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CBS Oxazaborolidines – Organocatalysts for Asymmetric Synthesis

A versatile reduction of ketones to alcohols is a powerful synthetic method in total synthesis

Corey-Bakshi-Shibata (CBS) catalysts (Fig. 1), represent proline based organocatalysts able to promote enantio- and diastereoselective transformations of highly functionalized ketones. The well-defined structure of the catalyst and the straightforward reaction pathway have allowed the rapid expansion of the scope of the CBS reduction, and the rational modification of catalyst structure to achieve optimal enantioselectivity for a particular type of substrate [1].


Fig. 1. CALLERY™ (R)-Methyl and CALLERY™ (S)-Methyl oxazaborolidines

This enantioselective catalytic reduction is now established as a major synthetic method for the asymmetric synthesis of chiral medicinals, catalytic ligands, complex natural products, and synthetic intermediates.

Figure 2 shows the basic CBS reduction reaction. In this reaction, the ketone in the presence of borane is enantioselectively reduced to the corresponding chiral, non-racemic alcohol.


Fig. 2. CBS enantioselective reduction of achiral ketone to non-racemic alcohol

CBS oxazaborolidine organocatalyst derives from the natural amino acid proline (please read our blog on prolines). Like  prolines, oxazaborolidine has a bifunctional nature. In the molecule boron acts as a Lewis acid, while nitrogen acts as a Lewis base. The presence of BH3 borane in the reaction is essential (usually ligated with THF 05-0175 or Me2S 05-0110). BH3 interacts with the lone pair on nitrogen as a Lewis acid, while the ketone, being a Lewis base, complexes to the boron. The borane is sufficiently reactive to reduce ketone only when bonded to the hydrogen atom.


The enantioselective reduction of ketones by the  CBS catalyst has found many applications in organic synthesis such as  kinetic resolution, Diels-Alder reactions, and cycloadditions. Most notably, it has recently gained prominent application in the total synthesis of a number of natural products. For more detailed information, please review the corresponding technical notes and review publication [1].

CBS Oxazaborolidine organocatalysts have already been utilized on a large scale in the chemical industry.


  1. Angew. Chem. Int. Ed. 1998, 37, 1986.
  2. Synthesis 2016, 48, 3890.
  3. Org. Process Res. Dev. 2018, 22, 574.
  4. Org. Biomol. Chem., 2016, 14, 4304.
  5. Synlett 2007, 6, 997.
  6. Molecules 2018, 23, 2408.
  7. Chem. Eur. J. 2007, 13, 215.


Featured Products:

05-1000 CALLERY™ (R)-Methyl oxazaborolidine, 1M in toluene (112022-83-0)
05-1001 CALLERY™ (S)-Methyl oxazaborolidine, 1M in toluene (112022-81-8)
05-0175 CALLERY™ Borane tetrahydrofuran, 1M in tetrahydrofuran (14044-65-6)
05-0110 CALLERY™ Dimethylsulfide borane, min. 93% (13292-87-0)

Related Materials & Products:

Corey Catalysts
Catalysis Product Families
Metal Catalysts for Organic Synthesis



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