Synthesis of L-α-amino-ω-bromoalkanoic acid for side chain modification

Article abstract of DOI:10.1016/j.tetlet.2003.11.007

L-α-Amino-ω-bromoalkanoic acids with side chain lengths varying from 4 to 10 methylene units have been conveniently synthesized as useful intermediates for the synthesis of functionalized non-natural amino acids.

Full text of DOI:10.1016/j.tetlet.2003.11.007

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 491–494
Synthesis of
L
-a-amino-x-bromoalkanoic acid for side chain
modification
a
Louis A. Watanabe, Binoy Jose, Tamaki Kato, Norikazu Nishino
b
c
b,c,
*
b,d
and Minoru Yoshida
a
Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan
CREST Research Project, Japan Science and Technology Agency, Saitama 332-001, Japan
b
c
Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu 808-0196, Japan
d
RIKEN, Saitama 351-0198, Japan
Received 18 October 2003; revised 30 October 2003; accepted 4 November 2003
Abstract— -a-Amino-x-bromoalkanoic acids with side chain lengths varying from 4 to 10 methylene units have been conveniently
L
synthesized as useful intermediates for the synthesis of functionalized non-natural amino acids.
Ó 2003 Elsevier Ltd. All rights reserved.
3
The synthesis of non-natural amino acids always
attracted attention of synthetic chemists due to the
improvement in the binding potency, chemical and
biological stability and pharmacokinetic characteristics
upon introduction into peptide based compounds. This
has resulted in the development of vast number of
resulting in low yields, and could not use 9-fluorenyl-
methoxycarbonyl (Fmoc) protection strategy. The use
4;5
of natural amino acids as the starting material would
not provide a-amino-x-bromoalkanoic acids with
varying side-chain lengths. Here we report a convenient
method for the synthesis of a-amino-x-bromoalkanoic
acids (Abn, n ¼ 6,7,8,9,12) from readily available
reagents.
1
methods for the synthesis of non-natural amino acids.
They include at least 100 derivatives of phenylalanine, in
which the phenyl group is widely modified, while ali-
phatic amino acids have been paid less attention. Opti-
cally pure aliphatic amino acids having easily
replaceable functional groups at the x-positions are
highly interesting synthetic targets, as the functional
group at the x-position can modify to the required
group, which provide a series of non-natural amino
acids. We are interested in the synthesis of optically pure
non-natural aliphatic amino acid containing a bromide
in the side chain that serves as a precursor of many non-
natural amino acid. Few methods for the synthesis of
The general method for the synthesis of a-amino-x-
bromoalkanoic acids (Abn) starting from commercially
available diethyl acetamidomalonate with dibromo
alkanes is illustrated in Scheme 1. The synthesis of
amino acids starting from diethyl acetamidomalonate
and bromide derivatives is a widely used method espe-
6
cially for the synthesis of phenylalanine derivatives.
However, to our knowledge, there was no report on the
synthesis of an amino acid containing a long alkyl chain
with a replaceable bromide at the x-position. Excess
amount of dibromoalkanes were reacted with diethyl
acetamidomalonate in the presence of sodium ethoxide
2–5
x-halo-a-amino acids are available.
However, the
scope of the reported methods for the synthesis of
x-halo-a-amino acids are limited due to many reasons
such as, the formation of racemic products with diffi-
7
in abs ethanol. One equivalent of 1 N NaOH was added
at 0–5 °C to the reaction mixture and obtained the
corresponding monoacid monoester as white solid after
work up. It was then decarboxylated by refluxing in
toluene. After evaporation of toluene the resulting Ac-
DL-Abn-OEt as an oil was dissolved in ethanol and
hydrolysed using NaOH at 0–5 °C to yield Ac-DL-Abn
as white powder in quantitative yield after work up.
Finally, Aspergillus genus aminoacylase (TCI) was
2
culty in purification, involve tedious procedures and
Keywords: Amino acid; Non-natural; Functional group; Side chain
modification.
*
Corresponding author. Tel./fax: +81-93-695-6061; e-mail: nishino@
life.kyutech.ac.jp
0
040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.11.007

