1. Department
of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein 9300, Republic of South Africa
2. Department
of Chemistry, University of Pretoria, Private Bag X20, Hatfield 0028, Republic of South Africa
3. Departamento
de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain