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Abstract

The type II photochemistry of nine diametric cyclic diketones (ten, twelve, fourteen, sixteen, eighteen, twenty, twenty-two, twenty-four and twenty-six membered rings), two non-diametric cyclic diketones (sixteen and seventeen membered rings) and two cyclic keto-alcohols (sixteen and eighteen membered rings) was studied in both solid and solution media in order to investigate the relationship between conformation and photo reactivity. Upon irradiation, most compounds investigated underwent smooth γ-hydrogen abstraction. The stereoelectronic requirements for the γ-hydrogen atom abstraction and the reactivity differences observed in the solid and solution reactions have been investigated with the help of X-ray crystallography and molecular mechanics calculations. Stereo selective cyclization was observed during the solid state photoreactions of all diketones investigated, with the exception of the ten, fourteen and seventeen membered ring compounds. The stereochemistry of the major solid state product correlated well with the hydrogen abstraction geometry found in the solid state conformation. Upon irradiation in solution, in rings larger than fourteen membered, the stereoselectivity is largely lost, but a slight preference for "trans" over "cis" cyclobutanol formation together with fairly large amounts of cleavage product formation were observed. In smaller rings, however, the product distributions in solution were quite similar to those observed in the solid state. In the above series of diketones, two groups of γ-hydrogen atoms can be distinguished with respect to their stereoelectronic dispositions in the solid state conformations. The γ-hydrogens having close O‧‧‧H contacts (