Synthesis, characterisation, stereochemistry and biological activity of N-formylpiperidin-4-ones

Article abstract of DOI:10.1016/j.molstruc.2014.05.053

A new series of N-formyl-2,6-bis(4-methoxyphenyl)piperidin-4-ones 5-8 has been synthesized and characterised using IR, mass and ¹H, ¹³C, DEPT and 2D (COSY and HSQC) NMR spectral techniques. The NMR spectral data indicated that the N-

Full text of DOI:10.1016/j.molstruc.2014.05.053

Accepted Manuscript
Synthesis, characterisation, stereochemistry and biological activity of N-for-
mylpiperidin-4-ones
P. Sakthivel, S. Ponnuswamy
PII:
S0022-2860(14)00567-5
DOI:
http://dx.doi.org/10.1016/j.molstruc.2014.05.053
MOLSTR 20652
Reference:
Journal of Molecular Structure
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Received Date:
Revised Date:
Accepted Date:
10 December 2013
21 May 2014
23 May 2014
Please cite this article as: P. Sakthivel, S. Ponnuswamy, Synthesis, characterisation, stereochemistry and biological
activity of N-formylpiperidin-4-ones, Journal of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/
j.molstruc.2014.05.053
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Synthesis, Characterisation, Stereochemistry and Biological
Activity of N-formylpiperidin-4-ones
P. Sakthivel and S. Ponnuswamy*
P. G. & Research Department of Chemistry, Government Arts College (Autonomous), Coimbatore 641 018,
Tamil Nadu, India
* Corresponding author. E-mail address: kspons2001@gmail.com
Mobile number: +919244645744
Abstract
A new series of N-formyl-2,6-bis(4-methoxyphenyl)piperidin-4-ones 5-8 has
1
been synthesised and characterised using IR, mass and H, 13C, DEPT and 2D
(COSY and HSQC) NMR spectral techniques. The NMR spectral data indicated that
the N-formylpiperidin-4-ones 5-8 prefer to exist in a conformational equilibrium
between a syn rotamer with a twist boat conformation (TB1) and an anti rotamer with a
twist boat conformation (TB2) in solution. The stereodynamics of these systems have
been studied by recording the dynamic 1H NMR spectra of compound 5, and the energy
barrier for the N-CO rotation was determined to be 64.3 kJ/mol. All of the synthesised
compounds (5-8) were screened for their biological activity.
Keywords :N-formylpiperidin-4-one; NMR spectra; twist boat conformation; syn and anti
rotamers; energy barrier; biological activity
1

1. Introduction
The conformational equilibrium between the syn and anti rotamers based on the
restricted rotation at the N-C bond in several N-acyl derivatives of azacycles is known to
be fast at RT, and their conformations can be drastically different due to the influence of
N-acyl functions [1-22]. Resonance stabilisation in the N-nitroso and N-acetyl
derivatives of cis-2,6-dimethylpiperidine has been observed [23,24]. Competition
between A1,3-strain [25] and resonance stabilisation in these cases leads to dynamic
equilibria due to restricted rotation around the N-N bond of the N-nitroso group or N-C
bond of the N-acetyl group. The ring has been shown to prefer flipped chair
conformation with diaxial methyl groups (Figure 1) [23,24]. However, when the cis-2,6-
substituents are aromatic groups, the rings prefer, twist chair, twist boat or flattened
boat conformations [1-22] even though the flipped chair conformation has also been
observed in a few cases in the solid state [2,22,26-28]. Because the presence of cis-
2,6-diaryl groups in N-acetyl and N-nitrosopiperidines exerts unpredictable
conformational changes [1-22] in the piperidine ring, there are significant differences
ꢀꢁꢂꢃꢁꢁꢄꢅ ꢂꢆꢁꢅ ꢂꢁꢇꢈꢉꢅ ꢈꢄꢊꢋꢌꢁꢄꢉꢁ ꢅ ꢍꢊꢅ ꢎꢄꢅ ꢏ-ꢎꢇꢐꢋꢅ ꢑꢇꢍꢌꢒꢅ ꢎꢄꢓꢅ ꢎꢄꢅ ꢏ-alkyl group on the
conformational preferences of 2,6-disubstituted piperidines containing heteroconjugate
groups (i.e., groups capable of delocalising the lone pair of electrons on nitrogen, such
as NO, CHO and COMe).
A comparison of the rotational barriers of various 2,6-dimethylpiperidines
containing hetero conjugate groups indicates that the rotational barrier for the N-CHO
derivatives [23,24] was lower than that of N-nitroso derivatives but larger than that of
any other derivatives (e.g., COCH3, COPh and CONHPh) [1-19,29]. The N-formyl
2

