N-Methylated Peptide Synthesis via Generation of an Acyl N-Methylimidazolium Cation Accelerated by a Br?nsted Acid

Article abstract of DOI:10.1002/anie.202002106

The development of a robust amide-bond formation remains a critical aspect of N-methylated peptide synthesis. In this study, we synthesized a variety of dipeptides in high yields, without severe racemization, from equivalent amounts of amino acids. Highly reactive N-methylimidazolium cation species were generated in situ to accelerate the amidation. The key to success was the addition of a strong Br?nsted acid. The developed amidation enabled the synthesis of a bulky peptide with a higher yield in a shorter amount of time compared with the results of conventional amidation. In addition, the amidation can be performed by using either a microflow reactor or a conventional flask. The first total synthesis of naturally occurring bulky N-methylated peptides, pterulamides I–IV, was achieved. Based on experimental results and theoretical calculations, we speculated that a Br?nsted acid would accelerate the rate-limiting generation of acyl imidazolium cations from mixed carbonic anhydrides.

Full text of DOI:10.1002/anie.202002106

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: N-Methylated Peptide Synthesis via Acyl N-Methylimidazolium
Cation Generation Accelerated by a Brønsted Acid
Authors: Yuma Otake, Yusuke Shibata, Yoshihiro Hayashi, Kawauchi
Susumu, Nakamura Hiroyuki, and Shinichiro Fuse
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10.1002/anie.202002106
Angewandte Chemie International Edition
RESEARCH ARTICLE
N-Methylated Peptide Synthesis via Acyl N-Methylimidazolium
Cation Generation Accelerated by a Brønsted Acid
Yuma Otake,[a,b] Yusuke Shibata,[c] Yoshihiro Hayashi,[c] Susumu Kawauchi,[c] Hiroyuki Nakamura,[a]
and Shinichiro Fuse*[d]
Abstract: The development of a robust amide bond formation
remains a critical aspect of N-methylated peptide synthesis. In this
study, we synthesized a variety of dipeptides in high yields without
severe racemization from equivalent amounts of amino acids. A highly
reactive N-methylimidazolium cation species were generated in situ
to accelerate the amidation. The key to success was the addition of a
strong Brønsted acid. The developed amidation enabled the synthesis
of a bulky peptide with a higher yield in a shorter amount of time
compared with the results of conventional amidation. In addition, the
amidation can be carried out using both a micro-flow reactor and a
conventional flask. The first total synthesis of naturally occurring bulky
N-methylated peptides, pterulamides I-IV, was achieved. Based on
experimental results and theoretical calculations, we speculated that
Brønsted acid would accelerate the rate-limiting generation of acyl
imidazolium cations from mixed carbonic anhydrides.
Introduction
Scheme 1. Amidations using highly active acyl intermediates. a) General
scheme
for
acylation
using
acyl
4-dimethylaminopyridinium/N-
N-methylated peptides have garnered considerable attention
as drug candidates because they can enhance oral bioavailability,
cell permeability, and stability against enzymatic degradation.[1]
However, amidation of these sterically hindered N-methyl amino
acids containing bulky side chains remains one of the most
challenging tasks in organic synthesis, because the reactivity of
N-methyl amino acids is ca. 1/10th-1/100th that of amino acids
without an N-methyl group.[2] In addition, slow amidation
enhances the risk of racemization. In fact, both a long reaction
time and an excess use of reagents and/or substrates to
overcome the insufficient reactivity of N-methyl amino acids are
usually required and often cause undesired racemization.
methylimidazolium cations and b) N-methylated peptide synthesis using the acyl
N-methylimidazolium cation investigated in this work.
Moreover, potentially explosive and/or expensive agents have
been used for coupling these N-methyl amino acids.[3]
Danishefsky et al. reported the synthesis of N-methylated
peptides from isonitriles and thioesters or carboxylic acids and
achieved a total synthesis of cyclosporin A,[4] although this
approach required premodification of the amino acids.
