Anionic cyclization of N-(trans-2,3-Diphenyl- aziridin-1-yl)imines and its application of sesquiterpene synthesis

Abstract

N-Aziridinylimines have found many useful applications in organic synthesis due to their ability to generate a variety of reactive intermediates such as cations, diazoalkanes and carbenes. We reported radical cyclization of N-aziridinylimines and its application to sesquiterpene synthesis. This approach is quite unique because it involves a consecutive carbon-carbon bond formation to construct a quaternary carbon center. However, an intermolecular addition of an alkyl radical onto the N-aziridinyl group did not proceed due to its inefficiency. As extension of this radical approach, we studied anionic cyclization of N-aziridinylimines. Previously, it was reported that N-(2-phenylaziridin-1-yl)-imines reacted with allyllithium, vinyllithium, allyl- and vinyl-Grignard reagents to give the addition products after liberation of styrene and nitrogen gas. However, we have found that the success of the reaction depended critically on the nature of the alkyllithiums. Treatment of 2-phenyl N-aziridinylimines with alkyllithiums resulted in the formation of the side product as a major product without giving the desired addition product. The use of 2,3-diphenyl N-aziridinylimines obviates the problem of the formation of the side product. Thus, treatment of 2,3-diphenyl N-aziridinylimines with alkyllithiums in ether/pentane at $-78^\circ C$ afforded the desired product in 60-75% yields. Next, We have studied the possibility of anionic cyclization of the carbanion intermediate onto unactivated alkenes and alkynes as acceptors. Anionic cyclization of N-(trans-2,3,-diphenylaziridin-1-yl)imines using unactivated alkenes and alkynes as acceptors proceeded smoothly, yielding cyclized products in good yields. Also, this consecutive carbon-carbon-bond formation approach was applied to synthesize naturally occurring dl-cedrene , dl-pentalene and dl-hirsutene. This synthesis was much simpler and more efficient than previously reported synthetic methods.