Improved Methodology for the Preparation of Chiral Amines

illion) in 1996, 29% (1997), 30% (1998), 32% (1999), 34% (2000), 38% (2001) to an
estimate of 39% (US $151.9 billion) in 2002. [24]
1.4. Chirality and Drug-Receptor Interaction
As mentioned before biological systems are based on chirality. For example, enzymes are
considered as chiral biological polymers consisting of solely L-amino acids. They are highly
structured compounds: their secondary and tertiary structure is determined by the amino acid
constituents. Enzymes function as molecular receptors by binding selectively to specific
molecules. Due to their chirality, they commonly interact much stronger with one enantiomer
of the ‘target’ molecule; or what is known as chiral recognition. ‘lock-and-key’ concept was
introduced in 1894 by Fischer to explain enzyme selectivity. This simple concept sates that
one enantiomer ‘fits’ in the enzyme cavity, the other enantiomer does not. This concept was
reformulated later to more complicated model (three point model). [25]
To get a high degree of enantioselection, a substrate must be held firmly in three dimensional
space. There must be at least three different points of attachment of the substrate onto the
active site. Variations and refinements to this rule have been reported. The most important
one is that the interactions may be attractive or repulsive. Steric hindrance often plays an
important role in chiral recognition.Chirality has also an important role in the field of fine
chemical industry. Large applications are found in agrochemicals, food and fragrance
industry. There are handful examples of enantiomers showing different fragrances because
one is (R) and one is (S).
For a chiral herbicide from the class of α-aryloxypropionic acids, the activity is present
almost solely in the (R) enantiomer. Although currently most herbicides are applied as
racemates, attempts towards the development of convenient large scale methodologies for the
synthesis of herbicides were awarded by development of metalochlor. This trend will result in
50% reduction of dosage, which means 50% less environmental pollution.
1.5. Sources of Enantiopure Substances
8