Efficient stereocontrolled synthesis of (S)-Fmoc-β-nitroalanine via oxidation of oxime

Article abstract of DOI:10.1016/j.tetlet.2010.04.083

Stereocontrolled synthesis of (S)-Fmoc-β-nitroalanine (20) was accomplished from (R)-Fmoc-Ser(^tBu)-OH (14) in a total of six steps via an oxime. The oxime (17) was obtained from (R)-Fmoc-Ser(^tBu)-H (16), which in turn was obtained by reduction of Weinreb amide (15). Oxidation of oxime was realized with peroxytrifluoroacetic acid at a neutral pH at 0 °C. After removal of the ^tBu protecting group with 90% TFA/H₂O, the hydroxyl group was oxidized with Jones reagent to afford (S)-Fmoc-β-nitroalanine (20) in overall good yield.

Full text of DOI:10.1016/j.tetlet.2010.04.083

Tetrahedron Letters 51 (2010) 3340–3343
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Tetrahedron Letters
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Efficient stereocontrolled synthesis of (S)-Fmoc-b-nitroalanine via oxidation
of oxime
*
Satendra S. Chauhan , Howard J. Wilk
Bachem Bioscience Inc., 3700 Horizon Drive, King of Prussia, PA 19406, USA
a r t i c l e i n f o
a b s t r a c t
Stereocontrolled synthesis of (S)-Fmoc-b-nitroalanine (20) was accomplished from (R)-Fmoc-Ser(tBu)-OH
(14) in a total of six steps via an oxime. The oxime (17) was obtained from (R)-Fmoc-Ser(tBu)-H (16),
which in turn was obtained by reduction of Weinreb amide (15). Oxidation of oxime was realized with
peroxytrifluoroacetic acid at a neutral pH at 0 °C. After removal of the tBu protecting group with 90%
TFA/H2O, the hydroxyl group was oxidized with Jones reagent to afford (S)-Fmoc-b-nitroalanine (20) in
overall good yield.
Article history:
Received 22 February 2010
Revised 19 April 2010
Accepted 20 April 2010
Available online 24 April 2010
Key words:
Amino acids
Ó 2010 Elsevier Ltd. All rights reserved.
Stereoselective synthesis
b-Nitroalanine
(S)-Fmoc-b-nitroalanine
Boc-b-nitro-Ala-OMe
1. Introduction
2-Chloro-3-nitropropionic acid (5) was treated with ammonium
hydroxide in methanol.10 3-Nitropropionic acid was prepared from
b-Nitroamino acids are a significant class of building blocks for
the synthesis of a variety of important target molecules. For exam-
ple, b-nitroamino acids are used as precursors for a,b-dehydroami-
acrylic acid and nitryl chloride, (b) protected bromoglycine (6) was
reacted with methylnitronate,2 and (c) Easton’s three-component
coupling method.14 The latter procedure utilized dipotassium salt
of nitroacetic acid (8), glyoxalic acid (9), and ammonia to produce
b-nitroaspartate (7), which upon decarboxylation afforded b-nitro-
alanine. In this communication, we describe a stereocontrolled
no acids,1,2 which are commonly found in naturally occurring
peptides.3–5 In addition, nitro group can be efficiently converted
into other valuable functional groups, such as amines, aldehydes,
or acid moieties, making it a versatile functional group for further
modifications into a variety of other chemical structures.6,7 Fur-
thermore, b-nitroamino acids are important as enzyme inhibi-
tors8,9 and are also useful as modified enzyme substrates.10,11
Among b-nitroamino acids, b-nitroalanine has been particularly
useful in studying aspartate metabolism because it is an isostere of
aspartic acid.10,12,13 However, the main reason for its biological
activity is that it is isoelectronic with aspartic acid/aspartate
(1,2) in its neutral (3) and nitronate (4) anionic states (Fig. 1).10
O
O
-H+
O
O
+H+
OH
OH
H2N
H2N
O
O
1
2
It is important to note that
sized by utilizing racemic b-nitroalanine as a surrogate amino acid
derivative since the stereochemistry at the -carbon is lost during
a,b-dehydroalanine can be synthe-
a
dehydronitration (elimination of HNO2). However, other chemical
applications, such as peptide synthesis, require use of a single iso-
mer of b-nitroalanine.
