A convenient synthesis of amino acid methyl esters

Article abstract of DOI:10.3390/molecules13051111

A series of amino acid methyl ester hydrochlorides were prepared in good to excellent yields by the room temperature reaction of amino acids with methanol in the presence of trimethylchlorosilane. This method is not only compatible with natural amino acids, but also with other aromatic and aliphatic amino acids.

Full text of DOI:10.3390/molecules13051111

Molecules 2008, 13, 1111-1119; DOI: 10.3390/molecules13051111
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.org/molecules
Communication
A Convenient Synthesis of Amino Acid Methyl Esters
Jiabo Li and Yaowu Sha*
The Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology, Department of
Chemistry, Tsinghua University, Beijing 100084, P. R. China; E-mail: lijb05@mails.tsinghua.edu.cn
* Author to whom correspondence should be addressed; E-mail: shayw@tsinghua.edu.cn
Received: 29 April 2008; in revised form: 5 May 2008 / Accepted: 6 May 2008 / Published: 8 May
2008
Abstract: A series of amino acid methyl ester hydrochlorides were prepared in good to
excellent yields by the room temperature reaction of amino acids with methanol in the
presence of trimethylchlorosilane. This method is not only compatible with natural amino
acids, but also with other aromatic and aliphatic amino acids.
Keywords: Amino acid methyl ester hydrochlorides; amino acids; trimethylchlorosilane;
esterification.
Introduction
Amino acid methyl esters are important intermediates in organic synthesis, which have been used
in various areas such as peptide synthesis [1], medicinal chemistry [2-3], as chiral sources [4-7] and
polymer materials [8-9].
A variety of reagents have been reported for the transformation of amino acids into amino acid
methyl esters, which include protic acids (gaseous hydrochloric acid [10], sulfuric acid and p-toluene-
sulfonic acid), thionyl chloride [11], 2,2-dimethoxypropane [12] and ion-exchange resins
(Amberlyst™-15, [13]). There are other methods which require multistep reactions to obtain the
products, such as the sequence of N-protection, esterification and deprotection. Although some of them
are widely used, they still have several disadvantages, including tedious workup procedures, safety and
waste disposal problems and harsh reaction conditions. Methanol/trimethylchlorosilane has been

Molecules 2008, 13
1112
shown to be a convenient system for the preparation of methyl esters of various carboxylic acids [14,
15]. This method has been used in the transformation of N-Boc-α-amino acids into N-unprotected α-
amino methyl esters [16] and some other amino acid methyl esters have been prepared using this
system [17-20]. In order to demonstrate the general applicability of the method, we have examined a
series of amino acids as substrates, including natural, aromatic and aliphatic amino acids and in this
communication we report that trimethylchlorosilane (TMSCl) with methanol at room temperature is an
efficient reagent for esterification of amino acids of all classes. Compared to the methods mentioned
above the use of TMSCl/MeOH was more advantageous due to the following features: easy operation,
mild reaction conditions, simple workup and good to excellent yields.
Results and Discussion
The synthesis of acid methyl ester hydrochlorides is shown in Scheme 1. A series of amino acids,
including natural amino acids, aromatic amino acids and aliphatic amino acids was transformed to
corresponding amino acid methyl ester hydrochlorides in good to excellent yields, which are
summarized in Table 1.
Scheme 1
TMSCl
R
OH
R
O
H2N
H2N
HCl
O
O
MeOH
Compared with other methods, the yields obtained with the TMSCl/MeOH system were in most
cases comparable to or even higher than those obtained with the thionyl chloride/MeOH and
HCl(SO4H2)/MeOH systems and the method is certainly more convenient from an operational point of
view. For example, for best results the temperature of the thionyl chloride/MeOH system should be
strictly maintained between –5~0 ˚C and HCl gas must be continuously passed through the refluxing
mixture in the MeOH/HCl method, making the TMSCl/MeOH system obviously more convenient.
In general, two equivalents of TMSCl were used. However, the substrates in entries 2, 12 and 18
have two carboxyl groups, so four equivalents of TMSCl were used in the esterifications of these
substrates. In general the reaction time was 12 h. Because of the poor solubility in methanol of both of
substrates and products in entries 1, 2, 13, 14, 19, the reaction time for these substrates was 24 h.
Racemization is a common problem in the synthesis of amino acid esters. According to a published
report esterification of protected amino acids by TMSCl showed little racemization [16]. It would of
course be more interesting if free amino acids could be directly esterified with little racemization and
work to determine if this is possible is currently under way in our lab and the results will be reported in
due course.

