Lanthanide-catalyzed endo- and enantioselective 1,3-dipolar cycloaddition reactions of nitrones with alkenes

The 1,3-dipolar cycloaddition reaction of alkenes with nitrones was catalyzed by Yb(OTf)(3) or Sc(OTf)(3) giving isoxazolidines in high yields and selectivities. The catalyst Yb(OTf)(3) induce a high endo-selectivity in the reaction of up to 94% de, whereas Sc(OTf)(3) show the highest rate accelerations. Both the conversion and endo-selectivity of the Yb(OTf)(3) and Sc(OTf)(3) catalyzed reactions proved to be dependent on the amount and type of molecular sieves added. The application of different chiral ligands in 1,3-dipolar cycloaddition reactions of a series of alkenes with nitrones catalyzed by Yb(OTf)(3) or Sc(OTf)(3) has been studied and it was found that high endo-selectivities and an ee's of up to 73% could be obtained by the use of Yb(OTf)(3) and 2,6-bis[4(S)-isopropyl-2-oxazolidin-2-yl]pyridine (PyBOX) as the chiral ligand. (C) 1997 Elsevier Science Ltd.