Studies directed toward the total synthesis of azaspiracid. Construction of the C1–C19 carbon backbone and synthesis of the C10, C13 nonnatural transoidal bisspirocyclic ring system

Total synthesis of (+)-azaspiracid-1. An exhibition of the intricacies of complex molecule synthesis

The synthesis of the marine neurotoxin azaspiracid-1 has been accomplished. The individual fragments were synthesized by catalytic enantioselective processes: A hetero-Diels-Alder reaction to afford the E- and HI-ring fragments, a carbonyl-ene reaction to furnish the CD-ring fragment, and a Mukaiyama aldol reaction to deliver the FG-ring fragment. The subsequent fragment couplings were accomplished by aldol and sulfone anion methodologies. All ketalization events to form the nonacyclic target were accomplished under equilibrating conditions utilizing the imbedded configurations of the molecule to adopt one favored conformation. A final fragment coupling of the anomeric EFGHI-sulfone anion to the ABCD-aldehyde completed the convergent synthesis of (+)-azaspiracid-1.

Synthetic and Computational Studies on the ABC Trioxadispiroketal Subunit of the Marine Biotoxin Azaspiracid-1

The trioxadispiroketal residue in the marine biotoxin azaspiracid-1, which exists in a configuration capable of exhibiting a double anomeric effect, is believed to be the thermodynamically most stable bis-spiroketal diastereomer. In order to get insight into how structural factors affect this equilibrium, a simplified ABC trioxadispiroketal analog of azaspiracid-1 was synthesized and subjected to equilbration and computational studies. Compound 7, which represents a double anomeric effect was obtained as the major isomer, together with diastereomers 14 and 15, in a respective ratio of 62:22:16. DFT calculations for 7, 14 and 15 qualitatively matched this observation. These results suggest that while a double anomeric effect may play a major role in the stability of the trioxadispiroketal configuration in the more complex natural product, the substitution pattern of the C ring is also a contributing factor.

Synthesis of the ABCD Trioxadispiroketal Subunit of Azaspiracid-1: An Iodoetherification−Dehydroiodination Strategy for Complex Spiroketals

An unusual spiroketalization strategy in which a hydroxyalkene serves as a precursor to a cyclic enol ether was applied to the synthesis of the ABCD trioxadispiroketal subunit of azaspiracid-1. The trioxadispiroketal product, which represents a double anomeric effect, was obtained as a single trioxadispiroketal diastereomer. A key ploy in the synthesis of the CD segment was the use of a cyclopropane as a synthon for the C-14 methyl group.

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.

Synthesis of the Azaspiracid-1 Trioxadispiroketal

The de novo analysis, design, and synthesis of the azaspiracid-1 trioxadispiroketal system is described. A revised structural model was developed on the basis of an independent analysis of the NMR spectral data of the natural product that fit all of the data and the thermodynamically favored spiroketal paradigm. This model was then tested via synthesis using a novel trioxadispiroketalization process and supported by spectroscopic correlation.

Azaspiracid poisoning, the food-borne illness associated with shellfish consumption

Azaspiracid poisoning (AZP) is a recently discovered toxic syndrome that was identified following severe gastrointestinal illness from the consumption of contaminated mussels (Mytilus edulis). The implicated toxins, azaspiracids, are polyethers with unprecedented structural features. Studies toward total toxin synthesis revealed that the initial published structures were incorrect and they have now been revised. These toxins accumulate in bivalve molluscs that feed on toxic microalgae of the genus Protoperidinium, previously considered to be toxicologically benign. Although first identified in shellfish from Ireland, azaspiracid contamination of several types of bivalve shellfish species has now been confirmed throughout the western coastline of Europe. Toxicological studies have indicated that azaspiracids can induce widespread organ damage in mice and that they are probably more dangerous than previously known classes of shellfish toxins. The exclusive reliance on live animal bioassays to monitor azaspiracids in shellfish failed to prevent human intoxications. This was a consequence of poor sensitivity of the assay and the fact that azaspiracids are not exclusively found in the shellfish digestive glands used for toxin testing. The strict regulatory control of azaspiracids in shellfish now requires frequent testing of shellfish using highly specific and sensitive methods involving liquid chromatography-mass spectrometry.

Synthesis of the ABC ring system of Azaspiracid. 2. A systematic study into the effect of C(16) and C(17) substitution on bis-spirocyclization

[reaction: see text] A systematic study into the effect of C(16) and C(17) substitution on the stereochemical outcome of bis-spirocyclization to form the ABC ring system of azaspiracid is disclosed. Successful construction of the natural 10R,13R bis-spirocyclic stereochemistry has been accomplished on the C(16) benzyloxy-containing precursor.

Synthesis of the ABC ring system of azaspiracid. 1. Effect of D ring truncation on bis-spirocyclization

[reaction: see text] Synthesis of a spirocyclization precursor with a truncated D ring has been accomplished. Subsequent bis-spirocyclization induced the formation of equal amounts of the natural transoidal 10R,13R bis-spirocycle and its cisoidal 10R,13S epimer under an apparent thermodynamically controlled process.