Development of efficient methodology to produce an optically pure enantiomer is of fundamental importance, particularly for the synthesis of biologically active natural products. Optically active compounds can be obtained by three different approaches, that is, resolution of racemates, use of a chiral pool or “chiron” (which are enantiomerically pure synthons), or asymmetric synthesis. Of these, one of the more challenging tasks involves asymmetric synthesis, which may be carried out either enzymatically or nonenzymatically. Whereas the nonenzymatic method enables us to introduce a chiral center with either a stoichiometric or catalytic amount of a chiral compound, the enzymatic method uses biological systems, such as microorganisms or isolated enzymes, to create the center of asymmetry. The purpose of this chapter is to survey asymmetric synthesis via the production of enantiomerically pure or enriched organic molecules by the enzymatic method focusing on the use of a hydrolytic enzyme, pig liver esterase (PLE; Enzyme Commission classification number, E.C. 220.127.116.11).
Enzymes are classified into the following six groups based on the reactions they catalyze:
- Oxidoreductase (oxidation–reduction reactions);
- Transferase (transfer of functional groups);
- Hydrolase (hydrolysis reactions);
- Lyase (addition to double bonds or the reverse);
- Isomerase (isomerization reaction); and
- Ligase (formation of bonds coupled with pyrophosphate bond cleavage of ATP).
Virtually all biochemical transformations are catalyzed in vivo by these six groups, and the biochemical aspects of these enzymes have been studied in detail. However, the practical utility of these enzymes in organic synthesis remains to be further exploited and refined. The ability of a substrate to associate with an enzyme is also one of the most significant problems. In some cases, these problems have been overcome and several enzyme reactions have been successfully used on a large scale.
An enzyme reaction generally takes place when an intimate interaction between the reactant and the chiral catalyst (enzyme protein) is realized. This enzyme–substrate complex is designated as the Michaelis complex. Certain amino acid residues of the enzyme form a three-dimensional structure as the active site. This site often contains reactive groups of the amino acids such as amino, mercapto, hydroxyl, carbonyl, carboxyl, guanidino, or imidazolyl. When the substrate is bound to the active site in a specific orientation, enantiotopic groups or faces of the substrate molecule are discriminated by the chiral enzyme. This discrimination is sufficiently sensitive to differentiate between the two hydrogen atoms in a methylene group. Thus the reaction proceeds stereospecifically. The enzyme can also distinguish among several substrates competing for an active site. Such substrate specificity is sometimes strict and sometimes broad. Synthetically useful enzymes should accept a wide range of substrates and exhibit high stereospecificity. Many enzymes meet these criteria, and PLE is one of them.