Synthesis and NMR spectral studies of some 2,6-diarylpiperidin-4-one O-benzyloximes

Article abstract of DOI:10.1016/j.saa.2007.07.059

Variously substituted 2,6-diarylpiperidin-4-one O-benzyloximes were synthesized by the direct condensation of the corresponding 2,6-diarylpiperidin-4-ones with O-benzylhydroxylamine hydrochloride. All the synthesized compounds are characterized by IR, Mass and NMR spectral studies. NMR spectral assignments are made unambiguously by their one-dimensional (¹H NMR and ¹³C NMR) and two-dimensional (¹H-¹H COSY, NOESY, HSQC and HMBC) NMR spectra. All the synthesized compounds are resulted as single isomer, i.e., exclusively E isomer (9-14). The conformational preference of 2,6-diarylpiperidin-4-one oxime ethers with and without alkyl substituents at C-3 and C-5 has also been discussed using the spectral studies. The observed chemical shifts and coupling constants suggest that compounds 8-13 adopt normal chair conformation with equatorial orientation of all the substituents while compound 14 contributes significant boat conformation along with the predominant chair conformation in solution. The effect of oximination on ring carbons, their associated protons, alkyl substituents and ipso carbons are studied. Every proton in the piperidone ring of the oxime ether is observed as distinct signal due to oximination. The order of chemical shift magnitude in compound 8 is H-2a > H-6a > H-5e > H-3e > H-3a > H-5a. For 9-12, the order is H-6a > H-5e > H-2a > H-3a > H-5a, for 13, H-6a > H-2a > H-5e > H-3a > H-5a and for 14, the order is H-2a > H-6a > H-5e > H-3a > H-5a while the ¹³C chemical shift magnitude for 8-14 due to oximination is C-2 > C-6 > C-3 > C-5.

Full text of DOI:10.1016/j.saa.2007.07.059

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Spectrochimica Acta Part A 70 (2008) 11–24
Synthesis and NMR spectral studies of some
2,6-diarylpiperidin-4-one O-benzyloximes
P. Parthiban, S. Balasubramanian1, G. Aridoss, S. Kabilan∗
Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamil Nadu, India
Received 20 May 2006; accepted 11 July 2007
Abstract
Variously substituted 2,6-diarylpiperidin-4-one O-benzyloximes were synthesized by the direct condensation of the corresponding 2,6-
diarylpiperidin-4-ones with O-benzylhydroxylamine hydrochloride. All the synthesized compounds are characterized by IR, Mass and NMR
spectral studies. NMR spectral assignments are made unambiguously by their one-dimensional (1H NMR and 13C NMR) and two-dimensional
(1H–1H COSY, NOESY, HSQC and HMBC) NMR spectra. All the synthesized compounds are resulted as single isomer, i.e., exclusively E
isomer (9–14). The conformational preference of 2,6-diarylpiperidin-4-one oxime ethers with and without alkyl substituents at C-3 and C-5 has
also been discussed using the spectral studies. The observed chemical shifts and coupling constants suggest that compounds 8–13 adopt normal
chair conformation with equatorial orientation of all the substituents while compound 14 contributes significant boat conformation along with
the predominant chair conformation in solution. The effect of oximination on ring carbons, their associated protons, alkyl substituents and ipso
carbons are studied. Every proton in the piperidone ring of the oxime ether is observed as distinct signal due to oximination. The order of chemi-
cal shift magnitude in compound 8 is H-2a > H-6a > H-5e > H-3e > H-3a > H-5a. For 9–12, the order is H-6a > H-5e > H-2a > H-3a > H-5a, for 13,
H-6a > H-2a > H-5e > H-3a > H-5a and for 14, the order is H-2a > H-6a > H-5e > H-3a > H-5a while the 13C chemical shift magnitude for 8–14 due
to oximination is C-2 > C-6 > C-3 > C-5.
© 2007 Elsevier B.V. All rights reserved.
Keywords: 2,6-Diarylpiperidin-4-one; O-Benzylhydroxylamine hydrochloride; Oxime ether; 1H NMR; 13C NMR; 2D NMR technique; Conformation; Electroneg-
ativity
1. Introduction
Substituted 2,6-diarylpiperidin-4-ones [3] have been sub-
jected to quite a large number of synthetic [4–8] and
physico-chemical studies [9–11]. 2,6-Diarylpiperidin-4-ones
normally adopt chair conformation with equatorial orientation
of all the substituents [12–15]. However, introduction of cer-
tain heteroconjugate groups such as NO, CHO, COCH3,
COC6H5, etc., at the ring nitrogen of 2,6-disubstituted piperi-
dine ring system have reported to cause a major change in ring
conformation [16–24]. Similarly, the conversion of the carbonyl
group into C N OH, C N NH2, C N N CH2, C N NHPh,
C NNHCSNH2 [26–35] cause an abrupt change in the chemical
shifts of the ring carbons and associated protons and also exhibit
a change in conformation of the compound and orientation of
the substituents. In the present study, we have converted the car-
bonyl carbon into bulkier C N O CH2–Ph in order to establish
the impact of the bulkier oximino group on the chemical shift
and conformation of the piperidone ring and substituents.
Generally, thedecreaseinelectronegativityofagrouporatom
on the ring cause major change in chemical shifts on the ring
Nuclear magnetic resonance spectroscopy is a versatile tool
for the structural analysis of most of the organic compounds. The
NMR techniques are also useful for the conformational analysis
too. 1H NMR and 13C NMR techniques have been extensively
applied in deriving stereodynamical information about a wide
variety of systems. It gives information about the influence of
electronic and conformational effects on chemical shifts and
coupling constants. Vicinal coupling constant values have been
used for the conformational analysis as it can give an idea about
the orientation of the substituents [1,2].
∗
Corresponding author.
E-mail addresses: skabilan@rediffmail.com, prskabilan@rediffmail.com
(S. Kabilan).
1
Present address: Department of Chemistry, Bowman Oddy Laboratories, The
University of Toledo, 2801, W. Bancroft Street, MS 602, Toledo, OH 43606,
USA.
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.07.059

