Studies On The [3+2] Cycloaddition Reactions Of N-alkyl Indole With Ketene Dithioacetals

Indole derivatives are important organic compounds.Because of their structural diversity and important biological activities,indole derivatives have been widely used in medicine,perfume,agrochemicals,pigments and materials science.Among them,cyclopenta[b]indole is the core structure of many alkaloids.In these alkaloids,Fischerindole I is an important cyanobacter alkaloid,Terpendole E is a kind of KSP inhibitors,and Enitrem and Paxilline could be used for selective blocking in pharmacology.It was found that most of the methods for synthesizing cyclopenta[b]indoles are from the transition metal catalyzed C-H bond functionalization of 2-or 3-substituted indoles.The development of simple,efficient and environmentally friend method for the synthesis of cyclopenta[b]indoles is still expected.In this paper,the [3+2] cycloaddition reaction of N-alkyl fluorene with dithioketal with 2,3 unsubstituted groups promoted by trifluoromethanesulfonic acid was developed,a series of cyclopenta[b]indoles were synthesized under mild conditions,which expands the application of ketene dithioacetals in the synthesis of heterocyclic compounds and also provides a new synthetic strategy for the synthesis of hydrazine derivatives.This method has the advantages such as from easily available substrates,no transition metal catalysis,environment-benign,convenient operation etc.In this paper,the structure of the series of cyclopenta[b]indoles synthesized was confirmed by 1H-NMR,13C-NMR,high-resolution mass spectrometry and single crystal X-ray diffraction experiments.