Trans-sinapyl alcohol

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. 

Figure 1. Structure of trans coniferyl alcohol and trans sinapyl alcohol (reproducedfrom reference 30. CAS Registry database records duplicated with permission of CAS, a division of the American Chemical Society).

Quantification of Pyrolysis Products. The pyrolysis must be performed separately ( off-line approach, see Fig. 4.7.5), and the products are trapped completely so that an internal standard can be added. Accurate quantification of the pyrolysis products is achieved using a gas chromatograph equipped with a flame ionization detector (Faix et al. 1987). Fluoranthene is a suitable internal standard as it elutes from the column after the last detectable lignin pyrolysis product (trans-sinapyl alcohol). 

Lignins, a major component of wood (15-30 wt%), are phenolic polymers based on three structural units, the content of which depends on the type of wood trans-p-coumaryl alcohol (I), trons-coniferyl alcohol (II), and trans-sinapyl alcohol (III) (see Chart 8.3). 

In the purest sense, lignin is a complex racemic consisting from aromatic heteropolymers produced by free radical coupling reactions initiated by enzymatic dehydrogenation of the three primary precursors, the hydroxycinnamyl alcohol monomers differing in their degree of methoxylation trans-4-coumaryl alcohol [3-(4-hydroxyphenyl)-2-propenol)], trans-coniferyl alcohol [3-(4-hydroxy-3-methoxyphenyl)-2-propenol)] and trans-sinapyl alcohols [3-(4-hydroxy-3,5-dime-thoxyphenyl)-2-propenol)] (Fig. 8.4). 

Woody plants synthesize lignin fiom 3-(4-hydroxyphenyl)-2-propenol (trans-4-coumaryl alcohol = 1.1, grasses), 3-(4-hydroxy-3-metho5OThenyl)-2-propenol (trans-cotriferyl alcohol = 1.2, pines), and3-(4-hydroxy-3,5-dimethoxyphenyl)-2-propenol (trans-sinapyl alcohol = 1.3, deciduous) by flee radical crosslinking initiated by errzymatic dehydrogenation (4). Structures of these alcohols and the notation for the carbon atoms of the C, repeat unit... 

OCH3 OCH3 trans-Sinapyl alcohol Syringin... 

Gymnosperms Mostly trans-coniferyl alcohol, but some irafis-p-coumaryl alcohol and trans-sinapyl alcohol... 

Monocotyledons trans-Coniferyl alcohol, trans-sinapyl alcohol and trans-p-coumaryl alcohol... 

The second type of data is derived from biochemical experiments related to the three cinnamyl alcohols—/>-coumaryl alcohol (I), coniferyl alcohol (II), and sinapyl alcohol (III) and their phenolic glucosides— >-glucocoumaryl alcohol (IV), coniferin (V), and syringin (VI). These and the following formulas do not differentiate between cis and trans isomers. 

Figure 5.2 Molecules from which the characteristics units of lignin are obtained trans-coniferyI alcohol (A), tmns-sinapyl alcohol (B) and trans-p-cumaryl alcohol (C)...

Cycloaddition of orf/io-quinone 162 with ferf-butyldimethylsilyl ether of sinapyl alcohol (163) yielded the 1,4-dioxane-linked adduct 164, as shown in Scheme 10b, but no regioisomer was detected. The ketal group in 164 was removed under acidic conditions and the derived ketone, a trans-cis mixmre in the ratio of 2 1, on treatment with alkali, yielded the frans-isomer, 165, in a good yield. The ketone 165 was converted to the key intermediate aldehyde 166 through the iodoform reaction, esterification, LAH reduction, and oxidation with Dess-Martin periodinane after protection of the hydroxy groups by MOM groups (Scheme 10b). 

Lignins are derived from the glucosides of irans-p-coumaryl alcohol, trans-coni-feryl alcohol and frans-sinapyl alcohol (Table 50). These compounds are incorpo-... 

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