An Efficient Synthesis of Enantiopure (2 R,3 R)-β-Methoxytyrosine

Article abstract of DOI:10.1055/s-0034-1381048

Enantiopure (2R,3R)-β-methoxytyrosine was stereoselectively synthesized from ethyl 3-(4-hydroxyphenyl)-3-oxopropanoate protected by 2-methoxyethoxymethyl (MEM) (ee >98%). l-Aminoacylase-catalyzed resolution of the corresponding erythro-N-acetyl derivatives afforded (2S,3S)-(4-MEM)-β-methoxytyrosine (ee >99%). The conversion increased to 98% by optimizing the synthesis to yield enantiopure N-acetyl-(2R,3R)-(4-MEM)-methoxytyrosine. N-Acyl cleavage was accomplished under mild conditions.

Full text of DOI:10.1055/s-0034-1381048

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
© Georg Thieme Verlag Stuttgart · New York
2015, 26, 2553–2556
letter
2553
S.-M. Fan et al.
Letter
Synlett
An Efficient Synthesis of Enantiopure (2R,3R)-β-Methoxytyrosine
Shi-Ming Fana
Xia Tianb
Yi-Hua Yangb
Long yi Jin*a
Shou-xin Liu*a,b
a Department of Chemistry, College of Science, Yanbian
University, 977 Gongyuan Road, Yanji 133002, P. R. of
China
lyjin@ybu.edu.cn
b State Key Laboratory Breeding Base-Hebei Province
Laboratory of Molecular Chemistry for Drug, Hebei
University of Science & Technology, 70 Yuhua Road,
Shijiazhuang 050018, P. R. of China
chlsx@hebust.edu.cn
Received: 18.05.2015
CONH2
Accepted after revision: 21.06.2015
Published online: 01.09.2015
DOI: 10.1055/s-0034-1381048; Art ID: st-2015-w0381-l
OH
NH
OH
O
O
H
N
O
H2N
N
N
N
NH
NH2
H
H
H
NH OH
O
O
HN
Abstract Enantiopure (2R,3R)-β-methoxytyrosine was stereoselective-
ly synthesized from ethyl 3-(4-hydroxyphenyl)-3-oxopropanoate pro-
tected by 2-methoxyethoxymethyl (MEM) (ee >98%). L-Aminoacylase-
catalyzed resolution of the corresponding erythro-N-acetyl derivatives
afforded (2S,3S)-(4-MEM)-β-methoxytyrosine (ee >99%). The conver-
sion increased to 98% by optimizing the synthesis to yield enantiopure
N-acetyl-(2R,3R)-(4-MEM)-methoxytyrosine. N-Acyl cleavage was ac-
complished under mild conditions.
N
O
H
O
O
Me
NH
HN
O
N
O
O
Me
N
MeO
O
N
OH
H
CONH2
OH
callipeltin A
Key words amino acids, biosynthesis, asymmetric synthesis, enantio-
selectivity, enzymes, natural products
Figure 1 Structure of callipeltin A
(2R,3R)-β-Methoxytyrosine is a common non-proteino-
genic amino acid in a number of novel cyclic depsipeptides
isolated from marine sponges.1 These marine metabolites
possess the similar structure, which includes callipeltins A
(Figure 1) and B,2 stellatolide A,3 papuamides A–F,4 mira-
bamides A–H,5 and neamphamides B–D.6 Many of these
have been reported to inhibit HIV-1 viral entry in vitro as
well as cytotoxicity against a number of human cancer cell
lines and fungi. β-Methoxytyrosine may also play an im-
portant role in the biological activity of these depsipep-
tides.5a,7 Recently, tremendous synthetic efforts have been
dedicated to the preparation of β-methoxytyrosine. The
strategies include the stereoselective addition reaction of
arylmetal reagents with serine aldehyde equivalents fol-
lowed by methylation,8 photo-assisted bromination of D-
and L-tyrosine derivatives followed by methanolysis,9 and
Sharpless asymmetric aminohydroxylation or dihydroxyl-
ation of cinnamyl ester derivatives.10
amounts of chiral auxiliaries. In contrast, biocatalysis is an
interesting alternative that leverages the enzymatic speci-
ficity and efficiency.11 For the first time, we report here a
coupling approach for the synthesis and aminoacylase reso-
lution to synthesize enantiopure β-methoxytyrosine as
part of efforts in the total synthesis of these natural cyclic
depsipeptides.
