1. Contribution from the Instituto de Quimica-Fisica “Rocasolano”, C.S.I.C., C/Serrano 119, 28006 Madrid, Spain, the N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospekt 47, 117913 Moscow, Russian Federation, and the Department of Chemistry, University of Reading, Whiteknights, P.O. Box 224, Reading RG6 6AD, UK