ORGANIC-PHOTOCHEMISTRY .95. ANTENNA-INITIATED PHOTOCHEMISTRY OF DISTAL GROUPS IN POLYFUNCTIONAL STEROIDS - INTRAMOLECULAR SINGLET AND TRIPLET ENERGY-TRANSFER IN 3-ALPHA-(DIMETHYLPHENYLSILOXY)-5-ALPHA-ANDROSTAN-17-ONE AND 3-ALPHA-(DIMETHYLPHENYLSILOXY)-5-ALPHA-ANDROSTANE-11,17-DIONE

Photolysis of 3-alpha-(dimethylphenylsiloxy)-5-alpha-androstane-11,17-dione (1) in acetonitrile with triethylamine, using 266-nm light absorbed by the dimethylphenylsiloxy (DPS) chromophore, leads to reduction of the C17 keto group as the principal photoreaction. This contrasts with the direct photolysis of the ketone moieties with 308-nm light, wherein epimerization of ring D is the major consequence and reduction is minimal. Triplet quenching studies with cis-1,3-pentadiene confirm that the reduction is derived from the C17 keto triplet state, while the epimerization originates with the C17 excited singlet state. Photolysis of 3-alpha-(dimethylphenylsiloxy)-5-alpha-androstan-17-one (3) under similar conditions gives qualitatively similar results but with a higher fraction of its epimer using 266-nm light and complete absence of reduction upon direct ketone excitation. Intramolecular singlet/singlet energy transfer (intra-SSET) from the DPS antenna to the carbonyl groups is demonstrable in both substrates by the reduced fluorescence quantum efficiencies and singlet lifetimes of the DPS group in these steroids. The rates of energy migration are ca. 2 x 10(8) and 29 x 10(8) s-1 for 3 and 1, respectively, reflecting the greater efficiency of transfer to the more proximal C11 keto group. Intramolecular triplet/triplet energy transfer (intra-TTET) is also evidenced in 3, for example, by the triplet-derived reduction chemistry at C17 which is uniquely characteristic of the antenna excitation; a through-bond exchange mechanism is proposed. Additional triplet chemistry observed at C17 in the diketone, 1, is rationalized by a conversion from the singlet to the triplet manifold at the C11 ketone (i.e., C11 acts as a singlet/triplet switch) followed by triplet energy migration from C11 to C17.