A simple method for the oxidation of α-amino acid esters to α-oximino esters

Article abstract of DOI:10.1016/j.tetlet.2009.11.045

Magnesium bis(monoperoxyphthalate) hexahydrate (MMPP) was found to be an effective reagent for the oxidation of various α-amino acid esters to the corresponding α-oximino acid esters. This transformation could be completed under mild conditions within 2.5 h using 1.1 equiv of MMPP in THF. Clean oximino esters were obtained after quenching and extracting the reaction from sodium thiosulfate solution. The O-phosphorylated derivative of 2-oximinoglutarate exhibited slow binding inhibitory potency for the metallopeptidase prostate-specific membrane antigen (PSMA) with an IC₅₀ value of 58 nM.

Full text of DOI:10.1016/j.tetlet.2009.11.045

Tetrahedron Letters 51 (2010) 402–403
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A simple method for the oxidation of
a-amino acid esters to a-oximino esters
*
Lisa Y. Wu, Joseph K. Choi, Krit Y. Hatton, Clifford E. Berkman
Department of Chemistry, Washington State University, Pullman, WA 99164-4630, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnesium bis(monoperoxyphthalate) hexahydrate (MMPP) was found to be an effective reagent for the
oxidation of various -amino acid esters to the corresponding -oximino acid esters. This transformation
Received 23 October 2009
Revised 6 November 2009
Accepted 10 November 2009
Available online 14 November 2009
a
a
could be completed under mild conditions within 2.5 h using 1.1 equiv of MMPP in THF. Clean oximino
esters were obtained after quenching and extracting the reaction from sodium thiosulfate solution. The
O-phosphorylated derivative of 2-oximinoglutarate exhibited slow binding inhibitory potency for the
metallopeptidase prostate-specific membrane antigen (PSMA) with an IC50 value of 58 nM.
Ó 2009 Elsevier Ltd. All rights reserved.
Our recent research efforts have been aimed at developing
potent inhibitors for glutamate carboxypeptidases, specifically
prostate-specific membrane antigen (PSMA). The most potent
inhibitor for this enzyme that we have identified to date is a
relatively simple molecule; N-phosphoryl glutamic acid which
exhibited an IC50 value of 70 pM against PSMA.1,2 While direct
tetrahedral intermediate analogs of peptide hydrolysis serve as
potent inhibitors of metalloproteaseas and metallopeptidases, we
have begun to explore late transition state analogs as putative
slow-tight binding inhibitors of the metallopeptidase PSMA.
Toward that end, we have initiated the development of phosphor-
general, these methods provide variable yields as a result of side
reactions, in particular, the overoxidation and nitroso dimer forma-
tion.5 In our initial attempts to oxidize 1a with MCPBA, overoxida-
tion to the nitro analog was indeed observed. Consequently, we
turned our attention to MMPP, which can be used to carry out a
wide variety of oxidation reactions.7
After optimizing the reaction conditions (solvent, temperature,
reaction time, and concentration) for the transformation of 1a to
the corresponding a-oximino acid ester 2a, we applied these con-
ditions to various amino acid esters (Table 1).8,9 In all cases, clean
oximes were obtained after quenching the reaction with saturated
Na2S2O3 and extracting from NaHCO3, without the need for
chromatographic purification. It is noteworthy to mention that
the oxime moiety was extremely sensitive to acidic conditions,
leading to rapid hydrolysis.10 While most amino acid esters listed
ylated derivatives of
of compounds, it requires the ready access of oxime analogs of var-
ious amino acids, that is, -oximino acids. However, there is little
a-oximino acids. In order to prepare this class
a
precedent for the preparation of this class of compounds from ami-
no acids. In this Letter, we describe the simple transformation of
amino acid esters into the corresponding
a-oximino acid esters
Table 1
Synthesis of
a
-oximino acid esters8,9
in high yield and the subsequent synthesis and evaluation of a
phosphorylated derivative of 2-oximinoglutarate as an inhibitor
of PSMA.
Our initial studies were aimed at the preparation of the 2-oxi-
minoglutarate diester (2a–c). There are few reports on the trans-
O
O
MMPP
H2N
HON
OR
OR
THF, 2hr
-40 ºC to r.t.,
aa
aa
1
2
formation of amino acids to
use of oxidants such as dimethyldioxirane, Na2WO4, and MeR-
a-oximino acids. These include the
aa = amino acid side chain
R = Me, tBu, Bn
eO3/H2O2, in addition to multi-step transformations in which tyro-
sine was the amino acid substrate.3,4
A
similar example
Amino acid esters 1
a-Oximino acid esters 2 (% yield)
1a Glu(OBn)-OBn
1b Glu(OMe)-OMe
1c Glu(OtBu)-OtBu
1d Gly-OBn
1e Leu-OBn
1f Phe-OBn
1g Asn-OtBu
1h Lys(Fmoc)-OMe
1i Ser-OBn
89
69
77
71
78
79
27
54
68
30
demonstrated that tyrosine methyl ester could be oxidized to the
corresponding
a-oximino acid ester with Na2WO4 and 30% H2O2,
which required column purification.5 It has also been reported re-
cently that primary amines could be effectively oxidized to oximes
with DPPH and WO3/Al2O3 but required long reaction times.6 In
* Corresponding author. Fax: +1 509 335 8389.
E-mail address: cberkman@wsu.edu (C.E. Berkman).
1j Trp-OMe
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.045

