Structural and theoretical study of four novel norcantharidine derivatives: two new cases of conditional isomorphism.

Structural and theoretical studies of four novel 5,6-dehydronorcantharidine (DNCA)/norcantharidine (NCA) derivatives, namely (3aR,4S,7R,7aS)-2-phenyl-3a,4,7,7a-tetrahydro-4,7-epoxy-1H-isoindole-1,3(2H)-dione, CHNO (DNCA-A), (3aR,4S,7R,7aS)-2-(4-nitrophenyl)-3a,4,7,7a-tetrahydro-4,7-epoxy-1H-isoindole-1,3(2H)-dione, CHNO (DNCA-NA), (3aR,4S,7R,7aS)-2-(4-nitrophenyl)-3a,4,5,6,7,7a-hexahydro-1H-4,7-epoxyisoindole-1,3(2H)-dione, CHNO (NCA-NA), and (3aR,4S,7R,7aS)-2-(2-hydroxyethyl)-3a,4,5,6,7,7a-hexahydro-1H-4,7-epoxyisoindole-1,3(2H)-dione, CHNO (NCA-AE), are reported. The supramolecular interactions and single-crystal structural characteristics of these molecules, together with the crystal structures of four other similar molecules, i.e. NCA-A (the 4-phenyl derivative of NCA-NA), DNCA-AE (the 5,6-unsaturated derivative of NCA-AE), DNCA and NCA, were analysed. Surprisingly, DNCA-A and NCA-A, as well as DNCA-NA and NCA-NA, proved to be isomorphic, while DNCA-AE and NCA-AE, as well as DNCA and NCA, have very different crystal structures. These are very rare isostructural examples between unsaturated and saturated oxanorbornene/oxanorbornane derivatives. To further explore how noncovalent interactions (NCIs) affect the degree of isomorphism in this particular series of rigid molecules where there is a fairly limited conformational degree of freedom, all four pairs of crystal structures were analyzed in parallel. The differentiation in NCIs which entails the packing mode of similar molecules is supported by energy calculations based on real or exchanged crystal structures. Our results show that minor structural differences may result in very different supramolecular interactions, and so lead to altered packing modes in the crystalline solids. Even if isostructurality sometimes occurs, the possibility of various molecular packing types cannot be ruled out. On the other hand, isomorphism may just be the result of kinetic possibilities instead of relative thermodynamic stabilities. Though crystal structure prediction is formidable, the comparison method based on existing crystal structures and quantum calculations can be used to predict the probability of isomorphism. This understanding will help us to design new norbornene derivatives with specified structures.