A Facile Method to Determine the Absolute Structure of Achiral Molecules: Supramolecular-Tilt Structures

Achiral compounds 4‐methoxy‐4‐(p‐methoxyphenyl)cyclohexanoneethylene ketal (2), 4‐hydroxy‐4‐(p‐methoxy phenyl)cyclohexanoneethylene ketal (3), and 3,5‐dimethyl‐4‐nitropyrazole (4) crystallized in chiral structures and the samples showed an enantiomeric excess. We have determined the absolute structures of these compounds by using X‐ray diffraction with copper radiation at low temperatures. Moreover, we have also established the prevalent absolute structures in these samples, by comparing their calculated and solid‐state vibrational circular dichroism (VCD) spectra. The consistency of this method was confirmed by using (R,R)‐2,8‐diiodo‐4,10‐dimethyl‐6 H,12H‐5,11‐methano‐dibenzo[b,f][1,5]diazocine, Tröger′s base, (R,R)‐1, as a chiral compound of known absolute configuration. The crystal maze: Two achiral cyclohexanoneethylene ketals and 3,5‐dimethyl‐4‐nitropyrazole crystallized in chiral structures and showed an enantiomeric excess. The calculated and solid‐state VCD spectra revealed the prevalent absolute structure (see figure).