C O M M U N I C A T I O N S
Table 1. The Ratios of [1-D-aa] to [1-L-aa] at Equilibrium
Many interesting models of pyridoxamine including chiral ones
have been developed to convert R-ketoacids to R-amino acids.
1
8
amino acids
D/
L
ratio
amino acids
D/L ratio
Receptor 1 may be considered a chiral model of pyridoxal. A
rational design has led to the development of a general receptor
(1) that binds a wide range of amino acids with remarkably good
and even predictable stereoselectivity. Unlike other amino acid
receptors, 1 can be used for deracemization of amino acids by
epimerization of the imine intermediates that are activated by
RAHBs.
threonine
glutamine
histidine
arginine
asparagine
tyrosine
20/1
15/1
14/1
14/1
13/1
12/1
11/1
methionine
glutamic acid
serine
leucine
tryptophan
alanine
11/1
11/1
11/1
9/1
8/1
7/1
phenylalanine
between the urea group and the carboxylate group over the
H-bonding between the urea group and the alcohol group may
contribute to the greater receptor streoselectivity for binding
R-amino acids than for binding 1,2-amino alcohols.
Acknowledgment. This research was supported by the Ministry
of Science and Technology of Korea (NRL M1060000269-06-
J0000-26910), SRC programs of MOST/KOSEF (R11-2005-008-
00000-0 and the Research Center for Women’s Diseases), and by
The stereoselectivity of 1 for binding a wide range of R-amino
acids is remarkably high for such a simple organic receptor. Amino
acids with basic (His) and acidic (Glu) side chains are tolerated as
well as those with hydrophobic (Tyr, Trp, Phe, Leu, Ala) and
the Natural Sciences and Engineering Research Council of Canada.
1
Supporting Information Available: Experimental details and H
NMR spectra for the conversion of L-amino acids to D-amino acids.
This material is available free of charge via the Internet at http://
pubs.acs.org.
8
hydrophilic (Ser, Thr, Gln, Arg, Asn, Met) side chains. It is likely
that the stereoselectivity is the highest for threonine because of the
steric bulk next to the R-carbon (see 1-L-aa of Scheme 1).
Furthermore, the hydroxyl side chain of threonine is electron
withdrawing and makes the R C-H more acidic for facile
epimerization. Over the years, there has been much interest in
References
(
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(
1
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2
57. (c) Reeve, T. B.; Cros, J.-P.; Gennari, C.; Piarulli, U.; Vries, J. G.
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qualitative agreement with the experimental results (Table 1; 7/1
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Figure S15). In contrast, there is less steric hindrance between the
R-proton of alanine group and the imine C-H of the receptor in
(
(
13) See Supporting Information.
14) Chin, J.; Kim, D. C.; Kim, H.-J.; Panosyan, F. B.; Kim, K. M. Org. Lett.
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(
(
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stronger H-bonds to form between the two urea N-Hs and the
(
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1
carboxylate group (Scheme 1b). The H NMR signals of the more
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observed (see urea N-H signals in Figure 1).
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