Bitter Principles

Lecture-1

By

Dr. Ahmed Metwaly

Objectives: I. Introduction

1. Definition

2. General characters

3. Classification

II. Terpenoid bitters

• Sesquiterpene Lactones

I. (−)-α-Santonin

II. Picrotoxin

III. Artemisinin

IV. Elephantopin

• Diterpenes

I. Forskolin

• Triterpenes

I. Quassin

II. Limonin

INTRODUCTION

Definition:

The bitter principles are heterogeneous compounds that doesn’t

belong to the class of alkaloids, but they have a characteristic

bitter taste.

General Characters:

•The term Bitters or Bitter principles is usually used to indicate

a group of natural products that have an intensely bitter taste

and were traditionally used in liquid medicaments to stimulate

appetite.

•Many of these products and drugs containing them are still

included in tonic formulations and are usually administered

before meals.

Bitter principles are mainly of vegetative origin, rarely of

animal origin and essentially comprise of C, H, and O, but

are rarely have or free from N.

•They are abundant in certain plant families especially

Compositae, Labiatae, Gentianaceae and Umbellifereae.

•Extracts of the following drugs have been used as bitter

stomachic: gentian, quassia, calumba, cinchona (or

quinine), nux vomica (or strychnine), hops, centaury,

condurago, quebracho and Taraxacum. Many of these drugs

are now mainly used for other pharmacological activities.

Classification

Two major classes of bitters could be distinguished: the

terpenoid or isoprenoid bitters and the non-terpenoid bitters.

I.Terpenoid bitters

This group includes isoprenoid bitters of different structures

such as:

1.Monoterpenoids (C10) e.g. iridoids (aucubin), secoiridoids

(gentopicrin); mostly in glycosidic forms.

2.Sesquiterpenoids (C15) containing a lactone ring and

subclassified to different groups.

3.Diterpenoids (C20) having labdane, kaurane and pimarane

structures e.g. marrubiin.

4.Triterpenoids (C30) e.g. cucurbitacins and quassinoids and

Monoterpenoid two isoprene units (C10H16)

Sesquiterpenoids three isoprene units (C15H24)

Diterpenoids four isoprene units (C20H32)

Sesterpenes five isoprene units (C25H40)

Triterpenoids six isoprene units (C30H48)

Tetraterpenoids eight isoprene units (C40H64)

II. Non terpenoid bitters

Compounds of this group are classified according to their

chemical structure into:

1.Phenolic bitters e.g. humulone and lupulone.

2.Chromone bitters e.g. khellin and visnagin.

3.Coumarin bitters e.g. xanthotoxin, imperatorin and bergapten.

4.Coumarone bitters e.g. rotenone.

5.Anhydride bitters e.g. cantharidin.

O O

O

-Pyrone-Pyrone

O O

O

ChromoneCoumarin

OO

Terpenoid Bitters

• Sesquiterpene Lactones

(−)-α-Santonin

They are C-15 lactone derivatives derived from farnesyl

pyrophosphate. The latter is a condensation product of three

isoprene units.

OH

O

CH3

COOH C

CH3

O

O

CH3

-SantoninSantonic acidO

Biological Sources: It is

obtained from the dried

unexpanded flower heads of

Artemisia cina (Wormseed);

(family: Compositae).

Artemisia cina

Chemical name:1, 2, 3, 4, 4α, 7-Hexahydro-1-hydroxy-α, 4α-8-

trimethyl-7-oxo-2-naphthaleneacetic acid γ-lactone;

(C15H18O3).

Isolation:

•The powdered flower heads are treated with milk of lime [Ca

(OH)2] where upon the insoluble calcium santonicate is formed

and collected by filtration.

•On treatment with sodium carbonate or hydroxide the soluble

sodium santoninate is formed.

•Ca (OH)2 released from the reaction is precipitated as carbonate

by passing CO2. The suspension is filtrate.

•The filtrate (Na santoninate) is treated with sulfuric acid to

precipitate the crude santonin that is purified by recrystallization.

1.Santonin + Ca (OH)2 Ca santoninate

2.Ca santoninate + NaOH Na santoninate

3.Na santoninate +H2SO4santonin.

Proposed Biosynthesis:

Characteristic Features The three different forms of santonin

have the following characteristic features:

(a) (–)-Form of Santonin:

1. It may be obtained either as tabular crystals or as sphenoidal

crystals having mp170-173°C.

2. It is found to be practically tasteless with a positive bitter after taste.

3. Its specific optical rotation [α]D 25 ranges between – 170° to 175° (

in ethanol).