12
P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
Scheme 1.
carbons and the attached protons [33]. So, the C N O Bn in
place of C O of the piperidone ring is expected to cause a severe
change in their chemical shifts of ␣-, ␤- and ␥-carbon and their
associated protons in addition to the conformational preference
of the piperidone ring.
2. Results and discussion
All the oxime ethers were synthesized by the direct
condensation of corresponding piperidin-4-ones with O-
benzylhydroxylamine hydrochloride in the presence of sodium
Table 1
Analytical data for compounds 8–14
Compound
Molecular formula
mp (◦C)
Yield (%)
Elemental analysis
Calculated (%)
C
Found (%)
C
H
N
H
N
8
9
C
24H24N2O
C25H26N2O
25H24N2OCl2
C27H30N2O
27H30N2O3
C26H28N2O
27H30N2O
84
96
117
132
130
102
137–138
95
94
92
92
94
91
89
80.87
81.05
68.34
81.37
75.32
81.21
81.37
6.79
7.07
5.51
7.59
7.02
7.34
7.59
7.86
7.56
6.38
7.03
6.51
7.29
7.03
80.85
81.06
68.31
81.39
75.30
81.20
81.39
6.71
7.06
5.52
7.55
7.04
7.32
7.58
7.89
7.57
6.40
7.04
6.51
7.27
7.04
10
11
12
13
14
C
C
C

Table 2
Proton chemical shift values (δ, ppm) of compounds 8–14
Compound H-2a (d)
H-3a (m) H-3e (ddd) H-5a (dd) H-5e (dd)
H-6a (dd) Aromatic (m)
p-CH3/p-OCH3 (s) CH3at C-3(d)
NH (bs) O-CH2-Ph (s)
a
8
9
3.93 (dd)
3.41
3.33
3.35
3.34
2.34 (dd)
2.40
2.28
2.36
2.33
2.58
–
–
–
–
2.01
1.93
1.80
1.89
1.89
2.01
2.18
3.52 (ddd)
3.51
3.43
3.43
3.46
3.85
3.74
3.65
3.68
3.67
3.78
3.81
7.25–7.48
7.12–7.35
7.13–724
6.99–7.03, 7.16–7.27 2.19, 2.22
6.73–6.77, 7.18–7.28 3.66, 3.68
–
–
–
–
0.80
0.75
0.79
5.10
1.79
1.69
1.74
5.03
5.00
5.01
5.02
5.04
5.03
10
11
12
13
14
0.78
1.71
b
3.59
3.82
2.38
2.40 (dd)
–
–
3.50
3.28
7.11–7.45
7.12–7.35
–
–
0.72 (CH3, t) 1.17, 1.49 (–CH2–, m)
0.79 (CH3) 0.94 (CHꢀ3) 1.68 (–CH–, m)
c
a
b
c
Merged with H-5a signal.
Merged with CH2 signal.
Merged with H-7 signal.
Table 3
13C chemical shift values of compounds 8–14 (δ, ppm)
Compound C-2 C-3 C-4 C-5 C-6
–O–CH2– CH3 at C-3
C-2ꢀ
C-6ꢀ
O–Bn ipso Aryl carbons
8
9
10
11
12
13
61.98 40.41 157.78 34.20 60.67 75.40
69.50 43.37 160.15 34.77 60.94 75.65
68.24 43.04 158.79 34.43 59.81 75.48
69.02 43.20 160.16 34.68 60.47 75.43
68.61 43.29 160.19 34.67 60.10 75.40
67.35 49.45 157.93 34.36 60.73 75.46
–
143.61 143.55 137.99
142.84 143.90 138.45
140.90 141.96 138.03
139.77 140.86 138.29
134.93 136.00 138.25
141.92 142.95 138.36
128.54, 128.31, 127.96, 127.65, 126.74, 126.61
128.52, 128.47, 128.29, 128.01, 127.81, 127.61,126.78
132.94, 133.21 (C2ꢀꢀꢀꢀ, C6ꢀꢀꢀꢀ), 129.05, 128.41, 128.10, 127.88, 127.49
136.90, 137.12 (C2ꢀꢀꢀꢀ, C6ꢀꢀꢀꢀ), 129.01, 128.95, 128.14, 128.09, 127.72, 127.47, 126.50
158.86, 159.00 (C2ꢀꢀꢀꢀ, C6ꢀꢀꢀꢀ), 128.77, 128.13, 128.10, 127.66, 127.47, 113.69, 113.61
128.67, 128.31, 128.10, 127.72, 127.50, 127.44, 126.72
11.91
11.63
11.85
11.79
11.71 (CH3)
18.89(CH2)
18.38 (CH3)
21.03(CHꢀ3)
28.01 (CH)
14
65.16 53.30 157.98 34.69 59.74 75.50
143.61 144.00 138.70
128.46, 128.41, 128.22, 128.16, 127.93, 127.61, 127.47, 127.10, 126.67

