1. Visiting Professor, on sabbatical leave from Wayne State University, Detroit, MI 48202, U.S.A.
2. For an excellent recent review see: H. G. Viehe, Z. Janousek, R. Merenyi, L. Stella, Acc. Chem. Res., 18, 148 (1985).
3. A. T. Balaban, Rev. Roum. Chim., 16, 725 (1971). (b) R. W. Baldock, P. Hudson, A. R. Katritzky, F. Soti, J. Chem. Soc., perkin I, 1422 (1974).
4. M. J. S. Dewar, J. Am. Chem. Soc., 74, 3353 (1952). (b) D. Crans, T. Clark, P. v. R. Schleyer, Tetrahedron Lett., 21, 3681 (1980). (c) G. Leroy, D. Peeters, Theochem., 2, 133 (1981).
5. M. T. Caproiu, M. Elian, N. Grecu, N. Negoita, A. T. Balaban, J. Chem. Soc., Perkin II, 591 (1983).