Coniferyl alcohol dehydrogenation

Ralph, J., Hehn, R. F., Quideau, S., and Hatfield, R. D. (1992) Lignin-feruloyl ester cross-links in grasses. Part 1. Incorporation of feruloyl esters into coniferyl alcohol dehydrogenation polymers. J. Chem. Soc., Perkin Trans. / (21), 2961-2969. 

According to a widely accepted concept, lignin [8068-00-6] may be defined as an amorphous, polyphenoHc material arising from enzymatic dehydrogenative polymerization of three phenylpropanoid monomers, namely, coniferyl alcohol [485-35-5] (2), sinapyl alcohol [537-35-7] (3), and /)-coumaryl alcohol (1). 

Recent work by Atalla(H) supports the idea that lignin is at least a semi-ordered substance in wood with the plane of the aromatic ring parallel to the cell wall surface. Woody plants synthesize lignin from trans-coniferyl alcohol (pines), trans-sinapyl alcohol 2 (deciduous), and trans-4-coumaryl alcohol 3 by free radical crosslinking initiated by enzymatic dehydrogenation(l2). Structures of these alcohols are given in Figure 1. 

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. 

Figure 7. NMR solution spectra of dehydrogenative polymerizates formed from [2- C]coniferyl alcohol with H2O2 in the presence of (a) lignin peroxidase at pH 4.0 and (b) horseradish peroxidase at pH 6.5 (51). S denotes solvent while A-E refer to substructural assignments depicted on left.

Figure 8. Dehydrogenative po erization (1) of [2- C]coniferyl alcohol with H2O2 at the hands of lignin peroxidase during 72 h at pH 4.0 effect of further enzyme action (2) upon dehydropolymerizate for subsequent 20 h period in the presence of H2O2 (51). Sephadex GlOO/aqueous O.IOM NaOH elution profiles monitored at 280 nm (dotted line blue d ran).

Moreover, no NMR spectral changes were detected as a consequence of treating dehydropolymerizates from [1- C], [2- C] and [3- C]coniferyl alcohol, respectively, or a dehydrogenative copolymer of /7-[/ing-4 -i C]coumaryl alcohol and coniferyl alcohol, at pH 3.0 with racellular P, chrysosporium culture fluid, or purifled lignin peroxidase, in the presence of H2O2 nor was the outcome affected by prior methylation of the substrates (52). Thus the result originally encountered with the purified spruce wood extract (13) is not representative of polymeric lignin-like preparations at all. 

Figure 1. Removal of 3H at position 5 of the guaiacyl ring of coniferyl alcohol (I) by formation of ring substituted structures (V, VI, VII) during dehydrogenative polymerization.

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].

Figure 1. Formation of guaiacyl lignin and lignin-carbohydrate complexes (LCC) via dehydrogenative polymerization of coniferyl alcohol.

Meanwhile, Freudenberg (17) was the first person who demonstrated the formation of an addition compound from a quinonemethide and sucrose during enzymatic dehydrogenation of coniferyl alcohol in a concentrated sucrose solution. Thereafter, Tanaka (18) observed the formation of a benzyl ester between the quinonemethide of a dilignol and a uronic... 

Freudenberg realized the importance of investigating this possibility. In 1937 he found that dehydrogenation of coniferyl alcohol with ferric chloride seemed to proceed in a way comparable with that of isoeugenol, and in 1943 he started his studies on the enzymatic dehydrogenation of coniferyl alcohol. It would not be possible here to give even a brief survey of the outstanding work which he has done since then. 

When coniferyl alcohol is dehydrogenated, it loses its phenolic hydrogen atom to form first an aroxyl radical Ra (XI), which is in eflFect also present as the mesomeric radicals Rb (XII), Rc (XIII), and Rd (XIV). Of these limiting structures, Rb is the most favored. The existence of the radicals in these forms is recognized by their reaction products. In very dilute dioxane-water solution (1 1 vol.), the half-life of the radicals is about 45 seconds 13). 

Other monolignols formed during the dehydrogenation of coniferyl alcohol are conifer aldehyde (VII), trans- and m-ferulic acid (XV, XVI), vanillin (traces), and vanillic acid (traces). 

Guaiacylglycerol 0,7-bisconiferyl ether (XLII) (16) is a labile trilignol formed by addition of coniferyl alcohol onto the dimeric quinonemethide (XXI). It readily loses a molecule of coniferyl alcohol by hydrolysis but is stabilized somewhat as soon as its phenolic group is etherified by further dehydrogenation and interaction with other radicals. 

Guan, S. Y., Mylnar, J., and Sarkanen, S., 1997, Dehydrogenative polymerization of coniferyl alcohol on macromolecular lignin templates,... 

Fig. 4-4. Formation of resonance-stabilized phenoxy radical by the enzymic dehydrogenation of coniferyl alcohol (Adler, 1977).

Fig. 4-7. Endwise polymerization (Adler, 1977). A guaiacylglycerol-/3-aryl ether structure (1) is dehydrogenated and after resonance, radical c is coupled with a coniferyl alcohol radical b (cf. Fig. 4-4). The /3-5 coupling product (3) is tautomerized and undergoes intramolecular ring closure (a phenylcoumaran structure, 5).

This product may be derived from a glyceraldehyde-2-aryl ether unit formed via a displacement reaction in the dehydrogenative polymerization of coniferyl alcohol (Lundquist et al 1967)... 

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