In 1887, German Chemist Otto Wallach introduced isoprene rule and mentioned that thermal decomposition of almost all the terpenoids produce isoprene as one of the products, and this led to suggestion that the skeleton structures of all naturally occurring terpenoids can be built up of isoprene units.
It was recognized that an isoprene unit could be connected in different ways to produce the variety of carbon atom arrangements found in terpenoids.
An isoprene unit is the carbon skeleton of isoprene (ignoring the double bonds).
For example: Myrcene contains two isoprene units.
Formally, in biosynthesis of terpenoids, two or more isoprene molecules are linked to one another. Linking between two isoprene molecules could occur in three ways, given that the head and tail of the molecules are primarily involved in the linkage.
1. Head to Head or 1-1 Link
The head of one isoprene molecule could link with the head of another isoprene molecule.
2. Head to Tail or 1-4 Link
In this arrangement, head of one isoprene unit could link with the tail of another isoprene molecule.
3. Tail to Tail or 4-4 Link
Special Isoprene Rule
Ingold in 1925 pointed out that isoprene units in natural terpenoids were joined ‘head to tail’. The head is being the branched end of isoprene unit. It should be noted however, that this rule, which has proved very useful, can only be used in guiding principle and not as a fixed rule, several exceptions are reported, for e.g., carotenoids are joined tail to tail (4-4 link) at their centre.
Cyclic terpenes also contain links that are neither 1-1, 1-4 or 4-4, which are called cross links.
Special isoprene rule states that in most naturally occurring terpenes, there are no 1-1 or 4-4 links.