2.1 Hydroxycinnamic Acids
The typical examples of hydroxycinnamic acids are, namely: p-coumaric acid, caffeic acid, ferulic acid, and sinapic acid, which shall be enumerated in the sections that follow:
2.1.1 Para-Coumaric Acid
Synonym p-Hydroxycinnamic acid.
Chemical Structure
Biological Sources It is present in a variety of medicinal plant, namely: Aloe barbadensis Mill. (Liliaceae)-Barbados Aloe, Mediterranean Aloe, Curaeao Aloe; Euphorbia lathyris L. (Euphorbiaceae)-Mole plant, Petroleum plant Caper spurge; Hedra helix L. (Araliaceae)-Ivy; Hura crepitans L. (Euphorbiaceae)-Sandhox Tree; Malus sylvestris Mill. (Rosaceae)-Apple; Melilotus officinalis Lam. (Fabaceae)-Yellow Sweetelover; Trifolium pratense L. (Fabaceae)-RedClover, Pavine Clover, Cowgrass.
Characteristic Features It occurs as needles having mp 210-213°C. It may be crystallized in its anhydrous form from concentrated hot aqueous solution, but as the monohydrate from dilute aqueous solution on gradual chilling. Its uvmax (in 95% ethanol) are 223 and 286 nm (ε 14,450, 19,000). It is practically insoluble in ligroin and benzene, slightly soluble in cold water, and freely soluble in ethanol, ether and hot water.
2.1.2 Caffeic Acid
Synonym 3, 4-Dihydroxycinnamic acid.
Chemical Structure
Biological Source It occurs widely in more than twenty different species of plants as detailed below: Aconitum napellus L. (Ranunculacea)-Aconite, Monkshood, Blue Rocket; Arctium lappa L. (Asteraceae)-Edible Burdock, Great Burdock, Lappa; Arnica montana L. (Asteraceae)-MountainTobacco, Leopard’s-bane; Cinnamonum camphora (L.) J.S. Presl. (Lauraceae)-Camphor, Hon-Sho; Citrullus coloeynthis (L.) Sehrad. (Cucurbitaceae)-Colocynth, Bitter Apple, Wild Gourd Clematis vitalba L. (Ranunculaceae)-Traveler's Joy; Coniun maculatum L. (Apiaceae)-Hemlock; Convalaria majalis L. (Liliaceae)-Lily of the Valley; Crataegus oxycantha L. (Rosaceae)-Howthorn; Digitalis purpurea L. (Serophulariaceae)-Common Foxglove, Digitalis; Equisetum hyemale L. (Equisetaceae)-Shavegrass, Great Scouring Rush; Euphorbia pulcherrima Wild ex Klotsch (Euphorbiaceae)-Poinsettia; Euphrasia officinalis L. (Scrophulariaceae)-Eyebright; Gaultheriaprocumbens L. (Ericaceae)-Wintergreen, Teaberry, Boxberry; Leonurus cardiaca L. (Lamiaceae)-Motherwort, Santolina charnaecyparissus L. (Asteraceae)-Lavender-Cotton, Seopolia carniolica Jacq. (Solanaceae)-Seopolia; Solanum tuberosum L. (Solanaceae)-Potato; Solidago virgaurea L. (Asteraceae)-European Goldenrod, Woundwort; Stachys officinalis (L.) Trevisan (Lamiaceae)-Betony; Trifolium pratense L. (Fabaceae)-Red Clover, Pavine Clover, Cowgrass; Valeriana officinalis L. (Valerianaceae)-Valerian; and Viscum album L. (Loranthaceae)-European Mistletoe.
Preparation It occurs in plants only in conjugated forms e.g., chlorogenic acid. It has also been isolated from green coffee,* and from roasted coffee.**
It can also be prepared by the hydrolysis of chlorogenic acid in an acidic medium as shown below:***
Characteristic Features Caffeic acid has yellow crystals obtained from concentrated aqueous solutions and the corresponding monohydrate from dilute solutions. It gets softened at 194°C and decomposes at 223-225°C. It is sparingly soluble in cold water, but freely soluble in cold ethanol and hot water.
Chemical Tests
1. It changes colour from yellow to orange in an alkaline medium.
2. It readily forms the methyl ester (C10H10O4) which are obtained as colourless crystals from water (mp 152-153°C).
2.1.3 Ferulic Acid
Synonyms Caffeic acid 3-methyl ether; 4-Hydroxy-3-methoxycinnamic acid.
Biological Sources Ferulic acid is widely distributed in small amounts in a variety of plants, namely: seeds of Citrullus colocynthis (L.) Schrad. (Cucurbitaceae)-Colocynth, Bilter Apple, Wild Gourd; flowers of Convallaria majalis L. (Liliaceae)-Lily-of-the-Valley; leaves of Digitalis purpurea L. (Scrophulariaceae)-Common Foxglove, Digitalis; young shoots of Equisetum hyemale L. (Equisetaceae)-Shavegrass, Great Scouring Rush; leaves of Euphorbia lathyris L. (Euphorbiaceae)-Mole Plant, Petroleum Plant, Caper Spurge; dried herb of Euphrasia officinalis L. (Scrophulariaceae)-Eyebright; gum-resin of Ferula assafoetida L. (Apiaceae)-Asafoetida; volatile oil of Gaultheria procumbens L. (Ericaceae)-Wintergreen, Teaberry, Boxberry; twigs of Hedera helix L. (Araliaceae)-Ivy; leaves of Hura crepitans L. (Euphorbiaceae)-Sandbox Tree; leaves of Plantago major L. (Plantaginaceae)-Plantain; volatile oil of Rheum officinale Baill. (Polygonaceae)-Chinese Rhubarb, Canton Rhubarb, Shensi Rhubarb; shrubs of Serenoa repens (Bartel.) Small (Arecaceae)-Saw Palmetto.
Preparation It has been isolated from Ferula foetida Reg. (Umbelliferae)* and from Pirus laricio Poir. (Abietineae)**. It may also be prepared by the interaction of vanillin, malonic acid and piperidine in pyridine for three weeks and then precipitating ferulic acid with dilute HCl.
Chemical Structure
Characteristic Features
cis-form : Yellow oil; uvmax (in ethanol): 316 nm.
trans-form : Orthorhombic needles obtained from water; mp 174°C UVmax (in ethanol): 236, 322 nm. It is soluble in hot water, ethanol and ethyl acetate; moderately soluble in ether; and sparingly soluble in benzene and petroleum ether.
Identification Test It forms the corresponding sodium salt by treatment with NaOH solution whereby the solubility gets enhanced appreciably.
Uses It is used as a preservative of food products.
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* Wolfrom et al. J. Agr. Food Chem., 8, 58 (1960).
** Krasemann, Arch. Pharm, 293, 721 (1960).
*** Fiedler, Arzneimittel–Forseh, 4, 41 (1954); Whiting, Carr, Nature 180, 1479 (1957), Guren, Chemical Abstracts, 61,
9965h (1964).
2.1.4 Sinapic Acid
Biological Source It is obtained from the leaves and twigs of Viscum album L. (Loranthaceae)-European Mistletoe.
Preparation It may be prepared by the hydrolysis of sinapic acid choline ester obtained from the black mustard seeds of Brassica nigra Koch (Cruciferae) either in acidic medium or by enzymatic hydrolysis as given below:
