SPECTROPHOTOMETRIC STUDY OF DIENESTROL PHOTOISOMERIZATION

— Photolysis of synthetic estrogen dienestrol resulted in formation of a 4a,4b‐dihydrophenanthrene, a rare example of a stable intermediate of the classical stilbene → phenanthrene photocyclization. Time‐lapse spectrometric investigation suggested an initial cis‐trans isomerization in the dimethylbuta‐dime moiety of dienestrol followed by a photochemical [1, 51 sigmatropic proton transfer which converted the two cross‐conjugated p ‐hydroxystyrene halves into a vinyl‐stilbene. A subsequent electrocyclic ring‐closure in an electronically excited state generated a cyclic nonaromatic heptenediol. This last photoproduct tautomerized rapidly in the dark to a stable, crystalline, tricyclic pentenedione. The final diketone product is an example of a 4a.4b‐dihydrophenanthrene self‐stabilized by a double ketoenol tautomerism. The three sequential photoisomerizations of dienestrol are made possible by a fortuitous coincidence of steric and orbital symmetry requirements.