derivatives of 2,6-diphenylpiperidines were reported to favour flattened boat
conformations [8].
To study the relative influences of allylic strain, torsional strain, resonance energy
due to the delocalisation of the lonepair electrons on nitrogen ꢂꢍꢅꢂꢆꢁꢅꢉꢎꢇꢀꢍꢄꢐꢋꢅ4-cloud
and 1,3-diaxial strains over the preferred conformations of the piperidine ring, four new
N-formyl-cis-2,6-bis(4-methoxyphenyl)piperidin-4-ones 5-8 were synthesised and their
1
stereochemistry was studied using H, 13C, DEPT and 2D (COSY and HSQC) NMR
1
spectral data and dynamic H NMR spectra. In addition, the antibacterial and
antifungal activities have also been tested.
2. Experimental
2.1 Materials, methods and instruments
All of the reported melting points were taken in open capillaries and are
uncorrected. The IR spectra were recorded using a SHIMADZU FT-IR 88400s
spectrometer using KBr pellets. The 1H NMR and 13C NMR spectra were recorded in a
CDCl3 solution at 243 K with a Bruker AV 300 & 75 MHz, Bruker DRX 500 & 125 MHz
and Bruker (Avance III) 500 & 125 MHz NMR spectrometers, and the chemical shifts
were referenced to TMS. A 0.05 M solution of the sample prepared in CDCl3 was used
for obtaining the 2D NMR spectra. The tubes used for recording the NMR spectra were
5-mm diameter. Electron impact mass spectra were recorded using a JEOL GS mate
spectrometer, and microanalyses were performed on a Carlo Erba 1108 CHN analyser.
Unless otherwise stated, all of the reagents and solvents were of high grade and
purchased from Sigma-Aldrich chemicals, Bangalore, India and Merck chemicals, Worli,
3

Mumbai. All of the solvents were distilled prior to use. The parent piperidin-4-ones
were prepared by following the literature procedure [30-33].
2.2 General procedure for the synthesis of compounds 5-8
An ice-cold solution of acetic-formic anhydride was prepared from acetic
anhydride (10ml) and 85% formic acid (5ml), and this solution was slowly added to a
cold solution of piperidin-4-ones [1: (1.625g, 5mmol), 2, 3 and 4: (1.69g, 5mmol)] in
benzene (30ml) to synthesise compounds 5-8, respectively. The reaction mixture was
allowed to stir at room temperature for 5 hrs. The organic layer was separated, dried
over anhydrous Na2SO4 and concentrated. The resulting product was purified by
crystallisation from benzene-petroleum ether (333-335K) in a 1:1 ratio. The analytical
data of compounds 5-8 are reported in Table 1.
3. Results and discussion
In the IR spectra of the compounds 5-8, the amide >C=O stretching bands were
observed at approximately 1662-79 cm-1. In addition, the ring carbonyl stretching band
was observed at approximately 1704-1716 cm-1.
The NH stretching band at
approximately 3300 cm-1 that was observed for parent compounds 1-4 was absent in N-
formylpiperidin-4-ones 5-8 (Table 2). In the mass spectra, the presence of molecular
ion peaks at m\z 353, 367, 367 and 367 for compounds 5-8, respectively, and their
fragmentation pattern confirmed the structures.
The RT NMR spectra showed the doubling of signals with broadening.
Therefore, all of the NMR spectra were recorded at 243 K for N-formylpiperidin-4-ones
5, 7 and 8, which showed well resolved signals for each of the protons and carbons
4

