P-Hydroxycinnamyl alcohol

Freudenberg, K. Lignin. Its constitution and formation from p-hydroxycinnamyl alcohols. Science 1965, 148, 595-600. 

The data presented in a recent communication by Freudenberg et al. (32) show that the methoxyl content of the dehydrogenation polymers of coniferyl alcohol do not change with condensation time. However, their reference to p-hydroxycinnamyl alcohols seems to indicate their appreciation of the significance of a p-hydroxyphenylpropane unit in the mechanism of lignin formation. 

As the benzyl (a) carbon is the most reactive site on the propanoid side chain (Allan 1971), it is not surprising that many substitution reactions are documented as occurring at this position and at the vinylogous y-carbon atoms of p-hydroxycinnamyl alcohol units. The variety of substitution reactions occurring at the benzyl carbon atom has been thoroughly reviewed by Allan (1971). 

The determination of catechol groups in various lignins is of practical and fundamental importance (Chap. 5.2.3.3). That such compounds have not been detected in acidolysis studies (Gellerstedt et al. 1984) may be attributed to their unstability under the acidolysis conditions. The indicated detection of catechol groups implies that structures such as p-hydroxycinnamyl alcohol end groups and catechol units, which are unstable in acidic aqueous media, may be determined by thioacidolysis. 

Support for the occurrence arylglycerol units (54) was obtained in mild hydrolysis studies of lignin [102]. According to studies of the formation of formaldehyde on periodate oxidation [103] only a few percent units with glycerol chain could be present in MWLs. The fact that arylglycerols were formed on enzymatic oxidation of p-hydroxycinnamyl alcohols provided further support for the occurrence of units of type 54 in lignins [104]. Final confirmation of this was later obtained in 2D NMR spectroscopic studies [74]. 

It is usually assumed that the covalent bonds between lignin and carbohydrates in LLCs are formed during the biosynthesis of lignin, through the addition of nucleophiles to quinone methides, formed as intermediates in the oxidation of p-hydroxycinnamyl alcohols [67,146,147],... 

T Higuchi, F Nakatsubo, Y Hceda. Enzymic formation of arylglycerols from p-hydroxycinnamyl alcohols. Holzforschung 28 189-192, 1974. 

Landucd L.L., Reaction of p-hydroxycinnamyl alcohols with transition metal salts. 3. Preparation and NMR characterization of improved DHPs, J. Wood Chem. Technol., 20, 2000, 243-264. 

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