Bis-amino methylation for the synthesis of spiro-fused piperidines using iron(III) trifluroacetate in aqueous micellar medium

Article abstract of DOI:10.1007/s11164-015-2363-0

An environmentally benign, multicomponent integrated chemical process has been developed for the synthesis of 3,5-dispirosubstituted piperidines by cyclo-condensation reaction of amines, formaldehyde, and dimedone using iron(III) trifluroacetate [Fe(F₃CCO₂)₃] Lewis acid in aqueous micellar medium at ambient temperature (2530 °C). The heterogeneous solid acid catalyst conveniently promotes this double amino methylation reaction in which six molecules condense in one pot to form six new covalent bonds. The synthesized 3,5-dispirosubstituted piperidines have been screened for their in vitro antibacterial activity using agar well method. Many of these compounds showed satisfactory antibacterial activity as compared to standard drugs against all the bacteria tested.

Full text of DOI:10.1007/s11164-015-2363-0

Res Chem Intermed
DOI 10.1007/s11164-015-2363-0
Bis-amino methylation for the synthesis of spiro-fused
piperidines using iron(III) trifluroacetate in aqueous
micellar medium
Trushant Lohar1 Sanjay Jadhav1 Arjun Kumbhar1
•
•
•
Ananda Mane1 Rajashri Salunkhe1
•
Received: 1 August 2015 / Accepted: 12 November 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract An environmentally benign, multicomponent integrated chemical pro-
cess has been developed for the synthesis of 3,5-dispirosubstituted piperidines by
cyclo-condensation reaction of amines, formaldehyde, and dimedone using iron(III)
trifluroacetate [Fe(F3CCO2)3] Lewis acid in aqueous micellar medium at ambient
temperature (25–30 °C). The heterogeneous solid acid catalyst conveniently pro-
motes this double amino methylation reaction in which six molecules condense in
one pot to form six new covalent bonds. The synthesized 3,5-dispirosubstituted
piperidines have been screened for their in vitro antibacterial activity using agar
well method. Many of these compounds showed satisfactory antibacterial activity as
compared to standard drugs against all the bacteria tested.
Keywords Multicomponent reactions Á Iron(III) trifluroacetate Á
3,5-Dispirosubstituted piperidines Á Aqueous SDS micellar medium
Introduction
Multi-component reactions (MCRs) become one of the fundamental aspects of
‘‘Green Chemistry’’ since, it involves construction of novel and structurally
important complex molecules. It combines several starting materials in a single pot
ensuring high atom economy, good yields with low costs, minimizing waste, labor,
and energy, short reaction time, simple work-up, and purification of products [1].
Therefore, MCRs synthetic strategy are currently very popular in organic,
medicinal, and combinatorial chemistry for rapid and efficient generation of
different biologically active compounds [2].
&
Rajashri Salunkhe
rsschem1@gmail.com
1
Department of Chemistry, Shivaji University, Kolhapur, M.S. 416004, India
123