corresponding to the syn and anti rotamers. For compound 6, a well-resolved doubling
of the NMR signals was observed at 283 K. These observations indicated the existence
of conformational equilibria in compounds 5-8.
1
The H NMR signals of N-formylpiperidin-4-ones 5-8 were assigned based on
their chemical shifts, multiplicities and intensities as well as in comparison with those of
parent piperidin-4-ones 1-4, respectively. Additional COSY spectra were also used to
perform the assignments. The 13C NMR spectral data of compounds 5-8 were assigned
by comparison with its parent amines 1-4, respectively, and within themselves. In
addition, DEPT and HSQC spectra were used for unambiguous assignments.
3.1 Orientation of -N-C=O group
Each of the protons and carbons in N-formylpiperidin-4-ones 5-8 exhibited
1
13
anisochronous nature in their H and C NMR spectra at RT. Doubling of the proton
and carbon signals in their NMR spectra indicated that the delocalisation of the lone pair
of electrons on the nitrogen of compounds 5-8 inꢂꢍꢅꢂꢆꢁꢅꢉꢎꢇꢀꢍꢄꢐꢋꢅ4- cloud is sufficient to
create a substantial double bond character along the -N-C=O bond. This non-
equivalence arises due to the syn and anti orientations of the formyl group. Therefore,
the presence of the anisochronous nature of the proton and carbon signals in
compounds 5-8 indicated the presence of rotational equilibrium between the syn and
anti rotamers in these systems, and the -N-C=O moiety adopts a coplanar orientation
with the C2-N-C6 plane of the piperidine ring.
To confirm the presence of restricted rotation around the N-CO bond, variable
temperature 1H NMR spectra were recorded for formamide 5, and the energy barrier for
5

N-CO rotation (Dꢔ#) was calculated using the modified Eyring equation [34-36]. Dꢔ# =
1.914 × 10-2 × Tc[9.97 + log Tcꢕꢖꢗꢘꢅꢙꢚꢅ ꢍꢋ-1 where Dꢔ# is the free energy of activation, Tc
ꢈ ꢅꢂꢆꢁꢅꢉꢍꢎꢋꢁ ꢉꢁꢄꢉꢁꢅꢂꢁ ꢒꢁꢇꢎꢂꢌꢇꢁꢅ!"#ꢅꢎꢄꢓꢅꢖꢗꢅꢈ ꢅꢂꢆꢁꢅꢉꢆꢁ ꢈꢉꢎꢋꢅ ꢆꢈꢊꢂꢅꢓꢈꢊꢊꢁꢇꢁꢄꢉꢁꢅꢈꢄꢅ$% at Tc.
The change in the shapes of the signals of the benzylic protons was followed (Chart 1)
to calculate the energy barrier for N-CO rotation. The chemical shift difference was
plotted as a function of the temperature, ꢎꢄꢓꢅꢂꢆꢁꢅꢉꢆꢁ ꢈꢉꢎꢋꢅ ꢆꢈꢊꢂꢅꢓꢈꢊꢊꢁꢇꢁꢄꢉꢁꢅ!ꢖꢗ#ꢅꢎꢂꢅthe
coalescence temperature (Tc) was determined by extrapolating the plot. The Tc ꢎꢄꢓꢅꢖꢗꢅ
were determined to be 325 K and 114 Hz, respectively. The energy barrier for the N-
CO rotation in compound 5 was calculated to be 64.3 kJ mol-1. A comparison of the
rotational barriers of N-formylpiperidin-4-one 5 with that of N-nitroso analogue 5 (Dꢔ# =
76.3 kJ mol-1) [37] indicated that the formyl derivatives exhibit lower rotational barriers
than the nitroso derivatives. This result may be due to the greater polarisation of the
carbonyl group than the nitroso group resulting in an increase in the repulsion between
ꢂꢆꢁꢅꢉꢆꢎꢇꢑꢁꢅꢉꢋꢍꢌꢓꢅꢍꢄꢅꢍ&ꢐꢑꢁꢄꢅꢎꢄꢓꢅꢂꢆꢁꢅ4-electrons of the aromatic rings.
3.2 Assignment of the 1H and 13C NMR signals to the syn and anti rotamers
For N-formylpiperidin-4-ones 5-8, when N-C=O is syn to the C2 carbon, it is
designated as a syn rotamer, and when N-C=O is anti to the C2 carbon, it is designated
as an anti rotamer (Figure 2). For N-formyl derivatives 5-7 (unsymmetrical H2+H6), the
H2 and H6 protons are expected to exhibit doubling of the signals, and in compound 8
(symmetrical H2=H6), only two signals are expected for the H2 and H6 protons of the syn
and anti rotamers.
6

