A C‐to‐B Atom Swap on Coumarins and Dibenzolactones

Abstract We report a carbon‐to‐boron “C‐to‐B” atom swap reaction to transform readily available coumarins into their isosteric benzoxaborins via a net replacement of the C═O group with a B─OH moiety. These conditions were applied to coumarin natural products and other 6–7‐membered lactones (25 examples, 29%–93%). We leverage this methodology to transform a flat polyaromatic hydrocarbon into three‐dimensional tribenzo[b.d.f]oxepines through a series of atom‐swapping reactions followed by ring expansion via the oxaborin intermediate.