Isoprene units, in terpenes

Q Identify the isoprene units in terpenes, and classify them according to the number of carbon atoms they contain. Problems 25-27, 30, and 31... 

Locating Isoprene units in terpenes was discussed in Section 29.7. 

Terpenes are characterized as being made up of units of isoprene in a head-to-tail orientation. This isoprene concept, invented to aid in the stmcture deterrnination of terpenes found in natural products, was especially useful for elucidation of stmctures of more complex sesquiterpenes, diterpenes, and polyterpenes. The hydrocarbon, myrcene, and the terpene alcohol, a-terpineol, can be considered as being made up of two isoprene units in such a head-to-tail orientation (1). 

Although the term terpene once refened only to hydrocarbons, cunent usage includes functionally substituted derivatives as well, grouped together under the general term isoprenoids. Figure 26.6 (page 1086) presents the structural formulas for a number of representative exanples. The isoprene units in some of these are relatively easy to identify. The three isoprene units in the sesquiterpene farnesol, for exanple, are indicated as follows in color. They are joined in a head-to-tail fashion. 

Problem 9.35 Pick out the isoprene units in the terpenes limonene, myrcene and a-pheliandrene, and in vitamin A, shown below,... 

Carefully circle the isoprene units in the following terpenes, and label each compound as a monoterpene, sesquiterpene,... 

Natural rubber is a terpene composed of repeating isoprene units, in which all the double bonds have the Z configuration. Because natural rubber is a hydrocarbon, it is water insoluble, and thus useful for waterproofing. The Z double bonds cause bends and kinks in the polymer chain, making it a soft material. 

Volatile organic compounds are contributed to the atmosphere by many forms of plant life, by way of conifers such as cedar, pine, and eucalyptus, and aromatic plants such as lavender, mint and sage. The pine forests of New England and the eucalyptus forests of the Blue Mountains in Australia contribute large masses of terpenes to the air above them. Terpenes are plant products biosynthetically derived from isoprene and have a formula of the type (C5H8)n where n is based on the number of isoprene units in the compound (Eq. 2.1). 

As a general rule, natural products derived from isoprenoid units arise from head-to-tail reactions of that synthon. The majority of the structures of such products built up by condensation of IPP will as a result display a branched methyl on every fifth atom in the chain. Thus, reaction of IPP (1-5) with the isomer with the internal double bond (1-6) proceeds by head-to-tail coupling with expulsion of a pyrophosphate ion. The free alcohol from the product 2-1 (Scheme 2.2) is the fragrant terpene geraniol. Reaction of 2-1 with a second isoprene unit in this case again takes place by head-to-tail reaction to afford 2-2. The free alcohol from this 15-carbon triene is known as farnesol and is generally classed as a sesquiterpene (Latin sesqui-, one and a half). The molecule is displayed in the unlikely conformation 2-2b, in anticipation of the next reaction. 

Isoprene (EYE-so-preen) is a clear, colorless, volatile liquid that is both very flammable and quite explosive. It is classified as a diene compound because its molecules contain two ( di- ) double bonds ( -ene ). It is also a member of the terpene family. The terpenes are a large family of organic compounds that contain two or more isoprene units. An example of a terpene is vitamin A, whose molecular formula is C2oH3O0. Vitamin A contains four isoprene units. The terpenes occur abundantly in nature in both plants and animals. 

Terpenes are formed by the head to tail joining of isoprene units. In nature, this is accomplished via the formation of isopentenyl pyrophosphate (IPP) from mevalonic acid, followed by the polymerization of the isopentenyl-pyrophosphate. 

Isoprene A colourless liquid diene, CH2 C(CH3)CH CH2. The systematic name is 2-methylbuta-l,3-diene. It is the structural unit in terpenes and natural rubber, and is used in maldng synthetic rubbers. 

Terpenes occur in many plants and flowers. They have 10,15, 20, or more carbon atoms and are formed by linking isoprene units in various ways. 

Notice (from your answer to Problem 15.8) that all of the isoprene units in farnesol and retinal are joined in a head-to-tail manner, but that they are not all arranged that way in squalene and jS-carotene. Where does the head-to-tail arrangement break down in these two terpenes Hint Count carbons to locate the terpene units.) What does this suggest about the way these two terpenes might be bio synthesized ... 

During 1884-1914, Wallach wrote about 180 articles that are summarized in his book Terpene und Campher (Wallach, 1914) compiling all the knowledge on terpenes at that time, and already in 1887, he suggested that the terpenes must be constructed from isoprene units. In 1910, he was honored with the Nobel Prize for Chemistry in recognition of his outstanding research in organic chemistry and especially in the eld of alicyclic compounds (Laylin, 1993). 

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