**3.1 Esters with aromatic acids**

Aromatic acids, along with flavonoids, belong to the group of phenols of secondary metabolism of living organisms. The described secondary metabolites represent a store of biologically active compounds, displaying various biological activities. We can therefore assume that physicochemical and biological properties of the initial flavonoids may be improved by acylation of flavonoids with aromatic acids. However, by this reaction a new compound can also gain novel activities provided by the aromatic acids.

Flavonoid acylation with aromatic acids was reported to improve physiological activities, such as UV-absorbing capacity, radical scavenging ability (Delazar et al., 2005; Ishihara & Nakajima, 2003; Harborne & Williams, 2000; Alluis & Dangles 1999; Jungblut et al., 1995) pigment stabilization (especially anthocyanins) (Ishihara & Nakajima 2003), and interaction with cellular targets (Ferrer et al., 2008).

Flavonoid esters acylated with *p*-coumaric acid were found to increase antioxidant (Pajero et al., 2005) and anti-inflammatory activities (Harborne & Williams, 2000), as well as antiproliferative and cytotoxic effects on various cancer cell lines (Mitrokotsa et al., 1993). Moreover, *p*-coumaroyl esters of quercetin and kaempferol were reported to have positive effects on cerebrovascular disorders (Calis et al., 1995). Similarly, flavonoid esters esterified with cinnamic acid were shown to exhibit antiproliferative activity against several human cancer cell lines (Duarte-Almeida et al., 2007). Flavonoid acylation with caffeic acid contributes to the enhancement of antioxidant properties (Pajero et al., 2005). Flavonolignans acylated with truxinic acid were shown to possess hepatoprotective as well as anticancer activity (Sharma et al., 2003).
