A remarkably simple protocol for the N -formylation of amino acid esters and primary amines

Article abstract of DOI:10.1021/ol201475j

A simple, convenient, and wide scope protocol for the N-formylation of amino acid esters and primary amines has been developed utilizing only imidazole in warm DMF.

Full text of DOI:10.1021/ol201475j

ORGANIC
LETTERS
2011
Vol. 13, No. 15
3952–3955
A Remarkably Simple Protocol for the
N-Formylation of Amino Acid Esters and
Primary Amines
ꢀ
Mojmır Suchy, Adam A. H. Elmehriki, and Robert H. E. Hudson*
´
Department of Chemistry, The University of Western Ontario, London,
Ontario N6A 5B7, Canada
rhhudson@uwo.ca
Received May 31, 2011
ABSTRACT
A simple, convenient, and wide scope protocol for the N-formylation of amino acid esters and primary amines has been developed utilizing only
imidazole in warm DMF.
The protection of reactive amino groups is very com-
monly required in organic synthesis. The formyl (CHO)
group may be used for this purpose, and numerous
methods are available for the formylation of amines and
amino acids.1 N-Formylated amino acid esters have been
utilized for peptide synthesis2 and may also serve as
precursors of isocyanides, which find use in multicompo-
nent reactions to furnish a wide variety of products.3
Many of the currently available methods for the N-
formylation of amino acid derivatives require harsh reaction
conditions resulting in tedious purification1f,k (e.g., triethy-
lorthoformate at 140 °C followed by vacuum distillation) and
use of toxic1g (e.g., cyanomethylformate) or somewhat un-
stable reagents (e.g., pentafluorophenyl formate).1c,e In addi-
tion, some reagents require the reaction to be performed
under an inert atmosphere using rigorously dried solvents.1d,h
Herein, we report an alternative method for the
N-formylation of amino acid esters based on treatment
with imidazole in warm (60 °C) DMF. The reagents used
are inexpensive and neither inert atmosphere nor dry
solvent are required for the reaction to proceed. Moreover,
the reaction work up and purification of the products is
straightforward and provides the corresponding N-formy-
lated amino acid esters in good yields (47ꢀ79%). We also
present evidencethat DMF is the formyl donor and that an
acyl transfer agent, preferably imidazole, is required rather
than just an organic base. This method does not require
additional reagents, such as chlorotrimethylsilane, to acti-
vate the DMF.1d
(1) Forthe review on acylating agents, see: (a) Olah, G. A.; Ohannesian,
L.; Arvanaghi, M. Chem. Rev. 1987, 87, 671. For selected examples of
N-formylating agents, see:(b) Martinez, J.; Laur, J. Synthesis 1982, 979. (c)
€
€
Kisfaludy, L.; Otvos, L., Jr. Synthesis 1987, 510. (d) Berry, M. B.; Blagg, J.;
Craig, D.; Willis, M. C. Synlett 1992, 659. (e) Akikusa, N.; Mitsui, K.;
Sakamoto, T.; Kikugawa, Y. Synthesis 1992, 1058. (f) Chancellor, T.;
Morton, C. Synthesis 1994, 1023. (g) Duczek, W.; Deutsch, J.; Vieth, S.;
ꢀ
Niclas, H. J. Synthesis 1996, 37. (h) Giard, T.; Benard, D.; Plaquevent, J. C.
Synthesis 1998, 297. (i) Meinnel, T.; Patiny, L.; Ragusa, S.; Blanquet, S.
Biochemistry 1999, 38, 4287. (j) Yang, D.; Jeon, H. B. Bull. Korean Chem.
Soc. 2010, 31, 1424. (k) Lei, M.; Ma, L.; Hu, L. Tetrahedron Lett. 2010, 51,
4186.
(2) For selected examples see: (a) Sheehan, J. C.; Yang, D. D. H.
J. Am. Chem. Soc. 1958, 80, 1154. (b) Boulay, F.; Tardiff, M.; Brouchon,
L.; Vignais, P. Biochem. Biophy. Res. Commun. 1990, 168, 1103.
(c) Stephenson, K. A.; Zubieta, J.; Banerjee, S. R.; Levadala, M. K.;
Taggart, L.; Ryan, L.; McFarlane, N.; Boreham, D. R.; Maresca, K. P.;
Babich, J. W.; Valliant, J. F. Bioconjugate Chem. 2004, 15, 128.
