Effect of the n_O → π*_CO Interaction on the Conformational Preference of 1,3-Diketones: A Case Study of Riolozatrione Derivatives

The cyclopropane ring-opening reaction of riolozatrione, a natural product obtained from Jatropha dioica, afforded a 2,2-disubstituted 1,3-cyclohexandione displaying an alkyl methyl ether group at position 5. The conformational analysis of this product showed a high preference for the trans-diaxial conformation in both solution and solid state. Such conformation was possible from the noncovalent intramolecular n_X → π*_CO interactions (X = an element having an unshared electron pair), allowing the determination of the interaction energies. Since the n_X → π*_CO interactions can be regarded as additive, the energy values ranged from 4.52 to 6.51 kcal mol^–1 for each carbonyl group with a strong dependency on the interatomic distances. The rigorous analysis of the electron density in the topological theory of atoms in molecules framework clearly shows that the origin of O–CO interactions are through the n_O → π*_CO electron transfer mechanism. Such interactions are slightly weaker than a canonical hydrogen bond but seemingly stronger than a van der Waals interaction. This interaction must be considered as a stereoelectronic effect due the electronic transfer between the interacting groups, which are limited by their relative stereochemistry and can be represented by a bond–no bond interaction, causing the pyramidalization of the carbonyl, which is the charge acceptor group.