Acylation via acyl 4-dimethylaminopyridinium cation
D
(generated from A and B) is one of the most powerful methods for
the coupling of poorly reactive substrates (Scheme 1a).
Reportedly, amidation via acyl N-methylimidazolium cation D’
(generated from A and B’) is comparably powerful to D (Scheme
1a).[5] In addition, with respect to cost and toxicity, the use of NMI
is superior to that of DMAP.[6] Only two examples of N-methylated
peptide synthesis via D’ were reported. Rapoport et al. reported
the generation of D’ using O=C(NMI+TfO-)2 and synthesis of one
N-methylated dipeptide.[7] Unfortunately, their conditions required
2 equiv of carboxylic acid against a nucleophile. The same group
reported the synthesis of one N-methylated tripeptide using
O=C(NMI+TfO-)2 and Cu salts.[8] However, these conditions
afforded unsatisfactory results (36% yield, ca. 6% of epimer). The
development of a robust, rapid, and inexpensive amide formation
for N-methylated peptide synthesis remains highly important.[9]
We reported micro-flow amide formation based on a rapid and
strong activation of carboxylic acids using inexpensive
triphosgene.[10] Our developed approach enabled the rapid
connection of various amino acids without severe racemization.
[a]
[b]
[c]
[d]
Dr. Y. Otake, Prof. Dr. H. Nakamura
Laboratory for Chemistry and Life Science, Institute of Innovative
Research, Tokyo Institute of Technology
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503 (Japan)
Dr. Y. Otake
School of Life Science and Technology, Tokyo Institute of
Technology
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503 (Japan)
Y. Shibata, Dr. Y. Hayashi, Prof. Dr. S. Kawauchi,
School of Materials and Chemical Technology, Tokyo Institute of
Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552
(Japan)
Prof. Dr. S. Fuse
Department of Basic Medicinal Sciences, Graduate School of
Pharmaceutical Sciences, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8601 (Japan)
E-mail: fuse@ps.nagoya-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/anie.202002106
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RESEARCH ARTICLE
However, our previous approach required 2.5 to 3.0 equiv of
carboxylic acids against nucleophiles because the active species
was
a
symmetric anhydride.[10b] This is inconvenient for
connecting expensive N-methyl amino acids. Here, we report a
robust, rapid, and inexpensive amidation of N-methyl amino acids
via the generation of acyl N-methylimidazolium cation D’ (Scheme
1b). Significant acceleration of amidation via the addition of
Brønsted acids was observed and an acceleration mechanism
was proposed. In addition, the first total synthesis of pterulamides
I-IV was successfully achieved using our originally developed
amidation.
Results and Discussion
In this study, we planned to generate D’ from corresponding
mixed carbonic anhydride A’ and NMI (B’) because A’ can be
prepared under ambient conditions with less waste generation
(Scheme 1b). In addition, excess use of carboxylic acids is not
required in contrast to our first-generation amide formation.[10b, 10c]
Lapshin et al. investigated the reactivity of acetyl N-
methylimidazolium cation D’ with different counter anions X- (Cl-,
Br-, or I-) in amidation with 4-nitroaniline (Scheme 1a).[11] The
highest level of activity was observed when the combination of D’
with the most basic X- (Cl-) was used. Those researchers
speculated that the proton of the nucleophile (4-nitroaniline) was
abstracted by X- in a transition state that accelerated the
amidation. Therefore, we anticipated that carbonate counter
Scheme 2. Plausible cause for the lower yield from the amidation of mixed
carbonic anhydride without the addition of a Brønsted acid and proposed
acceleration mechanism for formation of 2a with the addition of a Brønsted acid.
generates i-PrOCO2- as a counter anion (Table 1). Unexpectedly,
the amidation from 1b resulted in a lower yield (entry 2) compared
with that from 1a (entry 1). We speculated that the generation of
acyl N-methylimidazolium cation 2a from 1b (Scheme 2, bottom)
would be slower due to the lower electrophilicity of 1b compared
with that of 1a (Scheme 2, up) and that the generation of 2a would
be a rate-limiting step in the amidation from 1b. We carried out
numerous attempts to accelerate the generation of 2a, which
coincidentally revealed the significant effect of a Brønsted acid.
