Convenient access to L-3,4,5-trioxygenated phenylalanine compounds from L-tyrosine

Article abstract of DOI:10.1016/j.tet.2020.131243

A convenient and efficient synthesis of L-3,4,5-trioxygenated phenylalanine derivatives from L-tyrosine was developed. Dibromo phenylalanine is converted easily to bis-phenol via copper-catalyzed hydroxylation. The synthetic potential of this methodology has been demonstrated by efficient synthesis of L-3,4,5-trimethoxyphenylalanine methyl ester and one key intermediate of Trabectedin.

Full text of DOI:10.1016/j.tet.2020.131243

Tetrahedron 76 (2020) 131243
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Convenient access to L-3,4,5-trioxygenated phenylalanine compounds
from L-tyrosine
Shengfeng Zhou, Chen Zhou, Qian Lu, Xintong Liu, Jie Yuan, Xinhong Yu*
Engineering Research Center of Pharmaceutical Process Chemistry, Ministry of Education, Shanghai Key Laboratory of New Drug Design, State Key
Laboratory of Bioengineering Reactors; School of Pharmacy, East China University of Science & Technology, Shanghai, 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient and efficient synthesis of L-3,4,5-trioxygenated phenylalanine derivatives from L-tyrosine
Received 1 March 2020
Received in revised form
21 April 2020
Accepted 24 April 2020
Available online 29 April 2020
was developed. Dibromo phenylalanine is converted easily to bis-phenol via copper-catalyzed hydrox-
ylation. The synthetic potential of this methodology has been demonstrated by efficient synthesis of L-
3,4,5-trimethoxyphenylalanine methyl ester and one key intermediate of Trabectedin.
© 2020 Published by Elsevier Ltd.
Keywords:
Trioxygenated phenylalanine
L-tyrosine
Bishydroxylation
Trabectedin
1. Introduction
acids are commercially available, cheap and optically pure. There-
fore, the development methods for natural amino acid derivatiza-
L-3,4,5-trioxygenated phenylalanine derivatives are important
sectors in biologically active molecules. Specifically, L-3,4,5- tri-
methoxygenated phenylalanine 1a is one of the most important
nonproteinogenic amino acid. For example (Fig. 1), as a new class of
covalent ClpXP inhibitors, amino acid phenyl ester 2 [1] shows
potent inhibition, as well as stimulation of ClpXP-mediated prote-
olysis. Tripeptide 3 [2] shows high activity in biochemical and
cellular assays as a new class of noncovalent proteasome inhibitors.
A series of compounds 4 [3] were reported that have high anti-
hepatitis B activity in 2018.
Protected L-3,4,5-trioxygenated phenylalanine 1b [4] is also a
star molecule because it is one key segment in the total synthesis of
Trabectedin 5 (Fig. 2), which is the first representative of a marine
natural product to receive marketing authorization for the treat-
ment of patients with liposarcoma and leiomyosarcoma that
cannot be removed by surgery (unresectable) or is advanced
(metastatic) [5].
tion has attracted the attention of organic chemists. Till now two
synthetic routes have been established. In 2001 [9], Deboves et al.
reported the conversion of L-serine-derived organozinc reagent to
L-3,4,5-trimethoxyphenylalanie ester derivative using organome-
tallic chemistry without loss of stereochemical integrity. Chen
group published another synthetic route of L-3,4,5-trioxygenated
phenylalanine derivatives from L-tyrosine in 2013 [10]. Oxidative
hydroxylation of aryboronic acids as a key step was employed in the
synthesis. However, in order to avoid the side effection of active NH
group, extra synthetic steps were required. Thus, the amino acid
was reduced to amino alcohol and transformed to oxazolidine with
2,2-dimethoxypropane.
After
bishydroxylation,
L-3,4,5-
trioxygenated phenylalanine derivatives could be obtained by hy-
drolysis of oxazolidine 11 and oxidation of amino alcohol to amino
acid.
Given the significance of L-3,4,5- trimethoxygenated phenylal-
anine, we started to explore an easier and more convenient method
accessing this motif. Herein we report an efficient synthesis of L-
As of now, many synthetic methods have been developed for the
preparation of L-3,4,5-trioxygenated phenylalanine derivatives
[1,2,6e10]. Resolution, chiral synthesis and natural amino acid
derivatization are three typical approaches (Fig. 3). Natural amino
3,4,5-trioxygenated phenylalanine compounds through
a
key
copper-catalyzed hydroxylation from dibromo
tive directly without amino acid reduction.
Letyrosine deriva-
2. Result discussion
* Corresponding author.
E-mail address: xhyu@ecust.edu.cn (X. Yu).
In order to identify the proper precursor for bishydroxylation,
https://doi.org/10.1016/j.tet.2020.131243
0040-4020/© 2020 Published by Elsevier Ltd.

2
S. Zhou et al. / Tetrahedron 76 (2020) 131243
firstly, as outlined in Scheme 1, 3,5-dibromo-O-methyltyrosine
ester 12 was prepared from -tyrosine according to known pro-
L
cedures [10e12]. This 4-step synthesis could be easily scaled up on
decagram scale in 70% overall yield.
Dawei Ma’s group [13] reported an efficient method on trans-
formation of aryl halides to the phenols via a powerful catalytic
system combined by Cu(acac)2 and N,N’ -bis(4-hydroxyl-2,6-
dimethylphenyl)oxalamide (BHMPO). Considering the condition
of hydroxylation by using lithium hydroxide at above 80 ꢀC,
dibenzyl group was chosen as amine protection. The protection
with benzyl bromide went smoothly in high yield under the con-
dition of K2CO3 and DIPEA as base in acetonitrile. We then
attempted to use this dibenzyl amino ester 13 to run bishydrox-
ylation directly with Ma’s method. Unfortunately, the amino ester
13 was only hydroxylized to the amine acid 14 and compound 15
derived from the phenol hydroxylation was not found. Thus, we
isolated the amino acid 14 by column chromatography and try the
bishydroxylation under the same conditions. To our delight, mon-
ohydroxylated product 16 was produced and small amount of bis-
phenol 15 was detected. Then a systematic optimization of bishy-
droxylation was investigated. After extensive experiments,
increasing the loading of catalyst to 10 mol % and raising the re-
action temperature to 110 ꢀC, the conversion of dibromo amino acid
14 to bis-phenol 15 was improved to above 85% and the isolated
yield was 77% (Table 1).
Fig. 1. Trimethoxygenated phenylalanine derivatives.
With establishment of optimal conditions for the key bishy-
droxylation of 3,5-dibromo-O-methyltyrosine ester, we started to
explore the synthesis of 1a and 1b. Firstly, o-methylation of bis-
phenol 15 and methyl esterification by dimethylsulfate afforded
ester 17 in 91% yield. Debenzylation of 17 gave the L-3,4,5- trime-
thoxyphenylalanine methyl ester 1c in 96% yield, which is the
advanced intermediate one step away from 1a. On the other hand,
protection of bis-phenol 15 with TBSCl gave silyl ether 18. Deben-
zylation by hydrogenation of 18 in the presence of a catalytic
amount of Pd/C furnished the amino acid 19. Finally alloc protection
to afford the desired amino acid 1b [14] with high yield (see
Scheme 2).
Fig. 2. Trabectedin intermediate and Trabectedin.
3. Conclusions
In summary, a new, convenient and efficient synthesis of pro-
tected L-3,4,5-trioxygenated phenylalanine derivatives from
commercially available L-tyrosine was developed. The key step is
Ma’s copper-catalyzed bishydroxylation of dibromide using
Scheme 1. Preparation of bis-phenol 15.
Fig. 3. Synthetic methods for L-3,4,5-trioxygenated phenylalanine derivatives.

