Plant Alkaloids Atallah F. Ahmed

Plant Alkaloids
Atallah F. Ahmed, PhD
Atallah F. Ahmed, PhD 2010
Lecture 9
6. Alkaloids of the Indole Groupp
Atallah F. Ahmed, PhD 2010
1
2
5/30/2010
1

6.1. Calabar bean alkaloids (pyrroloindole ring system)
Atallah F. Ahmed, PhD 2010
6.1. Calabar bean alkaloids
Calabar beans are the seeds of
Physostigma venenosum, Fabaceae) yields
several alkaloids of Indole type e.g. eserine
or physostigmine (major, 0.3%) and other
minor alkaloids ee.g. g eseramine
Atallah F. Ahmed, PhD 2010
3
4
5/30/2010
2

Physostigmine (Eserine = eseroline methyl carbamate)
Tasteless, levorotatory, m.p. 105°
With traces of KOH (oxidation catalyst)
easily oxidized by air (O 2) rubreserine (red).
Eserine +KOH+ Heat (Hydrolysis)
eseroline + CH 3NH 2 +CO 2.
Tests of identity
Warming with strong NH 4OH
yellowish red color (?) …… evaporation
blue residue (eserine blue) …….
dissolves in ethanol blue soln.
Heating with 1 d. fuming HNO3 in a
porcelain dish yellow color ….
evaporation green residue
(chloreserine) ……. dissolves in ethanol
green soln
Atallah F. Ahmed, PhD 2010
Action and Uses of Physostigmine
Mechanism: Reversible cholinesterase inhibitor
cholinergic activity.
This activity resides primarily in the carbamate portion.
Uses
Mi Miotic ti ( (main i use) ) to t combat b t mydriatic d i ti effect ff t of f atropine t i
(anticholinergic poisons as antidote).
Increases drainage of aqueous humor reduces IOP
Treatment of glaucoma (often in combination with
Pilocarpine).
Reverses the effects of d-tubocurarine and Atracurium.
Preservation of acetylcholine significant memory
enhancement in Alzheimer patients. Rivastigmine
(physostigmine analogs) is prefered due to its longer
duration of action and less toxicity.
Atallah F. Ahmed, PhD 2010
5
6
5/30/2010
3

Synthetic analogs e.g. neostigmine and pyridostigmine
enhancing neuromuscular transmission in myasthenia gravis (rare
autoimmune condition of muscle weakness caused by faulty
transmission of nerve impulses). They are used in diagnosing and
treatment.
Atallah F. Ahmed, PhD 2010
6.2. Alkaloids of Nux Vomica
Atallah F. Ahmed, PhD 2010
7
8
5/30/2010
4

6.2. Alkaloids of Nux Vomica
Strychnine and brucine occur in the endosperm of the seeds
of Strychnos nux-vomica (Loganiaceae), ~5% yield. Strychnine
comprises usually from one 30-50% of the total alkaloids.
Theyy are dihydroindole y (indoline) ( ) alkaloid biosynthesized
y
from tryptophan and a monoterpene.
The alkaloids occur combined naturally with malic acid and
igasuric acid.
N N
N O
O
Strychnine
H H3CO 3CO
H 3CO
N O
O
Brucine
Atallah F. Ahmed, PhD 2010
Strychnine and Brucine
Strychnine and brucine are mono-acidic bases,
although they contain two nitrogen atoms (why?).
Strychnine white powder, m.p. 288 288°C, C, intensely bitter
taste and has very stable structure.
It is very powerful poison.
Brucine is dimethoxy strychnine.
The alkaloid is easily soluble is EtOH, ether, and
acetone (much more than strychnine).
It is much less poisonous than strychnine.
It is used in nitrate limit test and as denaturant agent for
alcohols and oils.
Atallah F. Ahmed, PhD 2010
9
10
5/30/2010
5

