Phenylpropane unit

In the presence of sulfide or sulfhydryl anions, the quinonemethide is attacked and a benzyl thiol formed. The P-aryl ether linkage to the next phenylpropane unit is broken down as a result of neighboring-group attack by the sulfur, eliminating the aryloxy group which becomes reactive phenolate ion (eq. 2). If sulfide is not present, a principal reaction is the formation of the stable aryl enol ether, ArCH=CHOAr. A smaller amount of this product also forms in the presence of sulfhydryl anion. 

Some cleavage takes place even if the phenoHc hydroxyl is blocked as an ether link to another phenylpropane unit and quinonemethide formation is prevented. If the a- or y-carbon hydroxyl is free, alkaH-catalyzed neighboring-group attack can take place with epoxide formation and P-aryloxide elimination. In other reactions, blocked phenoHc units are degraded if an a-carbonyl group is present. 

Evidence for this consideration lies in the fact that the basic structure of methyl-p-methoxycinnamate is analogous to that of a p-hydroxy-phenylpropane unit from which p-hydroxybenzaldehyde could arise upon oxidation. This consideration supports and amplifies the experimental findings embodied in the phase sequence devised previously (79). 

Lignin is a polymer built up of phenylpropane units substituted with hydroxy and methoxy groups, see Figure 48. Lignin is a three-dimensional molecule like a 3-D matrix, see Figure 48 [65]. 

Suzuki S, UmezawaT, ShimadaM (2001) NorUgnan biosynthesis in zlipuragMi officinalis L. the norlignan originates from two nonidentical phenylpropane units. J Chem Soc Perkin Trans 1 3252-3257... 

Lignin in wood may be considered to be a random three-dimensional network polymer comprised of phenylpropane units linked together in different ways. When wood is delignified the properties of the macromolecules made soluble reflect the properties of the network from which they are derived. 

If lignin is heated with phenol, the phenol condenses with lignin in the a-position of the side chain. Phenol generally couples in its para- position (15). Under optimum conditions, from 2.5 to 3 moles of phenol or phenolic derivative per phenylpropane unit are added to the protolignin (18). 

Figure 5. Mechanisms responsible for the higher electron density on positions 2 and 6 on lignin phenylpropane units in acidic medium, (a) the induction effect of the alkyl group at position 1 (b) the resonance effects of the electron pairs on the methoxy oxygen.

Various attempts have been made to prepare adhesives from lignin. The preparation of resol resin adhesives has been studied especially extensively. The introduction of phenols into the a or /2-position of the sidechain of the phenylpropane unit (phenolation of lignin) has been considered a key reaction for the formulation of these types of adhesives with adequate glu-ability. 

The ozonization was performed at the three levels of molar equivalents to phenylpropane unit shown in Table I. The highest yield of over 100% in sample No. 1 was caused by the addition of ozone to lignin. On the other hand, the yield decrease in samples No. 2 and No. 3 was considered to be the result of removal of a considerable amount of ozonized lignin by... 

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. 

Reactivity of Aromatic Sites in Lignin. It is of considerable interest to estimate the significance of the protodedeuteration data in predicting the reactivities of the phenylpropane units in lignin. 

Lignin is an amorphous non-polar macromolecule, constituted of phenylpropane units. The structure of lignin depends on the source. Moreover, the extraction method modifies the structure of the lignin prior to analysis. Recent studies (Banoub et al., 2007 Forss and Fremer, 2000) have shown fundamental differences with the known structures of lignin. 

Lignins are polymers of phenylpropane units. Many aspects in the chemistry of lignin still remain unclear, for example, the specific structural features of lignins located in various morphological regions of the woody xylem. Nevertheless, the principal structural elements in lignins have been largely... 

Methods based on classical organic chemistry led to the conclusion, already by 1940, that lignin is built up of phenylpropane units. Examples of typical reactions used in these studies are illustrated in Fig. 4-1. However, the concept of a phenylpropanoid structure failed to win unanimous acceptance, and as late as 30 years ago, some scientists were not convinced that lignin in its native state was an aromatic substance. Finally, the problem was solved by Lange in 1954, who applied UV microscopy at various wavelengths directly on thin wood sections, obtaining spectra typical of aromatic compounds. 

Fig. 4-8. The most common linkages between phenylpropane units. For proportions, see Tables 4-1 and 4-2.

More than two thirds of the phenylpropane units in lignin are linked by ether bonds, the rest by carbon-to-carbon bonds. Figure 4-8 shows the principal bonds in lignin. Their proportions are seen from Table 4-1 (spruce lignin) and Table 4-2 (birch lignin). 

Fig. 4-9. Prominent structures of softwood lignin (Adler, 1977) comprising 16 phenylpropane units.

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