2.6Miscellaneous Bitter Principles
There are some bitter principles which normally do not fall into the various categories already discussed from Sections 2.1 through 2.5. A few important potent drugs used as bitter principles that belong to this group are, namely: picrotoxin, quassin, cantharidin, which shall now be described separately as under:
2.6.1 Picrotoxin
Synonym Cocculin.
Biological Sources It is obtained as the bitter principle from the seed of Anamirta cocculus L. Wight & Arn. (Menispermaceae); and also found in Tinomiscium philippinense Diels.
Chemical Structure Picrotoxin is a molecular compound of one mole picrotoxinin (C15H16O6), q.v., and one mole picrotin (C15H18O7), q.v., into which it is readily separated. Thus, picrotoxin may be resolved into the two components by boiling with 20 parts of benzene. In this manner, picrotoxinin remains dissolved in benzene whereas picrotin that is practically insoluble in benzene can be separated easily. Likewise, this cleavage may also be accomplished by chloroform more efficiently. Thus, we have:
Isolation Various steps involved in the isolation of picrotoxin are:
1. The seeds are dried, powdered coarsely, sieved and defatted with petroleum ether in a Soxhlet apparatus.
2. The defatted powder (marc) is subsequently extracted by boiling with ethanol or with water.
3. The filtrate thus obtained is treated with lead acetate solution (5% w/v), filtered and the excess of ‘lead’ is removed by passing freshly generated H2S gas (i.e., Pb is precipitated as PbS).
4. The resulting solution is filtered, residue discarded and the filtrate is concentrated to a syrupy consistency in a Rotary Thin Film Evaporator. The syrupy liquid is kept in a refrigerator overnight.
5. Picrotoxin crystallizes out as a crude substance.
6. It may be further purified by treating with ethanol or boiling water and activated charcoal powder to obtain the pure substance.
Characteristic Features
1. It is obtained as shiny rhomboid leaflets mp 203°C.
2. It has an intense bitter taste and is extremely poisonous.
3. It has specific optical rotation [α]16D – 29.3° (C = 4 in absolute ethanol).
4. Solubility Profile: 1 g dissolves in 150 ml cold water; 45 ml boiling water, in 13.5 ml 95% ethanol, in 3 ml boiling ethanol; sparingly soluble in ether, chloroform; and readily soluble in aqueous solution of NaOH and in strong NH4OH.
5. It is highly toxic to fish.
6. It is stable in air, but is affected by light.
7. Picrotoxin is almost neutral to litmus.
Identification Tests These are as stated below:
1. Sulphuric Acid Test: Dissolve 2-3 mg of picrotoxin in a few drops of sulphuric acid, a goldenyellow-colour is produced that gets changed to reddish-brown gradually.
2. Anisaldehyde Test: Moisten a few crystals of picrotoxin with H2SO4 and just add 1-2 drops of a solution of anisaldehyde in dehydrated ethanol (1 : 5), a permanent blue colouration is produced.
3. Potassium-Cupric Tartrate Test: Add about 5-10 mg of picrotoxin to 2 ml of potassiumcupric tartrate solution (0.5%) with 10 ml of water, a red precipitate is formed gradually in the
cold, but a little faster on warming.
4. Vanillin HCl Test: A few mg of picrotoxin when boiled with vanillin hydrochloride solution (0.1% w/v) it gives rise to a green colouration.
5. Reduction Tests: Picrotoxin reduces Fehling’s solution to give a brick-red precipitate; and Tollen’s reagent to give a silver mirror.
6. Mix 0.2 g KNO3 with four drops of H2SO4 in an evaporating dish. Sprinkle a few crystals of picrotoxin on the resulting mixture and add dropwise NaOH solution (2N), until it is present in a little excess quantity. The crystals of picrotoxin shall initially acquire a red colouration that fades out slowly.
Uses
1. It is used as a CNS-stimulant. Therefore, it may be employed intravenously as an antidote in barbiturate poisoning and other narcotics also.
2. It also finds its application as an effective respiratory stimulant.
3. Very small quantities of the powdered seeds are sufficient to stupify fish.
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* Rotenoids: The rotenoids take their name from the first known example rotenone and are usually generated by ring
cyclization.