T. Lohar et al.
It is well known that spiro-fused piperidine represents one of the most ubiquitous
heterocyclic motifs and is present in many naturally occurring alkaloids, animal
toxins, biologically active synthetic molecules, and organic fine chemicals [3–5].
These heterocyclic scaffolds possess broad biological activities such as anti-
hypertensive, anti-convulsant, anti-inflammatory, anti-HIV-1, analgesic, leishmani-
cidal, antimicrobial, and cytotoxic activities [6–12]. The neurotoxic 2-spiro-fused
piperidines were isolated from the neotropical poisonous frog [13–16] and their
3-spirofused analogues alkaloids such as sibirine, nitramine, isonitramine, and
nitrabirine were isolated from plants of the genus Nitraria [17–19].
Conventionally, 3,5-dispirosubstituted piperidines are synthesized by using
several modified procedures such as FeCl3 in DCM [20], Fe3O4 nanoparticles [21],
silica supported tungstic acid in DCM [22], In(OTf)3 in ethanol [23], and in ethanol
solvent only [24]. Although some of these reported methods afford good to high
yields of the corresponding products, many of these methods suffer from various
disadvantages such as long reaction time, moisture sensitivity, high cost, and toxic
reagents. Hence, it is necessary to develop an efficient and convenient method to
construct these biologically active heterocyclic compounds.
The classical Lewis acids such as FeCl3, AlCl3, BF3, ZnCl2, TiCl4 , and SnCl4
used in organic synthesis are susceptible to fast hydrolysis by means of atmospheric
moisture and accordingly should be treated strictly in dry conditions to avoid the
loss of catalytic activity. Quite often, these acids are required in stoichiometric
amounts, are not reusable, and lead to secondary reactions, making the reaction
work-up and product isolation process tedious. Because of the increasing awareness
of environmental problems in chemical research and industry, the search for a more
efficient Lewis acid catalyst still continues.
Iron trifluroacetate [Fe(CF3CO2)3] is a non-hygroscopic, non-corrosive, stable,
commercially available Lewis acid with high catalytic activity, economic viability,
and easy recoverability from water. It has been used as a catalyst for solvolytic and
non-solvolytic nucleophilic ring opening of epoxides [25], O-silylation of a-
hydroxyphosphonates, alcohols, and phenols [26], synthesis of 3,4-dihydropyrim-
idinones and 1,4-dihydropyridines [27].
From a viewpoint of ecological advantage and greenness of water, it is of
considerable interest to perform organic reactions in water because water is
abundant in nature, has virtually no cost, and is safest among the available solvents,
thus leading to environmentally benign chemical processes [28]. Recently, some
attempts to achieve organic reactions in aqueous media have been performed and
consequently some successful examples have appeared in the literature [29, 30]. The
use of water in organic reactions has serious limitations. Most organic compounds
are immiscible in water and many are reactive, while catalysts, reagents, and
products are decomposed or deactivated in aqueous media. To overcome these
drawbacks, surface active compounds (surfactants) can be used successfully in the
water, which solubilize organic materials or form emulsions with them in water
[31]. The solubilization of water-insoluble reactants and products inside the
micelles results not only in high concentration within the small volume, but also in
different orientations of the soluble molecules that influence the reaction
123

Bis-amino methylation for the synthesis of spiro-fused…
mechanism, resulting in remarkable differences in the reaction rate and selectivity
that would be observed in a homogeneous system [32].
Considering the significance of anionic sodium dodecyl sulphonate surfactant
(SDS) [33] and in continuation of our research work to develop new and convenient
synthetic protocols for the construction of bioactive heterocycles [34], we aim to
investigate the efficiency of [Fe(F3CCO2)3] solid Lewis acid for the synthesis of 3,5-
dispirosubstituted piperidines in aqueous SDS micellar medium.
Results and discussion
Initially, to establish the feasibility of the strategy and to optimize the reaction
conditions, reaction of dimedone, formaldehyde, and 4-chloroaniline as a model
substrate was investigated (Scheme 1).
The reaction was carried out in 10 mol% aqueous SDS micellar medium, only
30 % of resulting product obtained after 10 h at room temperature (Table 1, entry
1). To improve the yield, a representative selection of iron(III) metal salts including
FeCl3, Fe(NO3)2, Fe(OH)3, and [Fe(CF3CO2)3] were tested in aq. SDS micellar
medium. Among the tested metal salts, [Fe(CF3CO2)3] was found to be a more
efficient and effective Lewis acid for this bis-amino methylation (Table 1, entries
2–5). The promising activity of iron(III) trifluoroacetate [Fe(CF3CO2)3] as
compared to other metal complex salts can be explained on the basis of the
spectrochemical series and hard-soft acids and bases (HSAB) principle [23].
Possibly, the trifluoroacetate anion coordinates with enol form of dimedone better
than other ions such as Cl-, NO3-, and OH- ions, which enhances the rate of
reaction.
With [Fe(CF3CO2)3] as catalyst, we next investigated the effect of solvents and
found that aq. SDS micellar medium was superior to organic solvents such as DCM,
CHCl3 , and acetonitrile (Table 1, entries 6–8). Moreover, in protogenic solvents
such as water and ethanol, protonation of amines prior to attack may slow the
nucleophilicity of the amine moiety, thus resulting in lower yields (Table 1, entries
9, 10). Performing the reaction with a higher catalyst loading 10 mol%
[Fe(CF3CO2)3] catalyst had no significant effect on yield. However, if the amount
of the catalyst was reduced to 2 and 1 mol%, the product yield was reduced to 76
and 72 %, respectively (Table 1, entries 11, 12). To our delight, the reaction worked
Cl
NH2
Cl
O
Iron(III)
N
trifluroacetate
O
O
+
3H-CHO +
2
O
aq. SDS, R.T
O
O
Scheme 1 Synthesis of 3,5-dispirosubstituted 4-chloro-phenyl piperidine
123