Stereoselective synthesis of b-nitroalanine is not reported in the
literature. However, racemic b-nitroalanine has been synthesized
by exploiting the following strategies as shown in Scheme 1. (a)
O
N
O
N
-H+
O
O
+H+
OH
OH
H2N
H2N
O
O
3
4
* Corresponding author. Tel.: +1 610 239 0300; fax: +1 610 239 0800.
E-mail address: schauhan@usbachem.com (S.S. Chauhan).
Figure 1. Anionic states of aspartic acid (1, 2) and b-nitroalanine (3, 4).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.083

S. S. Chauhan, H. J. Wilk / Tetrahedron Letters 51 (2010) 3340–3343
3341
NO2
OH
NO2
Br
a
b
O-tBu
OH
Boc-HN
Cl
H2N
O
O
O
3
6
5
c
O
OK
N
NO2
OK
KO
H2N
O
NH3 (aq)
+
+
O
CO2H
OK
9
O
O
8
7
Scheme 1.
synthetic strategy for the preparation of (S)-Fmoc-b-nitroalanine
starting from (R)-Fmoc-Ser(tBu)-OH.
NO2
OtBu
O
O
NH2
OtBu
X
2. Results and discussion
Acetone/H2O
Boc-HN
Boc-HN
O
O
b-Nitroalanine derivatives can be synthesized in various ways.
However, two methods can be considered particularly useful: (1)
displacement of a good nucleofuge with a nitrite anion15,16 and
(2) oxidation of nitrogenous derivatives, such as amine,17 oxime,18
or isocyanate.19
13
12
Scheme 3.
Based on above methodologies, we designed a short three-step
synthesis of (S)-Fmoc-b-nitroalanine starting from protected forms
of either (S)-b-iodoalanine or (S)-2,3-diaminopropionic acid (Dap)
via substitution or oxidation, respectively. Cleavage of the protect-
ing groups followed by Fmocylation should afford the target com-
pound. An important aspect of this strategy was the choice of the
protecting groups. Since the nitro group is reduced under catalytic
hydrogenation conditions and also undergoes elimination under
basic conditions, the protecting groups should be cleavable under
acidic conditions.
the desired nitro compound (13) could not be isolated as shown
in Scheme 3.17 Dimethyldioxirane was freshly prepared from ox-
one (2KHSO5ꢀKHSO4ꢀK2SO4) in buffered aqueous acetone following
a reported procedure.21 Therefore, we tried a different strategy that
utilized oxidation of oxime. In this case, we started with (R)-Fmoc-
Ser(tBu)-OH (14) as the starting material.
As shown in Scheme 4, (R)-Fmoc-Ser(tBu)-OH (14) was reacted
with N,O-dimethylhydroxyl amine hydrochloride using a mixed
anhydride method with isobutyl chloroformate to afford Weinreb
amide (15) in quantitative yield. The amide (15) was dissolved in
anhydrous ether/THF (3:2) and was added to lithium aluminum
hydride suspension in ether at ꢁ40 5 °C dropwise. After stirring
at the same temperature for 1 h, the reaction mixture was allowed
to warm to 0 °C and the stirring was continued at that temperature
for additional 2 h. The reaction was quenched with 1 N aq KHSO4
and the aldehyde was extracted with ether. (R)-Fmoc-Ser(tBu)-H
(16) was obtained as a light yellow solid in 91.8% yield.22 The
unpurified aldehyde (16) was treated with 5 equiv of hydroxyl-
amine hydrochloride and 5 equiv of pyridine in refluxing ethanol
for 30 min. Aldoxime (17) was obtained as white solid after
work-up and trituration with hexane in 82% yield and showed
two close spots on TLC (5% MeOH/DCM), perhaps due to E/Z iso-
mers. Before proceeding further, the structure of aldoxime (18)
was confirmed by spectroscopic methods and was consistent with
the structure.23
In the next step, oxime was oxidized into NO2 function. For this
purpose we utilized peroxytrifluoroacetic acid at a neutral pH. The
peroxytrifluoroacetic acid was freshly generated from urea-hydro-
gen peroxide (UHP) complex and trifluoroacetic anhydride (TFAA)
in acetonitrile at 0 °C. The reaction was complete within 2 h. The
reaction mixture was washed with 5% aq Na2SO3 to decompose
any residual peroxy acid/hydrogen peroxide.18 After silica gel col-
umn chromatography the nitro compound (18) was obtained in
68% yield as a viscous light yellow oil.24
In our first attempt, we utilized a commercially available Boc-b-
iodo-Ala-OMe (10) since following the substitution reaction with
silver nitrite both the Boc and methyl ester protecting groups can
be removed by aqueous HCl hydrolysis yielding (S)-b-nitroalanine.
