Aucuparins and Structurally Related Biphenyls

Aucuparin (22 a) was first isolated from the heartwood of mountain ash (Sorbus aucuparia, Rosaceae) where it co-occurs with methoxyaucuparin (22 b). They constitute up to 1% (dry-weight) of the wood (39, 40, 83). Their structures have been firmly established by syntheses (83). The compounds have later been found to be common wood constitutents of many Sorbus spp. (H. Erdtman, T. Norin, E. von Rudloff, unpublished results). 4-Hydroxyaucuparin (22 c) has been isolated from S. aucuparia. Chromatographic studies have shown that this compound is also present in many other Sorbus spp. (B. Kimland, T. Norin, unpublished results). 

Aucuparin has also been isolated from the wood of Kielmeyera spp. (Gut-tiferae), where it occurs together with xanthones. This observation has led Gottlieb (47) to propose that the biosyntheses of the two types of compounds are related. 

Aucuparin has been found to be a fungicidic phytoalexin of the loquat (Eriobotry japonica, Rosaceae) (119) and the compound also occurs in the sap-wood of the apple tree (Malus pumila, Rosaceae) when infected with the fungus that causes silver leaf disease (Chondrostereum purpureum) (70). The infected wood of the closely related pear tree (Pyrus communis, Rosaceae) (69) and the infected wood of Cotoneaster lactea (Rosaceae) (20) contain phytoalexins of the biogenetically related dibenzofuran type (a-, P-, and y-pyrufuran, and cotone-furan, 23 a-d). 

The aucuparins appear to be significant constituents of only a limited number of genera and families and are therefore useful taxonomic markers. So far they have only been found in species belonging to the Rosaceae family. They are part of the defense system and are formed in response to infections or as a consequence of heartwood formation. Further members of this class of plant phenolics will certainly be found in the future. 

Plant phenolics of biphenyl type but of different biogenetic origin to those of the aucuparins are constituents of the sap of the lac tree (Rhus vernicifera, Anacardiaceae). The sap is the raw material of the Japan lacquer. It dries into a tough and brilliant film and has been used in the Orient as a coating material for thousands of years (75) (see also Chap. 1.1). The major constituent of the sap is urushiol (24). The drying process is believed to be an enzymatic oxidative coupling of urushiol under the influence of oxidoreductases - e.g. laccase to form biphenyls (e.g. 25), dibenzofurans (e.g. 26), oligomers, and polymers (88). 

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