Adducts of Anthrahydroquinone and Anthranol with Lignin Model Quinone Methides. 3. Independent Synthesis of Threo and Erythro Isomers

Article abstract of DOI:10.1021/jo00170a003

A key intermediate responsible for anthrahydroquinone- (AHQ-) catalyzed delignification of wood under alkaline conditions is an adduct between AHQ and a quinone methide (QM) of a phenylpropanoid lignin subunit represented by models such as 1-(3-methoxy-4-hydroxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol.Unlike adduct formation via QMs, which give only threo products with AHQ or anthranol (9-hydroxyanthracene), both isomers were successfully prepared by alkylating 9,10-bis(trimethylsiloxy)anthracene or 9-(trimethylsiloxy)anthracene with α-bromo derivatives of ligninmodels in the presence of the Lewis acid zinc bromide.It is demonstrated by 1H NMR spectroscopy that the solution conformations of erythro adducts are marcedly different from those of the threo adducts and that the conformations vary with substitution and changes in solvent.Also, the erythro isomers are relatively unstable and readily isomerize under the alkylation conditions.