C2-symmetric copper(II) complexes as chiral Lewis acids. Catalytic enantioselective aldol additions of silylketene acetals to (benzyloxy)acetaldehyde

Journal of the American Chemical Society

Volumn 118, Issue 24, 1996, Pages 5814-5815

  Evans, David A ^a   Murry, Jerry A ^a   Kozlowski, Marisa C ^a  

^a HARVARD UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ALDEHYDE;

ARTICLE; CATALYSIS; CHIRALITY; DRUG SYNTHESIS; REACTION ANALYSIS; TECHNIQUE;

EID: 0030017687     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja960712i     Document Type: Article

References (32)

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18. Other substituted pybox complexes gave lower enantioselectivity: tert-butyl pybox (9% ee), isopropyl pybox (85% ee), benzyl pybox (67% ee).
19. 2 at room temperature for 4 h followed by filtration through a cotton plug. The resulting solution is stable to air and moisture and may be stored for up to 1 week without any special precautions. (Benzyloxy)acetaldehyde (0.50 mmol) and silylketene acetal (0.60 mmol) were added sequentially to a 12.5 mM solution of 2b at -78° C. After the reaction was complete (≤ 12 h), the mixture was filtered through silica and the silyl ether was hydrolyzed with 1 N HCl in THF to yield the hydroxy ester.
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21. The addition of this nucleophile to aldehydes has recently been reported. See: Singer, R. A.; Camera, E. M. J. Am. Chem. Soc. 1995, 117, 12360-12361.
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24. Five-coordinate Cu(II) complexes exhibit a strong tendency toward either square pyramidal or trigonal bipyramidal geometries, see: Hathaway, B. J. In Comprehensive Coordination Chemistry; Wilkinson, G., Ed.; Pergamon Press: New York, 1987; Vol. 5, Chapter 53.
25. In the square pyramidal geometry, the strong coordinating site resides in the ligand plane with a weaker coordination site in the axial position. For maximal RCHO activation, we presume that carbonyl coordination occurs in the ligand plane.
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31. The appropriate control experiments were performed ruling out catalyst-promoted silicon scrambling of the silylketene acetals or of the product silyl ethers.
32. Olah, G. A.; Heiliger, L.; Li, X.-Ya; Prakash, G. K. S. J. Am. Chem. Soc. 1990, 112, 5991-5995.