Terpenoid Indole Alkaloids

2.8.3 Terpenoid Indole Alkaloids

Terpenoid indole alkaloids is perhaps one of the major groups of alkaloids in the plant kingdom which comprise of more than 3000 recognized alkaloids till date Interestingly, they are found to be confined to eight different natural orders (i.e., families), of which the Apocynaceae, the Loganiaceae, and the Rubiaceae are predominantly the best known sources.

However, it is pertinent to mention here that practically in all the structure a tryptamine residue is strategically located in the molecule; while the remaining fragment is invariably recognized as a C9 or C10 residue.

The wisdom, relentless efforts and meticulous in-depth studies carried out by numerous groups of researchers dealing with plant substances across the globe ultimately led to three main structural variants entirely based on their good judgement and understanding namely:

(a) Coryanthe Type e.g., ajmalicine and akuammicine,

(b) Aspidosperma Type e.g., tabersonine, and

(c) Iboga Type e.g., catharanthine.

It has since been established beyond any reasonable doubt that the C9 or C10 component present in the aforesaid three types of structural variants i.e., Carynanthe, Aspidosperma and Iboga groups was definitely of the terpenoid origin. Besides, it was also confirmed that the secoridoid secologanin was duly proclaimed to be the terpenoid derivative, which perhaps must have initially combined with the tryptamine residue of the molecule. From these scientific and logical evidences one may safely infer that the three above mentioned groups of alkaloids might be not only related but also rationalized in terms of rearrangements taking place exclusively in the terpenoid portion of the various structural variants as shown in the pathway given below.

Salient Features The salient features of the above pathway are as follows:

1. Secologanin (a secoridoid and a terpenoid derivative) is formed through geraniol via loganin, which essentially contains the 10C-framework a typical characteristic feature of the Coryanthe moiety.

2. The resulting Coryanthe C-skeleton undergoes subsequent rearrangements to give rise to Aspidosperma and Iboga groups.

3. This intra-molecular rearrangement may be represented by detachment of a 3C-unit, which is subsequently reunited to the remaining C7 fragment in one of the two different manners as shown in the pathway.

4. Interestingly, where C9 terpenoid units are complied with, the alkaloids usually, seem to have lost a C-atom marked in the circle, which exactly corresponds to the carboxylate function of secologanin molecule. Therefore, its ultimate elimination by way of hydrolysis/decarboxylation is now understood without any reasonable doubt.

5. Thus, the Coryanthe type of C-skeleton yields ajmalicine and akuammicine.

6. The Aspidosperma type of C-skeleton yields tabersonine and vindoline.

7. The Iboga type of C-skeleton gives rise to catharanthine.

A few typical examples of terpenoid indole alkaloids, namely: Ajmalicine (Raubasine); Akuammicine; Vindoline; and Catharanthine shall be discussed below:

A. Ajmalicine

Synonyms Raubasine; Circolene; Hydrosarpan; Lamuran; Isoarteril;

Biological Sources It is obtained from the plants of catharanthus lanceus Pichon (Boj.) (Apocynaceae) (Lanceleaf Periwinkle); Catharanthus roseus (L.) G. Don (Apocynaceae) (Periwinkle, Madagascar or Cape Periwinkle, Old Maid]; leaves of Mitragyna speciosa Korth. (Rubiaceae)

(Katum, Kutum, Krantum); Rauvolfia scrpentina (L.) Benth. (Apocynaceae) (Rauvolfia, Chandra, Sarpaganda); and bark of Corynanthe johimbe K. Schum., (Rubiaceae).

Chemical Structure

(19α)-16, 17-Didehydro-19-methyl-oxayohimban-16-carboxylic acid methyl ester; (C₂₁H₂₄N₂O₃).

Isolation Ajmalicine may be isolated either from the bark of Corynanthe johimbe by the method suggested by Heinemann*, or from the roots of Rauwolfia serpentina by the procedure adopted by Hofmann.**

Characteristic Features

1. It is obtained as prisms from methanol which decompose at 257°C.

2. It has specific optical rotation [α]20D – 60° (C = 0.5 in chloroform); [α]²⁰_D – 45° (C = 0.5 in pyridine); and [α]²⁰_D – 39° (C = 0.25 in methanol).

