Halogen-lithium exchange diastereoselective

Halogen-lithium exchange of iodide 71 and subsequent addition of 2-acetyl-furan (72) to the resultant organolithium intermediate yielded two diastereomeric tertiary alcohols (dr=l l), which were converted to (E)-olefin 73 with complete diastereoselectivity upon brief exposure to catalytic amounts of concentrated aqueous hydrogen chloride (Scheme 11) [18]. Diastereoselective hydroboration/oxidation of 73 gave largely the desired stereoisomer 74 due to... 

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. 

A single diastereoisomer was also obtained by Ireland in the same context but using a chiral lithium derivative obtained from 216 by metal-halogen exchange [155]. However, the presence of magnesium bromide is needed to achieve up to 20 1 diastereoselectivity. 

An asymmetric lithium halogen exchange is possible by the reaction of metalated bislactim ether with electrophiles. The diastereoselectivity of this transformation is high in many cases176 and the resulting chlorides are useful synthons for chiral cationic amino acids. 

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