|dc.description.abstract||A range of l,2-diarylpropane-1,3-diols and 2,3-dihydro-2-aryl-7- methoxy-benzofurans were synthesised as model compounds for the B-5 (phenylcoumaran) structural unit found in wood lignin. The syntheses were performed following either established, or modifications of established procedures. Preliminary studies were carried out which showed the viability of a new general synthesis for B-aryl lignin model compounds.
Phenylboronates of three of the l,2-diarylpropane-1,3-diols were synthesised. The ¹H NMR coupling constants observed between the vicinal protons on the 6-membered ring confirmed the previously inferred erythro stereochemistry. Single crystal X-ray diffraction structures were obtained of 2,3-dihydro-2-(4-hydroxy-3- methoxyphenyl)-3-methyl-5-propyl-7-methoxybenzofuran and l-(4-hydroxy-3-methoxyphenyl-2-(2,3-dimethoxyphenyl)-propane-l,3- diacetate.
Quinone methides (4-methylene-cyclohexa-2,5-dienones) derived from the B-5 lignin model compounds were treated with anthranol (a reduced form of anthraquinone, AQ, found in soda-anthraquinone pulping liquors) to form a range of adducts. Anthrahydroquinone, the major catalytic species in the soda-AQ process, would not add to the quinone methides. The presence of erythro and threo isomers of the anthranol adducts was observed by ¹H and ¹³C NMR, and in some cases a preparative chromatographic separation of the two isomers was possible. The addition of anthranol to the quinone methides was shown to be reversible. The erythro adduct was found to be kinetically preferred, but over long reaction times, the threo isomer predominated. It was shown that for quinone methides possessing the B-hydroxymethyl group, the retro-aldol elimination of formaldehyde to give stilbenes took precedence over adduct formation. Thus, stilbene formation was believed to be that which occurs for phenylcoumaran and B-C-1 structures when reacted under soda-AQ pulping conditions, and the reactions of these structures would not contribute to delignification. Conversely, it was not expected that the phenylcoumaran and B-C-1 structures would contribute to the loss of AQ from the process.
The stabilised quinone methides were characterised by ¹H and ¹³C NMR. It was found that these and previously synthesised quinone methides derived from the B-aryl ether lignin structure, existed in solution as a mixture of two isomers in a ratio of 2:1, SYN:ANTI. The preference for the formation of the SYN isomer was believed to be either product-controlled or from conformational equilibrium control in the transition-state leading to the quinone methide. It was found by a ¹H NMR method that the ANTI isomers of all the quinone methides studied reacted with amines approximately 40% faster than the SYN isomers. It was not possible to determine if this difference in reactivity would be exhibited by quinone methides reacted under pulping conditions.||