C O M M U N I C A T I O N S
Table 2. Asymmetric anti Selective Hydrogenationa
In conclusion, we have developed an efficient asymmetric
synthesis of anti aromatic â-hydroxy-R-amino acid esters from the
readily available R-amino-â-keto esters using the Ir-MeOBIPHEP
catalyst for the first time. Our hydrogenation is the first example
of dynamic kinetic resolution using the Ir axially chiral phosphine
catalyst and can be carried out in an environmentally friendly
solvent using commercially available chiral phosphines. The product
anti aromatic â-hydroxy-R-amino acids are useful as building blocks
for the synthesis of various pharmaceuticals and natural products.
Further studies on the mechanism and the expansion of the scope
and applicability are now in progress.
Acknowledgment. Dedicated with great appreciation to Profes-
sor Takayuki Shioiri of Meijo University on the occasion of his
70th birthday (Koki).
Supporting Information Available: Experimental procedures and
spectral data for all new compounds. This material is available free of
References
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a The reaction was carried out by using Ir-(S)-MeOBIPHEP (prepared
from [IrCl(cod)]2 (0.015 equiv), (S)-MeOBIPHEP (0.04 equiv), and NaI
(0.06 equiv) prior to the hydrogenation) and NaOAc (1 equiv) in AcOH.
b (R)-MeOBIPHEP was used instead of (S)-MeOBIPHEP. c As the N-Boc
protected derivative. d (2R,3R)-Isomer was obtained.
conditions (entries 8 and 11). The heteroaromatic substrates are
also suitable for the hydrogenation (entries 9 and 10).
The method opens up a new efficient access to 3-methoxytyrosine
with anti stereochemistry, a stereochemically undefined component
of papuamides.11 For instance, O-methylation of the obtained
benzyloxyphenyl derivative (entry 3) using trimethyloxonium
tetrafluoroborate in the presence of Proton Sponge and subsequent
hydrolysis furnished (2R,3R)-3-methoxytyrosine, a building block
for papuamides, in good overall yield and excellent stereoselection.
As part of our study on the potential of the Ir-MeOBIPHEP
catalyst, we carried out asymmetric hydrogenation of â-keto ester,
R-amino acetophenone, and N-benzoyl R-amino-â-keto ester, which,
surprisingly, gave apparently different results, namely, no or low
conversion, no diastereoselectivity, and poor enantioselectivity,
compared to those of the Ru-catalyzed hydrogenation.12 This failure
suggests that the Ir-catalyzed hydrogenation might proceed with a
different mechanism from that of the Ru-catalyzed hydrogenation.
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(12) See the Supporting Information.
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