Coumarins from cinnamic acids

Figure 4.4 Proposed formation of coumarins from cinnamic acids.

Members of this group biogenetically originate from cinnamic acid derivatives via the shikimic acid pathway. They are all C-9 compounds having the 2H-l-benzopyran-2-one moiety in their skeleton. Coumarin itself has an unsubstituted basic skeleton. Compounds with hydroxyl or meth-oxyl substituents are described as simple coumarins. More... 

The biosynthesis of the coumarins can be illustrated by considering the case of coumarin itself (Fig. 98). It starts from cinnamic acid. 

Chlorogenic acid forms a 1 1 complex with caffeine, which can be crystallized from aqueous alcohol and yields very little free caffeine on extraction with chloroform. Other compounds with which caffeine will complex in this way include isoeugenol, coumarin, indole-acetic acid, and anthocyanidin. The basis for this selection was the requirement for a substituted aromatic ring and a conjugated double bond in forming such a complex. This kind of complex does modify the physiological effects of caffeine.14 Complex formation will also increase the apparent aqueous solubility of caffeine in the presence of alkali benzoates, cinnamates, citrates, and salicylates.9... 

Simple phenolic compounds include (1) the phenylpropanoids, trans-cinnamic acid, p-coumaric acid and their derivatives (2) the phenylpropanoid lactones called coumarins (Fig. 3.4) and (3) benzoic acid derivatives in which two carbons have been cleaved from the three carbon side chain (Fig. 3.2). More complex molecules are elaborated by additions to these basic carbon skeletons. For example, the addition of quinic acid to caffeic acid produces chlorogenic acid, which accumulates in cut lettuce and contributes to tissue browning (Fig. 3.5). 

It is also important to understand that most allelopathic effects apparently result from the combined actions of several allelochemicals, often with each below a threshold concentration for impact. In allelopathic situations which implicate phenolic acids, soil concentrations have ranged from below 10 to above 1000 ppm for each compound. The lower end of the spectrum is below a concentration required for an effect in current bioassays. Additive and synergistic effects have been demonstrated, however, for combinations of cinnamic acids (102), benzoic acids (103), benzoic and cinnamic acids (10 ). and -hydroxybenzaldehyde with coumarin (105). It appears that such combined interactions may be very important under field conditions. 

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

Two mechanisms have been proposed for the Knoevenagel reaction. In one, the role of the amine is to form an imine or iminium salt (378) which subsequently reacts with the enolate of the active methylene compound. Under normal circumstances elimination of the amine would give the cinnamic acid derivative (379). However, when an o-hydroxy group is present in the aromatic aldehyde intramolecular ring closure to the coumarin can occur. The timing of the various steps may be different from that shown (Scheme 118). 

In plants coumarins and hydroxycoumarins are believed to be synthesized from /ram-cinnamic acid (3.29) and /ram-/ -coumaric acid (3.30), respectively, but the exact mechanism for its synthesis is still unknown. One possible biosynthetic route toward coumarin is via o-hydroxylation of 3.29 to give coumaric acid (3.94), followed by glycosylation to result in /ram-coumaric acid-2-O-glucosidc (3.95) (Figure 3-14). 

Tannins and lignins are also derived from these pathways but are not included in Table 1. To make the list as simple as possible, all compounds of aromatic nature, viz., simple phenols, benzoic and cinnamic acid derivates, coumarins, flavonoids and quinones are condensed into one group - aromatic compounds. Thus I will attempt to cover systematically the secondary plant growth substances that fall into 11 major groups as shown in Table 1. 

Cinnamic acids can be created from the methylation of coumarins. Methoxy substituted coumarins produce lower yields than do non-substituted and hydroxy substituted coumarins. 

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