14
P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
Table 4
IR and Mass spectral data of compounds 8–14
Compound
IR (cm−1
)
EIMS (m/z)
8
2912 (N–H stretching), 2793 (C–H stretching), 1638 (C N stretching),
1600, 1493, 1453, 1426, 1368, 1299, 1198, 1105, 1043, 936, 878, 757,
700, 574, 494
356 (M+), 265, 249, 234, 208, 194, 167, 160, 144, 129, 115,
104, 91 (base peak)
9
10
11
12
3310 (N–H stretching), 2810 (C–H stretching), 1640 (C N stretching),
1601, 1494, 1453, 1371, 1303, 1263, 1208, 1104, 1043, 1018, 983,
921, 863, 783, 748, 699, 556, 476
3313 (N–H stretching), 2812 (C–H stretching), 1636 (C N stretching),
1596, 1490, 1451, 1368, 1303, 1262, 1089, 1015, 923, 827, 751, 697,
582, 511
3312 (N–H stretching), 2808 (C–H stretching), 1647 (C N stretching),
1597, 1513, 1452, 1369, 1305, 1261, 1208, 1100, 1022, 921, 867, 806,
748, 697, 663, 512.
3313 (N–H stretching), 2808 (C–H stretching), 1635 (C N stretching),
1601, 1512, 1454, 1361, 1303, 1247, 1175, 1106, 1034, 921, 831, 805,
751, 699, 549
3312 (N–H stretching), 2856 (C–H stretching), 1649 (C N stretching),
1604, 1514, 1459, 1413, 1262, 1096, 1023, 875, 802, 698, 470
3306 (N–H stretching), 2803 (C–H stretching), 1642 (C N stretching),
1601, 1493, 1454, 1361, 1301, 1262, 1096, 1023, 801, 752, 699, 662,
607, 490
371 (M + 1), 263, 194, 174, 156, 143, 129, 117, 106, 91 (base
peak)
438 (M+), 331, 262, 208, 192, 169, 165, 140, 128, 103, 91
(base peak)
13
14
385 (M + 1)a
a
Recorded on electron spray mass spectrometer (ESMS), which predominantly gave (M + 1) only.
acetate trihydrate (Scheme 1). All the synthesized compounds
are obtained in good yield. The analytical data are represented
in Table 1. Proton and carbon NMR spectra of all the synthe-
sized compounds 8–14 were recorded at 400 MHz and 100 MHz,
respectively, and are reproduced in Tables 2 and 3, respectively.
For compound 8, HOMOCOSY, NOESY, HSQC and HMBC
were also recorded. Mass spectra were recorded for compounds
8–10 and 13. All the obtained Mass spectral data are in agree-
ment with the proposed molecular formula of the respective
compounds. IR spectra of all the synthesized compounds sup-
port the formation of oxime ether by the appearance of a band
at about 1636–1649 cm−1. The IR and Mass spectral values are
furnished in Table 4. The numberings of the target compounds
are shown in Fig. 1.
which may be due to the benzylic protons H-2a and H-6a
and are having correlation, respectively, with 2.58/2.34 and
3.52/2.01 ppm signals of the methylene protons at C-3 and C-5.
Of the four methylene proton signals, the two lower frequency
3
2
signals (2.01 ppm Jaa = 11.8 Hz, Jae = 13.9 Hz and 2.34 ppm,
3Jaa = 11.6 Hz, 2Jae = 13.5 Hz) are appeared as double doublets,
which is due to the splitting of one methylenic proton by another
methylenic proton on the same carbon and also with the benzylic
proton of the adjacent position, while rest of the two are appeared
as doublets of double doublets (ddd) instead of the expected dou-
ble doublets due to the long-range coupling between them. This
is possible only when both the protons are in “W” arrangement
presumably with equatorial disposition at C-3 and C-5. This is
substantiated by the observed weak HOMO correlation (Fig. 3)
between these two signals (2.58 and 3.52 ppm). Consequently,
the double doublets at 2.01 and 2.34 ppm are due to the axial
methylenic protons H-3a and H-5a, which is confirmed by its
nOe with the ortho protons of the phenyl group at C-2 and C-
6 (Fig. 4). Of the two sets of axial and equatorial protons at
C-3 and C-5, the higher frequency ddd at 3.52 ppm is assigned
to the H-5e proton owing to the interaction of the N O bond
with the C H(5e) bond (Fig. 5). Due to this interaction, the
C H(5e) bond is polarized consequently the syn ␣-proton H-
5e gets positive charge and the syn ␣-carbon C-5 gets negative
charge. Further, the H-5a proton is highly shielded due to the
transmittance of negative charge from C-5 to H-5a. From this,
the higher frequency ddd at 3.52 ppm is assigned to the H-5e
whereas the lower frequency ddd at 2.58 ppm is assigned to the
anti ␣-equatorial proton H-3e. From the HOMO correlations of
the signals at 3.52 and 2.58 ppm, respectively, with double dou-
blets at 2.01 and 2.34 ppm, it is concluded that the former one
(2.01 ppm dd) is due to H-5a and the latter one (2.34 ppm dd) is
due to H-3a proton.
2.1. 1H NMR spectral study of 2,6-diphenylpiperidin-4-one
O-benzyloxime (8)
The signals (double doublet and multiplet) appeared in the
region of 7.25–7.48 ppm with 15 protons integral are due to the
phenyl group protons at C-2, C-6 and in O-benzyl group. Of
the two signals, the double doublet at 7.46–7.48 ppm has strong
nOe with the double doublets at 3.93 and 3.85 ppm, which sug-
gest that the double doublet at 7.46–7.48 ppm with four protons
integral is due to the ortho protons of the phenyl groups at C-2
and C-6. Deshielding of the ortho protons has been attributed to
the lone pair of electrons on the nitrogen in the piperidone het-
erocycle. The rest of the phenyl protons and O-benzyl moiety
phenyl protons are merged together to give multiplet at about
7.25–7.36 ppm.
The two doublets at 3.93 ppm and 3.85 ppm (Fig. 2)
3
are appeared with the coupling constant of Jaa = 11.5 Hz,
3Jae = 2.9 Hz and Jaa = 11.7 Hz, Jae = 3.0 Hz, respectively,
3
3