S. Zhou et al. / Tetrahedron 76 (2020) 131243
3
Table 1
Optimization of reaction conditionsa.
Entrya
condition
14
15
16
Other impurities
1
2
3
4
5
6
7
1%Cu(acac)2, BHMPO, 80 ꢀC, 24 h
3%Cu(acac)2, BHMPO, 80 ꢀC, 24 h
3%Cu(acac)2, BHMPO, 90 ꢀC, 24 h
7%Cu(acac)2, BHMPO, 90 ꢀC, 24 h
7%Cu(acac)2, BHMPO, 110 ꢀC, 24 h
10%Cu(acac)2, BHMPO, 110 ꢀC, 24 h
20%Cu(acac)2, BHMPO, 110 ꢀC, 24 h
49
17
0.5
1
2
10
45
76
90
82
14
3
5
6
8
8
10
11
11
76
86(77b)
86
3
a
Unless specified, all reactions were analyzed by HPLC.
Isolated yield.
b
[a]2D7 -27.7 (c 1.31, in MeOH); 1H NMR (400 MHz, CDCl3):
d (ppm)
7.12e7.29 (m, 12H), 3.90e3.93 (m, 5H), 3.79 (s, 3H), 3.51e3.57 (m,
3H), 2.8e3.0 (m, 2H); 13CNMR (100 MHz, CDCl3): d (ppm) 172.3,
152.5, 138.7, 137.2, 133.6128.6, 128.4, 127.2, 117.5, 61.7, 60.7, 54.4,
51.4, 34.3.ESIMS 548.0 [MþH]þ
4.2. (S)- 2-(dibenzylamino)-3-(3,5-dibromo-4-methoxyphenyl)
propanoic acid 14
LiOH$H2O (8.4 g, 200 mmol) and water (84 mL) was added to the
solution of compound 13 (51.0 g, 93 mmol) in THF (168 mL). The
solution was allowed to warm to 65e70 ꢀC and stirred overnight.
THF was evaporated in vacuo. The pH of the reaction was adjusted
to 4e6 by 1 M HCl solution. The mixture was extracted with DCM
(400 mL). The organic layer was dried with anhydrous Na2SO4 and
concentrated. The residue was purified by silica gel column chro-
matography (gradient elution: 5%e30% ethyl acetate in hexane) to
afford compound 14 (44.3 g, 89%) as a viscous oil.
Scheme 2. Preparation of trimethoxygenated phenylalanine methyl ester 1c and
Trabectedin intermediate 1b.
[a]2D7 -22.9 (c 0.84, in MeOH); 1H NMR (400 MHz, CDCl3):
d
(ppm) 7.14e7.37 (m, 12H), 3.77e3.96 (m, 7H), 3.69 (m, 1H), 3.11
(dd, J ¼ 12, 4 Hz, 1H), 2.98 (dd, J ¼ 12, 4 Hz, 1H); 13CNMR (100 MHz,
CDCl3): d (ppm) 176.7, 152.7, 138.0, 137.0, 133.5128.8, 128.5, 127.5,
117.7, 61.7, 60.7, 54.4, 33.4. ESIMS: 534.0 [MþH]þ
BHMPO as the ligand. Two typical L-3,4,5-trioxygenated phenylal-
anine derivatives 1b and 1c have been readily obtained. Notable,
the key building block L-trimethoxyphenyl-alanine methyl ester 1c
was prepared in nine steps with 39% overall yield.
4.3. (S)-2-(dibenzylamino)-3-(3,5-dihydroxy-4-methoxyphenyl)
4. Experimental
propanoic acid 15
All solvents and reagents were obtained from commercial sup-
pliers and used without further purification. Optical rotations were
reported as follows: [a]DT (c g/100 mL, in solvent). 1H and 13C NMR
spectra were recorded on Varian Mercury plus 400 MHz
spectrometer.