As shown in Scheme 1, the intermediate oxime 2
was successfully achieved through the oximation of ethyl
3-(4-hydroxyphenyl)-3-oxopropanoate protected with me-
thoxyethoxymethyl (MEM) with ethyl nitrite in the pres-
ence of nano-K2CO3 previously developed by our group.12
The diastereoselective hydrogenation of oxime 2 yielded
ethyl erythro-β-hydroxy-β-arylalanine in the presence of
Pd/C and AcOH,13 and N-acetylation occurred in Ac2O/AcONa.
A heterogeneous methylation of erythro-3 with iodometh-
ane and silver oxide base afforded 4.14 After removal of the
ethyl ester, the selective hydrolysis of the N-acetyl from the
Although each of these approaches offer elegant advan-
tages, they still suffer from poor stereoselectivity, the need
for expensive chiral catalysts or the need for stoichiometric
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2553–2556

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S.-M. Fan et al.
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nano-K2CO3
ethyl nitrite
O
O
O
O
1. H2, Pd/C,
AcOH/EtOH
OEt
OEt
EtOH, 10 °C
81%
2. Ac2O, NaOAc
90%
NOH
MEMO
MEMO
2
1
OMe
OH
O
COOEt
MeI, Ag2O
CH2Cl2
69%
NaOH
OEt
NHAc
NHAc
MEMO
MEMO
DL-erythro
DL-erythro
4
3
OMe
OMe
L-aminoacylase
COOH
COOH
OMe
NH2
NHAc
COOH
MEMO
MEMO
6
7
NHAc
conversion 85%
MEMO
DL-erythro
D-aminoacylase
no reaction
5
Scheme 1 The route to the synthesis of β-methoxytyrosine
Table 1 Aminoacylase-Catalyzed Hydrolysis of erythro-N-Acetyl-β-me-
thoxy-(4-MEM)-tyrosine
DL-erythro-5 using L-aminoacylase derived from Aspergillus
oryzae as a catalyst gave enantiopure (2S,3S)-β-methoxy-β-
arylalanine.
OMe
OMe
HN
COOH
COOH
O
L-aminoacylase
We studied the enzyme-catalyzed hydrolysis of
(DL)-erythro-5 at 37 °C in aqueous solution (pH 7.5).
The (2S,3S)-N-acetyl-β-methoxy-(4-MEM)-tyrosine could
be enantioselectively hydrolyzed by L-aminoacylase to af-
ford the corresponding (2S,3S)-6 with excellent ee values
(>99%), but low conversion (85%). Hence, the synthesis con-
ditions were iteratively optimized by changing the tem-
perature and pH (Table 1). Both high and low pH values de-
creased the conversion (Table 1, entries 2 and 3). Higher
temperatures were preferable, but not higher than 40 °C
(Table 1, entries 5 and 6). However, the enzyme activity
likely decreases with increasing time at 40 °C due to the de-
naturation of L-aminoacylase at the higher temperature,
and thus fresh aminoacylase was added three times at 12
hours intervals to give 98% conversion (Table 1, entry 8).
Unfortunately, no product was obtained when D-amino-
acylase was used to catalyze the hydrolysis of the corre-
sponding isomer substrate.
5
NH2
MEMO
MEMO
6
7
Entry
pH
7.5
Temp (°C)
Time (h)
Conv. (%)a
1
2
3
4
5
6
7
8
37
37
37
35
40
45
40
40
36
85
81
73
71
89
62
90
98
7.0
8.0
7.5
7.5
7.5
7.5
7.5
36
36
36
36
36
40
12 × 3
a The conversion was calculated at 0.5 molar quantity of rac-substrate and
determined by HPLC on a chiral stationary phase.
