Nitrogen analogues of the Nazarov cyclization reaction ( known as imino @-@ Nazarov cyclization reactions ) have few instances ; it is one example of a generalized imino @-@ Nazarov cyclization reported ( shown below ) , and several iso @-@ imino @-@ Nazarov reactions in the literature . Even these tend to suffer from poor stereoselectivity , poor yields , or narrow scope . The difficulty stems from the relative over @-@ stabilization of the pentadienyl cation by electron donation , impeding cyclization .