Occurrence and distribution of free flavonoid aglycones in plants

The aim of this review is to summarize the various naturally occurring chalcone compounds which have been isolated from different plants. Chalcones considered as obligate intermediated in flavonoid biosynthesis but they do not accumulate to appreciable degree in most plants. The largest number of natural chalcones has been isolated from species of the Leguminosae, Asteraceae and Moraceae families. Chalcone accumulating plants have often been used in traditional medicine and chalcones have therefore been studied and reported to possess many beneficial biological effects including anti-inflammatory, antimicrobial, antifungal, antioxidant, cytotoxic, antitumor and chemopreventive activities. Previously published reviews on this topic survey the biological activities of natural and synthetic chalcones but there are no comprehensive contributions on occurrence and biological activities of natural chalcones. The present study provides an overview of hydroxy or/and methoxy-substituted chalcones, methylated, prenylated, geranylated and other monomeric derivatives, chromeno- and furanochalcones, dimeric chalcones and dihydrochalcone derivatives focusing on their biosynthesis, natural sources and biological activities. On the basis of 608 references, this review covers the phytochemistry and biological activity of natural chalcones, describing 646 compounds have been appeared in the literature since 1975. The summary is aimed to initiate further pharmacobotanical, biotechnological and medicinal studies on the field of chalcone research. This contribution reviews some of the general aspects of naturally occurring chalcones including their chemical categories, focusing on plant sources and biological activities. The review covers the major works appeared in the literature from 1975 up to 2014, relating to 608 references and 646 compounds.

Naturally occurring chalcones and their biological activities

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

The present chapter is the first compilation in which the flavonoid chemistry of all lower plants has been brought together in a comprehensive manner. In a chapter of this size it is not possible to be exhaustive in a historical sense, and reference to early work is only made when such work is considered to be of continuing significance. Nevertheless this summary attempts to establish a reasonably complete database representing work published prior to early 1986 and upon which future updates can be based. The emphasis throughout is on published work in which flavonoid structures have been defined. Thus some extensive surveys in which (for example) only presence or absence data appear are not fully represented. Discussion is focused primarily on the distribution of flavonoids and the possible phylogenetic significance (if evident) of this distribution. Limited space precludes all but passing comment on the myriad of taxonomic implications addressed in the literature.

Distribution of flavonoids in the lower plants and its evolutionary significance

Biologists generally agree that species are to be delimited on the basis of genetic discontinuities. The two species concepts that depend on such discontinuities to delimit species are the biological and the evolutionary species concepts. A biological species is a group of interfertile populations that is reproductively isolated from other such groups and that occupies a specific niche in nature (following Mayr, 1982). An evolutionary species is a single lineage of ancestor-descendant populations that maintains its own identity from other such lineages, that fits into its own ecological niche, and that has a unique evolutionary history (Simpson, 1961; Grant, 1981; Wiley, 1981). It thus differs from the biological species concept in that it is equally applicable to both sexually and asexually reproducing organisms. Under both the biological and the evolutionary species concepts, genetic discontinuities between sister species are thought to arise stochastically following speciation. It is assumed that as time passes, the two diverge progressively in a suite of morphological, physiological, and ecological attributes. Although most botanists espouse an evolutionary species concept in their theoretical writings, in their classifications they often recognize only species that have distinctive structural characters by which the taxa can be identified. In so doing, they are employing the morphological species concept of their

Aura E. Star

Papers

Flavonoid and ceroptin pigments from frond exudates of Pityrogramma triangularis

Internal flavonoid patterns of diploids and tetraploids of two exudate chemotypes of Pityrogramma triangularis (Kaulf.) Maxon

Triangularin, a new chalcone from Pityrogramma triangularis