Sample chapter alkaloids - Pharmaceutical Chemistry of Natural Products by V. Alagarsamy--order +91 8527622422

�INTRODUCTI0N �

Alkaloids are pharmacologically active, complex organic compounds containing one or more nitrogen atoms, characteristically as primary, secondary or tertiary amines, which provide basicity to the alkaloid. The term alkaloid (alkaloid means alkali-like) cannot be defi ned exactly, as there is no clear-cut boundary between alkaloids and naturally occurring complex amines. In practice, those substances obtained from plant sources and answer for the standard qualitative tests are called alkaloids. The name protoalkaloid is applied to compounds such as ephedrine and colchicine which are not having certain properties of typical alkaloids. They are found mainly in plants, but to lesser extent in animals and microorganisms.

By keeping the knowledge of alkaloids in view, alkaloids may be defi ned as basic nitrogen containing (usually in a heterocyclic system) plant products, possessing marked pharmacological action at very low dose level.

Generally, from an amino acid, the nitrogen atoms in alkaloids are instigated and the carbon skeleton of the particular amino acid precursor is also mostly retained in the structure of alkaloid. However, through the transamination reactions a major group of alkaloids are found to obtain their nitrogen atoms, incorporating only the nitrogen from an amino acid, while the remaining part of the molecule may be derived from acetate or shikimate, or may be terpenoid or steroid in origin.

Alkaloids are generally bitter in taste and optically active (except papaverine), usually levorotatory in nature (exception is coniine, which is dextrorotatory), colourless (except berberine which is yellow; harmaline and betanidine which are reddish), crystalline solids (except nicotine and coniine which are liquids) and soluble in organic solvents like chloroform and ethanol but insoluble in water. As they are basic in nature and form salts with the acids, some of the alkaloids exist as salts called quaternary amines (e.g. cinchona alkaloids with quininic acid) while some of them exist as free bases (e.g. nicotine). Some of the alkaloids also occur as glycosides (e.g. solanum alkaloids) and esters (e.g. atropine). Biological activity of the alkaloids frequently depends on the amine function being transferred into a quaternary amine at physiological pH by protonation.

Alkaloids

��5

CHAPTER

Chapter-05.indd 149Chapter-05.indd 149 6/29/2012 10:19:07 AM6/29/2012 10:19:07 AM

150 �� Pharmaceutical Chemistry of Natural Products

Alkaloids were fi rst isolated successfully in the nineteenth century and the alkaloid-containing drugs were marketed. The structure of the fi rst alkaloid, namely coniine was established in the year 1870 by Schiff. In search of plant drugs with anticancer activity, catharanthus alkaloids and paclitaxel came into the market.

� � NOMENCLATURE OF ALKALOIDS

There is no systematic nomenclature of alkaloids because of their complex molecular structure and some historical reasons. They are named by adopting different methods as described below:

1. According to their physiological action: Examples include emetine (Greek word: emetikos means to vomit), morphine (German word: morphine means God of Dreams) and narcotine (Greek word: narkoo means benumb).

2. According to the plants from which they are obtained: Examples include papaverine from Papaver somniferum and berberine from Berberis vulgaris.

3. Prefi xes like epi, iso, neo, pseudo or Greek letters are used to name isomeric or slightly modifi ed structures: The prefi x nor indicates the structure which does not have a methyl group attached to the nitrogen atom (e.g. norephedrine).

4. According to the name of the discoverer: For example, pelletierine (discovered by P.J. Pelletier). 5. The minor alkaloids are named by adding one prefi x or suffi x to the name of principal alkaloids: For

example, cinchonidine derived from cinchonine.

� � CLASSIFICATION OF ALKALOIDS

Alkaloids are classifi ed in different methods. Some of these methods are described briefl y and the chemical classifi cation is described in detail:

1. Pharmacological classifi cation: It is based on the clinical use or pharmacological activity. Examples include analgesic alkaloids and cardioactive alkaloids.

2. Taxonomic classifi cation: It is based on the family or genus, without reference to the chemical type of alkaloid present (e.g. solanaceous alkaloids). However, the most common classifi cation is according to the genus in which they occur (e.g. rauwolfi a alkaloids and cinchona alkaloids).

3. Biosynthetic classifi cation: It is based on the type of precursors or building block compounds used by plants to synthesize alkaloids. This is a more fundamental method than chemical classifi cation. For example, morphine, papaverine, narcotine and colchicine may be listed as phenylalanine- and tyrosine-derived bases.

4. Chemical classifi cation: It is based on the chemical structure of the alkaloid. The chemical classifi cation of alkaloids is universally adopted and depends on the basic ring structure present. For example, atropine is a tropane alkaloid; quinine is considered as a quinoline-type alkaloid; papaverine is an isoquinoline and reserpine, strychnine and ergometrine are indole alkaloids.