The addition of 0.10 equiv of HCl dramatically improved the yield
(Table 1, entry 2 vs. 3). Increasing the amount of HCl did not
improve the yield (entry 3 vs. 4). The addition of a weaker acid,
CF3CO2H, also improved the yield, although the positive effect
was lower than that of HCl (entry 3 vs. 5). The addition of a mild
acid, AcOH, had no positive effect (entry 2 vs. 6). These results
indicated that the addition of a catalytic amount of a strong
Brønsted acid is key to accelerating the reaction.[13] HCl is
generally regarded as an inhibitor of amidations due to its
undesired protonation of nucleophiles.[14] However, the addition of
a catalytic amount of HCl seemed to accelerate the formation of
2a rather than inhibiting the attack of NMI and/or 3a under our
conditions. The added HCl should form corresponding ammonium
salts with organic amines such as NMI and/or 3a and they would
work as a proton source. We speculated that the mixed carbonic
anhydride 1b coordinated to the proton, which enhanced the
electrophilicity of 1b (Scheme 2, bottom). Thus, the nucleophilic
substitution of NMI against 1b was accelerated to afford 2a. The
results shown in Table 1 (entries 3, 5, and 6) suggest the
importance of the acidity of the in situ-generated ammonium salts.
These results are reasonable because the ammonium salts must
be sufficiently acidic to activate 1b, which is a weak base. On the
other hand, the ammonium salts were not too acidic to completely
deactivate nucleophiles(NMI and/or 3a). For more detailed
-
anion (ROCO2 is more basic than Cl-) would enhance the
nucleophilicity of sterically hindered N-methyl amino acid C’
(Scheme 1b). In this study, micro-flow technology[12] was used to
precisely control both reaction time and temperature while
examining the reaction conditions.
In order to confirm an expected acceleration of amidation via
the use of a carbonate counter anion, we initially compared
amidations from isolated acyl chloride 1a which generates Cl- as
a counter anion and mixed carbonic anhydride 1b which
Table 1. Comparison between amidations from acid chloride and mixed
carbonic anhydride and the positive effect of the addition of a Brønsted acid.
entry
X
Brønsted acid (equiv)
yield[a] (%)
1
2
3
4
5
6
Cl
-
59
24
90
88
51
21
OCO2i-Pr
OCO2i-Pr
OCO2i-Pr
OCO2i-Pr
OCO2i-Pr
-
HCl (0.10)
HCl (0.20)
CF3CO2H (0.10)
AcOH (0.10)
discussion using DFT calculation, see supporting information 8.
The use of isolated mixed carbonic anhydride for amidation is
inconvenient because it requires an additional step for the
purification of unstable mixed carbonic anhydride.[15] Therefore,
we planned to generate the mixed carbonic anhydride in situ and
[a] Combined yield of desired 4a and its epimer. It was determined by HPLC-
UV analysis.