To understand the preferred conformation of the individual rotamers (i.e., syn and
anti) the identification of the NMR signals corresponding to the syn and anti rotamers
was necessary (Figures 3, 4 and 5). The assignment of the syn and anti signals could
be better achieved using HSQC spectra. This method is based on the observation that
the syn carbons to the –X=Y functions in compounds containing N-X=Y functions, such
as N–NO, N-COR and,N-CHO are always more shielded than the anti carbons [38-40].
For example, for compound 5, between the two benzylic 13C NMR signals (i.e., 58.1 and
62.8), the signal that was more shielded (58.1) was assigned to the C2 carbon of the syn
rotamer (C2 syn to C=O), and the signal at 62.8 was assigned to the C2 carbon of the
anti rotamer (C2 anti to C=O). Because these two carbon signals at 58.1 and 62.8
correlated with the benzylic doublets at 5.26 and 4.66 in the HSQC spectrum,
respectively, the former signal (5.26) was assigned to the A - proton syn to the formyl
group and the latter signal (4.66) to the anti A - proton. From the benzylic carbons and
protons signals, the C3 and C5 NMR signals were assigned using COSY and HSQC
spectra. In addition, the population of the rotamers was also used for the assignment.
Similar assignments were extended to N-formylpiperidin-4-ones 6-8.
As a result of the attachment of the N-acyl groups at the nitrogen of the
heterocyclic systems, the benzylic proton that is syn to the N-C=O function would be
more deshielded compared to the proton that is anti based on the Paulsen and Todt’s
model for anisotropy of amides [41]. According tꢍꢅ ꢂꢆꢈ ꢅ ꢍꢓꢁꢋ(ꢅ ꢂꢆꢁꢅ ꢐꢄꢅ ꢏ-protons are
within the deshielding cone of the amide (in-plane region) and are deshielded.
Therefore, the signals corresponding to the H2 proton of the syn rotamer and the H6
proton of the anti rotamer are more deshielded than that of the anti and syn isomers,
7

respectively. This fact also supports the assignment of the benzylic proton signals
1
13
using the HSQC spectra. The complete assignment of the H and C NMR signals of
5-8 corresponding to the syn and anti rotamers is presented in Tables 3 and 4.
3.3 Preferred conformations of N-formylpiperidin-4-ones 5-8
The coupling constant data of N-formylpiperidin-4-ones 5-8 extracted from their
1H NMR spectra are listed in Table 5 along with the dihedral angles estimated using
DAERM [42] (Dihedral Angle Estimation By Ratio Method). The possible conformations
of N-formylpiperidin-4-ones 5-8 are shown in Figure 6. Compounds 5-8 may prefer to
adopt any one of the conformations where the destabilising interactions, such as A1,3
strain, 1,3-diaxial interactions and torsional strain, are minimised while retaining the
coplanarity of the N-C=O moiety, which results in the maximum resonance energy for
the molecule.
The conformational preference for N-formyl derivatives 5-8 was based on the
following factors (i) the relative strain factors involved in each of the possible
conformations (Figure 6); (ii) the observed vicinal coupling constant and estimated
dihedral angle values; (iii) the orientation of protons derived from the anisotropic
influence of the formyl group based on Paulsen and Todt’s model for the anisotropic
effects of the amides (Table 6) and (iv) shielding of the C2 and C6 carbons (Table 7).
Parent piperidin-4-ones 1-4 have been previously reported to exist in a chair
conformation with equatorial orientation of the alkyl and aryl groups [33,43-45].
8

