Vinyllithiums acetylenic

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). 

Transmetallations of vinylic tellurides deserve particular attention. These tellurides (prepared by anti-addition of tellurols to acetylenes, see Section 3.16.1.2) exhibit the Z configuration and therefore generate (Z)-vinyUithiums. These results are in sharp contrast to the earlier tin-lithium exchange performed with vinylstannanes (characterized by the E configuration), giving (ii)-vinyllithiums. ... 

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). 

A new approach to /3-alkyl substituted a-methoxy vinyllithiums 540 with Z-configuration involved the stereoselective metallation of a-bromo vinyl ethers 539, prepared from acetylenes 538, with f-BuLi at — 78 °C (Scheme 145)821. These anions react with different electrophiles to give the corresponding vinyl ethers in good yields. The /3-isobutyl substituted derivative as cuprate has been added to an enone in the total synthesis of the anticancer natural product OSW-1822. 

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... 

P. Quayle and co-workers utilized the Dotz benzannulation reaction for the synthesis of diterpenoid quinones." The authors developed a novel synthetic approach to 12-O-methyl royleanone using a simple vinyl chromium carbene complex along with a disubstituted oxygenated acetylene. The bicyclic hydrazone was converted to the corresponding vinyllithium derivative by the Shapiro reaction and then functionalized to give the desired crude Fischer chromium carbene complex. The benzannulation took place in refluxing THF with excellent regioselectivity, and the natural product was obtained in 37% overall yield from the hydrazone. 

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