Concurrent esterification and N-acetylation of amino acids with orthoesters: A useful reaction with interesting mechanistic implications

Article abstract of DOI:10.1016/j.tetlet.2010.10.081

The concurrent esterification and N-acetylation of amino acids has been studied with triethyl orthoacetate (TEOA) and triethyl orthoformate (TEOF). In a surprising finding, only 1 equiv of TEOA in refluxing toluene was necessary to convert l-proline and l-phenylalanine into the corresponding N-acetyl ethyl esters in good yield. The same transformation using TEOF was not effective. Stereochemical outcome and stoichiometric studies as well as structural variation of the amino acids in this reaction provided unexpected mechanistic insight.

Full text of DOI:10.1016/j.tetlet.2010.10.081

Tetrahedron Letters 51 (2010) 6737–6740
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Concurrent esterification and N-acetylation of amino acids with orthoesters: a
useful reaction with interesting mechanistic implications
⇑
Sarah Gibson, Dickie Romero, Hollie K. Jacobs, Aravamudan S. Gopalan
Department of Chemistry and Biochemistry, New Mexico State University Las Cruces, NM 88003-8001, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The concurrent esterification and N-acetylation of amino acids has been studied with triethyl orthoace-
tate (TEOA) and triethyl orthoformate (TEOF). In a surprising finding, only 1 equiv of TEOA in refluxing
toluene was necessary to convert L-proline and L-phenylalanine into the corresponding N-acetyl ethyl
esters in good yield. The same transformation using TEOF was not effective. Stereochemical outcome
and stoichiometric studies as well as structural variation of the amino acids in this reaction provided
unexpected mechanistic insight.
Received 10 August 2010
Revised 11 October 2010
Accepted 13 October 2010
Available online 21 October 2010
Keywords:
Ó 2010 Elsevier Ltd. All rights reserved.
Orthoester
Amino acid
Esterification
Acetylation
Mechanism
Orthoesters have been widely used in organic synthesis for a
variety of reactions.1 However, the difference in reactivity between
the most commonly used orthoesters, triethyl orthoacetate (TEOA)
and triethyl orthoformate (TEOF), has not been clearly appreciated.
A while ago, we reported that TEOA and trimethyl orthoacetate
(TMOA) were more effective and superior in the esterification of
carboxylic acids and sulfonic acids under mild reaction conditions
than TEOF.2 Recently, esterification of carboxylic acids using TMOA
or TEOA under solvent-free conditions in a microwave to yield the
corresponding methyl or ethyl esters has been reported.3 The use
of TEOA or TMOA for the esterification of carboxylic, phosphinic,
and phosphonic acids in ionic liquids (80 °C, 100 min) has also
been reported.4 Orthoesters have been used in the esterification
of N-acylamino acids.5 In the microwave-assisted esterification of
various N-protected amino acids, TEOA was found to better effect
the esterification than TEOF.6
on the acetylation of DL-leucine ethyl ester with TEOA to give the
N-acetyl derivative.9
Little is known on the reactivity of TEOF and TEOA with free
amino acids. In one isolated report, it was disclosed that proline
upon reflux in dimethyl acetamide with TEOA (1.5 equiv) for
15 min gave N-acetyl proline ethyl ester in 80–85% yield.10 Unfor-
tunately, this study was limited to one example, which prevents
understanding the usefulness and limitations of this type of trans-
formation. In this Letter, we would like to report the results of our
study on the reaction of amino acids using TEOA and TEOF. This
study not only discloses some interesting mechanistic implications
of this reaction but also clearly delineates the synthetic utility and
differences in reactivity between TEOA and TEOF.
We began our investigation by trying to study and understand
the differential reactivity, if any, of TEOA and TEOF on the acetyla-
tion/formylation of amino acid esters (Scheme 1). L-Proline methyl
The reaction of amines with orthoesters is quite complicated
and is dependent on a number of variables including the nature
of the amine and the orthoester, stoichiometry of the reagents,
presence of an acid catalyst, and the reaction conditions, leading
to the formation of a variety of nitrogen-containing products
including amides, imidates, amidines, and orthoamines.7 Formyla-
tion of several amino acid ester hydrochlorides using TEOF
(3 equiv) at reflux (145 °C) for 1 h in the absence of solvent without
racemization has been reported.8 There has also been a brief report
ester hydrochloride was reacted with 1 equiv of TEOA or TEOF in
toluene at room temperature for 24 h. In the case of TEOF, the reac-
tion did not proceed but led to the isolation of unreacted starting
material. None of the N-formyl product was isolated. To our sur-
prise, the corresponding reaction with TEOA was successful even
under such moderate reaction conditions. The desired N-acetyl
derivative was isolated in 73% yield. When the same reactions
were carried out using refluxing toluene for 24 h, both formylation
and acetylation could be accomplished in good yields. It is impor-
tant to mention that our attempts to acetylate underivatized pro-
line at room temperature using TEOA were not successful. This
lack of reactivity may be due to the absence of a strong acid cata-
lyst in the reaction media.
⇑
Corresponding author. Tel.: +1 575 646 2589; fax: +1 575 646 2649.
E-mail address: agopalan@nmsu.edu (A.S. Gopalan).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.10.081

