Phenylpropanoid aldehydes

Phenylpropanoid Aldehydes (Phenylacryl Aldehydes), Phenylpropanoid Alcohols (Phenylallyl Alcohols), Phenylpropenes... 

Lignin has a complex structure that varies with the source, growing conditions, etc. This complex and varied structure is typical of many plant-derived macromolecules. Lignin is generally considered as being formed from three different phenylpropanoid alcohols— coniferyl, coumaryl, and sinapyl alcohols, which are synthesized from phenylalanine via various cinnamic acid derivatives and commercially is sometimes treated as being composed of a Cg repeat unit where the superstructure contains aromatic and aliphatic alcohols and ethers, and aliphatic aldehydes and vinyl units. 

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]. 

The Diabrotica spp. com rootworm beetles are specifically attracted to a variety of plant-produced phenylpropanoids, eg, (E)-cinu am aldehyde [14371-10-9] for the southern com rootworm D. undecimpunctata howardi (E)-cinnamyl alcohol [4407-36-7] for the northern com rootworm D. barberi and indole [120-72-9] for the western com rootworm, D. virgifera virgifera. Especially powerful lures for these rootworm beetles are 2-(4-methoxyphenyl)ethanol for the northern com rootworm and 4-methoxycinnamaldehyde [71277-11-7] (177) for the western com bootworm. 

Sinapate is synthesized via the oxidation of sinapaldehyde (3.79) by an aldehyde dehydrogenase, as described in Section 13 of this chapter. Sinapaldehyde, in turn, is derived from the amino acid phenylalanine (3.27) via the general phenylpropanoid pathway (see Section 7), followed by a number of the hydroxylation and methylation reactions described in Section 10. 

The branch pathway of lignin biosynthesis is shown in Fig. 2. The first steps are shared with the general phenylpropanoid pathway. Cinnamic acid is transformed by hydroxylation and methylation to produce acids with different substitutions on the aromatic ring. The 4-coumaric, ferulic and sinapic acids are then esterified by hydroxycinnamate CoA ligase to produce cinnamyl-CoAs, which are reduced by cinnamyl-CoA reductase (CCR) to produce the three aldehydes. These in turn are reduced by CAD to the three cinnamyl alcohols which are then polymerised into lignins. 

L-Phenylalanine,which is derived via the shikimic acid pathway,is an important precursor for aromatic aroma components. This amino acid can be transformed into phe-nylpyruvate by transamination and by subsequent decarboxylation to 2-phenylacetyl-CoA in an analogous reaction as discussed for leucine and valine. 2-Phenylacetyl-CoA is converted into esters of a variety of alcohols or reduced to 2-phenylethanol and transformed into 2-phenyl-ethyl esters. The end products of phenylalanine catabolism are fumaric acid and acetoacetate which are further metabolized by the TCA-cycle. Phenylalanine ammonia lyase converts the amino acid into cinnamic acid, the key intermediate of phenylpropanoid metabolism. By a series of enzymes (cinnamate-4-hydroxylase, p-coumarate 3-hydroxylase, catechol O-methyltransferase and ferulate 5-hydroxylase) cinnamic acid is transformed into p-couma-ric-, caffeic-, ferulic-, 5-hydroxyferulic- and sinapic acids,which act as precursors for flavor components and are important intermediates in the biosynthesis of fla-vonoides, lignins, etc. Reduction of cinnamic acids to aldehydes and alcohols by cinnamoyl-CoA NADPH-oxido-reductase and cinnamoyl-alcohol-dehydrogenase form important flavor compounds such as cinnamic aldehyde, cin-namyl alcohol and esters. Further reduction of cinnamyl alcohols lead to propenyl- and allylphenols such as... 

By contrast, the 3D structural analysis of a bona fide CAD has been reported only for the Arabidopsis AtCAD4 and AtCAD5, as two other reports of putative CADs from Saccharomyces cerevisiae and Populus tremuloides (aspen) are not currently considered as CADs proper. Saccharomyces cerevisiae does not produce metabolites in the phenylpropanoid pathway, thereby making it unclear as to why this specific metabolic role was being contemplated. In addition, the putative sinapyl aldehyde dehydrogenase (SAD) from aspen was not established to have the specific function claimed (see Davin etal and Anterola and Lewis " for a full discussion). 

Cinnamic (1), p-coumaric (2), and related acids may be activated by conversion to CoA esters by CoA ligases [e.g., 4-coumarate CoA ligase (EC 6.2.1.12)] in much the same way that fatty acids are activated. The reduction of the CoA esters of cinnamic acids to cinnamyl alcohols involves two enz)mies cinnamoyl-CoA oxidoreductase (which forms the aldehydes) and cinnamyl alcohol dehydrogenase (Grisebach, 1981). Phenylpropanoids appear to be synthesized from the CoA esters of this series of acids by conversion to the corresponding aldehydes, then to the alcohols, and finally, by elimination of a phosphate group, to allyl and propenyl compounds. In many plants, mixtures of all t3q>es co-occur (Fig. 8.7) (Gross, 1981 Mann, 1987). Reduction of the side chain to produce dihydrocinnamic acids and related compounds is also known to occur in nature. 

The volatile fractions of many plants are isolated by steam distillation, distillation, or by solvent extraction (Banthorpe, 1991). The resulting volatile compounds that make up the essence or aroma of plants are called essential oils. Altiiough essential oils are comprised of many types of compounds, monoterpenes are major among them. These oils may also contain metabolically modified fatty acids, aldehydes, hydrocarbons, esters, phenylpropanoid compounds, acetylenic compounds, volatile alcohols, volatile alkaloids, phenylpro-panoids, and other shikimic acid derivatives. 

Other phenylpropanoids, so named due to their common phenylalanine 2 precursor, include hydroxycinnamic acids, cinnamic aldehydes and monoUgnols, coumarins, and stUbenoids 8. 

The Olefinic Substrates The highly iso-regioselective hydroformylation of allyl- or propenylarenes (phenylpropanoids) gives aldehydes with numerous applications in flavors and perfumes. Several 2-aryl-prop-1-enes required as substrates can be extracted from natural sources in a rather pure form (Figure 6.8). For example, eugenol, with its typical spicy, clove like aroma, can be isolated from clove oil, nutmeg, cinnamon, basil, and bay leaf. Safrol (shikimol) is isolated from sassafras plants and has a typical sweet-shop aroma. Estragole (methyl chavicol) is produced from basil oil and chavicol from betel oil. 

Hydroformylation of a series of aryl-substituted phenylpropanoids, among them eugenol, safrol, and estragole, in the presence of a Rh monophosphite catalyst at 300 psi (about 21 bar) afforded the w-aldehydes in 78-84% yield (Table 6.3) [178]. Substituents on the aryl ring had only a marginal effect. 

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