A Hetero Diels–Alder Approach to the Synthesis of Chromans (3,4-Dihydrobenzopyrans) Using Oxonium Ion Chemistry: The Oxa-Povarov Reaction
Bibliographic record
Abstract
An oxa analogue of the well-known Povarov reaction has been developed for the synthesis of 3,4-dihydrobenzopyrans (chromans). The reaction involves the formal inverse electron demand [4 + 2] cycloaddition reaction of in situ-generated cationic aryl 2-oxadiene oxocarbenium ions with alkenes. The oxonium ion intermediates are generated through Lewis acid (SnCl(4))-promoted reactions of phenol-derived Rychnovsky-type mixed acetals. The yield and diastereoselectivity of the chroman products are found to depend upon the substitution pattern of the precursor alkene (i.e., monosubstituted, trans- or cis-disubstituted and cyclic alkenes). Generally, the reactions afford the endo-diastereomers as the major products, except for the reactions of trans-β-methylstyrene, which afford exo-chromans. A comparison of the product distributions from the reactions of trans- and cis-β-methylstyrene reveal that the reaction proceeds, at least in part, by a nonconcerted ionic pathway. Just as for the aza-Povarov reaction, there are two potential mechanisms for the reaction. The first mechanism involves a direct asynchronous [4 + 2] cycloaddition pathway, while the second occurs through the stepwise Prins addition of the alkene to the aryl 2-oxadiene oxonium ion, followed by an intramolecular aromatic substitution reaction of the resultant cation (i.e., a domino Prins/intramolecular Friedel-Crafts reaction).
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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 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| 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".