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S. Gibson et al. / Tetrahedron Letters 51 (2010) 6737–6740
Table 1
a, b, c, d
Reactions of amino acids with TEOA
CO2Me
CO2Me
N
N
Entry Amino acid
Orthoester Product
Yield
(%)
.
H HCl
R
O
a) 1 eq. TEOA, toluene, rt, 24 h, 73%
79a
84b
CO2Et
CO2H
b) 1 eq. TEOA, toluene, 110°C, 24 h. 89%
c) 1 eq. TEOF, toluene, rt, 24 h, no reaction
d) 1 eq. TEOF, toluene, 110°C, 24 h, 81%
1
TEOA
N
N
H
Ac
CO2Et
CO2Et
CO2Me
CO2Et
Ph
Ph
2
TMOA
84b
L
-Pro
N
a, b, c, d
.
HN
NH2 HCl
R
Ac
CO2H
NH2
O
Ph
Ph
3
4
TEOA
87a
83a
a) 26% b) 100% c) no reaction d) 69%
NHAc
CO2Me
NHAc
Scheme 1. Reaction of amino acid esters with TEOA and TEOF.
Ph
TMOA
TEOA
L
-Phe
In the case of L-phenyalanine ethyl ester hydrochloride, acetyla-
tion with TEOA was observed at room temperature; however, the
conversion was low and the desired product was isolated along
with unreacted starting material (Scheme 1). In contrast, the use
of TEOF under identical conditions gave no desired formylation.
The same reactions when carried out in refluxing toluene gave
the desired products in good yields, with the acetylation with
TEOA being quantitative under these conditions. As in the case of
59a
61c
CO2H
DL
CO2Et
NHAc
5
NH2
CO2H
DL
CO2Et
6
7
8
TEOA
TEOA
TEOA
85a
NH
N
Ac
L
-proline, no acetylation was observed when unprotected L-phenyl-
7a
41b
94c
DL
alanine was treated with TEOA at room temperature. Our studies
clearly establish that acetylation of amino acid esters with TEOA
occurs more readily than the corresponding formylation with
TEOF. Further, it is important to note that the selective acetylation
or formylation of the unprotected amino acids could not be accom-
plished with either of these reagents.
HN
HN
N
CO2H
CO2H
CO2Et
CO2Et
Ac
Ac
14a
91d
N
Given the greater reactivity of the amino group over the carbox-
ylic acid, one would predict that when amino acids are treated
with TEOA, initial reaction would occur at the amine site. The ques-
tion was whether amino acids such as proline could be converted
into the corresponding N-acetyl ester derivatives stepwise, via
the intermediacy of the N-acetyl intermediate. Our study of this
transformation resulted in some surprising findings.
CO2H
CO2Et
0a
9
TEOA
94b
NH2
N
OEt
When L-proline was heated with TEOA (1 equiv) in refluxing tol-
a
b
c
1–1.2 equiv TEOA, toluene, 110 °C, 24 h.
2 equiv TEOA, toluene, 110 °C, 24 h.
5 equiv TEOA, toluene, 110 °C, 24 h.
5 equiv TEOA, 125 °C, 24 h.
uene for 24 h, to our surprise the major product that was isolated
in good yields was the N-acetyl ethyl ester (Table 1). Aqueous
work-up followed by distillation under reduced pressure, gave N-
acetyl proline ethyl ester as the only product in 79% yield. Later,
it was found that the product could be obtained in high yield
and purity by removal of the reaction solvent in vacuo followed
by filtration of the crude product through a short silica gel column
using chloroform.11 The corresponding reaction with TEOF even
when carried out with 5 equiv of the reagent gave a complex mix-
ture of products which were not easily separated. The same reac-
d
conducted (Figure 1). The reactions of proline and phenylalanine
with varying equivalents of TEOA were conducted in refluxing tol-
uene for 24 h. In the case of proline, good yields of product could be
obtained with 1 equiv of TEOA. However, the yield was slightly
tion was then carried out with
TEOA (1 equiv) in refluxing toluene, the N-acetyl ethyl ester of
phenylalanine was isolated in 87% yield. As in the case of -proline,
the reaction of -phenylalanine with TEOF did not give the corre-
sponding product. It is noteworthy that the TEOA reaction with
L-phenylalanine. Once again, using
100
90
80
70
60
50
40
30
20
10
0
L
L
L
-proline gave the N-acetyl ethyl ester of proline without racemiza-
tion ([
a
]
D
À95° (c1.0, EtOH), lit.10
[
a]
À90° (c1, EtOH). However,
D
Proline
the product obtained from the reaction of L-phenylalanine with
TEOA exhibited no optical rotation, indicating that it was racemic!
The observed stoichiometry and stereochemical outcome of the
TEOA reaction with proline and phenylalanine was surprising. We
had anticipated that an excess of TEOA would be needed for the
simultaneous acetylation and esterification reaction. In order to
gain some mechanistic insight into the reaction of TEOA with
amino acids, a study of the stoichiometry of these reactions was
Phenylalanine
0
0.5
1
1.5
2
2.5
3
Equivalents of TEOA
Figure 1. Reaction stoichiometry of phenylalanine and proline with TEOA.

