A facile chiral pool synthesis of (S)-6-nitroindoline-2-carboxylic acid from l -phenylalanine

Article abstract of DOI:10.1055/s-0029-1217122

(S)-6-Nitroindoline-2-carboxylic acid, a substructure occurring in numerous biologically active natural products, was synthesized with moderate yield (53%) and high enantiomeric excess (>99.5%) starting from the nitration of l-phenylalanine, which is a commercially available chiral pool compound, followed by successively bromination and intramolecular cyclization. The route was carried out in gram quantities and it is suitable for industrial application due to its convenient reaction conditions and low cost. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0029-1217122

PAPER
403
A Facile Chiral Pool Synthesis of (S)-6-Nitroindoline-2-carboxylic Acid from
L-Phenylalanine
C
hira
l
Poo
i
l
Synth
n
esis of
(
S
)-6
-
-Nitroin
Q
doline-2-carboxyli
i
c
A
cidang Liu, Chao Qian,* Xin-Zhi Chen
Department of Chemical Engineering, Zhejiang University, Hangzhou 310027, P. R. of China
Fax +86(571)87951615; E-mail: supmoney@163.com
Received 28 August 2009; revised 13 October 2009
its facile availability and, thus, it is widely used in the syn-
Abstract: (S)-6-Nitroindoline-2-carboxylic acid, a substructure oc-
curring in numerous biologically active natural products, was syn-
thesized with moderate yield (53%) and high enantiomeric excess
(>99.5%) starting from the nitration of L-phenylalanine, which is a
thesis of natural and bioactive molecule.14c,15 The molec-
ular similarity between L-phenylalanine and (S)-indoline-
2-carboxylic acid prompted us to prepare (S)-indoline-2-
commercially available chiral pool compound, followed by succes- carboxylic acid via intramolecular cyclization of ortho-
sively bromination and intramolecular cyclization. The route was
halo-L-phenylalanine, which, in the literature, was not ob-
carried out in gram quantities and it is suitable for industrial appli-
cation due to its convenient reaction conditions and low cost.
tained by the halogenation of L-phenylalanine. Here we
report our efforts aimed at the chiral pool synthesis of (S)-
6-nitroindoline-2-carboxylic acid (1) starting from L-phe-
nylalanine (2) (Scheme 1).
Key words: chiral pool synthesis, (S)-6-nitroindoline-2-carboxylic
acid, L-phenylalanine, 2-bromo-4-nitro-L-phenylalanine, intramo-
lecular cyclization
COOH
NH2
COOH
NH2
a
b
Indolines1 can be found as a building block in numerous
biologically active alkaloid natural products2 and pharma-
ceuticals.3 Recently, highly efficient indoline-based
organic dyes for dye-sensitized solar cells have also been
developed.4
O2N
2
3
COOH
NH2
COOH
c
NH
O2N
Br
O2N
4
1
Indoline-2-carboxylic acid (2,3-dihydro-1H-indole-2-car-
boxylic acid) has received much attention since the angio-
tensin converting enzyme (ACE) inhibitor perindopril
was discovered3k,l and its analogues can be utilized in the
catalysis of different organic reactions.5 A number of ele-
gant and useful approaches toward the synthesis of indo-
line-2-carboxylic acid have been developed, including the
reduction of indole derivatives6 and transition-metal-cata-
lyzed intramolecular amination of ortho-halophenylala-
nine derivatives7 or ortho-aminophenylpropanoic acid
derivatives;8 chirally pure enantiomers could be obtained
after resolution of the racemic mixture of its derivatives
with chiral amines9 or acids6b and/or enzymes.10 Recently,
a new procedure was proposed using a radical rearrange-
ment method.11 However, these previous routes had yields
and enantiomeric excesses that were too low for industrial
applications; a facile synthesis was needed to fulfill phar-
maceutical demand. (S)-6-Nitroindoline-2-carboxylic
acid (1) can easily be converted into other 6-substituted
(S)-indoline-2-carboxylic acids that can be utilized in the
synthesis of pharmaceuticals.12 It can be obtained from the
nitration of (S)-indoline-2-carboxylic acid;13 however,
this gives a mixture of 5-nitro and 6-nitro derivatives,
which cannot be easily separated.13 Moreover, few of the
methods mentioned above are chiral pool methods. L-Phe-
nylalanine is a convenient chiral pool compound14 due to
Scheme 1 Synthesis of 6-nitro-(S)-indoline-2-carboxylic acid (1)
with L-phenylalanine (2) as a chiral pool compound. Reagents and
conditions: (a) HNO3, H2SO4, 0–20 °C, 3 h, 81%; (b) TBCA, H2SO4,
r.t., 5 h, 73%; (c) H2O, K2CO3, CuCl (5 mol%), reflux, 4 h, 91%.
