GENERATION OF 2-AZAALLYL ANIONS BY THE TRANSMETALATION OF N-(TRIALKYLSTANNYL)METHANIMINES - PYRROLIDINE SYNTHESIS BY [3+2] CYCLOADDITIONS WITH ALKENES

Treatment of N-(trimethylstannyl)methanimines or N-(tri-n-butylstannyl)methanimines with methyllithium or n-butyllithium, respectively, affords 2-azaallyl anions by tin-lithium exchange. These anions undergo intermolecular or intramolecular [pi-4s + pi-2s] cycloadditions with alkenes and alkynes to generate pyrrolidines or pyrrolines after quenching with water or other electrophiles. The tin-lithium exchange method allows unstabilized 2-azaallyl anions to be generated for the first time. The lifetime of the anions is limited by a competing intermolecular side reaction. Therefore, relatively reactive alkenes and alkynes must be used, such as stilbene, styrenes, enynes, diphenylacetylene, vinyl sulfides, vinyl selenides, and vinyl silanes. The latter three types of anionophiles afford functionalized cycloadducts which may be transformed into more useful pyrrolidines by reduction, elimination, or oxidation. A synthesis of the alkaloid (+/-)-mesembrane was accomplished using an intramolecular 2-azaallyl anion cycloaddition.