Synthesis, structure, and chiroptical properties of the first 4‐Oxa[7]Paracyclophane

The synthesis of the chiral 9,12‐dimethyl‐4‐oxa[7]paracyclophane 3 was achieved by the dithia route with subsequent sulfone pyrolysis. The conformational flexibility of the oxamethylene bridge with local C s symmetry is evident from low temperature NMR experiments. Experimentally, an activation enthalpy of 11.4 kcal/mol is found for this process, which is in good agreement with the rotational barrier derived from semiempirical AM1 calculations (10.2 kcal/mol). Ab initio Hartree‐Fock geometry optimizations have been performed for 3 and the corresponding hydrocarbon 9,12‐dimethyl[7]paracyclophane 4. The distance of the O‐atom to the center of the benzene ring in the structure of 3 is only 2.784 Å, i.e., significantly closer than the analogous C4‐benzene ring distance in the slightly more strained 4 (3.112 Å). The enantiomeric separation of (±3) has been achieved by HPLC and the circular dichroism (CD) spectrum is reported. Ab initio all‐electron DFT/SCI calculations of the CD spectrum are in good agreement with the experimental data and reveal the importance of p(O‐atom)→ π* charge‐transfer type excited states at relative low energies (6.6 eV, 188 nm) responsible for an intense negative CD band. The benzene type π → π* states are energetically lowered by 0.4–0.8 eV due to the boat‐type deformation of the benzene moiety. By comparison of theoretical and experimental CD data the absolute configuration of 3 is assigned as (−)‐(S). Chirality 10:147–153, 1998. © 1998 Wiley‐Liss, Inc.