A detailed account regarding formal aza-[3 + 3] cycloaddition reactions of tetronamides with alpha,beta-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.
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Fused pyridine derivatives R 0450Tetronamides 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. -It is found that the temperature strongly affects the outcome of the [3 + 3] cycloadditions. When it is less than 130°C, the expected cycloadducts of type (III) and (VIII) are mainly formed, while at 150 < t < 220°C, considerable amounts of head-to tail regioisomers like (IV) and (IX) are isolated. The latter were found in previous studies. At higher temperature 220°C, rearrangement of the expected cycloadducts to pyridine derivatives of type (V) or (XII) takes place. The cycloaddition proceeds without stereoselectivity and the products can smoothly be transformed into substituted piperidines. -(SYDORENKO, N.; HSUNG*, R. P.; DARWISH, O. S.; HAHN, J. M.; LIU, J.; J.
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