Pyrrole, furan, and thiophene molecules each contain five-membered rings, composed of four atoms of carbon and one atom of nitrogen, oxygen, or sulfur, respectively 1. Pyridine and pyrrole are both nitrogen heterocycles-their molecules contain nitrogen atoms along with carbon atoms in the rings. The molecules of many biological materials consist in part of pyridine and pyrrole rings, and such materials yield small amounts of pyridine and pyrrole upon strong heating. In fact, both of these substances were discovered in the s in an oily mixture formed by strong heating of bones.
Today, pyridine and pyrrole are prepared by synthetic reactions. Their chief commercial interest lies in their conversion to other substances, chiefly dyestuffs and drugs. Pyridine is used also as a solvent, a waterproofing agent, a rubber additive, an alcohol denaturant, and a dyeing adjunct 2. Furan is an oxygen-containing heterocycle employed primarily for conversion to other substances including pyrrole.
Furfural, a close chemical relative of furan, is obtained from oat hulls and corncobs and is used in the production of intermediates for nylon. It is a frequent contaminant of the benzene obtained from natural sources and was first discovered during the purification of benzene.
Like the other compounds, it is used primarily for conversion to other substances. Furan and thiophene were both discovered in the latter part of the 19th century 3. In general, the physical and chemical properties of heterocyclic compounds are best understood by comparing them with ordinary organic compounds that do not contain heteroatoms. Heterocyclic chemistry deals with heterocyclic compounds which constitute about sixty-five percent of organic chemistry literature 4.
Heterocyclic compunds are widely distributed in nature and exxential to life; they play a vital role in the metabolism of all living cells. Genetic material DNA ia also composed of heterocyclic bases-pyrimidines and purines. A large number of heterocyclic compunds, both synthetic and natural, are pharmacologically active and are in clinical use. They also find applications as sentizers, developers, antioxidants, as corrosion inhibitors, as copolymers, dyestuff 6.
Many heterocycles cab ve fitted into one of a few broad groups of structures that have overall similarities in their properties but siginificant variations within the group. Such variations can include differences in acidity or bascity, different polarity 7. The possible structural variations include the change of one heteroatom for another ring and different positioning of the same heteroatoms within the ring.
History of Heterocyclic Chemistry: The history of heterocyclic chemistry began in the s, in step with the development of organic chemistry. Some noteworthy developments. An important feature of the structure of many heterocyclic compunds is that it is possible to incorporate functional groups either as subsituents or as part of the ring itself.
For example, basic nitrogen atoms can be incorporated both as amino substituents and as part of a ring. This means that the structures are particularly versatile as a means of providing, or of mimicking a functional group. Tetrazole group is superior in terms of metabolic stability, bioavailibilty and four nitrogen atoms present in the tetrazole ring can create a greater charge distribution. Another example , ocatanol 9 can be used to mimic the amphiphilic nature of lipid, because it has a polar head group primary alcohal and a long hydrocarbon chain as tail, such as that oh fatty acids which make up part of a lipid membrane.
Heterocyclic compunds are also finding an increasing use as intermediate in organic synthesis .
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Very often this is because a relatively stable ring system can be carried through a number of synthetic steps and then cleaved at the required stage in a synthesis to reveal other functional groups. For e. More ever they are the logical intermediate to prepare 4- phospho-ranylidene -5 4 H -oxazolones, that are very important and useful ligands. Heterocyclic compunds are widely distributed in nature. Many are of fundamental importance to living systems: it is striking how often a heterocyclic compund is found as a key component in biological processes.
For example the nucleic acid bases, which are derivatives of the purine, namely adenine, guanine and pyrimidine, namely thymine, cystosine as being crucial to the mechanism of replication. Some purine and pyrimidines can act as antibiotics, by interference with DNA synthesis. Puromycin is an example of such an antibiotic. Chlorophyll and heme, which are derivatives pf porphyrin ring system are the components required for the photosynthesis and for oxygen transport in higher plants and in animal.
