New organometallic approaches to heterocycles
Several heterocyclic ring systems have been prepared by two new organometallic methods. The first approach involved the mercuration of alkyl aryl acetylenes. This reaction resulted in the formation of mercurated heterocycles. Mercurated benzofurans, isocoumarins, chromones, benzothiophenes and benzopyrans were all prepared by this method. The mercury moiety in these compounds was readily substituted by various functional groups. By making a slight modification in the acetylenic substrate, the intramolecular cyclization was prevented. The reaction then produced the (beta)-acetoxy vinylmercurial which was used to synthesize the coumarin ring system. The second approach involved the intramolecular attack of an oxygen or a nitrogen nucleophile on a (pi)-allylpalladium intermediate. The (pi)-allylpalladium intermediate was generated by the reaction of an organomercurial with a diene or an olefin in the presence of a palladium(II) salt. This method was used for the preparation of lactones, dihydrobenzofurans, dihydroindoles and pyrans.