Versuche zur Synthese von Calicen aus trisubstituierten Cyclopropanen und Cyclopentenon

Attempted Synthesis of Calicene from Trisubstitued Cyclopropanes and Cyclopentenone The Li carbenoids 4 , prepared by treatment of substituted 1,1-dihalocyclopropanes with BuLi, are reacted with cyclopent-2-enone under thermodynamic and kinetic control (Scheme 1). In general, the latter procedure gives better yields of cyclopropylcyclopentenols 5a – e , but the reaction seems to be controlled mainly by the steric and electronic properties of the substituent Y. So, with 4b and 4e , the main reaction is the attack of the carbenoid at C(1) of cyclopent-2-enone, while 4a (Y = PhS) predominantly deprotonates the ketone (Scheme 4). Whereas 5d and 5e can easily be converted to the dihydrocalicenes 6d and 6e (Scheme 6), the attempted elimination of H₂O from 5a – c leads to the rearranged products 13 – 2 due to the opening of the cyclopropane ring (Scheme 5). Finally, the generation of the parent compound 2 from the silylated precursor 6d is attempted: treatment with MeO^? gives the addition products 18A/18B , while the reaction with Br₂ provides 19 by a bromination/dehydrobromination sequence (Scheme 7).