CHIRAL TRIPTYCENES OWING THEIR CHIRALITY TO HINDERED ROTATION OF A BRIDGEHEAD SUBSTITUENT - CRYSTAL-STRUCTURE, CHROMATOGRAPHIC ENANTIOMER RESOLUTION, AND CIRCULAR-DICHROISM SPECTRA

The complete enantiomer separation of the sc forms of two methyl 3-(1, 4-disubstituted 9-triptycenyl)-3-methylbutanoates by chromatography on swollen microcrystalline triacetylcellulose is described. The structu re of the 1,4-dimethyl derivative 1 was determined by X-ray crystallog raphy. The triptycene part was found to be nearly symmetrical with alm ost planar benzene rings, and the ester group was rotated away from th e 1-methyl group. The structure of the global energy minimum calculate d by the empirical force-field MM2-87 was similar to the structure in the crystal. The CD spectra of the first eluted enantiomers were recor ded and compared with spectra calculated by a semiempirical method, us ing monopolar transition charges from CNDO/S calculations. The calcula tions show that rotational strengths of the right order of magnitude m ay arise through interactions between the n --> pi and pi --> pi* tra nsitions in the ester group and the pi --> pi transitions in the benz ene rings. However, the calculated spectra show only partial resemblan ce with the experimental ones, and no safe conclusions regarding absol ute configurations are possible.