492
L. A. Watanabe et al. / Tetrahedron Letters 45 (2004) 491–494
Scheme 1. Reagents and conditions: (1) (i) EtOH, sodium ethoxide, reflux, 30 min; (ii) Br(CH
5
2
)
nꢀ2Br, reflux, 3 h, 80–87%; (2) NaOH aq, EtOH, 0 °C,
O, pH 7, 38 °C, 24 h, 66–80%.
h, 70–79%; (3) toluene, reflux, 3 h, 96–100%; (4) NaOH aq, EtOH, 0 °C, 3 h, 70–90%; (5) aminoacylase, H
2
8
applied to resolve the racemic mixture. The precipitated
-Abns were obtained as crystalline powder by the
L
9
filtration of the aqueous solution. The unreacted Ac-D-
Abns were recovered from the acidified aqueous solu-
tions by extraction.
Scheme 3. Reagents and conditions: (i) EtOH, sodium ethoxide,
reflux, 30 min; (ii) Br(CH Br, reflux, 3 h, 78%.
Eventhough, the method is a traditional one, working
well for the synthesis of Abns with n P 7. On the other
hand, the amino acids with lower chain length encoun-
tered some difficulty during the synthesis. In the case of
2 3
)
Ab6, we could not obtain any
step due to the cyclization of
during the enzymatic separation by aminoacylase. To
overcome the problem of cylization, we prepared Boc-
DL-Ab6-OEt using the same procedure, with diethyl
L
-Ab6 after resolution
-Ab6 to pipecolic acid
L
10
(
Boc-amino)malonate as one of the starting material
instead of diethyl acetamidomalonate as shown in
Scheme 2. Boc-DL-Ab6-OEt was subjected to the action
of subtilisin Carlsberg from Bacillus licheniformis
Scheme 4. Reagents and conditions: (1) (Boc)
2
O, dioxane, water, 8 h,
0
°C, 100%; (2) Fmoc-OSu, Na CO , water, dioxane, 8 h, 98%.
2
3
(
Sigma) in a mixture of DMF and water (1/1, v/v). Boc-
-Ab6 was isolated as a colourless oil, which was
deprotected using 4 N HCl/dioxane for the character-
L
11
ization of
L
-Ab6 hydrochloride.
The bromide at the x-position is labile, and therefore a
wide variety of functional group transformation is pos-
sible with Abns to synthesize several interesting non-
natural amino acids. For example, we illustrate here the
synthesis of 2-amino-7-phosphonatoheptanoic acid
starting from Boc-Ab7. The C-terminal of Boc-Ab7 was
protected with 2-(trimethylsilyl)ethanol and the Boc-
The syntheses of Abns (n 6 5) with short side chain
lengths were failed in our hands. During the first step of
the reaction the expected bromopropyl group was
changed to allyl group as shown in Scheme 3 and the
simultaneous treatments afforded L-allylglycine.
Ab7-OTmse was treated with P(OMe) in the presence
3
The Abns synthesized can be used for both Boc and
Fmoc methods for the synthesis of peptides. To verify
this we protected the N-terminal of the new amino acids
using Boc and Fmoc groups. The Boc protection of
of KI in THF at 70 °C to yield the desired compound in
almost quantitative yield (Scheme 5). a-Amino phos-
phonate derivatives have been found to be highly potent
inhibitors of a number of enzymes such as protein-
12
13
L
-Ab7 yielded Boc-
tive yield. Similarly the Fmoc protection of
yield the Fmoc- -Ab7 in quantitative yield as white
solid (Scheme 4).
L
-Ab7 as colourless oil in quantita-
tyrosine phosphatases, metalloproteases and serine
proteases. Also amino phosphonopeptides are known
14
L
-Ab7 also
15
L
for their significant antibacterial activity. Due to the
reasons, our method for the synthesis of amino acid
Scheme 2. Reagents and conditions: (1) (i) EtOH, sodium ethoxide, reflux, 30 min; (ii) Br(CH
3%; (3) toluene, reflux, 3 h, 95%; (4) subtilisin, H
O/DMF ¼ 1/1, pH 8, 38 °C, 3 h, 96%.
2 4
) Br, reflux, 3 h, 78%; (2) NaOH aq, EtOH, 0 °C, 5 h,
8
2

