Medicinal substances from group of alkaloids
Alkaloids are biologically active organic substances of natural origin, which contain in structure an atom (atoms) of nitrogen and have basic properties.
Most of the alkaloids according to their chemical structure are derivatives of various heterocyclic compounds with heteroatom of nitrogen; they belong to tertiary amines.
Typical alkaloids are derived from plant sources, they are basic, contain one ore more nitrogen atoms (usually in a heterocyclic ring) and they usually have a marked physiological action on human or animals.
Physical properties
Alkaloids are solid crystalline substances, colourless, odourless, with bitter taste. Some alkaloids in the base form are fluids and have unpleasant odour (colchicine, nicotine, and physostigmine).
Most of the alkaloids are optically active. Alkaloids-bases are slightly soluble or practically insoluble in water and freely soluble in various organic solvents (ethanol, ether, benzene, etc.).
Salts of alkaloids, as a rule, are soluble in water and slightly soluble in organic solvents. Exception is ethanol, which dissolves many salts of alkaloids. However, bases of caffeine, ephedrine and pilocarpine are very soluble in water; some salts (cocaine hydrochloride) are soluble in chloroform.
Knowledge of the solubility of alkaloids and their salts is of considerable pharmaceutical importance. Not only alkaloidal substances are often administered in solution, but also the differences in solubility between alkaloids and their salts provide methods for the isolation from the plant and their separation from the nonalkaloidal substances.
Tests of alkaloids
For identification of alkaloids are usually used general, group and specific reactions. Most alkaloids are precipitated from neutral or slightly acid solution by some reagents, which according to the specificity of reaction are named precipitative reagents.
General precipitative reagents:
1. Solution of iodine in potassium iodide (Wagner reagent).
2.Solution of potassium bismuth iodide (Dragendorff reagent).
3.Potassium mercury-iodide solution (Mayer reagent).
4.Solution of cadmium iodide in potassium iodide (Marme reagent).
5.Phospho-molybdic acid (Sonnenschein reagent H3PO4.12MoO3.2H2O) – very sensitive reagent for alkaloids; it gives amorphous precipitates with yellowish colour, which in time become blue or green (reducing of molybdic acid).
6.Phospho-tungstic acid (Schebler reagent H3 PO4.12WoO3.2H2O).
7.Silico-tungstic acid (Bertran reagent SiO2.12WoO3.4H2O).
8. 5% tannin solution.
9.Saturated solution of picric acid (Hager reagent).
Reagents mentioned above, do not give precipitates with all alkaloids. It is necessary to carry out reaction at least with 4-5 reagents.
Only the negative result has analytical importance, as the reagents also give precipitates with proteins and some other nitrogen-containing substances.
Colour tests
1. Concentrated sulphuric acid.
2. Concentrated nitric acid.
3. The mixture of concentrated nitric and concentrated sulphuric acids (Erdmann reagent).
4. Solution of ammonia molybdate in concentrated sulphuric acid (Frede reagent).
5.Solution of formaldehyde in concentrated sulphuric acid (Marquis reagent).
6.Solution of p-dimethylaminobenzaldehyde in concentrated sulphuric acid (Wazicki reagent).
Classification
1.Derivatives of quinolisine (1) and quinolizidine (nor-lupinane) (2) (sparteine, cytisine):
2.Derivatives of tropine (atropine, hyosciamine, cocaine):
3.Derivatives of quinoline (quinine) (1) and isoquinoline (opium alkaloids) (2):
1 2
4.Derivatives of imidazole (pilocarpine):
5.Derivatives of indole (physostigmine, strychnine, ergot alkaloids):
6.Derivatives of purine (caffeine, theobromine, theophylline):
7.Derivatives of pyrrolizidine (platiphylline):
Quinine and quinolizidine (nor-lupinane) alkaloids
Cytisine (Cytisinum)
It can be represented as derivative of 1,2,3,4-tetrahydroquinolizone-6, which is condensed with piperidine.
Properties
White or slightly yellowish crystalline powder; freely soluble in water, alcohol and chloroform.
Identification
1. Melting point 154-157оС.
2. Specific optical rotation: from 120оto 123о(for 2% aqueous solution).
3. Nitration of aromatic ring with the further reducing nitro-group to amino-group and obtaining azo-dye.
4. With Co(NO3)2 a bluish-green precipitate appears.
3. With FeCl3 solution a red colour appears; it is discharged by water addition.
4. Reactions with general precipitative reagents.
Assay Acidimetry. Titrate with hydrochloric acid using methyl red solution as an indicator; s=1.
Usage Respiratory and blood circulation stimulant.
Storage In well-closed containers protected from light.
Pachycarpine hydroiodide (Pachycarpini hydroiodidum)
-Sparteine hydroiodide
Properties
White crystalline powder, freely soluble in chloroform, soluble in alcohol and in water.
Identification
1. Specific optical rotation: +8,6оto +9,6о(in 7% alcohol solution).
2. Reactions of iodides.
3. Obtaining of the pachycarpine base, which is determined by:
а) the product of reaction with picric acid (melting point);
b) interaction with bromine and ammonia vapours – pink colour is obtained.
4. With alkaline sodium nitroprusside solution a reddish-brown crystalline precipitate appeared dissolves in the excess of hydrochloric acid.
5. Reactions with general precipitative reagents.
Assay
- Non-aqueous titration in the medium of anhydrous acetic acid in the presence of mercury acetate (II); s=1/2:
2. Argentometry by HI; s=1.
3. Alkalimetry by HI; s=1.
4. Photocolorimetry.
Usage Stimulant of myometrium.
Storage Protected from light.