Initially, we intended to synthesize an ortho-halo-substi-
tuted L-phenylalanine by halogenation of L-phenylalanine
(2) (Scheme 2).
Unfortunately, both the regioselectivity and the yields of
the halogenations were low regardless of the chloro and
bromo substitution and, moreover, the main product was
the para-substitution product regardless of the reagent
employed. To obtain higher regioselectivity for the ortho-
halo-substitution of L-phenylalanine, the para-substitu-
tion had to be blocked by an auxiliary, which could be re-
moved readily and had a meta-directing effect. Thus, the
nitro group was chosen for its strong meta-directing effect
and because after its reduction to the amino group, the
amino group undergoes facile conversion into either a het-
ero, carbon, or hydrogen atom, via a diazotization reaction
or other reactions. However, the nitro group was suffi-
ciently electron-withdrawing that it deactivated the ben-
zene ring to halogenation by the electrophilic substitution
process, but recently reported methods for the efficient
and simple bromination of deactivated aromatics16 made
the route feasible as the intramolecular cyclization
(Ullmann reaction) catalyzed by transition metal has been
well documented to give high yields and enantiomeric ex-
cess.
SYNTHESIS 2010, No. 3, pp 0403–0406
x
x
.
x
x
.2
0
0
9
Advanced online publication: 13.11.2009
DOI: 10.1055/s-0029-1217122; Art ID: F17709SS
© Georg Thieme Verlag Stuttgart · New York

404
J.-Q. Liu et al.
PAPER
COOH
COOH
NH2
COOH
NH2
NH
O2N
O2N
O2N
X
1
3
COOH
COOH
N
N
H
H
H
2
X
COOH
NH2
COOH
NH2
X
X
Scheme 2 The retrosynthesis of (S)-6-nitroindoline-2-carboxylic acid (1)
We began our synthesis from the nitration of L-phenylala- hypobromite;20 protection of the amino group seemed in-
nine (2), which has been well documented in literature de- sufficient to prevent this side reaction. Furthermore, de-
spite its low yield,17 which was easily overcome using a carboxylation in the bromination with N-bromo-
previously modified procedure.18 In the nitration of L-phe- succinimide in sulfuric acid was not the major side reac-
nylalanine (2) with mixed acid as nitrating reagent, a tion compared to dimerization. In other words, only traces
byproduct with a much lower melting point was found. of 2-bromo-4-nitrophenylacetaldehyde or 4-nitropheny-
After confirmation by IR and 1H and 13C NMR spectros- lacetaldehyde resulting from decarboxylation were found
copy, it was regarded as the coupling product of two L- in the products. The product distribution of the bromina-
phenylalanine molecules with sulfuric acid. To suppress tion of unprotected 4-nitro-L-phenylalanine (3) with N-
the side reaction, we use a tubular reactor instead of the bromosuccinimide in sulfuric acid was 2-bromo-4-nitro-
traditional batch reactor and this gave 3 in 81% yield.
