H
H
(+)-Limonene
(in orange)
(−)-Limonene
(in lemon)
*
*
N
H
Me
H
F 3 C
(S)-Fluoxetine
(prevents migraine)
*
Fluoxetine, commonly known as Prozac
1 , as a racemic mixture is an
antidepressant drug, but has no effect on migraine. The pure S-enantiomer
works remarkably well in the prevention of migraine and is now under
clinical evaluation.
3.4 Synthesis of chiral molecules
3.4.1 Racemic forms
On many occasions, a reaction carried out with achiral reactants results in
the formation of a chiral product. In the absence of any chiral influence, the
outcome of such reactions is the formation of a racemic form. For example,
hydrogenation of ethylmethylketone yields a racemic mixture of 2-hydroxybutane.
O
OH
+ H 2
Ni
Ethylmethylketone
(±)-2-Hydroxybutane
*
Similarly, the addition of HBr to 1-butene produces a racemic mixture of
2-bromobutane.
Br
1-Butene
HBr
Ether
(±)-2-Bromobutane
*
3.4.2 Enantioselective synthesis
A reaction that produces a predominance of one enantiomer over other is
known as enantioselective synthesis. To carry out an enantioselective
reaction, a chiral reagent, solvent, or catalyst must assert an influence on
the course of the reaction. In nature, most of the organic or bioorganic
reactions are enantioselective, and the chiral influence generally comes from
various enzymes. Enzymes are chiral molecules, and they possess an active
site where the reactant molecules are bound momentarily during the
3.4 SYNTHESIS OF CHIRAL MOLECULES
55