Synthesis of a novel category of pseudo-peptides using an Ugi three-component reaction of levulinic acid as bifunctional substrate, amines, and amino acid-based isocyanides

Article abstract of DOI:10.3762/bjoc.15.82

The synthesis of a novel category of pseudo-peptides via intramolecular Ugi reaction of levulinic acid (4-oxopentanoic acid), aromatic and aliphatic amines, and amino acid-based isocyanides is reported. Levulinic acid was applied as a bifunctional substrate containing both carbonyl and acid moieties suitable for the Ugi reaction. This article provides a facile and convenient one-pot procedure for the synthesis of peptide-like heterocyclic molecules containing 2-pyrrolidone (γ-lactam), amide and ester functional groups with good to excellent yields.

Full text of DOI:10.3762/bjoc.15.82

Synthesis of a novel category of pseudo-peptides using an
Ugi three-component reaction of levulinic acid as bifunctional
substrate, amines, and amino acid-based isocyanides
Maryam Khalesi1, Azim Ziyaei Halimehjani*1 and Jürgen Martens*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 852–857.
1Faculty of Chemistry, Kharazmi University, P.O. Box 15719-14911,
doi:10.3762/bjoc.15.82
49 Mofateh Street, Tehran, Iran and 2Institut für Chemie (IfC), Carl
von Ossietzky Universität Oldenburg, P.O. Box 2503, 26111
Oldenburg, Germany
Received: 22 February 2019
Accepted: 29 March 2019
Published: 04 April 2019
Email:
Azim Ziyaei Halimehjani* - ziyaei@khu.ac.ir; Jürgen Martens* -
juergen.martens@uni-oldenburg.de
This article is part of the thematic issue "Multicomponent reactions III".
Guest Editor: T. J. J. Müller
*
Corresponding author
©
2019 Khalesi et al.; licensee Beilstein-Institut.
Keywords:
License and terms: see end of document.
amino acid-based isocyanides; levulinic acid; multicomponent;
pseudo-peptides; Ugi reaction
Abstract
The synthesis of a novel category of pseudo-peptides via intramolecular Ugi reaction of levulinic acid (4-oxopentanoic acid), aro-
matic and aliphatic amines, and amino acid-based isocyanides is reported. Levulinic acid was applied as a bifunctional substrate
containing both carbonyl and acid moieties suitable for the Ugi reaction. This article provides a facile and convenient one-pot pro-
cedure for the synthesis of peptide-like heterocyclic molecules containing 2-pyrrolidone (γ-lactam), amide and ester functional
groups with good to excellent yields.
Introduction
The multistep synthesis of complex molecules normally cesses has fascinated considerable attention in the fields of drug
requires a large number of repetitive synthetic operations, such discovery, organic synthesis of natural products, and materials
as extraction, separation, chromatography and other purifica- sciences [10].
tion steps. These disadvantages encouraged chemists to synthe-
size complex molecules using multicomponent reactions Undoubtedly, one of the most prominent and studied MCRs is
(
MCRs). MCRs transform three or more starting materials into the Ugi reaction. The Ugi four-component condensation reac-
a single product in an atom- and step-economical way in diver- tion (U-4CC) between an aldehyde, an amine, a carboxylic acid
sity- and target-oriented syntheses in modern organic synthesis. and an isocyanide provides a rapid preparation of α-aminoacyl
In addition, MCRs are characterized by high yields, time effi- amide or pseudo-peptide derivatives. These biologically active
ciency, low waste production, and reduced energy consumption peptide-like molecules can be utilized to circumvent some of
[
1-9]. So, the design of novel MCRs with facile and green pro- the problems associated with several natural peptides such as
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Beilstein J. Org. Chem. 2019, 15, 852–857.
stability against proteolysis, poor bioavailability, receptor selec- aliphatic amines and ordinary isocyanides with excellent yields
tivity, and short duration of action [11]. The Ugi reaction allows [24]. In 2003, Mironov et al. reported the Ugi reaction of
the introduction of several substituents in its adducts to prepare levulinic acid, isocyanides and primary amines in aqueous
novel peptidomimetics with potential pharmaceutical applica- media with high yields [25]. In addition, Banfi et al. have
tions. Therefore, the development of innovative Ugi reactions is shown that by using levulinic acid in multicomponent reactions
crucial for the synthesis of novel chemical libraries for various and post-multicomponent reactions, diversities of bicyclic drug-
purposes [12].
like heterocyclic compounds can be obtained [26].
In recent years, one of the modifications for Ugi reactions is Results and Discussion
the introduction of bifunctional substrates into the Ugi In continuation of our interest on the synthesis of novel pseudo-
condensation reaction in order to keep the multicomponent se- peptides [27-29] via multicomponent reactions, herein we in-
quence as short as possible which makes it less complicated vestigate the Ugi-4C-3CR of levulinic acid, aromatic and ali-
[
13-17].
phatic amines and amino acid-based isocyanides. First of all,
racemic α-amino acids such as DL-tryptophan, DL-phenylala-
Levulinic acid or 4-oxopentanoic acid, is an organic compound nine and DL-leucine were used as amine source for the synthe-
which is classified as a ketoacid. It can be easily prepared in sis of isocyanide esters 3 through three sequential reactions
industrial scale and low price by acid catalysis from renewable [30,31]. The first reaction is esterification of the α-amino acid
resources, such as sugars, lignocellulosic biomass and waste using thionyl chloride in methanol as reagent and solvent. The
materials [18]. It can be used as a bifunctional precursor for the second reaction is the formylation of the corresponding amino