L-phenylalanine (4), 2-bromo-4-nitrophenylacetonitrile,
and the dibromination dimerization product. Protection of
the carboxylic acid group reduced the side reaction to the
maximum extent, but deprotection made the entire synthe-
sis too complicated. Thus, we turned to another bromina-
tion reagent, tribromoisocyanuric acid (TBCA) (entry 3),
which gave 4 in high yield and enantiomeric excess and
used mild reaction conditions with an easy workup. Bro-
mination with sodium tribromoisocyanurate16j,19 (entry 4)
was carried out several times, but the yield was scarcely
more than 45% and the reasons for this were unclear.
As 4-nitro-L-phenylalanine (3) has poor solubility in com-
mon organic solvent, concentrated sulfuric acid was cho-
sen as the solvent for the halogenations. Bromination,
rather than chlorination, was chosen due to the lower re-
action temperature required and the resultant bromine
atom is easier to cleave. Different bromination reagents
were used to improve the yield and enantiomeric excess,
including N-bromosuccinimide,16a sodium bromate,16h,i
and tribromoisocyanuric acid (TBCA)16j and sodium tri-
bromoisocyanurate (Na-TBCA)19 (Table 1).
The intramolecular cyclization of 2-bromo-4-nitro-L-phe-
nylalanine (4) catalyzed by copper(I) bromide in water,
similar to the conditions used by De Vries et al.,7 gave the
(S)-6-nitroindoline-2-carboxylic acid (1) in 91% yield and
>99.5% ee. The solvent employed in the cyclization was
crucial for the reason that the 2-bromo-4-nitro-L-phenyl-
alanine (4) can potentially undergo a racemization reac-
tion that would reduce the enantiomeric excess when an
organic solvent was chosen (Table 2). It was believed that
water suppressed the racemization and decomposition of
2-bromo-4-nitro-L-phenylalanine (4), which was in good
Table 1 Bromination of 4-Nitro-L-phenylalanine (3) with Different
Reagents
Entry
Bromination reagent
NaBrO3, H2SO4
NBS, H2SO4
Yielda (%) ee (%)
1
2
3
4
11
14
73
44
55
63
TBCA, H2SO4
>99
95
Na-TBCA, H2SO4
a Isolated yield.
Table 2 Different Solvents Employed in the Cyclization of
2-Bromo-4-nitro-L-phenylalanine (4)
Results showed that the bromination of 4-nitro-L-phenyl-
alanine (3) did not give a high yield with N-bromosuccin-
imide in sulfuric acid (entry 2). The unprotected 4-nitro-
L-phenylalanine (3) was prone to oxidative decarboxyla-
tion and dimerization with N-bromosuccinimide under the
acidic conditions,20 which could also be found in the case
of sodium bromate in sulfuric acid (entry 1). According to
Gopalakrishnan, oxidative decarboxylation with N-bro-
mosuccinimide is followed by the formation of the acyl
Entry
Solvent
H2O
Yield (%)
ee (%)
>99.5
45.2
13.2
0
1
2
3
4
91
77
65
36
DMSO
NMP
DMF
Synthesis 2010, No. 3, 403–406 © Thieme Stuttgart · New York

PAPER
Synthesis of (S)-6-Nitroindoline-2-carboxylic Acid
405
agreement with Yokoyama et al.19,21 The base used in the
cyclization was either potassium carbonate or tripotassi-
um phosphate; this made little difference. Other stronger
bases may cause racemization even in water.
tered to remove the impurities as soon as the reaction finished. The
pH of the filtrate was adjusted to 3–4 with 1 M HCl and the resultant
soln was extracted with EtOAc (3 × 50 mL). The combined organic
phases were successively washed with dilute HCl, H2O, and brine,
decolorized with charcoal, dried (MgSO4), and concentrated in vac-
uo. Recrystallization (EtOAc–petroleum ether) gave 1 as a yellow
solid; yield: 9.4 g (91%); >99.5% ee; mp 168.2–169.7 °C (dec.).
[a]D20 –39.8 (c 0.1, DMSO).
