Small and Medium Rings, 75. Syntheses, Photoelectron Spectra, and Photoreactivity of Polycyclic 1,5‐Diketones: Transannular Interaction in the Cyclooctane‐1,5‐dione Fragment

Five polycyclic 1,5‐diketones 1 – 4 and 6 are synthesized and studied by means of He(Iα) PE spectroscopy. Comparison of the spectra indicates significant differences in Orbital Interaction Through Bond, which is explained by computational methods, especially by analyses of the precanonical MOs. The lone pair splitting observed in diketone 1, ΔI(n)=0.75 eV, seems to be the largest one ever found in a 1,5‐diketone, obviously due to a favourable zigzag topology of the σ frame. In addition, coupling reactions and photoreactivity are studied in compound 6, which seemed to be well suited for Orbital Interaction Through Space. The crystal structure of 6 and its photoproduct 21 is solved by X‐ray crystallography.