Cycloadditions go with the crowd
A new mechanism for the control of cycloaddition stereochemistry by a well known chiral auxiliary has been discovered. The findings may prompt organic chemists to rethink their synthetic routes to achieve the product they want.
Cycloadditions are important routes for making organic ring compounds, which appear in a variety of natural products. Chiral auxiliaries, such as the widely used Evans’ oxazolidinone, are often temporarily added to syntheses to direct the stereochemistry. In (4+3) cycloadditions of oxyallyls to furans, the oxazolidinone auxiliary is thought to block one face of the oxyallyl substrate, forcing the furan to add to the less-crowded face.
But when the oxazolidinone has an aryl substituent in place of the more common isopropyl group, the cycloaddition occurs on the more crowded face. The unexpected stereoselectivity results from an attractive interaction (known as a CH–pi interaction) between the furan and the aryl group.
Other oxazolidinones will also promote the opposite selectivities than expected, the authors predict with the help of computational calculations. Further examples of stereocontrol via CH–pi interactions are expected to be reported in the future.
Original publication: Elizabeth H. Krenske, K. N. Houk, Andrew G. Lohse, Jennifer E. Antoline and Richard P. Hsung, Chem. Sci. 2010.
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