Alkaloids �� 151

Alkaloids

Derived from amino acids

�

True alkaloids Proto/amino alkaloids Pseudo alkaloids Example: Steroidal, terpenoidal alkaloids

Contain heterocyclic nitrogen Simple amines Not derived from amino acids but from acyl CoA units

Example: Conestine, caffeine

Based on the chemical nature, alkaloids are further classifi ed into two major groups as mentioned below:

1. Heterocyclic or typical alkaloids 2. Nonheterocyclic or atypical alkaloids [protoalkaloids (or) biological amines]

They are further subdivided as follows:

Heterocyclic Alkaloids

1. Pyridines and piperidines

O

O

C C C C C

H H O

N

H H

Piperine Nicotine

N

N

CH3

Arecoline

COOCH3

N

CH3

NH

O

Pelletierine

OCH3

CN

ON

CH3

RicinineLobeline

N

CH3

O

OH

MeNH

Conine



2. Quinolines

H CO3

NQuinine

H

CH2

H C2

HOH

N

CH CH� 2

H CO3

NQuinidine

H

CH2

H C2

HOH

N

CH CH� 2

H

NCinchonine

H

CH2

H C2

HOH

N

CH CH� 2

NCinchonidine

H

CH2

H C2

HOH

N

CH CH� 2

HH

H

3. Isoquinolines

H CO3

H CO3

N

CH2

OCH3

OCH3

PapaverineNoscapine

N

OCH3

O

OH C2

CH3

H

H CO3O

CO

H CO3

Berberine chloride

N�O

O

Cl�

OCH3

OCH3

H



N

H CO3

H CO3

H

HH

H

CH3

HN

OCH3

OCH3

Emetine

N

H CO3

H CO3

H

HH

H

CH3

HN

OCH3

OH

Cephaline

H C3

N�

CH3

H

O

OHH CO3

OH

H C3

N�

H

H

OOCH3

( )-Tubocurarine� Galanthamine

H CO3

O

NCH3

OH

4. Phenanthrenes

CH3

N

OHO

H

HO

Morphine

CH3

N

H

OHO

H CO3

ThebaineCodeine

H CO3OCH3

O

H

N

CH3



5. Indole alkaloids

Ergometrine

N

CH3

NH

HNOCC

H

CH OH2

H C3

Lysergic acid

HOOC

N

CH3

NH

N

HN

CONHH

HCH3

CH3

O

N

OH

O

H

N

O

CH2H

Ergotamine

R1

N

R2

CH3

H

N

R3

Physostigmine

R = CH NHCOO —R = CHR = CH

1 3

2 3

3 3

N

H

N

HH CO3

H

H

C

H CO3

O

OCH3

O

C

O

OCH3

OCH3

OCH3

Reserpine



H

NN

H

H

H

C

O

H CO3 OH H

N

N CH CH2 3

OH

H

COOCH3

N

Vinblastine R CHVincristine R CHO

��

3

Yohimbine

R

NH CO3

HO C

O

OCH3

OCOCH3

HCH CH2 3H

N

R1

R2

O

N

O

Strychnine R = R = HBrucine R = R = OCH

1 2

1 2 3

��

6. Pyrrole and pyrrolidines

N

CH3

CH2 C CH3

O

Hygrine

N+

Me

COO�

Stachydrine

Me

7. Tropane alkaloids

N CH� 3OCOCH

C H6 5

CH OH2

O OCO CH�

Atropine Hyoscine

N CH� 3

C H6 5

CH OH2



N CH� 3OOC CH�

C H6 5

CH OH2

HyoscyamineR= CHR’= C H CO (Benzoyl)

3

6 5

Cocaine

COOR

OR’

H

N CH� 3

8. Imidazole or glyoxalines

H C3

O O

H H

N CH3�

Pilocarpine

N

9. Purines

O

CH3

H C N3 �

CN

ON

N CH3� H C.N3

C

ON

CH3

N

O

NH HN

C

ON

CH3

N

N CH3�

O

Caffeine Theophylline Theobromine

10. Terpenoid alkaloids OH

OCH3

H C3

OCH3

OCOC H6 5

OH

OCOCH3

OCH3

Aconitine

OCH3

H C2

HO

N



11. Steroidal alkaloids

H C3

CH3

CH3

HO

NCH3

Solanidine ConessineH C3

H C3 N

CH3

CH3

CH3N

Nonheterocyclic Alkaloids

HO

C

C

H

N

H

CH3

HH C3

OH

OH

CH2 NH

CH3

HC

OH

( Adrenaline�)-Ephedrine

C

O

NH

OH

C

O

O

H C3

H COCO3 O OHCH3

CH3

CH3

HOAcO

O C O

Taxol

O



MeO

MeO

OMe

NHCOCH3

O

OMeColchicine

HO CH2 CH2 N

CH3

CH3

Hordenine

OCH3

OCH3H CO3

CH2

CH2

NH2

H COOC3

O

N( )�

N( )�

SerpentineMescaline

�QUALITATIVE CHEMICAL TESTS FOR ALKALOIDS �

General tests answered by all alkaloids are as follows:

1. Dragendorff’s test: To 2–3 mL of the alkaloid solution add few drops of Dragendorff’s reagent (potassium bismuth iodide solution). An orange brown precipitate is formed.

2. Mayer’s test: To 2–3 mL of the alkaloid solution add few drops of Mayer’s reagent (potassium mercuric iodide solution). White brown precipitate is formed.

3. Hager’s test: To 2–3 mL of the alkaloid solution add few drops of Hager’s reagent (saturated solution of picric acid). Yellow precipitate is formed.

4. Wagner’s test: To 2–3 mL of the alkaloid solution add few drops of Wagner’s reagent (iodine–potassium iodide solution). Reddish brown precipitate is formed.

5. For opium alkaloids: These alkaloids are present as salts of meconic acid. Opium is dissolved in water, fi ltered and to the fi ltrate, ferric chloride solution is added by which deep reddish purple colour is obtained. The colour persists even upon adding hydrochloric acid.

