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Mar 17, 2016
Cyclic compounds with at least two different atoms in the rin g
are known as heterocyclic compounds. The ring itself is call ed
a heterocycle
A hetero atom is an atom other than carbon. The name comes from
the Greek word heteros, which means "different ".
The N-atom is the most common heteroatom . Next in the importance
are O- and S-atoms. Heterocycles with Se-, Te-, P-, As-, Sb-, Bi-, Si-,
Ge-, Sn-, Pb- or B-atoms are less common .
Heterocyclic compounds are widely distributed in nature. M any are of
fundamental importance to living systems, for examples Chl orophyll
and heme
Structure of chlorophyll c1 Structure of Heme B
Essential ingradients such as thiamin (Vitamin B1), riboflavin (Vitamin
B2), pyridoxal (Vitamin B6), nicotinamide (Vitamin B3), and ascorbic
acid (Vitamin C) are heterocyclic compounds.
Among the twenty amino acids commonly found in prot eins,
three of them are heterocyclic.
H2N CH C
CH2
OH
O
HN
2-amino-3-(1H -indol-3-yl)propanoic acid
Trptophan (Trp)
HN
C
OH
O
Proline (Pro)
pyrrolidine-2-carboxylic acid
H2N CH C
CH2
OH
O
N
NH2-amino-3-(1H-imidazol-4-yl)
propanoic acidHistidine (His)
It has been found that the introduction of hetero-atom in carbocyclic
molecules makes predictable changes in their physical and c hemical
characteristics.
These changes are not only in the behavior of the parent ring b ut also
in the chemical behavior of the functional groups attached t o these
rings X
monosubstituted rings
N N N NX
X
X
There are several methods to nominate heterocycles
which can be summarized as follow:
I)Trivial names
These names are given to heterocycles and usually
arising from the origin of compounds or of their ut ility
and they are approved by the IUPAC
Five membered rings containing two heteroatoms
NH
N
N
NH
1H-Pyrazole
N
O
isoxazole
N
S
isothiazole
N
O
oxazole
N
S
thiazoleImidazoleNN
O
Farmazone
NH
NH
Pyrazoline
NH
NH
pyrazolidine
O
NH
isoxazoline
O
NH
isoxazolidine
S
NH
isothiazoline
S
NH
isothiazolidine
Uns a
tura
ted
PartiallyUns at ura
t ed
Satura
ted
Six membered rings
N
N
N
N
N
N
PyrazinePyrimidinePyridazine
NH
NH
O
Piperidine Morphiline
NH
HN
Piperazine
N
Pyridine
O
2H-Pyran
O
4H-Pyran
� The basis for this system were introduced independently by A.
Hantzsch and O. Widman .
� This system was suggested at the beginning for six and five
membered rings and then was extended to include rings from 3 to 10
atoms.
� The names of the rings is derived by combining a prefix and a suffix
( i.e Name = prefix + suffix )
� According to this system a prefix is used to indicate the nature
of heteroatom in the ring (O, N, S, P,……)
� a suffix is used to indicate the size and the degree of saturation
in the ring .
(i) Prefix : is used to indicate the nature of heteroatom in the ring (O,
N, S, P,……)
When different heteroatom is present the highest pr iority will be as follow:
PrefixValenceElementsPrefixValenceElements
phospha310)Phosphorous (P)floura11) Flour (F)
arse311)Arsenic (As)chlora12) Chlore (Cl)
stilba312)Antimony (Sb)broma13) Bromine (Br)
bisma313) Bismuth (Bi)ioda14) Iodine (I)
sila414) Silicon (Si)oxa25) Oxygen (O)
germa415) Germnium (Ge)thia26) Sulphur (S)
stanna416) Tin (Sn)selena27) Selenium (Se)
plumba417) Lead (Pb)tellura28) Tellurium (Te)
bora318) Boron (B)aza39) Nitrogen (N)
mercura219) Mercury (Hg)
� The Latin numbers (di, tri, tetra,….) are used to indicate the number of
heteroatoms.