3. It exhibits uvmax (methanol): 227, 292 nm (log e 4.61, 3.79).

Identification Tests

1. Ajmalicine Hydrochloride (C₂₁H₂₄N₂O₃.HCl): It is obtained as leaflets from ethanol having mp 290°C (decomposed); [α]²⁰_D – 17° (C = 0.5 in methanol); and is sparingly soluble in water or dilute HCl.

2. Ajmalicine Hydrobromide (C₂₁H₂₄N₂O₃.HBr): It is obtained as diamond-shaped plates from methanol having mp 295-296°C.

Uses

1. It is mostly used as antihypertensive and anti-ischemic agent (both ceretral and peripheral).

2. It has a broad application in the relief of obstruction of normal cerebral blood flow.

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* Heinemann, H., Ber. 67, 15 (1934).

** Hofmann, A, Helv. Chim. Acta. 37, 849, (1954).

B. Akuammicine

Biological Source It is obtained from the plant substance of Catharanthus roseus (L.) G. Don (Apocyanaceae) (Periwinkle, Madagascar or Cape Periwinkle, Old Maid); and also from the seeds of Picralima klaineana, Pierre, belonging to the natural order (Apocyanaceae).

Chemical Structure

2, 16, 19-20-Tetradehydrocuran-17-oic acid methyl ster; (C₂₀H₂₂N₂O₂).

Characteristic Features

1. It is obtained as plates from a mixture of ethanol and water having mp 182°C.

2. Its physical parameters are: [α]¹⁶_D – 745° (C = 0.994 in ethanol); pK_a 7.45; and uv_max (ethanol): 227, 330 and 330 nm (log ε 4.09, 4.07, 4.24).

Identification Tests It forms the following derivatives:

1. Akuaminicine Hydrochloride Dihydrate (C₂₀H₂₂N₂O₂.HCl.2H₂O): It is obtained as leaflets from ethanol or water having mp 171°C; and has [α]²¹_D – 610° (C = 1.430 in ethanol).

2. Akuaminicine Perchlorate Monohydrate (C₂₀H₂₂N₂O₂.HClO₄.H₂O): It is obtained as needles from a mixture of ethanol and water having mp 134-136°C.

3. Akuammicine Hydroiodide Monohydrate (C₂₀H₂₂N₂O₂.HI,H₂O): It is obtained as square plates from water having mp 128°C.

4. Akuammicine Methiodide: It is obtained as crystals from water with mp 252°C.

5. Akuammicine Nitrate: It is obtained as needles from hot water having mp 182.5°C.

Uses The drug exhibits a slight digitalis-like reaction; and is, therefore, believed to act as a heart poison.

C. Vindoline

Biological Sources It is obtained from the plant Catharanthus roseus (L.) G. Don (Apocynaceae) (Periwinkle, Madagascar or Cape Periwinkle; Old Maid). It is found to be the major alkaloid from the leaves of Vinca rosea Linn. (Apocynaceae).

Chemical Structure

(2β, 3β, 4β, 5α,12β, 19α)-4-(Acetyloxy)-6, 7-didehydro-3-hydroxy-16-methoxy-1 methylaspidospermidine-3-carboxylic acid methyl ester; (C₂₅H₃₂N₂O₆).

Isolation It is isolated from the leaves of Vinca rosea by the method suggested by Gorman et al.*

Characteristic Features

1. Vinodoline is obtained in two forms: first, as needles from a mixture of acetone and petroleum ether having mp 164-165°C; and secondly, as prisms having mp 174-175°C.

2. It has [α]²⁰_D - 18° (chloroform) and dissociation constant pKa 5.5 in 66% DMF.

3. It has uv_max (ethanol): 212, 250, 304 nm (log ε 4.49, 3.74, 3.57).

Identification Tests It gives specific derivatives as.

