Enantioselective Synthesis of Spiro[indoline-3,2′-pyrroles] through N-Heterocyclic-Carbene-Catalyzed Formal [3+2] Annulation

An N-heterocyclic-carbene-catalyzed asymmetric formal [3+2] annulation reaction of isatin N-Boc ketimines (Boc = tert-butyloxycarbonyl) and 3-bromoenals was developed for the construction of spiro[indoline-3,2'-pyrrole] derivatives with one quaternary chiral center in good yields (up to 81 % yield) with excellent enantioselectivity (up to 99 % ee).