2.6.2 Quassin
Synonym Nigakilactone D
Biological Sources It is obtained from the wood of Quassia amara L., (Simaroubaceae) commonly known in commerce as Surinam quassia. It is also obtained from the stem wood of Picrasma excelsa (Sw.) Planch. (Aeschrion excelsa or Picroena excelsa) known in commerce as Jamaican quassia. All these species belong to the natural order Simaroubaceae).
Chemical Structure
2, 12-Dimethoxypicrosa-2, 12-diene-1, 11, 16-trione; (C22H28O6).
Isolation The following steps may be adopted in a sequential manner for the isolation of quassin.
1. The quassia wood is chopped into small pieces and subjected to aqueous decoction, which is filtered and concentrated to the original weight of the wood taken; and finally neutrallized carefully with Na2CO3.
2. Tannic acid solution (5% w/v) is added slowly until no more precipitate is obtained.
3. The precipitate thus obtained is filtered, collected and transferred to a pestle and mortar, triturated with solid lead carbonate (or with freshly prepared lead oxide), so as to liberate quassin and form lead tannate; and the resulting mass is dried on a water bath.
4. The dried mass is powdered and then subjected to extraction with 80% (v/v) ethanol successively.
5. The combined ethanolic extract is filtered and concentrated under vacuo and left for cooling overnight, when the crystals of quassin would separate out.
Quassin may also be obtained by the resolution of the mixture of bitter constituents of quassia
wood by the method of London et al.*
Characteristic Features
1. Quassin is obtained as rectangular plates from dilute methanol having mp 222°C.
2. Its specific optical rotation [α]20D + 34.5° (C = 5.0 g in CHCl3).
3. It has uvmax: ~ 255 nm (ε ~ 11,650).
4. It is extremely bitter; and it has the bitterness threshold 1 : 60,000.
5. It is found to be freely soluble in benzene, acetone, ethanol, chloroform, pyridine, acetic acid, hot ethyl acetate; and sparingly soluble in ether and petroleum ether.
Identification Tests
1. Add to a few crystals of quassin 2-3 drops of concentrated, H2SO4 and sucrose when a red colouration is produced.
2. Phloroglucin Test: Dissolve 2-3 mg quassin in 1-2 ml ethanol, and add to it a few crystals of phloroglucin and a few drops of concentrated. HCl, when a crimson red colour is obtained.
Uses
1. It possesses insecticidal properties.
2. The quassia wood extract is used as a bitter tonic.
3. Quassin exhibits anthelmintic properties, and on being administered as enema expels thread worms specifically.
2.6.3 Lactucin
Biological Sources It is obtained from the dried milky juice of Lactuca virosa L. (Asteraceae) (Bitter Lettuce; Wild Lettuce); and from the plant of Cichorium intybus L. (Compositae).
Chemical Structure
[3aR – [3aα, 4β, 9aα, 9bβ)]-3,3a,4,5,9a,9b-Hexahydro 4-hydroxy-9-(hydroxymethyl)-6-methyl-3-methyl-eneazuleno [4, 5-b] furan-2, 7-dione; (C15H16O5).
Isolation Lectucin may be isolated by the method suggested by Schenck et al.*
Characteristic Features
1. It is obtained as crystal from methanol which sinters at 218°C and has mp 228-233°C.
2. It exhibits specific optical rotation [α]D + 49° (C = 0.90 in methanol); and +77.9° (C = 3.44 in pyridine).
3. It has uvmax: 257 nm (ε 14,000).
4. It is found to be soluble in water, ethanol, methanol, ethyl acetate, anisol and dioxane.
Identification Tests It may be identified from its derivative:
Lectucin para–hydroxyphenylacetate hydrate (C23H22O7) (Intybin; Lactucopicrin;): It is obtained as crystals from water which get decomposed at 148-151°C. It shows specific optical rotation [α]D17.5 + 67.3° (pyridine).
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* London et al., J. Chem. Soc., 3431, 1950.
