Generation and trapping of non-aromatic cycloimines via diazotization/dediazotization of N-amino cyclic amines: theoreti
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ORIGINAL RESEARCH
Generation and trapping of non-aromatic cycloimines via diazotization/dediazotization of N-amino cyclic amines: theoretical and experimental results Minita Ojha 1 & Raj K. Bansal 1 Received: 7 May 2020 / Accepted: 12 June 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract A theoretical investigation of the model diazotization/dediazotization of N-aminopiperidine and N-aminomorpholine at the DFT (B3LYP/6-31+G(d)) level indicated that the corresponding cycloimines can be generated transiently, which can be trapped with dimethyl acetylenedicarboxylate (DMAD) to form a 1,4-dipole followed by cycloaddition of the latter with a second molecule of DMAD to give the corresponding pyrido-annelated products. All steps have low activation free energy barriers and are thermodynamically favoured. Based on the theoretical results, we carried out successfully diazotization of N-amino cyclic amines, namely N-aminopiperidine, 4-aminomorpholine and 1-amino-4-methylpiperazine with tert.-butyl nitrite followed by dediazotization to generate transiently the corresponding cycloimines, which could be trapped with dimethyl acetylenedicarboxylate. to afford new annelated pyridine derivatives, namely tetramethyl 9H-5,6,7,8-tetrahydroquinolizine1,2,3,4-tetracarboxylate, tetramethyl 5,6,8,9-tetrahydropyrido[2,1-c][1,4]oxazine-1,2,3,4-tetracarboxylate and tetramethyl 9H5,6,7,8-tetrahydro-7-methylpyrido[1,2-a]pyrazine-1,2,3,4-tetracarboxylate which were duly characterized. Keywords 1,4-Dipoles . Annelated pyridines . Tetramethyl 9H-5,6,7,8-tetrahydroquinolizine-1,2,3,4-tetracarboxylate . Tetramethyl 5,6,8,9-tetrahydropyrido[2,1-c][1,4]oxazine-1,2,3,4-tetracarboxylate . Tetramethyl 9H-5,6,7,8-tetrahydro-7-methylpyrido[1,2-a]pyrazine-1,2,3,4-tetracarboxylate . DFT calculations
Introduction Diels and Alder [1] in 1932 reported the formation of an unstable red product from the reaction of pyridine with dimethyl acetylenedicarboxylate (DMAD), whose correct structure as tetramethyl 4H-quinolizine-1,2,3,4-tetracarboxylate (3) could be assigned almost three decades later by Acheson and coworkers [2–4]. However, Huisgen and co-workers [5, 6] for the first time postulated it as an example of 1,4-dipolar cycloaddition reaction wherein a 1,4-dipole (1) was generated in situ from the reaction of pyridine with DMAD which
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11224-020-01567-z) contains supplementary material, which is available to authorized users. * Raj K. Bansal [email protected] 1
Department of Chemistry, The IIS University, Jaipur 302020, India
subsequently reacted with a second molecule of DMAD acting as dipolarophile to afford the final product (3) (Scheme 1) [5, 6]. Furthermore, it was pointed out that in contrast to 1,3-dipoles, 1,4-dipoles are not isolable and are produced in situ from the reaction of a nucleophilic substrate (a = b) with an electrophilic reagent (c = d) followed by the reaction with the dipolarophile e = f (Scheme 2) [5,
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