“…In addition, their benzylidene analogues can serve as a molecular switch based on rhodopsin‐like E / Z photoisomerization [2] . These compounds have been synthesized by intramolecular aza‐Wittig reactions of α ‐azidoethyl‐ α , β ‐enones using Ph 3 P, [1a,2a,b] by cyclization reactions of nitrilium ions (nitrilium cyclization) derived from homoallyl carboxamides, [2c,3a] by metathesis reactions of alkynyl Fischer carbene complexes with imines [2d] and by condensation reactions of dihydropyrroles with aldehydes [1b,4] . As the alternative methods, the cyclization of other acyclic precursors, [3] nitrilium cyclization of 1,3‐diols [5a,b] or vinylidenecyclopropanes [5c] with nitriles, and miscellaneous methods [6] have been known.…”