Introduction
<P>Asymmetric synthesis represents a challenging topic in modern
organic chemistry. The asymmetric deprotonation of a prochiral carbon
by a chiral base offers attractive access to a chiral carbanion,
which may react to give enantioenriched products. (-)-Sparteine
is a chiral bidentate ligand with broad applicability. Hoppe was
the first to use a mixture of alkyllithium and (-)-sparteine
(Figure 1) for very effective asymmetric deprotonations.
[
1]
Beak examined enantioselective
deprotonations of
N-Boc-pyrrolidines
and
N-Boc-allylamines.
[
2]
Furthermore, it was used
for dynamic resolutions
[
3]
and
deprotonations
[
4]
of phosphine-boranes,
for asymmetric additions of alkyllithiums to imines,
[
5]
for asymmetric carbometallations
of cinnamyl derivatives,
[
6]
for palladium-catalyzed
oxidative kinetic resolutions of secondary alcohols,
[
7]
and for enantioselective syntheses
of ferrocenes with planar chirality.
[
8]
</P><P>The title compound is an alkaloid, which can be isolated from
certain
papilionaceous plants such as
Scotch broom.
[
9]
Its
antipode is also naturally occurring but can be obtained far less
easily. An 18 steps asymmetric total synthesis of (+)-sparteine
starting from norbornadiene has been reported.
[
10]
A
(+)-sparteine surrogate is readily available from (-)-cytisine.
[
11]
</P><P>(-)-Sparteine is commercially available as a free base
or as the sulfate-pentahydrate. The chiral ligand can usually be
recovered from the reaction mixtures by alkaline extraction.</P>
Figure 1
