Aldrichimica Acta Volume 30 No 4 (pdf) from Sigma-Aldrich The annellation of the pyrimidine ring onto alicyclic alkenes has been achieved using Diels-Alder chemistry. 2,4-Dimethoxy-1,3-diazaanthracene (DDA) reacts with Smith’s diene under forcing thermal conditions (sealed tube at 180 °C) to form a mixture of stereoisomeric adducts which can be separated by fractional crystallization.
Source: wiktionary