Intramolecular Hydrogen Bond in Biologically Active o-Carbonyl Hydroquinones

Intramolecular hydrogen bonds (IHBs) play a central role in the molecular structure, chemical reactivity and interactions of biologically active molecules. Here, we study the IHBs of seven related o-carbonyl hydroquinones and one structurally-related aromatic lactone, some of which have shown anticancer and antioxidant activity. Experimental NMR data were correlated with theoretical calculations at the DFT and ab initio levels. Natural bond orbital (NBO) and molecular electrostatic potential (MEP) calculations were used to study the electronic characteristics of these IHB. As expected, our results show that NBO calculations are better than MEP to describe the strength of the IHBs. NBO energies (∆E(ij)((2))) show that the main contributions to energy stabilization correspond to LP→σ* interactions for IHBs, O(1)(…)O(2)-H(2) and the delocalization LP→π* for O(2)-C(2) = C(α(β)). For the O(1)(…)O(2)-H(2) interaction, the values of ∆E(ij)((2)) can be attributed to the difference in the overlap ability between orbitals i and j (F(ij)), instead of the energy difference between them. The large energy for the LP O(2)→π* C(2) = C(α(β)) interaction in the compounds 9-Hydroxy-5-oxo-4,8, 8-trimethyl-l,9(8H)-anthracenecarbolactone (VIII) and 9,10-dihydroxy-4,4-dimethylanthracen-1(4H)-one (VII) (55.49 and 60.70 kcal/mol, respectively) when compared with the remaining molecules (all less than 50 kcal/mol), suggests that the IHBs in VIII and VII are strongly resonance assisted.