3.4 r-2, c-6-Bis(4-methoxyphenyl)-t-3-methyl-N-formylpiperidin-4-one (5)
In N-formylpiperidin-4-one 5, four broad signals were observed for benzylic
protons H2 and H6 at RT. When the temperature was lowered to 243 K, four resolved
signals were obtained for benzylic protons H2 and H6 at 5.26 and 4.66 and 5.22 and
6.02 corresponding to the syn and anti rotamers (Figure 3). The signals at 5.26 and
4.66 appeared as doublets (J=8.5 Hz and J=10.5 Hz, respectively) and were assigned
to the H2 proton of the syn rotamer (H2 is syn to C=O) and the H2 proton of the anti
rotamer (H2 is anti to C=O), respectively. The signal at 5.22 that appeared as a triplet
(J=5.5 Hz) was assigned to the H6 proton of the syn rotamer (H6 is anti to N-C=O), and
the signal at 6.02 was not resolved well even at 243 K. This signal was assigned to the
H6 proton of the anti rotamer (H6 is syn to N-C=O). Because the signal for the H6 proton
of the anti rotamer at 6.02 was broad, the trans and cis vicinal coupling constants were
calculated from the corresponding coupling partners at C5 (3J5HA6H= 6.0 Hz and 3J5HB6H
=
2.0 Hz).
If the molecule is in equilibrium between the chair conformations CE (syn and
anti rotamers), the trans and cis coupling constants are expected to be approximately
10-12 Hz and 2-4 Hz, respectively. In addition, the observed deshielding of the benzylic
ꢒꢇꢍꢂꢍꢄ ꢅ !ꢛꢖ*+,-./-2.23) from those of the parent cannot be convincingly explained
based on the Paulsen and Todt’s model [41]. Therefore, the possibility of equilibrium
between chair conformations CE has been ruled out. In the flipped chair conformation
CA (syn and anti rotamers), the A1,3-strain is completely relieved because the aryl
groups are in the axial position. However, the 1,3-diaxial interaction between the two
9

anisyl groups at the C2 and C6 positions would destabilise the conformations. Benzylic
protons H2 and H6 have equatorial orientations, and the trans and cis coupling constants
are expected to be in the range of 2 to 4 Hz. Although the observed deshielding can be
reasonably explained based on the Paulsen and Todt’s model [41], the observed vicinal
coupling constants (J2H3H=10.5 Hz) for the H2 proton of the anti rotamer ruled out the
possibility of an equilibrium between the flipped chair conformations (CA). In boat
conformations B5 (syn and anti rotamers), although the A1,3-strain is relieved, the 1,3-
diaxial interaction between the anisyl groups would destabilise the conformations. The
vicinal coupling constants between the H2 and H3 protons (=H5 and H6 protons) are
expected to be approximately 8 Hz and 4 Hz. However, the observed vicinal coupling
constant (J2H3HA=10.5 Hz) for the H2 proton of the anti rotamer ruled out the possibility of
an equilibrium between the boat conformations B5 (syn and anti rotamers). A1,3-strain
and bond eclipsing interactions destabilise boat conformations B6 (syn and anti
rotamers) and similar to B5, the observed vicinal coupling constant ruled out the
possibility of an equilibrium between boat conformations B6 (syn and anti rotamers).
In boat conformations B1 (syn and anti rotamers), the H2 and H6 benzylic protons
occupy the equatorial and axial orientations, respectively, and are not chemical shift
equivalent. If the molecule exhibits equilibrium between boat conformations B1 (syn
and anti rotamers), the trans and cis coupling constants are expected to be
approximately 10-12 Hz and 2-4 Hz, respectively. However, the observed vicinal
coupling constants ruled out the possibility of equilibrium between boat conformations
B1. In boat conformations B2 (syn and anti rotamers), the benzylic protons at C2 and
C6 occupy the axial and equatorial orientations, respectively. If an equilibrium exists
10

between the syn rotamer with B1 conformation and anti rotamer with B2 conformation,
the signals for the H2 proton in the syn rotamer of B1 and the H2 proton in the anti
rotamer of B2 would appear as doublets with vicinal coupling constants at
approximately 2-4 Hz and 10-12 Hz, respectively, and the signals for the H6 proton in
the syn rotamer of B1 and the H6 proton in the anti rotamer of B2 would appear as a
doublet of doublets with vicinal coupling constants at approximately 10-12 and 2-4 Hz
and 2-4 and 2-4 Hz, respectively. The signals corresponding to the H2 proton of the syn
rotamer and the H6 proton of the anti rotamer should be more deshielded. From the
vicinal coupling constants and dihedral angles r-2, c-6-bis(4-methoxyphenyl)-t-3-methyl-
N-formylpiperidin-4-one (5) preferred to adopt an equilibrium between the syn rotamer
with B1 conformation and the anti rotamer with B2 conformation with a twist along the
C6-N1-C2-C3 portion of the syn rotamer and C2-N1-C6-C5 portion of the anti rotamer
(Figure 7).
Therefore, equilibrium would be established between twist boat
conformations TB1 and TB2. The X-ray crystal structure of 5 also corresponds to the
anti rotamer of boat conformation B2 with a twist along C2-N1-C6-C5 [46]. In addition,
the more populated anti rotamer (70%) crystallised in the solid state.
3.5 r-2, c-6-Bis(4-methoxyphenyl)-t-3-ethyl-N-formylpiperidin-4-one (6)
In the 1H NMR spectrum of 6, the H3, H5a and H5e proton signals of both the syn
and anti rotamers were merged. The signals at 5.90 and 4.89 correspond to the syn
and anti rotamer of H2. The signal at 4.89 appeared as a doublet (J=6.0 Hz). The
signals at 5.10 (J6a,5a=7.5 Hz and J6a,5e=7.5 Hz) and 5.90 were assigned to H6 of the syn
and anti rotamer, respectively. The signal at 5.90 (H6 of the anti rotamer) was merged
11

