ABSTRACT
Flavonoids are widely spread plant secondary metabolites called C6-C3-C6 phenolics, which are classified in three groups, depending on the nature of the C3 fragment and the type of the heterocyclic ring, as follows: 1) chromone derivatives (flavones, flavonols, flavanones, and flavanonols); 2) chromane derivatives (catechines and antocyanidines); and 3) flavonoids with open propane chain (chalcones) and with a furane ring (aurones). From all of these, flavones, flavonols, and flavanones are the most abundant in the plant kingdom and their skeleton is given in Fig. 1. Substitution in the positions 3, 5, 6, 7, 8, 2′, 3′, 4′, 5′, and 6′ gives all the compounds from these groups, with hydroxylation, methoxylation, and glycosylation being the most common substitution. Thousands of various flavonoids with various substitution patterns are recognized today as free flavones, flavonols, and flavanones, i.e., aglycones, and as flavonoid glycosides, which consist of flavonoid, nonsugar component aglycone, connected to the sugar moiety (mostly monosaccharides and disaccharides). Bonding to sugars makes flavonoids soluble in water and enables their easy transport within plants.
