Photochemistry in Polyfunctional Steroids: Photorearrangement vs. Photoepimerization. Preparative and Mechanistic Studies

MATÍAS I. QUINDT; GABRIEL F. GOLA; JAVIER A. RAMIREZ; SERGIO M. BONESI

Congreso; XIV Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2019

Recently, we have studied the photochemistry of some 3-acyl estrone1 and as part of our ongoing research program we decided to study the photochemical behavior of some 3-arylsulfonyl- and 3-alkylsulfonyl estrone derivatives. Thus, direct irradiation of 3-sulfonylestrone 1 (see Scheme) with 254 nm in different solvents and under inert atmosphere (Ar) was carried out by means of steady-state conditions. Selective irradiation of the aryl chromophore of sulfonyl compounds underwent the photo-Fries rearrangement, with concomitant homolytic fragmentation of the sulfonic ester group and [1;3]-sulfonyl migration. This pathway afforded the ortho-sulfonyl estrone derivatives 2 and 3 as the main photoproducts. These steroidal photoproducts were obtained for the first time and were isolated by column chromatography and fully characterized by NMR spectroscopy. On the other hand, estrone and the ortho-regioisomer 3 are found to epimerize upon irradiation with 254 nm light through a Norrish type I photoreaction. This reaction involves the triplet excited state of the carbonyl group (C-17) at the D ring of the steroids that is populated from the singlet excited state of the phenolic moiety through an intramolecular singlet-singlet energy transfer process with a high value of ET (ca. 0.96)2. Photosensitization with acetone and chemical quenching with N,N,N,N-tetramethyldiazetinedioxide (TMDO) of the photo-Fries reaction confirmed that the photoreaction took place from the singlet excited state while Norrish type I reaction proceeds efficiently from the triplet excited state. Solvent effects, as well as the nature of the sulfonyl group on the photoreactions, were also studied.