with H2 of the syn rotamer. The observed deshielding of the benzylic protons and
shielding of the benzylic carbons are similar to that in compound 5. Therefore,
compound 6 also prefers to exist in equilibrium between the syn rotamer with boat
conformation B1 and anti rotamer with boat conformation B2 with a twist along the C6-
N1-C2-C3 portion of the syn rotamer and the C2-N1-C6-C5 portion of the anti rotamer
(Figure 7). However, there is more twisting in compound 6 based on the following
observations: (i) the deshielding of H2 syn ꢏ-protons (2.23) is more than H6 (1.89); (ii)
shielding of C2 syn ꢏ-carbon (13.1) is more than that of C6 (11.1) compared to those of
compound 5 and (iii) significant changes in the coupling constant and dihedral angle
values. N-formylpiperidin-4-one 6 exhibited a well-resolved doubling of the signals even
at 283K. Therefore, the energy barrier (Dꢔ#) of the N-C=O rotation in this compound is
expected to be higher than that in the remaining compounds.
3.6 r-2, c-6-Bis(4-methoxyphenyl)-c-3-t-3-dimethyl-N-formylpiperidin-4-one (7)
For N-formylpiperidin-4-one 7, four broad signals were observed for benzylic
protons H2 and H6 at RT. When the temperature was decreased to 243 K, four well-
resolved signals were observed for benzylic protons H2 and H6 at 5.79, 4.78, 5.21 and
5.70 corresponding to the syn and anti rotamers. The signals at 5.79 and 4.78, which
appeared as singlets, were assigned to H2 in the syn rotamer and H2 in the anti rotamer,
respectively. The signals at 5.21 and 5.70, which appeared as triplets (J=8.5 and 6.5
Hz and J=7.0 & 7.0 Hz, respectively), were assigned to H6 of the syn rotamer and H6 of
the anti rotamer, respectively. Based on the discussion for N-formylpiperidin-4-one 5,
equilibrium between the syn rotamer with boat conformation B1 and the anti rotamer
12

with boat conformation B2 with a twist along the C6-N1-C2-C3 portion of the syn
rotamer and the C2-N1-C6-C5 portion of the anti rotamer (Figure 7) as considered for
compound 7. In addition, the signals corresponding to the H2 proton of the syn rotamer
and the H6 proton of the anti rotamer are more deshielded. The X-ray crystal structure
of 7 also corresponds to the syn rotamer with boat conformation B1 with a twist along
the C6-N1-C2-C3 portion [47]. Similar to 5, the molecule crystallised in a more
populated syn rotamer (61%) in the solid state.
3.7 r-2, c-6-Bis(4-methoxyphenyl)-t-3-t-5-dimethyl-N-formylpiperidin-4-one (8)
For compound 8, broad signals were observed at RT, and when the temperature
was decreased to 243 K, two doublets were observed for benzylic protons H2 and H6 at
5.41 and 4.66 corresponding to the syn and anti rotamers. The signal at 5.41 was
assigned to the H2 proton of the syn rotamer (=H6 proton of anti rotamer), and the signal
at 4.66 was assigned to the H2 proton of the anti rotamer (=H6 proton of syn rotamer).
The coupling constant for the H2 proton of the syn rotamer (=H6 proton of anti rotamer)
was calculated to be 5.0 Hz. The coupling constant for the H2 proton of the anti rotamer
(=H6 proton of syn rotamer) was calculated to be 8.0 Hz. The coupling constant value
for the anti rotamer was larger than the syn rotamer. In boat conformations B1 and B2,
benzylic protons H2 and H6 are not chemical shift equivalent. If an equilibrium exists
between boat conformations B1 or B2 of the syn and anti rotamers, four signals would
be expected for benzylic protons H2 and H6 in both cases. The observations of only two
signals for the benzylic protons eliminated the possibility of the above equilibrium.
Therefore, the observation of only two signals for the benzylic protons, the vicinal
13

