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1
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11544346529
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For asymmetric cycloaddition reactions of 2-isoxazolines, see: a
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For asymmetric cycloaddition reactions of 2-isoxazolines, see: a) K.V. Gothelf, K. A. Jørgensen, Chem. Rev. 1998, 98, 863-909;
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Chem. Rev
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Gothelf, K.V.1
Jørgensen, K.A.2
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2
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44649151639
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for selected examples of 2-isoxazolines for the construction of aldol and 1,3-amino alcohol functionalities, see
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b) A. Bãdoiu, Y. Brinkmann, F. Viton, E. P. Kündig, Pure Appl Chem. 2008, 80, 1013-1018; for selected examples of 2-isoxazolines for the construction of aldol and 1,3-amino alcohol functionalities, see:
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(2008)
Pure Appl Chem
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, pp. 1013-1018
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Bãdoiu, A.1
Brinkmann, Y.2
Viton, F.3
Kündig, E.P.4
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7
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0348241047
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g) J. W. Bode, N. Fraefel, D. Muri, E. M. Carreira, Angew. Chem. 2001, 113, 2128-2131;
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Angew. Chem
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Bode, J.W.1
Fraefel, N.2
Muri, D.3
Carreira, E.M.4
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8
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0001386311
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for the use of isoxazolines to generate β-amino acids, see
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Angew. Chem. Int. Ed. 2001, 40, 2082-2085; for the use of isoxazolines to generate β-amino acids, see:
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(2001)
Angew. Chem. Int. Ed
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, pp. 2082-2085
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9
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17644402745
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h) A. A. Fuller, B. Chen, A. R. Minter, A. K. Mapp, J. Am. Chem. Soc. 2005, 127, 5376-5383.
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J. Am. Chem. Soc
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Fuller, A.A.1
Chen, B.2
Minter, A.R.3
Mapp, A.K.4
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13
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2342633120
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M. P. Sibi, K. Itoh, C. P. Jasperse, J. Am. Chem. Soc. 2004, 126, 5366-5367;
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J. Am. Chem. Soc
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Sibi, M.P.1
Itoh, K.2
Jasperse, C.P.3
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14
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64549123539
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II-binaphtyldiimine catalyst, see: H. Suga, Y. Adachi, K. Fujimoto, Y. Furihata, T. Tsuchida, A. Kakehi, T. Baba, J. Org. Chem. 2009, 74, 1099-1113.
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II-binaphtyldiimine catalyst, see: H. Suga, Y. Adachi, K. Fujimoto, Y. Furihata, T. Tsuchida, A. Kakehi, T. Baba, J. Org. Chem. 2009, 74, 1099-1113.
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15
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34447509381
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Y. Brinkmann, J. M. Reniguntala, R. Jazzar, G. Bernardinelli, E. P. Kundig, Tetrahedron 2007, 63, 8413-8419.
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(2007)
Tetrahedron
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, pp. 8413-8419
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Brinkmann, Y.1
Reniguntala, J.M.2
Jazzar, R.3
Bernardinelli, G.4
Kundig, E.P.5
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22
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33846957738
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a) S. Bertelsen, P. Diner, R. L. Johansen, K. A. Jørgensen, J. Am. Chem. Soc. 2007, 129, 1536-1537;
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J. Am. Chem. Soc
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Bertelsen, S.1
Diner, P.2
Johansen, R.L.3
Jørgensen, K.A.4
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23
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38349178582
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for a review of iminium catalysis, see
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b) for a review of iminium catalysis, see: A. Erkkilä, I. Majander, P.M. Pihko. Chem. Rev. 2007, 107, 5416-5470.
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(2007)
Chem. Rev
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Erkkilä, A.1
Majander, I.2
Pihko, P.M.3
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25
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33845191735
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Angew. Chem. Int. Ed. 2006, 45, 7581-7584;
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Angew. Chem. Int. Ed
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26
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0001340008
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for a pioneering report of aniline-catalyzed transamination reactions, see
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b) for a pioneering report of aniline-catalyzed transamination reactions, see: E.H. Cordes. W. P. Jencks. J. Am. Chem. Soc. 1962, 84, 826-831;
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(1962)
J. Am. Chem. Soc
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, pp. 826-831
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Cordes, E.H.1
Jencks, W.P.2
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27
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0034685933
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for a related cyclization that proceeds through nitrilium ions, see
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c) for a related cyclization that proceeds through nitrilium ions, see: S. D. Lepore, M. R. Wiley, J. Org. Chem. 2000, 65, 2924-2932.
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(2000)
J. Org. Chem
, vol.65
, pp. 2924-2932
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Lepore, S.D.1
Wiley, M.R.2
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28
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65349118967
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Attempts to convert 1a to the corresponding 2-isoxazoline by using acetone oxime with 20 mol% of catalyst 5 together with various acids resulted in racemic products at room temperature. When the reaction was conducted at- 20°C and sulfonic acids (MsOH or p-TsOH) were used as cocatalysts, moderate (ca. 60 %) ee values were obtained, but the yields were below 20%.