6. For tropane alkaloids ( Vitalis–Morin reaction): Tropane alkaloid is treated with fuming nitric acid, followed by evaporation to dryness and addition of methanolic potassium hydroxide solution to an acetone of nitrated residue. Violet colouration takes place because of the presence of tropane derivative.

7. For purine alkaloids ( murexide colour reaction): Caffeine is taken in a Petri dish to which hydrochloric acid and potassium chlorate are added and heated to dryness. A purple colour is obtained by exposing the residue to vapour of dilute ammonia. The purple colour is lost upon addition of alkali. Caffeine (and other purine alkaloids) gives murexide colour reaction.



� � ISOLATION OR PRODUCTION OF ALKALOIDS

Alkaloid bearing plant usually contains a complex mixture of alkaloids, hence isolation and purifi cation of an alkaloid from a plant is always not a simple process. Further, the presence of organic acids, glycosides, etc., present in the plants may complicate the isolation process. Thus, isolation of pure alkaloid may often become a laborious procedure.

The steps involved in the isolation of an alkaloid may be summarized as follows:

1. The presence of an alkaloid in a plant is ascertained by using the various alkaloidal reagents. (Refer the qualitative tests mentioned above.)

2. The next step is the separation of relatively small amount of alkaloids from large amount of extraneous plant materials.

3. The fi nal step is the separation and purifi cation of individual alkaloids from the crude mixture.

The isolation of an alkaloid from the crude powdered drug is schematically represented below.

Powdered drug containing alkaloid salts (tannates, oxalates, etc.)

Free alkaloids

Total extract

1. Defat if necessary with petroleum ether2. Moisten and render alkaline with Na

2CO

3 or

K2CO

3 or NH

3 or Ca(OH)

2

Extract the alkaloid with organic solvent like CHCl3, ether

Concentrate and shake the extract with succes-sive quantities of acid like dil. H

2SO

4

Aqueous acid solution of alkaloid

Make alkaline with Na2CO

3

and extract alkaloids with an immiscible solvent like CHCl

3

Fractional crystallization, fractional precipitation, column chromatography, partition chromatography, gas chromatography or by counter current extraction

Residual aqueous fraction

Crude alkaloid mixture

Individual alkaloids

Distil off the solvent

Organic solution of the alkaloid bases

Residual organic fraction(Pigments, fats and occasionally very weak bases or chloroform soluble alkaloid sulphates)



� � DETERMINATION OF MOLECULAR STRUCTURE OF ALKALOIDS: GENERAL METHODS

As the molecular structure of alkaloids is complex, only recently few of the complex alkaloids’ structures were elucidated. The various chemical methods performed to determine the structure of alkaloids is as follows:

Molecular Formula Determination

The fi rst step in structural elucidation is the determination of molecular formula and optical rotatory power. Elemental composition and hence the empirical formula is found by combustion analysis.

Hydrolysis

Simple fragmentation by hydrolysis with water, acid or alkali yields simple fragments which are then analysed separately. For example, atropine on hydrolysis yields tropine and tropic acid.

Determination of Unsaturation

The unsaturation can be determined by adding bromine, halogen acids or by hydroxylation with KMnO

4 or by reduction (using either LiAlH

4 or NaBH

4).

Functional Group Determination

By using the usual standard chemical tests or by infrared (IR) spectroscopy, functional nature of the alkaloids is determined.

Functional nature of oxygen: The oxygen atom may be present in the form of alcoholic hydroxyl (–OH), phenolic hydroxyl (–OH), methoxyl (OCH

3), acetoxy (–OCOH

3), benzoxyl (–OCOC

6H

5),

carboxyl (–COOH), aldehyde (–CHO), ketone (C=O) and methylene dioxide group (–O–CH2–O–).

These groups are characterized by the chemical tests as follows:

Phenolic hydroxyl group (=C–OH)

It is identifi ed by the following tests:

Soluble in alkali and reprecipitation by CO �2.

Violet colouration with neutral ferric chloride. �Yields ester on acetylation. This reaction can be used to determine the number of phenolic �–OH. Yields ether on reaction with alkyl halide. �



Alcoholic hydroxyl group (–C–OH)

It yields ester on acetylation and benzoylation (but negative answer for phenolic –OH)—refer above tests.

This is confi rmed by oxidation, dehydration, dehydrogenation and by spectroscopy (IR and NMR). Alcohols are of three different types: 1°, 2° and 3°, and they are usually distinguished by their oxidation products.

Primary alcohol

R—CH —OH2

(O)R—CHO

(O)R—COOH

Aldehyde withsame no. of‘C’ as in alcohol

Carboxylic acidwith same no. of‘C’ as in alcohol andaldehyde

Secondary alcohol

R—CH CH CH2 3— —

OH

(O)R—CH C CH2 3— —

O

(O)R CH C OH2— — —

OKetone withsame no. of

'C' as in alcohol

Carboxylic acidwith fewer no. of'C' with respect toalcohol and ketone

But in cyclic structure, 2° alcohol yields different oxidation products as shown below:

OH OCOOH

COOH

Ketone withsame no. of

'C' as in alcohol

Acid with same no. of'C' as in alcohol

(O) (O)

Tertiary alcohol

R CH C CH2 3— — —

OH

CH3

(O)R CH C CH2 3— — —

O

(O)R CH C OH2— — —

O

Ketone withlesser no. of

'C' with respect toalcohol

Carboxylic acidwith fewer no. of'C' with respect toalcohol and ketone



The number of hydroxyl (OH) groups present in the compound is determined by the following methods:

Acetylation method:

H+

R—OH + (CH3CO)

2O R—O—CO—CH

3

By determining the amount of acetic anhydride that reacted with alcohol to form an ester, the number of hydroxyl groups is determined.

