Influence of the β-Alkoxy Group on the Diastereoselectivity of\nAldol Reactions of Tetrahydro-4<i>H</i>-thiopyran-4-one with\n4-Alkoxytetrahydro-2<i>H</i>-thiopyran-3-carboxaldehydes
Bibliographic record
Abstract
The diastereoselectivity of the aldol reaction of tetrahydro-4H-thiopyran-4-one (3) with 1,4-dioxa-8-thiaspiro[4.5]decane-6-carboxaldehyde (9a) under a variety of conditions is examined. Under\noptimized conditions, three of the four possible diastereomers from this aldol reaction can be obtained\nselectively (3−16:1). Reactions of 9a with the Li, B, Mg(II), and Ti(IV) enolates of 3 and with the\ncorresponding trimethylsilyl enol ether 4b in the presence of BF3·OEt2, SnCl4, or TiCl4 as promoters\ngave the Felkin adducts exclusively (>95%) as mixtures of syn (11a) and anti (12a) diastereomers.\nUse of the “amine-free” Li enolate of 3 gave 12a with a much higher diastereoselectivity (9:1) and\nyield (70%) than that obtained using the lithium diisopropylamide-generated Li enolate of 3 (2−3:1; 15−40%). The TiCl4-promoted reaction of 4b with 9a gave 11a with excellent selectivity (16:1). In contrast, the MgBr2·OEt2-promoted reaction of 4b with 9a gave the anti-Felkin adducts\nexclusively as a 3:1 mixture of syn (13a)/anti (14a) diastereomers. Similar aldol reactions of 3 with\nthe cis and trans isomers of 4-(methoxy)methoxytetrahydro-2H-thiopyran-3-carboxaldehyde (9b\nand 9c) were examined to probe the influence of the ketal protecting group in 9a on the observed\naldol diastereoselectivity. The results are rationalized by applying Evans' stereochemical model\nfor merged 1,2- and 1,3-asymmetric induction (nonchelation), with the exception of the MgBr2·OEt2-promoted reactions of 4b with 9a, 9b, and 9c, which are accommodated by assuming chelation\ncontrol. Comparison of the reactions of 9a, 9b, and 9c suggests that the ketal group in 9a uniquely\nallows high levels of either Felkin or anti-Felkin selectivity to be achieved.
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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) | 0.000 | 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) | 0.002 | 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".