Isochroman lignin trimers from DFRC-degraded Pinus taeda

Abstract A pair of new aryl isochroman lignin trimers, 7-acetoxy-1-{2-acetoxy-1-[4-(3-acetoxyprop-2-enyl)]-2-methoxyphenoxy}-3-(4-acetoxy-3-methoxyphenyl)-4-acetoxymethyl-6-methoxy-isochromans, along with three related dimers were isolated from DFRC degraded loblolly pine ( Pinus taeda L.) wood following gel permeation and reversed-phase TLC and HPLC. Their structures were elucidated by means of 2D–NMR, FAB–MS and GC–MS analyses. These two trimers contained a unique β–1 inter-unit linkage with a 6-membered di-α-ether ring and with the original sidechain migrated. The mechanistic plausibility of the lignification pathway, the isolation of expected DFRC degradation dimers and trimers, and the tentative identification of the aryl isochroman structure in pine milled wood lignins by NMR spectroscopy, all suggest that such structures, or their precursors, are present in native lignins (in their unacetylated forms). An unrelated trimer possessing 5–5/β–β interunit linkages was also isolated and characterized.