Thus, the iodo compound (10) was refluxed with 5 equiv of silver
nitrite in diethyl ether for 48 h as shown in Scheme 2.16 Boc-b-ni-
tro-Ala-OMe (11) was isolated in 11% yield after silica gel column
chromatography.20 Longer reaction time and addition of excess sil-
ver nitrite did not improve the yield of the nitro compound (11).
We also tried sodium nitrite in DMF at ambient temperature over-
night, but the yield could not be improved.15 Since the substitution
reaction gave a poor yield of nitro compound (11), we did not pro-
ceed further.
Oxidation of the side-chain NH2 group of Boc-Dap-OtBu (12;
Scheme 3) followed by TFA treatment would also yield (S)-b-nitro-
alanine. However, oxidation of the compound (12) with dim-
ethyldioxirane in wet acetone resulted in a complex mixture and
I
NO2
OMe
AgNO2
OMe
Et2O, reflux
Boc-HN
Boc-HN
O
O
11
10
The nitro compound (18) was treated with TFA/H2O (9:1) for
Scheme 2.
35 min to cleave the tBu protecting group. TFA was evaporated un-

3342
S. S. Chauhan, H. J. Wilk / Tetrahedron Letters 51 (2010) 3340–3343
OtBu
OH
OtBu
OtBu
H
1) LiAlH4, 0 oC
HN(Me)OMe
iBuOCOCl
Me
N
2) NH2OH/Pyridine
Fmoc-HN
Fmoc-HN
Fmoc-HN
OMe
O
O
X
16; X = O
17; X = N-OH
15
14
OtBu
NO2
OH
Jones reagent
acetone, rt
UHP/TFAA/MeCN
Na2HPO4, 0 oC
TFA/H2O
(9:1)
NO2
Fmoc-HN
Fmoc-HN
19
18
NH2
NO2
OH
NO2
H
N
(21)
DIC/HOBt
Fmoc-HN
Fmoc-HN
O
O
20
22
Scheme 4.
4. Rink, R.; Wierenga, J.; Kuipers, A.; Kluskens, L. D.; Driessen, A. J. M.; Kuipers, O.
P.; Moll, G. N. Appl. Environ. Microbiol. 2007, 73, 1792–1796.
5. Nickeleit, I.; Zender, S.; Sasse, F.; Geffers, R.; Brandes, G.; Sörensen, I.;
Steinmetz, H.; Kubicka, S.; Carlomagno, T.; Menche, D.; Gütgemann, I.; Buer,
J.; Gossler, A.; Manns, M. P.; Kalesse, M.; Frank, R.; Malek, N. P. Cancer Cell 2008,
14, 23–35.
6. Ono, N. The Nitro Group in Organic Synthesis; Wiley-VCH: New York, 2001.
7. Ballini, R.; Palmieri, A.; Righi, P. Tetrahedron 2007, 63, 12099–12121. and
references cited therein.
8. Alston, T. A.; Bright, H. J. FEBS Lett. 1981, 126, 269–271.
9. Alston, T. A.; Porter, D. J. T.; Bright, H. J. Acc. Chem. Res. 1983, 16, 418–424.
10. Porter, D. J. T.; Bright, H. J. J. Biol. Chem. 1980, 255, 4772–4780.
11. Foote, J.; Lauritzen, A. M.; Lipscomb, W. N. J. Biol. Chem. 1985, 260, 9624–9629.
12. Porter, D. J. T.; Rudie, N. G.; Bright, H. J. Arch. Biochem. Biophys. 1983, 225, 157–
163.
