G. Kubiak

General Approach for the Synthesis of Polyquinenes.

Abstract The syntheses of tetracyclo[6.6.0.0 1,5 .0 8,12 ]tetradeca-3,6,10,13-tetraene 5 and staurane-2,5,8,11-tetraene 6 are described. The key step was the diborane-mediated reduction of the labile β-diketones 13 and 22 to provide the tetrols 11 and 23 , respectively. This was followed by removal of four molecules of water from the corresponding tetrols on heating them in HMPA . This technique has also been employed for preparation of the triquinanetrienes 26 and 27 . The use of the diborane-mediated reduction of strained β-diketones in combination with the HMPA -promoted elimination of the resulting alcohols provides an extremely versatile route to polyquinenes.

General approach for the synthesis of polyquinenes via the Weiss reaction XV. Synthesis of the [5.5.5.5]fenestrane system via the aldol approach and studies directed toward the [5.5.6.6]fenestranes

Abstract The all cis-8a, 12b-diacetoxytetracyclo[5.5.1.0.4,13010,13]tridecane-2,6-dione 9 was successfully synthesized via aldolization of the diketodialdehyde 8 under conditions of equilibration, followed by trapping with acetic anhydride. Aldol cyclization of the dialdehyde 14 to provide the [5.5.6.6]fenestrane system 12 was also attempted. This resulted in the observation of tetracycle 25 with the desired ring system albeit as a minor product. The major products isolated from this approach were the undesired α,β-unsaturated aldehyde 22 and monoaldehyde 23, the latter of which contained a configuration opposite to that of C-5 in 12. In addition, the related bisacylation approach provided 26 as the major product with the undesired configuration at C-8. This cyclization furnished the transannular tetraone 27 at higher temperature.

General approach for the synthesis of polyquinanes

The diborane-mediated reduction of labile β-diketones 4, 5 and 6 has been employed as the key step in the synthesis of the three polyquinanes, all-cis-tetracyclo-[6.6.0.01,5.08,12]tetradecane 1, all-cis-tricyclo[6.3.0.01,5]undecane 2 and all-cis-tetracyclo-[5.5.1.04,13.O10,13]tridecane, staurane 3, respectively.

Studies on the reaction of 1,2-dicarbonyl compounds with dimethyl 3-ketoglutarate. Steric and electronic effects

The steric and electronic influences of substituents attached to the 1,2-dicarbonyl system on the success of the reaction of 1,2- diketones with dimethyl 3-ketoglutarate 2 have been examined. It is clear from the reaction of 2 with benzil, thienil, furil, and phenanthrenequinone 5, respectively, coupled with 13C NMR spectroscopy of the reaction intermediates, that steric effects play a major role in the overall success of the reaction to provide 4. This is analogous to the situation observed earlier with 1,2-diketones, R-CO-CO-R, where R represented an aliptiatic or alicyclic group.

Unexpected stereoselectivity in the Weiss-Cook condensation of dimethyl 1,3-acetonedicarboxylate with pentane-2,3-dione

The Weiss–Cook condensation of dimethyl 1,3-acetonedicarboxylate† with the unsymmetrical pentane-2,3-dione gives only two (1a and 1c) of the four possible epimers of tetramethyl 1-ethyl-3,7-dihydroxy-5-methyl-cis -bicyclo[3.3.0]octa-2,6-diene-2,4,6,8-tetracarboxylate in 86% isolated yield. The structures of 1a and 1c have been established by means of single crystal X-ray crystallography.

STERIC AND ELECTRONIC EFFECTS ON THE WEISS REACTION. ISOLATION OF 1:1 ADDUCTS

The mechanism of the Weiss reaction has been studied with respect to the intermediacy of 4-hydroxycyclopent-2-en-1-ones (1∶1 adducts) in this process. Analysis of these experiments provides additional evidence that 4-hydroxycyclopentenones are indeed key intermediates in the Weiss reaction. Based on the reaction of dimethyl 3-oxoglutarate with benzil, pyridil, thenil, furil and phenanthrenequinone, steric effects play the major role in the overall success of this condensation to provide substituted cis-bicyclo[3.3.0]octane-3,7-diones. Moreover a trihydroxyindene [5.6] system (see 26) has been isolated for the first time under the Weiss conditions which provides additional support for the existence of cyclopentenone intermediates in this process.