Antenna-Initiated Photochemistry in Polyfunctional Steroids. Intramolecular Singlet and Triplet Energy Transfer between Aryl, Ketone, and Alkene Groups in 6β-(Dimethylphenylsiloxy)-5β-androstanes1

Steroids have been prepared that bear a dimethylphenylsiloxy (DPSO) group and additional C3 and/or C17 ketone functionalities. The DPSO group has been used to harvest 266 nm photons and then activate the ketone functionalities through intramolecular singlet−singlet energy transfer (intra-SSET). Thus, the monoketones 3,3-(ethylenedioxy)-6β-(DPSO)-5β-androstan-17-one (6) and 6β-(DPSO)-5β-androstan-3-one (8) both exhibit DPSO-initiated photochemistry at the carbonyl groups. Irradiation of the diketone, 6β-(DPSO)-5β-androstane-3,17−dione (5), gives two ring D-derived photoproducts, an epimer (19) and an enal (18), both coming from the C17 ketone excited singlet state. Here Φintra-SSET from the aryl antenna to the carbonyl groups is ca. 88% efficients and occurs with a rate of ca. 6.5 × 109 s-1, with the chemistry indicative of facile intra-SSET between the C3 and C17 ketones. The alkylidene group at C3 (i.e., as in 6β-(DPSO)-3(E)-ethylidene-5β-androstan-17-one (33) and its Z isomer (34)) has no effect on the rate or efficiency of aryl activation of the C17 ketone.