synthesis of pharmaceuticals, plasticizers, and different addi- acid ester salt with ethyl formate in the presence of NaHCO3.
tives [19]. Furthermore, it is recognized as an excellent starting Finally, the formamide group was transformed to the corre-
material for Ugi reactions because it has two functional groups sponding isocyanide 3 using POCl3 and triethylamine
in its structure. By the way, using bifunctional chemicals in Ugi (Scheme 1).
four-component condensation reaction (4CC) converts it to an
Ugi three-component condensation reaction (3CC) and this is The prepared amino acid-based isocyanides 3 were applied in
identified as an Ugi-4-centre-3-component reaction (U-4C- an Ugi-4C-3CR (Scheme 2). For this purpose, the reactions of
3
CR) [20-23]. This reaction proceeds through an intramolecu- one equivalent of bifunctional substrate levulinic acid (1), an
lar mechanism which leads to the formation of heterocyclic amine 2 and an isocyanide ester 3 were carried out in methanol
products as a result of a ring-closure process.
as solvent at room temperature to produce the corresponding
pseudo-peptides 4. We observed that under the optimized reac-
In 1998, Ugi et al. reported the intramolecular Ugi-4C-3CR of tion conditions, good to high yields of products were obtained.
ketoacids such as levulinic acid and phthalaldehydic acid with The results are summarized in Figure 1. Various aromatic and
Scheme 1: Synthesis of amino acid-based isocyanides starting from α-amino acids.
Scheme 2: Synthesis of pseudo-peptides using levulinic acid, isocyanide esters and amines.
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Figure 1: Synthesis of functionalized 5-membered lactams using Ugi reaction. aIsolated yield for mixture of diastereomers. bThe ratio of diastereo-
mers was determined by 1H NMR analysis. cIsolated yield of one of the diastereomers as pure stereoisomer after several recrystallization steps.
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Beilstein J. Org. Chem. 2019, 15, 852–857.
aliphatic amines were applied in this protocol to give the corre- The structure of products was confirmed by IR, 1H and
sponding pseudo-peptides. For this purpose, aliphatic amines 13C NMR, CHN and HRMS analyses and by X-ray crystallog-
such as o-chlorobenzylamine and tryptamine and aromatic raphy for 4a. The IR spectra of the derivatives show character-
amines such as aniline, 4-chloroaniline, 4-aminophenol, p-tolui- istic absorbance bands at 3200–3500 cm−1 for the N–H bond
dine, 4-fluoroaniline, and m-anisidine were applied successful- stretching vibration and at 1640–1750 cm−1 for two carbonyls
ly in this protocol. In addition, all three prepared isocyanides of the amide groups and one carbonyl of the ester group. The
from DL-tryptophane, DL-leucine and DL-phenylalanine 1H NMR spectra of the products show a characteristic peak at
worked very well in this reaction to provide the corresponding 6–7 ppm for the amide hydrogen and a peak as multiplet at
pseudo-peptides 4 containing γ-lactam, amide and ester func- 4.40–5.00 ppm for the CH group in the stereogenic center.
tional groups in a single structure.
Carbons of the amide and ester moieties were observed around
70–177 ppm in 13C NMR spectra. In addition, the structure of
1
By starting from DL-amino acids, the corresponding racemic compound (R*,S*)-4a was confirmed by single crystal X-ray
isocyanides were obtained. By using the racemic isocyanides in diffraction and an ORTEP representation is shown in Figure 2
the Ugi-4C-3CR, a mixture of diastereomers were obtained in (CCDC 1896942); for details of the crystal structure data and
approximately 1:1 ratio (see NMR spectra in Supporting Infor- refinement of (R*,S*)-4a see Supporting Information File 1).
mation File 1). Attempts to find a suitable procedure for separa-
tion of the diastereomers without losing the yield was not suc-
cessful. In the case of 4a, one of the diastereomers [(R*,S*)-4a]
was obtained as a pure compound in low yield (20%) after
several recrystallization steps from MeOH.
A proposed mechanism for this reaction is depicted in
Scheme 3. The primary step in the mechanism is the condensa-
tion reaction of the carbonyl group of levulinic acid with an
amine component that leads to the formation of an imine inter-
mediate A. The formed Schiff base is in equilibrium with its
iminium cation B as a result of an intramolecular proton
exchange with the carboxylic acid moiety which activates the
iminium ion for the nucleophilic addition of isocyanide. Conse-
quently, the electrophilic centre of the iminium ion in B is sub-
jected to a nucleophilic attack of the isocyanide to furnish the
intermediate C. Then, a second nucleophilic addition takes
place at this nitrilium intermediate C with an intramolecular
nucleophilic addition of the carboxylate anion. The final step is
Figure 2: ORTEP representation of compound (R*,S*)-4a with ther-
mal ellipsoids at 50% probability. Opposite enantiomer is omitted for
clarity. The atom numbering does not follow IUPAC nomenclature.
an acyl transfer from oxygen to nitrogen (Mumm rearrange- Conclusion
ment) in D which completes the Ugi reaction accompanied by In conclusion, we have synthesized a novel category of pseudo-
formation of the corresponding bis-amides.
peptides containing γ-lactam, amide and ester moieties in a
Scheme 3: Proposed mechanism for Ugi-4C-3CR.
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Beilstein J. Org. Chem. 2019, 15, 852–857.
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as a bifunctional substrate and amines which led to the forma-
tion of novel peptidomimetics with potential biological activi-
ties. In addition, the presence of an ester functional group in the
structure of products makes them suitable substrates for further
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