The radical scavenging activity of abietane diterpenoids: Theoretical insights.

Salvia species are frequently used in traditional medicine and are a source of diterpenoid antioxidants. In this study, the hydroperoxide radical scavenging activity of seven known abietane diterpenoids (ADs), isolated from Salvia barrelieri, are investigated using a quantum chemical approach. The ADs are 7-oxoroyleanone-12-methyl ether (1), 7a-acetoxyroyleanone-12-methyl ether (2), royleanone (3), horminone (4), 7-acetylhorminone (5), cryptojaponol (6), and inuroyleanol (7). It was found that formal hydrogen transfer is the main mechanism of the antiradical activity of these ADs in nonpolar environments, whereas the single electron transfer mechanism of anion states is favored in aqueous environment. The antioxidant activity of compounds 1-5 involves H-abstraction at the C7(15)-H bonds whereas for the compounds 6 and 7 the H abstraction takes place at the O12-H bond. The HOO scavenging activity of compounds 1-5 is minor in all of the studied media, however 6 and 7 exhibit excellent antiradical activity in aqueous solution. Remarkably, the HOO scavenging activity of compound 7 is substantially higher than that of Trolox, the reference antioxidant: the calculated rate constant was 122.3 times higher in polar and 6.1 times higher in nonpolar environments, respectively. Consistently 7 is a promising radical scavenger in physiological environments.