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A small amount ca. 5% of the corresponding isoindoline was obtained along with the major reaction product
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A small amount ca. 5% of the corresponding isoindoline was obtained along with the major reaction product.
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To the best of our knowledge, the combinationCF3TMS/fluoride source (i.e., TBAT: tetrabutylammonium difluorotriphenylsilicate) has never been reported as a base
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To the best of our knowledge, the combinationCF3TMS/fluoride source (i.e., TBAT: tetrabutylammonium difluorotriphenylsilicate) has never been reported as a base.
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Control experiments with either reagent (CF3TMS or TBAT) have been carried out separately. In both cases unaltered starting materials were recovered, even after several hours at room temperature
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Control experiments with either reagent (CF3TMS or TBAT) have been carried out separately. In both cases unaltered starting materials were recovered, even after several hours at room temperature.
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33
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84055221162
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For details, see Supporting Information
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For details, see Supporting Information.
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34
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This transition state has been suggested according to the most stable conformation of the corresponding enamine, which was in turn obtained by an MM2 minimization. A stabilizing electrostatic interaction between the oxygen of the sulfinamide and the electrophilic ester carbonyl carbon leading to a rigid eight-membered boat-like transition state could explain the high diastereoselectivities observed. For similar eight-membered transition states in intramolecular Michael reactions, see ref 4
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This transition state has been suggested according to the most stable conformation of the corresponding enamine, which was in turn obtained by an MM2 minimization. A stabilizing electrostatic interaction between the oxygen of the sulfinamide and the electrophilic ester carbonyl carbon leading to a rigid eight-membered boat-like transition state could explain the high diastereoselectivities observed. For similar eight-membered transition states in intramolecular Michael reactions, see ref 4.
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84857796725
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In an attempt to rationalize the performance of these uncommon bases, we suggest that the bulky noncoordinating tetrabutyl ammonium cation leads to a more reactive "metaloenamine" intermediate than the corresponding metal cation which could be stabilized by the negatively charged oxygen of the chiral sulfoxide group via a six-membered chelate
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In an attempt to rationalize the performance of these uncommon bases, we suggest that the bulky noncoordinating tetrabutyl ammonium cation leads to a more reactive "metaloenamine" intermediate than the corresponding metal cation which could be stabilized by the negatively charged oxygen of the chiral sulfoxide group via a six-membered chelate.
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84055221161
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For details, see Supporting Information
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For details, see Supporting Information.
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For the use of δ -amino acids in the synthesis of β -turn containing peptide hairpins, see: Rai, R.; Vasudev, P.G.; Ananda, K.; Raghothama, S.; Shamala, N.; Karle, I. L.; Balaram, P. Chem.; Eur. J. 2007, 13, 5917.
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We also essayed the diastereodivergent reduction using L-selectride as the reducing agent, but low yields (30%) and diastereoselectivities (5:1 dr) were achieved. The reversal diastereofacial selectivity in the reductions of tert-butylsulfinilketimines by either NaBH4 or L-selectride has been described: Colyer, J. T.; Andersen, N. G.; Tedrow, J. S.; Soukup, T. S.; Faul, M. M. J. Org. Chem. 2006, 71, 6859.
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84055221159
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A cross-peak between the two methine protons in the NOESY spectrum of 4a allowed assignment of the relative stereochemistry of the new stereocenter. The stereochemical outcome is in agreement with the chelated transition state suggested for reductions of tert-butanesulfinimides with NaBH4 (see Supporting Information)
-
A cross-peak between the two methine protons in the NOESY spectrum of 4a allowed assignment of the relative stereochemistry of the new stereocenter. The stereochemical outcome is in agreement with the chelated transition state suggested for reductions of tert-butanesulfinimides with NaBH4 (see Supporting Information).
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