Total Synthesis of (±)-Cis-Trikentrin B via Intermolecular 6,7-Indole Aryne Cycloaddition and Stille Cross-Coupling.

An efficient total synthesis of the annulated indole natural product (±)-cis-trikentrin B was accomplished by means of a regioselectively generated 6,7-indole aryne cycloaddition via selective metal-halogen exchange from a 5,6,7-tribromoindole. The unaffected C-5 bromine was subsequently used for a Stille cross-coupling to install the butenyl side chain and complete the synthesis. This strategy provides rapid access into the trikentrins and the related herbindoles, and represents another application of this methodology to natural products total synthesis. The required 5,6,7-indole aryne precursor was prepared using the Leimgruber-Batcho indole synthesis.

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

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 anomolous 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.