To a solution of compound 14 (16.0 g, 30 mmol) in DMSO
(80 mL) and water (20 mL) at 25 ꢀC, LiOH$H2O (6.3 g, 150 mmol),
Cu(acac)2 (0.78 g,
3 mmol) and N1,N2-bis(4-hydroxy-2,6-
dimethylphenyl)oxalamide (0.98 g, 3 mmol) was added under
argon. The reaction mixture was allowed to warm to 110e115 ꢀC
and stirred overnight. After the reaction was completed, the
mixture was cooled to room temperature and added water
(330 mL) and acetic acid (6.0 g, 100 mmol). The crude product
(12.5 g) was collected by filtration. Purification by silica gel column
chromatography (gradient elution: 15%e50% ethyl acetate in hex-
ane) afforded pure compound 15 (9.4 g, 77%).
4.1. (S)-methyl 2-(dibenzylamino)-3-(3,5-dibromo-4-
methoxyphenyl)propanoate 13
To a solution of compound 12 (36.7 g, 100 mmol) in acetonitrile
(367 mL) at 25 ꢀC was added benzyl bromide (42.75 g, 250 mmol),
DIPEA (12.9 g, 100 mmol) and K2CO3 (41.4 g, 300 mmol). The re-
action mixture was refluxed 24 h. After completion, the reaction
mixture was filtered. The filtrate was concentrated and purified by
silica gel column chromatography (gradient elution: 1%e5% ethyl
acetate in hexane) to give compound 13 (51.4 g, 94%) as an oil.
[a]2D7 -32.8 (c 0.95, in MeOH); 1H NMR (400 MHz, CDCl3):
d
(ppm) 7.21e7.31 (m, 10H), 6.31 (s, 2H), 3.89 (s, 3H), 3.69e3.80 (m,
5H), 3.24 (dd, J ¼ 12, 4 Hz, 1H), 2.87 (dd, J ¼ 12, 4 Hz, 1H); 13CNMR
(100 MHz, CDCl3): d (ppm) 173.2, 149.6, 136.6, 134.0, 133.7, 129.0,
128.6, 127.8, 108.7, 63.7, 60.7, 54.6, 34.0. ESIMS: 408.1 (M þ H)þ

4
S. Zhou et al. / Tetrahedron 76 (2020) 131243
4.4. (S)-3-(3-bromo-5-hydroxy-4-methoxyphenyl)-2-
bicarbonate solution (60 mL). The organic layer was dried (sodium
(dibenzylamino)propanoic acid 16
sulfate) and concentrated, and the residue was purified by flash
column chromatography (gradient elution: 5%e30% ethyl acetate in
hexane) to afford 18 as an oil (5.1 g, 80%).
To a solution of compound 14 (12.8 g, 24 mmol) in DMSO
(76.8 mL) and water (19.2 mL) at 25 ꢀC, LiOH$H2O (4.8 g,120 mmol),
Cu(acac)2 (192 mg, 0.72 mmol) and N1,N2-bis(4-hydroxy-2,6-
dimethylphenyl)oxalamide (240 mg, 0.72 mmol) was added un-
der argon. The reaction mixture was allowed to warm to 90e100 ꢀC
and stirred overnight. After the reaction was completed, the
mixture was cooled to room temperature and added water
(288 mL) and acetic acid (4.8 g, 84 mmol). The crude product (9.8 g)
was extracted with Ethyl acetate (500 mL). Purification by silica gel
column chromatography (gradient elution: 15%e30% ethyl acetate
in hexane) afforded pure compound 16 (8.8 g, 78%).
