The synthesis of chiral compounds in enantiomerically pure form is one of the most important and one of the most challenging goals of modern organic synthesis. Any key step of synthetic importance, which leads to the creation of a stereogenic centre, requires to be studied with regard to its stereoselectivity.
Now it’s time to add a third dimension to our study. Stereochemistry is the branch of chemistry concerned with the three-dimensional aspects of molecules. We’ll see on many occasions in future chapters that the exact three-dimensional structure of a molecule is often crucial to determining its properties and biological behavior.
Many chiral objects, such as spiral staircases, do not have stereocenters. The same is true for many chiral molecules. Remember that the only criterion for chirality is the nonsuperimposable nature of object and mirror image.
How do we use this idea to distinguish a chiral molecule from an achiral one? Chiral molecules cannot have a plane of symmetry.
A compound that contains two (or, as we shall see, even more than two) stereocenters but is superimposable with its mirror image is a meso compound (mesos, Greek, middle). A characteristic feature of a meso compound is the presence of an internal mirror plane, which divides the molecule such that one half is the mirror image of the other half. For example, in 2,3-dibromobutane, the 2R center is the reflection of the 3S center.
One possible approach is to start with the racemate and separate one enantiomer from the other. This process is called the resolution of enantiomers. Some enantiomers, such as those of tartaric acid, crystallize into mirror-image shapes, which can be manually separated … However, this process is time consuming, not economical for anything but minute-scale separations, and applicable only in rare cases.
Depending on the substitution pattern, three principal interactions dictate the conformational equilibrium: 3. The interaction of a pair of cis-1,3-diaxial substituents (1,3-diaxial interaction)
The stereochemical course of reactions at three-, four-, and five-member rings can be reliably predicted by assuming the relative congestion of the two faces. As the ring size increases above six so does the conformational mobility and hence the uncertainty of the stereochemical outcome. Even with seven-member rings, predictions are generally difficult.