P(NMe2)3-Mediated Reductive (1 + 4) Annulation Reaction of Isatins with Nitroalkenes: An Access to Spirooxindolyl Isoxazoline N-Oxides and Their Corresponding Isoxazolines.

The reductive (1 + 4) annulation reaction of isatins and substituted nitroalkenes mediated by a trivalent phosphorus reagent has been realized for the first time, providing easy access to spirooxindolyl isoxazoline N-oxides in moderate to excellent yields with a flexible substrate scope. This reaction presumably proceeds through a Michael addition-intramolecular substitution sequence via active in situ generated Kukhtin-Ramirez zwitterions from isatins and P(NMe2)3. It is also demonstrated that the spirooxindolyl isoxazoline N-oxides can be readily converted into the corresponding isoxazolines in good yields.