3-Methoxycarbonyl-4-keto-2, 5-dihydrothiophene is a convenient starting point for synthesis of a range of 3, 4-and 2, 3-quinoline and quinolone and coumarin fused dihydrothiophene dioxides. With the exception of the 2, 3-quinoline derivatives these all lose sulfur dioxide on thermolysis to give the corresponding o-quinodimethanes which can be intercepted in Diels- Alder reactions.