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Phenols and Tannins
Pharmacognosy I
Mosul University/ College of Pharmacy
L.A. Dilbreen Barzanji
Phenols and phenolic glycosides
Phenols probably constitute the largest group of plant
secondary metabolites.
Widespread in nature, and to be found in most classes of
natural compounds having aromatic moieties, they
range from simple structures with one aromatic ring to
highly complex polymeric substances such as tannins
and lignins.
Phenols are important constituents of some medicinal
plants and in the food industry they are utilized as
colouring agents, flavouring, aromatizers and
antioxidants.
Phenolic classes of pharmaceutical interest,
namely:
1) simple Phenolic compounds
2) Tannins
3) Coumarins and their glycosides
4) Anthraquinones and their glycosides
5) Naphthoquinones
6) Flavones and related flavonoids glycosides
7) Anthocyanidins and anthocyanins
8) Lignans and lignin
Simple phenolic compounds
• The biosynthetic origin of some of these
compounds involving the shikimic acid pathway.
• Catechol (o-dihydroxybenzene) occurs free in kola
seeds and in the leaves of Gaultheria spp.
• Derivatives of resorcinol (m-dihydroxybenzene)
constitute the narcotic principles of cannabis and
the glucosides arbutin involves quinol
(hydroquinone, p-dihydroxybenzene).
• The Phenolic compounds in this group often also
posses alcoholic, aldehydic and carboxylic acid
groups; they include eugenol ( a phenolic
phenylpropane), vanillin (a phenylic aldehyde)
and various phenolic acids, such as salicylic,
ferulic and caffeic acids.
• Glycosides formation is common, and the widely
distributed glycoside coniferin and other
derivatives of phenolic cinnamic alcohols are
precursors of lignin.
Willow bark
• Various species of Salix wich include S. purpurea
L. (purple willow)
• The commercial drug occurs as thin, channeled
pieces of varying length, about 1.5 cm wide and
1.5 mm thick. It is easily fractured longitudinally
and, transversely, shows an inner inconspicuous
fiber fracture.
• Willow bark is a source of salicin; a phenolic
glycoside now seldom used but generally
regarded as the natural forerunner of aspirin.
• The composition of the glycoside mixture is variable in
the bark depending on the species, age of bark and
time of collection. The latter is usually made in spring
when the bark is easily removed from the branches.
• Other phenolic glycosides are salicortin (an ester of
salicin), acetylated salicin (fragilin).
• The BP requires the dried drug to contain a minimum
of 1.5% total salicylic acid derivatives, calculated as
salicin.
• Willow is employed as an anti-inflammatory in the
treatment of rheumatism, arthritis and muscular pains.
Tar (Pix Liquida)
• Wood tar is known in commerce as Stockholm
tar. It is prepared by destructive distillation of
various trees of the family Pinaceae.
• Wood tar is a blackish semiliquid with a
characteristic odour and taste.
• Pine tar is characterized by the large amount of
guaiacol and its homologues which are present.
• Tar is mainly used externally, in the form of
ointment or tar parogen, as a stimulating
antiseptic in certain skin diseases.
Coal tar
• Coal tar is prepared by destructive distillation
of bituminous coal.
• It is a nearly black viscous liquid and when
shaken with water gives an aqueous alkaline
solution.
• Both coal tar and wood tar are used in the
treatment of psoriasis.
Vanilla and vanillin
• Vanilla (Vanilla Pods) consists of the carefully
cured fully grown but unripe fruits of Vanilla
fragrans (Salis).
• The fruits are collected when the upper part of
the pod changes in colour from green to yellow.
The characteristic colour and odour of the
commercial drug are only developed as a result of
enzyme action during the curing.
• Vanilla pods are 15-25 cm long, 8-10 mm
diameter and somewhat flattened.
• Green vanilla contains glycosides, namely
glucovanillin (vanilloside) and glucovanillic
alcohol.
• Glucovanillic alcohol yields on hydrolysis
glucose and vanillic alcohol; the latter
compound is then by oxidation converted into
vanillic aldehyde (vanillin).
• Glucovanillin, as its name implies, yields on
hydrolysis glucose and vanillin.
Vanillin BP
• It is the aldehyde corresponding to methylprotocatechuic acid and has been synthesized in a
number of ways.
