Catalytic Diastereoselective [5 + 2] Annulation of N -Acryloyl Indoles with Cyclic Sulfonyl Enamides: Facile Access to Isoeburnamonine.

ZnBr2-catalyzed stereoselective [5 + 2] annulation of N-acryloyl indoles with cyclic sulfonyl enamides is reported, providing a concise and efficient synthesis of isoeburnamonine, which is the key intermediate for norvincamine. Both [2 + 2] and [4 + 2] cycloadducts, depending on the ring size of the enamides, have been shown to be the important intermediates for this [5 + 2] annulation.