Novel tetrahydrofuran structures derived from β–β-coupling reactions involving sinapyl acetate in Kenaf lignins

Abstract

Free radical coupling of sinapyl gamma-acetate or cross-coupling between sinapyl acetate and sinapyl alcohol yields novel tetrahydrofuran beta-beta-(cross-)coupled dehydrodimers. Such substructures are therefore anticipated in naturally acetylated lignins, e.g. in Kenaf, if sinapyl acetate is a component of the lignin monomer pool. The DFRC (derivatization followed by reductive cleavage) method, modified by replacing all acetyl reagents and solvents with their propionyl analogs (DFRC'), allows the analysis of naturally acetylated lignins. DFRC' treatment of the sinapyl acetate-derived dimers or crossed dimers gave diagnostic products that retain at least one acetate group on a sidechain gamma-position; the products have been authenticated by comparison of their mass spectra and GC retention times with those of synthesized compounds. DFRC' of Kenaf lignins produces the same diagnostic products as from the dimers, implicating the presence of the various tetrahydrofuran units in Kenaf lignins. With data from the model compounds in hand, NMR analysis of Kenaf lignins elegantly confirms the presence of such substructures in the polymer, establishing that acetates on Kenaf lignins arise through incorporation of sinapyl acetate, as a lignin precursor, via enzyme-mediated radical coupling mechanisms.