First Diels–Alder reaction of a N-nitrogen-substituted iminium ion as dienophile furnishing N-amino-substituted 1,2,5,6-tetrahydropyridines

• Published in: Monatshefte für Chemie 2023-12, Vol.154 (12), p.1419-1425
• Main Authors: Gamper, Michael, Gstach, Hubert, Schantl, Joachim G.
• Format: Article
• Language: English
• Subjects: Analytical Chemistry; Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Diels-Alder reactions; Dienes; Inorganic Chemistry; Nitrogen; Organic Chemistry; Original Paper for Special Issue; Physical Chemistry; Substitutes; Theoretical and Computational Chemistry
• ISSN: 0026-9247; 1434-4475
• DOI: 10.1007/s00706-023-03054-7

The exploratory and preliminary work on the reaction of 1-(methoxydiphenylmethyl)-2-methyldiazene with 1,3-dienes reveals a remarkable dichotomy furnishing different heterocyclic Diels–Alder products depending on the presence or absence of water. In contrast to the acid-induced hydrolysis of the N,O-ketal starting material with 1,3-dienes furnishing 1-methyl-1,2,3,6-tetrahydropyridazines (preceding communication), the same acid-promoted reaction albeit under exclusion of water affords N -(methyleneamino)-1,2,5,6-tetrahydropyridines. The serendipitous discovery of the novel 1,2,5,6-tetrahydropyridines was corroborated by an alternative and independent synthesis providing the in situ generated formaldehyde benzophenone azine and its conjugate acid, 2-(diphenylmethylene)-1-methylenehydrazin-1-ium ion. The protonated azine serving as the putative dienophile resembles the first example of an N-nitrogen-substituted iminium ion employed as a dienophile in a bona fide normal electron-demand Diels–Alder reaction. Graphical abstract