1. Contribution from the Departments of Chemistry and Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, California 92093, and Dipartimento di Chimica delle Sostanze Naturali, Universita' degli studi di Napoli “Federico II”, via D. Montesano 49, Napoli 80131, Italy