4. It turns yellow on being exposed to light.

5. It causes irritation to the mucous membranes.

(b) (+)-Form of Santonin:

1. It is obtained as colourless plates from methanol having mp 172°C.

2. Its specific optical rotation [α]D 20 + 165.9° (in ethanol).

(c) (±)-Form of Santonin:

1. It is obtained as colourless plates from methanol having mp 181°C.

2. It has uvmax (ethanol): 241 nm (log ε 4.10).

Uses:

1. It is mostly used as an anthelmintic (Nematodes).

2. It is very efficient in its action on round worms (e.g.

Ascaris) in doses of 60 to 200 mg daily for 3 days; but shows

less effect on the thread worms and none on taenia.

3. Due to its toxicity it is now replaced by other anthelmintics.

Toxicity:

•Santonin affects vision causing "xanthopsia" (white objects

look green, blue or yellow).

•It may also cause headache, vertigo, nausea, vomiting, apathy,

sweating and diarrhea.

•Large doses may give rise to epileptic convulsions followed

by coma, hearing disorders and heamaturia. Death may occur

from respiratory failure.

•It is generally used in combination with kainic (Digenic) acid

to reduce its toxicity. A mixture of the two drugs was found

more effective in the treatment of ascariasis than if each was

used separately.

kainic (Digenic) acid

Chromosantonin (Photosantonin): Santonin is fairly stable

in air, however, it turns yellow on

•Exposure to light whereby it gets converted into its isomeric

form chromosantonin, also known as photosantonin. The latter

may be rated into santonin by simply crystallisation from

Ethanol

“Photosantonin is inactive” that is why we use unexpanded

santonica flower heads

Test for identification:

• Picrotoxin (Cocculin) O

O

O C

O

O

OH

H3C CH2

O

O

O C

O

O

OH

H3C CH3

OH

Picrotoxinin Picrotin

H

C15H16O6 C15H18O7

CH3

H

CH3

Biological Sources:

Obtained from the seed of

the fish berries Anamirta

cocculus L.

(Menispermaceae).

•Picrotoxin is isolated from

the endosperm of the seeds

as a bitter, crystalline, highly

toxic substance in an

amount attaining 1.5 %.

The name "picrotoxin" is a combination of the Greek words

"picros" (bitter) and "toxicon" (poison)

Chemical structure: •Picrotoxin consists of equimolecular proportions of two

components: picrotoxinin and picrotin.

•These are readily separated on boiling picrotoxin with 20 parts

of benzene or CHCl3. Picrotoxinin is soluble in benzene,

while picrotin is precipitated.

•Both picrotoxinin and picrotin are highly oxygenated

sesquiterpenoid derivatives.

•Picrotin is non-toxic while picrotoxinin is toxic.

Chemical name: (1R,3R,5S,8S,13R,14S)-1-hydroxy-14-(2-hydroxypropan-2-yl)-13-methyl-4,7,10-

trioxapentacyclo [6.4.1.1⁹,¹².0³,⁵.0⁵,¹³]tetradecane-6,11-dione; (1R,5S,8S,13R,14R)-1-hydroxy-13-methyl-

14-(prop-1-en-2-yl)-4,7,10-trioxapentacyclo[6.4.1.1⁹,¹².0³,⁵.0⁵,¹³]tetradecane-6,11-dione

Isolation

•The powdered fruits are defatted with petroleum ether,

extracted, by boiling with alcohol or H2O and filtered.

•The filtrate is treated with lead acetate solution, then filtered

and the excess of lead acetate is removed as PbS by passing H2S

gas followed by filtration.

•The filtrate is concentrated to syrupy consistency and left in a

refrigerator, where upon picrotoxin is crystallized.

•Crude picrotoxin is purified by treatment with active charcoal

and re-crystallized from boiling water or alcohol.

•powdered fruits are defatted with petroleum ether

•+Alcohol Extract

•+ Lead acetate

•+ H2S gas

•Crystallization

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]16 D – 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

Characteristic Features:

Uses

•Picrotoxin used by intravenous injection as antidote in

poisoning with barbiturates and other narcotics.

•It acts as CNS and respiratory stimulant (analeptic).

•Picrotoxin is highly toxic to fish, very small amounts of the

powdered fruits are sufficient to stupefy the animals.

Mechanism of action:

competitive -non acts as aIt has a strong physiological action. It

chloride channels. It is receptor AGABA for the channel blocker

therefore a channel rather than receptor antagonist

As GABA tself is an inhibitory neurotransmitter, infusion of

picrotoxin has stimulant and convulsant effects. As such,

picrotoxin can be used to counter barbiturate poisoning, that

can occur during general anesthesia or during a large intake

outside of the hospital.

