Atropisomerism about the C(1)-N single bond in N,N-disubstituted-1-aminoanthracenes: isolation of conformational diastereomers

Article abstract of DOI:10.1007/s12039-018-1473-9

Abstract: Restricted rotation about the C(1)-N single bond in N,?N-disubstituted 1-aminoanthracenes has been investigated. The synthesis, isolation and characterization of diastereomers arising out of the hindered rotation about the CSP2-NSP2 single bond

Full text of DOI:10.1007/s12039-018-1473-9

J. Chem. Sci. (2018) 130:61
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1473-9
REGULAR ARTICLE
Atropisomerism about the C(1)-N single bond in
N,N-disubstituted-1-aminoanthracenes: isolation of
conformational diastereomers
RUHIMA KHAN , ANJANA NINGOMBAM and
MOIRANGTHEM DHANESHWAR SINGH∗
Department of Chemistry, Manipur University, Canchipur, Imphal 795 003, Manipur, India
E-mail: moirangthemds@yahoo.co.in; ruhimakhan@yahoo.in
MS received 24 November 2017; revised 26 April 2018; accepted 29 April 2018
Abstract. Restricted rotation about the C(1)-N single bond in N, N-disubstituted 1-aminoanthracenes has been
investigated. The synthesis, isolation and characterization of diastereomers arising out of the hindered rotation
about the CSP2 -NSP2 single bond has been demonstrated. With unsymmetrical substituents on the Nitrogen atom,
preferred conformations about the C(1)-N bond were observed.
Keywords. Diels-Alder reaction; steric interaction; restricted rotation; atropisomerism; conformational
diastereomers.
1. Introduction
about a single bond is generally due to the combined
effects of steric hindrance and electronic repulsion. The
effect of electronic repulsion on the control system
of the rotational barrier has been studied by prepar-
ing 4ꢀ-substituted 2-benzylamino-N-(2ꢀ-methylphenyl)
succinimides 1 (Scheme 1).8 The compound exists as
two isomers 1a and 1c. The two isomers are easily dis-
tinguished from each other by 1H NMR chemical shifts
of the 2ꢀ-methyl group. The two isomers exist in the ratio
1:1.
Stereoisomerism due to restricted rotation about sin-
gle bonds, i.e. atropisomerism is a well-known and
widespread phenomenon among the biaryls and have
been exploited in the form of the binaphthyl class of
chiral auxiliaries.9,10 Biaryls are commonly used as chi-
ral ligands and chiral auxiliaries.9,11 Clayden et al.,12
have reported that atropisomerism which arises due
to restricted rotation about single bonds of the non-
biaryl compounds results into the formation of new
chiral centres with stereoselectivity that can exceed
99:1.
Atropisomers are conformers which interconvert slowly
enough because of steric and electronic constraints that
they can be isolated.1–3 The result of the restricted
rotation about a single bond can be such that an appar-
ent single compound can actually be a mixture of two
compounds or, an achiral compound can be a racemic
mixture. Atropisomerism may give rise to geometri-
cal isomers, diastereoisomers, or enantiomers. It offers
new ways of thinking about stereochemistry, dynamic
processes, and about the relationship between structure
and activity in both the biological and stereoselective
sense. The conformational restriction has been used
by scientists to study the compounds having struc-
turally crowded features that cause decreased freedom
of intramolecular motion, particularly of rotation around
chemical bonds.
The restricted rotation about N-Ar single bond of
N-phenyleimide has been utilised for stereoselective
synthesis,4 molecular recognition5 and self assembly6
and for the preparation of functionalised polymers.7 The
importance of both the rotational barrier and the spatial
disposition of substituents on the N-phenyl group has
been investigated in these works. The restricted rotation
Various methods have been developed for the
diastereo- and enantioselective synthesis of atropiso-
meric molecules resulting from rotational barrier about
single bonds.13,14 The potential of the axial chirality of
non-biaryl atropisomeric molecules as a source of asym-
metric induction has been exploited.15–17
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1473-9) contains
supplementary material, which is available to authorized users.

