Second-Generation Total Synthesis of Azaspiracids-1, -2, and -3

The naturally occurring but scarce marine neurotoxins azaspiracids-1, -2, and -3 have been synthesized from five key building blocks by a convergent strategy that involved dithiane and Stille coupling reactions. The ABCD fragments were constructed through a cascade reaction involving deprotection/self-assembly of the precursors, while the FGHI fragment was forged by a neodymium triflate-induced cyclization. The final ring closure (ring G) was achieved, after the union of all fragments, through an iodoetherification reaction followed by reductive removal of the facilitating iodine residue. These improved, second-generation routes confirm the absolute structures and render all three azaspiracids readily available for biological studies.