1. Vindoline Hydrochloride (C₂₅H₃₂N₂O₆.HCl): It is obtained as crystals from acetone having mp 161-164°C.

2. Demethoxy Vindoline (C₂₄H₃₀N₂O₅) (Vindorosine, Vindolidine): It is obtained as needles from benzene and petroleum ether having mp 167°C. It has [α]¹⁶_D -31° (Chloroform); and uv_max (methanol): 250, 302 nm (log ε 3.98, 3.52).

D. Catharanthine

Biological Sources It is obtained in the plant of Catharanthus lanceus Pichon (Boj.) (Apocynaceae) (Lanceleaf Periwinkle); and Catharanthus roseus (L.) G. Don (Apocyanaceae) (Periwinkle, Madagascar or Cape Periwinkle, Old Maid). It is also found in Vinca rosea Linn. (Apocynaceae).

Chemical Structure

3, 4-Didehydroibogamine-18-carboxylic acid methyl ester; (C₂₁H₂₄N₂O₂).

Isolation It may be isolated from Vinca rosea Linn by the method recommended by Gorman et al.**

Characteristic Feature

1. Its crystals obtained from methanol has mp 126-128°C.

2. It has uvmax (ethanol): 226, 284, 292 nm (log ε 4.56, 3.92, 3.88).

3. It has specific optical rotation [α]²⁷_D + 29.8° (CHCl₃); and dissociation constant pKa′ 6.8.

Uses

1. Its pharmacological action resembles to that of R. serpentina.

2. It also shows beneficial growth inhibition effects in certain human tumors.

3. It is used as a diuretic.

Biosynthesis of Ajmalicine, Vindoline and Catharanthine The various steps involved in the biosynthesis of ajmalicine, vindoline and catharanthine are summarized below:

1. Condensation of secologanin with tryptamine in a Mannich-type reaction gives rise to the tetrahydro-b-carboline system and generates strictosidine.

2. The structural variations involved in converting the Coryanthe type skeleton into the corresponding Aspidosperma and Iboga types are evidently quite complex and are given in the pathway as under.

3. Preakuammicine is obtained from strictosidine via the enol-form of dehydrogeissoschizine.

4. Preakuammicine undergoes intramolecular rearrangement to produce stemmadenine, which subsequently gives rise to a hypothetical intermediate.

5. The hypothetical intermediate may be redrawn which undergoes Diel's-Alder type reaction to produce catharanthine.

6. Dehydrogeissoschizine yields ajmalicine.

7. The hypothetical intermediate gives rise to vindoline via tabersonine.

It is pertinent to mention here that the sequence of alkaloid formation has been proved initially by noting carefully which alkaloids become labelled as a feeding experiment progresses, but more recently it has been confirmed by suitable enzymatic experimental studies.

It is important to mention here that there exists a plethora of structural variants of terpenoid indole alkaloids which may be exemplified with the help of the following specific examples of certain potent alkaloids, namely:

(i) Yohimbine: It is a carboxyclic variant related to ajmalicine and appears to arise from dehydrogeissoschizine by an elaborated mechanism.

(ii) Reserpine: It is a trimethoxybenzoyl ester of yohimbine-like alkaloid. It has an additional-OCH₃ moiety at C.-11 of the indole nucleus.

(iii) Rescinnamine: It is a trimethoxycinnamoyl ester of yohimbine-like alkaloid. It also contains an additional methoxyl substituent on the indole-system at C-11.

(iv) Vinblastine: The nucleophilie vindoline, C-5 of the indole nucleous is being activated adequately by the OMe at C-6, besides the N-atom of the indole moiety. The resulting adduct is subsequently reduced in the dihydropyridinium ring by the NADH-dependent 1, 4-addition, giving the substrate for hydroxylation. Its ultimate reduction gives rise to vinblastine.

(v) Vincristine: It is the oxidized product of vinblastine whereby the inherent N-formyl group on the indoline fragment is transformed.

(vi) Strychnine: The loss of one C from a preakuammicine-like structure via hydrolysis/decarboxylation followed by an addition of the additional two C-atoms by means of aldolcondensation with the formyl moiety, complexed as a hemiacetal in the well-known Wieland-Gumlich aldehyde. The ultimate formation of strychnine from its hemiacetal is by virtue of the formation of both ether and amide linkages.

