1. Pharmacognosy Department, Faculty of Pharmacy, Sana'a University, P. O. Box 13150, Sana'a, Yemen
2. Department of Chemistry, University of Alabama in Huntsville, Huntsville, AL 35899, USA
3. Department of Medicinal Chemistry and Pharmacognosy, Faculty of Pharmacy, Aden University, Yemen
4. Leibniz Institute of Plant Biochemistry, Department of Bio-organic Chemistry, Weinberg 3, 06120 Halle/Saale, Germany