Iron-catalyzed three-component synthesis of α-amino acid derivatives

Article abstract of DOI:10.1002/ejoc.201301349

An efficient, iron-catalyzed, three-component synthesis of arylglycines starting from readily available amides, glyoxalates, and arenes or heteroarenes has been developed. This method provides a versatile, atom-economic, and cost-effective route to arylglycines with water as the only byproduct. With carbamates as the amide component, various synthetically useful N-protected arylglycine derivatives could be prepared. An iron-catalyzed three-component reaction between amides or carbamates, glyoxylic acid derivatives, and (hetero)aromatics has been developed. This method provides a versatile, atom-economic, and cost-effective route to arylglycines with water as the only byproduct.

Full text of DOI:10.1002/ejoc.201301349

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301349
Iron-Catalyzed Three-Component Synthesis of α-Amino Acid Derivatives
Juliette Halli[a] and Georg Manolikakes*[a]
Keywords: Iron / Homogeneous catalysis / Multicomponent reactions / Amino acids
An efficient, iron-catalyzed, three-component synthesis of
arylglycines starting from readily available amides,
glyoxalates, and arenes or heteroarenes has been developed.
This method provides a versatile, atom-economic, and cost-
effective route to arylglycines with water as the only byprod-
uct. With carbamates as the amide component, various syn-
thetically useful N-protected arylglycine derivatives could be
prepared.
Introduction
α-Amino acids are of outstanding importance in chemis-
try and biology.[1] They are used in the production of drugs,
nutritional supplements, fertilizers, and biodegradable plas-
tics. As a backbone of all proteins, α-amino acids are in-
volved in almost every biological process.[2] With the dis-
covery of protein-based drugs[3] and the recent develop-
ments in protein engineering,[4] non-proteinogenic amino
acids are becoming increasingly important. Among these
non-proteinogenic amino acids, arylglycine derivatives are
of particular significance. They are important building
blocks for various drugs, such as β-lactam antibiotics[3a]
and cardiovascular agents,[3b] and the arylglycine moiety is
found in several natural products, such as vancomycin.[3c]
As a result of their importance, various methods for the
synthesis of arylglycines have been developed.[5,6] Of these
methods, multicomponent reactions based on the in situ
formation of reactive imine derivatives, such as the Strecker
reaction[7] or Mannich-type reactions,[8–10] provide the most
reliable and rapid access to a wide variety of arylglycines
(Scheme 1).
Scheme 1. Different synthetic routes to arylglycines.
reactions also meet the requirements of modern, sustainable
organic synthesis.[12] However, these reactions are in general
limited to very reactive (hetero)arenes,[6e] and often
stoichiometric amounts of strong Lewis or Brønstedt acids
have to be employed.[13] Therefore, the scope of these reac-
tions is rather limited, and considerable amounts of waste
can be generated in these processes.
Nevertheless, these methods have some decisive draw-
backs. The Strecker reaction uses highly toxic cyanide
sources, and strongly acidic conditions are required for the
hydrolysis of the α-amino nitrile (Scheme 1, path A),
whereas the Petasis–(Borono–Mannich) reaction,[9] a very
powerful method, uses prefunctionalized and often expens-
ive boronic acid derivatives (Scheme 1, path B).
Results and Discussion
We envisioned that the development of a general, cata-
lytic version of such amino- or amidoalkylations of arenes
should be only a matter of choosing a suitable catalyst.
Early on we focused on the in situ generation of acylimines
from the corresponding amides and aldehydes.[14] Owing to
The direct amino- or amidoalkylation[10] of unfunction-
alized arenes represents a more atom-economic[11] approach
(Scheme 1, path B). With water as the only byproduct, these
the high electrophilicity of these imine derivatives,[10]
a
higher reactivity towards less electron-rich arenes and there-
fore a broader reaction scope could be envisioned. To ident-
ify an appropriate catalyst, we selected the reaction between
benzamide (1a), ethyl glyoxalate (2a), and m-xylene (3a) as
a moderately reactive arene.[15] Preliminary studies showed
that several Lewis acids could catalyze this reaction with
various degrees of efficiency. One of the most efficient cata-
lysts was FeCl3. Only 5 mol-% FeCl3 afforded the desired
[a] Institut für Organische Chemie und Chemische Biologie,
Goethe-Universität,
Max-von-Laue-Straße 9, Frankfurt am Main, Germany
E-mail: g.manolikakes@chemie.uni-frankfurt.de
http://www.org.chemie.uni-frankfurt.de/arbeitskreise/
Manolikakes/index.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301349.
