Building bridges out of azaarenes

Organic chemists tend to swoon at the sight of [4 + 2] cycloadditions, a chemical dance in which a diene and an alkene join via two new bonds to create a six-membered ring. While many dienes are willing to partner with alkenes this way, one source of dienes—aromatic molecules—has been reluctant to engage in such behavior. That’s largely because getting these molecules to give up their aromatic stability requires quite a reactive kick. Now, chemists have coaxed azaarenes, including quinolines, isoquinolines, and quinazolines, to break out of their aromatic ruts and form bridged bicyclic molecules by reacting with alkenes (Science 2021, DOI: 10.1126/science.abg0720). A team led by M. Kevin Brown of Indiana University Bloomington; Kendall N. Houk of the University of California, Los Angeles; and Frank Glorius of the University of Münster reports the transformation, which uses an iridium photosensitizer, blue light, and an acid (example shown). The regio- and diastereoselective reaction forms bridged bicyclic molecules that require multiple steps to make by other methods. The reaction could be useful to medicinal chemists making unique and complex chemical scaffolds.