Antenna-Initiated Photochemistry in Polyfunctional Steroids. Intramolecular Singlet and Triplet Energy Transfer between Aryl, Ketone, and Alkene Groups in 6beta-(Dimethylphenylsiloxy)-5beta-androstanes(1).
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)-6beta-(DPSO)-5beta-androstan-17-one (6) and 6beta-(DPSO)-5beta-androstan-3-one (8) both exhibit DPSO-initiated photochemistry at the carbonyl groups. Irradiation of the diketone, 6beta-(DPSO)-5beta-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 Phi(intra-SSET) from the aryl antenna to the carbonyl groups is ca. 88% efficients and occurs with a rate of ca. 6.5 x 10(9) 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 6beta-(DPSO)-3(E)-ethylidene-5beta-androstan-17-one (33) and its Z isomer (34)) has no effect on the rate or efficiency of aryl activation of the C17 ketone.
Jiang
S. Anna
SA
Department of Chemistry, Brown Building, Purdue University, West Lafayette, Indiana 47907-1393.
Xiao
Changhe
C
Morrison
Harry
H