2
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10.1002/anie.202002106
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RESEARCH ARTICLE
investigate the substrate scope (Figure 1).[17] Before this
examination, the amount of HCl was re-examined and 0.15 equiv
proved to be the optimal amount (for details, see supporting
information 4-3). The desired N-methylated dipeptide 4a was
obtained in a 94% yield without detectable racemization using
equivalent amounts of amino acids. The productivity of this
reaction could be easily improved via the use of a higher
concentration without a severe decrease in yield (for details, see
supporting information 4-4). The use of nucleophile 3 without an
N-methyl group afforded the desired peptide 4b in a quantitative
yield without detectable racemization. N-Cbz and N-Alloc
phenylalanine afforded the desired products 4c and 4d in good
yields, whereas N-Boc phenylalanine afforded the desired
product 4e in a lower yield probably due to undesired formation of
-amino acid N-carboxy anhydride. Ala, Trp, and Lys gave the
desired products 4f-4h in high to excellent yields. Racemizable
Ser, Asp, and Cys also afforded the desired products 4i-4k in
excellent yields without severe racemization. The use of sterically
hindered N-alkyl amino acids were examined with an application
to peptoid synthesis in mind. To our delight, the products 4l-4o
were obtained in good to high yields without detectable
racemization (incomplete conversion due to the steric bulkiness
of the nucleophile was observed in the synthesis of 4m). In
addition, although our conditions used HCl, the total amount of
HCl was equal to the total amount of bases. Therefore, no free
HCl was supposed to exist in the reaction medium and substrate
scope was not significantly limited. In fact, solid-phase synthesis
using acid-labile 2-chlorotrityl resin was compatible. (for details,
see supporting information 4-5). When the more challenging
amidation for the synthesis of 4p containing sterically hindered
Val was examined, we encountered a problem with the generation
of an undesired carbamate. In order to solve this problem, we
used a more sterically hindered mixed carbonic anhydride which
can be readily prepared from 2,4-dimethyl-3-pentyl chloroformate
(6b) (for details, see supporting information 4-7). In addition,
increased amounts of Val, Me2NBn, and DIEA were used with
longer reaction times to improve the yield. As a result, the desired
dipeptide 4p was obtained in an 89% isolated yield. Our
developed conditions employ inexpensive reagents and only emit
readily removable CO2, hydrogen chloride, alcohol and organic
bases.
In order to extend the range of applications for our originally
developed amidation, naturally occurring, bioactive peptides,
pterulamides I-IV (7)-(10), were synthesized (Scheme 3).
Pterulamides were isolated from Malaysian Pterula sp., and their
structures were reported by Munro et al. in 2006.[18] Pterulamides
contain a bulky -branched amino acid (Val) and more bulky -
branched N-methyl amino acids (two N-MeIle and one N-MeVal).
Pterulamides I and IV are known to exert potent cytotoxicity
against P388 murine leukemia cells with IC50 values of 0.79 M
(pterulamide I), and 1.33 M (pterulamide IV), respectively. No
total synthesis of pterulamide has previously been reported.
Pterulamides are attractive synthetic targets that demonstrate the
robustness of our developed amidation. Pterulamides I-IV (7)-(10)
contain the same pentapeptidic core structure with different acyl
groups (= R), and, therefore, we planned to synthesize them via
acylation of the common pentapeptide intermediate 12 in the final
stage of total synthesis (Scheme 3).
Figure 1. Investigation of amidation via acyl N-methylimidazolium cation
generation using various substrates. [a] 1.3 equiv. of Fmoc-Val-OH (5p), 0.065
equiv. of BnNMe2, 1.3 equiv. of DIEA, 1.4 equiv. of 2,4-dimethyl-3-pentyl
chloroformate (6b), 0.10 equiv. of HCl were used. Amidation time was extended
to 20 min. [b] The yield of epimer was determined by HPLC-UV analysis.
use it for the subsequent amidation in a micro-flow reactor. In
order to realize this goal, a mixed carbonic anhydride formation in
the presence of different nucleophilic amines was investigated
(see supporting information 3). Anderson et al. reported that a
nucleophilic amine with at least one methyl group is suitable for
the formation of a mixed carbonic anhydride, and NMM (1 equiv)
afforded the best results.[16] Our results demonstrated that the use
of a catalytic amount (0.05 equiv) of Me2NBn containing two
methyl groups is the most suitable for the reaction.
Peptide chain elongation was performed as shown in Scheme
3. The coupling between sterically hindered Val 5p and N-MeIle
13 afforded the desired dipeptide 18 in a high yield (83%) without
In situ preparation of the mixed carbonic anhydride and
subsequent amidation was performed via the generation of an
acyl N-methylimidazolium cation in a micro-flow reactor to
3
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10.1002/anie.202002106
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RESEARCH ARTICLE
Table 2. Comparison between our developed amidation and previously reported
conventional amidations.
entry
1
reagents
yield[a] (%)
98±3 (84[b]
Me2NBn, 2,4-dimethyl-3-pentyl chloroformate (6b),
NMI, HCl, DIEA, 1,4-dioxane/MeCN, flow
)
2
Me2NBn, 2,4-dimethyl-3-pentyl chloroformate (6b),
NMI, HCl, DIEA, 1,4-dioxane/MeCN, batch
81±1
3
4
5
6
7
8
EDCI, HOBt, DIEA, DMF, batch
EDCI, HOAt, DIEA, DMF, batch
HATU, DIEA, DMF, batch
n.d.
n.d.
