Page 1. Tetrahedron Letters,Vol.28,No.47,pp 5809-5810,1987 0040-4039/87 $3.00 + .OO Printed in Great Britain Pergamon Journals Ltd. ... 5- P Boulanger and G. Descotes, Tetrahedron Lett., 1986, 27, 2599. 6- a 1 E. Wunsch, W. Graf, 0. Keller, W.Keller and G. Wersin, Synthesis, 1986, 958. b) Gerard Sennyey, Gerard Barcelo and Jean-Pierre Senet, Tetrahedron Lett., 1986, 27, 5375. 7- L. Grehn and U. Ragnarsson, Synthesis, 1987, 275. ...