NMR Assignment for Diaryl Ether Structures (4-O5 Structures) in Pine Wood Lignin

A 4-O-5-tetramer lignin model compound carrying beta-O-4 linkages on each Of the side-chain Moieties was synthesized, as well as 4-O-5-coupled dehydrodiconiferyl alcohol. By comparison with theiriNMR,data, two cross signals in the HSQC spectrum Of pane milled wood lignin recorded in DMSO-d(6) were assigned-to H2/C2 and H6/C6 correlations on the aromatic rings of 4-O-5-linke.d units; Although the H2/C2 correlation peak- appeared in the same region as syringyl units, nitrobenzene-oxidation of the pine lignin did not yield any syringyl-type product, but did release a 4-O-5-type product.