L. Y. Wu et al. / Tetrahedron Letters 51 (2010) 402–403
403
6. Suzuki, K.; Watanabe, T.; Murahashi, S. I. Angew. Chem., Int. Ed. 2008, 47, 2079.
7. Heaney, H. Aldrichim. Acta 1993, 26, 35.
O
P
BnO
i. (BnO)2PN(iPr)2
HON
CO2Bn
CO2Bn
BnO
ON
CO2Bn
CO2Bn
5-(ethylthio)-1H-tetrazole
8. All solvents used in the reactions were both anhydrous and obtained as such
from commercial sources. All other reagents were used as supplied unless
otherwise stated. 1H, 13C, and 31P NMR spectra were recorded on either a
Varian Vx 300 or Inova 500 MHz instrument. 1H NMR chemical shifts are
relative to TMS (d = 0.00 ppm), CDCl3 (d = 7.26 ppm), CD3OD (d = 4.87 and
3.31 ppm), or D2O (d = 4.87 ppm). 13C NMR chemical shifts are relative to
CD3OD (d = 49.15 ppm) or CDCl3 (d = 77.00 ppm). 31P NMR chemical shifts in
CDCl3, CD3OD, or D2O were externally referenced to 85% H3PO4 (d = 0.00 ppm)
in CDCl3, CD3OD, and D2O, respectively. Maldi-High resolution mass
spectrometry (Maldi-HRMS) was performed by the Laboratory for Bioanalysis
and Biotechnology Center (LBB2) at Washington State University.
9. General procedure for -oximino acid esters 2: Amino acid HCl salts were
extracted in CH2Cl2 from saturated NaHCO3, washed with brine, and the
resulting organic layer was dried over MgSO4, then concentrated in vacuo.
MMPP (1.1 mmol, 1.1 equiv) was suspended in THF (3 mL), purged with Ar (g),
and stirred at À40 °C. A neutralized amino acid ester (1 mmol) was dissolved in
THF (2 mL) and was added via a syringe to the MMPP solution. The reaction
was stirred for 20 min at À40 °C, the cooling bath was removed, and the
reaction was stirred for an additional 2 h at room temperature. The reaction
mixture was dissolved in ethyl acetate (15 mL), washed thrice with saturated
sodium thiosulfate (15 mL each), washed with saturated sodium bicarbonate
(15 mL), washed with brine (15 mL), and the organic layer was dried over
Na2SO4 and concentrated in vacuo to either a white solid or colorless oil.
10. Yang, S. M.; Lagu, B.; Wilson, L. J. J. Org. Chem. 2007, 72, 8123.
ii. t -BuOOH
2a
3
21%
iii. Pd/C, H2,
NaHCO3
O
–
—O
P
ON
CO2
—O
–
CO2
4
Quant.
Scheme 1. Synthesis of phosphoryl oxime gluatamate.12,13
in Table 1 were neutralized by basic extraction prior to reaction
with MMPP, we were unable to obtain 1g in this form and
consequently we observed considerable overoxidation to the cor-
a-nitro analog. The low yield of 2j is presumably due
to the competing oxidation of the indole nitrogen. Overall, the oxi-
responding
11. Allen, J. F. J. J. Am. Chem. Soc. 1957, 79, 3071.
12. Synthesis of intermediate 3: In a 25 mL round-bottomed flask, acetonitrile
(12 mL) was added to
a mixture of 5-(ethylthio)-1H-tetrazole (216 mg,
dation of most protected amino acids to their respective
acid ester analogs with MMPP gave good yields.
a-oximino
1.658 mmol) and (Z)-dibenzyl 2-(hydroxyimino) pentanedioate (2a, 283 mg,
0.829 mmol) under an Ar(g) atmosphere. The solution was cooled to 0 °C and
dibenzyl N,N-diisopropylphosphoramidite (409 lL, 1.243 mmol) was added
There are limited examples for the preparation of protected
phosphoryl oximes.11 We prepared the phosphorylated
-oximino
acid as shown in Scheme 1.12,13 As a late stage transition state ana-
log peptitometic inhibitor of PSMA, phosphorylated -oximino
acid 4 was found to have an IC50 of 276 nM but when preincubated
with the enzyme for 10 min, the IC50 was enhanced to 58 nM.
slowly. The reaction was stirred for 1.5 h at room temperature, then filtered. To
the resulting filtrate was added 5-(ethylthio)-1H-tetrazole (110 mg,
0.845 mmol) and tert-butyl hydroperoxide solution (70 wt % in H2O, 1 mL) at
0 °C. The reaction mixture was stirred at 0 °C for 10 min, the ice bath was
removed, and the reaction was stirred for an additional 1 h at room
temperature. The reaction mixture was concentrated in vacuo, extracted
with ethyl acetate from saturated NaHCO3, and washed with brine. The organic
