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

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