Synthesis of Hemiindigo Derivatives via a Gold‐Catalyzed Intramolecular Cyclization Strategy

We described a gold‐catalyzed synthesis of Z‐hemiindigo frameworks from ortho‐propynol phenyl azides via the β‐hydroxy‐α‐imino gold carbene intermediates. The intermediates are trapped by water or acetone and then subsequently undergo enol tautomerization and dehydration, or release of acetone. The reaction has the advantage of being able to be carried out under mild conditions and in a gram‐scale. Importantly, this method enables a simple and efficient synthesis of the special hemiindigo derivatives for the development of environmentally sensitive photoswitches in biological systems. Hemiindigos, important indigo derivatives, could be synthesized through the gold‐catalyzed cyclization from ortho‐propynol phenyl azides via the β‐hydroxy‐α‐imino gold carbene intermediates.