Lewis acid-mediated [3+2] cycloaddition between hydrazones and olefins

[3+2] Cycloaddition between hydrazones and olefins was accelerated in the presence of a stoichiometric amount of BF3.OEt2 or a catalytic amount of Zr(OTf)(4), Hf(OTf)(4), or Sc(OTf)(3) under mild conditions. The corresponding pyrazolidine derivatives were obtained in moderate to high yields using this novel [3+2] Lewis acid catalysis. (C) 2003 Elsevier Science Ltd. All rights reserved.