Synthesis of Triazolodiazepinium Salts: Sequential [3⁺+2] Cycloaddition/Rearrangement Reaction of 1-Aza-2-azoniaallenium Cation Intermediates Generated from Piperidin-4-ones

Abstract

The bicyclic 1-aza-2-azoniaallenium salt intermediates, generated from the azoester species upon treatment with a Lewis acid, have been demonstrated to participate in Huisgen-type cycloaddition with nitriles to result in the formation of fused 6,7,8,9-tetrahydro-5 H-[1,2,4]triazolo[1,5- d][1,4]diazepinium salts. This transformation is interpreted as a regular [3+2] cycloaddition between intermediates as the reactive 1,3-monopole reactants and nitriles as the nucleophilic reagents followed by spontaneous [1,2]-cationic rearrangement. The azoester precursors were easily accessible via oxidation of the corresponding hydrazones using hypervalent iodine oxidant PhI(OAc) under mild conditions. The [1,2,4]triazolodiazepine compounds represent a class of N-containing biologically important heterocycles with a new type of scaffold.

Synthesis of Antimicrobial Benzo[1,2,4]triazoloazepinium Salts and Tetrahydronaphtho[1,2-e][1,2,4]triazines by Polar [3+ + 2] and [4 + 2]-Cycloaddition Reactions

“…A second strategy relies on the late-stage formation of the triazolo ring, such as treatment of hydrazineylideneazepanes with formic acid leading to [1,2,4]­triazoloazepines reported by Piao and co-workers. , Alternatively, in the 1990s, one of us and Jochims first reported participation of 1-aza-2-azoniaalene salts in formal cycloaddition reactions with a variety of multiple bond compounds such as nitriles, acetylenes, and olefins . Since one of the reacting partners, that is, 1-aza-2-azoniaalene salt, is ionic and behaves as a 1,3-monopolar molecule with three-center and 4π-electron, the reaction can be characterized as polar cycloadditions and termed as polar [3 + + 2] cycloaddition reactions. , The protocol allows rapid synthesis of [1,2,4]­triazolium salts and the related annulated azaheterocycles that are otherwise difficult to access. − …”

C–O Coupling of Hydrazones with Diacetyliminoxyl Radical Leading to Azo Oxime Ethers—Novel Antifungal Agents

“…In general, free-radical functionalization of hydrazones with the formation of azocompounds is less developed compared to methods based on electrophilic attack of hydrazone carbon atoms, such as Michael-type reactions [20][21][22][23], chlorination [24], alkoxylation, or cyanation [25]. Geminal azoacetates are synthesized by the oxidation of hydrazones with Pb(OAc) 4 [26][27][28]. In the present work (Scheme 1C), diacetyliminoxyl was used as the only necessary reagent for high-yielding oxidative C-O coupling with the broad scope of both ketohydrazones and aldehyde-derived hydrazones at room temperature.…”