The Diels-Alder adduct, produced from 3,4-di-tert-butylthiophene 1-oxide and dimethyl acetylenedicarboxylate, spontaneously extrudes sulfur monoxide (SO) at room temperature and serves as a new method for generation of SO under mild conditions. Thus, the SO generated underwent [1 + 2] cycloadditions to a series of alkenes and alkynes to produce thiirane oxides and thiirene 1-oxides, respectively, providing new syntheses of the sulfur-containing three-membered heterocycles. The SO also reacted with a variety of cyclic and acyclic dienes to give the corresponding 2,5-dihydrothiophene 1-oxides.