Terpenes isoprene rule

The German chemist Otto Wallach (Nobel Prize m chemistry 1910) determined the structures of many terpenes and is credited with setting forth the isoprene rule ter penes are repeating assemblies of isoprene units normally joined head to tail... 

Terpenes are said to have structures that follow the isoprene rule in that they can be viewed as collections of isoprene units... 

Exercise 30-1 a. Write out all of the possible carbon skeletons for acyclic terpene and sesquiterpene hydrocarbons that follow the isoprene rule. Do not consider doublebond position isomers. 

The sesquiterpenes, diterpenes, and poly terpenes can be considered most simply from a structural point of view as products composed of multiple units of isoprene. Structures currently assigned to some of the sesquiterpenes may have to be revised as the study of terpene chemistry advances, since many of these have been based on the validity of the isoprene rule for terpene structures and exceptions to this rule are becoming known (37). 

Figure 1.25 The isoprene rule for classifying compounds as terpenes. To apply the rule one ignores the double bonds (see myrcene) and in some cases it is also necessary to distort the isoprene structure (see a-pinene)...

Proposal of isoprene rule in terpene chemistry O. Wallach... 

Ruzicka, L. (1953). The isoprene rule and the biogenesis of terpenic compounds. Experi-entia 9 357-367. 

The isoprene unit is one of nature s favorite building blocks. It occurs not only in rubber, but in a wide variety of compounds isolated from plant and animal sources. For example, nearly all the terpenes (found in the essential oils of many plants) have carbon skeletons made up of isoprene units joined in a regular, head-to-tail way. Recognition of this fact—the so-called isoprene rule—has been of great help in working out structures of terpenes. 

All terpenes are related, regardless of their apparent structural differences. According to the isoprene rule proposed by Leopold Ruzicka, terpenes can be thought of as arising from head-to-tail joining of five-carbon isoprene 2-methyl-1,3-butadiene) units. Carbon 1 is called the head and carbon 4 is the tail. For example, myreene contains two isoprene units joined head to tail, forming an eight-carbon chain with two one-carbon branches. of-Pinene similarly contains two isoprene units assembled into a more complex cyclic structure. 

The isoprene rule is a convenient formalism, but isoprene itself is not the biological precursor of terpenes. Nature instead uses two isoprene equivalents —isopentenyl pyrophosphate and dimethylallyl pyrophosphate. These five-carbon molecules are themselves made from condensation of three acetyl CoA units (Section 21.9). 

Terpenes are most familiar, at least by odor, as compounds of the so-called essential oils obtained by steam distillation or ether extraction of various plants. Thousands of different terpenes are known. According to the isoprene rule proposed by L. Ruzicka in 1921, they can be considered to arise from head-to-tail joining of simple five-carbon isoprene (2-methyl-1,3-butadiene) units. Terpenes are subdivided into groups depending on the number of isoprene units [9]. For example, monoterpenes are 10-carbon substances biosynthesized from two isoprene units, which can be divided into aliphatic, monocyclic, or bicyclic species Some typical exponents of each monoterpene subgroup are shown in Fig 1... 

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