S. Gibson et al. / Tetrahedron Letters 51 (2010) 6737–6740
6739
O
O
H
H
O
R
H
R
EtO
EtO
EtO
O
-2 EtOH
R
OH
+
CH3
OH
HN
N
OEt
OEt
NH2
O
EtOH
O
O
OH
H
H
H
tautomerization
racemization
R
R
R
R
O
O
O
O
N
I
N
HN
N
OEt
I
EtOH
O
H
O
R
H
OEt
O
II
HN
N+
O
Scheme 2. Proposed mechanism for the reaction of TEOA with amino acids.
higher when the reaction was conducted with 2 equiv of TEOA or
more. A different trend was observed for the reaction of phenylal-
anine with TEOA. Optimal yields were observed with approxi-
mately 1 equiv of TEOA. A decrease in yield was observed when
an excess of TEOA was used indicating that the product was under-
going further reaction. These studies clearly show that only 1 equiv
of TEOA is required for both esterification and acetylation of pro-
line or phenylalanine.
the desired products could be obtained using 5 equiv of TEOA in
refluxing toluene or neat TEOA at high temperatures. These condi-
tions are more similar to those required for esterification of simple
carboxylic acids with TEOA.2 It is interesting to point out that in
the case of 3-aminobenzoic acid, the product isolated on reaction
with TEOA (2 equiv) was the imidate ethyl ester, and not the N-acet-
yl ester. This supports our mechanistic hypothesis that an imidate
ester is involved in this process.
To rationalize the results of our studies of the reactions of TEOA
with L-proline and L-phenylalanine, we propose the following
In conclusion, we have shown that TEOA is an effective reagent
for the concurrent and one pot N-acetylation and esterification of
amino acids under neutral conditions. TEOF exhibits lower reactiv-
ity and performs poorly in this type of transformation. In the case
of a-amino acids, 1 equiv of TEOA in refluxing toluene is sufficient
to give the corresponding N-acetyl ethyl esters in good yields. Stoi-
mechanism (Scheme 2). TEOA first reacts with the more reactive
amino group to give an imidate ester, which undergoes cyclization
to give the oxazolidinone I. Subsequent ring opening of I with eth-
anol formed in the first step results in formation of the N-acetyl es-
ter product. In the case of primary amino acids like phenylalanine,
the intermediate oxazolidinone undergoes rapid racemization
through tautomerization to the enol form. Racemization of amino
acids through their oxazolidinone is known.12 For proline, the reac-
tion with TEOA proceeds with no loss of enantiomeric purity. Pre-
sumably, the bicyclic oxazolidinone II derived from proline
undergoes ring opening with ethanol much faster than any racemi-
zation process, which could be disfavored due to energy con-
straints offered by the bicyclic ring of the intermediate. In any
case, only 1 equiv of TEOA is necessary in this reaction to achieve
both N-acetylation and esterification.
chiometric studies indicate that the mechanistic pathway in the
reaction of
a-amino acids involves the intermediacy of an oxazo-
lidinone. This is not so where geometric constraints disallow the
formation of the oxazolidinone intermediate. In these cases, the
desired N-acetyl esters can be prepared in good yields by using a
larger excess of TEOA. It is important to emphasize that the choice
of TEOA or TEOF is critical and may govern the success of the de-
sired transformation. It is clear that further studies are required
to delineate the differences in reactivity between TEOA and TEOF
with other substrates and transformations. Also, the use of other
orthoesters may allow variation of the acyl or ester groups that
are incorporated.
In general,
a-amino acids (Table 1, entries 1–6) react with
1 equiv of TEOA to give good yields of the corresponding N-acetyl
esters. The corresponding methyl esters can be obtained using
TMOA.13
Acknowledgments
If our proposed mechanism is valid, one would predict that the
reaction of b-amino acids and other amino acids with TEOA could
follow a different pathway that would involve independent acetyla-
tion and esterification events. If the formation of oxazolidinone I is
not feasible, the reaction should require a minimum of 2 equiv of
TEOA for completion under comparable reaction conditions. In order
to test our hypothesis, the reaction of TEOA with racemic 2-, 3-, and
4-piperidinecarboxylic acidwas examined(Table 1). Only in the case
of piperidine 2-carboxylic acid formation of the five-membered
oxazolidinone intermediate I is feasible. The reaction of piperidine
2-carboxylic acid with 1 equiv of TEOA gave 85% of the desired
N-acetyl ethyl ester. In contrast, 3- and 4-piperidinecarboxylic acids
gave very little of the desired products upon treatment with 1 equiv
TEOA under similar reaction conditions. In both cases, good yields of
This research was supported by grants from the National Insti-
tutes of Health 1SC3GM084809-01 and GMO7667-34 (fellowship
to S.G. and D.R.). The NSF GRFP is also thanked for a fellowship
to S.G.
References and notes
1. (a) Taylor, R. T. Triethyl orthoformate In Encyclopedia of Reagents for Organic
Synthesis; Paquette, L. A., Ed.; John Wiley & Sons: New York, 1995; Vol. 7, pp
5102–5105; (b) Ziegler, F. E.; Belema, M.; Harran, P. G.; Kover, R. X. Triethyl
Orthoacetate In Encyclopedia of Reagents for Organic Synthesis; Paquette, L. A.,
Ed.; John Wiley & Sons: New York, 1995; Vol. 7, pp 5099–5102; (c) Pavlova, L.
A.; Davidovich, Y. A.; Rogozhin, S. V. Russ. Chem. Rev. 1986, 55, 1026–1041.
2. Trujillo, J.; Gopalan, A. S. Tetrahedron Lett. 1993, 34, 7355–7358.
3. Lee, J. C.; Song, I.; Park, J. Y. Synth. Commun. 2002, 32, 2209–2213.
4. Yoshino, T.; Imori, S.; Togo, H. Tetrahedron 2006, 62, 1309–1317.