Ex: Tetra + aza ► tetraza
� When different heteroatom is present the highst periority will be as follow:
O > S > N
For a ring has O + N ► Oxaza
For a ring has S + N ► thiaza
For a ring has O + S ► oxathia
Suffix is used to indicate the size and the degree of
saturation in the ring
Suffix = Stem + end
StemStemStemStemRing sizeRing sizeRing sizeRing size
ir3
et4
ol5
in6*
ep7
The end refers to the degree of unsaturation in the ring
( as shown in the next table)
2: System for Hantzsch-Widman names
Rings containing no NitrogenRings containing Nitrogen
Ring size SaturatedUnsaturatedSaturatedUnsaturated
IraneIreneiridineirine3
etaneeteetidineete4
olaneoleolidineole5
ane***ineane**ine6*
epaneepine****epine7
ocaneocine****ocine8
onaneonine****onin9
ecaneecine****ecin10
Rings containing no NitrogenRings containing no NitrogenRings containing no NitrogenRings containing no NitrogenUnsaturated SaturatedUnsaturated SaturatedUnsaturated SaturatedUnsaturated Saturated
Rings containing NitrogenRings containing NitrogenRings containing NitrogenRings containing NitrogenUnsaturated SaturatedUnsaturated SaturatedUnsaturated SaturatedUnsaturated Saturated
Ring sizeRing sizeRing sizeRing size
IraneIreneIridineIrine3
oxirane
thiirane
Oxirene
thiirene
Aziridineazirine
etaneEteetidineete4
Thietane
oxitane
Thiete
oxete
AzetidineAzete
HNH
NO O
NNH S S
O O
S SO
NH
Oxaziridine
O
NH
2H-[1,2]Oxazete
O
NH
[1,2]Oxazetidine
O
N
Oxazirine
OlaneOleOlidineOle5
Oxolane
thiolane
oxole
thiole
ane***ineane**ine6*
Oxane
thiane
Oxine
thiine
O O
S S
O O
S S
NH
AzoleO
NH
[1,3]-Oxazolidine
N
Azine
� The rings that have one heteroatom, the rings are numbered so that
the heteroatom is given number 1.
� When different heteroatom is present the highest priority will be
O > S > N and move in the direction that gives the other heteroatoms
the nearest possible numbers.
NH1
2
34
5
O1
2
34
5
O
N
1
2
3
4 5
NS1
2
3
45
For partially saturated 5-membered rings containing
Nitrogen
For partially saturated rings we must designate the location of the nearest
unsaturated atom by ∆n (where n = the number of the first unsaturated atom ).
NH
NH
NH
azole azoline azolidine
NH
O
N
O
N
O1
2
34
5
-1-oxa-3-azoline4
-1-oxa-3-azoline3
-1-oxa-3-azoline2
For full unsaturated rings which contain extra hydr ogens, the
positions of extra hydrogens are indicated as nH in t he prefix
When a heterocycle contains two similar
atoms, the one carrying the heavier
substituent is numbered first.
When a heterocycle contains two or more N atoms, we begin numbering
from SP3 nitrogen without deviation from the priority
N
NH
NH
1
2
34
5
1. Heterocyclic systems are named by considering
them to be derived from cyclic hydrocarbons by
replacing a carbon corner by heteroatom.
2. The heteroatoms are indicated by the same
prefixes used in the Hantzsch-Widman
nomenclature.
3. When more than one heteroatom is present
citation numbers are used (O>S>N).
NH
O
1-Oxa-4-azacyclohexane
O
Oxacyclopent-2,4-diene
N
Azabenzene
S
N
1-Thia-2-azacyclopent-2,4-diene
O
N
Oxazacyclopropene
S
1-thiacyclohexa-2,5-diene
N
O N
O
1,7-Dioxo-2,8-diazacyclododeca-2,8-diene
2,3-Dihydro-1,4-dithianaphthalen
N
S
S
N
O
N
2-Azanaphthalene
1
1
2
3
4
1
2
3 4
5
1
2
3
4
4a
5
6
7
4H-4a-Azanaphthalene 1-Oxa-3-azacyclopenta-2,4-diene
2
(iv) Naming substituted monocyclic compounds � In case of heterocycles with one heteroatom the hetero atom is given No. 1.