P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
15
Fig. 1. Numberings of the target compounds 8–14.
2.2. 1H NMR spectral study of
The H-5a proton signal has almost two protons integral,
contrary to other ring proton signals (each signal corresponds
to one proton integral). This may be due to the overlapping
of NH proton signal with that of H-5a at 2.01 ppm. Gener-
ally, in 2,6-diarylpiperidin-4-ones the NH proton appeared at
about 2 ppm. Moreover, this NH proton signal is unambigu-
ously identified by D2O exchange. Besides, the sharp singlet
appeared at 5.10 ppm, which integrates as two protons, is
obviously assigned to the methylene protons of the O-benzyl
moiety. Moreover, this singlet has nOe with its phenyl pro-
tons merged with the meta protons of the phenyl group at
C-2 and C-6 in the range of 7.34–7.36 ppm. The HOMO-
COSY and NOESY spectra correlations of 8 are reproduced in
Table 5.
3-alkyl-2,6-diarylpiperidin-4-one O-benzyloximes (9–14)
1
In the H NMR spectrum of compound 9 (Fig. 6), besides
the aryl protons signal at 7.12–7.35 ppm [7.33–7.35 (t) and
7.12–7.30 (m), the triplet is due to the ortho protons of the
phenyl at C-2 and C-6 while the multiplet is due to the rest
of the phenyl protons] and the O-benzyl methylene protons sig-
nal at 5.03 ppm, all the piperidone ring protons signals (splitting
pattern and chemical shifts) are varied drastically due to the
introduction of an equatorial methyl group at C-3. This methyl
protons signal appeared as a doublet at 0.80 ppm with coupling
constant of 6.5 Hz. The NH proton signal is appeared separately
as a broad singlet at 1.79 ppm.
Fig. 2. 1H NMR spectrum of compound 8.

16
P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
Fig. 3. HOMOCOSY spectrum of compound 8.
In the higher frequency region there are two double dou-
Fig. 4. NOESY spectrum of compound 8.
blets and one doublet at 3.74 (3J = 11.7 Hz, 2.8 Hz), 3.51
(2J = 13.7 Hz, 3J = 2.8 Hz) and 3.41 ppm (3J = 10.1 Hz), respec-
tively. Of the three signals, 3.41 ppm doublet and 3.74 ppm
doublet of doublet are assigned to the H-2a and H-6a
benzylic protons, respectively, while 3.51 ppm (2J = 13.7 Hz,
3J = 12.1 Hz) double doublet is assigned to H-5e (syn ␣) pro-
ton on the basis of their splitting pattern and vicinal coupling
constant. Consequently, the other double doublet at 1.93 ppm
is assigned to the methylenic proton of H-5a on the basis of
their position nature of splitting, coupling constants and in com-
parison with 8. Obviously, the sextet centered at 2.40 ppm is
designated to the H-3a. This is deshielded by about 0.06 ppm
than 8 due to the introduction of an equatorial methyl group at
C-3 [35].
Unlike 9, compound 10 gave a single multiplet with inte-
gral value for 13 protons (p-chlorophenyl at C-2 and C-6). But
in compound 11 (p-methylphenyl at C-2 and C-6) and 12 (p-
methoxyphenyl at C-2 and C-6) two sets of aryl signals are
observed with 13 protons integral value. Of the two set of aryl
signals, the upfield triplet is due to the protons ortho to the sub-
stituentin11, whilein12thetripletappearedintheupfieldregion
is due to the protons meta to the methoxy substituents i.e., H-2ꢀꢀ
and H-6ꢀꢀ. Contrary to 11, in 12, the ortho protons are observed
in the downfield region due to the ortho effect of the methoxy
group. In parent piperidones 4 and 5, only one singlet with six
protons integral was observed for the CH3/OCH3 at C-2ꢀꢀꢀꢀ and
C-6ꢀꢀꢀꢀ whereas, two closely merged singlets (appeared as like
that of a doublet), each corresponds to three protons appeared at
2.19, 2.22 and 3.66, 3.88 ppm, respectively, for the methyl and
methoxy protons at C-2ꢀꢀꢀꢀ and C-6ꢀꢀꢀꢀ due to the oximino effect.
Compound 13 also has similar kind of splitting as like that of
9. The side chain ethyl at C-3 gave three kinds of signals (Fig. 7)
viz., a septet and a multiplet centered, respectively, at 1.49 and
1.17 ppm for the methylene protons (H-7 and Hꢀ-7) in the ethyl
group while the methyl group gave a triplet with three protons
integral in the most upfield region of about 0.72 ppm.
Fig. 5. H(5e)–NO bond interaction.
three protons are assigned to the two-methyl group protons in
the isopropyl moiety. The separate signals for the two-methyl
groups are due to their diastereotopic nature. Further, the iso-
propyl methine proton and the ring NH proton were merged
to give a multiplet centered at 1.68 ppm (Fig. 8). All the ring
protons are assigned similar to that of compound 9.
2.3. Analysis of coupling constants
All the observed coupling constant values for 8–14 are fur-
nished in Table 6.
Table 5
Correlations in the HOMOCOSY and NOESY spectra of compound 8 (δ, ppm)
Signal
Correlations in the
HOMOCOSY spectrum
Correlations in the
NOESY spectrum
7.46–7.48 (dd, 4H)
7.25–7.36 (m, 11H)
5.10 (s, 2H)
3.93 (dd, 1H)
3.85 (dd, 1H)
3.52 (ddd, 1H)
2.58 (ddd, 1H)
2.34 (dd, 1H)
7.28–7.36
7.46–7.48
–
2.58, 2.34
3.52, 2.01
3.85, 2.58, 2.01
3.93, 3.52, 2.34
3.93, 2.58
3.85, 3.52
3.93, 3.85, 2.34, 2.01
–
7.28–7.36
2.58, 2.34
3.52, 2.01
2.01, 3.85
2.34, 3.93
2.58, 2.01, 3.85
3.52
In compound 14, all the phenyl ring protons are merged to
give a multiplet in the region 7.12–7.35 ppm. Two doublets at
0.94 ppm (J = 6.9 Hz) and 0.79 ppm (J = 6.9 Hz), each represent
2.01 (dd, 1H)

