Isoprene rulePrepared by

Dr. N.GOPINATHANASSISTANT PROFESSOR

DEPARTMENT OF PHARMACEUTICAL CHEMISTRYFACULTY OF PHARMACY

SRI RAMACHANDRA UNIVERSITYCHENNAI-116TAMILNADU

INDIA

• 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

CH22-methylbuta-1,3-diene

• The empirical formula is C5H8.• On thermal decomposition yield

isoprene.• Isoprene undergoes polymerisation to

yield various terpenoid.

• 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

• Another end as tail

According to special isoprene rule the isoprene unit in terpenoids are linked in head to tail fashion

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

The skeleton which donot follow isoprene rule are generally rejected

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

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