(3) For the recent review on Ugi multicomponent reaction, see: (a)
During the course of other studies in our laboratory, we
wished to prepare a 1-Me-histidine (1-Me-His-OH). The
synthesis of 1-Me-His-OH (in 3 steps from 1a) has been
described by Jain and Cohen,4a and involves the reaction
of His-OMe 2HCl (1a) with carbonyldiimidazole (CDI) in
DMF as an initial step. A cyclic urea 3 (Scheme 1) is
3
€
Domling, A. Chem. Rev. 2006, 106, 17. For selected examples, see:(b)
Urban, R.; Marquarding, D.; Seidel, P.; Ugi, I.; Weinelt, A. Chem. Ber.
1976, 110, 2012. (c) Mayer, J.; Umkehrer, M.; Kalinski, C.; Ross, G.;
Kolb, J.; Burdack, C.; Hiller, W. Tetrahedron Lett. 2005, 46, 7393.
(4) (a) Jain, R.; Cohen, L. A. Tetrahedron 1996, 52, 5363. (b) Guillen,
F.; Bregeon, D.; Plaquevent, J. C. Tetrahedron Lett. 2006, 47, 1245.
ꢀ
r
10.1021/ol201475j
Published on Web 06/28/2011
2011 American Chemical Society

formed during this step, which is subsequently trans-
formed to 1-Me-His-OH.4a Initial attempts to repeat the
synthesis of 3 failed. As indicated in their work,4a crude
product 3 was expected to crystallize after a standard
extractive workup. In our hands, no crystallization oc-
curred despite many attempts and the product was ulti-
mately obtained by flash column chromatography (FCC)
(see Supporting Information). Analysis of the compound
treated with imidazole in warm (60 °C) DMF, N-formyl
Phe-OEt (2b, Scheme 2) was isolated in 79% yield. Un-
fortunately treatment of phenylalanine (Phe-OH) under
the same conditions only afforded an intractable mixture
of products, suggesting, that amino acids need to be
protected as esters in order to achieve the N-formylation.
The protecting groups present in amino acid esters 1c, 1g,
1h and 1l are retained in the corresponding N-formyl
derivatives 2c, 2g, 2h and 2l. The corresponding N-formyl
derivatives 2bꢀ2n were obtained in moderate to good
yields (47ꢀ79%, Table 1) and were characterized by
1H NMR, 13C NMR and HRMS (see Table 1 and SI).6
The reactions were performed at 1 mmol scale (2 eq of
1
by H NMR revealed the presence of additional signals
(δ 11.87, s, D2O exch., 1H) and (δ 8.01, s, 1H), which were
not consistent with the structure of cyclic urea 3. The signal
at 8.01 ppm suggested the presence of a formyl group; this
hypothesis was further supported by the presence of the
signalatδ 161.2 ppm in 13C NMR spectrum (DMSOꢀD6).
The unknown product was assigned the structure of
N-formyl His-OMe (2a, Scheme 1);5 a proposal consistent
with the high resolution mass spectral data (HRMS)
obtained.
imidazole, 60 °C, 48 h). The N-formylation of Phe-OEt
3
HCl (1b) was also performed at 10 mmol scale (see SI for
details), providing 2b in 55% yield indicating scalability of
the method. Importantly, no racemization was observed
during the reaction as indicated by the comparison of [R]D
values for compounds 2b, 2d, 2f and 2i with those available
in the literature (Table 1).
Scheme 1. Reaction of 1a with CDI and Imidazole/DMF
Scheme 2. N-Formylation of Amino Acids 1aꢀ1n and Forma-
tion of 4
The unexpected formation of N-formyl His-OMe (2a)
prompted us to investigate the quality of reagents used in
the reaction which revealed that the CDI had completely
hydrolyzed to imidazole (1H NMR inspection) during
prolonged storage. When 1a was treated with “genuine”
imidazole, N-formyl His-OMe (2a) was isolated as a sole
product in 51% yield. It is worth noting, that successful
transformation of 1a to the cyclic urea 3 was achieved
using fresh CDI and the procedure described by Guillen
et al.4b which afforded better yields than Jain and Cohen
procedure.4a
The observed formation of 2a suggested that DMF in
the presence of imidazole, without any additional reagents,
acts as an acylating agent for the synthesis of N-formyl
amino acid derivatives. To investigate the scope of this
reaction, a variety of amino acid esters were screened. The
results of these studies are depicted in Scheme 2 and
One noticeable exception was found when Glu(OMe)-
OMe (1o) was subjected to the reaction conditions; no
product of N-formylation was obtained. Instead, the par-
tially racemized lactam 4 (Table 1) was isolated in 30%
(6) Due to the restricted rotation, the presence of two rotamers was
consistently observed in the NMR spectra acquired in CDCl3. Although
the amount of minor rotamer is small (<5%) in most of the cases, it is
more prevalent in the case of N-formyl Ile-OMe (2i, ca. 10%) and N-formyl
Pro-OMe (2n, ca. 40%).
