The title reaction leads to diastereomeric lithium carbanion acylation with acid chlorides, stannylation, and silylation take place at the α-position with stereoinversion (79-86% pairs that are configurationally unstable and equilibrate even at temperatures below -50 °C. The initially formed epimer ee). Methylating agents attack the γ-position; here, the stereochemical course depends on the leaving group, anti-S EЈ (1S)-epi-10 is rapidly converted to the thermodynamically more stable (1R)-10 (in toluene solution). Carboxylation, for the iodide (50% ee) and syn-S EЈ (48% ee) for the tosylate.
Chiral lithium/(؊)-sparteine carbanion pairs B and epi-B, 3. In case 3, a kinetic resolution of the diastereomers B and epi-B by very different rates k SS and k SR in the substi-which are accessible by deprotonation of prochiral precursors A, are valuable tools in enantioselective synthesis [1] [2] . tution step is responsible for the ratio of enantiomers. This typical Curtin-Hammett situation applies, e.g. if k SS > k SR Once produced, each of the epimeric ion pairs reacts stereospecifically with electrophiles to yield the enantioenriched << k epi .
Case 1 was found to be valid for lithiated alkyl carba-products C or ent-C, respectively. The direction of stereospecificity may differ depending on the nature of the car-mates 2 [4] , N-Boc-pyrrolidine 3 [5] , and the cinnamyl amine 8 [6] (Scheme 2), while route 2 was found to be operative in banionic species and the electrophile. In Scheme 1, a reaction proceeding with stereoretention is illustrated.
the case of lithiated crotyl carbamate 4 [7] , alkylindenides 5 [8] , and N-Boc-benzylamines 6 [9] . Markedly different reac-Three different mechanisms can be operative in the step responsible for the stereoselection (Scheme 1):
tion rates for the two epimers were observed by Beak et al. [10] in the case of the benzyllithium derivatives 7. Scheme 1. Pathways of chiral induction in lithium carbanion pairs Scheme 2. Examples of chiral lithium/(Ϫ)-sparteine complexes of carbamates