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Alkaloids

When we study organic chemistry which is a branch of chemistry mainly deals with organic compounds; we can see different organic compounds. Do you know about organic compounds? Organic compounds are chemical compounds mainly composed of carbon atoms. Although there are different elements such as H, O, N and S are bonded with carbon atoms, yet the majority of atoms will be carbon in such compounds. The simplest organic compounds are hydrocarbons which are mainly composed of Carbon and hydrogen. 

On the basis of structure, hydrocarbons can be classified as open chain compounds ad closed chain compounds. Open chain compounds have straight or branched chain of carbon atoms. Here carbon atoms can be bonded with either single or multiple bonds. The single bonded open chain hydrocarbons are called as alkanes whereas with multiple bonds, it can be alkene or alkyne. Alkenes are unsaturated hydrocarbons with double bond between carbon atoms whereas alkynes are unsaturated hydrocarbons with triple bonds between carbon atoms. 

Classification of Hydrocarbons

Closed chain compounds have cyclic structures which can be homocyclic or heterocyclic. Homocyclic hydrocarbons have same type of atom in the ring which is usually carbon atom. They can be aromatic and alicyclic hydrocarbons. Aromatic hydrocarbons are characterized by presence of delocalized pi-electron system in the ring such as benzene. Alicyclic compounds have ring structure but they do not have such delocalization of electrons that makes them less stable or more reactive than aromatic compounds. Another type of closed chain compounds are heterocyclic hydrocarbons. They have some heteroatom in ring structure with carbon atoms such as N, O and S etc. They may or may not aromatic in nature but presence of heteroatom change their physical and chemical properties. Some common heterocyclic compounds are given below. 

Heterocyclic Compounds

These compounds can be classified as aromatic and non-aromatic on the basis of presence of 4n + 2 pi electron system. For example; THF and 1, 4-dioxane are non-aromatic compounds. 

Five Membered Compounds

On the other hand, pyridine and pyrrole are aromatic compounds due to presence of delocalized pi electron system. 

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On the basis of ring size, they can be three, four, five and six membered rings. 

 Three Membered Compounds 

Heterocyclic Aromatic Compounds

 

Alkaloids Definition

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Alkaloids can be defined as the class of nitrogenous heterocyclic compounds which mainly originated from plants and exhibit physiological actions on humans. Some common examples are drugs like morphine, quinine and poisons such as atropine, strychnine. The name alkaloids means alkaline nature but it is not necessary that they exhibit alkaline behavior. 

They are mainly poison and medicines such as curarine is found in the extract curare. It acts as muscle relaxant. Similarly atropine is used to dilate the pupils and also involve in the treatment of muscular diseases. Narcotic alkaloids such as morphine, codeine are used in medicine for the relief of pain. Cocaine is a good local anesthetic. Some other examples of alkaloids are quinine, caffeine, nicotine, strychnine and serotonin.
  •  Aconitine - Alkaloid of aconite
  • Cinchonine and quinine- Derived from cinchona
  • Coniine – Derived from poison of hemlock
  • Reserpine- Extract of rauwolfia roots
  • Emetine- Alkaloid of ipecac.

Classification of Alkaloids

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Alkaloids are naturally occurring nitrogenous plant substances which have most diversified pharmacological properties. There are various basis of classification of alkaloids purposed by different biologist. 

In 1963, Hegnauer classified alkaloids into six groups on the basis of six precursor amino-acids; anthranilic acid, histidine, lysine, ornithine phenylalanine and tryptophan. Today we have different basis of classifications as given below.

