Study On The Synthesis Of Cyclopenta[b]indoles By Rhodium-catalyzed Annulation Of N-sulfonyl-1,2,3-triazoles And Indoles

N-sulfonyl-1,2,3-triazole is now becoming one of the most important precursors of metallocarbene,which could equilibrate with diazoimine through Dimroth rearrangement,then in the catalyzed of transition metal,it is able to be efficiently converted to ?-imino metallocarbene.Because of the electrophilicity of the carbene carbon and the nucleophilicity of the imino nitrogen,the ?-imino metallocarbene intermediate can be to participate all kinds of organic reactions,such as cycloaddition,X-H bond insertion reactions(X=C,N,or O),ylide formation and so forth.As early as 2012,Fokin reported the reaction of N-sulfonyl-1,2,3-triazole,in which ?-imino rhodium carbene underwent rearrangement to form aza-diene.Recently,our group also found that ester-substituted N-sulfonyl-1,2,3-triazole led to aza-diene intermediate through ring-opening and 1,2-migration reaction under rhodium catalysis,and this intermediate can undergo a[4+2]cyclization reaction with 3-substituted indole to form pyridine[b]indoles derivatives.Following the research on this subject,we found that the 3-unsubstituted indole can not undergo a similar[4+2]cyclization reaction,yet the formal[3+2]cyclization can occur by changing the reaction conditions,and cyclopenta[b]indoles can be obtained conveniently.The structures of cyclopenta[b]indoles are frequently included in important natural products and pharmaceutical molecules.The development of synthetic methods for such compounds is of great significance in the fields of natural product chemistry,medicinal chemistry,materials chemistry,etc.This research focuses on the rhodium-catalyzed synthesis of cyclopenta[b]indoles from N-sulfonyl-1,2,3-triazole and indole.After a lot of investigation,we used the 1,2-acyloxy migration strategy of a-imino rhodium carbene to achieve[3+2]cyclization of N-sulfonyl-1,2,3-triazole and indole in a one-pot procedure for synthesis of cyclopenta[b]indoles.Optimization of reaction conditions illustrated that under nitrogen atmosphere,with 2 mol%of Rh2(OAc)4 as catalyst,CHCl3 as reaction solvent,heating in sealing tube at 100? for 4 h.The reaction was cooled,and the product of Michael addition between the aza-diene intermediate and indole was detected.Then CCl3CO2H(1.0 equiv)was added to the mixture under N2 at room temperature.The mixture was stirred at 100?for 4 h again,and the desired cyclopenta[b]indoles could be obtained via intramolecular cyclization.In this transformation,we have successfully used ?-imino rhodium carbene as a formal[3C]synthon in the newly-formed five-membered ring,this strategy developed an efficient method for the synthesis of cyclopenta[b]indoles structures.