Lignans are dimers or oligomers that result from the coupling of monolignols - p-coumaryl alcohol (1.68), coniferyl alcohol (1.69), and sinapyl alcohol (1.70), with coniferyl alcohol being the most common monolignol used in lignan biosynthesis. Lignans are present in ferns, gymnosperms and angiosperms. They are localized in woody stems and in seeds and play a role as insect deterrents. Some of these compounds have medicinal properties.
Lignan biosynthesis results from the reaction of monolignol radicals. The monolignol radicals (1.71), in this example derived from p-coumaryl alcohol (1.68), are generated enzymatically by activated cell-wall bound peroxidases (see also Chapter 2, Section 220.127.116.11), which eliminate the proton on the para-hydroxyl-group of the phenol. The radical electron can be delocalized along the phenol ring, but also along the propane tail, so that the carbon at the 1, 3 and 5 positions of the ring, as well as the P-carbon of the propane tail, become reactive.
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The term lignan typically refers to dimers of monolignols that are linked via an 8-8' ((-(') bond, whereas the term neolignan refers to dimers and oligomers that contain bonds other than the 8-8' bond. Most lignans are optically active, and typically only one enantiomer is found in a given species. Examples of lignans include (+)-pinoresinol (1.72), (+)-sesamin (1.73), and (-)-plicatic acid (1.74).
The stereo-selective formation of certain lignans has been shown to be mediated by 'dirigent' proteins. These proteins hold the monolignols in a specific orientation, but have no catalytic activity (Davin et al., 1997; see also Chapter 3).
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