Aldehydes as Photoremovable Directing Groups: Synthesis of Pyrazoles by a Photocatalyzed [3+2] Cycloaddition/Norrish Type Fragmentation Sequence

A straightforward methodology for the regioselective synthesis of pyrazoles has been developed by a domino sequence based on photoclick cycloaddition followed photocatalyzed oxidative deformylation reaction. Distinguishing features this protocol include an unprecedented photoredox-catalyzed Norrish type fragmentation under green-light irradiation and use α,β-unsaturated aldehydes as synthetic equivalents alkynes, where aldehyde is acting novel photoremovable directing group.