Organocatalytic Nazarov-type cyclization of 3-alkynyl-2-indolylmethanols: construction of axially chiral cyclopenta[b]indole scaffolds

In recent years, it has become an urgent task to design new types of indole-based platform molecules for Nazarov-type cyclizations and develop organocatalytic Nazarov-type cyclizations for synthesizing indole derivatives. To fulfill this task, in this work, by changing the alkynyl terminal substituent from t-Bu to an aryl group, the reactivity of 3-alkynyl-2-indolylmethanols is modulated and the new platform molecules serve as competent substrates for Brønsted acid-catalyzed Nazarov-type cyclization. Based on this new reactivity, the first organocatalytic Nazarov-type cyclization of aryl-substituted 3-alkynyl-2-indolylmethanols with 2-naphthols is accomplished, leading to the efficient construction of a new class of axially chiral 3, 4-dihydrocyclopenta[b]indole scaffolds. This preliminary investigation of organocatalytic asymmetric Nazarov-type cyclization provides an optional strategy for the atroposelective construction of this new class of axially chiral cyclopenta[b]indole scaffolds. In addition, the first preparation of axially chiral 3, 4-dihydrocyclopenta[b]indole with optical purity is established through chiral resolution, which could serve as a complementary method to catalytic asymmetric approaches.