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DOI: 10.1055/s-2007-968725
Quaternary Centers Formation via Alkene Isomerization/Claisen Rearrangement
Contributor(s):Mark Lautens, Frédéric MénardUniversity of Pittsburgh, USA
Stereoselective Olefin Isomerization Leading to Asymmetric Quaternary Carbon Construction
Org. Lett. 2007, 9: 2325-2328
Publication History
Publication Date:
24 July 2007 (online)
Key words
iridium(I) - Claisen rearrangement - [3,3]-sigmatropic rearrangement - quaternary center
Significance
Chiral quaternary centers can be obtained through a one-pot reaction catalyzed by a cationic iridium complex. The reaction is highly regioselective, as simple bis-allylic ethers 1 undergo isomerization only at the most accessible allylic position. A [3,3]-sigmatropic Claisen rearrangement then takes place under the reaction conditions. The reaction tolerated a wide variety of substituents on the acyclic precuror 1. The method benefits from many good methods to prepare enantiopure allylic alcohols. Importantly, it was demonstrated that chiral information can be transfered to the aldehyde 3b without erosion of ee.
Comment
Study of the iridium isomerization step showed that it was stereospecific. Therefore, the vinyl ether 2 required for the [3,3]-pericyclic reaction is formed with stereodefined geometry. It is proposed that the vinyl ether stereochemistry is dictated by A1,3 allylic strain, favoring the η1-complex A. The reaction allows the synthesis of sterically demanding vicinal quaternary and tertiary centers from very simple starting materials. An enantioselective variant where the iridium complex would bear chiral ligands can be imagined.