Brønsted acid-catalyzed (3 + 2) cycloaddition of 2-indolylmethanols with o-alkynylnaphthols toward the construction of cyclopenta[b]indoles.

A Brønsted acid-catalyzed (3 + 2) cycloaddition of 2-indolylmethanols with o-alkynylnaphthols has been developed, leading to the efficient synthesis of cyclopenta[b]indoles with structural diversity in generally high yields. This study presents the first application of alkynes as 2C reaction partners in cycloadditions of 2-indolylmethanols. Furthermore, a chiral resolution strategy was developed using (R)-(-)-O-formylmandeloyl chloride as a resolution reagent to obtain optically pure cyclopenta[b]indole atropisomers from racemic substrates. Additionally, a catalytic asymmetric version of this (3 + 2) cycloaddition has been preliminarily explored, which disclosed the potential of the reaction for the enantioselective synthesis of axially chiral cyclopenta[b]indoles. This work not only offers a practical approach for the synthesis of cyclopenta[b]indoles bearing indole-fused five-membered rings but also expands the scope of indolylmethanol chemistry.