(-)-Sparteine inAsymmetric Synthesis

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Abstract
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

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