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

[GRAPHICS] The photolysis of 17 alpha-bromo-3 alpha-(triphenylsilyloxy)-5 alpha-andros tane (2; 3 alpha TPSO/17 alpha Br) and 17 alpha-bromo-3 alpha-(triphenylsil yloxy)-5 alpha-androstan-6- one (3; 3 alpha TPSO/6ketone/17 alpha Br) is de scribed. Irradiation of 2 with 266 nm light leads to debromination via intr amolecular transfer of triplet excitation energy with a quantum efficiency of 0.0011, Photolysis of 3 with both 266 and 308 nm light leads to debromin ation with quantum efficiencies of ca. 0.0066, The debromination of 3 is at tributed to activation via the ketone excited singlet state, with singlet e nergy transfer from C6 to C17 ca, 35% efficient and occurring with a rate c onstant of 1.4 x 10(8) s(-1).