Introduction
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]
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]
(-)-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.
Figure 1
