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Rearrangement of bicyclo[2.2.1]heptane ring systems by titanocene alkylidene complexes to bicyclo[3.2.0]heptane enol ethers. Total synthesis of (.+-.)-.DELTA.9(12)-capnellene
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1990
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A variety of ester-substituted norbornenes react with titanamethylene complex (Tebbe's reagent) to yield stable \ntitanacyclobutanes. Endo esters do not react with the reagent in competition with the norbornene double bond. \nThe 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 \nto a carbene-olefin complex. The ratio of productive opening, cleavage of the bicycloheptane ring system, to \nnonproductive opening, regeneration of the starting materials, is controlled by a variety of steric factors that \nwere studied and analyzed. The productive opening was detected by the formation of the product resulting from \nthe intramolecular trapping of the intermediate titanium alkylidene by the endo ester functionality in a Wittig-like \nreaction to yield substituted bicyclo[3.2.0]heptenes. Rearrangement of the titanacycle formed from 4,4-dimethyltricyclo[ \n5.2.1.0^(1,5)]dec-8-ene-6-carboxylic acid tert-butyl ester yielded 10,10-dimethyl-3-methoxy-7- \nvinyltricyclo[5.3.0.0^(2,5)]dec-2-ene, which was transformed into Δ ^(9(12))-capnellene in good yield.