Title:Diastereoselective Reduction of Selected α-substituted β-keto Esters and the Assignment of the Relative Configuration by 1H-NMR Spectroscopy
Volume: 8
Issue: 2
Author(s): Christian Trapp*, Corinna Herrmann, Chris Drewniok, Dieter Greif and Martin Hofrichter
Affiliation:
- Department of Organic Chemistry, Faculty of Natural and Environmental Sciences University of Applied Science Zittau/ Goerlitz, Theodor-Körner-Allee 16, Zittau 02763,Germany
Keywords:
Diastereoselectivity, Keto esters, β-Hydroxy esters, Stereoselective reduction, NMR Spectroscopy, α-substituted.
Abstract:
Background: Chiral β-hydroxy esters and α-substituted β-hydroxy esters represent versatile
building blocks for pheromones, β-lactam antibiotics and 1,2- or 1,3-aminoalcohols.
Objective: Synthesis of versatile α-substituted β-keto esters and their diastereoselective reduction
to the corresponding syn- or anti-α-substituted β-hydroxy esters. Assignment of the relative configuration
by NMR-spectroscopy after a Curtius rearrangement of α-substituted β-keto esters to 4-substituted
5-methyloxazolidin-2-ones.
Methods: Diastereoselective reduction was achieved by using different Lewis acids (zinc, titanium
and cerium) in combination with complex borohydrides as reducing agents. Assignment of the relative
configuration was verified by 1H-NMR spectroscopy after Curtius-rearrangement of α-substituted
β-hydroxy esters to 4-substituted 5-methyloxazolidin-2-ones.
Results: For the syn-selective reduction, titanium tetrachloride (TiCl4) in combination with a
pyridine-borane complex (py BH3) led to diastereoselectivities up to 99% dr. High anti-selective reduction
was achieved by using cerium trichloride (CeCl3) and steric hindered reducing agents such
as lithium triethylborohydride (LiEt3BH). After Curtius-rearrangement of each α-substituted β-hydroxy
ester to the corresponding 4-substituted 5-methyloxazolidin-2-one, the relative configuration
was confirmed by 1H NMR-spectroscopy.
Conclusion: We have expanded the procedure of Lewis acid-mediated diastereoselective reduction
to bulky α-substituents such as the isopropyl group and the electron withdrawing phenyl ring.