T. Lohar et al.
Table 1 Optimization of reaction conditions for the synthesis of 3,5-dispirosubstituted 4-chloro-phenyl
piperidine
Entry
Solvent
Catalyst
Mol (%)
Time (h)
Yielda (%)
1
Aq. SDS
Aq. SDS
Aq. SDS
Aq. SDS
Aq. SDS
CH2Cl2
CHCl3
–
10
5
5
5
5
5
5
5
5
5
2
1
10
30
80
70
68
86
80
78
75
75
68
76
72
2
FeCl3
5.5
6.0
6.5
4.5
5.5
5.5
6.0
6.0
6.5
5.5
5.5
3
Fe(NO3)3
4
Fe(OH)3
5
[Fe(CF3CO2)3]
[Fe(CF3CO2)3]
[Fe(CF3CO2)3]
[Fe(CF3CO2)3]
[Fe(CF3CO2)3]
[Fe(CF3CO2)3]
[Fe(CF3CO2)3]
[Fe(CF3CO2)3]
6
7
8
CH3CN
C2H5OH
Water
9
10
11
12
Aq. SDS
Aq. SDS
Reaction conditions: 4-chloroaniline (1 mmol), dimedone (2 mmol), formaldehyde (3 mmol) stirred at
R.T.
very well in the presence of 10 mol% SDS under the influence of 5 mol% iron(III)
trifluroacetate catalyst in water that led to the desired condensed adduct in 86 %
isolated yield after 4.5 h (Table 1, entry 5). It seems that the Lewis acid part of the
iron(III) trifluroacetate catalyst activates the substrate molecules and the surfactant
part of SDS increases the solubility of the substrates in water at the same time.
Because of this synergistic effect, a drastic enhancement of the reaction rate and
yield of the product was observed.
With these encouraging results in hand, we turned to explore the scope of the
reaction using different amines as substrate under the optimized reaction conditions
(Table 2). It was observed that aromatic amines with electron-donating as well as
electron-withdrawing groups reacted successfully to furnish the final product in
good to excellent yields, while the presence of a strong electron-withdrawing group
such as –NO2 on the phenyl ring results in lower yield (Table 2, 4f). o-nitro aniline
does not yield a spiro product due to relatively weak nucleophilicity and, therefore,
decreased activity in the Mannich step of the reaction (Table 2, 4k). Aliphatic
amines such as n-butyl amine afforded a corresponding product in 82 % (Table 2,
4i), but cyclohexyl amine does not yield any spiro adduct after 24 h due to its ring
strain effect (Table 2, 4j). It should also be noted that sterically hindered
1-naphthylamine (Table 2, 4l) did not give the corresponding spiropiperidine.
From the mechanistic point of view, a plausible mechanism for the formation of
target compound is outlined in (Scheme 2) [21]. The spirocyclization looks to
proceed as a domino sequence of Knoevenagel, Michael, and double Mannich
reactions. The well-known reaction of dimedone with formaldehyde leads to the
formation of the standard dimedone–formaldehyde adduct. This undergoes two
consecutive Mannich reactions with aniline to produce the bis-spiro piperidine. Here
123

