C-13 NMR SPECTROSCOPIC STUDIES OF PHENYLCOUMARAN AND 1,2-DIARYLPROPANE TYPE LIGNIN MODEL COMPOUNDS

A number of phenylcoumaran (cyclic beta-5') type lignin model compounds were synthesized from dehydrodiisoeugenol via cleavage of C-alpha-C-beta double bond in the side chain at C-1' of ring B by partial and selective ozonolysis to the corresponding aldehyde derivatives, followed by decarbonylation or reduction of the resulting aldehyde groups. In addition, C-alpha-reduced, noncyclic beta-5' type 1,2-diarylpropane and 2-aryl-3-methylcoumarone derivatives were obtained as by products in addition to the major products, the corresponding cyclic beta-5' type 2-aryl-3-methylcoumaran derivatives, when the corresponding 2-aryl-3-methyl-5-formylcoumaran derivatives were decarbonylated in the presence of 5% Pd-C as catalyst at high temperature under carbon dioxide atmosphere. Evidently, the byproducts were produced from the corresponding 2-aryl-3-methylcoumarans by disproportionation reaction under the reaction conditions. The possible reaction mechanism for the reaction is discussed. These compounds were synthesized in order to assign C-13 chemical shifts for aromatic carbons of phenylcoumaran type lignin substructures, as well as substituent effects on the aromatic carbons of these substructures.