1. Dipartimento di Chimica Organica, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy, Natural Products Research Laboratories, School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, and Division of Medicinal Chemistry and Natural Products, School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599