Synthesis of the ABCD Ring System of Azaspiracid We thank Drs. D. H. Huang and G. Siuzdak for NMR spectroscopic and mass spectrometric assistance, respectively. This work was financially supported by the National Institutes of Health (USA), The Skaggs Institute for Chemical Biology, a predoctoral fellowship from Bristol-Myers Squibb (to F.B.), postdoctoral fellowships from The Skaggs Institute for Research (to W.Q.), the Academy of Finland, the Ella and Georg Ehrnrooth Foundation and the Tauno Tönning Foundation (all to P.M.P.), and Bayer AG (to J.H.), as well as grants from Abbott, Amgen, ArrayBiopharma, Boehringer-Ingelheim, Glaxo, Hoffmann-La Roche, DuPont, Merck, Novartis, Pfizer, and Schering Plough.