Short and Stereoselective Total Synthesis of Δ‐11,13‐Didehydroguaianes and ‐guaianolides: Synthesis of (±)‐Achalensolide and (±)‐Pechueloic Acid; Revision of the Structure of (+)‐Rupestonic Acid

Abstract (±)‐Pechueloic acid ( 1 ), (±)‐rupestonic acid ( 3 ), and (±)‐achalensolide ( 5 ) (guaian‐8–12‐olide class) were prepared stereoselectively in only nine steps from the commercially available tropylium cation via central intermediate 6 , which is used as a general and efficient precursor to bicyclo[5.3.0]decane sesquiterpenes. The method does not require function protection. It is highly regio‐ and stereoselective thanks to an efficient C‐1 epimerization, a selective C‐8,9 hydrogenation, and a stereocontrolled 1,6 conjugate addition of an acrylate equivalent. These Δ‐11,13‐didehydroguaianes and‐guaianolides are good Michael acceptors and hence biologically active. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)