[a]2D7 -29.0 (c 1.02, in MeOH); 1H NMR (400 MHz, CDCl3):
d (ppm)
7.21e7.33 (m, 10H), 6.38 (s, 2H), 3.73 (s, 3H), 3.60e3.79 (m, 5H),
3.28 (m, 1H), 2.88 (m, 1H), 1.01 (s, 18H), 0.16 (s, 12H); 13CNMR
(100 MHz, CDCl3): d (ppm) 175.7, 149.6, 141.5, 138.0, 133.4, 128.9,
128.5, 127.5, 115.6, 62.9, 60.0, 54.5, 33.8, 25.7, 18.3, ꢁ4.6. ESIMS:
636.3 (M þ H)þ
4.8. (S)-2-amino-3-(3,5-bis((tert-butyldimethylsilyl)oxy)-4-
methoxyphenyl)propanoic acid 19
[a]2D7 -26.7 (c 0.57, in MeOH); 1H NMR (400 MHz, CDCl3):
d (ppm)
A solution of 18 (636 mg, 1.0 mmol) and 10% palladium on
activated charcoal (64 mg) in methanol (20 mL) was stirred under
1 atm of hydrogen gas at 40 ꢀC for 6 h. The reaction mixture was
filtered, and the filtrate was concentrated to afford 19 (450 mg, 99%)
which could be used directly in the next step without further
purification.
7.22e7.34 (m, 10H), 6.85 (s, 1H), 6.79 (s, 1H), 3.91 (s, 3H), 3.75e3.96
(m, 5H), 3.21 (dd, J ¼ 12, 4 Hz, 1H), 2.96 (dd, J ¼ 12, 4 Hz, 1H);
13CNMR (100 MHz, CDCl3): d (ppm) 175.0, 149.9, 143.2, 137.0, 136.1,
129.0, 128.6, 127.8, 125.0, 116.4, 115.8, 62.6, 60.1, 54.6, 33.5. ESIMS:
470.1 (M þ H)þ
[a]2D7 -10.0 (c 1.38, in MeOH); 1H NMR (400 MHz, CDCl3):
d (ppm)
4.5. (S)-methyl 2-(dibenzylamino)-3-(3,4,5-trimethoxyphenyl)
propanoate 17
6.47 (s, 2H), 3.70 (m, 1H), 3.64 (s, 3H), 3.20 (m, 1H), 2.80 (m, 1H),
0.94 (s, 18H), 0.12 (s, 12H); 13CNMR (100 MHz, DMSO): d (ppm)
170.2, 149.3, 141.4, 132.9, 115.9, 60.0, 55.6, 36.8, 26.7, 18.4, ꢁ4.3.
ESIMS: 456.2 (M þ H)þ
Dimethyl sulfate (630 mg, 5.0 mmol) was added to a mixture of
15 (407 mg,1.0 mmol) and K2CO3 (700 mg, 5.1 mmol) in acetonitrile
(15 mL), and the resulting suspension was allowed to warm to
65e70 ꢀC and stirred for 3 h. Acetonitrile was evaporated in vacuo.
The mixture was partitioned with DCM (30 mL) and water (20 mL).
The organic layer was dried with anhydrous Na2SO4 and concen-
trated. The residue was purified by silica gel column chromatog-
raphy (gradient elution: 5%e20% ethyl acetate in hexane) to afford
compound 17 (409 mg, 91%) as an oil.
4.9. (S)-2-(((allyloxy)carbonyl)amino)-3-(3,5-bis((tert-
butyldimethylsilyl)oxy)-4-methoxyphenyl)propanoic acid 1b
To a solution of 19 (455 mg, 1.0 mmol) in THF (6 mL) at 10 ꢀC was
added 1 M NaOH solution (2.5 mL) and allylchloroformate (180 mg,
1.5 mmol), and the reaction was stirred at 25 ꢀC for 50 min. The
mixture was quenched with acetic acid (120 mg, 2 mmol) and
concentrated. The residue was partitioned between water (20 mL)
and dichloromethane (20 mL). The organic layer was dried (sodium
sulfate) and concentrated, and the residue was purified by flash
column chromatography (15% ethyl acetate in hexane) to give 1b
(501 mg, 93%) as an oil.
