Cinnamic acids, coumarins

The biochemical mechanisms through which allelochemicals exert deleterious or toxic effects on plants are, for the most part, unknown (1). Some phenolic acids, cinnamic acids, coumarins, and flavonoids have been reported to inhibit photosynthesis and respiration of intact plants and microorganisms. However, the mechanisms, at the molecular level, through which the compounds interfere, remain to be ascertained. Some phenolic acids, coumarins, and flavonoids were reported to Inhibit C02-dependent 0 ... 

N.A. Coumarins, bioflavonoids, mucilage, vitamins , , C, benzoic acid, cinnamic acid, coumarins, carotenoids.99121 Antiseptic, antirheumatic, antibacterial, antioxidant. 

Coumarins are quantitatively hydrolysed to give yellow solutions of the salts of the corresponding cis-cinnamic acids (coumarinic acids), which cannot be isolated, since acidification brings about immediate re-lactonisation prolonged alkali treatment leads to isomerisation and the formation of the trans- c A (coumaric acid) salt. 

Phenylpropane amino acids Cinnamic acid, coumarins, lignin, lignans, flavan derivatives, stilbenes, phenolcarboxylic acids, phenols, components of volatile oils. 

Phenylalkylamines. isoquinoline alkaloids, amaryllidaceae alkaloids, aristolochic acids, melanins, gliotoxin, betalains, thyroxine, cinnamic acids, coumarins. lignins, lignans, benzoic acids, stilbenes. flavonoids, hydroquinone derivatives etc.. 

Hydroxide ions open the lactone ring of coumarin forming the dianion 6 of (Z)-(2-hydroxy)cinnamic acid (coumarinic acid), which recyclizes to coumarin on addition of acid. On methylation with dimethyl sulfate, the dianion 6 is transformed to the (Z)-methoxy ester 8 however, on prolonged reaction with base it isomerizes to the ( )-dianion 7 ... 

Phenylpropanolds, phenylpropenes, chromones, Isochromenes, cinnamic acids, coumarins... 

Ds-Cinnam c add o-Hydroxyw-cinnamic acid Coumarin Fig. 97. A memory aid for the structure of the coumarins. 

Coumarin is usually prepared by heating salicylaldehyde with acetic anhydride and sodium acetate (i.e., the Perkin cinnamic acid synthesis, p. 23 6), whereby the 0" hydroxy-cinnamic acid (I) undergoes cyclisation to coumarin. Coumarins having substituents in the benzene ring can often be similarly prepared. 

Hydrolysis. The lactone is easily hydroly2ed by alkaUes to the corresponding salts of coumarinic acid or o-hydroxy-i j -cinnamic acid [495-79 ]. Coumarinic acid salts are odorless. Coumarinic acid and salts revert to coumarin upon acidification with inorganic acids. Alkaline fusion of coumarin yields salts of sahcyhc and acetic acids. 

The couplings of vicinal protons in 1,2-disubstituted alkenes lie in the range 6-12 Hz for cis protons (dihedral angle 0°) and 12-17 Hz for trans protons (dihedral angle 180°), thus also following the Karplus-Conroy equation. Typical examples are the alkene proton AB systems of coumarin (16a, cis) and tra 5-cinnamic acid (16b), and of the cis-trans isomers 17a and b of ethyl isopente-nyl ether, in addition to those in problems 3, 4, 8, 11, 13 and 38. 

Arcmatic compounds phenols, phenolic acids, cinnamic acid derivatives, coumarins, flavonoids, quinones, and tannins, all of which are aromatic compounds, comprise the largest group of secondary plant products. They are often referred to as "phenolics" and have been identified as allelopathic agents in more instances than all of the other classes of compounds combined 5). 

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). 

This limitation is well Illustrated by the failure of those methods to uncover the presence of another important group, the flavanoid, coumarin and cinnamic acid phenols (85 - 117). On the other hand, some 32 of these compounds were identified when isolation procedures specific for these types of compounds were employed. Using isolation and identification techniques which are particularly useful for alkaloids, it would be possible to determine whether any representatives of this class are present and, if so, to conduct subsequent studies for structure determination. 

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). 

Deacetylation of 4-[(o-acetoxy)benzylidene]-2-phenyl-5(47/)-oxazolone also immediately affords 602. The starting oxazolone was obtained by cyclodehydration of the corresponding cinnamic acid precursor or by condensation of hippuric acid with 2-acetoxybenzaldehyde in the absence of base. In examples using 2-hydroxy-acetophenone, 4-methyl-3-(acylamino)coumarins are obtained. ... 

The biosynthesis of coumarins begins with traw5 -4-cinnamic acid, which is oxidized to ort/io-coumaric acid (2-hydroxy cinnamic acid) followed by formation of the glucoside. This glucoside isomerizes to the corresponding cA-compound, which finally through ring closure forms... 

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