Lithium-halogen exchanges tert-butyllithium

Simple proton abstraction is more difficult in the case of alkenyl substrates (p/iTa 36, but see Scheme 1.110) and a more-common method for the formation of alkenyllithium (vinyllithium) species is halogen-lithium exchange with n- or tert-butyllithium. The exchange reaction is stereospecific, such that the stereochemistry of the starting alkenyl halide determines the stereochemistry of the product alkenyllithium species (1.114). Subsequent reaction with electrophiles also occurs with retention of configuration and this of course is important in order to prepare stereodefined alkenyl products. 

In view of three excellent recent reviews on the Parham cyclization, " this section focuses mainly on the more recent literature. While -butyllithium and tert-butyllithium are commonly used to effect halogen-lithium exchange, these reagents do present problems as delineated earlier. Mesityllithium does not pose such problems since the resulting mesityl halides are not susceptible to the side reactions inherent with -butyl and tert-butyl halides. Mesityllithium has been used to prepare the camptothecin precursor 28. This base can also be used with added electrophiles, as for the conversion of 29 to phthalides 30. A more conventional Parham synthesis of phthalides, i.e., 31—>32, has been applied to the synthesis of the aristocularine alkaloids. ... 

---