Sparteines as Chiral Ligands for Asymmetric Synthesis

The alkaloid (−)-sparteine (1) has found widespread use as a chiral ligand for asymmetric reactions.¹⁾ The complex formed from 1 and organolithium has recognized enantiotopic sides in its carbanions and prochiral reaction partner’s protons.^2,3) Asymmetric aldol additions using 1 and TiCl₄ have provided aldol products with excellent chiral selectivities.⁴⁾ Palladium-catalyzed oxidation and the following resolutions of secondary alcohols assisted by 1 as a ligand have provided the optically activated alcohols.⁵⁾ In addition, 1 has been used for enantiomer-selective polymerizations.⁶⁾

(+)-Sparteine (2) is not easily obtained from natural source compared with 1. However, 2 has potential for a chiral ligand which affords the products having an opposite configuration to those obtained by using 1.³⁾