61 Page 2 of 7
J. Chem. Sci. (2018) 130:61
solid at 8% ethyl acetate in hexane. Yield: 79% (550 mg);
◦
M.p.: 150−153 C; C23H19NO: Anal. Found: C, 84.85; H,
5.84; N, 4.27 Calc.: C, 84.89; H, 5.89; N, 4.30; IR (cm−1):
1
3024, 3003, 2952, 2837, 1726, 1708, 1654,1271; H NMR
(δ, ppm in CDCl3): 8.51 (s, 1H), 8.33 (s, 1H), 7.97–8.04 (m,
3H), 7.5–7.55 (m, 2H), 7.2–7.33 (m, 6H), 6.92–6.93 (d, J =
5.4 Hz, 1H), 4.31 (s, 2H), 1.84 (s, 3H); 13C NMR (δ, ppm
in CDCl3): 171.2, 139.3, 137.8, 129.7, 129.5, 128.2, 128.0,
127.4, 127.3, 127.0, 126.5, 126.2, 126.0, 125.2, 124.1, 47.2,
21.2.
X- NEt2, OMe, Me, H, Cl, F, CO2Me, NO2
2.2b N-acetyl-N-(anthracen-1-yl) benzamide 7: To a
suspension of 1-acetamidoanthracene (500 mg) in 5% NaOH
(5 mL) was added few drops of THF. Then benzoyl chloride
(0.5 mL) was added and the reaction mixture was stirred at
room temperature for 5 h. The solid material formed was fil-
tered, washed with water and dried. Then it was recrystallized
from ethanol to get N-acetyl-N-(anthracen-1-yl) benzamide
as off-white solid. Yield: 85% (620 mg); M.p.: 175−178 ◦C;
C23H17NO2: Anal. Found: C, 81.37; H, 5.02; N, 4.11 Calc.:
Scheme 1. Rotational isomerization of 1b to 1a
and 1b.
We wish to report here the stereochemical outcome
of the Diels-Alder reaction of 1-N, N-disubstituted
aminoanthracenes with symmetrical dienophiles. It has
been observed that introduction of the bridge at the 9,10-
positions results in the restricted rotation of the aryl C, 81.40; H, 5.05; N, 4.13; IR (cm−1): 3093, 2994, 2868,
1
1703, 1679, 1418, 932; H NMR (δ, ppm in CDCl3): 8.44–
8.47 (d, J = 8.1 Hz, 2H), 7.97–8.01 (m, 3H), 7.65–7.67
(m, 2H), 7.22–7.52 (m, 7H), 2.45 (s, 3H); 13C NMR (δ, ppm
in CDCl3): 175.0, 172.8, 139.9, 133.0, 131.4, 131.2, 130.0,
129.7, 129.2, 129.1, 128.5, 128.3, 127.9, 126.7, 126.2, 125.1,
124.0, 123.7, 121.4, 22.4.
C(1)-N single bond, paving the way for the isolation
of conformational diastereomers.
2. Experimental
2.1 Materials and Physical measurements
2.3 General procedure for the Diels-Alder reaction
of 1-N-disubstituted anthracenes with
dimethylacetylenedicarboxylate
The melting points were recorded on ‘Veego’ melting point
apparatus and are uncorrected. IR spectra were recorded
in KBr on a Shimadzu FT-IR 8400. The 1H NMR and
13C NMR spectra were recorded on 400 MHz Jeol ECS-400
(or 100 MHz for 13C NMR) spectrometers using either resid-
ual solvent signals as an internal reference or from internal
tetramethylsilane on the δ scale (CDCl3δH, 7.24 ppm, δC 77.0
ppm and for DMSO-d6δH, 2.52 ppm, δC 41.23 ppm). The
chemical shifts (δ) are reported in ppm and coupling constants
To a 100 mL round bottom flask, the diene (1 equiv.) was
dissolved in toluene. To it was added dimethylacetylenedi-
carboxylate (3 equiv.) dropwise at room temperature. The
reaction mixture was then refluxed for 6 h. TLC showed com-
pletion of the reaction and two new products were observed.
Then the reaction mixture was cooled and the solvent was
evaporated under reduced pressure. The residue was purified
by silica gel column chromatography. The two isomers were
eluted at different solvent systems.
1
(J) in Hz. Abbreviations for H NMR multiplicities are as
follows: s-singlet; d-doublet; dd-doublet of doublet; bs-broad
singlet; m-multiplet. The compounds were purified by col-
umn chromatography on silica gel using petroleum ether and
ethyl acetate as a solvent mixture, followed by crystallization
from different solvents.