The above mentioned six structural variants of the terpenoid indole alkaloids shall now be discussed individually in the sections that follow.

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* Gorman et al. J. Am. Pharm. Assoc. 48, 256, (1959).

** M. Gorman et al., J. Arn. Pharm. Assoc. Sci. Ed. 48, 256 (1959).

A. Yohimbine

Synonyms Quebrachine; Corynine; Aphrodine;

Biological Sources It is found in the root bark of Alchornea floribunda Muell. Arg. (Euphorbiaceae) (Niando); plant* of Catharanthus lanceus Pichon (Boj.) (Apocyraceae) (Lanceleaf Periwinkle); bark of Pausinystalia johimbe (K. Schum.) (Rubiaceae) (Yohimbe); root of Rauvolfia serpentina (L.) Benth. (Apocynaceae) (Rauvolfia, Chandra, Sarpaganda); and plant of Rauvolfia tetraphylla L. (Apocynaceae) (Pinque-Pinque).

Chemical Structure

(16a, 17a)-17-Hydroxyyohimban-16-carboxylic acid methyl ester; (C21H26N2O3).

Characteristic Features

1. It is obtained as orthorhombic needles from dilute alcohol having mp 234°C.

2. Its specific optical rotations are: [α]²⁰_D + 50.9° to + 62.2° (ethanol); [α]²⁰_D + 108° (pyridine); and [α]²⁰₅₄₆ + 129° (C = 0.5 in pyridine).

3. It has uvmax (methanol): 226, 280, 291 nm (log ε 4.56, 3.88, 3.80).

4. It is freely soluble in ethanol, chloroform, hot benzene; moderately soluble in ether; and sparingly soluble in water.

Identification Tests

Yohimbine Hydrochloride (C₂₁H₂₆N₂O₃.HCl) (Aphrodyne, Yocon, Yohimex, Yohydrol): It is obtained as orthorhombic plates or prisms from ethanol which decompose at 302°C. Its specific optical rotation [α]²²_D + 105° (water). It is found to be soluble in nearly 120 ml water, 400 ml ethanol, and the aqueous solution is almost neutral.

Uses

1. It is an aderenergic blocking agent, which has been used extensively in angina pectoris and arteriosclerosis.

2. It has been used successfully for the treatment of impotency in patients with vascular or diabetic problems.

3. It is invariably employed as a pharmaiological probe for the study of α2-adrenoreceptor.

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* Emboden reported that this plant contains upto 5% yohimbin.

B. Reserpine

Synonyms Crystoserpine; Eskaserp; Rau-sed; Reserpoid; Rivasin; Serfin; Sandril; Sedaraupin; Serpasil; Serpine; Serpasol; Serpiloid.

Biological Sources It is obtained from the plant Catharanthus roseus (L.) G. Don (Apocynaceae) (Periwinkle, Madagascar or Cape Periwinkle, Old Maid); root of Rauvolfia serpentina (L.) Benth (Apocynaceae) (Rauvolfia, Chandra, Sarpaganda); root of Rauvolfia tetraphylla L. (Apocynaceae) (Pinque-Pinque); and from the plant of Vinca minor L. (Apocynaceae) (Periwinkle).

Chemical Structure

(3β, 16β, 17α, 18β, 20α)-11, 17-Dimethoxy-18-[(3, 4, 5-trimethoxy benzoyl)oxy] yohimban-16-carboxylic acid methyl ester; (C₃₃H₄₀N₂O₉);

Isolation Reserpine may be isolated by adopting the following steps in a sequential manner:

1. The powdered and sieved roots are allowed to swell in a NaHCO3 solution (10% w/v) for a period of 10-12 hours. The resulting solution is extracted with benzene, until the extracts give a weak positive reaction with HgI₂.

2. The combined benzene extracts are concentrated and ether is added to the benzene solution. The resulting mixture is extracted with dilute HCl. The combined acidic solution is washed with ether, filtered and extracted with chloroform in a successive manner.

