Br(?)nsted Acid-Mediated Tandem Diels-Alder/Aromatization Reactions Of Vinylindoles

The development of novel and efficient catalytic strategies for the construction of structural motifs presented in biologically active compounds or natural isolates is of considerable current interest.Domino,cascade and tandem reactions involve the rapid formation of structurally complex molecules from simple starting materials in only one operation,thereby minimizing the cost,waste,manual efforts,as well as avoiding the purification of the intermediates.Recently,significant efforts have been made to develop organocatalytic tandem transformations in settings.In this context,we document the preparation of synthetically important tetrahydrocarbazole derivatives by trifluoroacetic acid-catalyzed tandem Diels-Alder/aromatization reactions of 2-vinylindoles.1.A series of ethyl 1H-indole-2-carboxylate were prepared from various anilines through Fischer synthesis of indoles.The N-protected ethyl 1H-indole-2-carboxylates were then converted to 2-vinylindole derivatives in good yields by sequential reductions, oxidations and Wittig olefinations.2.The superior levels of reaction efficiency observed with 10 mol%of trifluoroacetic acid in dichloromethane at room temperature was selected to carry out the Br0nsted acid-catalyzed tandem Diels-Alder/aromatization reactions of 2-vinylindoles, providing a highly concise route to 3-indolyl-substituted tetrahydrocarbazole derivatives with up to 93%yield.The structures of all target compounds were characterized by ¹H NMR,¹³C NMR,MS,and elemental analysis.3.The deuterium-labeling experiment was performed to get the insight of the activation model of catalyst.Furthermore,cross Diels-Alder/aromatization reactions of two 2-vinylindoles with similar activity as well as two different 2-vinylindole components gave the primary explanation to the proposed mechanism.The structure of compounds was unambiguously confirmed by X-ray diffraction.