Nitrone and Alkyne Cascade Reactions for Regio- and Diastereoselective 1-Pyrroline Synthesis.

The synthesis of 1-pyrrolines from N -alkenylnitrones and alkynes has been explored as a retrosynthetic alternative to traditional approaches. These cascade reactions are formal [4+1] cycloadditions that proceed through a proposed dipolar cycloaddition and N -alkenylisoxazoline [3,3']-sigmatropic rearrangement. A variety of cyclic alkynes and terminal alkynes have been shown to undergo the transformation with N -alkenylnitrones under mild conditions to provide the corresponding spirocyclic and densely-substituted 1-pyrrolines with high regio- and diastereoselectivity. Mechanistic studies provide insight into the balance of steric and electronic effects that promote the cascade process and control the diastereo-and regioisomeric preferences of the 1-pyrroline products. Diastereoselective derivatization of the 1-pyrrolines prepared by the cascade reaction demonstrate the divergent synthetic utility of the new method.