Acetylenic vinyllithiums cyclization

By simple incorporation of a leaving group at the distal propargylic position of the acetylenic vinyllithium, the cyclization methodology described was slightly modified to allow preparation of otherwise relatively inaccessible exocyclic, conjugated allenenes (s-annulated 1,2,4-trienes)108. Thus, the vinyllithium 142 generated from bromide 141 cyclizes via 143 to afford the five-membered exocyclic allenene 144 in 97% isolated yield (Scheme 46). 

An extension of this intramolecular carbolithiation was recently disclosed for the preparation of bis-exocyclic 1,3-dienes by the 5-exo-dig cyclization of (5-acetylenic vinyllithiums. Through subsequent Diels-Alder chemistry, they serve as useful precursors of polycyclic ring systems [14a] (Scheme 7-13). 

Although the chemistry of anionic cyclizations of organolithium moieties derived from acetylenic systems has been less investigated38, studies of some acetylenic alkyllithiums, e.g. 131 (formed from 130), have shown that the ring closure proceeds in a regiospecific and highly stereoselective syw-fashion to give exocyclic vinyllithiums 132 (Scheme 43). 

C. Cyclization Reaction of Acetylenic Allenyl Lithiums, Vinyllithiums and... 

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... 

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