Studies on the base-promoted conversion of conjugated alkynyl esters to alpha-substituted alpha-allenyl esters.

We report the development of an efficient method for the conversion of a variety of conjugated alkynyl esters to alpha-substituted conjugated allenyl esters (racemic) through the use of strong amide bases. Substantially improved yields over typical enolate formation conditions were observed with the use of 2 equiv of lithium diisopropylamide. Trapping studies indicate that the second equivalent of base likely leads to the dianion intermediate, which upon addition of methyl iodide, trimethylsilyl chloride, or tributyltin chloride gives mixtures of alpha-substituted conjugated allenyl and beta,gamma-alkynyl deconjugated esters. Further optimization revealed that additive salts such as LiCl lead primarily to the allenyl product while the use of HMPA as a cosolvent gives the beta,gamma-alkynyl deconjugated alkylation product. The role of base, base concentration, and electrophile on product yield and selectivity is also discussed.