L. A. Watanabe et al. / Tetrahedron Letters 45 (2004) 491–494
493
Scheme 5. Reagents and conditions: (1) TmseOH, DCC, DMAP, DCM, 0 °C, 90%; (2) KI, P(OMe) , 70 °C, THF, 98%.
3
Scheme 6. Reagents and conditions: (1) (i) AcSK, DMF, 100%; (ii) NH
3
/MeOH, 90%; (2) n-benzyloxyformamide, KI, K
2
CO
3
, acetone, reflux, 90%.
containing a phosphonate side chain is useful for the
discovery of novel enzyme inhibitors.
References and Notes
1
. For lead references, see: Asymmetric Synthesis of Novel
Sterically Constrained Amino Acids; Symposium-in-print,
Hruby, V. J.; Soloshonok, V. A. Eds.; Tetrahedron, 2001,
To exemplify the application of the
L-Abns, we syn-
thesized cysteine homologues by replacing the bromide
at the side chain with thiol group. This was achieved by
the reaction of fully protected x-bromo-a-amino acid
with potassium thioacetate and subsequent treatment
with ammonia or methylamine in methanol as shown in
Scheme 6. These cysteine homologues have immense
5
7, 6329–6650.
. Nollet, A. J. H.; Huting, C. M.; Pandit, U. K. Tetrahedron
969, 25, 5971–5981.
. (a) Lundquist, J. T., IV; Dix, T. A. Tetrahedron Lett. 1998,
9, 775–778; (b) Lundquist, J. T., IV; B u€ llesbach, E. E.;
Dix, T. A. Tetrahedron: Asymmetry 1998, 9, 2739–2743.
2
3
1
3
16
potential as enzyme inhibitors and as peptidomimetics.
Further, we synthesized N-formylhydroxylamine (ret-
4. (a) Yamaguchi, J.; Ueki, M. Chem. Lett. 1996, 25, 621–
622; (b) Ciapetti, P.; Mann, A.; Shoenfelder, A.; Taddei,
M. Tetrahedron Lett. 1998, 39, 3843–3846.
. (a) Adamczyk, M.; Johnson, D. D.; Reddy, R. E.
Tetrahedron: Asymmetry 1999, 10, 775–781; (b) Adam-
czyk, M.; Johnson, D. D.; Reddy, R. E. Tetrahedron 1999,
rohydroxamate) derivatives in one step starting from
L
5
-Abns. Retrohydroxamates are reported as inhibitors
of metalloproteinases. In extension of this work, we
have synthesized lysine homologues by the reaction of
17
5
5, 63–88; (c) Adamczyk, M.; Akireddy, S. R.; Reddy,
fully protected
subsequent reaction with hydrazine hydrate. The Abns
may be reacted with NaN to the corresponding azides,
which is useful for Staudinger ligation.
L-Abns using phthalimide reaction and
R. E. Tetrahedron 2000, 56, 2379–2390; (d) Sugiyama, H.;
Yokokawa, F.; Shioiri, T.; Katagiri, N.; Oda, O.; Ogawa,
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3
18
6
. (a) Redman, J. E.; Ghadiri, M. R. Org. Lett. 2002, 4,
4467–4469; (b) Fugita, T.; Nose, T.; Matsuhima, A.;
Okada, K.; Asai, D.; Yamauchi, Y.; Shirasu, N.; Honda,
T.; Shigehiro, D.; Shimohigashi, Y. Tetrahedron Lett.
In conclusion, we have synthesized optically pure
L
-a-
amino-x-bromoalkanoic acids (Abns with n P 6) for
side chain modification. The N-terminals of these amino
acids can be easily protected with Boc or Fmoc groups
quantitatively. The bromide group at the x-position can
be transformed to several other functionalities to syn-
thesize a wide variety of non-natural amino acids.
2
000, 41, 923–927; (c) Stirling, I. R.; Freer, I. K. A.;
Robins, D. J. J. Chem. Soc., Perkin Trans. 1 1997, 5, 677–
80; (d) Rogers, L. M.-A.; Rouden, J.; Lecomte, L.;
6
Lasne, M.-C. Tetrahedron Lett. 2003, 44, 3047–3050.
. For the characterization and to determine the yield the
product was purified by column chromatography after the
evaporation of ethanol.
7
8
9
. Nishino, N.; Arai, T.; Ueno, Y.; Ohba, M. Chem. Pharm.
Bull. 1996, 44, 212–214.
. Typical experimental procedure: Metallic sodium
(
(
0.19 mol, 0.95 equiv) was dissolved in abs EtOH
200 mL) and diethyl acetamidomalonate (0.2 mol) was
Acknowledgements
added to the solution. The mixture was refluxed for 30 min
for the complete dissolution and then dibromoalkane
This work was supported in part by a Grant-in-Aid for
Scientific Research (no. 13450380) from the Ministry of
Education, Culture, Sports, Science and Technology,
Japan.
(
3 equiv) was added and refluxed for 3 h. The reaction
mixture was cooled on an ice bath and hydrolysis was
carried out by the addition of 1 N NaOH (aq, 1 equiv in five

