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

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-5H-[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)2 under mild conditions. The [1,2,4]­triazolodiazepine compounds represent a class of N-containing biologically important heterocycles with a new type of scaffold.