1) Large quantities of it is prepared from eugenol
isolated from oil of gloves or from guaiacol (methyl
catechol).
2) It can also be produced by microbial oxidation of
eugenol.
3) In the plant glucovanillin is biosynthesized via ferulic
acid.
• Uses: vanilla pods are widely used in confectionary and
in perfumery. They have been replaced to some extent
by synthetic vanillin.
Bearberry leaves
(Uva Ursi)
• The dried leaves of Arctostaphylus uva-ursi; a small
evergreen shrub found in central and northern Europe and
in North America. The leaves are dark green to brownish
green.
• The drug is odourless but has astringent and somewhat
bitter taste.
• Bearberry contains the glycosides arbutin and methyl
arbutin, about 6-7% of tannin, catechol, ursone and
flavones derivative quercetin.
• The pharmacopoeial drug is required to contain at least
7.0% of hydroquinone derivatives calculated as arbutin.
• Uses: Bearberry is diuretic and astringent and during
excretion it exerts an antiseptic action on urinary tract.
Capsicum
• Consists of the dried ripe fruits of Capsicum annuum, also
refered as ‘African chillies’ which are oblong-conical in
shape, 12-25 mm long and up to 7 mm wide.
Constituents:
• Capsaicin; an intensely pungent compound.
• The pungent phenolic fraction of capsicum also contains a
proportion of 6,7-dihydrocapsaicin.
• The capsaicin contents of fruits varies appreciably in a
range of up to 1.5% and is much influenced by
environmental conditions and age of the fruit. It occurs
principally in the dissepiments of the fruits- for example,
entire fruit 0.49, pericarp 0.10, dissepiment 1.79, seed 0.07.
• The pungency of capsicum is not destroyed by treatment
with alkalis (distinction from gingerol, which also contains
the vinyllyl group) but is destroyed by oxidation with
potassium dichromate or permanganate.
• Chillies also contain ascorbic acid (0.1-0.5%), thiamine and
red carotenoids.
• Uses: the drug is given internally for atonic dyspepsia and
flatulence.
• It is used externally as counter-irritant, in the form of
ointment, plaster, medicated wool, etc.
• For the relief of rheumatism, lumbago, etc.
• Capsaicin cream are available for the relief of pain in
osteoarthritis, post-herpetic neuralgia and painful diabetic
neuropathy.
Tannins
• The term tannin was first applied by Seguin in 1796 to
denote substances present in plant extracts which were
able to combine with protein of animal hides, prevent their
putrefaction and convert them into a leather.
• Tannins comprise a group of compounds with a wide
diversity in structure that share their ability to bind and
precipitate proteins. Tannins are polyphenols and include
oligomerics and polymeric constituents and are widely
distributed in plants.
• Most true tannins have molecular weight from 1000-5000.
• Many tannins are glycosides.
• Two main groups of tannins are usually recognized; these
are the hydrolysable tannins and the condensed tannins
(proanthocyanidins).
Hydrolysable tannins
• These may be hydrolysed by acids or enzymes
such as tannase.
• They are formed from several molecules of
phenolic acids such as gallic and hexa-hydroxydiphenic acids which are united by ester linkages
to a central glucose molecule.
• A simple tannin illustrating this point is one
derived from a species of sumac (Rhus).
• They were formerly known as pyrogallol tannins,
because on dry distillation gallic acid and similar
components are converted into pyrogallol.
• Two principal types of hydrolysable tannins are
gallitannins and ellagitannins.
• Ellagic acid (the depside of gallic acid) can arise
by lactonization of hexa-hydroxy-diphenic acid
during chemical hydrolysis of the tannin; thus,
the term ellagitannin is a misnomer.
Condensed tannins
(proanthocyanidins)
• Unlike hydrolysable tannins, these are not readily
hydrolysed to simpler molecules and they do not
contain a sugar moiety.
• They are related to the flavonoid pigments and
have polymeric flavan-3-ol structures.
• On treatment with acids or enzyme condensed
tannins are converted into red insoluble
compounds known as phlobaphenes.
• On dry distillation they yield catechol and these
tannins are therefore sometimes called catechol
tannins.
‘Complex tannins’
• This term has been applied to a newlydiscovered group of tannins which are
biosynthesized from both a condensed tannin
and a hydrolysable tannin (mostly a Cglycoside ellagitannin).
