Synthesis of Enantiopure Piperazines via Asymmetric Lithiation–Trapping of <i>N</i>-Boc Piperazines: Unexpected Role of the Electrophile and Distal <i>N</i>-Substituent
Bibliographic record
Abstract
A new method for the synthesis of enantiopure α-substituted piperazines via direct functionalization of the intact piperazine ring is described. The approach utilizes the asymmetric lithiation-substitution of an α-methylbenzyl-functionalized N-Boc piperazine using s-BuLi/(-)-sparteine or (+)-sparteine surrogate and provides access to a range of piperazines (as single stereoisomers). Optimization of the new methodology required a detailed mechanistic study. Surprisingly, it was found that the main culprits affecting the yield and enantioselectivity were the electrophile (the last reagent to be added to the reaction flask) and the distal N-substituent. The mechanistic studies included optimization of lithiation times using in situ IR spectroscopy, identification of a ring-fragmentation of the lithiated piperazines (that could be minimized with sterically hindered N-alkyl groups), and use of a novel "diamine switch" strategy to improve enantioselectivity with certain electrophiles. The methodology was showcased with the preparation of an intermediate for Indinavir synthesis and the stereoselective synthesis of 2,5-trans- and 2,6-trans-piperazines.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
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 teacher head, 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".