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