ORGANIC-PHOTOCHEMISTRY .97. ANTENNA-INITIATED PHOTOCHEMISTRY IN POLYFUNCTIONAL STEROIDS - PHOTOEPIMERIZATION OF 3-ALPHA-(DIMETHYLPHENYLSILOXY)-5-ALPHA-ANDROSTANE-6,17-DIONE AND ITS 3-BETA-ISOMER BY THROUGH-BOND EXCHANGE ENERGY-TRANSFER

We have previously reported that photolysis of 3-alpha-(dimethylphenylsiloxy-5-alpha-androstane-11,17-dione, in the presence of triethylamine (TEA) and with 266-nm light so as to selectively excite the dimethylphenylsiloxy (DPS) "antenna", primarily gives rise to triplet-derived reduction at C17. We now report on an analogous set of steroids, with a DPS antenna-alpha or beta at C3 and carbonyl groups at C6 and C17 (the title compounds, 3 and 4). In these compounds, intramolecular singlet/singlet energy transfer (intra-SSET) from the DPS group to C6 is rapid (k(intra-SSET) = ca. 3 x 10(9) s-1) and efficient (phi(intra-SSET) ca. 76%), but this energy transfer is followed by a ca. 85% efficient intra-SSET from C6 to C17. The result is exclusively singlet chemistry (e.g., chemistry resulting from alpha-cleavage) at C17 (with 60 mM TEA: 3, phi = 0.23; 4, phi = 0.098; eqs 3 and 4). The lower efficiency for the beta-isomer reflects a more than 2-fold less efficient intra-SSET relative to 3-alpha-DPS. There is also a modest amount of triplet-derived reduction at C6 (phi(red) = 0.046 at 60 mM TEA) with a 3-alpha-DPS antenna, but no such reduction is observed when the DPS group is beta. Through-bond-mediated exchange energy transfer is proposed for both the DPS --> C6 and the C6 --> C17 intra-SSET.