Chemistry Reference
In-Depth Information
4 Isomers and
Building Blocks
Stereochemistry is a term used to describe the relative (and absolute) orientation of the
groups within a molecule. The stereochemistry of carbon-carbon bonds represented
by cis / trans isomerism is also known as geometric isomerism. The terms cis means
“on the same side” and trans means “across.” Double bonds in unsaturated compounds
are represented either in the cis or trans configuration (Figure 4.1). An example of a
naturally occurring terpene, occurring in two isomeric forms, highlights the stereo-
chemical differences of cis and trans isomers, shown in Figure 4.2. Although these
isomers of the compound, named β-ocimene, have identical molecular formulas, they
can be chromatographically separated and they have different spectral properties.
An alternative use of the cis and trans notation to indicate relative stereochemis-
try is the E - Z notation. A set of rules known as the Cahn-Ingold-Prelog rules gives
each substituent on a double bond an assigned priority. In the example shown in
Figure 4.3, as an extension of the use of the cis and trans notation to indicate relative
stereochemistry, the E- Z notation is generally accepted to describe the absolute con-
figuration of the double bonds. The higher priority is given to the methyl group. Thus,
when the two groups of higher priority are on opposite sides of the double bond, the
bond is assigned the configuration E . Conversely, if the two groups are on the same
side of the double bond, the bond is assigned the configuration Z (Figure 4.3).
4.1 TERPENES AND THE ISOPRENE RULE
Terpenes constitute the largest and most diverse class of natural products and are
found mostly in plants. Terpenes are made up of isoprene (isopentane) units: a five
carbon subunit from which the isoprene rule is derived. Terpenes are thus identified
by tracing the 5n carbon atoms in the molecule. However, it should be noted that
larger and more complex terpenes (e.g., squalene [Figure 4.4] and lanosterol) are also
found in animals.
Monoterpenoids represent a large class of terpene compounds. Biochemical mod-
ifications such as oxidation or a rearrangement reaction produce a variety of open
chain and cyclized monoterpenoids; and several structures are shown in Figure 4.5.
Monoterpenes consist of two isoprene units and have the general molecular formula
C 10 H 16 . They are low molecular weight compounds, are very volatile, and many can
be recognized by their distinctive odors.
Isoprenes are made from C-5 units possessing 5n carbon atoms; n is an inte-
ger. Two or more isoprene molecules are linked to one another to create terpenoids.
Linking between two isoprene molecules can occur in three ways, either with the
“head” or “tail” of the molecule, as shown in Figure 4.6.
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