Lignans phenylpropanoid dimers

Lignans are phenylpropanoid dimers in which the monomers are linked by the central carbon (C8) atoms of the propyl side chains (Fig. 12.1) [10]. Many lignans are formed from coniferyl alcohol, a typical lignin monomer, and the coupling of two coniferyl alcohol radicals proceeds under the control of a unique asymmetric... 

Lignans are dimers of phenylpropanoid units linked by the central carbons of their side chains while naturally occurring dimers that exhibit linkages other than this type of bond are known as neolignans. Much attentions has been devoted to the oxidative, acidic or alkaline dimerisations of arylpropenoid molecules and particularly 4-hydroxycinnamic acids such as ferulic and caffeic acids [24-34]. 

The lignans are secondary plant metabolites biosynthetically derived from the shikimate pathway. They possess diverse structures, usually dimers of phenylpropanoid units, although compounds containing three, four or even five such units, have been reported. The number of higher oligomers identified is currently increasing. 

There are few examples of lignans/neolignans in the non-woody monocotyledons, except for the formation of dihydroxytruxillic and truxinic acids in certain superorders. These metabolites are apparently formed via photochemical dimerization rather than by enzymatically engendered phenolic coupling i.e., the stereoselective and non-stereoselective coupling enzymes tentatively appear to be absent in the monocotyledons. This decreased reliance upon a full extension of the phenylpropanoid pathway presumably prevents the formation of woody tissue. This, in turn, helps to explain why monocotyledons are generally more readily amenable to biodegradation. 

To confuse matters further, several dimeric phenylpropanoid (neo)lignans have been implicated in lignin synthesis (12) (see later). Others splay potent antiviral (/j), antifungal (74), bactericidal (15), insecticidal (16) and putative cytokinin (17, 18) properties. Therefore, it is difficult at first inspection to determine whether phenylpropanoid metabolism initially evolved for water/nutrient conduction, structural support or defense. (Figure 1 shows the overall biochemical pathway to the different metabolic products from Phe 1/Tyr 2). 

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