A hetero-Diels-Alder reaction that proceeds rapidly and only requires a simple filtration to purify the product is presented. The dienophile, 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD), is prepared by the heterogeneous oxidation of 4-phenylurazole by the bromenium ion, Br[superscript +], generated in situ by the oxidation of potassium bromide by Oxone. PTAD is reacted with both cyclopentadiene and 1,3,5-cycloheptatriene, and the resulting products are analyzed by NMR spectroscopy. PTAD reacts with cyclopentadiene in the expected fashion; however, it reacts exclusively with the norcaradiene valence tautomer of 1,3,5-cycloheptatriene to form a single cycloadduct. The speed of these reactions allows for more of a three-hour laboratory period to be devoted to spectral characterization and data analysis than in similar experiments. This laboratory experiment is suitable for second- or third-year organic chemistry courses. (Contains 2 schemes.)