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 structure of the 1,4-dimethyl derivative 1 was determined by X-ray crystallography. The triptycene part was found to be nearly symmetrical with almost planar benzene rings, and the ester group was rotated away from the 1-methyl group. The structure of the global energy minimum calculated by the empirical force-field MM2-87 was similar to the structure in the crystal. The CD spectra of the first eluted enantiomers were recorded and compared with spectra calculated by a semiempirical method, using monopolar transition charges from CNDO/S calculations. The calculations show that rotational strengths of the right order of magnitude may arise through interactions between the n --> pi* and pi --> pi* transitions in the ester group and the pi --> pi* transitions in the benzene rings. However, the calculated spectra show only partial resemblance with the experimental ones, and no safe conclusions regarding absolute configurations are possible.