Role of metal salts in the synthesis of furan–ketone condensation macrocycles: an ‘apparent’ metal template effect

Abstract

The acid-catalysed condensation of furan and acetone has been investigated by systematically varying the parameters of the reaction. The dramatic yield enhancement for macrocycles found on adding metal salts, and hitherto assumed to be due to a metal ion template effect, has now been demonstrated to result from pH effects. Optimisation of the condensation reaction conditions has enabled a number of tetrameric furan–ketone macrocycles to be synthesized with improved yields and a tetrameric furan–cyclohexanone macrocycle has been synthesized in a one-step reaction for the first time. Hexameric and octameric furan–acetone macrocycles have also been prepared. The tetrameric furan–ketone macrocycles show no binding interaction with any metal ion but the tetrameric tetrahydrofuran–acetone macrocycle binds strongly enough with LiClO₄ to form an isolable 1:1 complex and gives very strongly perturbed ¹H n.m.r. spectra in the cases of LiSCN and Ni(ClO₄)₂·6H₂O. Increasing the number of donor sites, as in the hexameric furan–acetone and hexameric tetrahydrofuran–acetone macrocycle, did not lead to complexation of metal ions.