Regioselective Diels–Alder cycloadditions and other reactions of 4,5-, 5,6-, and 6,7-indole arynes

✍ Scribed by Neil Brown; Diheng Luo; David Vander Velde; Shaorong Yang; Allen Brassfield; Keith R. Buszek

• Publisher: Elsevier Science
• Year: 2009
• Tongue: French
• Weight: 204 KB
• Volume: 50
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/j.tetlet.2008.10.086

✦ Synopsis

The regioselectivity of Diels-Alder cycloadditions of indole arynes (indolynes) at all three benzenoid positions was examined. Cycloadditions with the 4,5-and 5,6-indolynes, derived via metal-halogen exchange from the corresponding o-dibromo indoles, showed essentially no selectivity with 2-t-butylfuran. In contrast, the 6,7-indolyne displayed virtually complete preference for the more sterically congested cycloadduct. This same cycloadduct undergoes a facile acid-catalyzed rearrangement to afford the annulated enone, or alternatively, undergoes hydrolysis and oxidation in the presence of air to give the indolobenzoquinone. The 5,6-difluoroindoles show anomalous behavior and give either 5-fluoro-6,7-indolynes with n-BuLi in ether, or 5,6-indolynes with n-BuLi in toluene. We have also demonstrated that benzenoid indolynes can be easily and conveniently generated by the fluoride-induced decomposition of o-trimethylsilyl triflates.