2.6.4 Erythrocentaurin
Biological Sources It is obtained from the plant Centaurium umbellatum Gilib. (Erythraea centaurium Pers.), Gentinaceae or Swertia japonica (Maxim.) Makino Gentianaceae. It is also accomplished by carrying out the hydrolysis of swertiamarinand erytaurin with emulsin.
Chemical Structure
5-Formyl-3, 4-dihydroisocoumarin; (C10H8O3).
Isolation Erythroceantaurin may be isolated from C. umbellatum by the method of Kariyone and Matsushima.**
Characteristic Features
1. It is obtained as long needles having mp 140-141°C.
2. It turns red on being exposed to sunlight.
3. It has uvmax: 223, 290 nm (log ∈ 4.30, 3.13).
Uses It is mostly employed as a bitter tonic.
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* Schenck et. al., Arch. Pharm., 294, 17 (1961).
** Kariyone and Matsushima J., Pharm. Soc., Japan, 47, 25 (1927).
2.6.5 Gentisin
Synonyms Gentianin; Gentiin; Gentianic Acid.
Biological Sources It is obtained from the roots of Gentiana lutea L. (Gentianaceae) (Yellow Gentian).
Chemical Structure
1, 7-Dihydroxy-3-methoxy-9H-xanthen-9-one; (C14H10O5).
Characteristic Features
1. It is obtained as yellow needles from ethanol having mp 266-267°C.
2. It has uvmax (methanol): 260, 275, 315, 410 nm (log ∈ 435, 4.30, 4.10, 3.70).
3. It is observed to be very slightly soluble in water or organic solvents.
Identification Test
Gentisin Diacetate (C18H14O7) It is obtained as crystals from ethanol having mp 196-197°C. Its absorption max (metanol): 240, 270, 300 nm (log ε 4.58, 4.05, 4.10).
Uses It may be used to stimulate gastric secretion, improve appetite and digestion, and alleviate debility.
2.6.6 Cantharidin
Synonym Cantharides Camphor.
Biological Sources It is the active vesicating principle of cantharides (q. v) and other insects, in notorious ‘Spanish Fly’ aphrodisiac, which essentially comprise of the dried insects (Beetles) Lytta(Cantharis) vesicatoria belonging to the order Coleoptera; and family Meloidae. It has been found that the soft parts of the insect are the chief seat of cantharidin. Besides, cantharidis contain 0.5 to 0.95 of cantharidin.
Chemical Structure
Exo-1, 2-cis-Dimethyl-3, 6-epoxy hexahydrophthalic anhydride; (C10H12O4).
Isolation The various steps involved in the isolation of cantharidin are:
1. The dried insects are collected and powdered. It is now treated with an acid whereby the cantharidin gets liberated in the form of its corresponding salts.
2. The resulting product is subjected to extraction, of both cantharidin and fat, by the help of ethyl acetate in a Soxhlet apparatus.
3. The solvent is removed carefully under reduced pressure and the crude cantharidin crystallizes out.
4. The fat may be removed by the help of petroleum ether, in which cantharidin is only negligibly soluble.
5. Ultimately, the crude defatted cantharidin is dissolved in a minimum quantity of hot ethanol and allowed to cool when cantharidin crystallizes out in its purest form.
Charactersitic Features These are as follows:
1. Cantharidin is obtained as orthorhombic plates or as scales having mp 218°C.
2. It sublimes at 110 °C (12 mm Hg, 3-5 mm distances).
3. It is practically insoluble in cold water and somewhat soluble in hot water. 1g dissolves in 40 ml acetone; 65 ml chloroform; 560 ml ether; 150 ml ethyl acetate; and soluble in oils.
Identification Tests
1. Formaldehyde Test: Add to a few crystals of cantharidin 1-2 drop of dilute formaldehyde solution mixed with H2SO4, the development of a brown to black colouration on warming identifies it.
2. A solution of cantharidin in olive oil is vesicant to the skin (i.e., sensitive upto an extent of 0.14 mg).
Uses
1. It is mostly used as a vesicant.
2. It is also employed as a rubefacient and counterirritant in veterinary practice.
