Total Synthesis of (±)-Culmorin and (±)-Longiborneol:  An Efficient Construction of Tricyclo[6.3.0.0^3,9]undecan-10-one by Intramolecular Double Michael Addition

The treatment of 4-[(5E)-6-methoxycarbonyl-5-hexenyl]-3,4-dimethyl-2-cyclopenten-1-one (5) with LHMDS, TMSI−HMDS, Bu₂OTf−HMDS, or TMSCl−NEt₃−ZnCl₂ caused the intramolecular double Michael addition to afford tricyclo[6.3.0.0^3,9]undecan-10-one 12 in high yields with perfect stereoselectivity. The methodology was further elaborated to achieve efficient total syntheses of (±)-culmorin (1) and (±)-longiborneol (2). The common precursor 13 of them was obtained from 14 in 94% yield as a single isomer by the treatment with LHMDS. After the conversion of 13 into the corresponding acid 24 by hydrolysis, oxidative decarboxylation using S-(1-oxido-2-pyridinyl)-1,1,3,3-tetramethylthiouronium hexafluorophosphate (HOTT, 27), followed by the Birch reduction, stereoselectively afforded (±)-culmorin (1). (±)-Longiborneol (2) was synthesized from 24 by the standard transformation. Additionally, the treatment of 24 with Pb(OAc)₄ led to 28 via uncommon migration. Its structure was determined by X-ray analysis after the transformation into the diketone 29.