| Abstrakt: |
Introduction: The 13C-NMR data described for perezone (1), a 3-hydroxy p-quinone stated as the first natural product isolated as crystals in the New World, and rigidone (2), a 4-hydroxy o-quinone isolated from a coral species, are essentially the same. Some years ago, we described, using theoretical calculations, that a 4-hydroxy-1,2-quinone is more than 11 kcal/mol less stable than a 3-hydroxy-1,4-quinone making coexistence in nature of this type of quinones. In the present study, we approach the situation by comparing of the experimental 13C-NMR data for 1 and those described for 2 with the calculated using computational methods. Additional evidence was obtained from a X-ray diffraction analysis for the reaction product of perezone with o-phenylendiamine. Methods;The 13C-NMR data for the quinoid rings were calculated using the GIAO and CSGT methods, density functional theory (DFT) and the functional/basis set pairs B3LYP/6-31 g(d,p) and MPW1PW91/6-31 g(d,p); and TPSSTPSS/cc-PVTZ and PBE1PBE/aug-cc-PVDZ. Perezone reaction with o-phenylenediamine was achieved using a described method in MeOH at room temperature. X-Ray diffraction analysis of phenazine from perezone reaction was done using MoKα radiation. The data were used to calculate the Flack parameter. Results; After conformational analysis, complete optimization of the geometry of the conformers found and, calculation of the 13C-NMR chemical shifts for the quinone ring of 1 and 2, in all cases a better agreement was observed between the experimental data for 1 versus 2. Perezone reaction with o-phenylenediamine afforded the corresponding phenazine in its amine-keto tautomeric form as evidenced from a X-ray diffraction study. Conclusion; The better agreement observed between the experimental and calculated data for 1 versus 2, along with the free energy difference of more than 11 kcal/mol in favor of the 3-hydroxy p-quinone versus 4-hydroxy o-quinone, previously established for us, allow to say that the structure described for rigidone corresponds to ent -perezone. [ABSTRACT FROM AUTHOR] |