Molecules 2008, 13
Table 1. Esterification of amino acids with methanol in the presence of TMSCl.
1113
Reported
yields (%) d
Entry
Substrate
Product a
Time (hr)b Yield (%) c
OH
O
H2N
H2N
1
2
24
24
96
89
89 [21]
O
O
O
O
O
O
H
N
H
N
98 [22]
97 [23]
O
OH
O
HO
NH2
NH2
OH
O
3
12
97
O
O
NH2
O
NH2
O
O
OH
4
5
12
12
96
88
95 [23]
97 [24]
OH
O
H2N
H2N
O
O
O
O
6
7
8
OH
12
12
12
96
85
76
88 [25]
65 [26]
29 [27]
O
NH2
O
NH2
O
HS
OH
HS
O
NH2
O
NH2
O
OH
O
N
N
H
H
O
O
OH
O
9
12
91
92 [28]
NH2
NH2
N
N
H
H
NH2
NH2
OH
O
10
11
12
12
94
89
93 [29]
71 [30]
H2N
H2N
H2N
O
NH2
O
NH2
H
N
H
H2N
OH
N
O
NH
O
O
NH
O
O
NH2
O
O
NH2
OH
12
13
14
12
24
24
86
86
90
100 [31]
72 [32]
78 [33]
HO
O
O
O
O
H2N
H2N
C OH
H2N
H2N
C O
O
O
C
O
C OH
O
O
15
16
12
12
96
86
99 [35]
93 [36]
H2N
H2N
OH
OH
H2N
H2N
O
O
O
O

Molecules 2008, 13
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Table 1. Cont.
O
O
C OH
C O
17
12
12
24
94
98
97
93 [36]
H2N
H2N
O
NH2
O
NH2
S
OH
S
O
18
19
HO
S
92 [37]
62 [38]
O
S
NH2
O
NH2
O
O
O
H2N
H2N
C OH
C O
a
All products are isolated as hydrochloride salts.
Reaction times are not optimized
All yields refer to isolated products, fully characterized by 1H- and 13C-NMR and MS.
b
c
d The yields of entries 1, 2, 9, 15,16, 17 were with the thionyl chloride/MeOH system, entry 13 in
H2SO4/MeOH system, and the remainder with the HCl/MeOH system.
Conclusions
We have developed a facile method to synthesize different amino acid methyl ester hydrochlorides
through the esterification of corresponding amino acids with methanol using trimethylchlorosilane.
The reaction offers convenience, mild conditions and good to excellent yields.
Experimental
General
1H-NMR (300 MHz) and 13C-NMR (75 MHz) spectra were recorded in D2O on a JEOL JNM-
ECA300 spectrometer. All ESI-MS experiments were done on a Bruker ESQUIRE-LC.
General procedure for the preparation of amino acid methyl ester hydrochlorides
Amino acid (0.1 mol) was taken in a round bottom flask. Freshly distilled chlorotrimethylsilane (0.2
mol) was added slowly and stirred with a magnetic stirrer. Then methanol (100 mL) was added and the
resulting solution or suspension was stirred at room temperature. After the completion of reaction (as
monitored by TLC), the reaction mixture was concentrated on a rotary evaporator to give the product
amino acid ester hydrochloride.
1
13
Glycine methyl ester (entry 1). H-NMR: δ 4.03 (s, 2H), 3.92 (s, 3H); C-NMR: δ 168.8, 53.5, 40.2;
ESI-MS: calcd. for (M+H)/z: 90.1, found: (M+H)/z: 90.1.
1
13
Dimethyl iminodiacetate (entry 2). H-NMR: δ 4.06 (s, 4H), δ 3.77 (s, 6H); C-NMR: δ 167.5, 53.6,
47.2; ESI-MS: calcd. for (M+H)/z: 162.1, found: (M+H)/z: 162.3.

