On the Origin of Siphonariid Polypropionates: Total Synthesis of Baconipyrone A, Baconipyrone C, and Siphonarin B via their Putative Common Precursor
Bibliographic record
Abstract
The hypothesis that siphonariid polypropionates originate from nonenzymatic processes on acyclic biosynthetic precursors was tested by examining the properties of the putative common precursor for the S. zelandica decapropionates siphonarin B, caloundrin B, baconipyrone A, and baconipyrone C, i.e., (4S,5S,6S,8RS,10S,11S,12R,14R)-14-(6-ethyl-3,5-dimethyl-4-oxo-4H-pyran-2-yl)-5,11-dihydroxy-4,6,8,10,12-pentamethylpentadecane-3,7,9,13-tetraone. The first synthesis of such a precursor was achieved in an efficient and fully enantioselective manner using a thiopyran-based strategy for polypropionate synthesis. This putative precursor, a complex mixture of ring-chain and keto-enol tautomers, was kinetically stable and isomerized exceedingly slowly to the thermodynamically more stable siphonarin B. In the presence of imidazole, the mixture reached apparent equilibrium within several hours giving siphonarin B as the predominant component (ca. 70%), thereby constituting its enantioselective total synthesis. In the presence of alumina, both siphonarin B and the dihydroxytetraone precursor underwent retro-Claisen rearrangements (via a hemiacetal tautomer) to give baconipyrones A and C among other products. This is the first total synthesis of baconipyrone A and "biomimetic" synthesis of baconipyrone C. Control experiments suggested that baconipyrone A was produced in an unprecedented cascade process where the intermediate enol(ate) of the retro-Claisen rearrangement was directly engaged in aldol cyclization while baconipyrone C resulted from simple ketonization of the enol(ate). These experiments provide the first unambiguous demonstration that the baconipyrones are plausible isolation artifacts and suggest they are most likely derived from siphonarins rather than an "acyclic" precursor. Caloundrin B was not detected among the products from any of the isomerization experiments, suggesting the possibility that it is an unstable biosynthetic product.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".