Essential diet ingredients such as thiamin vitamin B 1 , riboflavin vitamin B 2 , pyridoxol vitamin B 6 , nicotinamide vitamin B 3 and ascorbic acid vitamin C are heterocyclic compounds. Of the twenty amino acids commonly found in different proteins, three, namely histidine, proline and tryptophan, are heterocyclic. It is not surprising, therefore that a great deal of current research work is concerned with methods of synthesis and properties of heterocyclic compunds.
Many of the pharmaceuticals and most of the other heterocyclic compunds with practical applications are nor extracted from natural sources but are synthesized. The origins of organic chemistry do, however lie in the study of natural products. Three different group pf pharmaceutical with structures related to natural products are described briefly below as illustrations of the ways in which natural products chemistry interacts with synthetic heterocyclic chemistry.
Pharmaceuticals related to Histamine. Histamine, a monosubstituted imidazole, is derived in vivo from the amino acid histidine by decarboxylation by enzyme histidine decarboxylase. Hisatmine,  a biogenic amine involved in local immune responses and also acts as neurotransmitter It is involved in allergic reactions and is liberated from skin cells on injury and also participates in the regulation of gastric acid secretion.
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From the s onwards several synthetic drugs became available that can act as histamine antagonists. Two different types of histamine receptor were distinguished in the body; these were labeled as H 1 and H 2 receptors.
Cetrizine , chlorpheniramine  etc act as blockers of H 1 receptors. The search for a specific histamine H 2 receptor antagonist that could potentially control gastric acid secretion, thus provide the basis for treatment for peptic ulcers, started in Having little information to guide their intial choice, the chemists used the structure of histamine for the synthesis of new compunds.
Although it took a long period to discover active compounds, the work eventually resulted in was the introduction of a drug, cimetidine an imidazole derivative for the treatment of peptic ulcers One of the approaches that have been intensively explored is to investigate the use of analogues of the nucleosides. Analogues can be made in which the structure of the heterocyclic nitrogenous base is modified, or that of the sugar is modified, or boht.
Such a compuund might interfere in the replicative cycle of a virus, for example by incorporating in place of natural nucleoside.
The main problem is of selectivity, because majority of such compunds are also likely to prove toxic to normal cells. It is a useful drug for the treatment of herpes simplex 28 and herpes zoster viral infections due to high selectivity 29 and low cyptotoxicity. Ganciclovir  is a structurally related antiviral agent that has been used for the treatment of cytomegalovirus infections in AIDS patient and in recipients of organ transplants.
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Cladribine Leustatin 32  and Pentostatin 33  are drugs, used to treat hairy cel leukaemia leukemic reticuloendotheliosis and multiple sclerosis. Compounds related to Serotonin: Natural products often occur in small quantities and therefore difficult to investigate if the compunds have to be extracted from the natural source.
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Organic chemists can provide a solution to the problem by devising practicable laboratory synthesis. An example is provided by serotonin  commonly called 5-hydroxy tyrpamine, or 5-HT. This compund is widely distributed in nature but occurs only in low concentration. It is derived in nature from the amino acid tryptophan, by the short pathway including two enzymes, tryptophan hydroxylase and amino acid decarboxylase.
These include the contraction of smooth and blood platelet aggregation. It acts as a constrictor of arteries in the brain and is implicated in migraine The Chemistry of Heterocyclic Compounds, since its inception, has been recognized as a cornerstone of heterocyclic chemistry. Each volume attempts to discuss all aspects - properties, synthesis, reactions, physiological and industrial significance - of a specific ring system.
To keep the series up-to-date, supplementary volumes covering the recent literature on each individual ring system have been published. Many ring systems such as pyridines and oxazoles are treated in distinct books, each consisting of separate volumes or parts dealing with different individual topics.
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With all authors are recognized authorities, the Chemistry of Heterocyclic Chemistry is considered worldwide as the indispensable resource for organic, bioorganic, and medicinal chemists. She is using books varieties.
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