coupling constants and the observed deshielding of the protons as well as the shielding
of carbons can only be explained if an equilibrium exists between the syn rotamer with
boat conformation B1 and the anti rotamer with boat conformation B2 with a twist along
the C6-N1-C2-C3 portion of the syn rotamer and the C2-N1-C6-C5 portion of the anti
rotamer (Figure 7). The X-ray crystal structure of 8 corresponds to the anti rotamer with
boat conformation B1 with a twist along the C6-N1-C2-C3 portion [48].
3.8 13C NMR Spectra
The syn A-carbons (8.7 – 13.1) are more shielded than the anti A-carbons (1.1 –
6.0). The higher shielding of the syn ꢏ-carbons may be due to the G-eclipsing
interaction between the -N-C=O bond of the formyl group and the N1-C2/N1-C6 bonds.
In addition, this observation also supports the coplanar orientation of N-C=O with the
C2-N-C6 plane.
4. Antimicrobial Screening
N-formylpiperidin-4-ones 5-8 were subjected to preliminary screening for their
antibacterial and antifungal activities using a disc diffusion technique. Sterile Muller-
Hinton agar plates were prepared, and the agar surface was inoculated with the
following bacteria: Escherichia Coli, Bacillus cereus, Klebsiella pneumoniae, Proteus
vulgaris and Staphylococcus aureus. The antifungal activity of the test compounds was
determined against the following fungi: Aspergillus flavus, Fusarium oxysporum, Mucor
indicus, Penicillium chrysogenum and Trichoderma viride. Compounds 5-8 were
14

dissolved in 1ml of DMSO in various concentrations in separate tubes. Commercially
available sterile discs were soaked in the preparation for half an hour. Then, the discs
were placed in empty petri plates for air-drying. Using sterile forceps, the discs were
placed on the surface of the agar plates and gently pressed on to the agar surface. The
culture plates were inverted and incubated for 24-48 hrs at 37°C. After incubation, a
zone of clearance was observed, and its diameter was measured using a microscope.
The zone of inhibition of the extracts was compared with standard ciprofloxacin for
antibacterial activity and amphotericin-B for antifungal activity. The results are reported
in the Tables 8 and 9, and the results indicated that the synthesised compounds
possessed a broad spectrum of activity against the tested microorganisms and
exhibited relatively better activity against all five bacteria. In comparison to the
reference disks (ciprofloxacin 5 µg/disk), compounds 5, 7 and 8 exhibited greater
activity against E. coli. Compound 6 exhibited better activity against E. coli and
Staphylococcus aureus. As shown in Table 8, all of the tested compounds, which were
applied in 12-fold concentrations (60 µg) compared to the ciprofloxacin (5 µg) reference,
exhibited less activity against the bacteria. Compounds 5 and 7 exhibited better activity
against Trichoderma viride compared to other fungal strains but less activity than the
amphotericin-B reference.
5. Conclusion
Four new N-formylpiperidin-4-ones 5-8 have been synthesised and characterised
using IR, mass, 1H, 13C, DEPT and 2D (1H, 1H-COSY and 1H-13C-HSQC) NMR spectra.
N-Formylpiperidin-4-ones 5-8 existed in conformational equilibrium between a syn
15

