Chiroptical Properties of Cryptophane-223 and -233 Investigated by ECD, VCD, and ROA Spectroscopy.

Enantiopure cryptophane derivatives 1 and 2, possessing linkers of different nature (ethylenedioxy and propylenedioxy) connecting the two cyclotribenzylenes (CTB) units, were separated by HPLC using chiral stationary phases. X-ray crystallographic structures of the four enantiomers (+)-1, (-)-1, (+)-2, and (-)-2 have been obtained, allowing the unambiguous determination of their absolute configuration (AC) in the solid state. The chiroptical properties of compounds 1 and 2 were determined from polarimetry, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) experiments and were compared to those of cryptophane-A (3) derivative. VCD, ROA and ECD spectra of 1 and 2 were calculated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, respectively, to confirm the AC of the cryptophane derivatives in solution. The (+)-PP and (-)-MM configurations were established for compounds 1 and 2 in chloroform solution, as already reported for the two enantiomers of 3. This result is in agreement with the X-ray structures of the two enantiomers of 1 and 2.