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J. Halli, G. Manolikakes
SHORT COMMUNICATION
arylglycine derivative 4a in 86% yield (Table 1, Entry 1). Fluorescent-labeled proteins have become a powerful bio-
From an economic as well as an ecological perspective, analytical tool and are widely applied in life sciences.[25]
commercially available and cheap iron salts would be ideal With fluorescent polycyclic aromatic compounds, such as
catalysts for this transformation.[16,17] Therefore, we tested pyrene or anthracene, the corresponding glycine derivatives
the catalytic activity of several iron salts. Although 4n and 4o, useful building blocks for such labeled proteins,
FeCl3·6H2O showed comparable catalytic activity (Entries 2 could be synthesized in a very efficient manner.
[18]
and 3), Fe(ClO4)3
in general gave higher yields with
lower catalyst loadings (Entries 4 and 5). The correspond-
ing Fe2+ salts catalyzed the reaction with similar efficiency
(Entry 6).[19,20] To rule out a possible “hidden” catalysis by
Brønstedt acids,[21a] we conducted the reaction in the pres-
ence of the proton scavenger 2,6-di-tert-butylpyridine
(dbpy)[21b] and observed no significant decrease in catalytic
activity (Entry 7). Therefore, we assume an Fe3+ species to
be the active catalyst.
Table 2. Study of the substrate scope of the reaction with various
arenes. Reagents and conditions: 1a (1.0 equiv.), 2a (1.2 equiv.), and
3 (3 equiv.) in MeNO2 at 80–100 °C for 24 h. Yields are given for
isolated products. r.r. = ratio of regioisomers; Bz = benzoyl; Mes =
mesityl (2,4,6-trimethylphenyl); Piv = pivaloyl.
Table 1. Optimization of the reaction parameters for the synthesis
of arylglycine 4a.
Entry
Catalyst
Yield[a] [%]
1
2
3
4
5
6
7
FeCl3 (5 mol-%)
FeCl3·6H2O (5 mol-%)
FeCl3·6H2O (2 mol-%)
86
84
84
91
91
82
86
Fe(ClO4)3·xH2O (5 mol-%)
Fe(ClO4)3·xH2O (1 mol-%)
Fe(ClO4)2·xH2O (2 mol-%)
Fe(ClO4)3·xH2O (5 mol-%) + dbpy (10 mol-%)
[a] Yields are given for the isolated product. The product was ob-
tained as a 20:1 mixture of regioisomers. Only the major re-
gioisomer is shown.
With the optimized conditions in hand, we investigated
the scope of the reaction with various arenes. In most cases
the best results were achieved with 5 mol-% Fe(ClO4)3, but
in individual cases similar results could be obtained with
lower catalyst loadings or with FeCl3·6H2O (Table 2). The
reactions of benzamide (1a) and ethyl glyoxalate (2a) with
different electron-rich arenes, such as mesitylene or anisole
[a] 5 mol-% FeCl3·6H2O. [b] 2 mol-% FeCl3·6H2O. [c] Obtained as
a mixture of regioisomers; only the major regioisomer shown. [d]
5 mol-% Fe(ClO4)3·xH2O. [e] 2 mol-% Fe(ClO4)3·xH2O.
As shown in Table 3, primary aryl- and alkylamides were
and its derivatives, furnished the arylglycine derivatives 4b– suitable substrates for the three-component reaction (5a–f)
[26]
4i in good to excellent yields. In general, the regioselectivity
and even acid-sensitive functionalities, such as an acryl-
of the reaction is good to excellent; often, only one re- amide, were well tolerated (5f). Performing the reaction
gioisomer could be detected. In a few cases, for example, with carbamates as the amide component allowed the ef-
with anisole 4c, the regioselectivity was lower. Unfortu- ficient preparation of various N-protected arylglycines (5g–
nately, the reaction was limited to at least moderately active k), such as 5g bearing a photolabile protecting group[27] or
arenes, such as o- or p-xylene (products 4j and 4k).[15] Tolu- the Fmoc derivative 5k.
ene or benzene did not react under our reaction condi-
With heteroaromatics as the nucleophilic component,
tions.[22] However, this disadvantage proved to be a major lower reaction temperatures were necessary to avoid the di-
advantage for the practicability of our method. Commer- rect addition of the heteroarene to ethyl glyoxalate.[28] Sev-
cially available, technical ethyl glyoxalate, a solution of the eral electron-rich heterocycles, for example, thiophenes,
polymer form in toluene, could be used directly without benzofuran, N-tosylpyrrole, or N-tosylindole, gave the cor-
prior purification.[23,24] Interestingly, pivaloyl-protected an- responding glycines 7a–7h in yields of 50–83% (Table 4).[29]
ilines reacted chemoselectively and furnished the aryl- However, in general, better yields were obtained with carb-
glycines 4l and 4m in 89 and 80% yields, respectively. amates, such as urethane, as the amide component.
