Acta Phys. -Chim. Sin. ›› 1995, Vol. 11 ›› Issue (06): 537-540.doi: 10.3866/PKU.WHXB19950612

• ARTICLE • Previous Articles     Next Articles

ESR Studies on 2-Methyl-3-Acetyl Quinoxaline N,N-Dioxide and Its Cyclodextrin Inclusion Compounds

Feng Liang-Bo,Wang Han-Qing   

  1. Lanzhou Institute of Chemical Physics,Chinese Academy of Sciences,Lanzhou 730000
  • Received:1994-04-28 Revised:1995-01-26 Published:1995-06-15
  • Contact: Feng Liang-Bo


2-Methyl-3-acetyl quinoxaline N,N-dioxide (MAQO) is a novel medicine. However it easily undergoes photoreations such as deoxygenation, cyclization etc., resulting in the change of its pharmaceutical action. The problem about radical intermediates involved in the change process is still not well resolved, and the relevant studies are worthwhile.
In CCl4 and CS2 nonprotonic solvents, MAQO gives identical electron spin resonance (ESR) spectra at g=2.0063 with hyperfine (fh) constant aN=1.03mT under UV-irradiation at room temperature. The photochemical process can be depicted as formula(1).
As soon as the unimolecular nitroxide biradical produces, it rapidly couple into a dimmer biradical, 4.Because the two nitroxyl radical in the dimer are far from each other, no interaction between them is observed. Therefore, it can be recognized as two independent mono-radicals.
In protonic solvent, CHCl3, when MAQO was irradiated with ultraviolet light at room temperature, it gives a ESR signal at g=2.0058 with hyperfine constant aN=0.98mT, aH(5,7)=o.11mT, aH(6.8)=0.36mT. The hf structure from two groups of equivalent ringprotons can be clearly observed. According to formula (1), the unimolecular nitroxyl biradical formed from a excited intermediate may abstract a H atom from CHCl3 to generate a hydrogenated nitroxyl mono-radical 5, which can elucidate the above ESR spectrum.
The ESR results of MAQO-α-CD, MAQO-β-CD and MAQO parent are similar in protonic solvent. The only difference is that the aH values, hf constant from protons on benzo-ring, decrease while the line widths increase, compared with that of MAQO parent. This result indicates that the benzo-ring, part of MAQO molecules, has been included in CD and CD have a microenvironmental effect on the proton on the ring.
The nitroxyl radicals detected by ESR are very reactive, they will polymerize, deoxygenate, cyclize, etc,. resulting in the deterioration of the medicine.


Key words: MAQO, Cyclodextrin, ESR