Identification of side-chain structures in a poplar lignin using three-dimensional HMQC-HOHAHA NMR spectroscopy

The structure of an;acetylated C-13-enriched poplar wood lignin preparation was studied using three-dimensional HMQC-HOHAHA NMR spectroscopy. This method takes advantage of the large dispersion of C-13 chemical shifts to resolve individual H-1 chemical shifts. The whole spin system of a H-1-C-13 correlation observed in an HMQC spectrum, even for minor components and unknown structural units, can be traced out. It is shown here that both trans- and cis-isomers of 6,7-dihydrodibenzo(e,g)(1,4)-dioxocin, a recently discovered prominent linkage in softwood lignins, occur also in hardwood lignin. Moreover, the poplar lignin preparation contains small amounts of noncyclic alpha-aryl ether linkages. Signals from a side chain of unknown structure are tentatively assigned to a spiro-cyclohexadienone.