Separation of a mixture strychnine and brucine
Acetone dissolves brucine leaving strychnine, which is
sparingly soluble in acetone.
Brucine is decomposed when treated with a mixture of
sulfuric acid and nitric acid, while strychnine is not affected.
PPotassium i fferrocyanide id precipitates i i strychnine h i quickly i kl
from its acid solution, while brucine is precipitated slowly.
Tests of identity
The sulphuric acid-dichromate test: trace of strychnine +
conc. H2SO4 ….. stirr with a crystal of potassium dichromate
reddish violet to purple yellow color. Strychnine salts
(except strychnine nitrate) give also this test.
Brucine + trace of nitric acid intense orange-red color.
The detection and determination of nitrate level in water by
brucine is an applicationAtallah of this F. Ahmed, test. PhD 2010
11
Pharmacological action and uses of strychnine
Rodenticide.
Strychnine is CNS stimulant and causes tonic convulsions. It acts
by blocking glycine receptors in the spinal cord. Fatal poisoning
(consumption of about 100 mg by an adult) tonic convulsion
contraction of f the diaphragm asphyxia death.
Due to its augmentation to spinal cord reflexes, it is used as an
aphrodisiac in male and emmenagogue in female. It reflexly and
directly increases the tone of plain muscle help in treatment of
chronic constipation, atony of the bladder, etc. (ampoule 1 mg).
In very small doses appetite stimulant and general tonic, with
iron salts if the patient is anemic.
Antidote in barbiturate poisioning.
Atallah F. Ahmed, PhD 2010
12
5/30/2010
6

6.3.Vinca Alkaloids (Bis-indole Alkaloids)
Atallah F. Ahmed, PhD 2010
6.3.Vinca Alkaloids
They are the alkaloids (1 % yield) of
Catharanthus roseus (Apocynaceae).
In Madagascar, it was used as antidiabetic
tea. Although plant extracts had no
hypoglycemic activity in rabbits, the
animals surrendered to bacterial infection
due to depleted white blood cell levels
(leukopenia). This selective action
suggested the anticancer (oncolytic)
activity for the plant.
Useful antitumour activity was confind in a number of
unsymmetrical ti ldi dimeric i iindole d l alkaloids lk l id (i (i.e. they th possess iindole d l
and indoline moieties in their dimeric molecule). These dimers
include vincristine and vinblastine in addition to a number of
monomeric alkaloids with indole group e.g. catharanthine or
indoline e.g. vindoline. Other effective semisynthetic antitumor
analogs are vinorelbine and vindesine.
Atallah F. Ahmed, PhD 2010
13
14
5/30/2010
7

Atallah F. Ahmed, PhD 2010
catharanthine
Pharmacological action and uses
vindoline 15
Mode of action: these indole dimeric alkaloids inhibit cell mitosis by
binding to the tubulin protein of the mitotic spindle (shared mechanism
with other natural agents, e.g. colchicine and podophyllotoxin).
Vinblastine (Velban): treatment of Hodgkin’s disease, a cancer affecting
the lymph glands, spleen, and liver.
Vincristine (Oncovin) is superior antitumor but more neurotoxic relative
to vinblastine. Especially useful in treatment of childhood leukemia,
giving a high rate of remission.
Both alkaloids are also effective in some cancers e.g. lymphomas, small
cell lung cancer, cervical and breast cancers, also.
Semi-synthetic derivative: Vindesine from vinblastine for treatment of
acute lymphoid leukaemia in children. children Vinorelbine from
anhydrovinblastine. It is orally active and has a broader anticancer
activity and lower neurotoxicity than vinblastine or vincristine.
Monomeric alkaloids are inactive.
Atallah F. Ahmed, PhD 2010
16
5/30/2010
8

Problems and Solutions for Vinca dimeric alkaloids
1 g of vinblastine costs around $US 20000
Total alkaloid ~1%, vincristine 0.0001% (i.e. each
1000 kg of plant 1 g) g).
Considerable effort is expended in semisynthesis of
‘dimeric’ alkaloids from ‘monomers’ such as
catharanthine and vindoline, which are produced in C.
roseus in much larger amounts. Efficient,
stereospecific p coupling p g has eventually y been achieved, ,
and it is now possible to convert catharanthine and
vindoline into vinblastine in about 40% yield.
Atallah F. Ahmed, PhD 2010
Atallah F. Ahmed, PhD 2010
17
18
5/30/2010
9

