Unusual Chiroptical Properties of the Cryptophane-222 Skeleton

Enantiopure
cryptophane-222 derivative (1) devoid
of substituents was obtained via high-performance liquid chromatography
(HPLC) using chiral stationary phases. The chiroptical properties
of 1 were determined from polarimetry, electronic circular
dichroism (ECD), synchrotron radiation circular dichroism (SRCD),
vibrational circular dichroism (VCD), and Raman optical activity (ROA)
experiments and were compared to those of the cryptophane-A (2) derivative. Unusual polarimetric results were obtained
for 1 in CHCl3 solvent as the sign of the
optical rotation (OR) values changes in the nonresonance region above
365 nm, whereas no change was observed in the CH2Cl2 solvent. ECD spectra in the 1La and 1Lb regions were very similar for the two solutions
and could not explain these unusual polarimetric properties. In contrast,
SRCD spectra in the 1Bb region revealed spectral
differences for the two solutions, which have been associated with
conformational changes of the three linkers by time-dependent density
functional theory (TDDFT) calculations. DFT calculations of the OR
support that conformational changes may explain the polarimetric results
obtained for the two solvents. Finally, TDDFT calculations of the
ECD as well as DFT calculations of the VCD and ROA allowed the attribution
of the (−)589-PP absolute configuration
for 1 in solution, as determined from the X-ray structures
of 1.