Zerewitinoff active hydrogen determination method: When alcohol is heated with CH3MgI, methane

is obtained. By measuring the methane so formed, the amount of alcohol can be determined.

R—OH � CH MgI3 CH4 � MgI

OR

–OH = CH4 = 22.4 L of alcohol at normal temperature and pressure.

Carbonyl group

The presence of aldehydes and ketones is detected by their reaction with hydroxylamine, semicarbazide and phenylhydrazine to form the corresponding oxime, semicarbazone and phenylhydrazone, respectively.

C ONH OH. HCl2

�H O2

C N OH � HCl

Oxime

By determining the HCl formed, the ketones are estimated quantitatively.

C OPh—NHNH2

�H O2

C N NH — Ph � HCl

Phenylhydrazone

C O

O N2 NHNH2

NO2

O N2 NH

NO2

N � C

Dinitrophenylhydrazone

C OH N — NH — C — NH2 2

�H O2

C N NH C NH2

Semicarbazone

OO

The aldehydes and ketones are distinguished by their oxidation or reduction products.

The carbonyl groups of aldehydes, ketones and carboxyl are further confi rmed by their spectral data such as IR, ultraviolet (UV) and NMR.



Carboxyl group (–COOH)

The presence of carboxyl group is determined by the following:

Its solubility in weak bases such as NH �3, NaHCO

3 and Na

2CO

3.

Esterifi cation with alcohols. �Specifi c IR and NMR signals. �Quantitatively by acid–alkali titration: � Performed by titrating the carboxylic acid with NaOH using phenolphthalein as an indicator. By knowing the volume of NaOH consumed the number of –COOH groups are determined.

Ester group (RCOOR)

Esters and related groups like amides and lactones are detected by their reaction with water, dilute acids or alkali to the hydroxyl and acidic compounds. By elucidating the acid and alcohol, the nature of alkaloids is determined.

R COOR’— R COOH— R’OH�

R—CONH2 R COOH— NH3�

R — CH — CH — CH2 2

O CO

NaOHR — CH — CH — CH2 2

OH COONa

Alkoxy group (–OR)

Determined by Zeisel’s method—alkoxy group such as methoxy on reacting with hydroiodic acid followed by silver nitrate yields equal amount of silver iodide. From the amount of silver iodide formed, the number of alkoxy groups is calculated.

OCH3

126°C� HI –OH � CH I3

AgNO3AgI

Boil+ CH3NO2

Estimation of C-methyl group (Kuhn Roth method): By estimating the acetic acid formed upon oxidation, the C-methyl groups are quantifi ed.

C CH3

K Cr O / H SO2 2 7 2 4CH COOH3

Functional nature of nitrogen: Most alkaloids contain ‘N’ in their ring structure, which may exist as 2° or 3°.



The 2°and 3° amines are distinguished as follows:

2° Amines (acetylated or benzoylated) undergo Libermann’s nitroso reaction. �2° Amines take up 2 moles of alkyl halide to form 4° ammonium salt. �3° Amines take up 1 mole of alkyl halide to form 4° ammonium salt. �

N(O)

3° N30% H O2 2

N O

Amine oxide

Alkaloid Methylamine, Dimethylamine, Trimethylamine–(indicates the presence of 1 or 2 alkyl groupsattached to amine ‘N’)–Ammonia (indicates the presence of primary amine)

Distillation

Further, the nature of ‘N’ is confi rmed by degradation methods such as Hoffmann Exhaustive Methylation (HEM).

The N-alkyl groups are estimated by Herzig–Meyer method:

N CH3

HIN H CH I3

150–300°C

Under pressure*

AgNO3

EtOHAgI�

N C H2 5

HIN H C H I52

150–300°C

Under pressure*

AgNO3

EtOHAgI�

*Differs form –OR (alkoxy group) estimation

From the amount of silver iodide formed, the number of N-alkyl groups is calculated.

Degradation of Alkaloids

Degradation of alkaloids gives rise to some identifi able products of known structure and hence by knowing structure of the degraded products and the changes occurred during the degradation it is convenient to know the structure of the original molecule. Different degradation reactions carried out in elucidating the structure of alkaloids are as follows:

1. HEM method 2. Emde method 3. Von Braun’s (VB) method for 3° cyclic amines 4. Reductive degradation 5. Oxidation 6. Zinc distillation 7. Alkali fusion 8. Dehydrogenation

1. HEM method: Originally this method was applied by Willstater in 1870 for naturally occurring alkaloids. It was further developed by Hoffmann and hence it is known as HEM. Principle of this method is that the quaternary ammonium hydroxides yield olefi n with the cleavage of carbon–nitrogen linkage upon heating with the loss of water molecule (H from β-carbon atom with respect to N and OH from the 4° ammonium hydroxide).



Quaternization is done by complete methylation of the amine followed by hydrolysis with moist Ag

2O or KOH.

R—CH —CH —NCH2 2 3

2 CH I3 R CH CH N (CH ) I2 2 3 3�— — —

Moist Ag O2

(AgOH)R—CH —CH —N (CH ) OH2 2 3 3

� �

–H ON(CH )

2

3 3–�

RCH CHOlefin

� 2

This method can be applied to the reduced ring system but fails with unsaturated analogues and hence, the unsaturated rings are fi rst saturated and then HEM is performed.