Bis-amino methylation for the synthesis of spiro-fused…
Table 2 Synthesis of 3,5-dispirosubstituted piperidines in aqueous SDS medium (4a–l)
R
O
1
NH2
Iron(III)
O
N
O
trifluroacetate
2
+
3H-CHO +
2
aq. SDS, R.T
O
R
O
O
(3a-l)
(4a-l)
Cl
N
OMe
N
Me
N
O
O
O
O
O
O
N
O
O
O
O
O
O
O
O
O
O
(4a, yield: 86 %)
(mp:190-1920C)
(4b, yield: 88 %)
(mp:195-1980C)
(4c, yield: 88 %)
(mp:202-2040C)
(4d, yield: 87 %)
(mp:198-2000C)
P
h
NO2
Br
N
O
O
O
N
O
N
N
O
O
O
O
O
O
O
O
O
O
O
O
(4e, yield: 81 %)
(mp:188-1900C)
(4f, yield: 75 %)
(mp:220-2220C)
(4g, yield: 80 %)
(mp:196-1980C)
(4h, yield: 82 %)
(mp:200-2020C)
Bu-n
NO2
N
O
O
N
O
O
N
O
N
O
O
O
O
O
O
O
O
O
O
O
(4i, yield: 82 %)
(m.p: 210-2120C)
(4j, n.r.)
(4k, n.r.)
(4l, n.r.)
Reaction conditions: Formaldehyde (3 mmol), amines (1 mmol), dimedone (2 mmol), 5 mol % Iron (III)-
trifluroacetate, 10 mol % aq. SDS micellar medium at R.T
n.r.: No Reaction
the acidic nature of iron(III) trifluroacetate facilitates both the enolization steps of
the dimedone.
Recycling of the catalyst is important for the large-scale operation and industrial
point of view. To check the possibility of the catalyst recycling, reaction of
4-chloroaniline, formaldehyde, and dimedone in the presence of 5 mol%
123

T. Lohar et al.
Fe3+
O
Fe3+
Fe3+
Fe3+
Fe3+
Fe3+
Fe3+
O
O
O
O
O
O
O
O
O
Michael addition
H
3+Fe
O
+
H
H
R
R
O
O
+3Fe
Fe3+
N
O
O
NH
O
O
Mannich reaction
Imine
Mannich reaction
O
O
O
H
H
O
O
O
O
Scheme 2 Plausible mechanism of 3,5-dispirosubstituted piperidine ring formation
[Fe(CF3CO2)3] in aq. SDS micellar medium was studied. In the model reaction,
after product isolation by simple filtration, the filtrate containing catalyst was further
applied by taking the same reactants in the aqueous phase to perform five successive
catalytic runs. The results in Table 3 shows that the catalytic system could be used
five times with only a slight decrease in the catalytic activity.
The synthesized 3,5-dispirosubstituted piperidines were screened for their in vitro
anti-bacterial activity against the growth of Gram negative bacteria and Gram
positive bacteria by using agar well diffusion and agar broth. As shown in the
Tables 4 and 5, the tested compounds showed moderate antibacterial activity
compared to standard drugs against each microorganism.
On the basis of zone of inhibition and MIC test against tested bacterial pathogens,
compound 4c shows good antibacterial activity against both Gram positive and
Gram negative bacterial pathogens at 30 lg/mL. Also, compound 4g possess good
activity against Gram negative S. aureus at 30 lg/mL. The remaining compounds
possess slightly moderate activity compared to the standard drug ampicillin.
The reason for different sensitivities between Gram positive and Gram negative
bacteria could be ascribed to the morphological difference between these micro-
organisms. The Gram positive bacteria had been found to be more susceptible since
they have only an outer peptidoglycan layer, which is not an effective permeability
barrier [35]. Gram negative bacteria have an outer phospholipidic membrane
carrying the structural lipopolysaccharide components. This makes the cell wall
Table 3 Reusability of catalyst
in the model reaction
Entry
No. of cycles
Yield (%)a
1
2
3
4
5
1
2
3
4
5
86
84
81
80
79
a
Yields are related to isolated
pure products
123