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Attempts to convert 1a to the corresponding 2-isoxazoline by using acetone oxime with 20 mol% of catalyst 5 together with various acids resulted in racemic products at room temperature. When the reaction was conducted at- 20°C and sulfonic acids (MsOH or p-TsOH) were used as cocatalysts, moderate (ca. 60 %) ee values were obtained, but the yields were below 20%.
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29
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65349112064
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It should be noted that a perfectly enantioselective catalyst could not be responsible for the nearly complete racemization of the intermediate. However, equilibration with a less-than-perfect catalyst will unavoidably result in the erosion of enantiomeric purity
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It should be noted that a perfectly enantioselective catalyst could not be responsible for the nearly complete racemization of the intermediate. However, equilibration with a less-than-perfect catalyst will unavoidably result in the erosion of enantiomeric purity.
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30
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65349133162
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1H NMR spectra and the decomposition of 1b was most reliably detected with the use of an internal standard (see the Supporting Information).
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1H NMR spectra and the decomposition of 1b was most reliably detected with the use of an internal standard (see the Supporting Information).
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31
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65349134324
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Apparently, the salts resulting from the combination of relatively weak bases, such as anilines or imidazolidinones, with moderately strong acids, such as TFA or diphenylphosphoric acid, are acidic enough to activate cyclization. Stronger bases, such as pyrrolidine or its derivatives, form salts that are weaker acids and thus poor catalysts for the cyclization process
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Apparently, the salts resulting from the combination of relatively weak bases, such as anilines or imidazolidinones, with moderately strong acids, such as TFA or diphenylphosphoric acid, are acidic enough to activate cyclization. Stronger bases, such as pyrrolidine or its derivatives, form salts that are weaker acids and thus poor catalysts for the cyclization process.
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32
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33845558494
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Previously reported examples of acid-promoted oxime transfer reactions require harsher conditions and longer reaction times, see: a P. A. Jacobi, M. Martinelli, E. C. Taylor, J. Org. Chem. 1981, 46, 5416-5417;
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Previously reported examples of acid-promoted oxime transfer reactions require harsher conditions and longer reaction times, see: a) P. A. Jacobi, M. Martinelli, E. C. Taylor, J. Org. Chem. 1981, 46, 5416-5417;
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34
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65349157297
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Other catalysts were also screened for this transformation. The diphenyl analoge of 6 Hayashi catalyst see: Y. Hayashi, H. Gotoh, T. Hayashi, M. Shoji, Angew. Chem. 2005, 117, 4284-4287;
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Other catalysts were also screened for this transformation. The diphenyl analoge of 6 (Hayashi catalyst see: Y. Hayashi, H. Gotoh, T. Hayashi, M. Shoji, Angew. Chem. 2005, 117, 4284-4287;
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35
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22144459070
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2COOH gave a significantly lower yield and enantioselectivity (40% yield, 45% ee).
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2COOH gave a significantly lower yield and enantioselectivity (40% yield, 45% ee).
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36
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65349109389
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2O/HCl system typically displayed slightly higher yields of the isoxazoline product. However, the isolated products were often contaminated with minor impurities that complicated the HPLC analysis.
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2O/HCl system typically displayed slightly higher yields of the isoxazoline product. However, the isolated products were often contaminated with minor impurities that complicated the HPLC analysis.
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37
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65349136583
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The yield of 4a is largely limited by competing polymerization of 1a in the reaction conditions.
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The yield of 4a is largely limited by competing polymerization of 1a in the reaction conditions.
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38
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33746463962
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Examples of asymmetric conjugate additions of oxygen nucleophiles to enals are rare. Addition of salicylaldehydes to enals preserves the aldehyde oxidation state but does not preserve the original functionality, see: a
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Examples of asymmetric conjugate additions of oxygen nucleophiles to enals are rare. Addition of salicylaldehydes to enals preserves the aldehyde oxidation state but does not preserve the original functionality, see: a) T. Govender, L. Hojabri, F. M. Moghaddam, P.I. Ar-vidsson, Tetrahedron: Asymmetry 2006, 17, 1763;
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(2006)
Tetrahedron: Asymmetry
, vol.17
, pp. 1763
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Govender, T.1
Hojabri, L.2
Moghaddam, F.M.3
Ar-vidsson, P.I.4
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39
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b) H. Sundén, I. Ibrahem, G.-L. Zhao, L. Eriksson, A. Cordova, Chem. Eur. J. 2007, 13,574-581.
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(2007)
Chem. Eur. J
, vol.13
, pp. 574-581
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Sundén, H.1
Ibrahem, I.2
Zhao, G.-L.3
Eriksson, L.4
Cordova, A.5
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