1. a Natural Products Division, Department of Chemistry , Sri Venkateswara University , Tirupati 517 502 , India
2. b Department of Chemistry , National Cheng Kung University , Tainan 701 , Taiwan
3. c Laboratoire de Chimie et Biochimie des Substances Naturelles , USM 0502 MNHN-UMR 5154 CNRS , 63 rue Buffon 75005 Paris , France