Being able to control the stereochemistry of organic reactions is important when making certain molecules, such as drug candidates or natural products. Although photochemistry allows chemists to create structural elements that are difficult to access in other ways, such as cyclobutanes, it can be tough to do photochemical reactions enantioselectively. Chemists at the University of Wisconsin, Madison, led by Tehshik P. Yoon now report an asymmetric [2 + 2] photocycloaddition of 2′-hydroxychalcones (Science 2016, DOI: 10.1126/science.aai8228). The reaction uses tris(bipyridyl) ruthenium(II) as a sensitizer and a chiral Lewis acid to catalyze the reaction (example shown). The chiral Lewis acid serves to dramatically lower the triplet energy state of the chalcone substrate. “This is an energy-transfer reaction instead of a photoredox reaction,” Yoon explains. The Wisconsin chemists expect this approach will help others apply a “detailed understanding of chiral Lewis acid catalysis to stereocontrol in reactions involving electronically excited states,” they write in their report.