Efficient activity of magnesium-aluminium hydrotalcite in the synthesis of amides

Article abstract of DOI:10.1039/c3ra42335b

The synthesis of amides by coupling benzotriazole esters and amines can be achieved conveniently in moderate to excellent yields (50-95%) using a commercial, synthesized, calcined or reconstructed Mg-Al hydrotalcite instead of one of the classic bases (tertiary amines). The experimental results demonstrated that commercial and synthesized hydrotalcite can be quantitatively recovered from the reaction by simple filtration and reused for a number of cycles and that the reconstructed hydrotalcite is the most active form for the amide bond formation. Finally, to test the scope of the protocol for the synthesis of biologically relevant molecules, the total synthesis of Sansalvamide A was carried out.

Full text of DOI:10.1039/c3ra42335b

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Efficient activity of magnesium–aluminium hydrotalcite
in the synthesis of amides†‡
Cite this: RSC Adv., 2013, 3, 23046
a
a
´
a
´
´
Jose Antonio Morales-Serna, Miguel Angel Jaime-Vasconcelos,
a
b
´
´
´
Erendira Garcıa-Rıos, Alejandro Cruz, Deyanira Angeles-Beltran,
Leticia Lomas-Romero, Guillermo Enrique Negron-Silva and Jorge Cardenas
c
b
a
*
*
´
´
The synthesis of amides by coupling benzotriazole esters and amines can be achieved conveniently in
moderate to excellent yields (50–95%) using a commercial, synthesized, calcined or reconstructed Mg–Al
hydrotalcite instead of one of the classic bases (tertiary amines). The experimental results demonstrated
that commercial and synthesized hydrotalcite can be quantitatively recovered from the reaction by
simple filtration and reused for a number of cycles and that the reconstructed hydrotalcite is the most
active form for the amide bond formation. Finally, to test the scope of the protocol for the synthesis of
biologically relevant molecules, the total synthesis of Sansalvamide A was carried out.
Received 12th May 2013
Accepted 5th September 2013
DOI: 10.1039/c3ra42335b
www.rsc.org/advances
heterogeneous catalysts that can be used instead of the classic
bases7 has become a major area of research recently. Hydro-
talcites (HTs) are one of the materials that have been used to
Introduction
Carboxylic acid amides1 are an important class of organic
compounds of biological, pharmaceutical and industrial
importance. Further, 25% of known pharmaceuticals contain in
their structures carboxylic acid amides,2 which play signicant
roles in biological processes. As a consequence of their biolog-
ical importance, acylation reactions of carboxylic acids with
amines to form amides represent one the most important and
commonly employed transformations in organic synthesis.3
Large scale processes involving acylation reactions are used in
the synthesis of products including high-value ne chemicals,
such as peptides, polymers, plasticizers, pheromones,
fragrances and organocatalysts, which require mild and strin-
gent fabrication protocols.4 In general, synthetic methods for
amide bond formation5 utilize the acylation of amines with
activated carboxylic acids or enzymatic reactions.1
replace the classic bases in the synthesis of ne chemicals and
can be easily prepared, handled and recycled. Hydrotalcites
3+
[M2+
M
x(OH)2]x+[Anꢀx/n$mH2O]xꢀ are hydrated aluminium–
1ꢀx
magnesium hydroxides with a lamellar structure, in which the
excess of positive charge, originating from the Mg2+ to Al3+
substitution, is compensated for by carbonate anions in the
interlayer space.8 In these compounds, also known as anionic
clays, the layers are built up by the condensation of octahedral
MO6 units (M ¼ Mg2+ or Al3+), as in brucite [Mg(OH)2]. There-
fore, hydrotalcite has OH groups that are shared by three
octahedral cations and point towards the interlayer space.
