The treatment of 4-[(5E)-6-methoxycarbonyl-5-hexenyl]-3, 4-dimethyl-2-cyclopenten-1-one (5) with LHMDS, TMSI-HMDS, Bu(2)OTf-HMDS, or TMSCl-NEt(3)-ZnCl(2) 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.
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Stereoselective syntheses of tricyclo[5.3.0.0(3,8)]decane 22 and tricyclo[6.3.0.0(3,9)]undecane 26, the basic skeletons of copaborneol and longiborneol, were achieved by the intramolecular double Michael reactions of 2-cyclopenten-1-ones 15-17. The substrates were prepared starting with tricyclo[5.2.1.0(2,6)]deca-4,8-dien-3-one (6). The intramolecular double Michael reactions were carried out under three different conditions: TMSCl-Et(3)N-ZnCl(2), TMSI-(TMS)(2)NH, and Bu(2)BOTf-(TMS)(2)NH. The framework 26 of longiborneol was constructed in good yields using the latter two reagent systems.
It was observed that the synthesis of tricyclo[5.2.1.01,5]decane 10 can be performed effectively by
the intramolecular double Michael reaction of 5-(5-methoxycarbonyl-4-pentenyl)-2-cyclopenten-1-one (9). Highly stereocontrolled total syntheses of (±)-8,14-cedranediol (2) and (±)-8,14-cedranoxide
(1) were accomplished by the application of this methodology. Heating 5-(1,5-dimethyl-5-ethoxycarbonylpent-4-enyl)-2-cyclopenten-1-one (15) with TMSCl, Et3N, and ZnCl2 in o-dichlorobenzene
at 150 °C provided (±)-(1R*,2R*,5R*,6R*,7S*)-2,6-dimethyl-6-ethoxycarbonyltricyclo[5.2.1.01,5]-decan-9-one (16) as a single isomer. The product 16 was stereoselectively converted into the above
cedranoids 2 and 1 through ring expansion chemistry.
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