Microwave-assisted gold(I) catalyzed pyran ring opening in brevifloralactone: synthesis of the hawtriwaic acid core

An efficient microwave-assisted Au(I) catalyzed cleavage of the pyran ring of brevifloralactone is described. We report one of the first combination reactions of gold and microwave irradiation. In base to this reaction, nine new ent-clerodane diterpene derivatives have been obtained by partial synthesis from brevifloralactone, a naturally occurring clerodane-type diterpene isolated in large quantities from the aerial part of Salvia breviflora. The clerodane diterpenes have very interesting biological activities and the semisynthetic approach described here represents an alternative to obtain them from other major diterpenes isolated from natural sources. The structures of these compounds were stablished from their physical and spectroscopic data.