2.3a (9R,10S)-dimethyl 1-(N-acetylacetamido)-9,10-
dihydro-9,10-ethenoanthracene-11,12-dicarboxylate 4:
◦
Colorless solid; Yield: 82% (620 mg); M.p.: 178−181 C;
2.2 Synthesis of dienes
C24H21NO6: Anal. Found: C, 68.71; H, 5.02; N, 3.31 Calc.:
C, 68.73;H, 5.05;N, 3.34;IR(cm−1):3010, 2958, 1714, 1695,
2.2a N-(anthracen-1-yl)-N-benzylacetamide 5: To a
solutionof1-acetamidoanthracene(500mg)inDMSO(4mL)
was added Na2CO3 (220 mg) and the mixture was stirred at
room temperature for 10 min. Then benzyl chloride (0.3 mL)
was added dropwise. The reaction mixture was then heated at
80−90 ◦C for 5 h. Then the reaction mixture was cooled and
water was added and extracted with ethyl acetate. The organic
layer was washed with water and dried over Na2SO4 and
concentrated to get the crude product. The product was then
1
1631, 1471, 1274, 1058, 758; H NMR (δ, ppm in CDCl3):
6.61–7.12 (m, 7H), 5.38 (s, 1H), 5.12 (s, 1H), 3.59 (s, 3H),
3.58 (s, 3H), 2.10 (s, 3H), 2.05 (s, 3H); 13C NMR (δ, ppm
in CDCl3): 171.3, 170.7, 167.0, 166.5, 141.3, 131.4, 131.0,
129.2, 129.1, 126.7, 126.2, 125.1,123.9, 53.0, 52.8, 45.1, 37.4,
22.7, 21.0.
The total yield of both isomers 6a and 6b: 78% (560 mg).
purified by silica gel column chromatography. The pure N- 2.3b (9R, 10S)-dimethyl 1-(N-benzylacetamido)-9,10-
(anthracen-1-yl)-N-benzylacetamidewaselutedascolourless dihydro-9,10-ethenoanthracene-11,12-dicarboxylate

J. Chem. Sci. (2018) 130:61
Page 3 of 7 61
Scheme 2. Diels-Alder reaction of 1-diacetamidoanthracene with DMAD.
6a: Colorless solid; M.p.: 169−171 ◦C; C29H25NO5: Anal. 1684, 1656, 1416, 1293; 1H NMR (δ, ppm in CDCl3): 7.90–
Found: C, 74.46; H, 5.37; N, 3.03 Calc.: C, 74.50; H, 5.39; N, 7.95 (m, 3H), 7.52–7.58 (m, 3H), 6.99–7.38 (m, 6H), 5.60
3.00; IR (cm−1): 3060, 3024, 2970, 2835, 1776, 1710, 1653, (s, 1H), 5.47 (s, 1H), 3.75 (s, 6H), 2.36 (s, 3H); 13C NMR
1587, 1477, 1383, 1189; 1H NMR (δ, ppm in CDCl3): 6.98– (δ, ppm in CDCl3):175.2, 172.3, 166.8, 165.6, 141.7, 132.5,
7.51 (m, 11H), 6.46–6.51 (m, 1H), 4.98 (s, 1H), 4.74 (s, 1H), 131.7, 130.9, 128.4, 127.9, 126.6, 125.8, 125.5, 125.2, 123.8,
4.30 (s, 2H), 3.49 (s, 3H), 2.94 (s, 3H), 2.24 (s, 3H); 13C NMR 119.6, 119.4, 107.7, 51.9, 51.6, 45.1, 35.8, 22.5.
(δ, ppm in CDCl3): 175.7, 167.4, 165.3, 143.5, 139.3, 138.6,
134.9, 129.1, 126.4, 125.3, 125.0, 52.3, 49.3, 45.8, 44.9, 37.8,
22.5.
