Phenylpropanoid ketone

Volatile compounds are often involved in long distance attraction and are especially important as attractants and repellents (as defined by Kogan, ). One major class of volatile materials, essential oils, is comprised of complex mixtures of terpenes, phenylpropanoid derived compounds and a number of esters, alcohols, aldehydes, ketones, acids, and hydrocarbons. The constituent compounds are mostly of low to medium molecular weight and generally not highly oxygenated. Some of the biological properties of these compounds have been reviewed (17,41,46,55,56). 

Essential oils may comprise volatile compounds of terpenoid or non-terpe-noid origin. All of them are hydrocarbons and their oxygenated derivatives. Some may also contain nitrogen or sulphur derivatives. They may exist in the form of alcohols, acids, esters, epoxides, aldehydes, ketones, amines, sulphides, etc. Monoterpenes, sesquiterpenes and even diterpenes constitute the composition of many essential oils. In addition, phenylpropanoids, fatty acids and their esters, or their decomposition products are also encountered as volatiles [1-16, 21-33, 36-38]. 

Fig. 4-1. Examples of classical methods indicating a phenylpropanoid structure of lignin. (A) Permanganate oxidation (methylated spruce lignin) affords veratric acid (3,4-dimethoxybenzoic acid) (1) in a yield of 8% and minor amounts of isohemipinic (4,5-dimethoxyisophtalic acid) (2) and dehydrodiveratric (3) acids. The formation of isohemipinic acid supports the occurrence of condensed structures (e.g., /3-5 or y-5). (B) Nitrobenzene oxidation of softwoods in alkali results in the formation of vanillin (4-hydroxy-3-methoxybenzaldehyde) (4) (about 25% of lignin). Oxidation of hardwoods and grasses results, respectively, in syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde) (5) and p-hydroxybenzaldehyde (6). (C) Hydrogenolysis yields pro-pylcyclohexane derivatives (7). (D) Ethanolysis yields so-called Hibbert ketones (8,9,10, and 11).

The precursors of some phenylpropanoid odor/flavor components of raspberry fruit occur as the 4 -0-p-D-gluco-pyranoside of 4-(4 -hydroxyphenyl)butan-2-one (raspberry ketone) and the S -O-p-o-glucopyranoside of 4-(3, 4 -dihy-droxyphenyl)butan-2-one (Pabst et al., 1990). 

Perilla frutescens can be class ed in several chemotypes as well according to the main mono-terpene components perillaldehyde, elsholtzia ketone, or perilla ketones and on the other side phenylpropanoid types containing myristicin, dillapiole, or elemicin (Koezuka et al., 1986). A comprehensive presentation on the chemotypes and the inheritance of the mentioned compounds was given by this author in Hay and Waterman (1993). In the referred last two examples, not only the sensorial but also the toxicological properties of the essential oil compounds are decisive for the (further) commercial use of the respective species biodiversity. 

The study involved the detection of altogether 125 volatiles. Beside monoterpenoids the following classes of metabolites were also found (species-dependent) Sesquiterpenoids (Sect. 7.3), phenylpropanoids (Sect. 6.3.3.1), aliphatic alcohols/ ketones/esters, and nitrogenous compounds such as nitriles or aldoximes (Raguso et al. 2003). 

The aromatic compound raspberry ketone, 4-(/ -hydroxyphenyl)-2-butanone, also known disframbinone, has been identified as the main fiavor impact component of raspberries, where it is contained in very low concentrations (0.1-0.2 ppm), only during the late maturation stage of the berry [140]. The biosynthetic pathway (shown in Scheme 9.6) stems from the general phenylpropanoid metabolism, starting from/)-coumaroyl-Co A and malonyl-CoA, involving at least two different enzymes (a synthase and a reductase) [141]. 

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