Numbers are then given in the way that gives substituents least possible
numbers.
� (When more than one heteroatom is present the heteroatom of highest priority is
numbered 1. Numbering is then made in the way that gives other heteroatoms
least possible numbers. This means least difference between the first two atoms
and not least algebraic sum of numbers.
� According to IUPAC rules the organic compounds are arranged in order of
decreasing priority as follow:
a. Cation and anions
b. Acids : Carboxylic, peroxycarboxylic, thiocarboxylic, sulfonic, sulfinic,… Etc.
c.Derivatives of acids : anhydrides, esters, acyl halides, amides, hydrazides,
imides, amidines, …… etc.
d. Nitriles : cyanides, isocyanides.
e. Aldehydes, thioaldehydes and their derivatives
f.Ketones, thioketones and their derivatives.
g. Alcohols, phenols, thioles , and their esters derivatives with inorganic acids.
h. hydroperoxides .
i. Amines, imines and hydrazines .
j. Ethers, thioethers .
k. Peroxides .
All other functional groups, which are included in one of these classes,
are then written as prefixes in form of the name in alphabetic order.
O
Cl
3-furanyl chloride
NH
NH2
3-Amino-1H-pyrrole
NH
N
N
COCH3
CN
5-Actyl-4H-1,2,4-triazole-3-carbonitrile
N
N
HN
C2H5
Ph
2-Ethyl-1-phenyl-1,2,4-triazolidine
N
O
NH2
C2H5O2C
Ethyl 5-aminoisoxazol-3-carboxylate
S
NN
SCN
1,3,4-thiadiazol-2-ylthiocyanate
NH
N
NH2
3-Amino-1H-pyrazole-4-carbonitrile
NC
N
NH
NH2NC
5-amino-1H-pyrazole-4-carbonitrile
B) Polycycles
(A) Trivial names
Trivial names approved to be utilized are given below.
These were drawn from the IUPAC (Blue Book).
N
N
Quinoline Isoquinoline
4
3
2
18
7
6
5
1
2
3
45
6
7
8
N
N
N
N
Peteridine
1
2
3
45
6
7
845
6
7
Phthalazine
N
N
8 1
N
N
Quinoxaline
2
3
45
6
7
8
N
N
NN
QuinazolineCinnoline
1
2
3
45
6
7
81
2
3
45
6
7
8
1
2
3
NH
1
2
3456
7
8
9
4a4b
8a 9a
Carbazole
1H-Indole 1H-Indazole
NH
N1
2
34
5
6
7
NH
12
34
5
6
7
1
2
34
56 7
8
9
Purine
N
HN
N
NN
Indolizine
N
3
2
18
7
6
5
2
1
5
1H-Pyrrolizine
434
6
7
NH
2H-Isoindole
1
2
34
5
6
7
Fusion names� When a heterocyclic ring is fused to a carbocyclic ring, the hetero
ring is chosen as the parent or base component. The n the
systematic name is formulated from the prefix benzo - and the
trivial name of the heterocyclic component as follo ws:
� The bonds between the ring atoms are denoted accord ing to the
successive numbers of the ring atoms by the letters a, b, c, etc.
� The letter must be as early as possible and hence
benzo[ d]thiophene is incorrect.
12
3
5
4
S S
+a
b
c
d
e
benzo[b]thiophene
b
Base component
O
NH
Benzo[b]furanBenzo[b]pyrrole
or Indole
12
3
b
1
2
3
5 4
SS
+a
b
c
d
e
naphtho[2,3-b]thiophenebase component
•Polycyclic systems with two or more heterocycles
The base component is chosen as that in the Hantzsch-Widmannomenclature except that of nitrogen is given the highest priority .
Prefixes are formed by changing the terminal (e) of the trivial or theHantzsch-Widman names of the components into (o)
Chose of the base component .
i- Is there only one component which contains nitrog en?
