Raman optical activity of enantiopure cryptophanes

Raman optical activity (ROA) and density functional theory (DFT) calculations were used to determine the absolute configuration of enantiopure cryptophane molecules and to obtain conformational information about their three ethylenedioxy linkers. ROA spectra recorded in chloroform solution for the two resolved enantiomers of cryptophanes derivatives bearing five (2), six (1), nine (3 and 4), and 12 (5) methoxy substituents are presented for the first time. The number of methoxy substituents (cryptophanes 1, 3, and 5) and the arrangement of the three linkers (anti for 3 and syn for 4) are two important parameters that significantly affect the ROA spectra. DFT calculations, at the B3PW91/6-31G** level, for cryptophane bearing six methoxy substituents establish, besides the absolute configuration, the preferential all-trans conformation of the ethylenedioxy linkers of the chloroform-cryptophane complex. This study shows that the ROA/DFT approach exhibits a higher selectivity for the conformation of the linkers than vibrational circular dichroism (VCD) associated with theoretical calculations.