Ruthenium-catalyzed glycine-selective oxidative backbone modification of peptides

Article abstract of DOI:10.1016/S0040-4039(00)01823-2

The reaction of N,C-protected peptides containing glycine residues with peracetic acid in the presence of a ruthenium catalyst gives α-ketoamides derived from the oxidation at the C(α) position of the glycine residues selectively. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01823-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 10245–10249
Ruthenium-catalyzed glycine-selective oxidative backbone
modification of peptides
Shun-Ichi Murahashi,* Akira Mitani† and Kyuuhei Kitao
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Machikaneyama 1-3,
Toyonaka, Osaka 560-8531, Japan
Received 11 September 2000; revised 10 October 2000; accepted 12 October 2000
Abstract
The reaction of N,C-protected peptides containing glycine residues with peracetic acid in the presence
of a ruthenium catalyst gives a-ketoamides derived from the oxidation at the Ca position of the glycine
residues selectively. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: oxidation; peptide; glycine; backbone modification; ruthenium catalyst.
Oxidation of proteins and peptides is of interest in view of aging and diseases.1,2 Oxidative
modification of peptides is also a useful tool for altering chemical and biological properties.3,4
However, there is no report on catalytic backbone modification of peptides except for the
modification by the side-chain scission at the serine and the threonine residues.3 We wish to
report a novel catalytic backbone modification at the glycine residue of peptides without
backbone fragmentation. Glycine selective reactions are limited to the glycine-selective NꢀCa
bond cleavage with nickel peroxide.5
During our study on a simulation of the enzymatic functions of cytochrome P-450,6 we found
that ruthenium-catalyzed oxidation of amines7 and amides8 with peroxides proceeds highly
efficiently. We wanted to apply this method to the oxidation of peptides, and found that the
ruthenium-catalyzed oxidation of N,C-protected peptides with peracetic acid gives a-ketoamide
derivatives which are formed by the selective oxidation at the Ca position of the glycine residue.
This is the first example of transition metal-catalyzed oxidative backbone modification of
peptides and also of the selective oxidation at the Ca position of glycine.
The catalytic oxidation of Ac–Gly–Ala–OEt (1) with peracetic acid in the presence of RuCl3
catalyst (20 mol%) in acetic acid gave Ac–NHCOCO–Ala–OEt (2) (81% yield, based on the
converted substrate), which was obtained by the oxidation at the Ca position of the glycine
* Corresponding author. Fax: +81 6 6850 6224; e-mail: mura@chem.es.osaka-u.ac.jp
† On leave from Odawara Research Center, Nippon Soda Co., Ltd., Takada 345, Kanagawa 250-0280, Japan.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01823-2

10246
residue, and ethyl pyruvate (3) (8%), obtained by the oxidation at the Ca position of the alanine
residue followed by hydrolysis, along with the recovered substrate (70% conversion) (Scheme 1).
The product ratio of the oxidation of the glycine residue and the alanine residue is 10 to 1. The
ruthenium-catalyzed glycine selective backbone modification thus occurs. The catalytic activity
of various ruthenium catalysts was examined for the oxidation of 1 with peracetic acid.
RuCl3·nH2O gave the best result. Ruthenium on charcoal also showed high catalytic activity;
however, other ruthenium complexes such as RuCl2(PPh3)3, [RuCl2(CO)3]2, RuCl2(bpy)2,
Ru3(CO)12, Ru(acac)3, and RuO2 gave unsatisfactory results.
O
O
O
O
O
H
N
H
N
RuCl3·nH2O
O
+
N
H
O
N
H
O
O
CH3CO3H / CH3CO2H
conv. = 70%
O
O
O
2
3
81%
8%
1
Scheme 1.
Typical procedure of the ruthenium-catalyzed oxidation of 1 is as follows. To a solution of
RuCl3·nH2O (0.1 mmol) and 1 (0.5 mmol) in acetic acid (1.0 mL) was added a 30% ethyl acetate
solution of peracetic acid (2.0 mmol) dropwise at room temperature over a period of 2 h. After
additional stirring for 2 h, acetophenone was added as an internal standard, and the conversion
of 1 (70%) and the yields of 2 (81%) and 3 (8%) were determined by HPLC and GLC analyses.
Isolation of 2 was carried out as follows. To the reaction mixture was added a saturated
aqueous solution of Na2SO3 (0.5 mL). The resulting solution was poured into a saturated
aqueous solution of NaHCO3 and extracted with ethyl acetate. Column chromatography on
silica gel gave 2 (48 mg, 60% yield) as a colorless solid.
The oxidation of Ac–Gly–Val–OEt (4) under the present reaction conditions gave the
corresponding a-ketoamide derivative (5) (48% conversion, 56% yield) (Scheme 2). Again, the
glycine residue was oxidized selectively, although the reactivity of 4 was decreased because of the
steric effect of the isopropyl group.
O
O
H
N
O
O
O
H
N
RuCl3·nH2O
N
H
O
N
H
O
CH3CO3H / CH3CO2H
conv. = 48%
O
O
56%
4
5
Scheme 2.
The oxidation of a mixture of 1 and Ac–Ala–Ala–OEt (6) (1:1) under the present reaction
conditions resulted in selective modification of the glycine residue of 1 (Scheme 3). Thus, the
relative reactivity towards the oxidation of the glycine residue of 1 and the acetyl protected
alanine residue of 6 is 10 to 1. This was calculated based on the yields of 2 (58%, based on 1)
and MeCOCO–Ala–OEt (7) (6%, based on 6) that were obtained by the oxidation at the Ca
position of the acetyl-protected alanine residue followed by hydrolysis. The low reactivity of 6
is ascribed to the steric effect of the methyl group at the alanine residue towards the
oxoruthenium species.8