P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
17
Fig. 6. 1H NMR spectrum of compound 9.
For compound 8, the observed vicinal diaxial coupling con-
stants J2a,3a and J5a,6a are 11.5 and 11.7 Hz, respectively (almost
same) and the vicinal axial–equatorial coupling constants J2a,3e
and J5e,6a are, respectively, 2.9 and 3.0 Hz, whereas the gem-
inal axial–equatorial coupling constants J3a,3e and J5a,5e are,
respectively, 12.9 and 14.0 Hz (slightly varied due to the bulkier
oximino group in place of the carbonyl carbon). Based on the
obtained coupling constant values, normal chair conformation
is proposed for compound 8 as depicted in Fig. 9.
In 9–12, due to the introduction of methyl group, J2a,3a is con-
siderably decreased by about 1.4 ppm. The difference between
J2a,3a and J5a,6a also in the same magnitude (1.4 Hz). Obviously,
this is due to the flattening of the ring about C(2)–C(3) bond
to decrease the Ph–Me gauche interaction while introducing an
equatorial methyl group at C-3 as depicted in Fig. 10. This is
also supported by the force field calculations [36] and X-ray
diffraction data [37]. The observed vicinal coupling constants
suggest that 9–12 adopts normal chair conformation.
substituent does not alter the conformation. To find the ethyl
group conformation at C-3, the coupling constants between the
methine proton H-3a and methylene protons H-7/Hꢀ-7 are found
ꢀ
as 2.7 Hz (J3a,7H) and 8.4 Hz (J3a,7H ). These values suggest that
the conformation of ethyl group exist as an equilibrium mixture
of the two forms as in Fig. 11. This conclusion is conformed
by the earlier report on the 3-ethyl 2,6-diphenylpiperidin-4-one
[28].
But in 14, the coupling constants are appreciably varied from
compound 9. The considerable decrease in J2a,3a (1.2 Hz) and
J5a,6a (0.4 Hz) and also an increase in J5a,5e (0.9 Hz) are observed
when compared to 9. The coupling constant is decreased about
1.4 Hz and 0.4 Hz, respectively, for J2a,3a and J5a,6a compared to
corresponding piperidone. Moreover the increasing magnitude
of J3a,7 is 1.7 Hz and J5a,5e is 0.9 Hz compared to piperidone.
These values strongly suggest that 14 contribute boat conforma-
tion (Fig. 12) along with the predominant chair conformation
in solution. The conformational preference of the isopropyl
substituent at C-3 is found from the vicinal coupling constant
between H-3a and isopropyl methine proton H-7. The observed
In 13, there is no appreciable variation in coupling constants
and are almost similar to that of 9, which suggest that the ethyl
Table 6
Coupling constant (Hz) values of compounds 8–14
Compound
3J2a,3a
3J2a,3e
2J3a,3e
4J3e,5e
3J5a,6a
2J5a,5e
3J5e,6a
3J3a,7
8
9
11.5
10.1
10.1
10.1
10.1
10.3
8.9
2.9
–
–
–
–
12.9
–
–
–
–
1.7
–
–
–
–
11.7
11.7
11.5
14.0
13.7
13.6
13.6
13.7
13.6
14.6
3.0
2.8
2.5
2.5
2.8
2.7
3.6
–
–
–
–
–
–
3.9
10
11
12
13
14
11.5
a
–
–
–
–
–
–
11.6
11.3
a
Could not be determined due to overlapped signal.

18
P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24

P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
19
Fig. 8. 1H NMR spectrum of compound 14.
J3a,7 = 3.9 Hz suggest that this value is probably due to only
gauche coupling. From this, we inferred that the preferred con-
formation of the isopropyl group is as depicted in Fig. 13. This
also supported by the earlier observations of 3-isopropyl-2,6-
diarylpiperidin-4-one [28].
On the basis of the observed vicinal coupling constants, we
inferred that the compounds 9–13 adopt normal chair conforma-
tion (Fig. 9) with equatorial disposition of all the substituents
at C-2, C-6 and also at C-3 while 14 contributes significant
boat conformation (Fig. 11) along with the predominant chair
conformation.
intense signals in the aliphatic region at 75.40, 61.98, 60.67,
40.41 and 34.20 ppm.
Of these five signals in the upfield region, 75.40 ppm shows
cross peak only with the O-benzyl moiety methylene protons
singlet at 5.10 ppm in HSQC spectrum (Fig. 15). The 61.98
and 60.67 ppm signals also having cross peaks only with,
respectively, H-2a and H-6a protons indicate these carbon
signals are, respectively, due to C-2 and C-6 of the piperidone
ring. From the methylenic protons correlation in HSQC, 40.41
and 34.20 ppm are, respectively, assigned to the methylenic
carbons C-3 and C-5.
The signals from 128.54–126.61 ppm show cross peak with
the aryl protons signal in HSQC, consequently these intense
aryl carbon signals are assigned to the phenyl ring carbon
at C-2 and C-6 and also in O-benzyl group. The absence of
correlation beyond 128.54 ppm in HSQC predicts those are quar-
tenery carbons. From the help of HMBC spectra (Fig. 16),
the 157.78 and 137.99 ppm resonances are assigned to the
C-4 (C N) and O-benzyl ipso carbon, respectively, without
ambiguity. Obviously the other two signals at 143.61 and
143.55 ppm are due to the ipso carbons C-2ꢀ and C-6ꢀ. The
2.4. 13C NMR spectral analysis of
2,6-diphenylpiperidin-4-one O-benzyloxime (8)
In the 13C NMR spectrum of compound 8 displayed
in Fig. 14, there are four low intense signals at 157.78,
143.61, 143.53 and 137.99 ppm and high intense signals at
128.54–126.61 ppm observed in the aryl region. There are five
Fig. 9. Chair conformation.
Fig. 10. Flattening about C(2)–C(3) bond.

