# Phenolic Compounds in Trees and Shrubs of Central Europe
## 3 Results
### Phenolic Compounds
#### 3.1.1. Phenolic Acids  
Phenolic acids, in their structure, contain a hydroxyl and a carboxyl group. Hydroxyl derivatives of benzoic and cinnamic acids are common in the plant world (Table 1). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{5}{c}{**(a) hydroxybenzoic acids**} \\ \hline \hline
**Acids** & **R1** & **R2** & **R3** & **R4** \\ \hline \hline
3-hydroxybenzoic acid & H & OH & H & H \\ \hline
4-hydroxybenzoic acid & H & H & OH & H \\ \hline Salicylic acid & OH & H & H & H \\ \hline Pyrocatechuric acid & OH & OH & H & H \\ \hline Genistic acid & OH & H & H & OH \\ \hline Procatechuric acid & H & OH & OH & H \\ \hline Vanillic acid & H & OCH3 & OH & H \\ \hline Isovanillic acid & H & OH & OCH3 & H \\ \hline Gallic acid & H & OH & OH & OH \\ \hline Syringic acid & H & OCH3 & OH & OCH3 \\ \hline \hline \(\alpha\)-Resorcylic acid & H & OH & H & OH \\ \hline \(\beta\)-Resorcylic acid & OH & H & OH & H \\  & R1 & O & & \\  & R2 & & & \\  & R3 & & & \\  & R4 & & & \\ \hline \hline \multicolumn{5}{c}{**(b) hydroxycinnamic acids**} \\ \hline \hline
**Acids** & **R1** & **R2** & **R3** & **R4** \\ \hline Cinnamic acid & H & H & H & H \\ \hline
2-cumaric acid & OH & H & H & H \\ \hline
3-cumaric acid & H & OH & H & H \\ \hline
4-cumaric acid & H & H & OH & H \\ \hline Caffe acid & H & OH & OH & H \\ \hline
4-metacyclcinnamic acid & H & H & OCH3 & H \\ \hline
3,4-dimetoxycinnamic & H & OCH3 & OCH3 & H \\ \hline Ferulic acid & H & H & OH & OCH3 \\ \hline Isoferulic acid & H & H & OCH3 & OH \\ \hline Synapic acid & H & OCH3 & OH & OCH3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The structures of phenolic acids [30; 31]. In plant tissues, other complexes of phenolic acids were also identified, e.g., complexes with flavonoids, fatty acids, sterols and polymers of cell walls. Phenolic acids may also be components of anthocyanins or flavones [30; 31]. A separate group is composed of depsides, being a complex of two or more molecules of phenolic acids. In plant organisms, including trees, they are formed mainly in the reaction of the so-called shikimate pathway or malate acid. Tyrosine and phenylalanine are precursors of most phenolic acids, from which, as a result of deamination, cinnamic acid and its hydroxy derivatives are formed [32]. In plants, phenolic acids are mostly found in the bound form as esters and glycosides contained in lignins and hydrolys tannins. Examples in this respect may be provided by hydroxycinnamic acids found in ester complexes with carboxylic acids or with glucose. They appear in ester complexes with the following acids: malonic, tartaric, \(\alpha\)-hydroxy-hydrocaffeic, hydroxycinnamic, tartronic, shikimic, galacturonic, glucaric (as caffeic acid glucuronide), gluconic (as feruloylgluconic acid, which main isomer is 2-O-feruloyl gluconic acid) and 4-methoxyaldaric (as 2-O-feruloyl-4-methoxyaldaric acid). In turn, hydroxybenzoic acids are primarily found as glycosides. In plant tissues, other complexes of phenolic acids were also identified, e.g., complexes with flavonoids, fatty acids, sterols and polymers of cell walls [30; 31]. Phenolic acids are found in wood of oak, pine, spruce, fir, walnut, willow, birch (leaves), and in the fruits of bird cherry (Table 2). A particularly interesting active compound from the group of hydroxybenzoic acids is ellagic acid, a dimer of gallic acid, found in plants in the free form and (more frequently) in an ester complex with glucose, forming hydrolysable tannins (ellagotannins) [33; 34]. It is found in the wood of oak, walnut and sweet chestnut, as well as in berry fruits, such as in strawberries and raspberries [35; 36], as well as in the loosestrife family (_Lythraceae_), particularly pomeranganate [10; 18; 37], and in certain nut seeds [38] and _Muscadine_ grapes. Ellagic acid exhibits, e.g., anti-cancer properties, thanks to which it may inhibit cell division and induce apoptosis in cancer cells [39; 40]. Moreover, its anti-inflammatory and antioxidant action [41; 42] were investigated and confirmed.