3. Results and Discussion
2.3c (9R, 10S)-dimethyl 1-(N-benzylacetamido)-9,10-
Hindered rotation and non-planar conformation about
dihydro-9, 10-ethenoanthracene-11,12-dicarboxylate
the C(1)-N bond in 1-diacetamidoanthrane (2) and 1-
6b: Colorless solid; M.p.: 192−195 ◦C; C29H25NO5: Anal.
diacetamidoanthraquinone (3) have been reported.17
Found: C, 74.48; H, 5.33; N, 3.01 Calc.: C, 74.50; H, 5.39; N,
The two acetyl methyls appear as a singlet in both 2
and 3 because the two acetyl methyls have a similar
environment (CS symmetry) in the non-planar confor-
mation.
3.00; IR (cm−1): 3052, 3025, 2948, 2863, 1729, 1714, 1649,
926; 1H NMR (δ, ppm in CDCl3): 6.94–7.56 (m, 11H), 6.41–
6.58 (m, 1H), 4.94 (s, 1H), 4.79 (s, 1H), 4.28 (s, 2H), 3.41 (s,
3H), 3.28 (s, 3H), 2.11 (s, 3H); 13C NMR (δ, ppm in CDCl3):
175.5, 166.8, 165.2, 143.5, 139.3, 138.6, 134.9, 130.8, 129.1,
126.4, 125.3, 125.1, 125.0, 52.9, 52.4, 46.8, 44.3, 37.8, 26.4.
The total yield of both isomers 8a and 8b: 81% (580 mg).
If the CS symmetry is destroyed, the two N-
substituents will have different environments and hence
1
will be non-equivalent in the H NMR spectrum. This
is exactly observed when a bridge was introduced
at the C(9) and C(10) positions by reacting
1-diacetamidoanthracene with dimethylacetylenedicar-
boxylate (DMAD) as shown in Scheme 2.
2.3d (9R, 10S)-dimethyl 1-(N-acetlylbenzamido)-9,10-
dihydro-9, 10-ethenoanthracene-11,12-dicarboxylate
8a: Colorless crystal; M.p.: 203−205 ◦C; C29H23NO6:
Anal. Found: C, 72.32; H, 4.76; N, 2.85 Calc.: C, 72.34; H,
4.81;N, 2.91;IR(cm−1):3003, 2952, 2922, 2837, 1726, 1708,
1654, 1452, 1271, 1059; 1H NMR (δ, ppm in CDCl3): 7.90-
8.05 (m, 3H), 7.48-7.88 (m, 6H), 7.19-7.46 (m, 3H), 5.57 (s,
1H), 5.48 (s, 1H), 3.89 (s, 3H), 3.47 (s, 3H), 2.22 (s, 3H);
13C NMR (δ, ppm in CDCl3): 176.6, 172.8, 167.2, 166.3,
143.0, 136.2, 132.1, 132.0, 131.6, 130.5, 128.7, 128.3, 128.0,
127.5, 127.2, 126.2, 126.1, 126.0, 124.3, 120.5, 52.1, 48.8,
45.1, 36.0, 22.6.
The creation of a C(9)-C(10) bridge by the Diels-
Alder reaction of 1-diacetamidoanthracene with DMAD
causes steric hindrance resulting into the restricted rota-
tion about the aryl C(1)-N bond. This also provides
dissimilar magnetic environments to the two N-acetyl
groups. Hence, the two acetyl groups become non-
equivalent in the Diels-Alder adduct. This is confirmed
1
by the H NMR spectrum of the product 4. The two
acetyl methyl groups appear as two different singlets at
2.05 δ and 2.10 δ. The probability of N-aryl resonance
due to delocalization of nitrogen lone pair onto the aro-
matic ring is very unlikely because planar arrangement
about the C(1)-N bond will result into high steric crowd-
2.3e (9R, 10S)-dimethyl 1-(N-acetlylbenzamido)-9,10-
dihydro-9, 10-ethenoanthracene-11,12-dicarboxylate
8b: Colorless crystal; M.p.: 219−221 ◦C; C29H23NO6:
Anal. Found: C, 72.31; H, 4.79; N, 2.87 Calc C, 72.34; H,
4.81;N, 2.91;IR(cm−1):3084, 2951, 2887, 2837, 1723, 1702, ing as the N-atom is attached to two carbonyl groups.

61 Page 4 of 7
J. Chem. Sci. (2018) 130:61
Scheme 3. Diels-Alder reaction of N-(anthracen-1-yl)-N-benzylacetamide (5) with DMAD.
