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Synthesis, Resolution, and VCD Analysis of an Enantiopure Diazaoxatricornan Derivative

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journal contribution
posted on 21.05.2008, 00:00 by Pierre Mobian, Cyril Nicolas, Eric Francotte, Thomas Bürgi, Jérôme Lacour
Using simple organic synthetic transformations, a novel diazaoxatricornan derivative, the 12c-methyl-12-phenyl-8-propyl-12,12c-dihydro-8H-4-oxa-8,12-diazadibenzo[cd,mn]pyrene (6a), was prepared. This novel chiral cup-shaped molecule was isolated in racemic form and in excellent yield after the addition of methyl lithium to the BF4 salt of a novel unsymmetrical diazaoxatriangulenium cation. Compound 6a was found to be stable under classical laboratory conditions—something not obvious considering the extreme stability of the carbenium ion precursor, the electron-rich nature of the core, and the strain induced by the pyramidalization of the central carbon. The enantiomers were readily separated by chiral stationary phase chromatography, and the absolute configuration of (−)-(S)-6a was determined by a comparison of the experimental and theoretical vibrational circular dichroism (VCD) spectra. This isolation of (−)-(S)-6a and (+)-(R)-6a constitutes thus the first report of a nonracemic closed-capped chiral bowl molecule for which the chirality is due to the intrinsic dissymmetry of the central core of the structure only.