Urushiol, phenolic lipid

As is true in the case of other phenolic lipids, urushiol is also a mixture of components varying mostly in the degree of unsaturation. Thus, the urushiol from Rhus vernicefera has structures shown in Fig. 6 [139]. Rhus toxicodendron is also known to give urushiol, but its... 

Although urushiol possesses an interesting structure for transformation into speciality polymers, no attempt has been reported. Notwithstanding its applications in a specified area, it appears that it is not properly put to use as it can be converted to polymers with better properties. The possibilities for such conversions into high-performance polymers are illustrated by cardanol, a phenolic lipid of related structure obtained from Ana-cardium occidentale. 

Figure I. Chemical structures of various phenolic lipids (A) sorgoleone, (B) heptadecenyl resorcinol (C) urushiol and (D) anacardic acid.

Besides urushiol, similar natural phenolic lipids, laccol from Taiwan and Vietnam and thitsiol from Thailand and Myanmar, are produced in Southeast Asian countries. They are inexpensive and often used for primer coating. The laccase-catalyzed crosslinking of urushiol, laccol, and thitsiol was examined in the presence of a protein hydrolysate [95]. Laccase (PCL and ML) efficiently catalyzed the crosslinking of urushiol and laccol to produce the film with high gloss surface and hardness. 

ABSTRACT This review is concerned with non-isoprenoid phenolic lipids typified by compounds biosynthesised by the polyketide pathway. Botanical, biological and entomological sources of such phenolic lipids are described which contain monohydric phenols, notably cardanol and relatives, dihydric phenols such as cardols, alk(en)ylresorcinols,urushiols and phenolic acids, particularly anacardic acids. Some recently investigated mixed types of dihydric phenolic lipids are included. Separatory methods are briefly reviewed. Synthetic methods for the saturated and unsaturated members of the three main classes of interest in structure/activity studies are summarised. Biological properties of members of the three main classes are given and discussed. 

Dihydric phenolic lipids are the most abundant phenolic lipids. They are of the cardol-or urushio 1-type, i.e. with hydroxyl groups in the metha position (1,3-dihydroxy) or in the ortho (1,2-dihydroxy) position respectively. Urushiol-type, dihydroxy phenolic lipids, are practically restricted to the Anacardiaceae family (Rhus genus) of poisonous plants. The most well-known are poison ivy, poison oak and the Indian marking nut. The sap from various plants belonging to Anacardiaceae from which Japanese, Chinese, Burmese, Formosan and Vietnamese lacs is one of the richest source of these phenols. The occurrence of dihydric phenolic lipid of the urushiol type is depicted in Table 3. 

Table 3. Occurrence of dihydric phenolic lipids of the urushiol type...

Fig. (3). Structures of known dihydric phenolic lipids Of urushiol (1,2-dihydroxy) type...

The Wittig reaction approach provides an alternative route to the synthesis of the unsaturated phenolic lipids in urushiol in Rhus vernicifera, which contains three C15 isomeric trienes, the major constituent the semi-conjugated Fig (3)-4, 8(Z),11(E),13(Z), (55.4%), the trace corresponding 13(E) isomer (1.7%) and the non-conjugated Fig (3)-5, 8(Z), 11(E), 14 (7.4%) which is also the chief component in urushiol from Rhus toxicodendron. Scheme 1 shows the routes to the first and last constituents [230-233]. 

Cardol (47), one of the phenolic lipids that characterise extracts of members of the plant family Anacardiaceae [118] showed pronounced antifilarial activity (LDioo 3.5 ppm) [119]. In rats, it was tolerated up to concentrations of 5 g/kg body weight. For activity, the phenolic hydroxyls and the alkyl side chain were necessary [119,120]. The related compound urushiol (48) showed toxicity to Ascaris suilla [121]. 

In earlier syntheses of the methyl ethers of (15 1)-cardanol (ref. 136) and urushiol (ref. 137) it is doubtful if the products were stereochemically pure. Nonetheless this work represents the first progress towards obtaining unsaturated methyl ethers of the phenolic lipids. Early difficulties centred on the lack of specificity in the reduction of the triple bond, in the removal of the protective group on the phenolic OH without affecting the double bond and, before kinetic and thermodynamic control was used, on lack of steric control in the Wittig reaction. 

The phenolic lipids of Anacardieum occidentale have been commercially exploited (ref. 174) and those in Rhus vernicifera to a lesser extent. Most of the technical cashew nut shell liquid (CNSL) which results from industrial processing is and has been employed as a phenolic source for formaldehyde polymerisation the products from which in compounded form have been the basis for friction dusts widely used throughout the world in vehicle brake and clutch linings (ref.175). Urushiol has had use over many centuries in the art of Japanese lacquering (ref. 176) and in more recent years has been sometimes supplemented with CNSL. Chemical uses are referred to later. 

A number of applications of commercial lacs and of separated urushiol have been referred to (ref. 2). As with the phenolic lipids of Anacardium occidentale a great deal of work has been carried out particularly in Japan and China to diversify the uses of lacs from Rhus vemicifera. It is widely employed in artistic decoration, building materials, textile equipment and furniture. The industrial utilisation of polyketide natural products including the phenolic lipid urushiol has been reviewed (ref. 314). The great number of uses largely comprise polymerisation reactions and some non-polymeric processes, some of both of which are described in the next sections. 

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