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

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.