13. Raushel, F. M. Arch. Biochem. Biophys. 1984, 232, 520–525.
14. Soapi, K. M.; Hutton, C. A. Amino acids 2006, 31, 337–339.
15. Farnandez-Mateos, A.; Coca, G. P.; Gonzalez, R. R.; Hernandez, C. T. J. Org. Chem.
1996, 61, 9097–9102.
der reduced pressure. The residual TFA was neutralized with aque-
ous NaHCO3 and (S)-Fmoc-b-nitro-Ala-ol (19) was extracted with
EtOAc. Evaporation of EtOAc and trituration of the residue with
hexane afforded compound (19) in 98% yield as a white solid,
which was taken to the next step without further purification.
The alcohol (19) was subjected to oxidation using Jones reagent
in acetone at ambient temperature for 16 h.25 After work-up and
silica gel column chromatography (S)-Fmoc-b-nitroalanine (20)
was obtained as a white solid in 66% yield. Its structure was con-
firmed by spectroscopic methods.26
In order to prove the chiral integrity of compound 20, we con-
verted it into a diastereomer by reacting with a chiral derivatizing
agent, (R)-(+)-a
-methylbenzylamine (21).27 Thus, (S)-Fmoc-b-
nitroalanine was activated as an HOBt ester by reacting with
DIC/HOBt and treated with chiral amine 21 to afford (S)-Fmoc-b-
16. Sekine, A.; Kumagai, N.; Uotsu, K.; Ohshima, T.; Shibasaki, M. Tetrahedron Lett.
nitroalanyl-(R)-a-methylbenzylamide (22) in 81% yield. There
2000, 41, 509–513.
was only a single product formed as checked by chromatographic28
and spectroscopic methods29 confirming that compound 20 was
obtained as a single isomer and no racemization occurred during
the synthesis.
17. Murray, R. W.; Jeyaraman, R.; Mohan, L. Tetrahedron Lett. 1986, 27, 2335–2336.
18. Ballini, R.; Marcantoni, E.; Petrini, M. Tetrahedron Lett. 1992, 33, 4835–4838.
19. Eaton, P. E.; Wicks, G. E. J. Org. Chem. 1988, 53, 5353–5355.
20. Compound 11. Maldi-Tof (DHB matrix): 249 (C9H16N2O6) [M+H]+. 1H NMR
(400 MHz, CDCl3): d 5.51 (1H, d, J = 6.8 Hz), 4.88 (1H, dd, J = 13.6 Hz, 3.2 Hz),
4.83 (1H, dd, J = 13.6 Hz, 3.2 Hz), 4.75–4.60 (1H, m), 3.85 (3H, s), 1.45 (9H, s).
In summary, a stereocontrolled synthesis of (S)-Fmoc-b-nitro-
alanine was accomplished in a total of six steps starting from
(R)-Fmoc-Ser(tBu)-OH in overall good yield of 33%. It is important
to mention that Fmoc-b-nitro-Ala-OH is suitable for solid phase
peptide synthesis using Fmoc chemistry. However, since the nitro
group is base labile, it can possibly undergo elimination reaction
(–HNO2) upon repeated use of piperidine during synthesis. There-
fore, a mild cleavage cocktail consisting of 2% HOBt, 2% hexameth-
yleneimine, and 25% N-methylpyrrolidine in NMP/DMSO (1:1) is
recommended for deprotection of the Fmoc group during solid
phase peptide synthesis using Fmoc chemistry.30
½ ꢂ -31.0 (c 1, MeOH).
a 2D4
21. Murray, R. W.; Jeyaraman, R. J. Org. Chem. 1985, 50, 2847–2853.
22. Fehrentz, J.; Castro, B. Synthesis 1983, 676–678.
23. Compound 17. Maldi-Tof (CCA matrix): 405 (C22H26N2O4) [M+Na]+ (100%). 1H
NMR (400 MHz, CDCl3): d 7.75 (2H, d, J = 7.2 Hz), 7.60 (2H, d, J = 7.2 Hz), 7.39
(2H, t, J = 7.2 Hz), 7.30 (2H, t, J = 7.2 Hz), 7.50 (0.6H, d, J = 4.8 Hz), 6.75 (0.4H, d,
J = 4.8 Hz), 5.56 (1H, d, J = 6.8 Hz), 5.05–4.94 (0.4H, m), 4.52–4.43 (0.6H, m),
4.42–4.35 (2H, m), 4.22 (1H, t, J = 6.8 Hz), 3.65–3.45 (2H, m), 1.16 (9H, s).
