Abstractmagnified imageMicrowave‐assisted [2+2] cycloadditions of acetylene mono‐ and acetylenedicarboxylates 2 to (5Z)‐5‐[(dimethylamino)methylene]imidazolidine‐2,4‐dione 4a or to the corresponding thioxo derivative 4b in MeCN furnished the highly functionalized imidazolidine‐2,4‐dione derivatives 5 and 6 or the corresponding thioxo derivatives 5–7 as single isomers or mixtures of two isomers (Schemes 2 and 3, Table 1). When the reaction of (5Z)‐5‐[(dimethylamino)methylene]imidazolidine‐2,4‐dione (4c) with acetylenedicarboxylate 2a was performed in DMF, hydrolysis of the (dimethylamino)methylene group took place to give (2E)‐2‐(2,5‐dioxoimidazolidin‐4‐ylidene)butanedioate 11 (Scheme 5).