Rathbone, Daniel Lee (1987) Synthetic and enzymatic studies related to branched-chain amino acid metabolism. PhD thesis, University of Warwick.

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Abstract

Several
sytheses of a-acetolactate
analogues
(2-hydroxy-3-oxo
carboxylic esters) and related compounds were
developed. The
action of
the enzyme acetolactate
decarboxylase
upon these compounds was
studied.
A
synthesis of
3-bromo-2-oxo
carboxylic acids, esters and amides
was
developed. Inhibition
studies using methyl
3-bromo-3-methyl-2-oxo
butanoate
with pig
liver
esterase
indicated that the esters were not
suitable mechanism-based
inhibitors
of the enzyme.
A synthesis of
3-hydroxy-2-oxo
carboxylic esters was
developed.
These
compounds were
found to isomerise,
via alkyl group migration, to
the corresponding
2-hydroxy-3-oxo
carboxylic esters upon treatment
with catalytic quantities of
dibutyl tin oxide.
Cyclic
substrates gave
access to
6,7
and
8-membered
ring-expanded products.
A one-step synthesis of
2-hydroxy-3-oxo
carboxylic esters,
in
high
yield,
from the corresponding c43-unsaturated esters was
developed. This involved the use of acidic manganate
(VII) in
aqueous
acetone and was
found
to
be
superior to any synthesis of acyclic
2-
hydroxy-3-oxo
carboxylic esters published to date.

The
enzyme acetolactate
decarboxylase (ADC)
was
found to
decarboxylate
the
(S)-isomers
of CY-acetolactate
and
its
analogues to
give the corresponding
(R)-(-hydroxyketones
in high
optical purity.
The (R)-substrates
were
decarboxylated
to
(R)-ß. 1'-hydroxyketones via
prior
isomerisation to
(S)-(-acetolactate
analogues
by
a tertiary
ketol
rearrangement
involving
carboxylate group migration.
As
a result
(-acetolactate
analogues with non-identical substituents at the
2-
and
3-positions
gave structurally
different OF-hydroxyketone
products upon
decarboxylation
with
ADC. In the case of
C-acetolactate,
a substrate
with identical
substituents at the
2-
and
3-positions, both
enantiomers were converted
into (R)-(-)-acetoin
with an enantiomeric
excess greater than 98%. This
represents an unusual example of the
enzymic conversion of
both
enantiomers of a racemic substrate
into
a
single enantiomer of product.
The behaviour
of the enzyme towards a
range of substrates gave some
insight into the nature of the enzyme
active site.

ADC
was
found to catalyse, stereospecifically, the incorporation
of
deuterium into the
(R)-isomer
of racemic acetoin at
the methine
position. The chirality of the deuterated
and non-deuterated
components of the partially
deuterated
mixture was analysed in situ
by
vibrational circular
dichroism
measurements.
Acetolactate
synthase
isozyme II (ALS II) was used to generate
QE-acetolactate
and its homologues from
simple
2-oxo
carboxylic acids.
These
were
decarboxylated in
situ
by ADC to give the corresponding
a-hydroxyketones
of
high optical purity. In one example
01-acetohydroxybutyrate
was generated
by
the action of
ALS II
upon
2-
oxobutanoate. The structure of the single
c-hydroxyketone
prepared
by
ADC-catalysed decarboxylation
of the 06-acetohydroxybutyrate indicated
that the
(S)-isomer
of
Qf-acetohydroxybutyrate had been
generated.
Model
studies were carried out on
2-hydroxycyclohexanone for the
stereospecific reduction of the enzymatically produced
Ck-hydroxyketones
to 1,2-diols. No
signifiant chiral
induction
was
observed with sodium
borohydride in
a range of solvents or with
lithium
aluminium
hydride.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Lyases , Enzymes, Decarboxylases
Official Date:	September 1987
Dates:	Date Event September 1987 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Crout, D. H. G.
Sponsors:	Science and Engineering Research Council (Great Britain) (SERC) ; Imperial Chemical Industries PLC
Extent:	182 leaves
Language:	eng

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