1. HETEROCYCLIC CYCLIC CHEMISTRY - NOMENCLATURE & RING SYNTHESIS

2. NOMENCLATURE OF HETEROCYCLIC COMPOUNDS
DEFINITION:
A cyclic organic compound containing all carbon atoms in ring formation is referred to as a carbocyclic compound.
If atleast one atom other than carbon forms a part of the ring system then it is designated as a heterocyclic compound.
Nitrogen , oxygen ,and sulfur are the most common heteroatoms .
An enormous number of heterocyclic compounds are known and this number is increasing rapidly.

3. Heterocyclic compounds may classified into ALIPHATIC and AROMATIC.
The Aliphatic heterocyclic's are the cyclic analogues of amines , ethers , thioethers , amides etc. their properties are particularly influenced by the presence of strain in the ring. These compounds generally consist of small(3-and 4-membered) and common(5 to 7 membered) ring systems.
The Aromatic heterocyclic compounds in contrast are those which have a hetero atom in the ring and behave in similar manner to benzene in some of their properties.
These compounds also comply with the general rule proposed by HUCKEL .This states that aromaticity is obtained in cyclic conjugated and planar systems containing (4n+2) Π electrons.

4. .
The chemistry of heterocyclic compounds is as logical as that of aliphatic or aromatic compounds. their study is of great interest both from the theoretical as well as practical stand point.
Heterocyclic compounds occur widely in nature and in a variety of non- naturally occurring compounds.
A large number of heterocyclic compounds are essential to life.
Various compounds such as alkaloids, antibiotics, essential amino acids, the vitamins, hemoglobin, the hormones and a large number of synthetic drugs and dyes contain heterocyclic ring systems.
A knowledge of heterocyclic chemistry is useful in biosynthesis and in drug metabolism.
Nucleic acids are important in biological processes of heredity and evolution.

5. NOMENCLATURE OF HETEROCYCLIC COMPOUNDS
PREFIX FOR HETERO ATOMS
HETERO ATOM
VALENCE
PREFIX O 2
Oxa N 3
Aza S
Thia Se
Selena Te
Tellura P
Phospha As
Arsa Si 4
Sila Ge
Germa

6. COMMON NAME ENDINGS FOR HETEROCYCLIC COMPOUNDS
RING SIZE
SUFFIXES FOR
UNSATURATED
FULLY
COMPOUNDS
SUFFIXES FOR
SATURATED
With N
Without N 3
-irine
-irene
-iridine
-irane 4
-ete
-etidine
-etane 5
-ole
-olidine
-olane 6
-ine
-in
-ane 7
-epine
-epin
-epane 8
-ocine
-ocin
-ocane

7. CLASSIFICATION OF HETEROCYCLIC COMPOUNDS
1. Three membered heterocyclic compounds with one hetero atom.
Aziridines Azirines
Oxiranes Oxirenes
Thiiranes Thiirenes
Three membered heterocyclic compounds with two hetero atoms.
Diaziridines
Diazirines
Oxaziridines
3. Four membered heterocyclic compounds with one hetero atom.
Azetines
Oxetanes
Thietanes

8. 5. Five membered heterocyclic compounds with two hetero atoms
Five membered heterocyclic compounds with one hetero atom
Pyrroles
Furans
Thiophenes
Five membered heterocyclic compounds with two hetero atoms
Pyrazole and Benzopyrazoles
Imidazoles and Benzimidazoles
Isoxazoles and Benzoisoxazoles
Oxazoles and Benzoxazoles
Isothiazoles and Benzoisothiazoles
Thiazoles and Benzoisothiazoles
SIX membered heterocyclic compounds with one hetero atom
Pyridines
pyrilium salts
α and ϒ -pyrones
Six membered heterocyclic compounds with two hetero atoms
Pyridazines Quinazolines
Cinnolines and Phthalazines Pyrazines
Pyrimidines Quinoxalines Q

9. Seven membered heterocyclic compounds .
Azepines
Oxepines
Thiepines
9. Bicyclic ring systems derived from Pyrrole , Furan , and Thiophene.
Indoles
Isoindoles
Indolizines
Dibenzopyrroles
Benzofuran , Isobenzofuran , and Dibenzofuran
Benzothiophene , Isobenzothiophene and Dibenzothiophene
Bicyclic ring systems derived from pyridine.
Quinolines Acridines
Isoquinolines Phenanthrides
Quinolizium salts

10. STRUCTURES OF VARIOUS HETEROCYCLIC COMPOUNDSII

15. RING SYNTHESIS FOR VARIOUS HETEROCYCLIC COMPOUNDS
1. AZIRIDINE – 3 MEMBERED COMPOUND
Gabriel ring closure : Aziridine was first obtained in 1888 by heating ᵦ -bromoethylamine in the presence of potassium hydroxide.

16. OXIRANE – 3 MEMBERED COMPOUND
From the oxidation of Alkenes: probably the simplest method of preparing the parent compound Oxirane is the direct oxidation of ethylene by air over a silver catalyst at elevated temperatures .