Molecules 2008, 13
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1
13
α -Alanine methyl ester (entry 3). H-NMR: δ 4.14 (m, 1H), 3.76 (s, 3H), 1.49 (m, 3H); C-NMR: δ
171.3, 53.7, 48.9, 15.2; ESI-MS: calcd. for (M+H)/z: 104.0, found: (M+H)/z: 104.1.
Leucine methyl ester (entry 4). 1H-NMR: δ 4.10 (t, 1H), 3.78 (s, 3H), 1.82 (m, 1H), 1.65 (m, 2H), 0.89
13
(m, 6H); C-NMR: δ 171.4, 53.6, 51.6, 38.9, 24.0, 21.6, 21.2; ESI-MS: calcd. for (M+H)/z: 146.0,
Found:(M+H)/z: 146.3.
1
6-Aminocaproic acid methyl ester (entry 5). H-NMR: δ 3.62 (s, 3H), 2.93 (t, 2H), 2.35 (t, 2H), 1.61-
13
1.55 (m, 4H), 1.33 (m, 2H); C-NMR: δ 177.4, 52.3, 39.4, 33.5, 26.5, 25.2, 23.8; ESI-MS: calcd. for
(M+H)/z: 146.1, found: (M+H)/z: 146.1.
Phenylalanine methyl ester (entry 6). 1H-NMR: δ 7.37-7.34 (m, 3H), 7.23 (d, 1H), 7.21 (d, 1H), 4.36 (t,
1H), 3.77 (s, 3H), 3.29-3.13 (m, 2H); 13C-NMR: δ 170.1, 133.8, 129.5, 128.2, 54.2, 53.7, 35.7; ESI-MS:
calcd. for (M+H)/z: 180.1, found: (M+H)/z: 180.1.
1
13
Methionine methyl ester (entry 7). H-NMR: δ 4.51 (t, 1H), 3.79 (s, 3H), 3.30 (d, 2H); C-NMR: δ
169.2, 54.0, 51.6, 35.7; ESI-MS: calcd. for (M+H)/z: 139.0, found: (M+H)/z: 139.2.
Proline methyl ester (entry 8). 1H-NMR: δ 4.39 (m, 1H), 3.76 (s, 3H), 2.35 (m, 2H), 2.03 (m, 2H), 1.98
13
(m, 2H); C-NMR: δ 170.5, 59.7, 53.9, 46.4, 28.4, 23.4; ESI-MS: calcd. for (M+H)/z: 130.0, found:
(M+H)/z: 130.1.
1
Tryptophan methyl ester (entry 9). H-NMR: δ 7.42 (d, 1H), 7.39 (d, 1H), 7.16 (d, 1H), 7.14 (d, 1H),
13
7.04 (t, 1H), 4.24 (t, 1H), 3.66 (s, 3H), 3.25 (m, 2H); C-NMR: δ 170.4, 136.4, 126.5, 125.4, 122.3,
119.6, 118.1, 112.1, 106.0, 53.7, 53.4, 25.7; ESI-MS: calcd. for (M+H)/z: 219.1, found: (M+H)/z:
219.2.
1
Lysine methyl ester (entry 10). H-NMR: δ 4.12 (t, 1H), 3.79 (s, 3H), 2.97 (t, 2H), 1.94 (m, 2H), 1.67
(m, 2H), 1.46 (m, 2H); 13C-NMR: δ 170.7, 53.7, 52.9, 39.2, 29.4, 26.4, 21.6; ESI-MS: calcd. for
(M+H)/z: 161.1, found: (M+H)/z: 161.0.
1
Arginine methyl ester (entry 11). H-NMR: δ 4.14 (t, 1H), 3.78 (s, 3H), 3.20 (m, 2H), 1.95 (m, 2H),
13
1.67 (m, 2H); C NMR: δ 170.5, 156.9, 53.8, 52.6, 40.4, 27.0, 23.9; ESI-MS: calcd. for (M+H)/z:
189.1, found: (M+H)/z: 189.1.

