Annulation of five-membered cyclic enols with 3-aryl-2H-azirines: Catalytic versus non-catalytic cycloaddition

Abstract A copper(I)-NHC-catalyzed annulation of 5-membered cyclic enols of furan, thiophene and indene family with 3-aryl-2H-azirines has been developed to provide a rapid access to furo[3,4-b]pyrrole, thieno[3,4-b]pyrrole, indeno[1,2-b]pyrrole derivatives. The reaction proceeds via a copper-initiated N C2 azirine bond cleavage with the retention of the C N double bond in the annulation product. The reaction of tetronic acids with 3-aryl-2H-azirines can also proceed under catalyst-free conditions, through a double addition of the enol to the azirine, but this route provides poor yields of the annulation products. This is the first example of the N C2 bond cleavage in 2-unsubstituted 2H-azirines in the absence of a transition metal catalyst.