"Enzymic Oxidative Coupling of Urushiol in Sap of the Lac Tree,Rhus vernicifera."

Dimerization of urushiol is a significant intial step of the laccase-catalyzed polymerization of natural lacquer.Urushiol dimers produced during the physiological oxidation of urushiol were thoroughly separated by liquid chromatography.Obtained have been classified into four types of compounds, viz., biphenyls, dibenzofurans, uncleus-side chain bound dimers,and their side chain oxidized products.Dibenzofurans are derived by successive oxidation of biphenyls and the last type of compounds by oxidation of parent dimers.Then it has been established that the dimerization of urushiol in natural sap proceeds through two predomainant reaction routes,i.e,phenol coupling and uncleus-side chain coupling.For the first and third types of compounds, product distribution with special regard ti the orientation of reactions has been compared to frontier electron densities of possible reaction species.It hasbeen inferred that an attack of urushiol-semiquinone on the urushiol uncleus affords biphenyls.Urushiol-quinone produced by disproportionation of the semiquinone abstracts from the side chain of triolefinic urushiol, 3-［8'(Z),11'(E),13'(Z)-pentadecatechyl］(3), to give the 1,7-disubstituted heptatrienyl cation 24.Electrophilic substitution of the cation to the urushiol uncleus-side shain bound dimers.In this reaction, C-C coupling occurs exclusively,and derivatives of dihydric phenols are given as dimers of urushiol.This regiospecificity may be due to the slightly acidic reaction medium of natural sap facilitates subsequent oxidation mediated by polyphenol oxidase laccase.