1) Biosynthetic Classification- It is based on the precursor from which the alkaloids are produced in the plants. 
  • Indole alkaloids- Derived from tryptophan.
  • Piperidine alkaloids - Derived from lysine.
  • Pyrrolidine alkaloids - Derived from ornithine.
  • Phenylethylamine alkaloids - Derived from tyrosine.
  • Imidazole alkaloids - Derived from histidine.
2) Chemical Classification- This classification is based on the presence of basic heterocyclic nucleus.  It is the most widely accepted and common mode of classification.
  • Pyrrolidine alkaloids - Hygrine
  • Piperidine alkaloids - Lobeline
  • Pyrrolizidine alkaloids - Senecionine
  • Tropane alkaloids - Atropine
  • Quinoline alkaloids - Quinine
  • Isoquinoline alkaloids - Morphine
  • Aporphine alkaloids - Boldine
  • Indole alkaloids - Ergometrine
  • Imidazole alkaloids - Pilocarpine
  • Diazocin alkaloids - Lupanine
  • Purine alkaloids - Caffeine
  • Steroidal alkaloids - Solanidine
  • Amino alkaloids - Ephedrine
  • Diterpene alkaloids - Aconitine
3) Pharmacological Classification- On the basis of pharmacological characteristics alkaloids can classified like analgesics, cardio-vascular drugs, CNS-stimulants, mydriatics, anticholinergics, sympathomimetics, antimalarials, purgatives etc.
  • Morphine- A Narcotic analgesic
  • Quinine - A  Antimalarial
  • Strychnine - A  Reflex excitability
  • Lobeline - A  Respiratory stimulant
  • Boldine - A  Choleretics and laxatives
  • Aconitine - A  Neuralgia
  • Pilocarpine - A  Antiglaucoma agent and miotic
  • Ergonovine - A  Oxytocic
  • Ephedrine - A  Bronchodilator
  • Narceine - A  Analgesic and antitussive
4) Taxonomic Classification- This classification is based on the taxonomic categories of plant kingdom. Some of the classes are listed below.
  • Cannabinaceous Alkaloids
  • Rubiaceous Alkaloids
  • Solanaceous Alkaloids
5) Structural classification: On the basis of structure, alkaloids can be classified as non-heterocyclic nucleus and heterocyclic nucleus.
  • Non-heterocyclic Alkaloids:  Ephedrine, Hordenine, Capsaicin, Narceine are some examples of non-heterocyclic alkaloids.
  • Heterocyclic Alkaloids: These alkaloids have heterocyclic nucleus such as Hygrine contains pyrrolidine ring whereas there is pyridine ring in Ricinine alkaloid.
6) On the basis of chemical reactions: On the basis of different chemical reactions alkaloids can be categorized. For example on the basis of amination reactions, alkaloids can be classified as; 
  • Acetate-derived Alkaloids
  • Phenylalanine-derived Alkaloids
  • Terpenoid Alkaloids
  • Steroidal Alkaloids
On the basis of derivation from Anthranilic Acid; 
  • Quinazoline Alkaloids
  • Quinoline Alkaloids
  • Acridine Alkaloids
On the basis of derivation from Histidine
  • Imidazole Alkaloids
On the basis of derivation from Lysine
  • Piperidine Alkaloids
  • Quinolizidine Alkaloids
  • Indolizidine Alkaloids
On the basis of derivation from Nicotinic Acid
  • Pyridine Alkaloids
On the basis of derivation from Ornithine
  • Pyrrolidine Alkaloids
  • Tropane Alkaloids
  • Pyrrolizidine Alkaloids
On the basis of derivation from Tyrosine
  • Phenylethylamine Alkaloids
  • Tetrahydro iso-quinoline Alkaloids
  • Benzyl Tetrahydro iso-quinoline Alkaloids
On the basis of derivation from Tryptophan
  • Indole Alkaloids
  • β-Carboline Alkaloids
  • Terpenoid Indole Alkaloids
  • Quinoline Alkaloids
  • Pyrroloindole Alkaloids
  • Ergot Alkaloids

Alkaloids Examples

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A large number of alkaloids which play an important role in pharmaceutical industries. They have a long-standing place in medicine. Let’s discuss some examples of alkaloids. In 1805; the pain killers derived from the poppy plant, named as morphine. The anti-malarial drug quinine is also an alkaloid which was derived by tribes in the Amazon. 

Morphine acts as powerful painkiller which mainly used in surgery and severe injury. It attaches to injured sites on our nerve cells. It prevents the transfer of messages about pain from injured sites to the body, causes euphoric state with no pain. The major limitation of this alkaloid is extremely addicting.

One of the most common derivatives of morphine is oxycodone. It is used as a pain medication and also extremely addictive like Morphine. Later Heroin was designed as a drug to replace morphine in medial areas but now it is only used illegally as users can quickly addictive to this drug.

Quinine is an anti-Malaria drug that has some side effects such as low blood sugar, nausea, nervousness or confusion. Atropine is extracted from Atropa belladonna. The purple flowers and berries of the belladona are beautiful but are lethal due to presence of this alkaloid.

Overall we can say that limited and legal use of alkaloids is useful for our society but can be hazardous when use illegally such as Atropine prevents messages from the brain to muscle so it is an anti-cholinergic medication as acetylcholine is used for the communication of messages from brain to muscle. So Atropine can be used to treat digestive symptoms and can also be used as an antidote for poisoning by mushrooms.

Alkaloids Extraction

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Today we have a large number of alkaloids extracted from various sources. There is no one way to extract all alkaloid from their sources. The extraction depends on chemical properties of alkaloids and also on the source, it originated. Most of the alkaloids are extracted on the basis of their solubility in organic solvents and insolubility in water. The mixture extracted from the plant contains different alkaloids in the form of salts of organic acids in different proportions. This mixture is treated with alkali to form alkaline solution which can be further extracted with organic solvents.

The impurities can be removed with the use of weak acids which converts alkaloid bases into salts. These salts are soluble in water and can be separated easily.

Types of Alkaloids in Plants

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Alkaloids mainly extracted from plants. Some common alkaloids with their sources and properties are listed below.

Alkaloid   Source  Pharmacological property  
Ajmalicine  Catharanthus roseus  hypotensive 
Atropine  Atropa belladonna  anticholinergic 
Berberine  Berberis, coptis japonica  antipasmodic, antiprotozoal
Codeine Papaver somniferum sedative,analgesic
Colchicine  Colchicum autumnale  Antimitotic 
Caffeine  Coffee arabica, camellia sinensis  Stimulant
Camptotheeine  Camptotheca acuminata Antitumor 
Emetine  Cephaelis ipecacuanha  Emetic 
Ellipticine  Ochrosia elliptica  antitumor 
Ephedrine Ephedra gerardians spasmolytic 
Morphine Papaver somniferum analgesic, sedative 
Papaverine Papaver sominiferum spasmolytic 
Quinine, quinidine Cinchona ledgeriana antimalaria 
Reserpine Rauwolfia serpentina hypotensive 
Vinblastine, vincristine Catharanthus roseus anticancer 
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