N-Formyl-3,3‘,4,4‘-tetrahydrospiro[naphthalene-1(2H),2‘(1‘H)-pyridine] (2) and N-Formyl-3‘,4‘-dihydrospiro[indan-1,2‘(1‘H)-pyridine] (3) were resolved into enantiomers, compound 3 partly and compound 2 completely, by chromatography on triacetylcellulose. The CD spectra were recorded and compared with theoretical spectra calculated by a semiempirical method, using geometries from empirical force-field calculations. In the observed CD spectra, the carbonyl n → π* transitions did not give rise to visible bands, but their signs and energies were found by analysis of CD spectra in solvents of different polarity. The theoretical spectra of 2 and 3 showed complete agreement with the observed spectra in sign and qualitatively in intensity, for the five transitions observed between 190 and 280 nm, which permitted a safe assignment of the absolute configurations. The enantiomers of 2 and 3 with the aryl rings located on the same side of the tetrahydropyridine ring showed CD spectra, which were nearly mirror images, thus demonstrating the risk of deducing absolute configurations by a direct comparison of CD spectra.

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