The DFRC method for lignin analysis - Part 5 - Isochroman lignin trimers from DFRC-degraded Pinus taeda

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-ac etoxymethyl-6-methoxy-isochromans, along with three related dimers were iso lated from DFRC degraded loblolly pine (Pinus taeda L.) wood following gel permeation and reversed-phase TLC and HPLC. Their structures were elucidate d by means of 2D-NMR, FAB-MS and GC-MS analyses. These two trimers containe d a unique beta-1 inter-unit linkage with a 6-membered di-alpha-ether ring and with the original sidechain migrated. The mechanistic plausibility of t he lignification pathway, the isolation of expected DFRC degradation dimers and trimers, and the tentative identification of the aryl isochroman struc ture in pine milled wood lignins by NMR spectroscopy, all suggest that such structures, or their precursors, are present in native lignins (in their u nacetylated forms). An unrelated trimer possessing 5-5/beta-beta interunit linkages was also isolated and characterized. (C) 1998 Elsevier Science Ltd . All rights reserved.