4-Hydroxycinnamyl alcohol

Reduction. Coumarin is reduced to o-hydroxycinnamyl alcohol by reaction with lithium aluminum hydride (21). By reaction with diborane coumarin gives o-aUylphenol [1745-81 -9] (22). 

Lignin polymerization is a natural process initiated by the enzymatic oxidation of hydroxycinnamyl alcohols such as /)-coumaryl (CM), coniferyl (CF), and sinapyl alcohols (SN), which are known as monolignols (Scheme 2.20).60... 

SCHEME 2.20 Hydroxycinnamyl alcohols involved in lignin polymerization. 

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

Quinone methides play an important role in lignification. They are produced directly, as intermediates, when lignin monomers, be they hydroxycinnamyl alcohols, hydroxy-cinnamaldehydes, or hydroxycinnamates, couple or cross-couple at their 8- positions. A variety of postcoupling quinone methide rearomatization reactions leads to an array of structures in the complex lignin polymer (Fig. 12.2). 

Takahama, U. Oxidation of hydroxycinnamic acid and hydroxycinnamyl alcohol derivatives by laccase and peroxidase—interactions among p-hydroxyphenyl, guaiacyl and syringyl groups during the oxidation reactions. Physiol. Plantarum 1995, 93, 61-68. 

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. 

Whilst it is not possible to give a completely detailed structure for lignin, a great deal is known about the molecule. All lignins appear to be polymers of 4-hydroxycinnamyl alcohol (/>-coumaryl alcohol) or its 3- and/or 3,5-methoxylated derivatives, respectively coniferyl and sinapyl alcohol (Figure 3.1). 

Lignin its Constitution and Formation from />-Hydroxycinnamyl Alcohols. Science [Washington] 148, 595—600 (1964). 

Dehydration of o-hydroxycinnamyl alcohols (94) may be achieved by pyrolysis or under acid catalysis. In the latter, formation of an allyl carbocation (95) is considered to precede nucleophilic attack by the phenolic oxygen atom, though this is once again the involvement of a quinoneallide in another guise (72CB863). 

Mn Peroxidase. The manganese peroxidase (MnP) is one of the two known enzymes capable of the oxidative degradation of lignin, an amorphous, random, aromatic polymer synthesized from p-hydroxycin-namyl alcohol, 4-hydroxy-3-methoxycinnamyl alcohol, and 3,5,-dime-thoxy-4-hydroxycinnamyl alcohol precursors by woody plants. Both enzymes contain the protoporphyrin IX heme prosthetic group, similar to the heme peroxidases with an L5-histidine and both use hydrogen peroxide as a substrate. However, the manganese peroxidase has an absolute requirement for Mn(II) to complete its catalytic cycle (50). The X-ray structure of this protein has recently appeared (51). 

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. 

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