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Synthesis

Enantioselective Photocycloadditions

Chiral Lewis acid guides enone in intramolecular reaction

by Bethany Halford
November 18, 2013 | APPEARED IN VOLUME 91, ISSUE 46

The light-activated cycloaddition reaction that weds the C=C bond of an enone with another alkene group to create a four-membered ring has been around for more than 100 years. Despite its age, chemists have found it challenging to create an enantioselective version of this transformation. In the past, chiral auxiliaries—chemical compounds that are temporarily added to the reactants—have been used, but attaching and removing these units require extra steps. Now, Thorsten Bach and Richard Brimioulle of Technical University of Munich have found a simpler way to guide the stereochemical outcome of this reaction (Science 2013, 10.1126/science.1244809). In the presence of a chiral Lewis acid and UV light, 5,6-dihydro-4-pyridones cleanly undergo enantioselective intramolecular cycloadditions (shown). Reported yields are fair to good, with enantiomeric excesses of 82–90%. The researchers used the reaction to synthesize two natural products, (+)-lupinine and (+)-thermopsine. This strategy may provide a general approach to similar enantioselective cycloadditions of enones, the researchers say.

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