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Iron-Catalyzed Synthesis of α-Amino Acid Derivatives
Table 3. Study of the substrate scope of the reaction with a variety (Scheme 3). The reaction with urethane gave the N-pro-
of amides and carbamates. Reagents and conditions: 1 (1.0 equiv.),
tected glycine 9a in 92% yield, whereas the reaction with
2a (1.2 equiv.), and 3b (3 equiv.) in MeNO2 at 80 °C for 24 h. Yields
9-fluorenylmethyl carbamate provided the Fmoc-protected
are given for isolated products. Mes = mesityl (2,4,6-trimeth-
arylglycine 9b, an ideal starting material for solid-phase
ylphenyl); Fmoc = [(9H-fluoren-9-yl)methoxy]carbonyl.
peptide synthesis, in 85% yield.
Scheme 2. Study of the substrate scope of the reaction with gly-
oxylic acid derivatives. Reagents and conditions: 1a (1.0 equiv.), 2
(1.2 equiv.), and 3b (3 equiv.) in MeNO2 at 60–80 °C for 24 h.
Yields are given for isolated products. [a] 5 mol-% Fe(ClO4)3·xH2O.
[b] 5 mol-% FeCl3·6H2O. Mes = mesityl (2,4,6-trimethylphenyl).
[a] 5 mol-% FeCl3·6H2O. [b] 2 mol-% FeCl3·6H2O. [c] 2 mol-%
Fe(ClO4)3·xH2O. [d] 5 mol-% Fe(ClO4)3·xH2O. [e] Obtained as a
Scheme 3. Three-component synthesis of N-protected arylglycines.
Reagents and conditions: 1a (1.0 equiv.), 2b (1.2 equiv.), 3b
(3 equiv.), and FeCl3·6H2O (5 mol-%) in MeNO2 at 80 °C for 24 h.
Yields are given for isolated products. Mes = mesityl (2,4,6-trimeth-
ylphenyl).
1:1 mixture of diastereomers.
Table 4. Study of the substrate scope of the reaction with different
heterocycles. Reagents and conditions:
1
(1.0 equiv.), 2a
(1.2 equiv.), 6 (3 equiv.), and Fe(ClO4)3·xH2O (5 mol-%) in MeNO2
at 40–80 °C for 24–48 h. Yields are given for isolated products. r.r.
= ratio of regioisomers; Ts = tosyl.
Concerning the mechanism of this reaction, we assume
the in situ formation of a reactive acylimine species fol-
lowed by an electrophilic aromatic substitution. However,
further studies are necessary to clarify the exact mecha-
nism.
Conclusions
By employing cheap iron(III) salts, we have developed an
efficient and versatile three-component synthesis of aryl-
glycines starting from readily available amides, glyoxalates,
and arenes or heteroarenes. This new method has a very
broad scope and, because it is not necessary to exclude air
or moisture, very simple to perform. By using carbamates
as the amide component, various synthetically very useful
N-protected arylglycine derivatives could be prepared. With
water as the only byproduct, this method provides a sus-
tainable route to arylglycines. Further applications as well
as the development of an enantioselective variant of this
method are currently being investigated in our laboratory.
[a] Obtained as a mixture of regioisomers; only the major re-
gioisomer is shown.
The reaction of mesitylene and benzamide with isopropyl
2-oxoacetate provided product 8a in 83% yield (Scheme 2).
This method proved to be very robust and insensitive
towards air or moisture. Indeed, the reaction of mesitylene
and benzamide with an aqueous solution of glyoxylic acid
directly furnished the free acid 8b. These free acids, bearing
different N-protecting groups, would be ideal starting mate-
rials for peptide synthesis. Therefore, we investigated the re-
Experimental Section
General Procedure for Reactions with Ethyl Glyoxalate: A 10 mL
screw-cap vial was charged with the amide (1.0 equiv.), iron salt (1–
5 mol-%), and MeNO2 (4 mL/mmol amide). Ethyl glyoxylate
(1.2 equiv.) and the aromatic compound (3.0 equiv.) were added un-
action of glyoxylic acid with different carbamates der vigorous stirring. The reaction mixture was heated to 40–
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J. Halli, G. Manolikakes
SHORT COMMUNICATION
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Supporting Information (see footnote on the first page of this arti-
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pounds.
[13]
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For an excellent overview of the nucleophilicity of arenes as
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This work was financially supported by the Fonds der Chemischen
Industrie (Liebig Fellowship to G. M.) and the Goethe University
(Nachwuchs im Fokus-Program). We would like to thank Prof.
Michael Göbel (Goethe University Frankfurt) for his support and
Rockwood Lithium (Frankfurt) and Evonik Industries (Darm-
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Iron-Catalyzed Synthesis of α-Amino Acid Derivatives
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Received: September 5, 2013
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Published Online: October 9, 2013
[26] Secondary amides or sulfonamides did not react under our re-
action conditions.
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