Synthesis of Saxitoxin Biosynthetic Intermediates: Reveal the Mechanism for Formation of its Tricyclic Skeleton in Biosynthesis.

The synthesis and biosynthesis of the complex saxitoxin (STX) structure have garnered significant interest. Previously, we hypothesized that the tricyclic skeleton of STX originates from the monocyclic precursor 11-hydroxy-IntC'2 during biosynthesis, although direct evidence has been lacking. In this study, we identified conditions to synthesize a proposed tricyclic biosynthetic intermediate, 12,12-dideoxy-decarbamoyloxySTX (dd-doSTX), along with its 6-epimer (6-epi-dd-doSTX) and a bicyclic compound, in a single step from di-Boc protected 11-hydroxy-IntC'2. The reaction mechanism involves successive aza-Michael addition of a guanidino amine to the conjugated olefin. Notably, both dd-doSTX and 6-epi-dd-doSTX were detected in a toxin-producing cyanobacterium, suggesting that the biosynthetic enzymes may generate these compounds via similar mechanisms.

Synthesis and Identification of decarbamoyloxySaxitoxins in Toxic Microalgae and their Reactions with the Oxygenase, SxtT, Reveal Saxitoxin Biosynthesis.

Saxitoxin (STX, 1) is a representative compound of paralytic shellfish toxins (PSTs) that are produced by marine dinoflagellates and freshwater cyanobacteria. Although several pathways have been proposed for the biosynthesis of STX, the order of ring and side chain hydroxylation, and formation of the tricyclic skeleton have not been well established. In this study, 12,12-dideoxy-decarbamoyloxySTX (dd-doSTX, 2), the most reduced STX analogue having the tricyclic skeleton, and its analogues, 12ß-deoxy-doSTX (12ß-d-doSTX, 3), 12α-deoxy-doSTX (12α-d-doSTX, 4), and doSTX (5), were synthesized, and these compounds were screened in the toxic microalgae using high-resolution LCMSMS. dd-doSTX (2) and 12ß-d-doSTX (3) were identified in the PSTs-producing dinoflagellates (Alexandrium catenella, A. pacificum, and/or Gymnodinium catenatum) and in the cyanobacterium Dolichospermum circinale (TA04). doSTX (5), previously isolated from the dinoflagellate G. catenatum, was also identified in D. circinale (TA04). Furthermore, the conversion of 2 to 3, and 4 to 5, by SxtT with VanB, a reported Rieske oxygenase and its redox partner in STX biosynthesis, was confirmed. These results support that 2 is a possible biosynthetic precursor of STX, and that ring and side-chain hydroxylations proceed after cyclization.

Sonographic findings in two cases of lymphangioma of the mesocolon in adults.

Lymphangioma of the mesocolon is very rare. We report two cases of surgically resected and histologically proven mesocolic lymphangioma in adults. In both cases, ultrasound revealed a large cystic mass with multiple thin septa in the lower abdomen. A peculiar finding was the large craniocaudal sliding movement of the mass synchronized with the patient's respiration, which was a clue to the diagnosis of mesenteric lymphangioma. © 2017 Wiley Periodicals, Inc. J Clin Ultrasound 46:78-81, 2018.