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S. Gibson et al. / Tetrahedron Letters 51 (2010) 6737–6740
5. Rogozhin, S. V.; Davidovich, Y. A.; Korshak, V. V. Izv. Akad. Nauk. SSSR, Ser. Khim.
1969, 12, 2858.
6. Zhang, S.; Arvidsson, P. I. Int. J. Pept. Res. Ther. 2008, 14, 219–222.
7. (a) Swaringen, R. A., Jr.; Eaddy, J. F.; Henderson, T. R. J. Org. Chem. 1980, 45,
3986; (b) Taylor, E. C.; Ehrhart, W. A. J. Org. Chem. 1963, 28, 1108; (c) Roberts, R.
M.; Vogt, P. J. J. Am. Chem. Soc. 1956, 78, 4778.
8. Chancellor, T.; Morton, C. Synthesis 1994, 1023.
9. Rogozhin, S. V.; Davidovich, Y. A.; Korshak, V. V. Izv. Akad. Nauk. SSSR, Ser. Khim.
1970, 3, 727.
10. Korshak, V. V.; Rogozhin, S. V.; Davidovich, Y. A. Izv. Akad. Nauk SSSR, Ser. Khim.
1969, 977.
11. Synthesis of N-acetyl proline ethyl ester: A mixture of L-proline (0.2 g, 1.7 mmol)
and triethyl orthoacetate (0.6 mL, 3.5 mmol) in toluene (1 mL) was refluxed
under nitrogen for 24 h. After cooling to room temperature, the toluene was
removed under reduced pressure. The residue was dissolved in CHCl3 and
filtered through a PrepSep solid phase extraction tube (Fisher Scientific, silica
gel, 1.0 g). The solvent was removed in vacuo to afford the product (0.265 g,
84%).
12. Wang, X.-J.; Yang, Q.; Liu, F.; You, Q.-D. Synth. Commun. 2008, 38, 1028.
13. Spectroscopic analyses of all compounds are in agreement with the assigned
structures.