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 c onformational flexibility of the oxamethylene bridge with local C-s sy mmetry is evident from low temperature NMR experiments. Experimentally , an activation enthalpy of 11.4 kcal/mol is found for this process, w hich is in good agreement with the rotational barrier derived from sem iempirical AM1 calculations (10.2 kcal/mol). Ab initio Hartree-Fock ge ometry optimizations have been performed for 3 and the corresponding h ydrocarbon 9,12-dimethyl [7]paracyclophane 4. The distance of the O-at om to the center of the benzene ring in the structure of 3 is only 2.7 84 Angstrom, i.e., significantly closer than the analogous C4-benzene ring distance in the slightly more strained 4 (3.112 Angstrom). The en antiomeric separation of (+/-3) has been achieved by HPLC and the circ ular dichroism (CD) spec trum is reported. Ab initio all-electron DFT/ SCI calculations of the CD spectrum are in good agreement with the exp erimental data and reveal the importance of p(O-atom) --> pi charge-t ransfer type excited states at relative low energies (6.6 eV, 188 nm) responsible for an intense negative CD band. The benzene type pi --> p i 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 e xperimental CD data the absolute configuration of 3 is assigned as (-) -(S). (C) 1998 Wiley-Liss, Inc.