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 H 2 O 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 Br 2 provides 19 by a bromination/dehydrobromination sequence ( Scheme 7 ).