1 Peason R G et al..J.Org.Che m.,1967;32:28992 W eaver W M et al..J.A mer.Chem.S oc.,1964;86:2613 G ordon J E e t al..J.Chem.Soc.Che m.Commu n.,1971:11604 W instein S et al..Tet rahed ron L et t.,1960;9:245 Parker A J e t al..J.Chem.Soc.,1961:43986 Pet er Sykes.A Guidebook t o M echanism in Organic Chemist ry,sixt h edit ion.97