Gezielte Konformationsänderungen an cyclischen Systemen, 3. Mitt.: Konformationsänderungen durch “Non-Bonded Interactions” in [2.2]Metacyclophanen

Changes of the interplanar angle in 2.2]metacyclophanes can be achieved by increasing the steric strain between the axial position at C-1 and H-8 as well as between the equatorial position and H-14 and H-12 resp.: These non bonded interactions are relieved by changing the torsional and bond angles with consequent rotation of one aromatic ring about an axis bisecting C-1 and C-10. This has been demonstrated with the aid of acetals4–10 derived from 1,10-dioxo-2.2]-metacyclophane (13). The conformational changes were deduced from the chemical shift difference between the intraanular protons 8 and 16 in the¹H-NMR spectrum as well as from an increase in allylic coupling of the axial proton at C-2 with H-8. For these investigations the¹H-NMR correlation of the protons of the bridge was indispensable. This problem was solved by deuterium labelling experiments and application of the INDOR-sweep technique on theABX-system of (e,e)-dihydroxy-2.2]metacyclophane (2).