Bis-amino methylation for the synthesis of spiro-fused…
Table 4 Antibacterial activity of 3,5-dispirosubstituted piperidine derivatives
Entry
Compound
Zone of inhibition (mm)
S. aureus
E. coli
50 (lg/mL)
100 (lg/mL)
50 (lg/mL)
100 (lg/mL)
1
2
3
4
5
6
7
8
9
10
a
4a
2a
2
5
4
1
–
3
–
4b
4c
3
7
4
9
4d
1
3
1
4
4e
–
1
–
–
4f
2
4
2
4
4g
4
9
–
–
Standard amphicilin
Solvent (ethanol)
Control
22
–
20
–
23
–
20
–
–
–
–
–
Zone inhibition values are given in millimeters, ‘–’ no inhibition
Table 5 MIC values of
Entry
Compound
S. aureus
MIC
E. coli
MIC
compounds against Gram
positive and Gram negative
bacteria
(lg/mL)
(lg/mL)
1
2
3
4
5
6
7
8
9
4a
40
40
30
50
100
40
30
10
–
50
–
4b
4c
30
50
–
4d
4e
4f
40
–
4g
Standard amphicilin
Control
10
–
‘–’ no inhibition
impermeable to lipophilic solutes, while porins constitute a selective barrier to the
hydrophilic solutes with an exclusion limit of about 600 Da [36].
Experimental
General
Solvents and reagents were of AR grade commercially sourced from Sigma Aldrich
and used without further purification. Melting points were determined in an open
capillary and are uncorrected. Infrared spectra were measured on a Perkin Elmer
FT-IR spectrophotometer. The samples were examined as KBr discs *5 %w/w.
123

T. Lohar et al.
1
NMR spectra were recorded on a Bruker AC (300 MHz for H NMR and 75 MHz
for 13C NMR) spectrometer using CDCl3 and DMSO as solvents.
General procedure for the synthesis of bis-spiropiperidines (4a-l)
In a round bottom flask, a mixture of dimedone (2 mmol), formaldehyde (3 mmol),
aniline (1 mmol), iron(III) trifluroacetate (5 mol%), and SDS surfactant (10 mol%)
in water (5 mL) was stirred at R.T. The progress of reaction was monitored by thin
layer chromatography. After the completion of the reaction, the formed product was
filtered and recrystallized from ethanol to afford the pure product. Identity of the
synthesized products was confirmed by comparing the physical and spectral data
1
(MS, H NMR, 13C NMR) with those of the compounds reported in literature.
Agar well method for antibacterial activity of synthesized compounds
Antibacterial activity was tested on nutrient agar plates using agar well diffusion
[37]. The organisms used in the present study were obtained from the laboratory
stock. The subculture was prepared by pouring 15–20 mL of molten media into each
sterile nutrient broth plate. The incubated plates were allowed to stand for 15 min.
Using a sterile cork borer, the bores are made and solutions of synthesized
compounds were prepared at different concentrations (lg/mL), and transferred into
the bores. The control was run under similar conditions to get the activity of blanks.
The plates were incubated at temperature of 37 °C. Antimicrobial activity was
evaluated after 24 h by measuring the diameter (mm) of zone of inhibition.
Minimum inhibitory concentration assay
The minimum inhibition concentration assay was performed for the compounds at
10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 lg/mL concentrations. A set of tubes with
multiple concentrations of compounds was prepared in growth medium (agar broth).
The tubes were then inoculated with the microorganisms, incubated for 24 h, and
examined for growth of bacteria. Broth tubes that appear turbid are indicative of
bacterial growth, while tubes that remain clear indicate no growth. Growth seems to
diminish as the concentration of some compounds increase and eventually appeared
to reduce at higher concentrations. The minimum concentration at which no growth
was observed was taken as the MIC value.
Physical and spectral data of representative compounds
Compound (4a)
1
mp: 190–192 °C (lit. 198–200 °C [21]); H NMR (300 MHz, CDCl3, d ppm): 1.00
(12H, s, CH3), 2.51 (2H, s, CH2), 2.70 (4H, d, J = 13.5 Hz, COCH2), 2.92 (4H, d,
J = 13.5 Hz, COCH2), 3.54 (4H, s, NCH2), 7.10 (2H, d, J = 8 Hz, ArH), 7.31(2H,
d, J = 8 Hz, ArH); 13C NMR (75 MHz, CDCl3, d ppm): 28.2, 28.4, 30.7, 32.2,
51.2, 54.2, 65.4, 113.4, 120.4, 131.2, 150.4, 205.6; MS (m/z): 443(M?).
123