Note: The chloroform will specifically extract the weakly basic alkaloids, such as: Reserpine and Rescinnamine.

3. The combined chloroformic extract is washed subsequently with 10% (w/v) sodium carbonate solution and followed by water so as to get rid of any free acids present. The resulting extract is finally evaporated to dryness under vacuo.

4. The residue is dissolved in anhydrous methanol and seeded with a pure crystal of reserpine and allowed to cool gradually when reserpine will crystallize out.

5. However, rescinnamine, deserpidine and other minor weakly basic alkaloids could be obtained from the mother liquor conveniently.

6. The mother liquor is evaporated to dryness, and the residue is dissolved in the minimum quantity of benzene and subjected to column chromatography over a column packed with acid-washed alumina. The alkaloids are eluted in the different fractions by making use of benzene, chloroform, methanol (10%) in a sequential manner.

Characteristic Features

1. It is obtained as long prisms from dilute acetone which get decomposed at 264-265°C; (decomposes at 277-277.5°C in an evac-tube).

2. Its specific optical rotations are: [α]²³_D - 118° (CHCl3); [α]²⁶_D - 164° (C = 0.96 in pyridine; [α]²⁶_D - 168° (C = 0.624 in DMF).

3. It has uvmax (CHCl3): 216, 267, 295 nm (61700, 17000, 10200).

4. Reserpine is weakly basic in nature, pKa 6.6.

5. It is found to be freely soluble in chloroform (~ 1g/6 ml), glacial acetic acid, methylene chloride; soluble in benzene, ethyl acetate; slightly soluble in acetone, methanol, ethanol (1g/1800 ml), ether, in aqueous solutions of citric and acetic acids; and very sparingly soluble in water.

Identification Tests

1. Most solutions of reserpine upon standing acquire a distanct yellow colouration and a marked and pronounced fluorescence; especially after the addition of an acid or upon exposure to light.

2. Reserpine Hydrochloride Hydrate (C₃₃H₄₀N₂O₉.HCl.H₂O): It is obtained as crystals which decompose at 224°C.

Uses

1. It is a hypotensive drug which exhibits strong hypotensive and sedative activity.

2. It is also employed to alleviate mild anxiety conditions i.e., the drug shows a mild tranquillizing effect.

C. Rescinnamine

Synonyms Reserpinine; Anaprel; Apoterin S; Cartric; Cinnaloid; Moderil;

Biological Sources It is obtained from the roots of Rauvolfia serpentina (L.) Benth. (Apocynaceae)

(Rauvolfia, Chandra, Sarpaganda).

Chemical Structure

3, 4 5-Trimethoxy-cinnamic acid ester of methyl reserpate; (C₃₅H₄₂N₂O₉).

Isolation Rescinnamine may be isolated from step (5) onwards as described under Morphine.

Characteristic Features

1. It is obtained as fine needles from benzene having mp 238-239°C (under vacuum).

2. Its specific optical rotation is [α]²⁴_D - 97° (C = 1 in chloroform).

3. It has uvmax (methanol): 228, 302 nm (log ε 4.79, 4.48).

4. Solubility Profile: It is moderately soluble in methanol, benzene, chloroform and other organic solvents; and practically insoluble in water.

Uses It is mostly used as an antihypertensive.

D. Vinblastine

Synonyms Vincaleukoblastine; VLB; 29060-LE;

Biological Source It is obtained from Vinca rosea Lin.. (Apocynaceae).

Chemical Structure

Isolation It may be isolated from Vinca rosea Linn., either by the method suggested by Noble et al*. or by Gorman et al.,**

Characteristic Features

1. It is obtained as solvated needles from methanol having mp 211-216°C.

2. Its specific optical rotation [α]²⁶_D + 42° (chloroform).

3. It has uvmax (ethanol): 214, 259 nm (log ε4.73, 4.21).

4. It is soluble in alcohols, chloroform, acetone, ethyl acetate and is practically insoluble in water and petroleum ether.

