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

A variety of ester-substituted norbornenes react with titanamethylene complex (Tebbe's reagent) to yield stable titanacyclobutanes. Endoasters 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-[2.2.1]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-di-methyltricyclo[5.2.1.01,5]dec-8-ene-6-carboxylic acid tert-butyl ester yielded 10,10-dimethyl-3-methoxy-7-vinyltricyclo[5.3.0.02,5]dec-2-ene, which was transformed into Δ9(12)-capnellene in good yield. © 1990, American Chemical Society. All rights reserved.