No Yes: choose this as base component
O
N
Base component pyrrole
ii- Is nitrogen absent from all the rings?
NO Yes: choose the ring containing a heteroatom W hich occurs
highest?
S OBase component furan
iii- Are there more than two rings presents?
No Yes: choose the component containing the grea ter
number of rings
N N
N
Base component quinoline
V- Do the rings contain different numbers oh heteroa toms?
No Yes: chose the one with the greater number
NO
Base component isoxazole
HN
Vi- Are there differences between the numbers of eac h kind of
heteroatom in the two rings?
No Yes: choose the ring with the greater num ber of atoms
listed first
O
N
Base component oxazole
N
S
Vii- In case of the two rings are of the same size an d contains
the same heteroatoms, how to select the base compon ent?
Rule is to select the base ring to be the one in which
heteroatoms has less separation between the heteroatoms.
Thus:
N
NN
NN
NN
N
NH
N
Base component Pyrazole
N
HN
Base component Pyridazine Base component Pyridazine
Viii- The ring containing the greatest Varity of het eroatom
arranged according to their order priority
Base component Oxazole
N
O
N
HN N
N
S
Base component Thiazole
1- When a heterocyclic ring is fused to a
carbocyclic ring (choose the hetero ring as
the parent or base component
2- One ring only contains N (choose the ring
containing N)
3- No, Nitrogen ( choose according to the
priority O ˃ S )
S
benzo[b]thiophene
b
Base component is thiophene
How to Choose of the base component (Summary)
O
N
Base component pyrrole
S OBase component furan
4- One consists of two or more rings
(Choose one with the greater number of rings)
5- Two rings of different size (choose the
larger)
6- Choose the one with more heteroatoms
N N
N
Base component quinoline
O
N
Base component azepine
7- Same number of heteroatoms (choose O >
S > N)
8- Same type and number of heteroatoms, and
same ring size (choose lower separation
between atoms)
9- Choose the ring containing the greatest
Varity of heteroatom.
O
N
Base component oxazole
N
S
NH
N
Base component Pyrazole
N
HN
Bridge Specification
� Adding prefixes to the name of a base component
� The base component is chosen as before
� Prefixes are formed by changing the terminal e of t he trivial name into <O>
(e.g. pyrazol e is converted to pyrazol o)
� Some prefixes are formed as in the following table
Acenaphthylene Acenaphtho Naphthalene Naphtho
Anthracene Anthra Perylene Perylo
Benzene Benzo Pyridine Pyrido
Furan Furo Pyrimidine Pyrimido
Imidazole Imidazo Quinoline Quino
Isoquinoline Isoquino Thiophene Thieno
� The position of fusion between the prefix and the b ase is indicated
by inserting (in square brackets) number for the pr efix and italic
letter for the base component.
�Name = prefix[number, letter ]base
N
N
N
4H-pyrimido[1,2-b]pyridazine
N
N
N
N
Pyrazino[2,3-d]pyrimidineor
pteridine
N
NN
N
Pyrazino[2,3-c]pyridazine
ON
N
O
oxazolo[5,4-d]isoxazole
SN
N
S
thiazolo[5,4-d]isothiazole
N
NN
N
Pyrimido[4,5-c]pyridazine
If a position of fusion is occupied by a heteroatom, the names of
the two component rings are chosen in a way that both contain
this heteroatom .