[a]2D7 -54.8 (c 1.12, in MeOH); 1H NMR (400 MHz, CDCl3):
d (ppm)
7.19e7.26 (m, 10H), 6.19 (s, 2H), 3.95 (d, J ¼ 14 Hz, 2H), 3.86 (s, 3H),
3.78 (s, 3H), 3.69 (s, 6H), 3.63 (m, 1H), 3.54 (d, J ¼ 14 Hz, 2H), 3.03
(dd, J ¼ 12, 4 Hz, 1H), 2.88 (dd, J ¼ 12, 4 Hz, 1H); 13CNMR (100 MHz,
CDCl3): d (ppm) 172.8, 152.8, 139.2, 136.2, 134.2, 128.7, 128.1, 127.0,
106.2, 62.5, 60.9, 55.8, 54.4, 51.2, 36.2. ESIMS: 450.2 (M þ H)þ
[a]2D7 9.7 (c 1.0, in MeOH), 25.4 (c 1.0, in DCM); reference [14] 18.8
(c 1.0, in DCM);
4.6. (S)-methyl 2-amino-3-(3,4,5-trimethoxyphenyl)propanoate 1c
1H NMR (400 MHz, CDCl3):
d (ppm) 6.32 (s, 2H), 5.87 (m, 1H),
A solution of 17 (449 mg, 1.0 mmol) and 10% palladium on
activated charcoal (45 mg) in methanol (20 mL) was stirred under
1 atm of hydrogen gas at 45 ꢀC for 6 h. The reaction mixture was
gravity filtered, and the filtrate was concentrated to afford amine
(260 mg, 96%).
5.28.
(d, J ¼ 16 Hz, 1H), 5.20 (d, J ¼ 12 Hz, 1H), 5.11 (d, J ¼ 8 Hz, 1H),
4.58 (m,1H), 4.55 (d, J ¼ 4 Hz, 2H), 3.70 (s, 3H), 3.02 (dd, J ¼ 12, 4 Hz,
1H), 2.96 (d, J ¼ 12, 4 Hz, 1H), 0.99 (s, 18H), 0.15 (s, 12H);
[a]2D7 9.2 (c 1.0, in MeOH); 1H NMR (400 MHz, CDCl3):
d (ppm)
Declaration of competing interest
6.39 (s, 2H), 3.80 (s, 6H), 3.78 (s, 3H), 3.70 (s, 3H), 3.77e3.81 (m,1H),
3.02 (dd, J ¼ 12, 4 Hz, 1H), 2.80 (d, J ¼ 12, 4 Hz, 1H), 2.68 (s, 2H);
13CNMR (100 MHz, CDCl3): d (ppm) 174.8, 153.2, 136.8, 132.5, 106.1,
60.7, 56.0, 55.6, 54.4, 52.0, 40.8.
☐ The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
4.7. (S)-3-(3,5-bis((tert-butyldimethylsilyl)oxy)-4-
methoxyphenyl)-2-(dibenzylamino)propanoic acid 18
Acknowledgments
To a solution of compound 15 (4.1 g, 10 mmol) in dichloro-
methane (41 mL) was added TBSCl (6 g, 40 mmol) and imidazole
(3.4 g, 50 mmol) and the resulting mixture was stirred at 25 ꢀC for
3 h. The reaction mixture was diluted with dichloromethane
(41 mL) and was washed sequentially with saturated aqueous so-
dium bicarbonate solution (60 mL) and saturated aqueous sodium
chloride solution (60 mL). To the organic layer was added tri-
fluoroacetic (2 mL) acid. The reaction mixture was stirred at 25 ꢀC
for 0.5 h and then quenched with saturated aqueous sodium
We are grateful to financial support from National Natural Sci-
ence Foundation of China (No.21476078) and Science and Tech-
nology Commission of Shanghai Municipality (No. 12431900902).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2020.131243.

S. Zhou et al. / Tetrahedron 76 (2020) 131243
5
M.D. Sintchak, C. Tsu, K.M. Gigstad, Bioorg. Med. Chem. Lett 20 (2010)
6581e6586.
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