(7) Amere, M.; Lasne, M. C.; Rouden, J. Org. Lett. 2007, 9, 2621.
(8) Only the optical rotation of unnatural R enantiomer of 2d has
been determined, see: Chu, K. S.; Negrete, G. R.; Konopelski, J. P.
J. Org. Chem. 1991, 56, 5196.
(9) (a) Andersen, T. P.; Ghattas, A. B. A. G.; Lawesson, S. O.
Tetrahedron 1983, 39, 3419. (b) Anderson, W. K.; Bhattacharjee, D.;
Houston, D. M. J. Med. Chem. 1989, 32, 119.
summarized in Table 1. When Phe-OEt HCl (1b) was
3
(10) Note that the values for the optical rotation reported for
N-formyl Ile-OMe (2i) in Aitali, M.; Allaoud, S.; Karim, A.; Mortreux,
A. Tetrahedron: Asymmetry 2000, 11, 1367 and in ref 1h have opposite
signs (Table 1). Our value (Table 1) indicates that 2i is dextrorotatory.
(11) (a) Cappon, J. J.; van der Walle, G. A. M.; Verdegem, P. J. E.;
Raap, J.; Lugtenburg, J. Rec. Trav. Chim. Pays-Bas 1992, 111, 517. (b)
Hinderaker, M. P.; Raines, R. T. Protein Sci. 2003, 12, 1188.
(5) We were unable to compare the spectral data obtained for 2a with
the literature. There is only one reference describing the use of 2a for the
preparation of polymeric polyoxyethylene catalysts, however neither the
procedure for the preparation nor the spectral data are provided, see:
Topicheva, I. N.; Solovieva, A. B.; Kabanov, V. A. Dokl. Akad. Nauk
SSSR 1971, 199, 1084.
Org. Lett., Vol. 13, No. 15, 2011
3953

Table 1. N-Formylation of Amino Acid Esters 1aꢀ1o, Selected Data
R1 (amino acid)
R2
yield (%)
[R]D
δ CHO 1H/13C (ppm)a
1af2a
1bf2b
His
Me
Et
51
79
þ20.1 (c 1, MeOH)
þ72.7 (c 0.55, CH2Cl2)
þ81.7 (c 1.78, CHCl3)1g
þ76.0 (c 0.55, CHCl3)7
þ57.5 (c 1, CHCl3)
þ52.1 (c 1.1, CHCl3)
ꢀ51.8 (c 1.1, CHCl3)8
optically inactive
8.01/161.2 (DMSOꢀD6)
Phe
8.17/160.4 (CDCl3), see ref 7
1cf2c
1df2d
Tyr(t-Bu)
Me
Me
73
48
8.14/160.4 (CDCl3)
Trp
8.12/160.7 (CDCl3), see refs 1k, 8
1ef2e
1ff2f
Gly
Ala
Et
55
53
8.24/160.9 (CDCl3), see ref 9
8.17/160.3 (CDCl3), see ref 1h
Me
ꢀ61.8 (c 0.6, EtOH)
ꢀ34.7 (c 0.6, EtOH)1g
ꢀ34.6 (c 0.6, EtOH)1h
þ13.4 (c 1, CHCl3)
þ6.1 (c 1, CHCl3)
1 gf2 g
1 hf2 h
1if2i
Ser(t-Bu)
Cys(Bn)
Ile
Me
Me
Me
67
47
62
8.22/160.7 (CDCl3)
8.10/160.5 (CDCl3)
8.26ma 7.99mi/163.6mi160.6ma (CDCl3),see refs 1h, 10
þ27.6 (c 1, CHCl3)
þ32.5 (c 1, CHCl3)10
ꢀ32.3 (c 1, CHCl3)1h
þ38.8 (c 1, CHCl3)
þ12.3 (c 1, CHCl3)
þ12.6 (c 1, CHCl3)
ꢀ1.2 (c 0.4, MeOH)
ꢀ89.5 (c 1, CHCl3)
þ3.6 (c 1, EtOH)
1jf2j
Met
Me
Me
Me
Me
Me
Me
59
50
69
56
76
30
8.21/160.7 (CDCl3)see ref 1i
8.18/160.7 (CDCl3)
1kf2k
1 Lf2 L
1 mf2 m
1nf2n
1of4
Asp(OMe)
Lys(Boc)
Arg(NO2)
Pro
8.19/160.8 (CDCl3)
8.06/161.1 (DMSOꢀD6)
8.26ma 8.23mi/161.5mi160.7ma(CDCl3), see ref 11
no CHO group in 4
Glu(OMe)
þ10.5 (c 1, EtOH)
a ma, major rotamer; mi, minor rotamer.