N NH

H –Ni22CH I3

AgOH

N�

(CH )3 2

�

�H O2

N

(CH )3 2

(i) CH I(ii) AgOH

3

Isomerism �

-H O-N(CH )

2

3 3 N�

(CH )3 3

OH�

OH�

As β-hydrogen atom is needed to cleave C–N bond and eliminate water molecule, the HEM fails on the ring system that does not have β-hydrogen atom. For example, in the degradation of isoquinoline, the cleavage of N does not occur at the fi nal step as there is no β-hydrogen with respect to ‘N’.

N

Na—EtOH

NH

(i) 2CH I3

(ii) AgOH N� (CH )3 2 OH�

�H O2�

N (CH )3 2

(i) 2CH I3

(ii) AgOHN� (CH )3 3 OH�



However, there are some cases in which HEM fails even if the β-hydrogen atom is present (the following reaction explains this).

NH

(i) 2CH I3

(ii) AgOH

N�OH�

(CH )3 2

�

�CH OH3

N

CH3(95%)

�H O2

N

(CH )3 2(5%)

2. Emde method: Emde modifi cation may be used in the above two cases, where HEM failed. In this method, 4° ammonium halide is reduced with sodium amalgam in aqueous ethanol or Na–liquid NH

3 or catalytically.

N

Na—EtOH

NH

(i) 2CH I3

(ii) AgOH N� (CH )3 2 OH�

N (CH )3 2

(i) 2CH I3

(ii) AgOH

�H O2

(CH ) OH3 2�N�

(Beta hydrogen absent)

Na–liq. NH(or)Na–HgH O–EtOH

3

2

(Alpha methyl styrene)

CH3

�



Tetrahydroquinoline is degraded as follows:

N I� �

(CH )3 2

Na liq. NH3–

3-Phenyl-N,N-dimethylpropylamine

N

H C3 CH3

N

H C3 CH3

O-Propyldimethyl aniline

Emde degradation on tetrahydroisoquinoline also proceeds as follows:

N� (CH )3 2 I�

N� (CH )3 2

H Pt(or)Na–Hg

2– (I) CH I(ii)Na Hg

3–

N (CH )3 2

CH3

HEM

CH3

3. VB method:

(a) For 3° cyclic amines: The 3° N atom in the ring upon reaction with CNBr followed by hydrolysis yields brominated 2° amine.

N

R

BrCN

N�

R CN

Br�

NCH Br2

Hydrolysis

HBrNH

R

CH Br2

2° Amine

R CN

This method is applied on compounds which do not respond to HEM. Ring opening takes place differently in VB and HEM method which is shown in the following degradation.

N (CH )3 2 N (CH )3 2

HEM VB

CH Br2

NCH3

CN



In general, CNBr cleaves the unsymmetrical amines to yield the bromides or shorter bromides. However, in the VB method only dealkylation may occur without ring cleavage in some cases.

N — CH3

COOCH3

OCOC6H5CNBr

N — CN

COOCH3

OCOC6H5

(i) HCl(ii) CO� 2 N — H

COOCH3

OCOC6H5

(b) For 2° cyclic amines:

NH

N

COC H6 5

C H COCl6 5 PBr –5 Br2

N

Br—C—C H6 5

Br

Distil under

reduced pressureBr—(CH ) Br + C H CN2 5 6 5

1,5-Dibromo pentane

4. Reductive degradation: Ring system is opened by treating with HI in many cases.

HI

N NH

(or)300°C

CH —(CH ) —CH + NH3 2 3 3 3

n-Pentane

5. Oxidation: Oxidation gives valuable information about the fundamental structure of alkaloids and the position and nature of functional groups, side chains, etc.

For example, picolinic acid obtained upon oxidation of coniine indicates that the coniine is an α-substituted pyridine.

(O)

N COOH

Picolinic acid

Coniine

By varying the strength of oxidizing agents, a variety of products may be obtained. Different types of oxidizing agents used are as follows:

1. For mild oxidation: H2O

2, O

3, I

2.

2. For moderate oxidation: acid or alkali KMnO4, CrO

3 in CH

3COOH.

3. For vigorous oxidation: K2Cr

2O

7–H

2SO

4, concentrated HNO

3 or MnO

2–H

2SO

4.

6. Zinc distillation: Distillation of alkaloid over zinc dust degrades it into a stable aromatic derivative.

PhenanthreneMorphineZinc dust

distillation



The reaction indicates that morphine is possessing phenanthrene nucleus.

7. Alkali fusion: Fusion of alkaloids with solid KOH gives simple fragments from which the nature of alkaloid can be derived.

Substituted isoquinolinePapaverineFusion with

KOH

The reaction indicates papaverine is containing isoquinoline nucleus.

AdrenalineKOH

fusion

COOHHO

HO

Protocatechuic acid

The reaction indicates adrenaline is a monosubstituted catechol derivative.

8. Dehydrogenation: Distillation of alkaloid with catalysts such as S, Se and Pd yields simple and recognizable products from which the gross skeleton of the alkaloid may be derived.

Thus with the help of degradation, nature of various fragments obtained, nature of nucleus and type of linkages are established. The fragments obtained are arranged in the possible ways with the possible linkages and the one that will explain all the properties is selected and confi rmed by synthesis. Optical activity of an alkaloid helps greatly in establishing the structure of alkaloid.

