P-Coumaryl alcohol

Tanahashi M. Takeuchi H. Higuchi T. Dehydrogenative polymerization of 3,5-disubstituted p-coumaryl alcohols. Wood Res. 1976, 61, 44—53. 

Kobayashi, T. Taguchi, H. Shigematsu, M. Tanahashi, M. Substituent effects of 3,5-disubstituted p-coumaryl alcohols on their oxidation using horseradish peroxidase-H202 as the oxidant. J. Wood Sci. 2005, 51, 607-614. 

Since numerous degradations of hgnin, e.g. ethanolysis according to Hibberi (77) or hydrogenation with a variety of catalysts 72, 77, 119, 135), had shown that it is in fact made up largely of phenylpro-panoid units, it seemed that the theory of its origin from the three -p-coumaryl alcohols could be considered reliable. It remained therefore tO establish the nature of the processes involved in the polymerization. 

Figure 49 The precursors of lignin biosynthesis, p-coumaryl alcohol (I), coniferyl alcohol (II) and sinapyl alcohol (III). [65]...

Condensed Structures in p-Hydroxyphenyl-Guaiacyl Type DHP. It has been difficult to determine the exact amount of p-hydroxyphenylpropane units in lignin. This can best be illustrated by an example nitrobenzene oxidation of a DHP prepared by the Zutropfverfahren method from a mixture of p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, gave no detectable p-hydroxybenzaldehyde on alkaline nitrobenzene oxidation... 

To determine the reasons for this, DHP s were prepared from a mixture of [ring-2-3H] p-coumaryl alcohol and [or-14C] coniferyl alcohol in the presence of carbohydrates by the procedure described above (35). The LCC fraction so obtained was subjected to combustion, and the exact p-hydroxyphenylpropane guaiacylpropane ratio was determined from the activity of 3H20 and 14C02 released (Figure 2). A portion of the same LCC fraction was also oxidized with nitrobenzene and alkali, and the resulting liberated aromatic aldehydes were then analyzed by HPLC. Results are shown in Table V. 

Figure 2. Dehydrogenative polymerization of a mixture of p-coumaryl alcohol-[ring-2-3H] and coniferyl alcohol-[U-14C], and nitrobenzene oxidation of the DHP to give p-hydroxybenzaldehyde-[ring-2-3H] and vanillin-[formyl-14C].

As can be seen, the molar ratios of p-hydroxyphenyl to guaiacyi units were slightly lower for the DHP s when compared to the original mixtures. This implies that coniferyl alcohol tends to be incorporated into the polymer slightly more readily than p-coumaryl alcohol. On the other hand, the much-reduced ratio of p-hydroxybenzaldehyde to vanillin, liberated during alkaline nitrobenzene oxidation, proved that this DHP contained a larger amount of condensed p-hydroxyphenylpropane units than condensed guaiacyi units. 

In principle, most types of biomass can be used as a raw material in the pyrolysis process [14]. Most of the research has been carried out using different wood as feedstock, although more than 100 different types of biomass have been tested [14]. Besides wood, these materials include forest residues, such as bark black liquor and agricultural residues such as straw, olive pits, and nut shells [12, 14], Additionally, pure biological polymers cellulose (linear polymer of D-glucose units), hemicellulose (heteropolymers of different hexoses and pentoses), and lignin (heteropolymer of p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol [19]) - have been tested as raw materials in the pyrolysis. 

Fig. 4-2. Simplified reaction route illustrating the formation of lignin precursors. 1, 5-Dehydroquinic acid 2, shikimic acid 3, phenylpyruvic acid 4, phenylalanine 5, cinnamic acid 6, ferulic acid (Ri=H and R2=OCH3), sinapic acid (R,= R2=OCH3), and p-coumaric acid (R1=R2 = H) 7, coniferyl alcohol (Ri = H and R2=OCH3), sinapyl alcohol (Rj = R2=OCH3), and p-coumaryl alcohol (R =R2=H) 8, the corresponding glucosides of 7.

Fig. 4-3. Lignin precursors. 1, p-Coumaryl alcohol, 2, coniferyl alcohol 3, sinapyl alcohol.

In normal softwood lignin, usually referred to as guaiacyl lignin, the structural elements are derived principally from coniferyl alcohol, (1) (more than 95% of the total number of structural units), with the remainder consisting mainly of p-coumaryl alcohol-type units (Fig. 1.1, unit 2) and trace amounts of sinapyl alcohol-derived units (Fig. 1.1, unit 13). 

Yamasaki T, Hata K, Higuchi T (1972) Chemical properties of dehydrogenation polymer from p-coumaryl alcohol Mokuzai Gakkaishi 18 361-366... 

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