Novel pyrromethenes. 1‐Oxygen and 1‐sulfur analogues; evidence for photochemical <i>Z‐E</i> isomerization

Abstract

Abstract Pyrromethenes [2‐(2‐pyrrylmethylene)‐2 H ‐pyrroles] and 1‐oxygen and 1‐sulfur analogues with a minimum number of alkyl substituents can be synthesized under very mild conditions. Even the parent unsubstituted pyrromethene system can be prepared this way. After synthesis in the dark a Z ‐configuration was established using N.O.E. measurements. A photochemical Z—E isomerization was detected (PMR spectroscopy) for derivates lacking an intramolecular hydrogen bridge.