To remove the protecting acetyl group, the transforma-
tion of N-Ac to N-Boc was accomplished with a one-pot
procedure.15 The acetamide 7 was treated with equivalent
amounts of triethylamine in THF, and then refluxed with
Boc2O and DMAP for four hours. The mixture was cooled to
room temperature, and the same volume of MeOH and ex-
cess hydrazine (4 equiv) were added in order to both
quench the unreacted Boc2O and cleave the acetamide. Fi-
nally, (2R,3R)-β-methoxytyrosine was obtained in a one-
step deprotection of the Boc and MEM groups mediated by
TFA with ee >98% (Scheme 2).16
OMe
HN
OMe
1. Et3N, Boc2O, DMAP,THF,
then hydrazine, MeOH
COOH
O
COOH
NH2
2. 30% TFA/CH2Cl2
MEMO
HO
68%
7
8
Scheme 2 Deprotection of the N-acetyl of 7
In the synthesis of enantiopure β-methoxytyrosine,
considerable effort has been directed toward the selection
of a suitable protecting group to protect the phenol hydroxyl.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2553–2556

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OH
O
O
O
O
COOH
NHAc
1. H2, Pd/C,
AcOH, EtOH
OEt
+
OEt
+
NHAc
HO
NOH
HO
RO
2. Ac2O, NaOAc
DL-erythro
10 41–48% yield
12
11
9 R = Bn, TMS, TBDMS, MOM
MeI, Ag2O
10
+
11
12
CH2Cl2
Scheme 3 The retro-aldol reaction
Initially, benzyl (Bn) was selected as the protecting group,
but no desired product was obtained. During the conversion
from 9 to 10, the reduced product decomposed partly with
yields of 41%. The methylation of compound 10 in the pres-
ence of base converts the products via a retro-aldol reac-
tion. Trimethylsilyl (TMS), tert-butyldimethylsilyl (TBDMS)
and methoxymethyl (MOM) were also tested as protecting
group for the phenol hydroxyl, but all of them were re-
moved during the hydrogenation of the corresponding ox-
imes (Scheme 3).
was hydrolyzed selectively by L-aminoacylase catalysis to
obtain the enantiopure β-methoxytyrosine.18 This reagent
will be very valuable in the future natural product synthesis
efforts developing callipeltin A and its congeners.
Acknowledgment
We are grateful for the financial assistance from the National Basic
Research Program of China (2011CB512007, 2012CB723501), the Na-
tional Natural Science Foundation of China (Grant No. 30873139), and
the
Hebei
Natural
Science
Foundation
(No.12966737D,
The methylation was selected to protect the phenol hy-
droxyl due to its stability. Following the procedure, N-ace-
tyl-13 was synthesized from ethyl 3-(4-methoxyphenyl)-3-
oxopropanoate (monitored by NMR and chiral HPLC). The
selective removal of the methyl from the methoxy attached
on benzene ring of 13 is a critical reaction. According to the
literature,17 BBr3 can be used to form the product 14. How-
ever, when 13 was treated with BBr3, the decarboxylation
and elimination formed compound 15 and simultaneously
removed the methyl group (Scheme 4). We then used NMR
to confirm the structure of 15, and the trans-configuration
was unambiguously determined by the coupling pattern
(J = 15.0 Hz).
B2015208134).
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1381048.
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OMe
COOH
OMe
COOH
HN
BBr3, CH2Cl2
HO
HN
O
MeO
14
unsuccessful
O
NHAc
13
HO
15
(5) (a) Plaza, A.; Gustchina, E.; Baker, H. L.; Kelly, M.; Bewley, C. A.
J. Nat. Prod. 2007, 70, 1753. (b) Lu, Z.; Van Wagoner, R. M.;
Harper, M. K.; Baker, H. L.; Hooper, J. N. A.; Bewley, C. A.; Ireland,
C. M. J. Nat. Prod. 2011, 74, 185.
Scheme 4 Demethylation of 13 with BBr3
In summary, we have reported a novel and efficient syn-
thetic method to prepare enantiopure (2R,3R)-β-methoxy-
tyrosine. The key intermediate 5 was successfully synthe-
sized using MEM as the protecting group of the phenol hy-
droxyl. The erythro-N-acetyl-(4-MEM)-β-methoxytyrosine
(6) Tran, T. D.; Pham, N. B.; Fechner, G.; Zencak, D.; Vu, H. T.;
Hooper, J. N. A.; Quinn, R. J. J. Nat. Prod. 2012, 75, 2200.
(7) Ratnayake, A. S.; Bugni, T. S.; Feng, X.; Harper, M. K.; Skalicky, J.
