Exciton coupling in geometrically well-defined spiro compounds - Comparison of theoretical and experimental CD-spectra

The conformationally rigid spirobilactone 1, obtained by acid-catalyzed cyc lization of o,o'-dicarboxy benzophenone, has been resolved into enantiomers by semipreparative, chiral stationary phase mediated, liquid chromatograph ic separation. The first eluted enantiomer, which showed positive optical r otation at 302 nm, gave a strong negative CD band at 207 nm and did not dis play the couplet around 230 nm expected from the usual exciton coupling pat tern for a pair of benzoate chromophores. The CD-spectrum of the second elu ted enantiomer was the expected mirror image of the first. The calculated C D-spectrum for (R)-1 was found to be in good qualitative agreement with the one found experimentally for the second eluted enantiomer. The calculations showed that the geometry of I gives an orientation of the transition moments for the L-1(a) and B-1(b) transitions that promotes a st rong interaction between them which will dominate over the coupling between the L-1(a) transitions. The sulfur analog 2, which has a trigonal bipyrami dal geometry and exhibits C-2- symmetry, shows a distinct exciton coupling centered at 220 nm. A spectral comparison with a structurally similar compo und of known absolute configuration shows (+)-2 to possess the S-configurat ion.