Synthesis and Identification of Key Biosynthetic Intermediates for the Formation of the Tricyclic Skeleton of Saxitoxin

Abstract

Saxitoxin (STX) and its analogues are potent voltage‐gated sodium channel blockers biosynthesized by freshwater cyanobacteria and marine dinoflagellates. We previously identified genetically predicted biosynthetic intermediates of STX at early stages, Int‐A′ and Int‐C′2, in these microorganisms. However, the mechanism to form the tricyclic skeleton of STX was unknown. To solve this problem, we screened for unidentified intermediates by analyzing the results from previous incorporation experiments with 15N‐labeled Int‐C′2. The presence of monohydroxy‐Int‐C′2 and possibly Int‐E′ was suggested, and 11‐hydroxy‐Int‐C′2 and Int‐E′ were identified from synthesized standards and LC‐MS. Furthermore, we observed that the hydroxy group at C11 of 11‐hydroxy‐Int‐C′2 was slowly replaced by CD3O in CD3OD. Based on this characteristic reactivity, we propose a possible mechanism to form the tricyclic skeleton of STX via a bicyclic intermediate from 11‐hydroxy‐Int‐C′2.

Thirty Years of (TMS)3SiH: A Milestone in Radical-Based Synthetic Chemistry

“…Deoxygenation via reaction by TTMSS and AIBN in 1,4-dioxane was carried out in position C-12 of the tricyclic intermediate thiocarbonyl imidazolide 80 for the synthesis of intermediate 81 in the synthesis of saxitoxin. 83 In the O-phenylthiocarbonate 82, the deoxygenation was found to be effective and achieved with a 88% yield of product 83. 84 In the angular triquinane ring of ventricos-7(13)-ene, the final deoxygenation step of precursor alcohol of 84 failed to occur via tosylation as well as via mesylate due to a significant formation of side products.…”

PEARL-catalyzed peptide bond formation after chain reversal during the biosynthesis of non-ribosomal peptides

“…Ethyl ketones are commonly observed motifs during PK/NRP biosynthesis, but the biosynthetic precursors of the ethyl group are usually S -adenosyl methionine (SAM) and malonate. For instance, the ethyl ketone derivative of arginine is a biosynthetic intermediate of saxitoxin 32,33 and is biosynthesized by a polyketide-like synthase SxtA. 46 Malonyl-CoA is loaded onto the ACP and is then methylated by the methyltransferase domain of SxtA.…”

Substrate Promiscuity of a Paralytic Shellfish Toxin Amidinotransferase

“…10 Intermediate 5 is anticipated to undergo spontaneous cyclization to deliver aminoimidazole 6, a molecule that has been previously identified in both PST-producing cyanobacteria and dinoflagellates. [17][18][19] Amidinotransferases are well-characterized for their activity on amino acid substrates; however, reactions of amidinotransferases on other classes of substrates are less well-studied. Understanding the reactivity and substrate promiscuity of SxtG provides the opportunity to identify a biocatalyst with a complementary substrate scope to known amidinotransferases.…”