Physical Methods in Conjunction with Chemical Methods

The developments in spectroscopic methods not only curtailed time consumption as compared to degradation studies, but also helped in determining the molecular structure of complex alkaloids. Morphine structure was established after the developments of the below-mentioned physical methods. The complete structure of vindoline (including confi guration) has been established by spectroscopic methods. Thelepogine, another alkaloid structure, has been established using X-ray analysis technique without performing any of the chemical analysis.

The important physical methods used in structural elucidation of alkaloids are as follows:

1. IR spectroscopy 2. UV spectroscopy 3. NMR spectroscopy 4. Mass spectroscopy 5. X-ray analysis 6. Optical rotatory dispersion (ORD) and circular dichroism 7. Conformational analysis

IR spectroscopy: This method is used to identify the presence of functional groups such as –OH, –NH

2, –NH and –C=O. The groups such as –OCH

3, –NCH

3, –OH, –NH

2 and –NH can be detected by

IR spectroscopy but quantifi ed by NMR spectroscopy.

NMR spectroscopy: This method helps to detect protons of alkyl, alkenyl, N-methyl, O-methyl, C-methyl, aryl and heteroaryl groups, etc. It also helps in quantitative estimation of these groups. Aromatic and heteroaromatic protons are exactly quantifi ed by using NMR spectroscopy.



UV spectroscopy: UV spectrum of a compound is characteristic of chromophoric system and not the whole compound. Hence, it helps to establish the likely structural type or class of the alkaloid under investigation.

Mass spectroscopy: This method is used to confi rm the proposed molecular structure of the alkaloid by determining the molecular weight of compounds and the fragments of the degradation products. It also helps to confi rm the side chain or attached groups by analysing the fragmentation pattern.

X-ray analysis: This method is used to distinguish the various possible structures of alkaloids.

ORD and circular dichroism: This method is used to confi rm the structure of optically active stereoisomers.

Conformational analysis: It is an experimental technique used to establish the stereochemistry as well as physical properties and chemical reactivity of alkaloids.

Synthesis

The above-mentioned chemical and analytical work helps to propose a tentative structure (or structures) of the alkaloid under investigation. Synthesis always gives additional evidence for the assigned structure even though the physical methods (mentioned above) provide fi nal proof of the proposed structure.

� � ALKALOIDS OF PHARMACEUTICAL IMPORTANCE

Source of medicinally important alkaloids with their pharmacological properties and uses are depicted in Table 5.1.

Table 5.1 Source pharmacological properties and uses of alkaloids

S. no. Alkaloids Source (Family) Pharmacological properties/uses

1. Piperine Piper nigrum and other Piper spp(Piperaceae)

Aromatic, stimulant, stomachic, carminative, condiment stimulates taste buds and gastric juice.

2. Nicotine Nicotiana tobaccum (Solanaceae) Stimulant effects on heart and nervous system.

3. Arecoline Areca catechu (Palmae)

Parasympathomimetic, anthelmentic drug.

4. Lobeline Lobelia nicotianaefolia (Campanulaceae or Lobeliaceae)

Used in asthma and as respiratory stimulant.

5. Pelletierine Punica granatum (Euphorbiaceae) Anthelmintic against tapeworm. Astringent in the treatment in diarrhoea.

6. Quinine, quinidine, cinchonine, cinchonidine

Cinchona offi cinalis and other Cinchona spp (Rubiaceae)

Antimalarial, bitter stomachics, antipyretic




7. Quinidine C. offi cinalis and other Cinchona spp (Rubiaceae)

Antiarrhythmic

8. Morphine Papaver somniferum (Papaveraceae) Hypnotic, sedative and analgesic. It sedates respiratory centre, emetic centre and the cough refl ux.

9. Codeine P. somniferum (Papaveraceae) Used in the treatment of cough.

10. Thebaine, papaverine, noscapine, narceine

P. somniferum (Papaveraceae) Hypnotic sedative and analgesic.

11. Berberine Various genera of Berberidaceae (Ranunculaceae, Papaveraceae)

In the treatment of cutaneous Leishmaniasis.

12. Emetine, cephaline Cephaelis ipecacuanha, Cephalis accuminata (Rubiaceae)

Expectorant in small doses and emetic in higher doses. Antiprotozoal against Entamoeba histolytica.

13. Tubocurarine Chondrodendeon tomentoscum (Menispermaceae)

Neuromuscular blocking agent, skeletal muscle relaxant.

14. Galanthamine Leucojum aestivum (Amaryllidaceae) Used in the treatment of Alzheimer’s disease

15. Ergometrine Claviceps purpurea (Clavicipitaceae) Oxytocic in obstetrics.

16. Ergotamine C. purpurea (Hypocreaceae) Specifi c analgesic in the treatment of migraine.

17. Lysergic acid amide Rivea corymbosa(Convolvulaceae)

As a psychotomimetic.

18. Physostigmine Physostigma venenosum (Leguminosae) Parasympathomimetic (ophthalmic) activity.

19. Reserpine Rauwolfi a serpentine(Apocynaceae)

As an antihypertensive and in the treatment of neuropsychiatric disorder.

20. Serpentine R. serpentine(Apocynaceae)

As an antihypertensive and in the treatment of neuropsychiatric disorder.

21. Yohimbine Aspidosperma spp(Apocynaceae)

In the treatment of erectile dysfunction.