IR (KBr): 3396 (s), 3113 (br s), 1727 (s), 1521 (s), 1335 cm–1 (s).
1H NMR (400 MHz, DMSO-d6): d = 12.84 (br s, 1 H, COOH), 7.43
(dd, J = 2.4, 8.0 Hz, 1 H, H5), 7.25 (d, J = 2.4 Hz, 1 H, H7), 7.22 (d,
J = 8.0 Hz, 1 H, H4), 6.67 (s, 1 H, NH), 4.44 (dd, J = 5.6, 10.4 Hz,
1 H, CH), 3.37 (ddd, J = 5.6, 10.8, 18.4 Hz, 2 H, CH2).
In conclusion, we have demonstrated a facile synthetic
proposal for the chiral pool synthesis of (S)-6-nitroindo-
line-2-carboxylic acid (1) starting from L-phenylalanine
in a total yield of 53%. The convenient reaction condition
makes the proposal readily for industrial application.
All reagents were obtained commercially and used without further
purification. Melting points were determined on a WSR-2 capillary
1
melting point apparatus and are uncorrected. H NMR and 13C
13C NMR (101 MHz, DMSO-d6): d = 175.0, 152.9, 148.3, 135.6,
124.8, 113.1, 101.6 (Ar-C7), 60.1, 33.3.
NMR data were recorded on a Bruker 400 MHz spectrometer oper-
ating near 400 (1H) or 100 (13C) MHz in D2O or DMSO-d6 solutions
and TMS as internal standard for calibration. IR spectra were re-
corded on a Nicolet E.S.P560 apparatus. LC-MS were recorded on
an Agilent 6000 apparatus. Optical rotations were measured with a
Perkin-Elmer 341 polarimeter in a 1 dm cell. Chiral HPLC were ob-
tained on a Waters Alliance 2695 HPLC using a Waters Atlantis
C18 column. Elemental analyses were recorded on a Vario E1 III el-
ementary analyzer.
LC-MS: m/z (%) = 209 (100%, [M + 1]).
Anal. Calcd for C9H8N2O4: C, 51.92; H, 3.85; N, 13.46. Found: C,
51.93; H, 3.86; N, 13.43.
Acknowledgment
We greatly acknowledge the generous financial support by a grant
from the National Natural Science Foundation of China (No.
20776127), the National Key Technology R&D Program (No.
2007BAI34B07), the Zhejiang Province Science and Technology
Program (No. 2008C01006-1), and the Natural Science Foundation
of the Zhejiang Province (Y4090045, R4090358).
4-Nitro-L-phenylalanine (3)
Prepared in 81% yield following our previous work;18 mp 241.5–
242.4 °C (dec) [Lit.17a 235–240 °C (dec)].
[a]D20 –7.9 (c 1.01, 1 M HCl) [Lit.17a –8.9 (c 2.41, 1 M HCl)].
IR (KBr): 3425(br s), 1695 (s), 1535 (s), 1345 cm–1 (s).
1H NMR (400 MHz, D2O): d = 8.05 (d, J = 8.8 Hz, 2 H, H3, H5),
7.36 (d, J = 8.8 Hz, 2 H, H2, H6), 4.27 (t, J = 6.0 Hz, 1 H, CH), 3.24
(ddd, J = 6.0, 7.2, 14.4 Hz, 2 H, CH2).
13C NMR (101 MHz, D2O): d = 170.7, 147.2, 141.9, 130.5, 124.1,
53.4, 35.2.
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Anal. Calcd for C9H10N2O4: C, 51.43; H, 4.76; N, 13.33. Found: C,
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and was recrystallized (H2O) to give 4 as a white solid; yield: 21.9
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(S)-6-Nitro-2,3-dihydro-1H-indole-2-carboxylic Acid (1)
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(7.0 g) and CuCl (0.3 g). The result mixture was heated to reflux and
kept at this temperature for an additional 4 h. The content was fil-
Synthesis 2010, No. 3, 403–406 © Thieme Stuttgart · New York

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