6.4. Ergot Alkaloids (lysergic acid-dervied Alkaloids)
Atallah F. Ahmed, PhD 2010
Ergot alkaloids
Consumption of flour contaminated with Ergot led to
many serious intoxications known as (Ergotism Ergotism- “Holy Holy
fire fire" " or " "Saint Saint Anthony's fire " ) in Europe.
E Ergot t can b be detected d t t d in i flour fl by b using i UV UV light li ht ( (366 ( (366 366 nm)
)
where contaminated flour will show violet spots spots.
318nm UV scan of ergot ergot-polluted polluted flour
Atallah F. Ahmed, PhD 2010
19
20
5/30/2010
10

Ergot alkaloids
Ergot is hard sclerotia formed by Claviceps sp. when infect plants of
family Graminae e.g. rye or wheat.
The poisonous properties of Ergot-containing grains are due to the
ergoline (indole-based) alkaloids.
Ergot yield up to 0.5% alkaloids (> 50 have been identified).
Medicinally useful Ergot alkaloids are ( (−)-lysergic )-lysergic acid derivatives
[exist as an amide with an amino alcohol as in ergometrine, or linked with
a small polypeptide as in ergotamine. The building block for lysergic acid
is tryptophan and an isoprene unit.
Atallah F. Ahmed, PhD 2010
Classification of Ergot alkaloids
Useful alkaloids of (−)-lysergic acid are two groups:
1. Water-soluble simple lysergic acid amide (~ 20% of total
alkaloids). Example: Ergometrine (known as ergonovine in
the USA and ergobasine in Switzerland) is an amide of (-)-
lysergic acid and 2-aminopropanol
2-aminopropanol.
Atallah F. Ahmed, PhD 2010
21
22
5/30/2010
11

2. Water-insoluble polypeptide alkaloids (~ 80% of total
alkaloids). They contain a cyclized tripeptide fragment
bonded to lysergic acid via an amide linkage. Based on the
nature of the three amino acids, these structures can be
subdivided into three groups:
Ergotamine gota e ggroup oup -Ergoxine go e group g oup -Ergotoxine goto e ggroup. oup
Hydrolysis Atallah F. Ahmed, products PhD 2010 of Ergotamine 23
The amino acids involved in the tripeptide part are alanine, valine,
leucine, isoleucine, phenylalanine, proline, and β-aminobutyric
acid, in various combinations. All contain proline and an αhydroxy-α-amino
acid.
Ergotamine group by hydrolysis (-)-lysergic acid + proline +
phenylalanine + alanine pyruvic acid + ammonia.
Ergotoxine group by hydrolysis ( (-)-lysergic ) lysergic acid + proline +
valine dimethylpyruvic acid + ammonia.
Atallah F. Ahmed, PhD 2010
24
5/30/2010
12

The isomeric series derived
from (+)-isolysergic acid (C-8
epimer of lysergic acid) as
ergometrinine, ergotaminine,
ergoxinine, and ergotoxinine
groups g p are less medicinally y
active.
Ergot alkaloids related to (-)-Iysergic acid have the suffix
–ine levorotatory more medicinally active.
Ergot alkaloids related to (+)-isolysergic acid have the
suffix –inine dextrotatory less medicinally active.
Conversion of -ine ine -inine inine isomer is done by refluxing
its methanolic solution or treatment with alc. alkali.
The conversion of -inine -ine isomer is done by
refluxing its AcOH solution or treatment with alc. H 3PO 4.
Atallah F. Ahmed, PhD 2010
Since (+)-inine isomers are less active, a decrease in potency may
occur during long storage of the active (-)-ine alkaloids.
The fresh active alkaloid show blue
fluorescence under UV light UV maxima
at 318 nm, while alkaloids suffered from
excessive light or UV exposure (in dil
acidic soln.) change into lumi-alkaloids
(addition of H 2O into 9,10-double bond)
UV maxima at 285 nm (no fluorescence)
inactive alkaloid.
H
C
8
O
R
25
N CH CH3 Ergot-ine Alkaloids Ergot-inine Alkaloids
(e.g. ergometrine) (e.g. ergometrinine)
Hydrolysis y y lysergic y g acid isolysergic y g acid
Optical activity Levorotatory (−) Dextrotatory (+)
Pharmaco. activity active inactive
+ hot alc. alkali Isomerized to ergot-inine −
+ hot H3PO4 − Isomerized to ergot-ine
Stability Unstable in light and
Atallah radiation F. Ahmed, PhD 2010
stable
26
HO
10
5/30/2010
13

