Organocatalytic (5+1) benzannulation of Morita–Baylis–Hillman carbonates: synthesis of multisubstituted 4-benzylidene pyrazolones†
4-Benzylidene pyrazolones are valuable scaffolds of numerous bioactive molecules, building blocks, ligands, dyes, and pigments. Therefore, the construction of 4-benzylidene pyrazolones with diverse substitution patterns is a desirable target of organic synthesis. We developed a (5+1) benzannulation strategy for the practical synthesis of multisubstituted 4-benzylidene pyrazolones, which are challenging to prepare through conventional condensation reactions. This organocatalytic cycloaddition of Morita–Baylis–Hillman carbonate undergoes an unprecedented α-double deprotonation pathway to de novo assemble the benzene ring, avoiding the use of a metal catalyst, large amount of base, and oxidant. This green approach features the use of a cheap catalyst (triethylenediamine), metal-free and mild conditions, readily available starting materials, and broad substrate scope. Moreover, several highly efficient one-pot approaches based on the (5+1) benzannulation reaction were developed, enabling the construction of diverse, interesting multisubstituted oxindole derivatives, which are also challenging to synthesize via existing methods.