Picrotoxin antagonizes the GABAA receptor channel directly,

which is a ligand-gated ion channel concerned chiefly with the

passing of chloride ions across the cell membrane. Therefore

picrotoxin prevents Cl- channel permeability and thus promtes

an inhibitory influence on the target neuron. Picrotoxin

reduces conductance through the channel by reducing not only

the opening frequency but also the mean open time. Picrotoxin

also antagonizes GABAC receptors (also called GABAA-rho

receptors) but the result of this action is not known. The

GABAC receptor is also linked to chloride channels, with

distinct physiological and pharmacological properties. In

contrast to the fast and transient responses elicited from

GABAA receptors, GABAC receptors mediate slow and

sustained responses.

Toxicity:

Oral, mouse: LD50 = 15 mg/kg. In large doses it is a powerful

poison, causing unconsciousness, delirium, convulsions,

gastro-enteritis and stimulation of the respiratory centre

followed by paralysis, from which death sometimes results.

Tests for identification

•Add few drops of H2SO4 to few crystals of picrotoxin, a

golden yellow color is developed that gradually changes to

reddish-brown.

•Sprinkle few crystals of picrotoxin onto a mixture of 4 drops of

H2SO4 containing about 0.2 gm KNO3 in an evaporating dish,

add NaOH solution drop wisely, the particles of picrotoxin

acquire a red color, which gradually fades.

•Moisten few crystals of picrotoxin with H2SO4, add one drop

of a solution of anisaldehyde in dehydrated alcohol (1:5). A

permanent blue color is developed.

• Picrotoxin gives a green color on boiling with vanillin

hydrochloride solution.

•Picrotoxin reduces Fehling’s and ammoniacal AgNO3

solutions.

Artemisinin

Structure;

A characteristic feature in the structure of artemisinin is the

presence of an endoperoxide moiety which is essential for the

antimalarial activity.

Chemical name; (3R,5aS,6R,8aS,9R,12S,12aR)-Octahydro-3,6,9-trimethyl-3,12-epoxy-

12H-pyrano[4,3-j]-1,2-benzodioxepin-10(3H)-one

Biological Source;

It is obtained from the

leaves and the closed,

unexpanded flower heads of

Artemisia annuna Linn.,

family Asteraceae.

Artemisia annuna

This particular herb has been used in the Chinese system of

medicine exclusively for the treatment of malaria since more

than one thousand years.

A characteristic feature in the structure of artemisinin is the

presence of an endoperoxide, artemisinin was isolated and

identified in 1972.

To date, Artemisinin and its simple derivatives have been

tested in China to treat more than 1.5 million, patients

suffering from malaria and particularly cerebral malaria; and it

has been shown to be valuable and effective against resistant

strains of Plasmodium

Biosynthesis in A. annua

Uses;

• Artemisinin is an excellent antimalarial, approximately

equal in potency to chloroquine. There are two reasons for

the great interest being shown in artemisinin and its

derivatives.

a- Active against chloroquine resistant strains of Plasmodium

falciparum.

b- The high lipid solubility ensures rapid penetration into CNS;

so it’s a first-line drug for the treatment of cerebral malaria

caused by P. falciparun, which is otherwise fatal.

Dosing;

Artemisinin and derivatives have half-lives on the

order of an hour. Therefore, they require at least

daily dosing over several days. For example, the

WHO-approved adult dose of co-artemether is

four tablets at 0, 8, 24, 36, 48, and 60 hours (six

doses)

Mechanis of Action;

The drug has a high affinity for hemozoin, a storage form of

haem which is retained by the parasite after digestion of

hemoglobin, leading to a highly selective accumulation of the

drug in the parasite. Artemisinin then decomposes in the

presence of iron, probably from hemozoin and releases free

radicals (hydrogen peroxide) which kill the parasite. The

peroxide bridge is therefore a crucial part of the drug molecule as

was suspected from structure activity studies

Modifications in Structure;

On account of the poor water solubility of artemisinin an

attempt was made to improve either its water solubility ir its

lipid solubility. In the former instance, Sodium artesunate i.e.,

the sodium salt of its hemisuccinate ester was developed; while

in the latter instance,

Artemether i.e., its corresponding methyl ether analogue was

produced. Evidently, sodium artesunate is employed for

intraveneous injections and artemether is used as a potent long

acting drug.

The WHO has recommended artemisinin combination

therapies (ACT) be the first-line therapy for P. falciparum

malaria worldwide. Combinations are effective because the

artemisinin component kills the majority of parasites at the

start of the treatment, while the more slowly eliminated

partner drug clears the remaining parasites.

Several fixed-dose ACTs are now available containing an

artemisinin component and a partner drug which has a long

half-life, such as mefloquine (ASMQ), lumefantrine

(Coartem), amodiaquine (ASAQ), piperaquine (Duo-

Cotecxin), and pyronaridine (Pyramax).

