The 6,7-indolyne shows remarkable regioselectivity in its cycloaddition with 2-substituted furans. Electron-donating groups give predominantly the more sterically crowded product, while electron-withdrawing groups display the opposite regioselectivity. By contrast, 4,5- and 5,6-indolynes show no regioselectivity. Optimized electronic structure calculations using the M06-2X density functional and 6-311+G(2df,p) basis set revealed that the 6,7-indolynes are highly polar structures and that their cycloadditions have substantial electrophilic substitution character that leads to the observed preference for contrasteric products.

An efficient nine-step total synthesis of the annulated indole natural products (±)-cis-trikentrin A and (±)-herbindole A was accomplished via an intermolecular Diels–Alder cycloaddition using our recently developed indole aryne (indolyne) methodology as the key step. This strategy provides rapid access into the trikentrins and the related herbindoles and represents the first application of this methodology to natural products total synthesis. The required 6,7-indolyne precursor was readily constructed by means of the Bartoli indole synthesis with substituted nitrobenzenes and vinyl magnesium bromide.