ANTENNA-INITIATED PHOTOCHEMISTRY IN POLYFUNCTIONAL STEROIDS - INTRAMOLECULAR SINGLET AND TRIPLET ENERGY-TRANSFER BETWEEN ARYL, KETONE, AND ALKENE GROUPS IN 6-BETA-(DIMETHYLPHENYLSILOXY)-5-BETA-ANDROSTANES

Steroids have been prepared that bear a dimethylphenylsiloxy (DPSO) gr oup and additional C3 and/or C17 ketone functionalities. The DPSO grou p has been used to harvest 266 nm photons and then activate the ketone functionalities through intramolecular singlet-singlet energy transfe r (intra-SSET). Thus, the monoketones 3,3-(ethylenedioxy)-6 beta-(DPSO )-5 beta-androstan-17-one (6) and 6 beta-(DPSO)-5 beta-androstan-3-one (8) both exhibit DPSO-initiated photochemistry at the carbonyl groups . Irradiation of the diketone, 6 beta-(DPSO)-5 beta-androstane-3,17-di one (5), gives two ring D-derived photoproducts, an epimer (19) and an enal (18), both coming from the C17 ketone excited singlet state. Her e 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 t he chemistry indicative of facile intra-SSET between the C3 and C17 ke tones. The alkylidene group at C3 (i.e., as in 6 beta-(DPSO)-3(E)-ethy lidene-5 beta-androstan-17-one (33) and its Z isomer (34)) has no effe ct on the rate or efficiency of aryl activation of the C17 ketone.