On the structure of reaction products of 2,3-disubstituted N-phthalimidoaziridines with dimethyl acetylenedicarboxylate

Abstract

1,3-Dipolar cycloaddition of azomethine ylides to various unsaturated compounds is widely used for nitrogen heterocycles construction [1, 2]. One of the general methods for azomethine ylides generation is a thermally or photochemically induced cleavage of strained aziridine cycle at the C–C bond. However Naminoaziridine derivatives almost were not involved in such reactions. The articles of Person et al. [3, 4] on the reactions between several threeand tetrasubstituted N-phthalimidoaziridines and one of the most active dipolarophiles, DMAD (dimethyl acetylenedicarboxylate), were an exception. Reactions there described gave N-phthalimidosubstituted 3-pyrrolines ex ante but their spatial structure was not established. Due to a possible convenience of this reaction for the synthesis of five-membered unsaturated N-aminoheterocycles the main goal of this research was an investigation of its applicability for less substituted Naminoaziridine derivatives and an attempt to elucidate its stereochemical features. To this end we have carried out thermolysis in the presence of DMAD of several N-phtalimidoaziridines with electron-withdrawing groups CN, COOMe facilitating the aziridine cleavage. Starting N-phthalimidoaziridines I, II, V, and VI were synthesized by N-aminophthalimide oxidation in the presence of the corresponding olefins using a standard procedure [5]. Their thermolysis in the presence of DMAD was carried out in sealed ampoules and/or in a hermetically closed thermostable vial with the 150–300% excess of dipolarophile. Benzene (aziridines V and VI) and more high-boiling chlorobenzene (aziridines I and II) were used as solvents. The optimal temperature for the reaction products formation was 220°С (for aziridines I and II), 150°С (for aziridine V), and 120°С (for aziridine VI), considerably higher than for threeand tetrasubstituted compounds [3, 4]. It is in agreement with the reaction conditions becoming more stringent with the decrease in the number of substituents stabilizing the intermediate azomethine ylide. The formation of products obtained in all cases may be understood as a result of 1,3-dipolar cycloaddition of the corresponding azomethine ylides to DMAD: 2pyrrolines III and VII, 3-pyrroline VIII, and pyrrole IV. An assortment of products was much wider than found earlier [3, 4]. 3-Pyrrolines initially formed after cycloaddition appear to undergo further transformations under the severe reaction conditions: A proton migration leading to 2-pyrrolines and/or a loss of phthalimide molecule leading to pyrrole. The formation only of products mentioned was confirmed by H NMR spectra of the reactions mixtures just after heating. The composition of compounds III, IV, VII, and VIII is confirmed by elemental analysis data; their structures agree with H and C NMR spectra and mass-spectra. A special feature of all N-phthalimidopyrrolines obtained is a retarded rotation around the N–N bond in the NMR time scale. A conformation with a DOI: 10.1134/S107036320904029X PhthN−N ~ CO2Me CO2Me CO2Me CO2Mw + 220 οC N CO2Me