24. Compound 18. Maldi-Tof (CCA matrix): 421 (C22H26N2O5) [M+Na]+ (100%). 1H
NMR (400 MHz, CDCl3): d 7.78 (2H, d, J = 7.2 Hz), 7.58 (2H, d, J = 7.2 Hz), 7.42
(2H, t, J = 7.2 Hz), 7.33 (2H, t, J = 7.2 Hz), 5.34 (1H, d, J = 7.2 Hz), 4.65–4.52 (2H,
m), 4.50–4.4.38 (3H, m), 4.26–4.20 (1H, t, J = 6.8 Hz), 3.56–3.45 (2H, m), 1.18
(9H, s).
25. Zhang, X.; Ni, W.; van der Donk, W. A. J. Org. Chem. 2005, 70, 6685–6692.
26. Compound 20. Maldi-Tof (CCA matrix): 378 (C18H16N2O6) [M+Na]+ (100%). 1H
NMR (400 MHz, CDCl3): d 7.77 (2H, d, J = 7.2 Hz), 7.57 (2H, d, J = 7.2 Hz), 7.41
(2H, t, J = 7.2 Hz), 7.32 (2H, t, J = 7.2 Hz), 5.82 (1H, d, J = 6.8 Hz), 5.02 (1H, dd,
J = 13.6 Hz, 3.2 Hz), 4.86–4.79 (2H, m), 4.45 (2H, d, J = 6.4 Hz), 4.22 (1H, t,
References and notes
J = 6.4 Hz). ½a 2D4
ꢁ13 (c 1, MeOH).
ꢂ
1. Coghlan, P. A.; Easton, C. J. Tetrahedron Lett. 1999, 40, 4745–4748.
2. Coghlan, P. A.; Easton, C. J. Arkivoc 2004, 10, 101–108.
3. Tomkinson, B.; Grehn, L.; Fransson, B.; Zetterqvist, O. Arch. Biochem. Biophys.
1994, 314, 276–279.
27. Rosen, T.; Watanabe, M.; Heathcock, C. H. J. Org. Chem. 1984, 49, 3657–3659.
28. Analytical HPLC was performed on a C18
, reversed-phase column (Vydac,
150 ꢃ 4.6 mM, 5 ). A linear gradient from 5% to 95% buffer B in 45 min was
l

S. S. Chauhan, H. J. Wilk / Tetrahedron Letters 51 (2010) 3340–3343
3343
used. Buffer A consisted of 0.1% TFA in water and buffer B consisted of 0.1% TFA
in acetonitrile. Flow rate was 1.5 mL/min. Compound 22 eluted at 25.7 min
under these conditions. Compound 20 eluted at 20.0 min under the same
conditions.
7.31 (2H, t, J = 7.2 Hz), 6.44 (1H, br), 5.58 (1H, br), 5.08 (1H, dd, J = 13.6 Hz,
3.2 Hz), 4.99 (1H, 1H, dd, J = 13.6 Hz, 3.2 Hz), 4.79 (1H, dd, J = 13.6 Hz, 3.2 Hz),
4.54 (2H, d, J = 5.4), 4.51 (1H, t, J = 5.4 Hz), 4.18 (1H, t, J = 6.0), 1.49(3H, d,
J = 6.0 Hz). ½a 2D4
ꢂ
5.66 (c 0.3, MeOH).
29. Compound 22. ESI-MS: 482 (C26H25N3O5) [M+Na]+ (100%). 1H NMR (600 MHz,
CDCl3): d 7.75 (2H, d, J = 7.2 Hz), 7.53 (2H, d, J = 7.2 Hz), 7.41 (2H, t, J = 7.2 Hz),
30. Li, X.; Kawakami, T.; Aimoto, S. Tetrahedron Lett. 1998, 39, 8669–8672.