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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. |
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