 6.14).
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