Chromatography of the complex reaction mixture gave two isomeric 1:2 adducts, shown by X-ray crystallography to be (E)- and (Z)-[1,2-bis(methoxycarbonyl) vinyl] thio-substituted thiopyran-4-spiro-2'-imidazolidines. Treatment of the spiro compounds with dilute nonaqueous acid led to isomers in which the imidazolidine ring had opened, and acid hydrolysis gave a thiopyrone and its enol tautomer. Continued chromatography of the original reaction mixture gave three new compounds, a thiopyran-4-thione, a tetrakis-(methoxycarbonyl)-1,4-dithiaphenalene, and a precursor to this with the same ring system. The formation of these compounds requires migration of a sulfur atom from the cyclohexene ring to the annelated thiopyran ring, and a possible mechanism is discussed. The dithiaphenalene structure was firmly established by the 2D IN-ADEQUATE technique