In 0.1 M
phosphate buffer, pH 3.0, and at 37 degrees C,
resveratrol ((E)-3,4',5-trihydroxystilbene, 1a), an
antioxidant and
cancer chemopreventive phytoalexin, reacted smoothly at 25 microM or 1 mM concentration with excess
nitrite ions (NO2(-)) to give a complex pattern of products, including two novel regioisomeric alpha-nitro (3a) and 3'-nitro (4) derivatives along with some (E)-3,4',5-trihydroxy-2,3'-dinitrostilbene (5), four oxidative breakdown products,
4-hydroxybenzaldehyde,
4-hydroxy-3-nitrobenzaldehyde, 3,5-dihydroxyphenylnitromethane, and 3,5-dihydroxybenzaldehyde, two dimers, the
resveratrol (E)-dehydrodimer 6 and restrytisol B (7), and the partially cleaved dimer 2. The same products were formed in the absence of
oxygen. 1H,15N HMBC and LC/MS analysis of the crude mixture obtained by reaction of 1a with Na (15)NO2 suggested the presence of 3,4',5,beta-tetrahydroxy-alpha-nitro-alpha,beta-dihydrostilbene (8) as unstable intermediate which escaped isolation. Under similar conditions, the structurally related catecholic
stilbene piceatannol ((E)-3,3',4,5'-tetrahydroxystilbene, 1b) gave, besides (E)-3,3',4,5'-tetrahydroxy-beta-nitrostilbene (3b),
3,4-dihydroxybenzaldehyde and small amounts of 3,5-dihydroxybenzaldehyde. Mechanistic experiments were consistent with the initial generation of the
phenoxyl radical of 1a at 4'-OH, which may undergo
free radical coupling with NO2 at the alpha- or 3'-position, to give eventually nitrated derivatives and/or oxidative double bond fission products, or self-coupling, to give dimers. The
oxygen-independent, NO2(-)-mediated oxidative fission of the double bond under mild, physiologically relevant conditions is unprecedented in
stilbene chemistry and is proposed to involve breakdown of hydroxynitro(so) intermediates of the type 8.