20
P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
Fig. 11. Ethyl group conformation in compound 13.
Moreover, the intensed signal at 75.65 ppm also assigned to
the O-benzyl methylene carbon as in the case of 8.
By considering the ␤-effect of the methyl sustituent at C-
3, the signal resonating at 69.50 ppm can be assigned to the
benzylic carbon C-2. Generally introduction of an equatorial
methyl substituent at C-3 of the piperidone moiety causes a large
␤-effect (≈7 ppm) at C-2 benzylic carbon [38]. Indeed, the 13C
resonance at 69.50 ppm is assigned to the C-2 benzylic carbon.
Obviously the other higher frequency signal is designated to the
C-6 benzylic carbon.
Of the three signals in the lower frequency region, the most
upfield signal at 11.91 ppm is ascribed to the methyl carbon at
C-3. In the remaining two signals, one appeared at 34.77 ppm
as like that of C-5 methylenic carbon of 8 is conveniently
assigned to C-5 of 9. Obviously the other signal at 43.37 ppm is
designated to the C-3 methylenic carbon, which is 2.97 ppm
deshielded than the C-3 of 8. Indeed, this is obviously due
to the ␣-effect of the methyl substituent at C-3. Similar kind
of ␤-effect of the methyl group at C-3 on C-2 benzylic car-
bon of 1-hetera-2,6-diaryl-3-methyl-4-cyclohexanones is also
noted [39].
Fig. 12. Boat conformation.
The 13C NMR spectra of 10–12 are also analysed and
assigned similarly in 9. Further in 10–12, two pair ipso car-
bons are observed due to the p-substituents in the phenyl rings
at C-2 and C-6. In 10 and 11 the substituent bearing ipso carbons
(C-2ꢀꢀꢀꢀ and C-6ꢀꢀꢀꢀ) are appeared in the upfield region than C-2ꢀ
and C-6ꢀ ipso carbons, where as in 12 the methoxy bearing ipso
carbons are appeared in the most downfield region.
Fig. 13. Isopropyl group conformation in compound 14.
1H–13C one and multiple bond correlations are represented in
Table 7.
2.5. 13C NMR spectral study of
3-alkyl-2,6-diarylpiperidin-4-one O-benzyloximes (9–14)
The methyl carbons present at C-2ꢀꢀꢀꢀ and C-6ꢀꢀꢀꢀ of 11 are
observed in the deshielding region compared to methyl group
at C-3 (21.01 and 21.05 ppm) due to ring current effect. The
methoxy carbons substituted at C-2ꢀꢀꢀꢀ and C-6ꢀꢀꢀꢀ of 12 are
observed at 55.09 ppm.
The 13C NMR spectrum of compound 9 is reproduced in
Fig. 17, which shows similar kind of resonances beyond the
aliphatic region and are assigned as like that of 8.
Table 7
Correlations in the HSQC and HMBC spectra of compound 8 (δ, ppm)
Signal
Correlations in the HSQC spectrum
Correlations in the HMBC spectrum
7.46–7.48 [ortho protons (i.e., H-2ꢀꢀ and H-6ꢀꢀ)]
126.61, 126.74
126.74, 126.61
7.25–7.36 (remaining aryl protons)
5.10 (–O–CH2–Ph)
3.93 (H-2a)
3.85 (H-6a)
3.52 (H-5e)
2.58 (H-3e)
2.34 (H-3a)
2.01 (H-5a)
127.65, 127.96, 128.31, 128.54
143.61, 143.55, 128.54–127.65
137.99, 128.54–127.65
126.74, 126.61
126.74, 126.61
157.78, 40.41
157.78, 34.20
157.78, 143.61, 61.98
157.78, 143.55, 60.67
75.40
61.98
60.67
34.20
40.41
40.41
34.20

P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
21
Fig. 14. 13C NMR spectrum of compound 8.
Table 8
Chemical shift difference between piperidone (1–7) and oxime ether (8–14) (δpiperidone − δoxime ether) (ꢀδ, ppm)
Compound
C-2
C-3
C-4
C-5
C-6
C-2ꢀ
C-6ꢀ
0.95
1.20
1.20
CH3 at C-3
8
9
−0.98
−1.00
−1.01
−0.98
−0.91
−0.65
−0.26
9.79
8.23
8.21
8.17
8.41
8.95
8.00
50.02
49.35
49.30
49.29
49.61
51.17
51.02
16.00
16.13
16.15
16.19
16.33
17.24
17.41
0.33
0.56
0.65
0.61
0.80
1.07
1.56
−1.01
−1.04
−1.01
−0.97
−0.83
−0.23
−1.71
–
−1.81
10
11
12
13
14
−1.81
1.21
−1.85
−1.00
−0.38
−1.21
−1.69
−1.05 (CH2), −0.47 (CH3)
−1.91(CH), −0.23 (CH3, CHꢀ3)
-
+: shielding; −: deshielding.
In compounds 13 and 14, due to the replacement of methyl
by ethyl or isopropyl substituents, respectively, at C-3 causes an
increase in ␣-effect on the C-3 methylenic carbon and decrease
in ␤-effect on C-2 benzylic carbons, Other signals are assigned
similar to that of 9 and their chemical shift values are furnished
in Table 3.
2.6. Effect of oximination
Generally in six-membered heterocycles, decrease in elec-
tronegativity of a particular group in the ring skeleton shield
the ␣-carbons and deshields the ␤- and ␥-carbons [40]. Inspite
of the less polar nature of the C N than C O bond, the elec-
tronegativity of the C N O Bn group must be less than that
of C O group. All the ꢀδ (δpiperidone − δoxime ether) values for
the piperidone ring carbons of the synthesized compounds 8–14
are represented in Table 8. A perusal of the data in Table 8, the
ꢀδ values of 14 are significantly deviate from other compounds.
This is due to the bulkier isopropyl group at C-3 of the oxime
ether 14 and conformational preference.
Generally in ketoximes, the syn ␣-carbon is shielded than the
anti ␣-carbon, probably the chemical shift difference between
Fig. 15. HSQC spectrum of compound 8.