With these features in mind, and as part of our research
program into the use of hydrotalcite in organic reactions9 we
investigated the synthesis of carboxylic acid amides from
carboxylic acids and amines in the presence of coupling
reagents, replacing the classic tertiary amine bases with
hydrotalcites, because of the following advantages: ease of
separation of the products, reduction of waste, possible regen-
eration of the hydrotalcite and low cost. Thus, in the present
paper we describe a highly efficient protocol for the synthesis of
amides using benzotriazole esters10 as the activated carboxylic
acids11 in the presence of hydrotalcite, demonstrating the effi-
ciency of these materials by high reaction yields. It is worth
noting that recently hydrotalcite supported nano-gold has been
reported as an efficient heterogeneous system for the dehy-
drogenative synthesis of an amide from an alcohol and amine.12
However, that process involves a catalytic oxidative amidation
in the presence of a metal, while this process is a condensation
of an amine with a carboxylic acid via an active ester. Finally, we
used the protocol in the total synthesis of Sansalvamide A to
Some of those protocols use stoichiometric quantities of
bases, principally tertiary amines, which are associated with the
generation of high quantities of waste products and expensive
end-products.6 In this context, the development of eco-friendly
a
´
´
´
Instituto de Quımica, Universidad Nacional Autonoma de Mexico, Circuito Exterior,
´
´
Ciudad Universitaria, Mexico D.F. 04510, Mexico. E-mail: rjcp@unam.mx; Fax: +52
55 5616 2217; Tel: +52 55 5622 4413
b
´
´
Departamento de Ciencias Basicas, Universidad Autonoma Metropolitana, Av. San
´
´
Pablo No. 180, Mexico D.F., C.P. 02200, Mexico. E-mail: gns@correo.azc.uam.mx;
Fax: +52 55 5318 9000; Tel: +52 55 5318 9593
c
´
´
Departamento de Quımica, Universidad Autonoma Metropolitana, Av. San Rafael
´
´
Atlixco No. 186, Mexico D.F., C.P. 09340, Mexico. E-mail: llr@xanum.uam.mx; Fax:
+52 55 5318 9000; Tel: +52 55 5318 9593
† Dedicated to Professor Sergio Castillon on the occasion of his 60th birthday.
´
‡ Electronic supplementary information (ESI) available: Experimental details and
characterization data of all compounds including 1H and 13C spectra for
compounds 3–9. See DOI: 10.1039/c3ra42335b
23046 | RSC Adv., 2013, 3, 23046–23050
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demonstrate its synthetic value. Notably, previous works have memory effect,18 from the calcined sample by rehydration in a
demonstrated the efficiency of clay13 and alumina14 for the liquid or gas phase generates Brønsted base sites between the
elongation of oligopeptide chains in the absence of coupling new layers, where carbonate anions are substituted by hydroxyl
reagents. However, this report represents the rst time that the anions. These reconstructed layered materials exhibit different
elongation of a peptide chain has been carried out in the morphologies and textural properties19 compared with their
presence of hydrotalcite and coupling reagents.
parent layered double hydroxide materials and oen demon-
strate higher catalytic activity than the Mg–Al mixed oxides in a
variety of reactions such as aldol condensations,20 Knoevenagel
and Claisen–Schmidt condensations,21 Michael additions,22 and
the transesterication of tributyrin with methanol.23
Results and discussion
Initially, we optimized typical reaction parameters including
the mole ratio of a commercial hydrotalcite (HTs Aldrichꢀ,
Mg6Al2(CO3)(OH)16$4H2O), temperature and solvents, using
N-Boc-leucine (1 mmol), L-alanine methyl ester hydrochloride
(1 mmol), N-(3-dimethylaminopropyl)-N0-ethylcarbodiimide
hydrochloride (EDC; 1.1 mmol) and 1-hydroxybenzotriazole
(HOBT; 1.1 mmol) as the model substrates (Scheme 1). Preliminary
reactions were carried out to identify the best concentration of
hydrotalcite, with CH2Cl2 as the solvent at room temperature.
Among the various mole ratios of hydrotalcite that we screened,
the most effective concentration of hydrotalcite was 250 mg per 1
mmol of N-Boc-leucine 1 and 1 mmol of L-alanine methyl ester
hydrochloride 2 giving peptide 3 in 95% yield at room tempera-
ture. Similar results were obtained when we carried out the reac-
tion using CHCl3, THF and DMF as solvents. It is worth noting that
The hydrotalcites investigated in the present study (Mg2+–
Al3+ in a ratio of x ¼ 0.33) were characterized by XRD, FTIR and
BET methods. The obtained XRD patterns are shown in Fig. 1.
