Photocyclization of diethylstilbestrol. Isolation of a stable, self-trapping dihydrophenanthrene intermediate

Near-ultraviolet irradiation of the synthetic estrogen diethylstilbestrol, a stilbene derivative, resulted in the isolation of the first stable 4a,4b-dihydrophenanthrene. The stability of this DHP is the result of a double tautomerism of the initially formed dienol to a diketone structure in which the tendency to rearomatization is lost. Furthermore, the reverse ring-opening photoreaction may be prevented by the conversion of the prerequisite ..pi..,..pi..* character of the lowest excited singlet to an n,..pi..* state in the diketone. NMR and mass spectral data provided direct spectrochemical evidence for the trans configuration of the intact 4a,4b-hydrogen atoms, in agreement with orbital symmetry considerations and with recent chemical proof. Time-lapse spectrometric determinations demonstrated the overall transformation to be free of side reactions and to give quantitative conversion to the DHP without further oxidation to the phenanthrene product characteristic of stilbene photocyclizations. The mechanism involves an initial trans reversible cis photoisomerization, competitive photocyclization of the cis isomer, and a fast dark tautomerization of the dienol DHP to the isolable dihydrophenanthrenedione product.