Pseudotannins
As already mentioned, pseudotannins are
compounds of lower molecular weight than
true tannins, e.g.:
• Gallic acid
• Catechins
• Chlorogenic acid
• Ipecachuanic acid
Occurrence of tannins:
• Tannins are of wide occurrence in plants and are usually
found in greatest quantity in dead or dying cells.
• They exert an inhibitory effect on many enzymes due to
protein precipitation and, hence, they may contribute a
protective functions in barks and heartwoods.
• Commercial tannins are used in the leather industry.
• Pharmaceutical tannin is prepared from oak galls and yields
glucose and gallic acid on hydrolysis; many commercial
samples contain some free gallic acid.
• Some plants (clove, cinnamon , etc.) contain tannin in
addition to the principal therapeutic constituents. This may
complicate extraction or produce incompatibilities with
other drugs (many alkaloids for example, are precipitated
by tannins).
Medicinal and biological properties:
• Tannin-containing drug will precipitate protein
and have been used traditionally as styptics
and internally for the protection of inflamed
surfaces of mouth and throat. They act as
antidiarrhoeals and have been employed as
antidotes in poisoning by heavy metals,
alkaloids and glycosides.
Oak bark
• Oak bark is the cut and dried bark from the
fresh young branches of Quercus robur.
• Principal constituents are phlobatannins,
elligatannins and gallic acid.
• A minimum requirement of 3.0% calculated as
pyrogallol being specified by the BP/EP.
• Oak bark is used medicinally for its astringent
properties .
Galls and tannic acid
• Turkish galls are vegetable growth s formed on
the young twigs of the dyer’s oak, Quercus
infectoria, as a result of deposition of the eggs of
gall-wasp.
• Abnormal development of vegetable tissue round
the larva is due to an enzyme-containing
secretion, produced by the young insect after it
has emerged from the eggs.
• Galls are collected and dried then they are graded
according to colour into three grades: blue green
and white.
Characters: Aleppo galls are globular in shape and from
10-25 mm in diameter. They have a short basal stalk
and numerous rounded projections on the surface.
Galls are hard heavy , usually sinking in water.
The so-called ‘blue’ variety are actually of a grey or
brownish-grey colour. These, and to a lesser extent the
olive-green ‘green’ galls are preferred to the ‘white’
variety, in which the tannin is said to have been partly
decomposed.
White galls also differ from the other grades in having a
circular tunnel through which the insect has emerged.
Galls without the opening have insect remains in the
central cavity.
Galls have a very astringent taste.
Constituents: galls contain 50-70% of the tannin
known as gallatonic acid (Tannic acid); this is a
complex mixture of phenolic acid glycosides
varying greatly in composition.
Galls also contain gallic acid (about 2-4%), ellagic
acid,etc.
Tannic acid is hydrolysable tannin yielding gallic acid
and glucose. Solutions of tannic acid tend to
decompose on keeping with formation of gallic
acid, a substance which is also found in many
commercial samples of tannic acid.
Uses:
• Tannic acid is used as an astringent and styptic.
Hamamelis leaf
• Hamamelis leaf (Witch Hazel leaves) consists of the
dried leaves of Hamamelis virginiana , a shrub or small
tree 2-5 m high, which is widely distributed in Canada
and the USA.
• Hamamelis contain gallitannins, ellagitannins, free
gallic acid, proanthocyanidins, bitter principles and
traces of volatile oil.
• Uses: hamamelis owes its astringent and hemostatic
properties to the tannins.
• Hamamelis Water or distilled Witch hazel, is widely
used as an application to sprains, bruises and
superficial wounds and as an ingredient of eye lotions.
Pomegranate rind
• Pomegranate rind consists of the dried pericarp of the
fruit of Punica granatum.
• It occurs in thin, curved pieces about 1.5 mm thick,
some of which bear the remains of the woody calyx or
a scar left by the stalk.
• The outer surface is brownish-yellow or reddish. The
inner surface bears impression left by the seeds.
• Pomegranate rind used in India as a herbal remedy for
non-specific diarrhea, it is very astringent and contains
about 28% of tannin (ellagitannins) and colouring
matters.
• It should be distinguished from the root bark , which
contains alkaloids.
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