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Abstract:
This thesis describes studies
on non-classical carbocations and enols in asymmetric
catalysis. In the first part, a novel catalytic system based on
asymmetric counteranion-directed
Brønsted acid organocatalysis
was developed, aiming for
the
enantiocontrol
of
highly
reactive intermediates such as non-classical carbocations.
This challenge led
to the
design and fine tuning
of a new
class of non-nucleophilic catalysts
and culminated
in
the
first
proof of
concept
that these intermediates can
indeed
be controlled asymmetrically
and
good enantioselectivities
were
obtained. In the second part, a novel activation mode for
organocatalysis is presented. Its key feature is the use of a chiral Brønsted acid to promote
the enolization of
carbonyl compounds
and to overcome some
intrinsic
limitations of
aminocatalysis.
Successfully applied to asymmetric α-alkylations
(Michael addition and α-allylation)
and α-oxidation reactions of ketones,
this concept
opens
versatile
and highly
enantioselective
new
routes
towards synthetically challenging quaternary and
tetrasubstituted chiral centers.