35±3
8±0.2
47±4
n.d.
PyBrop, DIEA, DMF, batch
Scheme 3. Total synthesis of pterulamides I-IV. Conditions: a) AcOt-Bu, HClO4
aq. 0 C to r.t., 24 h, 74%; b) 5p, 2,4-dimethyl-3-pentyl chloroformate (6b),
Me2NBn, DIEA, 1,4-dioxane, 60 °C, 80 s, then, 13, NMI, HCl, MeCN/1,4-dioxane,
60 °C, 40 min; c) DBU, CH2Cl2, 0 °C, 10 min, 97%; d) 14, 6b, Me2NBn, DIEA,
1,4-dioxane, 60 °C, 4 min, then, 19, NMI, HCl, MeCN/1,4-dioxane, 60 °C, 120
min; e) DBU, CH2Cl2, 0 °C, 15 min, 94%; f) 15, 6b, Me2NBn, DIEA, 1,4-dioxane,
60 °C, 4 min, then, 21, NMI, HCl, MeCN/1,4-dioxane, 60 °C, 120 min; g) DBU,
CH2Cl2, 0 °C, 25 min, quant.; h) 16, 6b, Me2NBn, DIEA, 1,4-dioxane, 60 °C, 4
min, then, 23, NMI, HCl, MeCN/1,4-dioxane, 60 °C, 120 min; i) 85w% H3PO4,
MeCN, 0 °C to r.t., 6 h, 89%; j) MeNH2, EDCI, HOBt, DIEA, CH2Cl2/THF, 0 °C
to r.t., 2.5 h; k) DBU, THF, 0 °C to r.t., 15 min, 2 steps 92%
COMU, DIEA, DMF, batch
CDI, MeOTf, NMM, MeNO2, THF, DMF, batch
[a] The yields were determined by HPLC-UV analysis and were the means ± SE
(standard error) of triplicate experiments. [b] Isolated yield. CDI 1,1’-
carbonyldiimidazole.
previously reported data. Munro et al. reported the cytotoxicity of
pterulamides against P388 murine leukemia cells, but cytotoxicity
against human cancer cells was not reported. Therefore, we
evaluated the cytotoxicity of pterulamides I-IV against HeLa cells.
Pterulamide I (7) exerted the most potent cytotoxicity against
HeLa cells (IC50 = 0.74 M). The other pterulamides 8-10 showed
moderate cytotoxicity (for details, see supporting information 6).
Our developed amidation was compared with previously
reported conventional amidations (Table 2).[22] The amidation of
sterically hindered substrates 15 and 21 proceeded to afford the
desired peptide 22 in a 98% HPLC-UV yield under our developed
conditions (entry 1). Although micro-flow conditions allow the
advantage of scaling up the reaction, the developed amidation is
not limited to micro-flow conditions. The developed amidation can
be performed using a conventional flask without a significant
decrease in yield (entry 2). The same reaction was performed
using conventional combinations of N-(3-dimethylaminopropyl)-
generating a detectable epimer. The next coupling between
hindered N-MeVal 14 and dipeptide 19 afforded the desired
tripeptide 20 in an 81% yield without generating a detectable
epimer. The next coupling was challenging because the
significantly hindered N-methylated -branched substrates 15
and 21 had to be coupled. Our developed amidation afforded the
desired tetrapeptide 22 in an 84% yield without severe
racemization (epimer = 0.1%). The final coupling of hindered
tetrapeptide 23 with N-MeAla 16 afforded the desired
pentapeptide core structure of pterulamides 11 in an 88% yield
(1.1 g) without generating a detectable epimer. The obtained
pentapeptide 11 was converted to the acylation precursor 12
using batch conditions (Scheme 3). Acidic removal of the t-butyl
group was examined according to the HCl in Et2O/CH2Cl2, the
CF3CO2H in CH2Cl2,[19] the formic acid without a solvent, and the
ZnBr2 in CH2Cl2.[20] These conditions, however, afforded multiple
products and resulted in low yields. The use of phosphoric acid[21]
dramatically improved the yields. The subsequent amidation and
base-mediated removal of the Fmoc group afforded the desired
acylation precursor 12 in a high yield. Then, the obtained amine
12 was acylated to obtain the desired pterulamides I-IV (7)-(10).
