Pyrimidine fused dihydrothiophene S,S-dioxides 7 undergo thermal elimi
nation of sulfur dioxide to give the pyrimidine o-quinodimethanes 8 wh
ich can be intercepted in Diels-Alder reactions to give substituted 5,
6,7,8-tetrahydroquinazolines; these form cyclooctapyrimidines on heati
ng with more dienophile.