Aldehydes or ketones react readily in one-pot with isatoic anhydride and ammonium salts in the presence of tris-(hydrogensulfato)boron to produce the corresponding 2,3-dihydroquinazolin-4(1H)-ones in high yields under solvent-free conditions. The use of tris(hydrogensulfato)boron makes this process quite simple, more convenient and environmentally friendly. 2,3-Dihydroquinazolin-4(1H)-one derivatives have interesting biological and pharmacological activities.