22. Vincristine Catharanthus roseus(Apocynaceae)

As an antineoplastic, for treating leukaemia in children, as hypotensive and in the treatment of diabetes.

23. Vinblastine C. roseus(Apocynaceae)

In the treatment of Hodgkin’s disease.

24. Strychnine Strychnos nux-vomica(Loganiaceae)

Stomachic, tonic, stimulant to CNS, CVS and respiratory. Increases blood pressure and hence used in the treatment of heart failure.

25. Brucine S. nux-vomica(Loganiaceae)

Stomachic, tonic, stimulant to CNS, CVS and respiratory. Increases blood pressure and hence used in the treatment of heart failure.

26. Hygrines Erythroxylum coca(Erythroxylaceae)

Local anaesthetic and stimulant.

27. Atropine, Hyoscine, Hyoscyamine

Atropa belladonna, Datura stramonium and Hyoscyamus spp(Solanaceae)

Parasympatholytic agent, decreases saliva, sweat, gastric juice.

28. Cocaine Cocca spp (Erythroxylaceae) Local anaesthetic, CNS stimulant.

29. Pilocarpine Pilocarpus jaborandi (Rutaceae) Physiological antagonist of atropine, increases sweating, used in the treatment of glaucoma.




30. Caffeine Coffea arabica (Rubiaceae) CNS stimulant and weak diuretic.

31. Theophylline Thea sinensis(Theaceae)

CNS stimulant and weak diuretic.

32. Theobromine Thea sinensis(Theaceae)

CNS stimulant and weak diuretic.

33. Aconitine Aconitum napellus and other Aconitum spp (Ranunculaceae)

In the treatment of neuralgia, sciatica, rheumatism, infl ammation, analgesic and cardiac depression.

34. Conessine Holarrhena antidysenterica (Apocynaceae)

Antiprotozoal, used in the treatment of dysentery.

35. Ephedrine Ephedra gerardianaand other Aconitum spp(Ephedraceae)

Sympathomimetic and used in the treatment of asthma.Used in the treatment of allergic condition such as hay fever.

36. Adrenaline Adrenal glands Sympathomimetic

37. Taxol(paclitaxel)

Taxus brevifolia(Taxaceae)

Anticancer

38. Colchicine Colchicum autumnaleand other Aconitum spp(Liliaceae)

In the treatment of gout and rheumatism.Antitumour activity.

Abbreviations: CNS, central nervous system; CVS, cardiovascular system.

Some of the pharmaceutically important alkaloids are described with their structural elucidation herein.

Tropane Alkaloids

Atropine

12

3

45

6

7

8N(CH )3 O CC

CH OH2

H

2

1

O

8-Methyl-8-aza-bicyclo[3.2.1]octan-3-yl-3-hydroxy-2-phenylpropanoate

3

Atropine is a naturally occurring belladonna alkaloid that is extracted from the belladonna plant. It is the racemic mixture of L-hyoscyamine, and hence it can also be called (±) hyoscyamine. It is the tropine ester of racemic tropic acid and occurs mainly in the roots of deadly nightshade (Atropa belladonna), thorn apple ( Datura stramonium) of the Solanaceae family along with L-hyoscyamine (an optically active form).

Mode of action: It is a competitive antagonist for the muscarinic acetylcholine receptor. Atropine reduces the parasympathetic activity of all muscles and glands regulated by the parasympathetic nervous system through muscarinic (M) receptors. M

1 and M

3 receptors function through Gq



protein and activate membrane-bound phospholipase C, generating inositol triphosphate and diacylglycerol which releases calcium ions to produce depolarization in glands and smooth muscles. M

2 receptors open through Gi proteins to activate potassium channels resulting in hyperpolarization

(decreases the cardiac function). Atropine equally blocks the M1, M

2 and M

3 subtypes of receptors

and antagonizes the actions.

Properties and uses: It is a white, crystalline powder or colourless crystals. Freely soluble in alcohol and well soluble in water. The greater molar potency of atropine helps it to block several moles of acetylcholine. The umbrella-like atropine molecule may mechanically or electrostatically inactivate adjacent receptors on the cell surface so that these receptors are also unavailable for acetylcholine or other parasympathomimetic stimulants. It is well absorbed from the gastrointestinal (GI) tract and distributed throughout the body. It crosses the blood–brain barrier to enter the central nervous system (CNS), where large doses produce stimulant effects and toxic doses produce depressant effects. Atropine is also absorbed systemically when applied locally to mucous membranes. The drug is rapidly excreted in the urine. Atropine is used in ophthalmology for its mydriatic action on eye, and it is also used to relieve night sweats as it diminishes salivary and gastric secretions.

Structural elucidationThe structure of atropine is established as follows:

1. Molecular formula: The molecular formula of atropine is found to be C17

H23

NO3.

2. Atropine is an ester: Atropine on hydrolysis yields tropine and (±) tropic acid. Therefore, atropine is a tropine ester of tropic acid (tropine tropate).

C H NO17 23 3 � H O2

Ba(OH)2 C H NO8 15 � C H O9 10 3

Atropine Tropine Tropic acid

Structure of tropic acid (C9H

10O

3)

Results of the usual standard tests reveal that tropic acid is found to possess one 1° alcohol �(–OH) and –COOH group.Upon strong heating tropic acid is converted to atropic acid, which on further oxidation yields �benzoic acid. This indicates atropic acid and tropic acid have benzene ring with a side chain.

