Page 21 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

4 hydrolysis, oxidation and reduction

was being accepted as a powerful method, especially for the production of
[3]
optically active products . At this time the whole field was given another
boost by Alexander Klibanov at the MIT who showed emphatically (but not
for the first time) that some enzymes (especially lipases) could function in
organic solvents, thus broadening the substrate range to include water-insol-
[4]
uble substances .
For a while, in the early 1990s, the interest in the use of enzymes in organic
synthesis increased at an almost exponential rate and two-volume works were
[5]
needed even to summarize developments in the field . Now, at the turn of the
century, it is abundantly clear that the science of biotransformations has a
significant role to play in the area of preparative chemistry; however, it is, by no
stretch of the imagination, a panacea for the synthetic organic chemist. Never-
theless, biocatalysis is the method of choice for the preparation of some classes
of optically active materials. In other cases the employment of man-made
catalysts is preferred. In this review, a comparison will be made of the different
[6]
methods available for the preparation of various classes of chiral compounds .
Obviously, in a relatively small work such as this it is not possible to be
comprehensive. Preparations of bulk, achiral materials (e.g. simple oxiranes
such as ethylene oxide) involving key catalytic processes will not be featured.
Only a handful of representative examples of preparations of optically inactive
compounds will be given, since the emphasis in the main body of this book, i.e.
the experimental section, is on the preparation of chiral compounds. The focus
on the preparation of compounds in single enantiomer form reflects the much
increased importance of these compounds in the fine chemical industry (e.g. for
pharmaceuticals, agrichemicals, fragrances, flavours and the suppliers of inter-
mediates for these products).
The text of this short review article will be broken down into the following
sections:

1. Hydrolysis of esters, amides, nitriles and oxiranes
2. Reduction reactions
3. Oxidative transformations
4. Carbon±carbon bond forming reactions.

In each of these areas the relative merits of biocatalysis versus other catalytic
methodologies will be assessed. Note that the text is given an asterisk (*) when
mention is made of a catalyst for a reduction or oxidation reaction that is
featured in the later experimental section of this book.

1.1 HYDROLYSIS OF ESTERS, AMIDES, NITRILES AND OXIRANES

The enantioselective hydrolysis of racemic esters to give optically active acids
and/or alcohols (Figure 1.1) is a well established protocol using esterases or
lipases. In general, esterases from microorganisms or animal sources (such as