Synthesis of Highly Reactive Ketenimines via Photochemical Rearrangement of Isoxazoles

[Image: see text] Ketenimines are highly electrophilic species with multiple applications as building blocks in organic synthesis; however, the effective preparation of these versatile entities remains a synthetic challenge. Here we report a continuous photochemical process that generates ketenimines via skeletal rearrangement of trisubstituted isoxazoles. The resulting flow process is noteworthy, as it provides for the first time a straightforward entry into these ketenimines that were previously only observed spectroscopically. The value of this methodology toward heterocyclic transposition reactions is demonstrated by converting isoxazoles via isolable ketenimines into pharmaceutically relevant pyrazoles.