Hence, the formation of a partial double-bond about chromatography using ethyl acetate and hexane as elu-
1
the C(1)-N bond cannot be the reason for the observed ent. The isomers were confirmed by H NMR and
results. The carboxylate methyl groups appear as sin- 13C NMR spectra of the adducts.
1
glets at 3.58 δ and 3.59 δ. The singlet peaks at 5.12 δ and
In the H NMR spectrum of 6a, one of the two car-
5.38 δ are for the protons at C(9) and C(10) positions. boxylate methyl groups appear more upfield (2.94 δ)
The aromatic protons appear in the range 6.61–7.12 δ. than the other. This corresponds to the CH3 group that
The non-equivalence of the two acetyl groups was fur- lies in the shielding zone of the phenyl ring of the ben-
ther confirmed from the 13C NMR spectrum of 4. They zyl group. The shielding effect is probably due to CH/
π
π
appear as two different peaks at 22.7 δ and 21.0 δ. interaction between the methyl protons and the elec-
This observation confirms that the introduction of C(9)- trons of the phenyl ring. Such effect is absent in the case
C(10) bridge in 1-diacetamidoanthracene affect the free of the other carboxylate group as it is far from the phenyl
rotation about the aryl C(1)-N bond. This provides a dif- ring and hence it appears more downfield (3.49 δ) in the
ferent magnetic environment to the two N-acetyl groups 1H NMR spectrum. The acetyl methyl group appears as
and hence they become non-equivalent. The product 4 a singlet at 2.24 δ. The singlet peak at 4.30 δ corre-
exhibits IR absorption at 3010 and 2958 cm−1 which are sponds to the benzylic methylene group. The protons
characteristics of the aromatic, aliphatic groups, respec- at C(9) and C(10) positions appear as a singlet at 4.74
tively. The peaks at 1714, 1695 cm−1 are for the two δ and 4.98 δ. The aromatic protons appear in the range
carboxylate carbonyl groups.
6.46–7.51δ. Thisobservationisfurtherconfirmedbythe
13C NMR spectrum. The acetyl carbonyl group appears
at 175.7 δ. The spectrum showed two different groups
for the two carboxylate carbonyl groups at 167.4 δ and
165.3 δ. The carboxylate methyl groups appear at 52.3
δ and 49.3 δ. Here, one of the carboxylate appears more
upfield at 49.3 δ than the other one. This shows that it
experiences shielding effect of the phenyl ring. In the IR
spectrum, the aromatic, aliphatic groups appear at 3060,
3024, 2970, 2835 cm−1. The two carboxylate group are
shown by the carbonyl peaks at 1776, 1710 cm−1. The
acetyl carbonyl group appears at 1653 cm−1. In the case
of 6b, since the benzyl group is far from the C(9)-C(10)
bridge, the carboxylate methyl group does not experi-
ence any shielding effect like that of 6a. Hence, the two
-CO2CH3 groups appear very near as two singlets at
3.28 δ and 3.34 δ in the 1H NMR spectrum. The singlet
at 2.11 δ is for the acetyl methyl group. In the 13C NMR
spectrum of 6b, the carbonyl group appears at 175.5 δ.
The peaks at 166.8 δ and 165.3 δ correspond to the two
carboxylate carbonyl groups. Here, since the carboxy-
late methyl group do not fall in the shielding zone of
the phenyl ring, the two methyl groups appear almost in
the same range at 52.9 δ and 52.4 δ. This observation
shows that the formation of C(9)-C(10) bridge results
in the restricted rotation of the aryl C(1)-N bond. The
groups attached to the N-atom being different give rise
to the two atropisomers 6a and 6b.
