Page 41 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

24 hydrolysis, oxidation and reduction

The biomimetic protocol was invented by Julia  and Colonna, and involves the
use of polyamino acids (such as poly-(l)-leucine) as the catalysts for peroxide
oxidation of chalcones, styryl alkyl ketones and conjugated alkenones. The
substrate range is broad, especially when using immobilized catalysts and an
organic solvent containing the substrate, urea±hydrogen peroxide and an or-
ganic base (Scheme 22) [101] .

O O
i O
Ph Ph Ph Ph
>95 % ee
>95 % yield

Scheme 22: Reagents and conditions: i) poly-L-leucine, urea±hydrogen peroxide, THF,
diazabicycloundecene.

Neither biocatalysts nor non-natural catalysts have yet been found that
provide a robust method of choice for stereocontrolled conversion of a ketone
to an ester (or lactone) via a Baeyer±Villiger reaction. Thus whole cell bioca-
talysts can be used for very elegant transformations (Scheme 23) [102] but the
microorganisms (such as Acinetobacter sp.) need to be grown and harvested
before use (an anathema to most organic chemists, particularly for organisms
which are potentially pathogenic) and the use of the relevant isolated enzymes
(Baeyer±Villiger monooxygenases) is plagued with the problem of cofactor
(NADH or NADPH) recycling and, often, progressive poisoning of the cata-
[103]
lytic action of the enzyme by the product as it is formed . The cloning of
useful Baeyer±Villiger monooxygenases into bakers' yeast may give, in time,
more widely-available, easily-used microorganisms [104] .

O
O
O
O O
ii i
H H
Cl Cl Cl
(R)-lactone (S)-lactone
>99 % ee 85 % ee

Scheme 23: Reactions and conditions: i) Acinetobacter calcoaceticus, H 2 O ii) Cunning-
hamella echinulata, H 2 O.

3-Phenylcyclobutanone has been a substrate for a copper-catalysed Baeyer±
Villiger oxidation. Thus the complex (35), (ca 1 mole%) in conjunction with