Microwave-assisted [2 þ 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).Introduction. -In the last few decades, there have been some reports of reactions of simple tertiary enamines with electron-poor acetylenes, such as alkyl propiolates (¼ alkyl prop-2-ynoates) and dialkyl acetylenedicarboxylates (¼ dialkyl but-2-ynedioates) [1]. It has been generally accepted that these reactions initially proceed by a [2 þ 2] cycloaddition. However, the structure of the final product is strongly dependent on the structure of the starting compound and reaction conditions [1d]. The geometry of the obtained products was, in some cases, unambiguously determined by Reinhoudt and co-workers in the early eighties [1d]. When these reactions were extended to other functionalized enamines and enaminones, in some cases, a Michael-type addition took place preferentially over the [2 þ 2] cycloaddition [2].The 3-(dimethylamino)propenoates and related enaminones have been demonstrated to be useful building blocks in the synthesis of many heterocyclic systems [3], including the preparation of natural products and their analogs, such as aplysinopsins [4], meridianines [5], dipodazines [6], and tryprostatins [7]. We have also reported an efficient method for the preparation and functionalization of highly substituted 1amino-2,3-dihydro-1H-pyrrole, 1-amino-1H-pyrrole, and fused pyrrolo[3,2-d]oxazole systems from 1,2-diazabuta-1,3-dienes and 3-(dimethylamino)prop-2-enoates [8] and the regio-and stereoselective one-pot synthesis of oxazolo-fused pyridazines via a Michael-addition -pyridazine-cyclization -oxazoline cyclization cascade reaction [9].As an extension of our research, we recently reported on microwave assisted regiospecific [2 þ 2] cycloadditions of electron-poor acetylenes 2 to (2Z)-2-(acylamino)-3-(dimethylamino)prop-2-enoates 1, which resulted in the formation of (1E,3E)-1-(acylamino)-4-(dimethylamino)buta-1,3-dienes 3 (Scheme 1) [10].