layer was dried over MgSO4, filtered, and concentrated in vacuo to give a pale
yellow oil. The product was purified by flash chromatography (2:8, acetone/
hexane, v:v; Rf = 0.18) to give a white solid (21% yield, mp 48–49 °C).
a
a
These results suggest that the phosphorylated
a-oximino acid 4
may exhibit the characteristics of slow-tight binding.14,15 Further-
more, the O-phosphoryloxime motif may serve as an alternative
zinc-binding group in the design of metalloprotease inhibitors.
13. Synthesis of phosphoryl oxime 4: THF (1 mL) was added to
a mixture of
precursor of 3 (46 mg, 0.083 mmol), 10% Pd/C (5.5 mg), and NaHCO3 (4.1 equiv,
28.4 mg, 0.338 mmol). The mixture was stirred vigorously, purged with argon
and hydrogen. Then, distilled H2O (0.4 mL) was added. The hydrogenolysis
reaction was run for 2.5 h at rt. The reaction solvent was filtered through a
Acknowledgments
0.2 lm PTFE micropore filtration disk (Whatman), removed in vacuo and
pumped overnight to yield white salts. Yield. Quantitative.
14. PSMA inhibition assay and IC50 determination: Inhibition studies were performed
as described previously with only minor modifications. Working solutions of
The authors would like to extend their gratitude to Greg Helms,
William Hiscox, and Jackie Zhu, and to the NMR facility and the
Laboratory for Bioanalysis and Biotechnology (LBB2) center at
WSU for their expert assistance. This work was supported by the
Krit Y. Hatton Memorial Fund and in fond memory is dedicated
to Krit Y. Hatton and the Hatton family.
the substrate (N-[4-(phenylazo)benzoyl]-glutamyl-
c-glutamic acid, PAB-Glu-
c
-Glu) and all inhibitors were prepared in Tris buffer (50 mM, pH 7.4). Working
solutions of purified PSMA were appropriately diluted in Tris buffer (50 mM,
pH 7.4) with 1% TritonX detergent to provide from 15% to 20% conversion of
substrate to product in the absence of inhibitor. A typical incubation mixture
(final volume 250
inhibitor solution or 25
(50 mM, pH 7.4) in a test tube. A volume of the 25
was added to the above-mentioned solution. The enzymatic reaction was
initiated by the addition of 25 L of the PSMA working solution. In all cases, the
final concentration of PABGlu- -Glu was while the enzyme was
incubated with five serially diluted inhibitor concentrations to provide
l
L) was prepared by the addition of either 25
L Tris buffer (50 mM, pH 7.4) to 175 L Tris buffer
L PAB-Glu- -Glu (10 M)
lL of an
l
l
c
Supplementary data
l
l
l
Supplementary data (NMR data for compounds 2a–j, 3, and 4)
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.11.045.
c
1 lM
a
range of inhibition from 10% to 90%. The reaction was allowed to proceed for
15 min with constant shaking at 37 °C and was terminated by the addition of
25
by vortexing. The quenched incubation mixture was quickly buffered by the
addition of 25 L K2HPO4 (0.1 M), vortexed, and centrifuged (10 min at 7000g).
An 85 L aliquot of the resulting supernatant was subsequently quantified by
HPLC as previously described. IC50 values were calculated using KaleidaGraph
3.6 (Synergy Software).
lL methanolic TFA (2% trifluoroacetic acid by volume in methanol) followed
References and notes
l
1. Rodriguez, C. E.; Lu, H.; Martinez, A. R.; Hu, Y.; Brunelle, A.; Berkman, C. E. J.
Enzyme Inhib. 2001, 16, 359.
l
2. Maung, J.; Mallari, J. P.; Girtsman, T. A.; Wu, L. Y.; Rowley, J. A.; Santiago, N. M.;
Brunelle, A. N.; Berkman, C. E. Bioorg. Med. Chem. 2004, 12, 4969.
3. Hoshino, O.; Murakata, M.; Yamada, K. Bioorg. Med. Chem. Lett. 1992, 2, 1561.
4. Boehlow, T. R.; Harburn, J. J.; Spilling, C. D. J. Org. Chem. 2001, 66, 8.
5. Kotoku, N.; Tsujita, H.; Hiramatsu, A.; Mori, C.; Koizumi, N.; Kobayashi, M.
Tetrahedron 2005, 61, 7211.
15. PSMA preincubation inhibition studies: This assay was conducted as described
above for IC50 determination except that PSMA was preincubated with
inhibitor 4 (60 nM) for 10 min with constant shaking at 37 °C prior to the
addition of PABGlu-c-Glu.