In the tricyclooctanone approach to polyquinane syntheses[ll the photochemical rearrangement of bicyclo[2.2.2]octenone, 1a 2a, [2J has been the initial key reaction providing tricyclo- 2 [3.3.0.0 ' S]octan-3-one as a chiral building block.[3-SJ This reaction - an oxadi-n-methane (ODPM) process - and the 1,3-acyl shift (AS) forming the cyclobutanone 3 are quite generally the characteristic re- arrangement paths undergone by, -unsaturated ketones (, -UKs) upon triplet sensitization and direct excitation with ultraviolet light, respectively. R' R' hv hv R O (sens) WO R ONR R 1a-c 2a-c 3a-c a R=R'=H b R = CH, R'= H 3 c R = R'= CH 3 As a polyvalent building block, the enantiomers of which are readily accessible from cheap bulk chemicals such as benzene and its derivatives, the parent system 1a and three younger generations of its class (Sections 3.1. - 3.4.) have no equal among synthetically useful photochemical rearrangements. 2. THE PHOTOCHEMISTRY OF, -UNSATURATED KETONES The literature dealing with the spectroscopy and photo- chemistry of, -UKs has been exhaustively and critically reviewed around 1975 by Dauben et al.[6l and Houk, [7J and in 1980 by Schuster.[SlI1 The 1,3-AS was first observed on direct excitation of 1) For more recent papers on i3, -UK photochemistry see [9-24]. 64 R' hv o hv ., .