10247
O
O
O
H
N
H
N
RuCl3·nH2O
+
1
2
+
N
H
O
O
O
CH3CO3H / CH3CO2H
O
O
6%
58%
6
7
Scheme 3.
Next, we examined the oxidation of the dipeptide Ac–Gly–Gly–OEt (8) bearing two glycine
residues (Scheme 4). The Ca position of the N-terminal glycine of 8 was oxidized to give the
corresponding a-ketoamide derivative 99 (81% yield); however, the Ca position of the C-terminal
glycine of 8 was not modified (67% conversion). The protecting groups of the glycine residues
are a crucial factor, and the glycine residues that are protected with two amide groups are
readily oxidized.
O
O
O
O
O
H
N
H
N
RuCl3·nH2O
N
H
O
N
H
O
CH3CO3H / CH3CO2H
conv. = 67%
O
O
8
81%
9
Scheme 4.
Accordingly, one can expect that inner glycine residues can be modified readily. Indeed, the
catalytic oxidation of Piv–Ala–Gly–Ala–OEt (10) gave the products, Piv–NH2 (11) (15%),
MeCOCO–Gly–Ala–OEt (12) (10%), Piv–Ala–NHCOCO–Ala–OEt (13) (21%), NH2COCO–
Ala–OEt (14) (19%), Piv–Ala–NH2 (15) (17%), and 3 (8%) (52% conversion) (Scheme 5). The
products derived from the oxidation at the glycine residue (b) were obtained predominantly
(13+14+15=57%). Selective backbone modification at the glycine residues can be carried out not
only at the terminal position of peptides but also at the inner position of peptides.
a
b
c
O
O
RuCl3·nH2O
H
N
O
CH3CO3H / CH3CO2H
conv. = 52%
N
H
N
H
O
O
10
O
H
O
N
O
a;
+
O
N
H
10 %
NH2
O
O
12
11 15 %
O
O
O
O
H
N
O
H2N
O
NH2
+
+
b;
c;
N
H
N
H
N
H
N
H
O
O
O
O
O
O
15
13 21 %
19 %
17 %
14
O
O
O
3
8 %
Scheme 5.

10248
The oxidation can be rationalized by assuming the cytochrome P-450 type mechanism as
shown in Scheme 6.6 The reaction of low-valent ruthenium complex LnRum (m=2 or 3) (16)
with peracetic acid gives oxoruthenium species LnRum+2ꢁO (17).7 Abstraction of hydrogen from
the Ca position of the glycine residue and subsequent single electron transfer would afford an
iminium ion complex 19. Rebounding of the hydroxyl ligand would afford a-hydroxyglycine
(20) and 16 to complete the catalytic cycle. The hydroxy compound 20 can be converted to
a-ketoamide derivative (21) under the reaction conditions. Iodosylbenzene, which is used for
generation of an oxo-metal species,10 was also effective for the oxidation at the glycine residue
of 1, although the conversion and the yield were low in comparison with peracetic acid. It is
noteworthy that the oxidation of glycine residues with RuO4 does not occur.3 Glycine selective
modification is due to the abstraction of hydrogen with the oxoruthenium species at the Ca
position of the glycine residues, which have a less sterically hindered secondary car-
bonꢀhydrogen bond.8
Scheme 6.
In summary, glycine-selective backbone modification of peptides can be performed by the
ruthenium-catalyzed reaction with peracetic acid.
Acknowledgements
This work was supported by the Research for the Future Program, the Japan Society for the
Promotion of Science.
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