Symmetric and Asymmetric Heterocycle
N N
N
N
pyrazolo[1,5-a][1,3,5]triazine
N
N
N
N
pyrazolo[5,1-c][1,2,4]triazine
12
3
45
a
a bc
12
3
45
N
N
N
pyrido[3,2-d]pyrimidine
1
2
3a
bcd
eN
NH
NN
abc
123 4
5
imidazo[4,5-c]pyrazole
N N
N
pyrido[2,3-d]pyrimidine
1 2
3a
bcd
e
N
N
NN
1
2
3
a
bcd
e
N
NN
N1
2
3a
b c
d
pyrazino[2,3-d]pyridazine
4
5N
NN
N 1
23a
b cd
pyrimido[4,5-d]pyridazinenot
pyrimido[5,4-d]pyridazine
pyrido[3,2-e][1,2,4]triazine
NN
HN
1H-imidazo[1,2-b]pyrazole
N
N
N
4H-pyrimido[1,2-b]pyridazine
N S
NS
thiazolo[5,4-d]isothiazole
N
O
furo[3,4-c]quinoline
ab
c2
3
4
1
Numbering fused heterocycles
The numbering of the complete fused structure does not depend on theoriginal numbering systems of its components .
1. The greatest number of rings should be on a horizontal line , thenthe maximum number of rings should be above and to the right ofthis a horizontal line.
2. Numbering is carried out usually in a clockwise (Not in all cases)starting with the atom not engaged in the ring fusion in the mostcounter-clockwise position of the uppermost ring.
1
2
3a b
c
d
N
benzo[h]isoquinoline
h e
fg
N
HN
1H-pyrido[3,2-c]carbazole
1
23
4
5
67
8
9
1011
4a
6a7a
11a 11b11c
1
2
3a b
c
d
N
4,9-dimethylbenzo[h]isoquinoline
CH3h e
fg
H3C
N
NH
4H-pyrido[2,3-c]carbazole
1
2 3
423
a
bc
N
1
23
4
5
6
7
8910
11
12
13
4a
4b
13a13b
a)- Give low numbers to the heteroatoms
O
Benzo[b]furan
NH O
S
4H-[1,3]oxathiolo[5,4-b]pyrrole
(N.B: 1,3,4 is lower than 1,3,6)
1
2
3 4
5
61
2
3
N
O
N
HN
6H-imidazo[4,5-d]oxazole
N
O
N
HN
Me
5-methyl-6H-imidazo[4,5-d]oxazole
1
2
3 4
5
6
1
2
3 4
5
6
c) Give hydrogen atom the lowest numbers
O
O6
4H-cyclopenta[d][1,3]dioxole
N
NH O
O
5
6
4H-[1,3]dioxolo[4,5-d]imidazole
1
2
34
5
1
2
34
A carbon atom common to two or more rings is given a lower case
letter after the numeral of the atom immediately pr oceeding. A
heteroatom common to two or more rings receives an individual
locant.
N
N
N
N
1
2
3
45
6
7 8
4a
Imidazo[1,2-b][1,2,4]triazine
12
3
456
N
O
3,9-dimethylfuro[3,4-c]quinoline
75a
CH3
CH3
3a8
99a
9b 1
2
3 N
N
N
7-methyl-4H-pyrimido[1,2-b]pyridazine
CH345 6
7
8
9
1
2 3 4
5
67
7a
4a
NN
S
NH2
thieno[3,4-d]pyridazin-5-amine
1
2
3
4
5 6
7
NNH
O
NH2NC
6-amino-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile
NN
N
CH3
3,6-dimethyl-3H-imidazo[1,2-b]pyrazole
H3C
1
2 3
4
5
67
8
NS NH2
COOC2H5NC
Ethyl 5-amino-8-cyano-7H-thiazolo[3,2-a]pyridine-6-carboxylate
N O
NN
Pyrrolo[3,2-e][1,3,4]oxadiazine
Pyrrolo[2,3-e][1,3,4]oxadiazine
O
NNN
This method is highly convenient for naming bicycli c saturated heterocycles.
The names originally for hydrocarbons , converted into names ofheterocycles by application of replacement principals .
prefix ( bicyclo )followed in square brackets by the number of carbon atomsseparating the bridgeheads on three possible routes from on e bridgehead tothe other , followed by the name of alkane containing the same number ofatoms in the chain as the whole bicyclic skeleton.
1
2
3
4
5
6bicyclo[3.1.0]hexane
O
3-oxabicyclo[3.1.0]hexane
12
3
45
6
NH
S
2-thia-4-azabicyclo[3.1.0]hexane