The synthesized hydrotalcite sample exhibited Mg–Al hydro-
talcite reections associated with the layered double hydroxide
crystal structure. The maxima correspond to typical diffractions
by the (0 0 3) and (0 0 6) planes (Fig. 1, blue line). Calcination of
hydrotalcite gives a Mg(Al)O mixed oxide with a periclase-like
structure with only two reections corresponding to the (4 0 0)
and (4 4 0) planes (Fig. 1, red line). Rehydration of hydrotalcite
in the liquid phase regenerates the layered structure as a
consequence of the memory effect (Fig. 1, green line), however it
is less crystalline than the synthesized hydrotalcite as can be
observed by comparing the intensity of the reections (Fig. 1).
The adsorption of N2 (BET method) was used to quantify the
surface area and pore size and the results are summarized in
Table 1. As shown in Fig. 2 the isotherms of each material
closely resemble a type II isotherm; nitrogen uptake mono-
tonically increases with p/po values due to sorption in the HT
mesopores and the hysteresis loops characteristic of type H III,
which describes materials with different adsorption and
desorption behaviours that are usually solids consisting of
aggregates or agglomerates of particles that form slit-like
structures.24 The synthesized hydrotalcite sample had a low
porosity, pore volume and surface area (Table 1, entry 1). The
surface area of the calcined hydrotalcite is higher as normally
occurs and the reconstructed material obtained from the
calcined hydrotalcite has a lower surface area than the calcined
hydrotalcite but larger cavities are formed aer the structural
reconstruction.
1
no epimerization products were observed in the H NMR spectra
and HPLC chromatogram of the crude reaction mixtures (see
ESI‡). Additionally, we observed that hydrotalcite can be easily
recovered and reused for at least three cycles with signicant
decreases in yield (1st run 92%; 2nd run 80% and 3rd run 75%).
Aer the third reaction the yield decreased notably by 20%.
At this point, we considered the possibility of using hydro-
talcite synthesized by our research group. The principal idea
was to study the behavior of synthesized, calcined, and recon-
structed hydrotalcites, in the formation of a peptide bond and
then to compare these results with those obtained using the
commercial hydrotalcite. By heating at a temperature of 500 ꢁC,
the hydrotalcite is converted into a homogeneous mixed oxide
of Al and Mg with a very small crystal size, high thermal stability
and large surface area. The basic property of the mixed oxide
comes from the oxygen anions and depends on the Mg–Al
ratio15 in the hydrotalcite precursor as well as the preparation
method.16 We have previously studied both types of basic solid
by infrared spectroscopy and demonstrated that there is a
variation in the distribution of the carbonate species between
the synthesized and calcined samples; the synthesized hydro-
talcite had 37% of monodentate carbonates and 63% of the
bidentate species. The calcined one had 26% of monodentate
carbonates and 74% of the bidentate species.17 The recon-
struction of the layered hydrotalcite, using the so-called
Scheme 1 Synthesis of an amide in the presence of hydrotalcite.
This journal is ª The Royal Society of Chemistry 2013
Fig. 1 Powder X-ray diffraction patterns of the hydrotalcites.
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Table
1
The nitrogen adsorption–desorption analysis parameters of the
Table 2 Synthesis of Sansalvamide A
materials
Entry
1
Product
Hydrotalcite (mg)
Yielda (%)
Pore volume Pore size
Entry Hydrotalcite
SBET (m2 gꢀ1
)
(cm3 gꢀ1
)
(A)
d003 (A)
˚
˚
Dipeptide 5
Commercial
Synthesized
Calcined
Reconstructed
Commercial
Synthesized
Calcined
Reconstructed
Commercial
Synthesized
Calcined
Reconstructed
Commercial
Synthesized
Calcined
90
90
92
95
91
92
95
95
88
86
90
90
90
90
92
95
65
60
65
65
1
2
3
Synthesized
Calcined
Reconstructed 95
44
176
0.36
0.73
0.53
165
85
175
7.58
—
7.64
2
3
4
5
Tripeptide 6
Tetrapeptide 7
Pentapeptide 8
Macrolactoneb 9
Reconstructed
Commercial
Synthesized
Calcined
Reconstructed
a
Yield of isolated product aer chromatographic purication. Yield of
b
isolated product aer the two reaction steps. Reagents and conditions
for the macrolactonization reaction: u-hydroxy acid 8 (1 mmol), EDC
(1.1 mmol), HOBT (1.1 mmol), CHCl3 (50 mL), reux, 18 h.
Fig. 2 Nitrogen adsorption–desorption isotherms of the hydrotalcites.
