First application of hexaaquaaluminium(III) tetrafluoroborate as a mild, recyclable, non-hygroscopic acid catalyst in organic synthesis: a simple and efficient protocol for the multigram scale synthesis of 3,4-dihydropyrimidinones by Biginelli reaction

For the first time hexaaquaaluminium(III) tetrafluoroborate has been used as a mild acid catalyst in organic synthesis. A simple method of its preparation based on the reaction of aluminium triisopropoxide and tetrafluoroboric acid in isopropanol afforded catalyst of high purity and activity. The three-component Biginelli condensation of acetoacetate esters, urea and aldehydes catalyzed by 10 mol% of [Al(H(2)O)(6)](BF(4))(3) in refluxing acetonitrile afforded 3,4-dihydropyrimidonones in good to high yields on multigram scales. The tolerance to acid sensitive reactants such as thienyl and furyl carbaldehydes, applicability to sterically hindered beta-ketoesters and simple recyclability without losing catalytic activity make this catalyst as good replacement to literature methods. The mechanism of the reaction includes formation of the so called 'ureido-crotonate' rather than corresponding acylimino intermediate as found with Bronsted type catalysts. (C) 2010 Elsevier Ltd. All rights reserved.