Bis-amino methylation for the synthesis of spiro-fused…
Compound (4b)
1
mp: 195–198 °C (lit. 190–192 °C [22]); H NMR (300 MHz,CDCl3 d ppm): 1.00
(12H, s, CH3), 2.31 (2H, s, CH2), 2.52 (4H, d, J = 13.5 Hz, COCH2), 2.83 (4H, d,
COCH2), 3.69 (4H, s, NCH2), 7.00–7.31 (5H, m, ArH); 13C NMR (75 MHz, CDCl3,
d ppm): 28.12, 28.21,30.55, 32.38, 51.20, 65.73, 118.80, 121.60, 129.20, 151.90,
205.80; MS (m/z): 409 (M?).
Compound (4d)
1
mp: 198–200 °C (lit. 196–198 °C [20]); H NMR (300 MHz,CDCl3 d ppm): 1.00
(12H, s, CH3), 2.01 (3H, s, CH3), 2.41 (2H, s, CH2), 2.68 (4H, d, J = 13.5 Hz,
COCH2), 2.87 (4H, d, J = 13.5 Hz, COCH2), 3.51 (4H, s, NCH2), 7.22 (2H, d,
J = 8.1 Hz, ArH), 7.43(2H, d, J = 8.1 ArH); 13C NMR (75 MHz, CDCl3, d ppm):
20.25, 28.55, 28.92, 30.71, 32.35, 51.52, 55.61, 66.12,119.62, 129.35, 131.92,
149.51, 206.81; MS (m/z): 423 (M?).
Compound (4h)
1
mp: 200–202 °C (lit. 202–204 °C [24]); H NMR (300 MHz, CDCl3 d ppm): 0.99
(12H, s, CH3),2.46 (2H, s, CH2), 2.70 (4H, d, J = 13.5 Hz, COCH2), 2.81 (4H, d,
J = 13.5 Hz, COCH2), 3.61 (4H, s, NCH2), 7.10 (2H, d, J = 8.5 Hz, ArH), 7.50 (2H,
d, J = 8.5 Hz, ArH); 13C NMR (75 MHz, CDCl3, d ppm): 28.12, 28.58, 30.65,
32.24, 51.25, 54.83, 65.16, 113.38, 120.61, 131.33, 150.93, 205.26; MS (m/z): 487
(M?).
Conclusion
In summary, we have developed an efficient synthesis of 3,5-dispirosubstituted
piperidines of antibacterial profiles through domino sequence of Knoevenagel,
Michael, and double Mannich reactions. The combination of highly stable and
reusable iron(III) trifluroacetate Lewis acid with aq. SDS micellar medium has
proven to be an effective catalytic system in contrast to other iron(III) metal salts for
the synthesis of bis-spiro piperidines. This methodology is endowed with several
advantages such as an inexpensive as well as nontoxic catalyst, easy work-up
procedure, use of green micellar medium, and ambient temperature.
Acknowledgments One of the author, Mr. Trushant Lohar is very much thankful to the Department of
Science and Technology (DST) for providing DST-INSPIRE fellowship (IF-140918).
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