Diastereoselectivity lithium carbenoids

Chelation is another driving force that provides diastereoselective bromine-lithium exchange reactions to give cyclopropyl carbenoids. Thus, the exo-bromine atom in dibro-mocyclopropane 25 is exchanged exclusively due to the methoxy substituent, which encourages the lithium to occupy the cis orientation (equation 16) ° Several representative examples of cyclopropyl bromo lithium carbenoids obtained by bromine-lithium exchange reactions are given in Table 1. 

Whereas the halogen-lithium exchange is of limited importance for the generation of a-lithiated ethers, the reductive lithiation of 0/S-acetals has been applied more frequently, the versatility being enhanced by remarkable diastereoselective variants. Thus, a single diastereomer of the lithium carbenoid 52 results from the diastereomeric mixture 51 (equation 34) . Representative examples of a-lithiated ethers generated by this method and their reactions with electrophiles are given in Table 4. 

The dibromoalkene S-40 can be prepared from S-ethyl lactate by introduction of the MEM (methoxyethoxymethyl) protecting group, reduction to the O-protected lactaldehyde and Corey-Fuchs carbonyl olefination (Scheme 19). The l -enantiomer of 40 is available analogously from f -isobutyl lactate and serves as the reagent in the enantiomeric series. The lithium carbenoid S-41 is generated from S-40 by treatment with n-butyllithium in diethyl ether and reacted with aliphatic and aromatic aldehydes in tetrahydrofuran. High diastereoselectivities are reached, as shown in Scheme 19 . ... 

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