Of phenolic lipids

Type III synthases, as a whole, employ a wider spectrum of physiological starter molecules than their type I and II counterparts, including a variety of aromatic and aliphatic CoA esters such as coumaiyl-CoA, methyl-anthraniloyl-CoA, as well as the recently identified medium- and long-chain fiitty acyl-CoA ester starters used by certain bacterial and plant type III enzymes involved in the biosyndiesis of phenolic lipids (22, 24, Cook et al., unpublished results). The most extensively studied type III en mie, chalcone synthase (Fig. 4), uses 4-coumaryl-CoA as the starter unit and catalyzes three successive condensation reactions with malonyi-CoA as the extender. Cyclization and aromatization of the linear tetraketide intermediate is performed within the same active site, yielding the final product 4 ,2 ,4 ,6 -tetrahydroxychalcone. 

The combination of a root hair specific EST approach and expression analysis was an effective strategy for isolating candidate polyketide synthases potentially involved in sorgoleone biosynthesis. As a result of these efforts, two novel type III polyketide synthases have been identified that preferentially use long chain acyl Co-A s and are potentially involved in sorgoleone biosynthesis. These candidate polyketide synthases can form pentadecatriene resorcinol, an intermediate in sorgoleone biosynthesis. Furthermore, these efforts may aid in the identification of other polyketide synthases responsible for the biosynthesis of phenolic lipids in other plant species. 

Supercritical CO2 extraction of AR or other phenolic lipids have been only sparcely reported (49,50). The solubility of a compound in a solvent depends on its physico-chemical properties. The ARs present in a crude extract, especially those with consecutive number of carbon atoms in the acyl chain (e.g. Cis and Ci7 homologues in rye alkyiresorcinols) have very similar physico-chemical properties. Therefore, when supercritical CO2 is used, the separation of individual homologues from crude extracts still remains a challenge. Illustrative studies on the extraction of phenolic lipids by supercritical CO2 are given in the following sections. 

Dihydric phenolic lipids of the cardol type are the most abundant in plant, fungal and bacterial kingdoms. The first species in which the members of the title subclass of phenolic lipids, resorcinolic lipids, were found was Ginkgo biloba (Ginkgoaceae) [6,9,10,17]. Later, the presence of resorcinolic lipids (5-n-alk(en)ylresorcinols) was shown also in other species, first, in the Anacardiaceae [14], which is an important source of various phenolic lipids, not only of alkylresorcinols but also of alkylphenols and alkylcatechols. For example, the cashew and the processing of cashew nuts are the main source of phenolic lipids for the formaldehyde-polymer industry. Aspects of Anacardium occidentale in relation to synthesis, semi-synthesis and chemical industry have been reviewed Tyman [1,11,14] as well as a recent book [2]. 

The synthesis of phenolic lipids of all types bearing saturated side-chains presents few problems and has been approached by the methodologies of classical organic chemistry [1,2,11,131]. 

An increasing number of phenolic lipids with hydroxy or oxo substituted side-chains has been isolated in recent years. Thus, the C17,5-(2-oxoalkylresorcinol, Fig (4)-70, found in Cereale secale, has been synthesised from 3,5-dimethoxyphenylacetic acid [181]. 

The properties of phenolic lipids have tended to be dominated by technological aspects and it is only comparatively recently that potential biological usefulness has come to the fore. For example, products derived from the Anacardiacae occidentale, notably cardanol obtained by semisynthesis through thermal decarboxylation were all directed to polymeric and technical applications and the vast industrial literature [246] contains only two references to insecticidal uses of chlorinated cardanol [247]. 

A wide range of botanical and biological families are sources of phenolic lipids. Notable classes of compounds present are certain dihydric phenols (the cardols), phenolic acids (the anacardic acids) and a variety of... 

Synthesis has been useful in the chemistry of phenolic lipids for structural confirmation and in enabling structure/property correlations to be made by the general applicability of syntheses to a range of non-natural isomers. With the polyunsaturated constituents synthesis is valuable since either argentation chromatography or preparative HPLC have usually proved laborious on the natural products and on transformed isomeric compounds required for evaluatory purposes in structure/activity studies. 

Since cardanols can be obtained from anacardic acids and cardols from orsellinic acids, the methods outlined have a general applicability to a range of phenolic lipids. Reference has been made largely to the phenols of the Anacardiacae but the methods are likely to be applicable to other phenolic systems, and those with methylene-interrupted structures at different side-chain positions. Alkynes and phosphorans have both proved invaluable in synthetic studies but attention should be drawn to the very elegant use of ailenic compounds in the polyethenoid (arachidonic) series (ref. 168) which has a potential application with phenolic lipids. Methods for the synthesis of leukotrienes are also relevant for the methylene group-interrupted structures of phenolic lipids (169). 

CNSL used in polymerisation with formaldehyde as for example in friction dusts may not require elaborate analysis. Nevertheless interest in the industrial chemical uses of phenolic lipids has led to a study of quantitative methods of analysis by a variety of chromatographic methods. For cashew phenols these were first based on GLC. Thus the (15 3), (15 2), (15 1) and (15 0) constituents of methyl anacardate, cardol and cardanol have been separated by GLC on PEGA columns (ref.206), the free phenols (anacardic acid as methyl anacardate) by GLC on SE30 (ref207) and the hydrogenated anf fully methylated phenols on Dexsil and PEGA columns (ref.208). A further number of stationary phases have been investigated... 

A prerequisite for quantitative analysis is a knowledge of the structure of phenolic lipids. An extensive amount of the work leading to the present accepted structures has been carried out with the phenols from Anacardium occidentale and the Rhus genus has been summarised in an earlier review (ref. 2) and more recently (ref.3).. A diagnostic of great importance in determining the position of substituents in monoalkyl phenolic lipids was the finding in mass spectroscopy (ref. 223) that in... 

The preceeding chemical and MS methods give information on the position of double bonds but not on their stereochemistry or indeed, if existent, their chirality (refe. 14,53). Infrared spectroscopy has been valuable. Thus In the early work on the configuration of the double bond in (15 1)-cardanol an initial trans assignment was later corrected when the natural product was found to possess an ir absorption band at 960cm (10.4p) characteristic of a c/s-configuration (ref. 2). The cis (Z) configuration of the unsaturated constituents of phenolic lipids from Anacardium occidentale is clearly demonstrable by the J constants in the NMR spectra of the... 

The resins in the friction dust area tend to be rigid and the flexibility and plasticity associated with the long alkyl chain of phenolic lipids have been used in natural rubber vulcanisation by for example incorporating crosslinking with phosphorylated cardanol (ref. 252). Unpolymerised CNSL phenols have been used in natural or diene rubber compositions for tyre treads to give an improved dynamic elastic modulus but with the same hardness as formulations without the phenolic addition (ref. 253). 

An inherent problem in the usage of phenolic lipids, particularly in surface coatings, is the discolouration which can impair products. Apart from colourants arising from the solvent action of CNSL on the shell in the industrial process, the dihydric phenols In CNSL notably the minor component 2-methylcardol (ref. 200) more than cardol appear to be the cause of this deterioration rather than the monohydric member, cardanol. The usage of purer cardanol, or the less unsaturated material by semi-hydrogenation or chemical reduction, as well as the Incorporation of an antioxidant are methods for colour stabilisation (ref. 277). Antioxidant applications and pharmaceutical uses of CNSL and its component phenols are referred to in the next section. 

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