In order to compare the efficiency of these hydrotalcites in
the amide bond formation, catalyst screening was carried out
using different concentrations of the hydrotalcite: synthesized,
calcined and reconstructed (see ESI‡). The synthesized hydro-
talcite and commercial hydrotalcite exhibited similar behav-
iours. The calcined hydrotalcite is less active than the
reconstructed hydrotalcite. However, these hydrotalcites are
more active than the synthesized hydrotalcite and
the commercial hydrotalcite (HTs Aldrichꢀ). These results
conrm previous observations reported in the literature about
the higher catalytic activity of reconstructed hydrotalcite in
organic reactions.18,23 The synthesized hydrotalcite was recov-
ered and reused in at least three cycles with signicant
decreases in yield (1st run 80%; 2nd run 75% and 3rd run 60%).
Unfortunately, attempts to reuse the reconstructed hydrotalcite
have not been successful.
To determine whether our methodology is useful for the
synthesis of complex molecules, we synthesized Sansalvamide A
(9), which is a cyclic pentadepsipeptide isolated from the
mycelium of a fungus of the genus Fusarium, which is collected
from the surface of the seagrass Halodule wrightii that is found
on San Salvador Island in the Bahamas. This cyclic depsipeptide
has an IC50 value of 27.4 mg mLꢀ1 against the National Cancer
Institute's 60-cell-line panel and an in vitro IC50 value of 9.8 mg
mLꢀ1 toward HCT-116 colon carcinoma cells.25 Furthermore, it
is an inhibitor of virus-encoded type-1 topoisomerase, which is
likely to be necessary for the replication of the Molluscum con-
tagiosum virus (MCV).26 With this stimulating biological back-
ground and the complications in obtaining Sansalvamide A
from natural sources, Silverman’s27 and McAlpine’s28 groups
have focused on synthesizing Sansalvamide analogues and
evaluating their antitumor activities. In the same context,
Jiang29 has developed a ionic-liquid-supported total synthesis of
this peptide analogue. Herein, the total synthesis of this cyclic
Scheme 2 Synthesis of Sansalvamide A.
23048 | RSC Adv., 2013, 3, 23046–23050
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pentadepsipeptide is described in solution. The enlargement of
the chain from 2-hydroxy-4-methylpentanoic acid 4,30 was
carried out by using the corresponding amino acid methyl ester
hydrochlorides, EDC, HOBT and commercial, synthetic,
calcined or reconstructed hydrotalcite as the base. Thus, the
residues of Val, Leu, Phe, and Leu were present in the u-hydroxy
acid 8 (Scheme 2). Finally, the macrolactonization of the u-
hydroxy acid 8 was carried out under conditions previously
reported by our group.9 In the four coupling reactions (Table 2,
entries 1–4) and macrolactonization to obtain Sansalvamide A
(Table 2, entry 5), the behaviour of the hydrotalcites was similar
to that described above. Again, the reconstructed hydrotalcite
was more active than the other hydrotalcites. The nal product
9 had a mp of 143–145 ꢁC (ref. 25, mp 143–152 ꢁC) and an [a]D of
ꢀ116 (c 0.001 in MeOH) (ref. 25, [a]D ꢀ115 (c 0.001 in MeOH)).
From these results, we consider that hydrotalcites can be very
useful for the elongation of peptide chains.
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Conclusions
In summary, the present study demonstrates the capacity of
hydrotalcite to replace typical tertiary amine bases in the
synthesis of amides. Compared with the traditional protocol,
this method possesses at least four advantages: (i) the hydro-
talcite can be easily removed from the reaction mixture, (ii)
reduction of waste, (iii) possibility for the reuse of the hydro-
talcite and (iv) low cost of the process. In addition, this study
demonstrates that hydrotalcite reconstructed using the memory
effect is the most active form for this amide bond formation.
Finally, considering the biological importance of peptides, the
use of hydrotalcites in the elongation of oligopeptide chains
introduces an attractive alternative for their synthesis.
´
´
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9 J. Cardenas, J. A. Morales-Serna, E. Sanchez, R. Gavino,
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Acknowledgements
LLR, GENS and JC wish to acknowledge SNI-CONACyT for the
distinction of their membership and the stipend received. We
´
~
wish to thank Ruben Gavino, for the determination of the NMR
spectra.
¨
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