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 met hod takes advantage of the large dispersion of C-13 chemical shifts to reso lve individual H-1 chemical shifts. The whole spin system of a H-1-C-13 cor relation observed in an HMQC spectrum, even for minor components and unknow n structural units, can be traced out. It is shown here that both trans- an d cis-isomers of 6,7-dihydrodibenzo(e,g)(1,4)-dioxocin, a recently discover ed prominent linkage in softwood lignins, occur also in hardwood lignin. Mo reover, the poplar lignin preparation contains small amounts of noncyclic a lpha-aryl ether linkages. Signals from a side chain of unknown structure ar e tentatively assigned to a spiro-cyclohexadienone.