Molecules 2008, 13
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Aspartic acid dimethyl ester (entry 12). 1H-NMR: δ 4.44 (dd, 1H), 3.77 (s, 3H), 3.68 (s, 3H), 3.10 (dd,
2H); 13C-NMR: δ 171.7, 169.4, 54.0, 53.1, 49.3, 33.7; ESI-MS: calcd. for (M+H)/z: 162.0, found:
(M+H)/z: 162.1.
Methyl 4-aminobenzoate (entry 13). 1H-NMR: δ 7.86 (d, 1H), 7.83 (d, 1H), 7.32 (d, 1H), 7.29 (d, 1H),
3.72(s, 3H); 13C-NMR: δ 167.9, 135.2, 131.2, 129.6, 123.0, 52.8; ESI-MS: calcd. for (M+H)/z: 152.1,
found: (M+H)/z: 152.2.
1
13
Methyl 3-aminobenzoate (entry 14). H-NMR: δ 7.90-7.83 (m, 2H), 7.51 (m, 2H), 3.77 (s, 3H); C-
NMR: δ 167.6, 131.4, 130.6, 130.2, 130.1, 128.0, 123.9, 53.0; ESI-MS: calcd. for (M+H)/z: 152.0,
found: (M+H)/z: 152.1.
1
13
β-Alanine methyl ester (entry 15). H-NMR: δ 3.68 (s, 3H), 3.22 (t, 2H), 2.77 (t, 2H); C-NMR: δ
173.2, 52.7, 35.2, 31.2; ESI-MS: calcd. for (M+H)/z: 104.0, found: (M+H)/z: 104.1.
γ-Aminobutyric methyl ester (entry 16). 1H-NMR: δ 3.63 (s, 3H), 2.97 (t, 3H), 2.45(m, 2H), 1.89 (t,
13
2H); C-NMR: δ 175.7, 52.4, 38.8, 30.6, 22.1; ESI-MS: calcd. for (M+H)/z: 118.2, found: (M+H)/z:
118.3.
1
Methyl 4-(aminomethyl)cyclohexanecarboxylate (entry 17). H-NMR: δ 3.59 (s, 3H), 2.79 (m, 2H),
2.29 (m, 1H), 1.94-1.90 (m, 2H), 1.78-1.75 (m, 2H), 1.57 (m, 1H), 1.38-1.26 (m, 2H), 1.04-0.92 (m,
2H); 13C-NMR: δ 179.4, 52.3, 44.9, 42.6, 34.8, 28.5, 27.7; ESI-MS: calcd. for (M+H)/z: 172.1, found:
(M+H)/z: 172.2.
Cystine dimethyl ester (entry 18). 1H-NMR: δ 4.50 (t, 1H), 3.79 (s, 3H), 3.37-3.25(m, 2H); 13C-NMR:
δ 169.2, 54.0, 51.6, 35.7; ESI-MS: calcd. for (M+H)/z: 269.0, found: (M+H)/z: 269.0.
1
Methyl 4-(aminomethyl)benzoate (entry 19). H-NMR: δ 7.96 (d, 2H), 7.64 (d, 2H), 4.08(s, 2H), 3.84
(s, 3H); 13C-NMR: δ 166.0, 147.8, 130.0, 128.3, 126.6, 51.5, 45.5; ESI-MS: calcd. for (M+H)/z: 166.0,
found: (M+H)/z: 166.1.
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