Okadaic acid, C44H68O13, is a toxin produced by several species of dinoflagellates. It is known to accumulate in both marine sponges and shellfish. One of the primary causes of diarrhetic shellfish poisoning, okadaic acid is a potent inhibitor of specific protein phosphatases, and has a variety of negative effects on cells. A polyketide, polyether derivative of a C38 fatty acid, okadaic acid and other members of its family have illuminated many biological processes both with respect to dinoflagellate polyketide synthesis as well as the role of protein phosphatases in cell growth.
History As early as 1961, reports of gastrointestinal disorders following the consumption of cooked mussels appeared in both the Netherlands and Los Lagos. Attempts were made to determine the source of the symptoms; however, they failed to elucidate the true culprit, instead implicating a species of microplanctonic dinoflagellates. In the summers of the late 1970s, a series of food poisoning outbreaks in Japan led to the discovery of a new type of shellfish poisoning. Named for the most prominent symptoms, the new Diarrhetic Shellfish Poisoning (DSP) only affected the northern portion of Honshu during 1976; however, by 1977 large cities such as Tokyo and Yokohama were affected. Research into the shellfish consumed in the affected regions showed that a fat-soluble toxin was responsible for the 164 documented cases, and this toxin was traced to mussels and scallops harvested in the Miyagi prefecture. In northeastern Japan, a legend had existed that during the season of paulownia flowers, shellfish can be poisonous. Studies following this outbreak showed that toxicity of these mussels and scallops appeared and increased during the months of June and July, and all but disappeared between August and October. Elsewhere in Japan, in 1975 Fujisawa pharmaceutical company observed that the extract of a black sponge, Halichondria okadai, was a potent cytotoxin; this was dubbed Halichondrine-A. In 1981, the structure of one such toxin, okadaic acid, was determined after it was extracted from both the black sponge in Japan, Halichondria okadai, for which it was named, and a sponge in the Florida Keys, Halichondria melanodocia. Okadaic acid sparked research both for its cytotoxicicity, and for being the first reported marine ionophore. Several years later, one of the toxins responsible for DSP, dinophysistoxin-1 (DTX-1) (named for one of the organisms implicated in its production, Dinophysis fortii) was compared to and shown to be very chemically similar to okadaic acid; okadaic acid itself was implicated in DSP around the same time. Since its initial discovery, reports of DSP have spread throughout the world, and are especially concentrated in Japan, South America and Europe.
Synthesis
Derivatives Okadaic acid (OA) and its derivatives, the dinophysistoxins (DTX), are members of a group of molecules called polyketides. The complex structure of these molecules include multiple spiroketals, along with fused ether rings.
Biosynthesis Being polyketides, the okadaic acid family of molecules are synthesized by dinoflagellates via polyketide synthase (PKS). However, unlike the majority of polyketides, the dinoflagellate group of polyketides undergo a variety of unusual modifications. Okadaic acid and its derivatives are some of the most well studied of these polyketides, and research on these molecules via isotopic labeling has helped to elucidate some of those modifications. Okadaic acid is formed from a starter unit of glycolate, found at carbons 37 and 38, and all subsequent carbons in the chain are derived from acetate. Because polyketide synthesis is similar to fatty acid synthesis, during chain extension the molecule may undergo reduction of the ketone, dehydration, and reduction of the olefin. Failure to perform one of more of these three steps, combined with several unusual reactions is what allows for the formation of the functionality of okadaic acid. Carbon deletion and addition at the alpha and beta position comprise the other transformations present in the okadaic acid biosynthesis. Carbon deletion occurs by way of a Favorskii rearrangement and subsequent decarboxylation. Attack of a ketone in the growing chain by enzyme-bound acetates, and subsequent decarboxylation/dehydration results in an olefin replacing the ketone, in both alpha and beta alkylation. After this the olefin can isomerize to more thermodynamically stable positions, or can be activated for cyclizations, in order to produce the natural product.
Laboratory syntheses
Three total syntheses of okadaic acid have been achieved, along with many more formal syntheses and several total syntheses of the other dinophysistoxins. The first total synthesis of okadaic acid was completed in 1986 by Isobe et al., just five years after the molecule's structure was elucidated. The next two were completed in 1997 and 1998 by the Forsyth and Ley groups respectively. In Isobe's synthesis, the molecule was broken into three pieces, along the C14–C15 bonds, and the C27–C28 bonds. This formed fragments A, B, and C, which were all synthesized separately, after which the B and C fragments were combined, and then combined with the A fragment. This synthesis contained 106 steps, with a longest linear sequence of 54 steps. The precursors to all three fragments were all glucose derivatives obtained from the chiral pool. Spiroketals were obtained from precursor ketone diols, and were therefore formed thermally in acid.
Similar to Isobe's synthesis, the Forsyth synthesis sought to reduce the number of steps and to increase potential for designing analogues late in the synthesis. To do this, Forsyth et al. designed the synthesis to allow for structural changes and installation of important functional groups before large pieces were joined. Their resulting synthesis was 3% yielding, with 26 steps in the longest linear sequence. As above, spiroketalization was performed thermodynamically with introduction of acid.
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