Res. Chem. Intermed.
, Vol. 29, No. 4, pp. 365–377 (2003)
Also available online - www.vsppub.com
EPR of free radicals formed from 3-hydroxyesculetin
and related derivatives
and HERVÉ VEZIN
Laboratoire de Chimie Organique Physique, UMR (CNRS) 8009, Associé à l’E.N.S.C.L., Université
des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq, France
Received 28 October 2002; accepted 21 November 2002
Abstract—EPR spectroscopy of 3-hydroxyesculetin (1, solid and in solution) and of the radicals
formed during the aerobic oxidationof alkaline solutionsof 1 and related compounds was investigated.
1 in the solid state was studied by pulsed EPR experiments and showed a radical character. The
aerobic oxidation of alkaline solutions of 1 was also followed by EPR spectroscopy. A ring
contractionoccurred leading to a 5,6-dihydroxybenzofuran-2-carboxylate radical. The autoxidationof
an alkaline solutionof (Z)-3-(3,4-dihydroxyphenyl)-2-hydroxypropenoic acid allowed the observation
of a spectrum attributable to 5,6-dihydroxybenzofuran-3-one-2-carboxylate radical. The formation
mechanisms of these radicals are discussed.
: EPR; 3-hydroxyesculetin;free radicals; isotropic hyper ne splitting constants.
Catechols and related compounds are widely distributed in nature. There is current
interest in the oxidative degradation of these materials, with clear evidence for
the toxicity of their degradation products. The recent isolation of the antioxidant
sagecoumarin (a 3-aryloxy-6,7-dihydroxycoumarin) [1, 2] and 3-hydroxycoumarin
 prompted us to investigate the aerobic oxidation of alkaline solutions of
3,6,7-trihydroxycoumarin (3-hydroxyesculetin) (1)  and related compounds 2– 5
(Scheme 1). The observation of an unusual broadening of all the signals (even
that of the solvent residue) on the
H-NMR spectrum of 1 led us to suspect that 1
may be paramagnetic. In this paper, we present the EPR spectroscopy of 1 (solid
and in solution) and of the radicals formed during the aerobic oxidation of alkaline
solutions of compounds 1–5. In order to assign the radical species formed during
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