One-pot synthesis of spirooxindoles bearing α-methylene-γ-butyrolactone moiety from Morita–Baylis–Hillman carbonates of isatins and paraformaldehyde

One-pot synthesis of spirooxindoles bearing α-metylene-γ-butyrolactone moiety has been carried out by the reaction of Morita–Baylis–Hillman (MBH) carbonates of isatins and paraformaldehyde in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in refluxing 1,2-dichloroethane in moderate yields. The reaction proceeded via (i) the formation of resonance-stabilized N-ylide from DBU and the MBH carbonate of isatin, (ii) a selective γ-attack of N-ylide to formaldehyde, (iii) lactonization to liberate methoxide ion, (iv) addition of methoxide ion to the iminium part of DBU, (v) intramolecular hydride transfer via a six-membered transition state, and finally (vi) elimination of DBU.