M.Sc. Part II Sem IV Heterocyclic compounds-II Nomenclature of heterocyclic compounds of bicyclic/tricyclic (5-6 Membered) fused heterocycles (up to three hetero atoms). (Common, systematic (Hantzsch-Widman) and replacement nomenclature)

1. Heterocyclic compounds
Nomenclature of
heterocyclic compounds
of bicyclic system.
Mr. Harshad R. Sonawane

2. Bicyclic Systems
 Fused Rings , Bridged Bicyclic Rings And Spiro Rings.
There are three ways to join two rings.
1. Fused rings share two adjacent carbon atoms.
2. If two rings share two or more atoms, then the structure is called a bicyclic compound.
3. If the two rings share a single atom, then the structure is called a spirocyclic compound.
Examples of each way to join rings is shown below
Bicyclic compound Spirocyclic compound
Fused rings
1.Two cycle
2.Two carbons which is common in two cycle
Single covalent bond
(fused bicyclo comp.)
1.Two cycle
2.Only one carbons which is
common in two cycle

3. • In “bridged bicyclic” molecules, the two bridgeheads are separated by “bridges” containing at
least one carbon.
R ing 1 Ring
2
Bridge Head Carbon
Ring 3

4. Ring strain & Van Der Waals strain in the “inside-outside” diastereomer
The picture shows a model of this molecule highlighting the angle
strain and
Van Der Waals strain resulting from this arrangement.

5. • This bicyclic ring can be drawn from several different position
• Naming Bridged Bicyclic Molecules (Fused rings can also be named using this system)
1. a) Find the highest priority functional group; this will be the suffix.
b) Count the total number of carbons in the molecule, which gives you the root name.
Decane (10 carbons total) (alkane-All single bond)
• Prefix- Bicyclo
• Root word- Dec

6. 2. a) Identify the two rings of the bicyclic molecule (they should share a perimeter) The
bridgehead carbons are where these rings meet.
b) Not necessary to give letters to the rings, just done to identify the two of them. Note that
A is a 7-membered ring and B is a 6-membered ring.
3. Numbering:
a) Begin numbering at one bridgehead and follow the longest path to the second
bridgehead. Continue numbering along the next longest path until all carbons are
numbered.
b) Numbering from C-2 to C-5 is the longest path between bridgeheads (4 carbons).
c) From C-7 to C-9 is along the second longest path (3 carbons]
d) C-10 is along the third longest path (1 carbon)

7. 4. Find all unique paths between bridgehead carbons and count the number of carbons along
each path. Then arrange in order: (4.3.1).
bridge length: 4 carbons 3 carbons 1 carbon

8. 5. Putting everything together, we start with "bicyclo", and then use the results from steps 4
[4.3.1) and 1 (decane) "bicyclo[4.3.1]decane"
Bicyclo[4.3.1]decane

9. • A Few More Examples Of Bicyclic Molecules
Top View Side View
1.
Seven carbon alkane: heptane
Bridge lengths: 2, 2, 1
bicyclo[2.2.1]heptane
2.
Eight carbon alkane: octane
Bridge lengths: 2, 2, 2
bicyclo[2.2.2]octane
3.
Nine carbon alkane: nonane
Bridge lengths: 3, 3, 1
bicyclo[3.3.1]nonane

10. 4. Six carbon alkane: hexane
Bridge lengths: 2, 1, 1
bicyclo[2.1.1]hexane
Ten carbon alkane: decane
Bridge lengths: 4, 4,0
bicyclo[4.4.0]decane
5.

