Electron-Transfer-Induced Diels–Alder Reactions of Indole and Exocyclic Dienes: Synthesis and Quantum-Chemical Studies

✍ Scribed by Udo Haberl; Eberhard Steckhan; Siegfried Blechert; Olaf Wiest

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 208 KB
• Volume: 5
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/(sici)1521-3765(19991001)5:10<2859::aid-chem2859>3.0.co;2-r

✦ Synopsis

Photoinduced electron transfer (PET) initiates radical-cation Diels ± Alder reactions between indole and exocyclic dienes. As catalysts, triarylpyrylium tetrafluoroborates are used. The products are [b]-anellated tetrahydrocarbazoles, which are not accessible through neutral cycloadditions. The resulting structures are of interest because of their DNA intercalating properties, their antitumor activity, and their importance as building blocks of numerous indole alkaloids. When the exocyclic dienes are substituted with an aromatic or heteroaromatic ring, the reaction proceeds with complete regioselectivity. The mechanism is discussed in detail and the potential-energy hypersurface of the radical-cation Diels ± Alder reaction is investigated with quantum-chemical methods by variation of the distances of the two newly formed bonds. The potential surface demonstrates the nonsynchronous and nonconcerted reaction pathway. The energies of the different long-bond intermediates, which are local minima on the energy surface, have been calculated with semiempirical, molecular orbital, and density functional methods. Regio-and diastereoselectivities can be explained and predicted by comparing these energies. Quantumchemical calculations lead to a reaction model that is consistent with the observed products.