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Published February 1, 1990 | public
Journal Article

Rearrangement of bicyclo[2.2.1]heptane ring systems by titanocene alkylidene complexes to bicyclo[3.2.0]heptane enol ethers. Total synthesis of (±)-Δ^(9(12))-capnellene


A variety of ester-substituted norbornenes react with titanamethylene complex (Tebbe's reagent) to yield stable titanacyclobutanes. Endo esters do not react with the reagent in competition with the norbornene double bond. The X-ray structure of the metallacycle formed from titanacene methylene complex and 1-methylbicyclo[2.2.l]hept-5-ene-2,3-dicarboxylic acid diisopropyl ester was determined. On heating, the metallacycle rearranged to a carbene-olefin complex. The ratio of productive opening, cleavage of the bicycloheptane ring system, to nonproductive opening, regeneration of the starting materials, is controlled by a variety of steric factors that were studied and analyzed. The productive opening was detected by the formation of the product resulting from the intramolecular trapping of the intermediate titanium alkylidene by the endo ester functionality in a Wittig-like reaction to yield substituted bicyclo[3.2.0]heptenes. Rearrangement of the titanacycle formed from 4,4-dimethyltricyclo[^(1,5)]dec-8-ene-6-carboxylic acid tert-butyl ester yielded 10,10-dimethyl-3-methoxy-7- vinyltricyclo[^(2,5)]dec-2-ene, which was transformed into Δ ^(9(12))-capnellene in good yield.

Additional Information

© 1990 American Chemical Society. Received September 20, 1988. Financial support for this work was provided by the National Institutes of Health (GM-31332). We gratefully acknowledge J. D. Meinhart for obtaining two-dimensional and difference NOE spectra. High-resolution mass spectral determination on 75 was performed by the Mass Spectroscopy Center at the University of California, Riverside. We thank Professor John K. Stille (Colorado State University) for providing spectral data of both 73 and 75. Upgrade of the Divisional X-Ray Diffraction Facility was supported by the National Science Foundation with Grant CHE-82-19039.

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