**Acknowledgements**

The research leading to these results has received funding from the European Union's Seventh Framework Programme FP7/2007–2013 under grant agreement n° 229893 (NatPharma) and from grant No. 01504-07 awarded by U.S. Center for Disease Control and Prevention and North Carolina Department of Health and Human Services to Carmelo Tomas. Mass and NMR spectra were recorded at the "Centro Interdipartimentale di Analisi Strumentale", Università di Napoli "Federico II". The assistance of the staff is gratefully acknowledged. 
