Chemo- and regioselective functionalization of nortrilobolide: application for semi-synthesis of the natural product 2-acetoxytrilobolide

The difference of reactivity of the hexaoxygenated natural product thapsigargin (1) and the pentaoxygenated nortrilobolide (3) is compared in order to develop a chemo- and regioselective method for the conversion of nortrilobolide (3) into the natural product 2-acetoxytrilobolide (4). For the first time, a stereoselective synthesis of the 2-acetoxytrilobolide (4) is described, which involves two key reactions: the first chemical step is a one-pot substitution-oxidation reaction of an allylic ester into its corresponding α,β-unsaturated ketone. The second process consists of a stereoselective α´-acyloxylation of the key intermediate α,β-unsaturated ketone to afford its corresponding acetoxyketone, which was converted into 2-acetoxytrilobolide (4) in a few steps. This innovative approach would allow the synthesis of a broad library of novel and valuable penta- and hexaoxygenated guaianolides as potential anticancer agents.