Tetronamides as Latent Acyclic Vinylogous Amides in Formal Aza-[3 + 3] Cycloaddition Reactions with α,β-Unsaturated Iminium Salts. An Unexpected Rearrangement and an Approach to Synthesis of Substituted Piperidines

A detailed account regarding formal aza-[3 + 3] cycloaddition reactions of tetronamides with α,β-unsaturated iminium salts is described here. This investigation uncovers regioisomeric cycloadducts that were not found in previous studies involving this formal cycloaddition and an unexpected rearrangement that led to pyridines and dihydropyridines. Both stereochemical and regiochemical issues raised in this study provide further mechanistic insights into this cycloaddition. With careful control of reaction temperatures, the desired formal cycloadducts are obtained. Ensuing transformation of these cycloadducts into functionalized piperidines establishes the concept of employing tetronamides as latent acyclic vinylogous amides for the formal aza-[3 + 3] cycloaddition.