Tests of Identity
Mostly depend on the presence of indole nucleus.
1. Van Urk test: Ergot alkaloids solution + Van Urk
reagent (p-dimethyl-aminobenzaldehyde + 5% FeCl 3 +
15% sulfuric acid) deep blue color.
The Van Urk reagent can be used as spray reagent for
PC or TLC of the ergot alkaloids (10% HCl is uesd in
place of sulfuric acid). The spectrophtometric assay
method for ergot alkaloids is based on this blue color.
2. Keller's test: Ergot alkaloids solution in acetic acid +
trace of FeCl3 + conc. sulfuric acid carefully added
intense blue colored band at the boundary of two layers layers.
3. Glyoxylic (formylformic) acid reagent + conc. sulfuric
acid + ergot alkaloids blue color.
4. The aqueous salts solution blue flourescence.
Atallah F. Ahmed, PhD 2010
Pharmacological action and uses
Mechanism: Ergot alkaloids act at α-adrenergic,
dopaminergic and serotonergic receptors. The
relationship of the alkaloid structure to those of
noradrenaline, dopamine, and and5-hydroxytryptamine 5 hydroxytryptamine (5- (5
HT, serotonin) is shown:
Atallah F. Ahmed, PhD 2010
27
28
5/30/2010
14

Obstetric use: Ergometrine should be injected during the final
stages of labor (induce uterine contractions during childbirth
= oxytocic effect) and immediately following childbirth to
reduce postpartum haemorrhage (vasoconstrictor effects). The
semisynthetic methyl-ergometrine (methergine) is now used.
Ergotamine is applied in treatment of acute attacks of migraine, migraine
where it reverses the dilatation of cranial blood vessels.
Ergotamine is effective orally, or by inhalation in aerosol form,
and may be combined with caffeine, which may enhance its
action. The semi-synthetic dihydroergotamine produced by
hydrogenation of the lysergic acid Δ9,10 double bond has
fewer side-effects.
The semisynthetic dihydro-ergotoxine mixture (1:1:1 mixture of
dihydroergocornine, dihydroergocristine, and
dihydroergocryptine) is employed medicinally for treatment of
hypertension. The reduction of the double bond in ergotoxine
(the natural mixture) reverse the vasoconstrictor effect
Atallah F. Ahmed, PhD 2010
29
cerebral vasodilator activity.
Methergine
Atallah F. Ahmed, PhD 2010
Erotamine
Hydergine
30
5/30/2010
15

A number of semi-synthetic lysergic acid derivatives act by
stimulation of dopamine receptors in the brain and are used in
Parkinson’s disease. Bromocriptine (2-bromo-α-ergocryptine) are
all used in this way.
Bromocriptine also inhibit release of prolactin by the pituitary,
and can thus suppress lactation and be used in the treatment of
breast tumours tumours.
Methysergide is a semi-synthetic analog of ergometrine, having
potent 5-HT antagonist and employed in prophylaxis of migraine.
Lysergic acid diethylamide or LSD, the widely abused
hallucinogen is the most active and specific psychotomimetic drug,
and is a mixed agonist–anatagonist at 5-HT receptors, interfering
with the normal processes. An effective oral dose is from 30 to 50
μg.
It intensifies perceptions and distorts them.. Experiences can vary
from beautiful visions to living nightmares, sometimes lasting for
days. Although the drug is not addictive, it can lead to
schizophrenia and there is danger of serious physical accidents
occurring whilst the user is under the influence of the drug. It has
Atallah F. Ahmed, PhD 2010
31
been used in psychoanalysis.
Atallah F. Ahmed, PhD 2010
LSD
32
5/30/2010
16