Phenylpropane and Phenol Derivatives

Among the substances which are treated in this section, the methoxy-phenylpropane derivatives are of more interest in medicine and in the perfume industry than are the steam-volatile phenols. Compounds of this type occur in a number of plants or in the essential oils obtained from them cloves, pimento, anise, fennel, parsley, dill, calamus and sassafras may be mentioned. 

Numerous compounds can be separated successfully on 250 (xm-silica gel GF254 or HF254 layers, prepared in the usual way and with benzene as solvent (Fig. 106) (cf. also [282]). Pastuska and Petrowitz [191a], Fishbedst and co-workers [63] and Fiori and Marigo [62] have recommended equipolar solvents like petrol ether (BP 50—75° C)-diethyl ether (85 + 15) or n-hexane-ethyl acetate (95 -j- 5). Catechol and the other di- and triphenols then remain at the start point (Table 31). They can, however, be separated with more polar solvents [70]. 

The following rules of thumb apply to the separation and identification of phenylpropane and phenol derivatives when standard conditions are used they are the result of systematic investigations and have been confirmed by Klouwen and Ter Heide [131]. 

Free phenol groups have a powerful influence on the adsorption affinity. Monophenols migrate but diphenols remain at the start point in benzene or chloroform solvents. 

Phenol ethers show distinctly lower adsorption affinity than the corresponding phenols (phenol-anisole resorcinol-resorcinol dimethyl ether). 

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Phenylpropanes are aromatic compounds with a propyl side chain attached to the benzene ring, which can be derived directly from phenylalanine. Naturally occurring phenylpropanoids often contain oxygenated substituents, e.g. OH, OMe or methylenedioxy, on the benzene ring. Phenylpropanoids with hydroxyl substituent(s) on the benzene ring belongs to the group of phenolics, e.g. caffeic acid and coumaric acid. 

Recent chemotaxonomlc investigations on phenylpropane derivatives show the existence of species-specific flavonoid patterns in bud exudates and the wide distribution of the secretion of lipophilic phenols in the... 

Nonflavonoid compounds comprise simple phenols, phenolic acids, coumarins, xanthones, stilbenes, lignins, and lignans. Phenolic acids are further divided into benzoic acid derivatives, based on a C6-C1 skeleton and cinnamic acid derivatives, which are based on a C6-C3 skeleton [6], The conmarins are phenolic acid derivatives composed of a benzene ring fnsed with an oxygen heterocycle. Xanthones consist of a C6-C1-C6 basic structure, and stilbenes are composed of a C6-C2-C6 skeleton with various hydroxylation patterns [13], Lignins are polymers of C6-C3 units, whereas lignans are made up of two phenylpropane units [13]. The structures of each of these classes are shown in Table 16.1. 

The majority of these phenols are also phenylpropane derivatives— many are prominent constituents of common spices such as cloves, aniseed, celery seed, basil and tarragon. 

Higher plants accumulate a wide range of phenolic compounds that are derivatives or metabolites of cinnamic acid. The phenyl-propanes, generally denoted as C -C compounds, are important intermediates in the biosynthesis of lignin and flavonoids. The common phenylpropane derivatives cinnamic acid, p-coumaric acid, caffeic acid, ferulic acid, 5-hydroxyferulic acid, and sinapic acid, are interrelated and are synthesized from phenylalanine (Fig. 5). 

The aromatic compounds are the derivatives of phenylpropane, which are aldehydes, alcohols, phenols, methoxy, and methylenedioxy in nature. A few nitrogen and sulfur compounds present in essential oils are also characterized as plant essential constituents. A list of selected and important antimicrobial components of essential oils has been presented in Fig. 132.1. 

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