22
P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
tronegativity effect. However shielding of syn ␣-carbon is more
pronounced than that of anti ␣-carbon. This is due to the interac-
tion of the N–O bond with the syn ␣ C–H(e) bond (Fig. 5). This
interaction induces a polarity in the syn ␣ C–H(e) bond so that
the syn ␣-equatorial proton (H-5e) gets slight positive charge
and the C-5 carbon gets slight negative charge, consequently the
proton is deshielded and the carbon is shielded significantly.
Totally the shielding magnitude on syn ␣-carbon is around
16–17 ppm for 8–14 whereas the shielding magnitude on anti
␣-carbon is only about 8–10 ppm. In the former (syn ␣-
effect), the magnitude is increased from 16.00 ppm (for 8)
to 17.24 ppm (for 14) whereas in the latter (anti ␣-effect) is
decreased from 9.79 ppm (for 8) to 8.00 ppm (for 14). This
variation is only due to the effect of the alkyl sustituent at the
anti ␣-carbon (C-3).
Due to the interaction of N–O bond with the syn ␣ C–He
bond, the syn ␣-equatorial proton (H-5e) gets a slight positive
charge and the C-5 carbon gets slight negative charge. Inspite
of this, H-5e deshielded about 1 ppm and appeared at around
3.5 ppm in all compounds while the syn ␣-axial proton (H-5a)
shielded by the negative charge on the syn ␣-carbon. The dif-
ference between the H-5e and H-5a is about 1.5 ppm in 8–13
whereas the difference is considerably decreased in 14 probably
due to its conformational preference. The difference between
the H-5e and H-5a is about 0.01 ppm in 3-methyl-2,6-diaryl
piperidin-4-one [44].
The anti ␣-protons (H-3a and H-3e) are shielded according to
the decrease in electronegativity at C-4. In 8–14, except H-5e, all
the remaining ␣-protons are shielded than the parent piperidone.
The chemical shifts are decreased in the order H-5e > H-3e > H-
3a > H-5a in 8. Similarly for the 3-alkyl compounds 9–14, the
chemical shift of the ␣-protons are in the order H-5e > H-3a > H-
5a. The differences between all the ␣-proton are shown in
Table 10.
Fig. 16. HMBC spectrum of compound 8.
the ␣-carbons (ꢀδ␣) varying from 2.0 to 7.5 ppm, depending
on the structure of the oximes [26,41–43]. The difference in the
chemical shift increases with a decrease in the dihedral angle
between the C N and C–␣-H bonds. If the ␣-hydrogen lies in
the NOC-␣ plane, the dihedral angle between them is zero. For
such oximes the ꢀδ␣ value is around 7 ppm [42]. The ꢀδ␣ values
of the synthesized compounds 8–14 are furnished in Table 9. The
ꢀδ␣ values of 8–12 are not much deviated from 7.0 ppm while
compound 13 exhibits a slight variation only, which suggest
there may be no appreciable possibility for the boat conforma-
tion. For 14, the deviation is appreciable and suggests that there
may be a possibility for the significant contribution to the boat
form in addition to the predominant chair form.
2.6.2. β-Effect
The ␤-effect can be studied by comparing the chemical shifts
of the ␤-carbon C-2 and C-6 of the piperidone oxime ether
with corresponding piperidones. In six membered heterocycles,
decrease in electronegativity of a group in the ring deshield
␤-carbons and shield ␤-protons [25].
2.6.1. α-Effect
The ␣-effect can be studied by comparing the chemical shifts
of the ␣-carbon C-3 and C-5 of the oxime ether with the corre-
sponding piperidones. In oxime ethers, the C N group shields
the ␣-carbon due to its decreased electronegativity than C O.
So shielding of ␣-carbon is in accord with the expected elec-
The deshielding of anti ␤-carbon is in accordance with the
expected electronegativity effect. The magnitude of deshield-
ing is about 1 ppm for compounds 8–12, but 13 and 14 were
deshielded, respectively, by 0.65 and 0.26 ppm. This is due
to the replacement of methyl by bulkier ethyl and isopropyl
group, respectively, in 13 and 14. But the syn ␤-carbons are
shielded. Moreover, the shielding magnitude is increased from
compounds 8–14. The reason for the shielding is due to the
negative charge on the syn ␣-carbon, which is transmitted to
small extent on the syn ␤-carbon. Thus the shielding produced
on the syn ␤-carbon by the transmittance of the negative charge
overcomes the deshielding produced by the electronegativity
effect.
Table 9
Chemical shift difference between the ␣-carbons (ꢀδ␣) in oxime ethers (8–14)
and piperidones (1–7)
Oxime ether
Piperidone
δanti − δsyn (ppm)
Oxime ether
Piperidone
Oxime
ether-piperidone
8
9
1
2
3
4
5
6
7
6.21
8.60
8.61
8.52
8.62
0
6.21
7.90
7.89
7.84
7.92
8.29
11.56
0.70
0.72
0.68
0.70
6.80
7.05
10
11
12
13
14
The ␤-protons are shielded according to electronegativity
effect. Shielding of syn ␤-proton is pronounced (about 0.1 ppm)
than the anti ␤-proton due to the negative charge on the syn ␤-
15.09
18.61

