Carbolithiation rearrangement

The tandem carbolithiation-rearrangement of silyl derivatives constitutes also a useful synthetic methodology. The carbon—carbon bond formation is triggered by anionic rearrangement of a silyl group from carbon to oxygen and constitutes a new methodology for... 

Taylor and colleagues have shown the sequential carbolithiation/[l,4]-Wittig rearrangement of styryl ethers 105 (e.g., 105a) which provide appropriately functionalized 2-alkylphenol 106 (e.g., 106a) in good yield . In this reaction, the alkenyl transfer occurs with retention of configuration (equations 59 and 60). 

The preparation of pyrrolidines by carbolithiation reaction of /V-homoally] a-amino-organolithium compounds has been extensively studied in the past ten years by Coldham and coworkers, who have used the tin-lithium exchange method for the generation of the organolithiums. For instance, aminomethylstannane 198 gives rise to pyrrolidine 200 probably by way of the lithiomethyl intermediate 199. The overall transformation of 198 into 200 is a rearrangement and can be promoted with only 0.2 eq of MeLi in the presence of SnMe4 even in better yield than the stoichiometric case (Scheme 54)92. 

Although the synthesis of dihydrobenzofuran derivatives seems to be not possible by this anionic cyclization methodology, there are some particular examples in which these heterocycles are prepared by an intramolecular carbolithiation reaction. In this respect Baldwin and coworkers described in 1980 the preparation of 350 by rearrangement of 349 when it was treated with BuLi in THF/TMEDA (Scheme 91)151. The most likely explanation starts with an orf/zo-lithiation giving a dilithium intermediate, which undergoes an intramolecular 5-exo carbolithiation reaction affording a 3-lithiomethyldihydrobenzofuran... 

On the other hand, the cyclization reaction of a vinyllithium onto an acetylenic unit provides an efficient route to five- and six-membered bis-exocyclic 1,3-dienes, which react stereoselectively with a wide range of dienophiles157. The 5-exo carbolithiation reaction of vinyllithiums 369, derived from the corresponding vinyl bromides, is syw-stereospecific giving, after hydrolysis, the /(-isomer of five-membered outer-ring dienes 370 and tolerates aryl-, silyl- or alkyl-substituents at the distal acetylenic carbon (Scheme 97). However, the alkyl-substituted alkynes are far more resistant to rearrangement than the aryl- or silyl-substrates and the addition of TMEDA and longer reaction times are needed for the latter... 

Wittig rearrangements [66],but also to perform intramolecular carbolithi-ations on a terminal C=C bond, or an internal C=C bond bearing an allylic leaving group [55d, 67] (Scheme 40). A trans-2,3-disubstituted tetrahydrofuran is formed with a total retention of configuration. 

S -Alkenyl-A-arylthiocarbamates have been obtained from allylic alcohols by sigmat-ropic rearrangement and isomerization or C=C bond cleavage. Carbolithiation generates... 

---