A BIOMIMETIC ROUTE TO LIGNIN MODEL COMPOUNDS VIA SILVER(I) OXIDE OXIDATION .1. SYNTHESIS OF DILIGNOLS AND NONCYCLIC BENZYL ARYL ETHERS

The oxidative coupling of (E)-coniferyl alcohol using 1.5 equiv. of si lver (I) oxide in methylene chloride led to the formation of beta-5-co upled dehydrodiconiferyl alcohol in 50% yield, whereas in 1 : 1 methyl ene chloride: water solution, the beta-beta-coupled dehydrodimer pinor esinol was obtained as the major reaction product in 25% yield. In aqu eous (pH 5-8) and non-aqueous solutions, the quinone methide generated by beta-O-4 coniferyl alcohol radical coupling rapidly led to the for mation of significant amounts of the non-cyclic alpha-O-4 benzyl aryl ethers; alpha-dehydrodiconiferyl-beta-coniferyl-bis-ether, erythro-gua iacylglycerol-alpha,beta-bis-coniferyl ether and ycerol-alpha-pinoresi nol-beta-coniferyl-bis-ether, via phenol addition. The addition of wat er became predominant only under acid catalysis. Non-cyclic alpha-O-ga mma benzyl alkyl ethers were not obtained. These findings, together wi th literature precedents, have implications for lignin biosynthesis.