Enolization of chiral α-silyloxy ketones with dicyclohexylchloroborane. Application to stereoselective aldol reactions

Organic Letters

Volumn 2, Issue 17, 2000, Pages 2599-2602

  Galobardes, Marta ^a   Gascon, Miguel ^a   Mena, Marisa ^a   Romea, Pedro ^a   Urpí, Fèlix ^a   Vilarrasa, Jaume ^a  

^a UNIVERSITY OF BARCELONA   (Spain)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000715599     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol006109t     Document Type: Article

References (37)

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18. (a) Martín, R.; Pascual, O.; Romea, P.; Rovira, R.: Urpí, F.; Vilarrasa, J. Tetrahedron Lett. 1997, 38, 1633.
19. (b) Martín, R.; Romea, P.; Tey, C.; Urpí, F.; Vilarrasa, J. Synlett 1997, 1414.
20. See, for instance: (a) Reetz, M. T.; Hüllmann, M. J. Chem. Soc., Chem. Commun. 1986, 1600.
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23. Brown et al. suggested (see ref 2d) that E-enolborinates are highly favored by the use of moderately sterically hindered amines, nonpolar solvents, and low temperatures.
24. 1H NMR analysis and/or HPLC. The yields and diastereomeric ratios for 5 and 6 are given in Table 1.
25. This isomer has not been isolated. Its stereochemistry has been assigned on the basis of NMR analysis.
26. 1H NMR analysis and/or HPLC. The yields and diastereomeric ratios for 7-9 are given in Table 1.
27. (a) Goodman, J. M. Tetrahedron Lett. 1992, 33, 7219.
28. (b) Goodman. J. M.; Paterson, I. Tetrahedron Lett. 1992, 33, 7223.
29. Hoffmann. R. W. Chem. Rev. 1989, 89, 1841.
30. Frenking and Reetz have postulated that conformational effects are responsible for a significant portion of the energy difference between competing transition states involved in 1,2-stereoinduction: Frenking, G.; Köhler, K. F.; Reetz, M. T. Tetrahedron 1991, 47, 8991; Tetrahedron 1993, 49, 3971. See also: Lecea, B.; Arrieta, A.; Cossío, F. P. J. Org. Chem. 1997, 62, 6485.
31. Frenking and Reetz have postulated that conformational effects are responsible for a significant portion of the energy difference between competing transition states involved in 1,2-stereoinduction: Frenking, G.; Köhler, K. F.; Reetz, M. T. Tetrahedron 1991, 47, 8991; Tetrahedron 1993, 49, 3971. See also: Lecea, B.; Arrieta, A.; Cossío, F. P. J. Org. Chem. 1997, 62, 6485.
32. Frenking and Reetz have postulated that conformational effects are responsible for a significant portion of the energy difference between competing transition states involved in 1,2-stereoinduction: Frenking, G.; Köhler, K. F.; Reetz, M. T. Tetrahedron 1991, 47, 8991; Tetrahedron 1993, 49, 3971. See also: Lecea, B.; Arrieta, A.; Cossío, F. P. J. Org. Chem. 1997, 62, 6485.
33. Dipolar interactions have also been invoked to rationalize the 1,3-asymmetric induction observed in the Mukaiyama aldol addition of enol silyl ethers to β-substituted aldehydes. See, for instance: (a) Evans, D. A.; Dart, M. J.; Duffy, J. L.; Yang, M. G. J. Am. Chem. Soc. 1996, 118, 4322.
34. (b) Bonini, C.; Esposito, V.; D'Auria, M.; Righi, G. Tetrahedron 1997, 53, 13419.
35. 2BOTf. Therefore, polar solvents and higher concentrations might favor the formation of Z-enolborinates through an alternative pathway.
36. α-H bond to eclipse the enolborinate double bond to minimize the allylic strain. See: (a) Vulpetti, A.; Bernardi, A.; Gennari, C.; Goodman, J. M.; Paterson, I. Tetrahedron 1993, 49, 685.
37. (b) Bernardi, A.; Gennari, C.; Goodman, J. M.; Paterson, I. Tetrahedron: Asymmetry 1995, 6, 2613.