Identification Tests It forms derivatives as given below:

1. Vinblastine Sulphate (C₄₆H₅₈N₄O₉.H₂SO₄) (Exal, Vebe, Velban): It is obtained as crystals mp 284-285°C. Its physical parameters are: [α]²⁶_D – 28° (C = 1.01 in methanol); pKa1 5.4; pK_a 27.4. It has uv_max (methanol): 212, 262, 284, 292 nm (log € 4.75, 4.28, 4.22, 4.18). One part is soluble in 10 parts of water, 50 parts of chloroform; very slightly soluble in ethanol; and practically insoluble in ether.

2. Vinblastine Dihydrochloride Dihydrate (C₄₆H₅₈N₄O₉.2HCl.2H₂O): It is obtained as crystals that decompose at 244-246°C.

Uses

1. The alkaloid is used for the treatment of a wide variety of neoplasms.

2. It is also recommended for generated Hodgkin’s disease, lymphocytic lymphoma, hystiocytic hymphoma, mycosis fungoides, advanced testicular carcinoma, Kaposi's sarcoma, and choriocarcinoma and lastly the breast cancer unresponsive to other therapies.

3. It is effective as a single entity, however, it is normally given along with other neoplastic agents in combination therapy for the increased therapeutic effect without any noticeable additive toxicity.

4. It arrests mitosis at the metaphase.

5. It is found to be effective in the acute leukemia of children.

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* Noble et al. Ann. N.Y. Acad. Soc. 76, Art 3, 892-894 (1958)

** Gorman et al. J. Am. Chem. Soc., 81, 4745, 4754, (1959).

E. Vincristine

Synonyms Leurocristine; VCR; LCR.

Biological Sources It is also obtained from Vinca rosea Lin., (Catharanthus roseus G. Don) belonging to the natural order Apocynaceae.

Chemical Structure Please see the chemical structure under Vinblastine. It may also be named as: 22-Oxovincaleukoblastine.

Isolation Vincristine may be isolated from Vinca rosea Linn., by the method suggested by Svoboda.*

Characteristic Features

1. It is obtained as blades from methanol having mp 218-220°C.

2. Its specific optical rotation [α]²⁵_D + 17°; [α]²⁵_D + 26.2° (ethylene chloride); pKa: 5.0, 7.4 in 33% DMF.

3. It has uv_max (ethanol): 220, 255, 296 nm (log am 4.65, 4.21, 4.18).

Identification Tests

Vincristine Sulphate (C46H56N4O10.H2SO4) (Vincrex, Oncovin, Vincosid, Kyocrystine): Its crystals are obtained from ethanol and is found to be unstable.

Uses

1. Vincristine sulphate is recommended for the treatment of acute lymphocytic leukemia, and in combination therapy in Hodgkin's disease, lymphosarcoma, reticulum cell sarcoma, neuroblastoma, Wilm's tumour and rhabdomyosarcoma.

Note: Viucristine sulphate being highly unstable; therefore, its refregerated storage in sealed ampules is absolutely essential.

2. It is broadly used as an antineoplastic agent.

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* Svobada, Lyoydia, 24, 173 (1961)

F. Strychnine

Biological Sources It is abundantly found in the seeds of Strychnos Nux Vomica L. (Loganiaceae) (Nux Vomica, Strychnine); beans of Strychnos ignatti Berg. (Loganiaceae); roots of S. cinnamomifolia Thw.; seeds, bark and wood of S. colubrina Linn.; and plant of S. malaccensis Benth. (Syn: S. gautheriana Pierre).

Chemical Structure

Strychnidine-10-one; (C₂₁H₂₂N₂O₂)

Salient Features

1. Strychnine contains two N-atoms even then it happens to be a mono-acidic base.

2. Strychnine readily forms a variety of salts, such as: nitrate, N6-oxide, phosphate and sulphate. Interestingly, the N-atom which is specifically involved in the salt formation is the one that is located farthest from the aromatic benzene ring.

3. The second N-atom is strategically positioned as an amide nitrogen; and, therefore, it does not exhibit any basic characteristics.

