Bicyclo[3.3.0]octanone and bicyclo[4.3.0]nonanone derivatives with a cyclopropane unit in the alpha-position and an unsaturated side chain in the gamma-position of the carbonyl group undergo fragmentation-cyclization processes leading to quasi-triquinane systems upon reductive photoinduced electron transfer (PET). For example, the new angular triquinane derivative 6 and the new propellane derivative 12 were synthesized in one step from these starting materials in moderate to good yields. (C) 2003 Elsevier Ltd. All rights reserved.