Isolation of Reserpine: Reserpine, an indole alkaloid, is obtained from roots of Rauwolfia serpentina, Rauwolfia micrantha, R. vomiforia, and R. tetraphylla belongs to the family Apocynaceae. Rauwolfia also contains ajmaline, ajmalicine, ajmalinine, rescinnamine, reserpinine, serpentine, and yohimbine.
The powdered root of the drug is extracted with alcohol (90%). The obtained extract is concentrated and dried under reduced pressure (below 60°C). This process will yield dried extract of rauwolfia which contains total alkaloids (about 4%). This extract is again extracted with ether-chloroform-alcohol (90%) in a ratio of 20:8:2.5. Then add dilute ammonia in little quantity to the extract with continuous shaking. The alkaloids will convert into a water-insoluble base. Then add water and keep the drug to settle after shaking and filtering it. The obtained residue is extracted with 4 volumes of 0.5N sulphuric acid in a separating funnel. The obtained total acidic extract is combined and filtered. To the filtrate dilute ammonia is added to make it alkaline. This will liberate the alkaloids. Finally, it is extracted with chloroform, and the obtained extract is filtered. The solvent chloroform is separated by distillation and the total extract is dried under vacuum. The alkaloids are isolated by column chromatography or fractionation method.
Melting point: 270°C.
Thin-layer chromatography of Reserpine
1 mg alkaloidal extract or purified Reserpine is dissolved in methanol. The silica gel-G TLC plates are spotted and eluted in solvent system chloroform-acetone-diethylamine (5:4:3). In the case of alumina-G plates, elute the plates in solvent system cyclohexane-chloroform (3:7). Then eluted plates are dried and sprayed with Dragondorff’s reagent. The alkaloidal component gives an orange spot in both cases. The Rf value of Reserpine is 0.72 (in silica gel-G stationary phase) and 0.35 (in alumina-G stationary phase).
Identification Test of Reserpine
- A red coloration along the medullary rays is observed when the freshly fractured surface is treated with concentrated nitric acid.
- Reserpine shows a violet red colour when treated with a solution of vanillin in acetic acid.
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