J.; Mohammed, K. A.; Andjelic, C. D.; Barrows, L. R.; Ireland, C. M.
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S.-M. Fan et al.
Letter
Synlett
(8) (a) Okamoto, N.; Hara, O.; Makino, K.; Hamada, Y. J. Org. Chem.
2002, 67, 9210. (b) Hansen, D. B.; Wan, X.; Carroll, P. J.; Joullié,
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(13) Chang, Y. T.; Hartung, W. H. J. Am. Chem. Soc. 1953, 75, 89.
(14) Characterization Data for 4: white solid; mp 59–60 °C.
1H NMR (500 MHz, CDCl3): δ = 7.20 (d, J = 8.5 Hz, 2 H), 7.05 (d,
J = 9.0 Hz, 2 H), 6.08 (d, J = 9.0 Hz, 1 H), 5.27 (s, 2 H), 4.92 (dd,
J = 9.0, 4.5 Hz, 1 H), 4.55 (d, J = 4.5 Hz, 1 H), 4.12 (m, 2 H), 3.83
(m, 2 H), 3.56 (m, 2 H), 3.37 (s, 3 H), 3.21 (s, 3 H), 2.01 (s, 3 H),
1.16 (t, J = 7.0 Hz, 3 H). 13C NMR (125 MHz, CDCl3): δ = 169.9,
169.7, 157.4, 130.3, 128.0, 93.6, 83.1, 71.7, 67.8, 61.4, 59.1, 57.7,
57.1, 23.2, 14.0. HRMS: m/z [M + H]+ calcd for C18H28NO7:
370.1866; found: 370.1857.
rate = 0.4 mL/min, tR = 7.9 min. 1H NMR (500 MHz, CD3OD con-
taining 1% TFA): δ = 7.17 (d, J = 8.5 Hz, 2 H), 6.82 (d, J = 8.5 Hz,
2 H), 4.80 (d, J = 4.0 Hz, 1 H), 4.28 (d, J = 4.0 Hz, 1 H), 3.34 (s,
3 H). 13C NMR (125 MHz, CD3OD containing 1% TFA): δ = 169.1,
159.4, 129.6, 125.8, 116.7, 81.4, 59.1, 57.4. HRMS: m/z [M + H]+
calcd for C10H14NO4: 212.0923; found: 212.0917.
(18) Aminoacylase Resolution: A solution of 4 (3.7 g, 10 mmol) in a
mixture of EtOH (36 mL) and 1 N NaOH (10.0 mL) was stirred for
1.5 h at 0 °C. This was then concentrated under reduced pres-
sure to 3 mL. Deionized H2O (84 mL) and 0.05 M CoCl2 (1.7 mL)
were then added to the residue, and the pH of the solution was
adjusted to 7.5 with 1 N HCl. The mixture was stirred at 40 °C.
The L-aminoacylase (40 mg) was added three times at 12 h
intervals to this magnetically stirred solution. The pH was
maintained at 7.5 with 0.1 N NaOH, and the mixture was con-
centrated under reduced pressure to 10 mL. The residue was
cooled to 0 °C and acidified with cold concentrated HCl to pH 1.
It was then extracted with EtOAc (3 × 50 mL), and these extracts
were washed with 2% HCl (2 × 30 mL), dried over anhyd Na2SO4,
and concentrated to yield N-acetyl enantiomer 7 (1.5 g, 88%).
The aqueous layer was adjusted to pH 6.5 with 4.0 N NaOH and
concentrated until the volume reached 5 mL. The precipitated
crystals were collected on a filter, washed with a small amount
of H2O, and dried to give a tan solid 6 (1.06 g, 71%).
(15) Burk, M. J.; Allen, J. G. J. Org. Chem. 1997, 62, 7054.
(16) Akbaba, Y.; Türker Balaydın, H.; Göksu, S.; Şahin, E.; Menzek, A.
Helv. Chim. Acta 2010, 93, 1127.
20
(17) Characterization Data for 8: white solid; [α]D +6.1° (c = 0.2,
MeOH). HPLC: column: Daicel Chem. Ind. Crownpak CR(+),
mobile phase: MeOH–H2O, 1:7 at pH 1.0 (HClO4), 25 °C; flow
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2553–2556