ChemFiles Volume 5 Article 9

Since 1987, the series of chiral oxazaborolidines known as CBS catalysts (after the work of Corey, Bakshi, and Shibata) have been used for catalytic reduction of prochiral ketones,1 imines,2 and oximes3 to produce chiral alcohols, amines, and amino alcohols in excellent yields and ee’s. Sigma-Aldrich is pleased to offer both enantiomers of 2-methyl-CBS-oxazaborolidine (Figure 1) as a dry reagent, in addition to our current offerings as a 1 M solution in toluene.

Figure 1

Recently, (R)-2-methyl-CBS-oxazaborolidine was employed in asymmetric syntheses of Cinacalcet congeners (Scheme 19). After much experimental effort with different chiral reducing agents, the authors determined that Me-CBS-catalyzed reduction afforded the best results.4

Scheme 19

Chung and co-workers have used (S)-2-methyl-CBS-oxazaborolidine in the asymmetric synthesis of N-tert-butyl disubstituted pyrrolidines (Scheme 20). Additionally, the authors determined that using controlled addition of the ketone at 40 °C allows the catalyst loading to be reduced to 0.5 mol %, while retaining >98% ee’s.5

Scheme 20

Steven Ley’s group has employed (S)-Me-CBS in their recent work on the total synthesis of 10-hydroxyasimicin (Scheme 21).6 This product is a member of the natural product class referred to as the annonaceous acetogenins, a class of products which exhibit a wide array of biological activity.

Scheme 21

Catalytic kinetic resolution can also be achieved using 2-methyl-CBS-oxazaborolidine. Partial reduction of the racemic bicyclic enone with (S)-Me-CBS and borane-dimethylsulfide complex affords the cis-alcohol (98% ee after crystallization from heptane) and unchanged (S)-enone in 97% ee (Scheme 22).7

Scheme 22

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  1. (a) Corey, E. J.; Bakshi, R. K.; Shibata, S. J. Am. Chem. Soc. 1987, 109, 5551. (b) Corey, E. J. et al. J. Am. Chem. Soc. 1987, 109, 7925.
  2. (a) Kirton, E. H. M. et al. Tetrahedron Lett. 2004, 45, 853. (b) Cho, B. T.; Chun, Y. S. Tetrahedron: Asymmetry 1992, 3, 337. (c) Cho, B. T.; Chun, Y. S. J. Chem. Soc., Perkin Trans. 1 1990.
  3. Tillyer, R. D. et al. Tetrahedron Lett. 1995, 36, 4337.
  4. Wang, X. et al. Tetrahedron Lett. 2004, 45, 8355.
  5. Nattrass, G. L. et al. Angew. Chem., Int. Ed. Engl. 2005, 44, 580.
  6. Chung, J. Y. L. et al. J. Org. Chem. 2005, 70, 3592.
  7. Fehr, C. et al. Eur. J. Org. Chem. 2004, 1953.

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