Highly efficient indium(iii)-mediated cyclisation of 5-hydroxy-1,3-diketones to 2,3-dihydro-4H-pyran-4-ones; mechanistic insights from in situFourier transform infrared spectroscopy

Abstract

5-Hydroxy-1,3-diketones have been synthesised in a facile one-pot reaction from the treatment of acid chlorides with non-substituted ketones and LiHMDS. Subsequent cyclisation to 2,3-dihydro-4H-pyran-4-ones occurs rapidly and in high yield (89–99%) when mediated by anhydrous indium(III) chloride. A spectroscopic study of the reaction using in situ Fourier transform infrared (FTIR) spectroscopy has shown the reaction to be highly dependent on temperature, metal complex formation and InCl₃ concentration. Since the reaction is deactivated by the precipitation of [InCl₃·(H₂O)₃], the concurrent use of a stronger drying agent, such as molecular sieves 4 Å or anhydrous MgSO₄, allows the reaction to be successfully carried out at relatively low loadings of InCl₃ (1–10%). In their absence, the optimum reaction conditions were found to be a diketone : InCl₃ ratio of 3 : 1 in toluene, and a reaction temperature of 80 °C.