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Note that since boronate ester 2 is purified by chromatography prior to oxidation and analysis of the enantiomeric purity by gas chromatography, we cannot rule out unintentional enrichment in ee by physical separation of the enantiomerically pure material from the racemate. This type of separation has been observed for certain systems, but considering the high yields of the boronate ester obtained, we consider this will contribute little to the ee, if at all. For examples of separation of enantiomers by chromatography see: Diter, P.; Taudien, S.; Samuel, O.; Kagan, H. B. J. Org. Chem. 1994, 59, 370. Matusch, R.; Coors, C. Angew. Chem., Int. Ed. Engl. 1989, 28, 626 and references therein.
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Kagan, H.B.4
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25
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84990137332
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and references therein
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Note that since boronate ester 2 is purified by chromatography prior to oxidation and analysis of the enantiomeric purity by gas chromatography, we cannot rule out unintentional enrichment in ee by physical separation of the enantiomerically pure material from the racemate. This type of separation has been observed for certain systems, but considering the high yields of the boronate ester obtained, we consider this will contribute little to the ee, if at all. For examples of separation of enantiomers by chromatography see: Diter, P.; Taudien, S.; Samuel, O.; Kagan, H. B. J. Org. Chem. 1994, 59, 370. Matusch, R.; Coors, C. Angew. Chem., Int. Ed. Engl. 1989, 28, 626 and references therein.
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Matusch, R.1
Coors, C.2
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27
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0010896926
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This may be attributed to in situ generation of the corresponding peroxide from the aldehyde as has been previously observed: Matteson, D. S.; Moddy, R. J. J. Org. Chem. 1980, 45, 1091.
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0343952741
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Ogilvie, W. Personal communication.
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Ogilvie, W.1
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38
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0003726028
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John Wiley & Sons: New York
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Oxygen is known to oxidize organoboranes in a stereorandom fashion: see Brown, H. C. Organic Syntheses via Boranes; John Wiley & Sons: New York, 1975.
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(1975)
Organic Syntheses Via Boranes
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Brown, H.C.1
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Nagai, U.; Shishido, T.; Chiba, R.; Mitsuhashi, H. Tetrahedron 1965, 21, 1701.
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Nagai, U.1
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Mitsuhashi, H.4
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40
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0342646682
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note
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A similar yield at up to 96% ee can be obtained by carrying out the reaction at -67 to -66°C for 6 h.
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42
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37049123633
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Okamoto, K.; Kinoshita, T.; Takemura, Y.; Yoneda, H. J. Chem. Soc., Perkin Trans. 2 1975, 1426.
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Okamoto, K.1
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Takemura, Y.3
Yoneda, H.4
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43
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0343952740
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note
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The solution should remain colorless. Color indicates decomposition of the homologation reagent (this will occur if the addition is too rapid and/or the internal reaction temperature exceeds -80°C) to the corresponding carbene, and colored reagents should not be used for the homologation. Precipitation of the homologation reagent may also be observed near the end of the 10 min period, but it can be used for the homologation without any deleterious effect on the yield or the enantiomeric excess.
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44
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0343080758
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note
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2 separates from the ether solvent at room temperature and thus should be added rapidly with a syringe previously chilled to -25°C in a desiccator.
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45
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0343080759
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note
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Sodium chlorite has a limited lifetime in water at room temperature, so the oxidant must be introduced in two batches to ensure quantitative conversion.
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46
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0343080757
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note
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Care should be taken to maintain the internal temperature below 35°C to minimize decomposition of product acid.
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47
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4243223188
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Aldrich Chemical Co.: Milwaukee
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(1974)
The Aldrich Library of NMR Spectra
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49
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0042599756
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0005343340
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Takano, S.; Ogasawara, K.; Nagayama, I.; Kutsuma, T. Synth. Commun. 1976, 6, 349.
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Takano, S.1
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Kutsuma, T.4
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