The structures of the obtained products were confirmed by 1H and
13C NMR, 2D NMRs, IR, and HRMS spectra as well as by specific
rotation. The observed spectra were consistent with the
N’-ethylcarbodiimide
(EDCI)[23]
and1-hydroxybenzotriazole
(HOBt) or 1-hydroxy-7- azabenzotriazole (HOAt), but neither
condition afforded a detectable product 22 (entries 3 and 4). Other
frequently used coupling agents in N-methylated peptide
synthesis,
tetramethyluronium
bromotripyrrolidinophosphonium
(PyBroP),[25]
and
oxoethylideneaminooxy)dimethylamino(morpholino) carbenium
hexafluorophosphate (COMU)[26] afforded the product 22, but the
O-(7-azabenzotriazole-1-yl)-N,N,N’,N’-
hexafluorophosphate
(HATU),[24]
hexafluorophosphate
(1-cyano-2-ethoxy-2-
4
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10.1002/anie.202002106
Angewandte Chemie International Edition
RESEARCH ARTICLE
yields were insufficient (entries 5-7). Rapoport’s conditions[7] via
acyl N-methylimidazolium cation generation afforded 22 in a 3%
yield. Acylimidazolium cation with a triflate counter anion (pka
=12)[27] was generated under their conditions and a low yield was
observed. Based on these results and DFT calculation (see
supporting information 8), we speculated that the carbonate
counter anion generated under our conditions accelerated the
desired amidation, as expected. It should be noted that the
reaction time of our developed amidation (entries 1 and 2) was 2
h, but the reaction time of previously reported amidations (entries
3-8) was 24 h. Although, these examinations were carried out at
60 C using DMF as a solvent, we also examined the same
combinations of coupling agents, bases, and solvents, as well as
the concentrations and temperatures that were reported in the
original manuscripts (for details, see supporting information 7).
None of the reported conditions exceeded a 60% yield, although
the reactions were carried out for 24 h. These results indicated
the power of the developed amidation for less-reactive N-methyl
amino acids under either micro-flow or batch conditions.
Keywords: amino acids • anhydrides • continuous flow • peptides
• acylation
[1]
a) J. Chatterjee, C. Gilon, A. Hoffman, H. Kessler, Acc. Chem. Res. 2008,
41, 1331-1342; b) J. Chatterjee, F. Rechenmacher, H. Kessler, Angew.
Chem. Int. Ed. 2013, 52, 254-269; Angew. Chem. 2013, 125, 268-283;
c) D. J. Craik, D. P. Fairlie, S. Liras, D. Price, Chem. Biol. Drug Design
2013, 81, 136-147.
[2]
[3]
We preliminary determined reaction rates in amidations using N-methyl
amino acids and an amino acid without N-methyl group.
a) Y. E. Jad, A. Kumar, A. El-Faham, B. G. de la Torre, F. Albericio, ACS
Sus. Chem. Eng. 2019, 7, 3671-3683; b) K. G. Varnava, V. Sarojini,
Chem. Asian J. 2019, 14, 1088-1097.
[4]
[5]
[6]
X. Wu, J. L. Stockdill, P. Wang, S. J. Danishefsky, J. Am. Chem. Soc.
2010, 132, 4098-4100.
