Synthesis of spiro(indoline-2,3 '-hydropyridazine) via an "on-water" [4+2] annulation reaction

An on-water [4 + 2] annulation reaction between the 2-methyl-3H-indolium salt and alpha-bromo N-acyl hydrazone has been developed. The environmentally friendly strategy provides the first facile access to spiro(indoline-2,3 '-hydropyridazine) scaffolds. Compared with the reaction in organic solvents, the use of water as the sole solvent remarkably improves the reaction efficiency as well as chemoselectivity. This green protocol features high yield, broad substrate scope, and mild reaction conditions.