Synthesis, crystal structure, and circular dichroism spectra of (1S)-4,8-diphenylbarbaralane-2,6-dicarbonitrile - Chiroptical properties of the transition state of a degenerate cope rearrangement

Diphenylbicyclo[3.3.1]nonane-2,6-dione rac-3 is resolved in 57 % overall yi eld by chromatographic separation of the diastereomeric (R)-N-(1-phenylethy l)carbamates 9 which are obtained from (R)-(1-phenylethyl) isocyanate (8) a nd the 6-hydroxydiphenylbicyclo[3.3.1]nonan-2-ones endo- and exo-4. The ena ntiomers (1R)- (e.r. = 98:2) and (1S)-3 (e.r. = 97:3) are regenerated from 9 by reduction with lithium aluminium hydride followed by Swern oxidation o f the resulting diols 5. The title compound (1S)-1 is synthesised in three steps from (15)-3 in improved yield on the route that had led to rac-1. The absolute configurations are established by X-ray diffraction analyses of t he carbamates endo-(1R)-9 and exo(15)-9. X-ray diffraction analyses were al so performed of the camphanoate (1R)-7, the intermediates rac-endo-4 and (1 S)-3, and the title compound (1S)-1. Hydroxy ketone rac-endo-4 adopts simil ar conformations in the solid state and in solution as shown by a compariso n of vicinal H-1,H-1 coupling constants from proton spectra with those calc ulated from torsional angles in the crystal. The molecular structures of (1 S)-1 and (1S)-3 closely resemble those of the corresponding racemates inves tigated previously. These results show (i) that intermolecular interactions in the solid state are of minor importance and (ii) that the unusually lon g C2-C8 distance of (1S)-1 and rac-1 (168 pm) is a molecular but not an ave raged property due to a non-degenerate Cope rearrangement in the crystal, C D spectra are reported for (1R)- and (1S)-3, the unsaturated dicarbonitrile (1S)-13, and (1S)-1. The CD spectrum of (1S)-1 exhibits a weak positive ba nd at 459 nm where rac-1 shows a temperature-dependent absorption which has been assigned to the higher, vibronic state represented by rac-1*. The int ensity of the weak CD band depends on the temperature and the solvents in t he same way as the UV/Vis absorption of rac-1. This supports the conclusion that both bands originate from the same source, viz. the transition state 1* of the degenerate Cope rearrangement 1 reversible arrow 1'.