yield after FCC.12 No trace of N-formyl Glu(OMe)-OMe
was detected (1H NMR inspection).
Scheme 3. N-Formylation of Amines 5aꢀ5c
With a general strategy for the N-formylation of amino
acid esters established, we were interested in applying it to
the N-formylation of amines (Scheme 3). Benzylamine (5a),
aniline (5b) and N,N-dicyclohexylamine (5c) were selected
as examples of primary aliphatic, primary aromatic and
secondaryamines. Treatmentof5aꢀcunder the previously
established reaction conditions showed the reactions to be
rather sluggish. Reaction of benzylamine (5a) resulted in
an unacceptable yield (ca. 20%) of N-formyl benzylamine
(6a) while a complete recovery of starting materials was
observed for aniline (5b) and N,N-dicyclohexylamine (5c).
When the temperature was increased to 120 °C, N-formyl
benzylamine (6a) was isolated in excellent yield (98%,
Scheme 3).13 On the other hand, massive decomposition
was observed when aniline (5b) and N,N-dicyclohexyla-
mine (5c) were treated with imidazole in DMF at 140 °C.
Only a trace amount (ca. 1%) of N-formylated product
6b13 was isolated from the reaction mixture (Scheme 3)
after extractive workup and FCC, while no N-formyl-N,
N-dicyclohexylamine (6c, Scheme 3) was isolated. The lack
of reactivity is likely caused by decreased nucleophilicity
of aromatic amine 5b and steric hindrance of the secondary
amine 5c. It appears that the presented synthetic methodology
can only be successfully extended to the N-formylation of
primary amines (see SI for details).
Several experiments with Phe-OEt HCl (1b) were car-
3
ried out in order to understand the mechanism of the
reaction. First, the reaction was carried out in the absence
of imidazole. The reaction did not proceed at all and no
N-formyl Phe-OEt (2b) was detected (1H NMR inspection).
Replacement of imidazole with other organic bases
(Et2NH, Et3N or piperidine) also resulted in a complete
recovery of starting material, with no trace of 2b detected.
We therefore deduced that the presence of imidazole is
required for the reaction to proceed.
It was also found, that 2 equiv of imidazole are required
for the reaction to proceed within a reasonable period of
time (48 h). Lowering the amount of imidazole slows the
reaction, while increasing the amount of imidazole (>2
equiv) does not seem to have a significant impact on the
rate of the reaction. Indeed, larger amounts of imidazole in
several instances complicate the purification of final pro-
ducts 2.
(12) Jain, R. Org. Prep. Proc. Int. 2001, 33, 405.
(13) Spectral data of 6a and 6b were in agreement with those
previously reported, see: Katritzky, A. R.; Chang, H. X.; Yang, B.
Synthesis 1995, 503.
To establish the origin of the formyl (CHO) group in
N-formyl Phe-OEt (2b), a formylation of Phe-OEt HCl (1b)
3
Org. Lett., Vol. 13, No. 15, 2011
3954

was carried out in DMF-D7 as a solvent (Scheme 4). The
product of formylation 2b-D1 was obtained in 70% yield
and was characterized by 1H NMR, 13C NMR and mass
by 1H NMR), respectively. N-Methylimidazole performed
better with 37% isolated yield at 48 h, yet still fell short of
the performance of imidazole (2b, 79%).