H. Ueki, T. K. Ellis, C. H. Martin, T. U. Boettiger, S. B. Bolene, V. A.
Soloshonok, J. Org. Chem. 2003, 68, 7104-7107.
a) G. Höfle, W. Steglich, H. Vorbrüggen, Angew. Chem. Int. Ed. Engl.
1978, 17, 569-583; Angew. Chem. 1978, 90, 602-615; b) K. Wakasugi,
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[7]
A. K. Saha, P. Schultz, H. Rapoport, J. Am. Chem. Soc. 1989, 111, 4856-
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[8]
[9]
F. S. Gibson, H. Rapoport, J. Org. Chem. 1995, 60, 2615-2617.
Synthesis of several peptides without N-methyl group via acyl N-
methylimidazolium cation was recently reported. The reported conditions
did not use a Brønsted acid, which was one of our key conditions. Details,
see: G. L. Beutner, I. S. Young, M. L. Davies, M. R. Hickey, H. Park, J.
M. Stevens, Q. Ye, Org. Lett. 2018, 20, 4218-4222.
Conclusion
[10] a) S. Fuse, N. Tanabe, T. Takahashi, Chem. Commun. 2011, 47, 12661-
12663; b) S. Fuse, Y. Mifune, T. Takahashi, Angew. Chem. Int. Ed. 2014,
53, 851-855; Angew. Chem. 2014, 126, 870-874; c) S. Fuse, Y. Mifune,
H. Nakamura, H. Tanaka, Nat. Commun. 2016, 7, 13491.
In conclusion, the developed amidation via acyl N-
methylimidazolium cation generation offered a solution for one of
the most challenging amidations in organic synthesis — the high-
yield coupling of sterically hindered N-methyl amino acids without
racemization. The key to success was the addition of a strong
Brønsted acid. A variety of dipeptides were synthesized in high
yields without severe racemization using equivalent amounts of
amino acids. The developed conditions also showed promise for
both peptoid and solid-phase syntheses. The developed
amidation uses inexpensive and less wasteful reagents, and is far
more powerful than conventional amidations. In addition, the
developed amidation is general and not limited to micro-flow
conditions. This process can be carried out using a conventional
flask. The first total synthesis of naturally occurring bulky N-
methylated peptides, pterulamides I-IV, was achieved using our
developed process for amidation. This process for amidation
would accelerate drug/material development based on N-
methylated peptides, and it has the potential to become a
powerful and general approach for the coupling of poorly reactive
and/or labile substrates.
[11] a) S. A. Lapshin, V. A. Dadali, Y. S. Simanenko, L. M. Litvinenko, Zh.
Org. Khim. 1977, 13, 586-594; b) S. A. Lapshin, V. A. Dadali, L. M.
Litvinenko, Y. S. Simanenko, Zh. Org. Khim. 1981, 17, 1938-1944.
[12] Recent selected reviews for continuous-flow synthesis, see: a) Gutmann,
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The authors thank Prof. Seiji Suga, Okayama University for fruitful
suggestions about the reaction mechanism. This work was
partially supported by Scientific Research on Innovative Areas
2707 Middle molecular strategy from MEXT (Grant Number
16H01138), JST-Mirai Program Japan (Grant Number
JPMJMI18G7), AMED (Grant Number JP19ak0101077h0003),
The Asahi Glass Foundation Research Grant Program, a Grant-
in-Aid for Specially Promoted Research (JSPS KAKENHI Grant
Number JP17H06092), a Grant-in-Aid for Young Scientists (B)
(JSPS KAKENHI Grant Number JP17K17720), and a JST
CREST (Grant Number JPMJCR1522).
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10.1002/anie.202002106
Angewandte Chemie International Edition
RESEARCH ARTICLE
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10.1002/anie.202002106
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
A variety of N-methylated peptides were synthesized in high yields without severe racemization via the generation of acyl N-
methylimidazolium cations. Brønsted acids dramatically accelerated the reaction. The developed amidation enabled the synthesis of a
bulky peptide with a higher yield in a shorter time compared with the results of conventional amidation. The first total synthesis of
pterulamides I-IV, was also achieved.
7
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