Isolation Strychnine may be isolated from the seeds of S. nux vomica by adopting the following steps sequentially:

1. The seeds of nux vomica are dried, ground and sieved which are mixed with an adequate quantum of pure slaked lime and made into a paste by adding a requisite amount of water. The wet mass thus obtained is dried at 100°C and extracted with hot chloroform in a continuous extractor till the extraction is completed.

2. The alkaloids are subsequently removed from the chloroform solution by shaking with successive portions of dilute sulphuric acid (2N). The combined acid extracts are filtered to get rid of any foreign particles or residue.

3. To the resulting acidic filtrate added an excess of ammonia to precipitate the alkaloids (strychnine + brucine).

4. The precipitate is extracted with ethanol (25% v/v) several times which exclusively solubilizes brucine, and ultimately leaves strychnine as an insoluble residue.

5. The residue containing strychnine is filtered off and is finally purified by repeated recrystallization from ethanol.

Characteristic Features

1. It is obtained as brilliant, colourless cubes from a mixture of chloroform and ether having mp 275-285°C, and d¹⁸ 1.359.

2. Its specific optical rotation [α]¹⁸_D-104.3° (C = 0.254 in ethanol); [α]²⁵_D-13° (C = 0.4 in chloroform).

3. Its dissociation constant pK_a (25°) 8.26.

4. It has uvmax (95% ethanol); 2550, 2800, 2900 Å (E1% 1cm 377, 130, 101).

5. Solubility Profile: 1g dissolves in 182 ml ethanol, 6.5 ml chloroform, 150 ml benzene, 250 ml methanol, 83 ml pyridine; and very slightly soluble in water and ether.

6. A solution of strychnine containing 1 part in 700,000 parts of water gives a distinct bitter taste.

Identification Tests Strychnine may be identified either by specific colour tests or by specific derivatives:

(a) Colour Tests

1. Sulphuric Acid-Dichromate Test: Strychnine (5-10 mg) when dissolved in a few drops of concentrated sulphuric acid and stirred with a crystal of pure potassium dichromate [K₂Cr₂O₇] it gives an instant reddish-violet to purple colouration.

Note: Strychnine derivatives will also give this test except strychnine nitrate.

2. Mandelin’s Reagent Test: Strychnine or its corresponding salt when treated with Mandelin’s Reagent* it gives rise to a violet to blue colouration.

3. Ammonium Vanadate (V) Test: Strychnine or its salt when treated with a saturated solution of ammonium vanadate, it produces a violet to blue colouration.

4. Nitric Acid Test: Strychnine on being treated with a trace of HNO3 (conc.) yields an instant yellow colouration.

Note: A similar test with Brucine gives an intense orange-red colouration. It may be used to differentiate between strychnine and brucine.

(b) Strychnine Derivatives: The various important strychnine derivatives are as given under:

1. Strychnine Nitrate (C₂₁H₂₃N₃O₅): It is obtained as colourless, odourless needles or while crystalline powder 1g dissolves in 42 ml water, 10 ml boiling water, 150 ml ethanol, 80 ml ethanol at 60°C, 105 ml chloroform, 50 ml glycerol; and insoluble in ether. It shows a pH ~ 5.7.

2. Strychnine N⁶– Oxide (C₂₁H₂₂N₂O₃): It is obtained as monoclinic prisms from water which decompose at 207°C. It has pK value 5.17. It is found to be freely soluble in ethanol, glacial acetic acid, chloroform; fairly soluble in water; sparingly soluble in benzene; and practically insoluble in ether and petroleum ether.

3. Strychnine Phosphate (C₂₁H₂₅N₂O₆P): It is usually obtained as its dihydrate salt (2O₆P.2H₂O) which is colourless or while crystals or white powder. 1g dissolves in slowly in ~ 30 ml water, more soluble in hot water, and slightly soluble in ethanol. The aqueous solution is acidic to litmus.

