Occurrence and distribution of free flavonoid aglycones in plants

Lignans are phenylpropanoid dimers, where the phenylpropane units are linked by the central carbon (C8) of their side chains. Ligans vary substantially in oxidation level, substitution pattern, and the chemical structure of their basic carbon framework. In addition to structural diversity, lignans show considerable diversity in terms of enantiomeric composition, biosynthesis, and phylogenetic distribution. In this review, these diversities are outlined and the phylogenetic distribution of plants producing 66 typical lignans is listed. The distribution is correlated with the putative biosynthetic pathways of the lignans and discussed from evolutionary aspects. 
Abbreviations: SIRD – Secoisolariciresinol dehydrogenase; PLR – pinoresinol lariciresinol reductase; DP – dirigent protein

Diversity in lignan biosynthesis

Most reports on the identification of known flavonoids in new plant sources as well as those on novel flavonoids naturally deal with the two most abundant classes, the flavones and flavonols. For this chapter we have listed all references to the occurrence of flavone and flavonol aglycones which have appeared from 1975 to 1980, and also include such earlier references as were overlooked in the relevant chapters of The Flavonoids’ (1975). Emphasis is given to the natural distribution of these compounds. The data are collected in Tables 4.1 and 4.2 which cite substances, plant sources and references. Since 1974, some 50 flavones and 60 flavonols with common substitution patterns have been described as novel compounds. Many are simply new methyl ethers of known compounds. The structures are arranged throughout the Tables by number and arrangement of substituents in ring A, followed by ring B. Wherever possible, the plant organs from which the compounds have been isolated are cited. In cases where authors have extracted the ‘whole plant’ or ‘aerial parts’, it does not necessarily follow that the compound is present in all parts, since these substances are often specifically located in certain organs only. In all cases where free aglycones are encountered, especially highly methylated flavonoids, it is likely that they occur externally, or are associated with secretory structures and are correlated with the production of other lipophilic secondary compounds, mainly of terpenoid origin.

Flavones and Flavonols

Section I. Introduction to the Sunflower Family.- 1. Biology and Distribution.- 2. Classification, Phylogeny and Biogeography.- Section II. Introduction to the Flavonoids.- 3. The Use of Flavonoids as Taxonomic Markers.- 4. Structural Variation of the Flavonoids of Asteraceae.- 5. Biosynthesis of Flavonoids.- 6. Biological Functions of Flavonoids.- Section III. Flavonoid Data.- 7. Flavonoids of Anthemideae.- 8. Flavonoids of Astereae.- 9. Flavonoids of Calenduleae and Cardueae.- 10. Flavonoids of Eupatorieae.- 11. Flavonoids of Heliantheae s.l.- 12. Flavonoids of Inuleae s.l.- 13. Flavonoids of Lactuceae.- 14. Flavonoids of Mutisieae and Barnadesioideae.- 15. Flavonoids of Senecioneae.- 16. Flavonoids of Tageteae.- 17. Flavonoids of Vernonieae and Liabeae.- Section IV. Efficacy of Flavonoids at Different Taxonomic Levels.- 18. Flavonoids at the Subfamilial Level.- 19. Flavonoids at the Tribal Level.- 20. Flavonoids at the Subtribal Level.- 21. Flavonoids at the Generic Level.- 22. Flavonoids at the Specific Level.- 23. Flavonoids at Infraspecific Levels.- Section V. Flavonoids as Indicators of the Evolutionary Process.- 24. Flavonoids and Populational Variation.- 25. Flavonoids and Hybridization and Introgression.- Section VI. Flavonoids and Phylogeny.- 26. Flavonoid Relationships with other Families.- 27. Evolution of the Flavonoid System in Asteraceae.- Addendum.- References.- Common Names of Flavonoids Used in This Book and Their Equivalents.- Chemical Index.- Taxon Index.

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Sheela Tandon

Papers

Wikstromol, a new lignan from Wikstroemia viridiflora

Strictic acid, a novel diterpene from Conyza stricta☆

Conyzatin—a new flavone from Conyza stricta☆

Wikstrosin, a tricoumarin from Wikstroemia viridiflora

Erioside, a new coumarin glucoside from Lasiosiphon eriocephalus