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16
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0000311605
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α-Deprotonierung eines α-chiralen 2-Alkenylcarbamats unter Retention und Lithium-Titan-Austausch unter Inversion - zur Homoaldol-Reaktion unter 1,3-Chiralitätsübertragung
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(1986)
Angewandte Chemie
, vol.98
, pp. 171-173
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Hoppe, D.1
Krämer, T.2
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21
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0000163012
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Die Homoaldol-Reaktion oder wie man Probleme der Regio- und Stereoselektivität in den Griff bekommt
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Review:, 946
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(1984)
Angewandte Chemie
, vol.96
, pp. 930
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Hoppe, D.1
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25
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84990132917
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We thank Professor Schneider and Dr. Laumen for help with the enzymatic ester hydrolysis.
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27
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84990132987
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A solution of (R)‐2 (1 mmol) and TMEDA (1.1 mmol) in hexane (2 mL) was treated dropwise at − 78°C with a ca. 1.4 M solution of see‐butyllithium (1.1 mmol) in cyclohexane/isopentane and the mixture stirred for 30 min at this temperature. The resulting yellow solution of (R)‐3 was treated at −78 °C with 1.1 mmol of the electrophile, stirred for 30 min, allowed to warm to room temperature, neutralized with 2 mL of 2N HCl, and extracted three times with ether. Purification was performed by flash chromatography on silica gel with ether/pentane (1:4 to 1:8).
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28
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84990132989
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We assign the lithium compound 3 the (R)‐configuration, since we assume a deprotonation with retention of configuration. Although this assumption is plausible [9a] and finds theoretical support [16], it has to our knowledge, never been proven, since the absolute configuration of the trapping product was always determined and not that of the organometallic intermediate. For simplification an equilibrium of monomeric 3 with dimeric or tetrameric aggregates has not been taken into consideration; cf. references cited in [27].
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30
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84990132940
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All the new compounds gave correct C, H analyses (C.H ± 0.3%).
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31
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84990118674
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3.
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34
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84990132997
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(R)‐7 was correlated with a sample obtained from (R)‐atrolactic acid
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37
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84990142528
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The (R)‐configuration of the stannane (+)‐9 is not strictly confirmed; it follows from the observed rotation using Brewsters rule [23]. In the sequence (+ )‐9 → (S)‐3 → (–)‐9 only one of the two steps can proceed with inversion, and according to the derived assignment of configuration it must be the delithiostannylation. This is also supported by the fact that retention has always been established for lithiodestannylations of α‐oxyalkylstannanes [24, 25].
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