rotamer with twist boat conformation TB1 and an anti rotamer with twist boat
conformation TB2 in solution. By performing variable temperature NMR spectral
studies, the barrier for the N-C rotation in N-formylpiperidin-4-one 5 was determined to
be 64.3 kJmol-1. The antibacterial and antifungal activities of formylpiperidin-4-ones 5-8
were assessed. The antibacterial activity of the test compounds was determined
against Escherichia Coli sp, Bacillus cereus sp, Klebsiella pneumonia sp, Proteus
vulgaris sp, Staphylococcus sp and Pseudomonas sp. Antifungal activity of the test
compounds was determined against Aspergillus sp, Fusarium sp, Mucor sp,
Trichoderma sp and Penicillium sp. The results clearly indicated that all four
compounds exhibited significant antibacterial and antifungal activity.
Acknowledgements
One of the authors (SP) wishes to thank UGC for financial assistance in the form
of a Major Research Project [No. F. 39-724/2010 (SR)]. We wish to thank MKU
Madurai, (SAIF) IIT Chennai and (SIF) IISC Bangalore for the NMR spectra.
16

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20

Scheme 1
Synthesis of N-formylpiperidin-4-ones 5-8.
21

Figure captions
Figure 1
Allylic strain or A1,3 strain.
Figure 2
Designation of syn and anti rotamers in 5-8.
Figure 3
1H NMR spectrum of 5 showing doubling of the signals (syn and anti rotamers).
Figure 4
13C NMR spectrum of 7 showing doubling of signals (syn and anti rotamers).
Figure 5
HSQC NMR spectrum of 8.
Figure 6
Possible conformations of the N-formylpiperidin-4-ones 5-8.
Figure 7
Conformational equilibrium between the syn and anti rotamers of N-formylpiperidin-4-
ones 5-8.
Chart 1
Dynamic 1H NMR spectra of 5.
22

Table captions
Table 1:
Analytical data for compounds 5-8.
Table 2:
IR spectral data for compounds 5-8.
Table 3:
0ꢇꢍꢂꢍꢄꢅꢉꢆꢁ ꢈꢉꢎꢋꢅ ꢆꢈꢊꢂꢅꢗꢎꢋꢌꢁ ꢅ!ꢖꢒꢒ #ꢅꢊꢍꢇꢅcompounds 5-8.
Table 4:
131ꢅ1ꢆꢁ ꢈꢉꢎꢋꢅ ꢆꢈꢊꢂꢅꢗꢎꢋꢌꢁ ꢅ!ꢖꢒꢒ #ꢅꢍꢊꢅꢂꢆꢁꢅꢉꢍ ꢒꢍꢌꢄꢓ ꢅ5-8.
Table 5:
Vicinal and geminal coupling constants and dihedral angles of N-formylpiperidin-4-ones
5-8 compared to parent piperidin- 4-ones 1-4.
Table 6:
Formyl induced 1$ꢅ234ꢅꢉꢆꢁ ꢈꢉꢎꢋꢅ ꢆꢈꢊꢂ ꢅꢍꢊꢅꢏ-protons in N-formylpiperidin-4-ones 5-8.
Table 7:
Formyl induced 131ꢅ234ꢅꢉꢆꢁ ꢈꢉꢎꢋꢅ ꢆꢈꢊꢂ ꢅꢍꢊꢅꢏ-carbons in N-formylpiperidin-4-ones 5-8.
Table 8:
Result of the antibacterial activity of N-formylpiperidin-4-ones 5-8.
Table 9:
Result of the antifungal activity of N-formylpiperidin-4-ones 5-8.
23

Table 6
Formyl induced 1H NMR chemical shifts of A-protons in N-formylpiperidin-4-ones 5-8
Compounds
syn rotamer
H2a
H6a
+1.69
+1.09
+2.23
+1.22
+2.03
+1.02
+1.86
+1.11
+1.19
+1.99
+1.09
+1.89
+1.22
+1.71
+1.11
+1.86
5
6
7
8
anti rotamer
syn rotamer
anti rotamer
syn rotamer
anti rotamer
syn rotamer
anti rotamer
+ = deshielding
Table 7
Formyl induced 13C NMR chemical shifts of A-carbons in N-formylpiperidin-4-ones 5-8
Compounds
syn rotamer
anti rotamer
syn rotamer
anti rotamer
syn rotamer
anti rotamer
syn rotamer
anti rotamer
- = shielding.
C2
C6
-10.2
-5.5
-5.9
5
6
7
8
-12.6
-4.8
-13.1
-6.0
-11.1
-4.4
-8.7
-1.1
-8.8
-11.5
-5.8
-5.8
-11.5