Chiroptical study of cryptophanes subjected to self‐encapsulation

In 1,1,2,2‐tetrachloroethane‐ d 2 , the 129 Xe NMR spectrum of the Xe@cryptophane‐223 complex bearing seven acetate groups (Xe@ 1 complex) shows an unusually broad signal compared with that of its congeners (Chapellet, LL. et al. J. Org. Chem . 2015 ;80:6143–6151). To interpret this unexpected behaviour, a 1 H NMR analysis and a thorough study of the chiroptical properties of 1 as a function of the nature of the solvent have been performed. The 1 H NMR spectra of 1 reveal that a self‐encapsulation phenomenon takes place in DMSO‐ d 6 and 1,1,2,2‐tetrachloroethane‐ d 2 solvents. Thanks to the separation of the two enantiomers of 1 by HPLC on chiral stationary phase, the two enantiomers of 1 have been studied in detail by polarimetry, electronic (ECD), and vibrational (VCD) circular dichroism spectroscopies. Except for ECD spectroscopy, these chiroptical techniques reveal spectroscopic changes as a function of the nature of the solvent. For instance, in DMSO and 1,1,2,2‐tetrachloroethane, in which the self‐encapsulation phenomenon takes place, the sign of the specific optical rotation of [CD(−) 254 ]‐ 1 and [CD(+) 254 ]‐ 1 is changed. These results have then been compared with those obtained with cryptophane‐223 bearing only one acetate group on the propylenedioxy linker (compound 2 ) and with cryptophane‐223 bearing six acetate groups (compound 3 ). A self‐encapsulation phenomenon is also observed with compound 2 . Finally, compounds 2 and 3 show different chiroptical properties compared with those obtained with the two enantiomers of compound 1 .