11. b
i
c
y
c
l
o
[
1
.
1
.
0
]
b
u
t
a
n
e
bicyclo[3.2.1]octane
b icy clo [5 .2 .0 ]n o n an e
b
icy
clo
[4
.3
.2
]u
n
d
ecan
e
bicyclo[2.2.2]oct-2-ene
bicyclo[4.2.0]octane
b
ic
y
c
lo
[4
.3
.0
]n
o
n
a
n
e
b ic y c lo [ 2 .2 .2 ] o c ta n e
bicyclo[4.3.1]decane

12. NOMENCLATURE OF FUSED RING HETEROCYCLES
• Fused heterocycles are systems constructed by combining two or more rings.
• The rings are fused if they are linked in such away that each ring has one common bond (fusion
bond) to each other.
• Moreover, each of the fused ring units should contain a maximum number of non-cumulative
double bonds.
• There are two broad types of fused heterocycles. They involve either a aromatic carbocycle linked to
a heterocycle or a heterocycle linked to another heterocycle

13. Rules for Naming Fused Heterocycles
• Many of these fused heterocycles occur naturally in plant and animal systems, and
some are used commercially.
• Due to their traditional uses, some of these heterocycles have trivial names that
reflect these uses, but also have IUPAC names that reflect their structure.
• The general format for the name of a fused heterocycle involving a carbocycle fused
to a heterocycle is:
• To generate this name, it is necessary to identify the fused or attached ring and the
base ring that will be the parent ring that will provide the suffix of the molecule.

14. • To derive the name of the fused heterocycle, the attached component is added as a
prefix to the name of the base component.
• The prefix in such names is generated by replacing the terminal ‘e’ in the name of the
attached component by ‘o’, i.e. benzo, naphtho and soon.
• The prefixes for the common aromatic heterocycles are listed below:
 Furan = furo
 Thiophene = thieno
 Pyridine = pyrido
 Pyrrole = pyrrolo
 Imidazole = imidazo
 Quinoline = quino

15. How to Identifying the Fused Components ?
• To identify the fused components of a fused heterocycle, dissect it along the fusion bond
into its component rings to set one as its base component and the other will be
designated as the attached component.
• Note that the base component is always a heterocyclic system. The components are
given their recognized trivial names (if possible) else, systematic name is used.
N
H
N
H
C
a
rb
o
c
y
c
le H
e
te
ro
c
y
c
le
B
a
s
ec
o
m
p
o
n
e
n
t=P
y
r
r
o
le
F
u
s
e
dh
e
te
ro
c
y
c
le

16. To name a fused heterocycle, derived from the fusion of a carbocycle to a heterocycle:
i. Derive the parent name (written at the end) from the name of the heterocyclic ring (base
component).
ii. Generate the attached name (written first) from the name of the attached benzene (i.e
Benzo).
iii. The bond of fusion of the parent ring with the attached benzene ring is indicated using
alphabetical letters ( a, b, c etc) placed in square brackets between the prefix and parent
name.
iv. To determine the alphabet for the bond of fusion, number the parent ring from the
heteroatom to the fusion bond such that side 1,2,takes letter (a),side 2,3 takes letter
(b),side3,4 takes letter (c)etc
Naming a Carbocycle Fused to Heterocycle

17. N
H
N
H
N
H
1
2
3 4
5
a
b
c
d
e
b
e.g. To nomenclature of indole illustrated below shows how fused heterocycles with a carbocycle fused to a
heterocycle are named:
Prefix = Benzo Fusion side = [b] Parent name = Pyrrole
Full name = Benzo[b] pyrrole
(Trivial name = Indole)

18. N
N
S
4,5 dihydrobenzo [c] thiophene
1
2
3
4
5
6
7
a
b
c
d
e
Benzo[b] pyridine
(Quinoline)
Benzo[c] pyridine
(Isoquinoline)
a
b
c d
a
b
c
• Note that when the fused heterocycles are partially saturated or substituted, the total numbering of the
complete fused system is done to determine the positions of saturations or substitutions.
• The numbering starts from the atom next to the fusion point giving the heteroatom the least possible
locant and the fused bond is given the fusion number using letters (a, b, c, etc).