C H O9 10 3 �H O2

�C H O9 8 2

(O)

COOH

Therefore, atropic acid is having one –COOH, one double bond and one benzene nucleus.

C H C6 5 H CH.COOH�C H C COOH6 5 �

CH2

I II

Structure II is known to be cinnamic acid, hence I is atropic acid.

As atropic acid is obtained by dehydration of tropic acid, addition of water molecule to atropic �acid yields tropic acid—compound III or IV.



CH3

C H C COOH6 5 ��

OHIII

CH OH2


HIV

The structure (IV) is found to be correct, as it has one 1° alcohol [fi rst point under the section ‘Structure of Tropic Acid (C

9H

10O

3)’] and it is confi rmed by synthesis.

Synthesis of tropic acid from acetophenone

C H C O6 5� �

CH3

HCNC H C6 5�

CH3OH

CN

HClC H C6 5�

CH3OH

COOH

Atrolactic acidAcetophenone

Heat

�H O2


CH2

ether

HClC H CH COOH6 5� �

CH Cl2

Na COH O

2 3

2

C H CH COOH6 5� �

CH OH2

( )-Tropic acid�

Structure of tropine (C8H

15NO)

Results of the usual standard tests reveal that the ‘N’ atom is found to be present as 3° N. �

By � Herzig–Meyer method, it is found to possess one N—CH3 group.

Results of the usual standard tests reveal that it is found to possess one 2° alcohol (–OH) group �(benzoylation and oxidation reaction).

Ladenburg performed the following reactions on tropine. �

C H NO8 15

TropineHI

Tropine iodide

C H NI8 14

(H)Tropane

Distil

hydrochlorideCH Cl3 � nor-Tropane

C H N7 13

Zn dust

C H2 5

2-Ethylpyridine

NC H N8 14

On this basis, Ladenburg proposed that tropine is a reduced pyridine derivative.

CH CH OH2 2N

CH3

CHOH CH� 3N

or

CH3



Oxidation of tropine: Tropinic acid is a dicarboxylic acid obtained upon oxidation (possessing same number of carbon atom as in alcohol) and hence the alcoholic group in tropine must be present in ring structure and hence, Ladenburg structure is discarded.

Tropine

CrO3

Tropinone

CrO3

( -Tropinic acid�)

C H NO8 15 C H NO8 13 C H NO8 13 4

Furthermore, it found that tropinone yields dibenzylidene derivative which is a characteristic of –CH

2–CO–CH

2. This confi rms that the ketone and alcohol in tropine is linked to a ring

structure.

In HEM, tropinic acid yields pimelic acid. Formation of pimelic acid confi rms the presence of seven-membered carbon chain in tropinic acid and tropine.

C H NO8 13 4

Tropinic acid

HEMC H NO7 8 4

4HHOOC (CH ) COOH� 2 5�

Piperylenecarboxylic acid

Pimelic acid

Presence of fi ve-membered ring is confi rmed on the basis of the formation of N-methylsuc-cinimide from tropinic acid on oxidation.

H C2

H C2

CH

N CH� 3

CH

CH COOH2

COOH

Tropinic acid

CrO3

H SO2 4

H C2

H C2

CO

CO

N-Methylsuccinimide

N CH� 3

Thus, the structure of tropinone and tropine can be proposed as follows:

H C2

H C2

CH

CH

N CH� 3

CH2

CH2

CO

H C2

H C2

CH

CH

N CH� 3

CH2

CH2

CHOH

2

3

45

6

71

2

3

45

6

7

N CH� 3OH

Tropinone Tropine

1

All the foregoing reactions of tropine are explained with the above structure as mentioned below:

Formation of 2-ethyl pyridine from tropine �

Tropine

N Me� OHHI

N Me� N Me�I[HI]

Dihydrotropidine(tropane)

HCl

DistilMeCl � NH

Nordihydrotropidine 2-Ethylpyridine

Zn

CH CH2 3N



Formation of tropinone and � tropinic acid from tropine

Tropine Tropinone Tropinic acid Dibenzylidenetropionone

PhCHO

N Me� N Me� N Me� N Me�OH O

CO H2

CH CO H2 2

CHPh

O

CHPh

Formation of tropilidene (cycloheptatriene) from tropine �

N Me� N Me� �N Me� 2

Tropine

OHH SO2 4

�H O2

(i) MeI

(ii) AgOH OH– Vacuum

distil

(i) MeI(ii) AgOH

(iii)Vacuumdistil

TropilideneNMe2

Formation of pimelic acid from tropinic acid �

N Me�

CH CO H2 2

CO H2

(i) MeI

(ii) AgOH

Tropinic acid

� �N Me OH� 2

CH CO H2 2

CO H2

Heat

NMe2

CHCO H2

CO H2

(i) MeI(ii) AgOH

(iii)Heat

CHCO H2

CO H2

Na–Hg

Pimelic acid

CO H2

CO H2

The proposed structure of tropine is confi rmed by the synthesis.

Willstatter’s synthesis:

Suberone Cycloheptene

(i) HI

(ii) [HI]KOH

C H OH2 5

Br2

Br

BrMe NH2

NMe2

Exhaustivemethylation

Cycloheptadiene

Br

Br

Br2

Cycloheptatriene

Quinoline

150°CHBrMe NH2

NMe2

(i) Na/EtOH(ii) Br /HBr2

O I

Br


Technology

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