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

Abstract 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.