P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
23
Table 10
Chemical shift differences between piperidone ring protons of compounds 8–14 (δ, ppm)
Compound
H(5e)–H(5a)
H(3e)–H(3a)
H(5e)–H(3a)
H(3a)–H(5a)
H(2a)–H(6a)
8
9
1.51
1.58
1.62
1.53
1.57
1.49
1.10
0.24
–
–
–
–
1.10
1.11
1.15
1.07
1.13
1.12
0.88
0.33
0.47
0.47
0.46
0.45
0.37
0.22
0.08
−0.33
−0.32
−0.33
−0.33
−0.19
0.01
10
11
12
13
14
–
–
Fig. 17. 13C NMR spectrum of compound 9.
carbon and is more (0.3 ppm) for 14 due to its conformational
preference. The chemical shift difference between the benzylic
protons is presented in Table 10.
stituent at C-3) and 14 (CH3 groups of the CH(CH3)2 at C-3)
and its deshielding magnitude is about 0.47 and 0.23 ppm,
respectively.
2.6.3. γ-Effect
3. Conclusion
The␥-effectisstudiedbycomparingthechemicalshiftsofthe
ipso carbons (C-2ꢀ and C-6ꢀ) in piperidones with oxime ethers.
In all compounds the ipso carbons are deshielded in accor-
dance with the electronegativity effect. The ␥-effect magnitude
is about 1 ppm for 8–12 and is decreased for 13 whereas highly
increased for 14 due to their substituent nature at C-3. All the
values are reported in Table 8.
All the synthesized 2,6-diarylpiperidin-4-one O-benzylo-
ximes (8–14) were characterized by their IR, Mass and NMR
spectra. All the spectral data support and confirm the formation
of the target compounds. Well-pronounced oximination effect
is observed on the piperidone ring carbons and the associated
protons. The oximination effect is extended up to ipso carbons
and alkyl substituents at C-3.
2.6.4. Oximino group effect on alkyl group
From the observed chemical shifts and coupling constants,
the conformation of the synthesized compounds is proposed
and conformational preference of the alkyl substituent at C-3
also established. The compounds 8–13 adopt normal chair con-
formation with equatorial disposition of substituents at C-2, C-6
and C-3 while the compound 14 contribute significant boat con-
In accordance with the electronegativity effect, the ␤-effect
on methyl carbon is about 1.8 ppm (deshielding) for 9–12
and methylene and methine carbon (C-7) of 13 and 14 are
1.05 and 1.91 ppm, respectively. The ␥-effect also observed
on the alkyl group in 13 (CH3 group of the CH2CH3 sub-

24
P. Parthiban et al. / Spectrochimica Acta Part A 70 (2008) 11–24
formation along with the predominant chair conformation in
solution.
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4. Experimental
All the reported melting points were taken in open capillaries
and are uncorrected. IR spectra were recorded in AVATAR-330
FT-IR spectrophotometer (Thermo Nicolet) and only notewor-
thy absorption levels (reciprocal centimeters) are listed. 1H
NMR spectra were recorded at 400 MHz on BRUKER AMX
400 MHz spectrometer using CDCl3 as solvent and TMS as
internal standard and 13C NMR spectra were recorded at
100 MHz on BRUKER AMX 400 MHz spectrometer in CDCl3.
1H–1H COSY, phase-sensitive NOESY, one-bond and multiple
bond 1H–13C correlations spectra were recorded on BRUKER
DRX 500 MHz NMR spectrometer using standard parameters.
0.05 M solutions of the sample prepared in CDCl3 were used
for recording 2D NMR spectra. The tubes used for recording
NMR spectra are of 5 mm diameter. Electron impact mass spec-
tra were recorded on mass engine HP 5989 series while ESMS
was recorded on an API 3000 series mass spectrometer and
microanalyses were performed on Heraeus Carlo Erba 1108
CHN analyzer. Unless otherwise stated, all the reagents and sol-
vents used were of high grade and purchased from Lancaster
and Merck. All the solvents were distilled prior to use.
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57 (1992) 6006.
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Chem. 39B (2000) 419.
All the parent 2,6-diarylpiperidin-4-ones were prepared by
the literature precedent of Noller and Baliah [3].
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4.1. Synthesis of 2,6-diarylpiperidin-4-one O-benzyloximes
A mixture of 2,6-diarylpiperidin-4-one 1 (0.1 mol), O-
benzylhydroxylamine hydrochloride (0.1 mol) and sodium
acetatetrihydrate(0.3 mol) in methanolwas refluxed till comple-
tion of the reaction. After completion of the reaction water was
added and extracted with ether, dried with anhydrous sodium
sulphate and evaporated. The residue was triturated with sol-
vent ether to get solid product 8. The compounds 9–14 were
also synthesized similarly.
[31] A. Manimekalai, J. Jayabharathi, L. Rufina, R. Mahendiran, IndianJ. Chem.
42B (2003) 2074.
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43B (2004) 1753.
Acknowledgements
[34] A. Manimekalai, J. Jeyabharathi, R. Sankardoss, Indian J. Chem. 43B
(2004) 1018.
[35] H. Booth, Tetrahedron 22 (1966) 615.
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38B (1999) 52.
[38] S. Balasubramanian, Ph.D. thesis, Annamalai University, 2003.
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Saundes, Can. J. Chem. 56 (1978) 1940.
We are thankful to Prof. K. Pandiarajan, Head, Department of
Chemistry, AnnamalaiUniversityforthefacilitiesprovided. One
of the authors SK is grateful to University Grants Commission,
New Delhi, India for financial support in the form of Major
Research Project. We also thank NMR Research Centre, IISc,
Bangalore for providing all the NMR facilities to record NMR
spectra.
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