The Diels–Alder reaction has after its initial description in 1928 quickly evolved to be one of the most frequently used and powerful transformations known to the organic chemist.^[1,2] Despite its widespread use and extensive mechanistic investigations, examples where the diene and the dienophile are part of the same π-conjugated system are scarce in literature.^[3,4] In connection to our ongoing efforts to design an organic, bistable magnetic photoswitch, we coincidentally discovered an example of such an intramolecular Diels–Alder reaction. This reaction additionally stands out because it proceeds with a half-life of 50 min at 293 K, which is exceptionally fast compared to other uncatalyzed Diels–Alder reactions of unactivated systems.^[5] Our findings offer insights that can pave the way for more step-efficient strategies to construct complex polycyclic molecular scaffolds and highlight that, nearly a century after its discovery, the Diels–Alder reaction still raises compelling fundamental questions.

[1] Diels, O.; Alder, K. Justus Liebigs Ann. Chem. 1928, 460 (1), 98–122.

[2] Nicolaou, K. C.; Snyder, S. A.; Montagnon, T.; Vassilikogiannakis, G. Angew. Chem. Int. Ed. 2002, 41 (10), 1668–1698.

[3] Fuchter, M. J.; Weimar, M.; Yang, X.; Judge, D. K.; White, A. J. P. Tetrahedron Lett. 2012, 53 (9), 1108–1111.

[4] Murase, T.; Matsuda, C.; Adachi, K.; Sawada, T.; Fujita, M. Commun. Chem. 2018, 1, 97.

[5] Houk, K. N.; Li, Y.; Evanseck, J. D. Angew. Chem. Int. Ed. 1992, 31 (6), 682–708.