4. Strychnine Sulphate (C₄₂H₄₆N₄O₈S): It normally crystallizes as pentahydrate [2C₂₁H₂₂N₂O₂.H₂SO₄.5H₂O]. It is colourless, odourless, very bitter crystals or white crystalline powder. It effloresces in dry air and loses all its water of crystallization at 100°C. It shows mp when anhydrous ~ 200°C with decomposition. 1g dissolves in 35 ml water, 7 ml boiling water, 81 ml ethanol, 26 ml ethanol at 60°C, 220 ml chloroform, 6 ml glycerol, and insoluble in ether. A 1 : 100 solution shows pH 5.5.

5. Strychnine Gluconate Pentahydrate (C₂₇H₃₄N₂O₉.5H₂O): Its crystals darken above 80°C. It is soluble in 2 parts water ~ 40 parts ethanol. The aqueous solution is found to be neutral.

6. Strychnine Glycerophosphate Hexahydrate (C₄₅H₅₃N₄O₁₀P.6H₂O): 1g dissolves in ~ 350 ml water, ~ 310 ml ethanol; slightly soluble in chloroform; and very slightly soluble in ether.

7. Strychnine Hydrochloride Dihydrate (C₂₁H₂₃ClN₂O₂.2H₂O): It is obtained as trimetric prisms which are efflorescent in nature. 1g dissolves in ~ 40 ml water, ~ 80 ml ethanol, and insoluble in ether. The pH of a 0.01 M solution is 5.4.

Uses

1. Strychinine is extremely interesting pharmacologically and is regarded as a valuable tool in both physiologic and neuroanatomic research.

2. It is extremely toxic, and functioning as a central stimulant.

3. It causes excitation of all parts of the central nervous system and blocks inhibitory spinal inpulses at the post synaptic level. This may lead to an exaggeration in reflexes ultimately leading to tonic convulsions.

4. The drug is rarely used in modern medical practice but is utilized as a vermin killer i.e., animal or insect killer.

5. It is used chiefly in poison baits for rodents.

Biosynthesis of Yohimbine, Reserpine, Rescinnamine, Vinblastine, Vincristine and Strychnine Dehydogeissoschizine (keto-form) undergoes isomerization by means of the nucleophilic attack on to carbonyl through a conjugated system, which subsequently forms an onium ion that upon reduction produces yohimbine as shown below:

Reserpine and deserpidine are essentially the trimethoxybenzoyl esters of yohimbine-type alkaloids; whereas, rescinnamine is a trimethoxycinnamoyl ester. Interestingly, both reserpine and rescinnamine contain an additional methoxyl moiety present strategically on the indole ring system at C-11, which is accomplished by virtue of hydroxylation and methylation at a late stage along the pathway. A predominant and characteristic feature of these alkaloids is that they exhibit the opposite stereochemistry at C-3 to yohimbine and strictosidine as depicted below:

The biosynthetic pathway leading to vinblastine and vincristine is supposedly involve the following vital steps:

1. An oxidative reaction on catharanthine, catalysed by an enzyme peroxidase, thereby producing a peroxide that aptly loses the peroxide as a leaving group, ultimately breaking a carbon-carbon covalent bond as shown in the diagram given below.

2. The intermediate electrophilic ion is attacked on to the conjugated iminium system by the vindoline, whereby C-5 of the indole nucleus being appropriately activated by the –OCH₃ moiety located at C-6, and also by the N-atom present in the indole ring.

3. The resulting adduct is subsequently reduced in the dihydropyridinium ring by NADH*-dependent 1, 4-addition thereby giving rise to the substrate for hydroxylation.

4. Ultimately, reduction of the above resulting product generates vinblastine.

5. The oxidized product from vinblastine, with its N-formyl moiety rather than N-methyl on the vindoline fragment, may finally yield vincristine.

The biosynthetic pathway leading to strychnine essentially comprise of the following steps, namely:

1. Preakuammicine loses one C-atom via hydrolysis followed by decarboxylation.

2. Addition of the two extra C-atoms is accomplished by means of Aldol-condensation reaction with acetyl-CoA, whereby it yields the Wieland-Gumlich aldehyde as a complexed hemiacetal form.

3. The subsequent construction of ether and amide linkages gives rise to the formation of stryctinine from the above hemiacetal as shown below.

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* NADH = Nicotinamide adenine dinucleotide (reduced form).
Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar