Meroterpenoid Synthesis via Sequential Polyketide Aromatization and Radical Anion Cascade Triene Cyclization: Biomimetic Total Syntheses of Austalide Natural Products

The first total synthesis of five austalide natural products, (+)-17S-dihydroaustalide K, (+/-)-austalide K, (+/-)-13-deacetoxyaustalide I, (+/-)-austalide P, and (+/-)-13-deoxyaustalide Q acid, was accomplished via a series of biomimetic transformations. Key steps involved polyketide aromatization of a trans,trans-farnesol-derived beta,delta-diketodioxinone into the corresponding beta-resorcylate, followed by titanium(III)-mediated reductive radical cyclization of an epoxide to furnish the drimene core. Subsequent phenylselenonium ion induced diastereoselective cyclization of the drimene completed the essential carbon framework of the austalides to access (+/-)-17S-dihydroaustalide K, (+/-)-austalide K, and (+/-)-13-deacetoxyaustalide I via sequential oxidations. Furthermore, (+/-)-13-deacetoxyaustalide I could serve as a common intermediate to be derivatized into other related natural products, (+/-)-austalide P and (+/-)-13-deoxyaustalide Q acid, by functionalizing the cyclic lactone moiety.