- In 1887, Wallach enunciated the isoprene rule, which states that the skeleton structures of all naturally occurring terpenoids are built up of isoprene units. - Isoprene, with the empirical formula C5H8, is the monomeric structural unit that all terpenoids are derived from and thus called isoprenoid compounds. - Ingold formulated the special isoprene rule in 1925 to describe how the isoprene units in terpenoids are linked together in a head-to-tail fashion, with the branched end considered the head and the other end the tail.

1. Isoprenerule
Prepared by
Dr. N.GOPINATHAN
ASSISTANT PROFESSOR
DEPARTMENT OF PHARMACEUTICAL CHEMISTRY
FACULTY OF PHARMACY
SRI RAMACHANDRA UNIVERSITY
CHENNAI-116
TAMILNADU
INDIA

2. • In 1887 wallach enunciated isoprene
rule i.e the skeleton structures of all
naturally occurring terpenoids are build
up of isoprene units.
• All terpenoids derived from one
monomeric structural unit, isoprene
hence they are referred as isoprenoid
compounds.
CH2
CH3
CH2
2-methylbuta-1,3-diene

3. • The empirical formula is C5H8.
• On thermal decomposition yield
isoprene.
• Isoprene undergoes polymerisation to
yield various terpenoid.

4. • There is a special type of arrangement
were found in all terpenoid.
• Ingold in 1925 formulated this
observation under rule.
• It is based on how isoprene units in
terpenoids are linked together.
• The branched end of the isoprene unit is
considered as head

5. • Another end as tail
According to special isoprene rule the isoprene
unit in terpenoids are linked in head to tail
fashion

6. The basic carbon skeleton of a mono terpenoid
Is as follows according to isoprene rule
Special isoprene rule is a very suitable
tool to limit the number of carbon
skeletons of the structure of known
terpenoid

7. The skeleton which donot follow isoprene rule
are generally rejected

8. It is only used as guiding principle and not as a
fixed rule.
It is summary of experience and natural law.
There are deviation and violation of it
Exception – Crypton a natural terpenoid
contains only nine carbon atom instead of ten
hence isoprene rule cannot be applied to this

9. THANK YOU