Phenohc glycosides

Several other naturally occurring antioxidants have been identified in oils. Sesamol [533-31-3] (6) occurs as sesamoline [526-07-8], a glycoside, in sesame seed oil. FemUc acid [1135-24-6] (7) is found esterified to cycloartenol [469-38-5] in rice bran oil and to 3-sitosterol in com oil. Although it does not occur in oils, rosemary extract has also been found to contain powerful phenoHc antioxidants (12). 

Glycosides, particularly of phenoHc compounds, are widely distributed in plant tissues (2,10). Glycosides of anthocyanidins, flavones, flavanols, flavanones, flavanonols, stilbenes and saponins, gaUic acid derivatives, and condensed tannins are all common. 

The use of various low-water reaction systems, including anhydrous organic solvents as well as imidazolium-based ionic liquids, is investigated for the biocatalytic preparation of lipophilic derivatives of various natural phenohc compounds (Figure 9.1) including phenohc and flavonoid glycosides (escuhn, salicin, helicin. 

Phenolic acids occur naturally in higher plants, usually as free adds, glycosides, esters, or in insoluble-bound form in a complex mixture of other phenolic compounds. The presence of phenolic acids in food products has been associated with astringency, discoloration, inhibition of enzyme activity, and antioxidant properties, among others [15]. A major portion of phenohc acids present in almond was found in the form of soluble esters. Wijeratne et al. [16] reported that the total amounts of identified free phenolic acids were 16.3, 14[ig/g and a trace amount in skin, shell, and whole seed extracts, respectively, whereas the amounts for total esterified phenolic acids were 279.6,967.1, and 40.3 [ig/g, respectively. 

Phenolic compounds are present separately from the substrates, predominantly located in the cell vacuoles. Particularly prominent substrates are caffeic acid and its esters, as well as some flavonoid substances, of which monomeric flavan-3-ols (catechins) are the most important. Other groups of phenolic compounds, such as condensed forms of flavan-3-ols and flavan-3,4-diols (tannins), flavonols, flavones, flavans, chalcones, dihydrochalcones and anthocyanins, are only partly oxidised. One of the reasons for this is probably the steric hindrance caused by the corresponding glycosides and substrate specificity of polyphenol oxidases. The content of phenohc compounds depends on genetic factors (on plant species and varieties), the degree of maturity and external (environmental) factors (hght, temperature, nutrients, use of pesticides and so on). The only substrate in animal tissues is the amino acid tyrosine. 

The main substrates of polyphenol oxidases in apples are hydrox-ycinnamic acid derivatives, especially caffeic add depsides, such as chlorogenic acid, and some groups of flavonoid compounds, such as flavan-3-ols, flavonols, dihydrochalcones and (in red-skinned apple varieties) also anthocyanins. In the flesh of apples, the main phenohcs are chlorogenic (3-caffeoylquinic) add, flavan-3-ol epicatechm, and procyanidin B2 representing condensed tannins. These three compounds represent more than 90% of the phenolic compounds present. The main substrates in apple peel are flavan-3-ols and flavonols (quercetin glycosides) and to a lesser extent derivatives of hydroxydnnamic acids are present (chlorogenic add and some other depsides). 

Phlorizin is a glycoside found in the root bark of certain fruit trees. Hydrolysis of phlorizin yields the following phenohc material. How many possible structures are possible for the glycoside Explain how methylation of the glycoside using dimethyl sulfate followed by hydrolysis of the product with dilute acid can establish the structure of the glycoside. 

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