Page 55 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

38 hydrolysis, oxidation and reduction

other forms of catalysis) when one is faced with the transformation of the novel
substrate.

. Enantioselective hydrolysis reactions, especially esters, amides and nitriles.
. Stereocontrolled oxidation of aromatic compounds (hydroxylation or dihy-
droxylation) and hydroxylation of some alicyclic compounds, especially at
positions remote from pre-existing functionality.
. Stereocontrolled oxidation of sulfides to sulfoxides.
. Formation of optically active cyanohydrins.

Biomimetic reactions should also be considered for the preparation of
optically active cyanohydrins (using a cyclic dipeptide as catalyst) and also
for the epoxidation of a, b-unsaturated ketones (using polyleucine or congener
as a catalyst).
In most other areas, especially in the field of carbon±carbon bond formation
reactions, non-natural catalysts reign supreme.
However, while it is clear that biocatalysts may only provide viable and
reliable methods in about 5±10 % of all transformations of interest to synthetic
organic chemists, it is also clear that in some cases the biotransformation will
provide the key step in the best method in going from a cheap substrate to a
high value, optically active fine chemical. Thus ignoring biotransformations
altogether means one may occasionally overlook the best pathway to a target
structure.
In addition there is at least one area where enzyme-catalysed reactions have
established themselves as the first line of attack for solving synthetic problems;
that area involves the transformations of carbohydrates. Indeed, biocatalysed
transformations of saccharides is becoming increasingly popular and roughly
10 % of the recent literature (Year 2000) on biotransformations involves the
preparation and modification of carbohydrates. Early literature on chemoenzy-
matic approaches for the synthesis of saccharides and mimetics has been
reviewed by a pioneer in the field, C.-H. Wong [158] . For one of the most popular
areas, enzyme-catalysed glycosylation reactions, a useful survey is also available,
penned by the same senior author [159] .
One advantage of using enzyme-catalysed reactions in this field is that exquis-
ite regio- and stereo-selectivity can be obtained, without recourse to long-winded
protection/deprotection strategies. Furthermore, it is perfectly feasible to use
different enzymes sequentially, quickly to produce complex polysaccharides. In
the example shown in Scheme 50; N-acetylglucosamine is appended by a linker
to a Sepharose bead: thereafter galactosyltransferase (with UDP-galactose),
sialyltransferase (with CMP-neuraminic acid 5-acetate) and fucosyltransferase
(with GDP-fucose) were used sequentially to prepare sialyl Lewis tetrasacchar-
ide (70) attached to the solid support; an impressive overall yield of 57% was
recorded [160] .