Effect of ring strain on the formation and pyrolysis of some Diels–Alder adducts of 2-sulfolene (2,3-dihydrothiophene 1,1-dioxide) and maleic anhydride with 1,3-dienes and products derived therefrom
7-Thiabicyclo[4.3.0]non-3-ene 7,7-dioxide 3 and the 2,5-bridged analogues 4–6 formed by Diels–Alder reaction of 2,3-dihydrothiophene 1,1-dioxide (2-sulfolene)2 with butadiene and cyclic 1,3-dienes are found, upon flash vacuum pyrolysis, either to lose SO2 and ethene, or to undergo a retro-Diels–Alder reaction, depending on the degree of ring strain present. The epoxides 10, 13 and 17 of these compounds decompose exclusively by loss of SO2 and ethene to give the mono-epoxides of cyclic 1,4-dienes which undergo further reactions under the pyrolysis conditions. The same final products can also be obtained by extrusion of CO and CO2 from the epoxides 12, 16 and 19 of the corresponding diene–maleic anhydride adducts. In the case of the furan–maleic anhydride adduct, its epoxide 20 fragments by a retro-Diels–Alder reaction to give 1,4-dioxine, which, at high temperatures, is transformed into acrylaldehyde. Pyrolysis of the aziridines 22–24, formed by photochemical reaction of the diene–maleic anhydride adducts with ethyl azidoformate, generally gives complex decomposition products, although for the methylene-bridged compound 22 there is good evidence for the generation of a 1,4-dihydropyridine via a retro-Diels–Alder reaction.