Isohemipinic acid

Fig. 4-1. Examples of classical methods indicating a phenylpropanoid structure of lignin. (A) Permanganate oxidation (methylated spruce lignin) affords veratric acid (3,4-dimethoxybenzoic acid) (1) in a yield of 8% and minor amounts of isohemipinic (4,5-dimethoxyisophtalic acid) (2) and dehydrodiveratric (3) acids. The formation of isohemipinic acid supports the occurrence of condensed structures (e.g., /3-5 or y-5). (B) Nitrobenzene oxidation of softwoods in alkali results in the formation of vanillin (4-hydroxy-3-methoxybenzaldehyde) (4) (about 25% of lignin). Oxidation of hardwoods and grasses results, respectively, in syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde) (5) and p-hydroxybenzaldehyde (6). (C) Hydrogenolysis yields pro-pylcyclohexane derivatives (7). (D) Ethanolysis yields so-called Hibbert ketones (8,9,10, and 11).

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. 

Isohemipinic Acid (4 6-Dimefhoxyiso-pMfudic acid, catechol- Micarhoxylic acid dimithyl ether, veratrol-3 5 dicarboxyltc add) COOH... 

---