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DOI: 10.1055/s-2007-992485
Enantioselective [2+2] Cycloaddition with Oxazaborolidine Catalysts
Contributor(s):Mark Lautens, Praew ThansandoteHarvard University, Cambridge, USA
Highly Enantioselective [2+2]-Cycloaddition Reactions Catalyzed by a Chiral Aluminum Bromide Complex
J. Am. Chem. Soc. 2007, 129: 12686-12687
Publication History
Publication Date:
23 January 2008 (online)
Key words
aluminum - oxazaborolidines - enantioselective [2+2] cycloaddition
Significance
The authors have reported the utility of an aluminum oxazaborolidine catalyst developed in their laboratory for the catalytic enantioselective [2+2] cycloaddition of enol ethers with trifluoroethyl acrylate. Two types of enol ethers were tested, with the TIPS-protected enol ethers having the greater endo selectivity compared to the TBS-protected analogues. The products are previously unknown chiral cyclobutanes and can be transformed into enones in four steps.
Comment
Compared to the catalyst reported, the use of oxazaborolidine salts was inferior due to side reactions involving the enol ether. Absolute configurations determined by X-ray diffraction analysis suggested that attack by the vinyl ether occurs at the si-face of the acrylate ester in all cases. This lead the authors to propose an asynchronous mechanism leading to the transition state shown above.