DOI: 10.1002/chem.201501783
Communication
&
Dipeptide Synthesis
Coupling-Reagent-Free Synthesis of Dipeptides and Tripeptides
Using Amino Acid Ionic Liquids
Shinya Furukawa,*[a] Takahide Fukuyama,[b] Akihiro Matsui,[b] Mai Kuratsu,[b]
Ryotaro Nakaya,[a] Takashi Ineyama,[a] Hiroshi Ueda,[a] and Ilhyong Ryu*[b]
spite the effectiveness of this standard approach, the require-
Abstract: A general method for the synthesis of dipepti-
des has been developed, which does not require any cou-
ments for protection/deprotection protocols, pre-activation or
the use of excess dehydrative coupling reagents are hardly op-
pling reagents. This method is based on the reaction of
timal. Herein, we report a simple, coupling-reagent-free syn-
readily available HCl salts of amino acid methyl esters with
thesis of dipeptides,[6] which uses HCl salts of amino acid
tetrabutylphosphonium amino acid ionic liquids. The isola-
methyl esters and amino acid ionic liquids (AAIL) (Scheme 1,
tion procedure of stepwise treatment with AcOH is easy
bottom).[7–9] The scope of the synthesis is wide, providing a vari-
to carry out. The method was extended to the synthesis
ety of dipeptides in good yields. We also discuss the applica-
of tripeptide, tyrosyl-glycyl-glycine, present in IMREG-1,
tion of the method to the synthesis of tripeptide, tyrosyl-
also.
glycyl-glycine, which possesses immune stimulation properties.
As a model reaction, the reaction between the HCl salt of
glycine methyl ester 1a and three different glycine ionic liq-
uids was investigated (Y+ =1-ethyl-3-methyl imidazolium,
Bu4N+, Bu4P+). When a mixture of 1a and 5.4 equivalents of
Dipeptides and tripeptides have attracted intense interest
owing to their wide range of bioactive properties, including
hypotensive effects, antiulcer properties, and analgesic effects,
just to name a few.[1] Therefore, ongoing efforts have been
made to pursue convenient methods for their synthesis.[2] Pep-
tides can be obtained by both chemical synthesis and enzy-
matic synthesis;[3] the chemical synthesis of peptides has been
studied since the early 20th century. In 1934, Bergmann and
Zervas introduced the benzyloxycarbonyl group as an N-pro-
tecting group for peptide synthesis.[4] Since that time, with few
exceptions, synthetic procedures typically involve a protection/
deprotection protocol, in which the protection of an amino
group (PG1 =Boc, Fmoc, Z, etc.) and a carboxylic acid group
(PG2 =tert-Bu, Bn, etc.) is used to suppress the formation of un-
desired homo-dimeric peptides. Pre-activation of carboxylic
acids as acid chlorides, activated esters, or related compounds
facilitates the condensation process, and in situ activation
methods using a stoichiometric amounts of coupling reagents,
such as carbodiimides, phosphonium salts, and uronium salts,
are in widespread use (Scheme 1, top).[5] Needless to say, de-
glycine-derived imidazolium ionic liquid 2a was heated to
608C for 3 h, we were pleased to find that indeed glycyl-gly-
cine ionic liquid 3a was formed. Subsequent treatment of 3a
with H3PO4 gave 4a in 50% yield (HPLC) (Scheme 2, entry 1).
Although tetrabutylammonium salt 2b gave inferior results
(entry 2), tetrabutylphosphonium salt 2c gave the desired
product 4a in 81% yield (entry 3). Reaction with a decreased
amount of 2c (2.4 equiv) gave slightly lower yield of 4a owing
to the formation of oligopeptides and diketopiperadine 5 as
byproducts (entry 4).
Having confirmed that the reaction indeed works, we then
focused on the isolation of dipeptide 4a from the reaction
mixture containing dipeptide ionic liquid 3c and excess gly-
cine ionic liquid 2c. The reaction mixture also contained gly-
cine and Bu4P+ClÀ, which were formed by anion exchange of
2c with HCl. We were pleased to find that free dipeptide 4a
could be isolated by a simple acid treatment using AcOH
(Scheme 3). Thus, when CHCl3 solution (6 mL) of the reaction
mixture of 1a (1 mmol) with 2c (5.4 equiv) was treated with
AcOH (3.2 mmol) at 08C, only glycine ionic liquid 2c reacted
with AcOH to give glycine as a white precipitate. Then, AcOH
(2.8 mmol) was added to the filtrate containing 3c and Bu4P+
XÀ (XÀ =ClÀ or AcOÀ) at room temperature and stirred for 3 h
to ensure complete precipitation of 4a. After filtration, glycyl-
glycine 4a was obtained as a white solid in 78% yield (95%
purity). Recrystallization of the product from H2O/MeOH gave
4a in pure form (>99% purity, 54% yield).
[a] Dr. S. Furukawa, R. Nakaya, T. Ineyama, H. Ueda
Research Institute for Bioscience Products & Fine Chemicals
AminoScience Division
Ajinomoto Co., Inc.
Kawasaki, Kanagawa 210-8681 (Japan)
[b] Prof. T. Fukuyama, A. Matsui, M. Kuratsu, Prof. I. Ryu
Department of Chemistry, Graduate School of Science
Osaka Prefecture University
To examine the generality of the present dipeptide synthe-
sis, a variety of amino acid methyl esters 1 and AAILs 2 were
exposed to similar conditions (Table 1). Methyl esters of alanine
(1b) and valine (1c) reacted with 2c to give alanyl-glycine (4b)
and valyl-glycine (4c) in good yields (entries 2 and 3). Dipep-
Sakai, Osaka 599-8531 (Japan)
Supporting information for this article is available on the WWW under
Chem. Eur. J. 2015, 21, 11980 – 11983
11980
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