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Reaction woods of three softwoods,  Pinus merkusii ,  Cryptomeria japonica  and  Cedrus deodara , were investigated by alkaline nitrobenzene oxidation (NBO) to characterize the condensed-type structures in compression wood lignins. A novel biphenyl-type NBO product carrying guaiacyl (G)- and  p -hydroxyphenyl (H)-units, dehydrovanillin- p -hydroxybenzaldehyde (HG-biphenyl product), was identified using the authentic standard compound. On the basis of the yield of this novel NBO product, as well as those of GG-biphenyl-, β-5-, and uncondensed-type products [e.g. dehydrodivanillin, 5-formylvanillin, vanillin and  p -hydroxybenzaldehyde], the compression wood lignins contained more HG-type biphenyl and H-type β-5 structures than the opposite wood lignins. The increase in the condensed-type structure content was largely offset by the decreases in the content of GG-biphenyl and G-type β-5 structures. Consequently, the relative yields of biphenyl, β-5 and uncondensed-type NBO products were very similar between the compression wood and the opposite wood, even though the H-unit having no methoxy group on its aromatic ring can be assumed to have a greater probability to form condensed-type structures during lignin biosynthesis than the G-unit.

Influence of the p -hydroxyphenyl/guaiacyl ratio on the biphenyl and β-5 contents in compression wood lignins