19. •The IUPAC nomenclature of such systems is based on:
i. The parent name (suffix) of the base component is given by the higher priority heterocycle.
ii. The attached component is the ring name of the lower priority ring and is used as the
prefix.
• The bond of fusion is indicated by numbers and an appropriate letter enclosed in a square
bracket and placed immediately after the prefix.
• The numbers (positions of attachment) of the attached component are placed in the
sequence in which they are attached to the base component.
• For substituted systems, the total numbering of the fused heterocycle is in the usual way
1,2,3….based on the principle of lowest possible numbering.
Heterocycle Fused to Another Heterocycle

20. • The general format for capturing the name of a fused heterocycle combining a heterocycle fused
to an other heterocycle is summarized below:
i. The parent or base name given by the higher priority heterocycle is used as suffix.
ii. The fused ring name is the lower priority ring and is used as prefix.
Heterocycle Fused to Another Heterocycle

21. • If there is a choice between different heterocyclic components, nitrogen-containing components take
singular precedence over all other heterocycles as the base component.
• If nitrogen is absent, then the ring with other heteroatom(s) is selected as base component based
on the order of preference (oxygen > sulphur).
S O
S O
F
u
s
e
d
h
e
t
e
r
o
c
y
c
l
e B
a
s
e
c
o
m
p
o
n
e
n
t=
F
u
r
a
n
Rules for Fused Aromatic Heterocycles
order of preference (Nitrogen > Oxygen > Sulphur).
N
H
O
N
H
O
F
u
s
e
d
h
e
t
e
r
o
c
y
c
l
e B
a
s
e
c
o
m
p
o
n
e
n
t=
P
y
r
r
o
l
e

22.  If components have the same heteroatom, but rings of unequal size are present, then the one
with largest size of the ring is selected.
Rules for Fused Aromatic Heterocycles
 If rings of equal size with different number of heteroatoms are present, then the ring with greater
number of heteroatoms of any kind is considered as a base component.
N
H
O
N N
O
F
u
s
e
d
h
e
t
e
r
o
c
y
c
l
e B
a
s
eC
o
m
p
o
n
e
n
t=
O
x
a
z
o
l
O
O
O
O
Fused heterocycle
Base component = pyran

23. 1
2
3 4
5
a
N
H
O N
H
O
F u s e d h e t e r o c y c l e B a s e C o m p o n e n t A t t a c h e d C o m p o n e n t
N
H
O
b
c
a
b
c
F u s i o n b o n d l e t t e r F u s i o n b o n d n u m b e r
P a r e n t n a m e = P y r r o l e P r e f i x = F u r o F u s i o n p o i n t = 2 , 3 - b
N
H
O
1
2
3
4
5
6
6 H - f u r o [ 2 , 3 - b ] p y r r o l e
Example

24. • In “spiro” fused molecules, the two rings are both joined at the same carbon.
• A spirobicycloalkane is a molecule in which only one carbon atom is shared by the two rings in the
molecule.
• The carbon atom shared by the two rings is called the spirocarbon.
• A chain of bonds originating and ending at the spirocarbon is called a bridge.
• The compound below is named spiro[4.5]decane to communicate the number of carbons in each bridge
with the spirocarbon.
Spirocyclic Compounds
Spirocyclic compound
1.Two cycle
2.Only one carbons which is common in two cycle

25. • Naming Spiro
Compounds
Since both “bridgehead” positions are on the same carbon, we won’t be able to use the same “bicyclo”
nomenclature as before- but the process is very similar. We simply substitute “spiro” for “bicyclo” , insert the
two bridge lengths, and place the suffix as before. So the molecule below is spiro[5.4]decane.
s
p
i
r
o
[
5
.
4
]
d
e
c
a
n
e

26. Numbering:- Numbering start from C-atom adjacent to 40 C then Smallest chain to largest chain.
sp
iro
[4
.3
]o
c
ta
n
e spiro[5.2]octane spiro[2.3]hexane
sp
iro
[3
.4
]o
c
ta
n
e
O
Br
1-bromo spiro[3.4]octane 6-carboaldehyde