17. THIIRANE – 3 MEMBERED COMPOUND
From 2- mercaptoetanol : probably the most useful method of preparing Thiirane is from 2-mercaptoethanol by treating it with phosgene in ethyl acetate and pyridine to give monothioethylene carbonate which on decarboxylation leads to Thiirane.

18. AZETIDINE – 4 MEMBERED COMPOUND
Cyclization method : Azetidine and its derivatives have been prepared by a number of methods which include intramolecular cyclization and cycloaddition .
In this method ϒ – bromopropylamines in the presence of base produces Azetidine compound

19. THIETANES- 4 MEMBERED COMPOUND
From 1,3- dihaloalkanes: The earliest and common method for the preparation of Thietane involved in the reaction of 1,3 –dihaloalkanes with sodium or potassium sulphide .this reaction is generally carried out in alcohol / water mixture at 70°.

20. OXETANE -4 MEMBERED COMPOUND
Cyclization reactions: It is a direct route for the preparation of oxetane. in this reaction halohydrin in the presence of base forms an 2,2-diethyloxetane.

21. PYRROLE- 5 MEMBERED COMPOUND
From furan : Commercially pyrrole is obtained by fractional distillation of coal tar or bone oil. Alternatively it can be obtained from furan by passing it over ammonia and steam and heated (400°) in the presence of aluminium oxide catalyst. A primary amine may be employed to prepare 1- substituted pyrrole.

22. FURAN – 5 MEMBERED COMPOUND
From furfural : The most important source of furan is furfural. The latter is available by acid hydrolysis of polysaccharides present in oat, husks and corn cobs. these are degraded to pentose's which are subsequently converted to furfural on further treatment with acid.
Furfural can also be converted directly to furan by its gas phase .decarbonylation in the presence of palladium and charcoal.

23. THIOPHENE – 5 MEMBERED COMPOUND
From sodium succinate : The classical method for preparing Thiophene in the laboratory consists of heating a mixture of sodium succinate in the presence of phosphorous trisulphide.

24. PYRIDINE – 6 MEMBERED COMPOUND
6- benzyl -3,5-dichloro-2H,1,4-oxazin-2- one on refluxing with phenyl acetylene in Toluene yields a highly substituted pyridine.

25. AZEPINE – 7 MEMBERED COMPOUND
From nitrobenzene : Nitrobenzene by its deoxygenation with tributylphosphine initially forms an arylnitrene .this is accomplished with an primary alcohol leads to the formation of 2-alkoxy -3H – azepines.

26. OXEPIN - 7 MEMBERED COMPOUND
Oxepin was first prepared by the treatment of 1,2-epoxy-4,5-dibromo cyclohexane with a mild dehdrohalogenating agent

27. THIEPIN - 7 MEMBERED COMPOUND
Wynberg and Helder reported that 2,3,4,5-tetramethylthiophene and dicyanoacetylene in the presence of aluminium chloride yielded Thiepin derivative.

28. INDOLE – BICYCLIC RING SYSTEMS DERIVED FROM PYRROLE
The Reissert synthesis : This procedure also offers a convenient method for the preparation of indole and its derivatives. It involves a base – catalysed condensation of nitrotoluene with oxalic acid ester in the presence of sodium ethoxide forms 0- nitro-phenylpyruvate this on hydrolysis gives a corresponding acid ,this on reductive (zn/CH3COOH) cyclization yields Indole.

29. BENZOFURAN – BICYCLIC RING SYSTEM DERIVED FROM FURAN
From coumarin: This method involves initially the bromination of coumarin to 3,4- dibromocoumarin followed by treatment with alkali to coumarillic acid. The acid decarboxylates to benzofuran.

30. BENZOTHIOPHENE-BICYCLIC RING SYSTEM DERIVED FROM THIOPHENE
From o-mercapto ᵦ -chlorostyrene : The o- mercapto ᵦ -chlorostyrene on refluxing with an alcoholic solution of alkali cyclizes to benzothiophene.

31. ACRIDINE- BICYCLIC RING DERIVED FROM PYRIDINE
From Diphenylamine-2-Carboxylic Acid: o-chlorobenzoic acid is first condensed with aniline in the presence of a base to yield diphenylamine-2-carboxylic acid. The acid is cyclized in the presence of sulphuric acid to acridone whereas reaction with phosphorous oxychloride gives 9-chloroacridine.reaction of acridone and 9-chloroacridine with appropriate reagents provide acridan.this on oxidation by air or ferric chloride leads to ACRIDINE.

33. PHENANTHRIDINE-BICYCLIC RING DERIVED FROM PYRIDINE
Cyclodehydration of an acyl o- aminodiphenyl in the presence of phosphorous oxychloride.

34. REFERENCES TEXT BOOK OF HETEROCYCLIC CHEMISTRY BY RAJ K.BANSAL.
TEXT BOOK OF THE CHEMISTRY OF HETEROCYCLIC COMPOUNDS BY R.MORRIN ACHESON.
WWW . GOOGLE .COM
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