Chiroptical properties of nona- and dodecamethoxy cryptophanes

Enantiopure cryptophane derivatives bearing nine (2, 3) and 12 (4) methoxy substituents attached on the six aromatic rings were separated by HPLC using chiral stationary phases. The chiroptical properties of compounds 2-4 were determined from polarimetry, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) experiments and were compared to those of cryptophane-A (1) derivative. ECD spectra of 1 and 4 were calculated by time-dependent density functional theory (TDDFT) to determine the absolute configuration (AC) of cryptophane derivatives. The (+)-PP absolute configuration was thus established for the anti-cryptophane-A (1) and its congeners 2 and 4. VCD experiments associated with DFT calculations confirmed the (+)-PP configuration of anti-compounds 2 and 4 and established the (+)-PM configuration of the syn-3 compound as well. This study revealed the preferential all-trans (TTT) conformation of the three ethylenedioxy linkers for the CHCl3@1, CHCl3@3, and CHCl3@4 complexes, whereas the GTT conformation was found the most favorable for the CHCl3@2 complex.