Cycloaddition of Heteroatom-Substituted 2-Azaallyl Anions with Alkenes. Synthesis of 1-Pyrrolines and Bridged Azabicyclic Compounds

Nonstabilized 2-azaallyl anions bearing heteroatom substituents [R¹CHNC(X)R²(−)Li(+), where R¹ and R² are hydrogen or alkyl groups and X = OMe, SPh, or NR₂] were generated and found to undergo efficient [3 + 2] cycloadditions with alkenes to provide 1-pyrrolines after loss of LiX. The 2-azaallyl anions were generated by tin−lithium exchange on stannyl imidates, thioimidates, or amidines R¹CH(SnBu₃)NC(X)R² with n-butyllithium. The initially formed 1-pyrrolines were found to be deprotonated under the reaction conditions to afford 1-metalloenamines, which could be quenched with alkyl halides, carbonyl compounds, or MeSSMe to provide further functionalized 1-pyrrolines. Cyclic methoxy-substituted 2-azaallyl anions were generated and were found to undergo cycloadditions with alkenes to afford bridged azabicyclic compounds (1-methoxy-7-azabicyclo[2.2.1]heptanes and 1-methoxy-8-azabicyclo[3.2.1]octanes). These are the first examples of cyclic nonstabilized 2-azaallyl anions.