Adducts of Anthrahydroquinone and Anthranol with Lignin Model Quinone Methides. 1. Synthesis and Characterization

Article abstract of DOI:10.1021/jo00139a020

Adduct formation of anthrahydroquinone (9,10-dihydroxyanthracene, AHQ) or anthranol (9-hydroxyanthracene) with lignin model quinone methides (4-methylenecyclohexa-2,5-dienones) was established.This reaction is thought to be the key step in AHQ-catalyzed delignification of wood under alkaline pulping conditions.Numerous quinone methides derived from both 1-aryl-2-O-arylethyl and 1-aryl-2-O-arylpropyl lignin models were used.A typical example is the reaction of the quinone methide derived from 1-(3-methoxy-4-hydroxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol with AHQto give the adduct threo-1-(3-methoxy-4-hydroxyphenyl)-1-(10-hydroxy-9-oxoanthracen-10-yl)-2-(2-methoxyphenoxy)propan-3-ol. 1H NMR spectra of the adducts revealed large diamagnetic shifts of the protons in the 1-aryl substituent due to its close approach to the shielding regions of the anthracenyl moiety.This effect dimished with increasing size of the 10-substituent (H to OH to OAc).In AHQ adducts, intense hydrogen bonding between the 10-OH and the ether oxygen of the 2-aryl ether substituent was indicated by a large paramagnetic shift of the hydroxyl proton.The unusually large diamagnetic and paramagnetic shifts reflect a distinct rigidity of the adduct conformation that is more pronounced in the adducts containing a propyl side chain.