494
L. A. Watanabe et al. / Tetrahedron Letters 45 (2004) 491–494
2
D
5
þ
portions) at intervals of 15 min. After selective hydrolysis,
ethanol was evaporated and the unreacted dibromoalkane
was removed by extraction with diethyl ether under basic
condition. Then the aqueous solution was extracted with
ethyl acetate (at pH ¼ 3–4, by adding citric acid) and
washed with brine, dried and evaporated to obtain white
solid of monoester monoacid (60–80%). It was then
dissolved in toluene (100 mL) and refluxed for 3 h and
toluene was evaporated to a colourless oil (96–100%). The
resulting oil was dissolved in ethanol (100 mL) and hydro-
lysis and work up was carried out as described above to get
Ac-DL-Abn as white solid (70–90%). The acid (0.1 mol) was
dissolved in water (400 mL) at pH ¼ 7 by the addition of
11.
L
-Ab6ÆHCl ½aꢁ +24, HRMS (M+H) found 210.0144
7
9
6 13 2
(calcd for C H O N Br 210.0130).
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1
N NaOH (aq). Then CoCl
lus genus aminoacylase (3 g) were added and incubated at
8 °C for 24 h. -Abn was solidified from the aqueous
2 2
Æ6H O (120 mg) and Aspergil-
3
L
solution and was filtered after cooling the reaction mixture
and washed with water and ethanol (68–80%). The aqueous
solution was extracted with ethyl acetate at pH ¼ 3 to
recover Ac-
D
-Abn (80–90%). Characterization data for:
L
-
2
5
þ
Ab7 ½aꢁ +25, HRMS (M+H) found 224.0286 (calcd for
D
7
9
25
D
C
(
7
H
15
O
2
þ
N Br 224.0286);
L
-Ab8 ½aꢁ
+24, HRMS
O
79
M+H) found 238.0466 (calcd for
C
8
H
17
þ
2
N Br
2
5
238.0442);
294.1104 (calcd for C12
L
-Ab12 ½aꢁ +23, HRMS (M+H) found
D
7
9
1
25 2
H O N Br 294.1069). H NMR
of the Boc-
were measured in Cl solution of
dioxane.
L
-Abns gave acceptable data. ½aꢁ Values
D
-Abns in 4 N HCl/
L
18. (a) Staudinger, H.; Meyer, J. Helv. Chim. Acta 1919, 2,
635–646; (b) Soellner, M. B.; Dickson, K. A.; Nilsson, B.
L.; Raines, R. T. J. Am. Chem. Soc. 2003, 125, 11790–
11791.
1
0. The synthesis of pipecolic acid and related compounds
from Ab6 will be described elsewhere.