1. Laboratory for Ecological Chemistry , Stellenbosch University , Stellenbosch 7600 , South Africa , E-mail:
2. Laboratory for Ecological Chemistry , Stellenbosch University , Stellenbosch 7600 , South Africa
3. Present address: National Center for Natural Products Research, Thad Cochran Research Center, School of Pharmacy , University of Mississippi , MS 38677 , USA
4. Present address: The Australian Wine Research Institute , Glen Osmond, SA 5064 , Australia
5. Present address: Department of Chemistry , Eritrea Institute of Technology , Asmara , Eritrea