1
spectroscopy. In H NMR spectrum, an absence of the
signal at δ 8.17 ppm (CHO proton) was observed; on the
other hand a presence of triplet at δ 160.3 ppm (CDO
carbon) was observed in 13C NMR spectrum (see SI). The
mass spectrum (EI) was also consistent with the structure
2b-D1. These results indicate that the N-formyl group
present in amino acid esters 2aꢀ2n and in formamides 6a
and 6b originates from DMF, which is used as a solvent in
these reactions.
Scheme 5. Plausible Mechanism for the N-Formylation of
Amino Acid Esters 1aꢀ1n and Amines 5a, 5b
Scheme 4. N-Formylation of Phe-OEt HCl (1b) in DMF-D7
3
In summary, we have developed a simple protocol for
the N-formylation of amino acid esters and primary
amines. The major advantages of our approach are
(i) the use of widely available and inexpensive reagents
and (ii) experimentally simple protocol, requiring neither
dry solvents nor an inert atmosphere. The described pro-
tocol could furnish N-formylated derivatives which are
suitable for conversion into amino acid or amine-derived
isocyanides, which themselves have synthetic use as sub-
strates in the Ugi multicomponent reaction, for example.15
As well, this work prompts us to make a cautionary note
for reactions taking place in warm/hot DMF in the pre-
senceof nucleophilic acyltransfer catalysts asthismay lead
to unwanted formylated byproducts.
A plausible mechanism for the N-formylation of amino
acid esters and amines, consistent with the results of our
experiments, is depicted in Scheme 5. Nucleophilic attack
of imidazole on DMF results in the formation of a tetra-
hedral intermediateA. Collapseof A leads tothe formation
of the reactive N-formyl imidazole (intermediate B,
Scheme 5). This intermediate has been previously postu-
lated to act as an acyl transfer reagent in the N-formylation
of amines by a mixture of formic acid, oxalyl chloride and
imidazole.14 Nucleophilic attack of the amino acid ester
1aꢀ1n or amine (5a and 5b) on intermediate B gives a
tetrahedral intermediate C, which collapses with the for-
mation of N-formylated products 2aꢀ2n, 6a and 6b.
This mechanism suggests that other acyl transfer cata-
lysts may also be compatible with the reaction. To this end,
pyridine, 4-N,N-dimethylaminopyridine (DMAP) and
N-methylimidazole were investigated in reaction with 1b.
None of these agents worked aswell as imidazole under the
same conditions (1 mmol scale, 60 °C, DMF). After 24 h of
reaction, the reactions in the presence of pyridine or
DMAP demostrated lower reactivity showing only 14
and 19% conversion (crude reaction mixture analyzed
Acknowledgment. The financial support from The
Ontario Institute for Cancer Research (OICR) is gratefully
acknowledged. We thank Prof. J. M. Chong (University
of Waterloo) for the assistance with optical rotation
measurements.
Supporting Information Available. Complete experi-
mental details, spectral characterization of compounds
2aꢀ2n, 4, 6a and 6b. This material is available free of
charge via the Internet at http://pubs.acs.org.
(15) Recently there has been a growing interest in the use of Ugi
multicomponent reaction as a key step in the formation of cyclen-
derived ligands, suitable for the preparation of, for example, MRI
contrast agents, see: (a) Main, M.; Snaith, J. S.; Meloni, M. M.; Jauregui,
M.; Sykes, D.; Faulkner, S.; Kenwright, A. M. Chem. Commun. 2008,
5212. (b) Piersanti, G.; Remi, F.; Fusi, V.; Formica, M.; Giorgi, L.;
Zappia, G. Org. Lett. 2009, 11, 417. (c) Tei, L.; Gugliotta, G.; Avedano,
S.; Giovenzana, G. B.; Botta, M. Org. Biomol. Chem. 2009, 7, 4406.
(14) (a) Kitagawa, T.; Arita, J.; Nagahata, A. Chem. Pharm. Bull.
1994, 42, 1655. (b) Kitagawa, T.; Ito, J.; Tsutsui, C. Chem. Pharm. Bull.
1994, 42, 1931. Our attepmts to isolate or detect the presence of N-formyl
imidazole in the reaction mixture containing no external nucleophile
(1aꢀ1n or 5a, 5b) failed, presumably due to the termal instability of
N-formyl imidazole.
Org. Lett., Vol. 13, No. 15, 2011
3955