Enantiomer resolution, barrier to ring inversion, circular dichroism spectrum and absolute configuration of cycloocta[2,1-b: 3,4-b′]dipyridine

Abstract

The chiral cycloocta[2,1-b:3,4-b′]dipyridine 3b has been cleanly resolved into enantiomers by chromatography on swollen, microcrystalline triacetylcellulose, using ethanol as the mobile phase. A trace of ammonia had to be added to prevent acid-catalysed racemization. The free energy barrier to ring inversion was found to be close to 25 kcal mol^–1, and rather insensitive to the temperature. The experimental CD spectrum could be reasonably well simulated by CNDO/S calculations, indicating that the first eluted enantiomer has the R configuration. According to the calculations, the long-wavelength CD band has no clear n→π* character.