Enantioselective addition of a chiral thiazolidinethione-derived titanium enolate to acetals

Organic Letters

Volumn 3, Issue 4, 2001, Pages 615-617

  Cosp, Annabel ^a   Romea, Pedro ^a   Talavera, Pere ^a   Urpí, Fèlix ^a   Vilarrasa, Jaume ^a   Font Bardia, Mercè ^a   Solans, Xavier ^a  

^a UNIVERSITY OF BARCELONA   (Spain)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0001583042     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol0070177     Document Type: Article

References (44)

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22. Stereoselective addition of metal enolates to acetals are scarce: (a) Pilli, R. A.; Riatto, V. B.; Vencato, I. Org. Lett. 2000, 2, 53-56 (cyclic hemiacetals).
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29. 3) were also investigated but turned out to be less suitable.
30. Flash silica gel chromatography can be easily visually monitored because all of the adducts prepared to date are bright yellow.
31. 4, filtered, and concentrated. Purification by flash column chromatography on silica gel (hexanes/EtOAc) afforded the pure major diastereomer 2.
32. Crystallographic data (excluding structure factors) for the structures 2a, 2d, and 3d reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication numbers CCDC-150269, 150271, and 150270, respectively. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax (+44) 1223-336-033; e-mail deposit@ccdc.cam.ac.uk).
33. Reactions of acetals with carbon nucleophiles have been shown to proceed via carbocation intermediates. See, for example: (a) Romero, J. A. C.; Tabacco, S. A.; Woerpel, K. A. J. Am. Chem. Soc. 2000, 122, 168-169.
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36. (d) Sammakia, T.; Smith, R. S. J. Am. Chem. Soc. 1992, 114, 10998-10999. For earlier mechanistic discussions, see:
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38. (f) Denmark, S. E.; Almstead, N. G. J. Am. Chem. Soc. 1991, 113, 8089-8110 and references therein.
39. It deserves to be mentioned that the C=S bond of the auxiliary constitutes a key feature of the process. 1,3-Oxazolidine-2-thione derived auxiliary also displays an excellent performance, although its final removal has been proven less efficient. However, 1,3-oxazolidin-2-one and camphorsultam-derived auxiliaries afforded worse results.
40. The stereochemical outcome of the reaction reveals complete facial control by the chiral titanium enolate (only products arising from the addition to the Si face of the enolate have been observed).
41. For a similar discussion of related Lewis acid-mediated aldol addition transition states, see: Mahrwald, R. Chem. Rev. 1999, 99, 1095-1120 and references therein.
42. (a) Su, D.-W.; Wang, Y.-C.; Yan, T.-H. Tetrahedron Lett. 1999, 40, 4197-4198.
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44. The chiral auxiliary can be recovered by flash column chromatography in ≥90% yield or, in certain cases, by washing the reaction mixture using 1 M NaOH and acidification.