Enantioselective Synthesis of Dihydrospiro[indoline-3,4′-pyrano[2,3-c]pyrazole] Derivatives via Michael/Hemiketalization Reaction

A new bifunctional squaramide organocatalyst derived from l-proline mediated the first enantioselective synthesis of dihydrospiro[indoline-3,4′-pyrano[2,3-c]pyrazole] derivatives in excellent enantioselectivity by reacting pyrazolones with isatylidine β,γ-unsaturated α-ketoester. This new catalyst outperformed widely used thioureas and squaramides in inducing enantioselectivity.