Long-range through-bond photoactivated sigma bond cleavage in steroids. Intramolecular sensitized debromination

[structure: see text] The photolysis of 17alpha-bromo-3alpha-(triphenylsilyloxy)-5alpha-androstane (2; 3alphaTPSO/17alphaBr) and 17alpha-bromo-3alpha-(triphenysilyloxy)-5-androstan-6-one (3; 3alphaTPSO/6ketone/17alphaBr) is described. Irradiation of 2 with 266 nm light leads to debromination via intramolecular transfer of triplet excitation energy with a quantum efficiency of 0.0011. Photolysis of 3 with both 266 and 308 nm light leads to debromination with quantum efficiencies of ca. 0.0066. The debromination of 3 is attributed to activation via the ketone excited singlet state, with singlet energy transfer from C6 to C17 ca. 35% efficient and occurring with a rate constant of 1.4 x 10(8) s(-1).