XRD and VCD: a marriage of love or convenience? Honeymoon around a cyclic urea derivative

The structures and absolute configurations of the enantio­mers (3aR,8aR)-2,2-dimethyl-4,4,8,8-tetra­phenyl-4,5,6,7,8,8a-hexahydro-3aH-1,3-dioxolo[4,5-e][1,3]diazepin-6-one 0.33-hydrate, C₃₂H₃₀N₂O₃·0.33H₂O, (Ia), and (3aS,8aS)-2,2-dimethyl-4,4,8,8-tetra­phenyl-4,5,6,7,8,8a-hexa­hydro-3aH-1,3-dioxolo[4,5-e][1,3]diazepin-6-one 0.39-hydrate, C₃₂H₃₀N₂O₃·0.39H₂O, (Ib), have been elucidated unambiguously using the complementary power of single-crystal X-ray diffraction (XRD) and vibrational circular dichroism (VCD). The enantio­mers crystallize in the Sohncke space group P2₁2₁2 and pack as dimers stabilized by two symmetric hydrogen bonds involving one amide group each of the cyclic urea moiety. This double inter­action is capped by a water mol­ecule that partially occupies a site lying on the twofold axis and forms an uncommon hydrogen bond between the two monomers. A comparison between the solid-state VCD characterizations and the Bayesian statistics on Bijvoet differences determined from the XRD measurements reveals a tendency towards the correct determination of the absolute configuration by this latter method.