Phenyl propanoid units

Flavonolignans are not merely products formed by oxidative coupling of a phenyl-propanoid unit with a typical flavone, since this term embraces all such products derived from other flavonoid types, viz. flavonols, chalcones, catechins, and others. However, plant constituents like rhodiolinin (103) (725) and the iryantherins (104) 126-129), in which a phenylpropanoid unit is linked with the A ring of the flavone nucleus, would be more appropriately termed lignoflavonoids (726) rather than... 

Figure 6.21. The Shikimate pathway in higher plants extension of the phenyl-propanoid unit and miscellaneous metabolites summary...

Flavonoids occur both in the free state and as glycosides. Their chemical structure is based on a C15 skeleton consisting of two benzene rings connected by a three-carbon chain, that is, C -Ci-C. The three-carbon chain is generally closed to form a heterocyclic ring (the C-ring). Flavonoids are products of both the shikimic acid and acetate pathways, being formed by the condensation of a phenyl-propanoid precursor with three malonyl coenzyme A units. 

With the failure to demonstrate that norbelladine or its relatives plays a role in the biosynthesis of the mesembrine alkaloids, a reevaluation led to a modified approach in which attempts to identify the sequence of occurrence of the post-tyrosine and post-phenylalanine intermediates were made. There is now a substantial body of information available to suggest that phenylalanine and tyrosine have separate metabolic roles in plants belonging to the order Dictolyoden. Not only do they lack the enzyme phenylalanine hydroxylase (phenylalanine 4-monooxygenase) which is necessary for the conversion of phenylalanine to tyrosine, but the metabolic pathways of these two amino acids are generally quite different in secondary metabolism (70). Phenylalanine is involved in initial conversion to cinnamic acid and subsequent transformation to structural units of the so-called phenyl-propanoid pathway, which include Ar—C3, Ar—C2, and Ar—Cj structural entities. On the other hand, the role of tyrosine in the biosynthesis of secondary metabolities is most frequently seen as the precursor of Ar—Cj—N and Cg—C2—N units, and somewhat less frequently, as Ar—C2 and Q—C2 units. 

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