Elephantopin

Structure;

Elephantopin is a sesquiterpene lactone containing two lactone

rings and an epoxide function.

Chemical name;

(1aR,8S,8aR,11aS,11bR)-1a-Methyl-9-methylene-5,10-dioxo-

2,3,5,7,8,8a,9,10,11a,11b-decahydro-1aH-3,6-(metheno)furo[2,3-

f]oxireno[2,3-d][1]oxacycloundecin-8-yl methacrylate

Biological source;

Elephantopin is obtained from Elephantopus elatus,

Family Compositae

Elephantopus elatus

Uses:

Elephantopin has been shown to have an antitumour activity.

Forskolin (Coleonol)

Forskolin is a recently discovered labdane diterpene that

originates from the Ayruvidic system of medicine.

(3R,4aR,5S,6S,6aS,10S,10aR,10bS)- 6,10,10b- trihydroxy- 3,4a,7,7,10a-

pentamethyl- 1-oxo-3-vinyldodecahydro-1H-benzo[f]chromen-5-yl acetate

Biological source;

from the Indian Coleus plant (Coleus forskohlii)

Coleus forskohlii

Uses;

• Forskolin has been demonstrated to have hypotensive,

cardiotonic and platelet aggregation inhibitory activity;

because of its adenylate cyclase stimulant activity, it is

considered a promising drug for the treatment of glaucoma,

congestive cardiopathy and asthma.

• Forskolin is now being marketed in Japan (hypotensive and

spasmolytic).

• Forskolin activates the enzyme adenylyl cyclase and

increases intracellular levels of cAMP. cAMP is an

important second messenger necessary for the proper

biological response of cells to hormones and other

extracellular signals.

Quassin

Chemical name: (3aS,6aR,7aS,8S,11aS,11bS,11cS) -1,3a,4,5,6a,7,7a,8,11,

11a,11b,11c-dodecahydro-2,10-dimethoxy-3,8,11a,11c- tetramethyldibenzo[de,g]

chromene-1,5,11-trione

Structure ;

Quassin is an intensely bitter oxygenated triterpenoid

characterized by having a lactone structure. Quassin and related

compounds constitute the group of quassinoids or amaroids

Biological source:

Quassin is the main constituent

of quassia wood, which is the

stem wood of Picrasma excelsa

known in commerce as Jamaica

quassia or of Quassia amara

known in commerce as Surinam

Quassia family Simarubaceae .

Quassia amara

Isolation;

• The aqueous decoction of t wood is concentrated and

neutralized with Na2CO3.

• Tannic acid solution is added gradually, until no more

precipitate is formed.

• The precipitate is collected, triturated with lead carbonate

(to form lead tannate and liberate quassin) then dried on a

water bath.

• The produced mass is powdered and repeatedly extracted

with alcohol 80 %.

• The combined alcoholic extracts are concentrated and left

to allow crystallization of quassin.

Characteristic Features;

Quassin is obtained as rectangular plates from dilute methanol

having mp 222°C.

2. Its specific optical rotation [α]20 D + 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.

Test for Identification;

• Add concentrated H2SO4 and sucrose to few crystals of

quassin, a red color is produced.

• An alcoholic solution of quassin gives crimson color with

phloroglucin and HCl.

Uses;

• Quassia wood extract is used as a bitter tonic.

• The drug has anthelmintic properties, and is administered as

enema for expulsion of threadworms.

• It also used as insecticide.

Limonin

Structure;

Limonin is a modified triterpenid compound belonging to

the class of limonoids with or derived from a 4,4,8-trimethyl-17-

furanylsteroid skeleton.

Limonoids constitute a group of secondary metabolites

which are commonly found in the order Rutales mostly in family

Meliaceae and less frequently in the Rutaceae

Biological source;

• Limonin is isolated from the pericarp of Citrus limonis and

other Citrus species (Rutaceae).

• It occurs in the fruits and its juice as a non-bitter monolactone

precursor which undergoes further lactonisation with

formation of a second lactone ring to yield limonin

Citrus limonis

Uses;

• Limonoids act as insecticides, insect growth regulators and

insect antifeedants.

• They have antibacterial, antifungal and antiviral properties.

• They have possible anticarcinogenic activity.

Summary: I. Introduction

1. Definition

2. General characters

3. Classification

II. Terpenoid bitters

• Sesquiterpene Lactones

I. (−)-α-Santonin

II. Picrotoxin

III. Artemisinin

IV. Elephantopin

• Diterpenes

I. Forskolin

• Triterpenes

I. Quassin

II. Limonin