Phenylcoumaran structures

Of the four remaining dimeric products, the phenylcoumarone (XXII) (m.p., 110°C.), isolated in a yield of about 0.5% of the lignin, and the 0,/> -dihydroxystilbene (XXIII) obviously are genetically interrelated both must be derived from a phenylcoumaran structure, as already shown above in the acidolysis of dihydrodehydrodiconiferyl alcohol. 

The postulation that the structure of one dimer is 2,4 -dihydroxy-3,3 -dimethoxy-5-ethylbibenzyl is, of course, based on NMR data, on the proposal that phenylcoumarane structures are common in lignin [see (/)], and on evidence that they are cleaved to stilbenes by alkali (14) and eliminate 7-methylol units (2) as formaldehyde by mechanisms quite analogous to those above. Although there are some modest surprises in the structure of dimers isolated so far, it is gratifying how well these results fit into our long-standing concepts of lignin structure and reactions. 

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).

F g- 7-27. Example of the base-catalyzed reactions of the free phenolic phenylcoumaran structures (1) (Gierer, 1970). Cleavage of the a-aryl ether bond results in a quinone methide intermediate (2) which after elimination of a proton from the /8-position is stabilized to a stilbene structure (3). Structures containing open a-aryl ether bonds react analogously. 

For lignin substructures containing a carbon substituent in the aromatic C-5 position, or for stilbenes originating from phenylcoumaran structures, the oxidative degradation leads to the formation of isohemipinic acid methyl ester (4) (Fig. 6.3.2, R=CH3). However, Gellerstedt and co-workers have shown that model compounds of the biphenyl type also give rise to isohemipinic acid in addition to the expected bis-vanillic acid structure (7). The reason seems to be incomplete alkylation (pH 11, 24 h) with the result that the substrate is alkylated at only one of the available positions. To achieve a more complete alkylation, it is necessary to increase the pH to 13 or alternatively, to extend the alkylation time to 84 h. In both cases, the yield of the bis-vanillic acid structure is substantially increased as shown in Table 6.3.1 nevertheless, the concomitant formation of isohemipinic acid cannot be completely avoided. 

FIGURE 2.11 Basic ozonation reactions on lignin strnctures (a) formation of erythronic and threonic acids from erythro and threo forms of lignin side chains involved in (3-0-4 bonds and (b) degradation of diarylpropane and phenylcoumaran structures. (Adapted from Akiyama, T., Sugimoto, T., Matsumoto, Y., and Meshitsuka, G., J Wood Sci., 48, 210-215, 2002 Aimi, H., Matsumoto, Y, and Meshitsuka, G., J Wood Sci., 51, 252-255, 2005.)... 

Ralph, J., Rde, R. M., and Wilkins, A. L. (1986) Synthesis of trimeric lignin model compounds composed of (3-aryl ether and phenylcoumaran structures. Holzforschung 40(1), 23-30. 

Units linked to an adjacent unit by a P-5 linkage (21) are present in phenylcoumaran structures. Other types of lignin structures containing a P-5 linked unit have been speculated to occur in lignin [61,62], but conclusive evidence for this has not been obtained. 

Since lignin is not a uniform entity, chemical criteria for its characterization have centred around analytical detenninations of its functional groups, e.g. total hydroxyl content, phenolic hydroxyls (56), methoxyl and other ether groups, benzyl alcohol groups (7a), carbonyl groups (6), etc., and estimations of its content of special structural features, e.g. phenylcoumaran units (5), biphenylyl linkages (123), etc. 

Min T, Kasahara H, Bedgar DL et al (2003) Crystal structures of pinoresinol-lariciresinol and phenylcoumaran benzylic ether reductases and their relationship to isoflavone reductases. J Biol Chem 278 50714-50723... 

Strong analytical support for the presence of the phenylcoumaran system (I) in lignin was obtained a few years ago (5) (Figure 1). Under the conditions of acidolysis, models for system I, namely dihydrodehydro-diconiferyl alcohol (III) 13) and its methyl ether (III, OCH3) were converted into phenylcoumarone derivatives (VIII and VIII, OCH3). The structure of the phenolic coumarone (VIII) was established by an inde-... 

Figure 1. Structure of phenylcoumaran (I) and arylglycerol P-aryl ether II)...

Phenylcoumarone (VIII) has a characteristic ultraviolet and ioniza-tion-Ae spectrum, which enabled us to detect dimeric structures of this type in reaction mixtures obtained when Bjorkman spruce lignin was subjected to acidolysis for 20 hours. From the spectrophotometric estimation of the amount of the phenylcoumarone systems formed, we concluded that from a total of 100 phenylpropane units of Bjorkman lignin, about 20 are involved in phenylcoumaran systems (I) in other words, about every 10th phenylpropane unit is linked to one of its neighbors by the cyclic benzyl aryl ether linkage characteristic of I. 

The structure of the phenylcoumarone (XXII) 26) was derived from analytical and spectral investigation and was confirmed by a synthesis 29) starting from dehydrodiconiferyl alcohol (XXVII). The latter compound (XXVII) was converted by monoperphthalic acid into an epoxide whose side chain was equivalent to that of an arylglycerol. By properly performed acidolysis, the epoxide side chain therefore was converted into the primary ketol structure, and at the same time the hydroxymethyl-substituted phenylcoumaran system see XXVII) was converted into the methyl-substituted phenylcoumarone system of XXII (Figure 8). 

Figure 8. Synthesis of phenylcoumaran XXII) and its formation from a trimeric structure in lignin...

Structures Containing a-Ether Bonds The a-ether bonds in phenolic phenylcoumaran (Fig. 7-27) and pinoresinol structures are readily cleaved by hydroxide ions, usually followed by the release of formaldehyde. Only in the case of open a-aryl ether structures does this reaction result in the fragmentation of lignin. In contrast, the a-ether bonds are stable in all etherified structures. 

Min, T., Kasahara, H., Bedgar, D.L., Youn, B., Lawrence, P.K., Gang, D.R., Halls, S.C., Park, H. Hilsenbeck, J.L., Davin, L.B. Lewis, N.G., Kang, C. (2003) Crystal structures of pinoresinol-lariciresinol and phenylcoumaran benzylic ether reductases and their relationship to isoflavone reductases. /. Biol. Chem., 278, 50714-23. 

Dimers 15-19 represent the ji-5 bonding pattern. Upon thioacidolysis, the ben-zylic ether linkage of phenylcoumaran-type models is cleaved with concomitant thio-ethylation. The C. -methylol group of ji-5 structures is lost to an extent that depends on thioacidolysis duration. In contrast, the total amount oi 16+ 17 or 18 +19 is stable when this duration varies between 3 and 6 hours. Minor dimers of the ji-5 series are observed as dimers with shortened side chains, similar to dimer 15 or with rearranged side chains similar to monomers 4-5 (Figure 2.4). 

Nakatsubo, R, and Higuchi, T. (1980) Synthesis of trimeric lignin model compound composed of phenylcoumaran and b-O-4 structures. Mokuzai Gakkaishi 26(1), 31-36. 

---