3.1 Variable-Temperature 1 H NMR Studies
In order to evaluate the barrier to rotation (ꢀG ) about
the C(1)-N bond, the compound 4 was subjected to
variable temperature 1H NMR studies. The two singlets
for the two N-acetyl methyl groups (2.05 δ and 2◦.10
δ) remain unchanged at elevated temperature (110 C,
toluene—d8) and there is no sign of coalescence even
◦
at 180 C (Nitrobenzene—d5). Using the Gutowsky–
Holm approximation, a lower limit of ꢀG more than
23 kcal mol−1 is estimated18 which is the required
energy barrier to the rotation for the rotational isomers
to be separated as stable species at room temperature.19
If the substituents at C(1)-N position are different,
then we can expect two stereoisomers, i.e. atropiso-
mers. So in order to study this, we focussed on the
Diels-Alder reaction of unsymmetrically substituted
aminoanthracenes with DMAD (Scheme 3). For this, N-
(anthracen-1-yl)-N-benzylacetamide (5) was prepared
by acetylation of 1-aminoanthracene followed by ben-
zylationandusedasdiene. TheDiels-Alderreactionwas
carried out by refluxing the diene and the dienophile
in toluene for 9 hours. The starting materials were
consumed and gave two new spots on thin layer chro-
matography which were separated by silica gel column

J. Chem. Sci. (2018) 130:61
Page 5 of 7 61
Scheme 4. Diels-Alder reaction of 7 with DMAD.
Table 1. Diels-Alder reaction of N, N-disubstituted-1-aminoanthracenes with
DMADa.
CO2CH3
CO2CH3
H3CO2C
H3CO2C
R
R'
R
N
R'
N
N
R'
R
CO2CH3
CO2CH3
toluene, reflux
6 h
+
+
4, 6a, 8a
4, 6b, 8b
Substrate
Time (h) Adduct Ratio (a:b) Total yield (%)b
R = Rꢀ- COCH3
6
9
6
4
6
8
-
82
78
81
R- CH2Ph, Rꢀ- COCH3
R- COPh, Rꢀ- COCH3
65:35
56:44
aReaction conditions: Diene (1 equiv.), DMAD (3 equiv.) in toluene (5 mL) at
120 ◦C for 6 h.
bisolated yields.
In order to further prove the formation of atropiso- to C(1)-N group are different, it gives rise to the two
1
mers, the Diels-Alder reaction between N-acetyl-N- diastereomers 8a and 8b. In the H NMR spectrum of
(anthracen-1-yl) benzamide (7) and DMAD was carried 8a, the singlet peak at 2.23 δ is for the acetyl methyl
out. The reaction was carried out by refluxing the reac- protons. The two carboxylate methyl groups are shown
tion mixture in toluene for 6 hours. In this case, also, thin by the two singlets at 3.47 δ and 3.89 δ. In this, one
layer chromatography showed the formation of two new of the carboxylate methyl groups which is near the N-
spots. They were separated by silica gel column chro- benzoyl group appears more upfield at 3.47 δ. This is
matography. Here also, from the 1H NMR and 13C NMR because the methyl group falls in the shielding zone of
spectrum, it was found that the two spots were the atropi- the phenyl ring. The protons at C(9) and C(10) positions
somers 8a and 8b which was resulted from the restricted appear as singlets at 5.48 δ and 5.56 δ. The aromatic pro-
rotation about the aryl C(1)-N single bond (Scheme 4). tons correspond to the multiplet in the range 7.19–8.05
In both the cases of 6 and 8 also, the possibility of hav- δ. In the 13C NMR spectrum of 8a, the acetyl and the
ing a planar arrangement about the C(1)-N bond which benzoyl carbonyl groups appear at 176.6 δ and 172.8 δ.
may lead to the partial double bond formation is ruled The two carboxylate carbonyl groups are shown by the
out as the N-atom is attached to two bulky groups. In peaks at 167.2 δ and 166.3 δ. Here the two carboxylate
amide systems like the most widely investigated N, N- methyl groups appear as two different peaks at 52.1 δ
dimethylformamide and related derivatives, the methyl and 48.8 δ. This is because one of the methyl groups fall
signals are different because of restricted rotation about in the shielding zone of the phenyl ring and hence appear
the N-CO bond, but conformers cannot be physically more upfield at 48.8 δ. The IR spectrum of 8a exhibits
isolated. Physical isolation of 6a and 6b and also 8a peaks at 3003, 2952, 2922, 2837 cm−1 corresponding
and 8b which are conformational diastereoisomers are to the aromatic and the aliphatic groups. The carbonyl
interesting examples of atropisomerism about the Aryl peaks at 1726 and 1708 cm−1 are for the two carboxy-
C-N bond.
late groups. The peak at 1654 cm−1 corresponds to the
The two isomers could be differentiated from the acetyl group. In the 1H NMR spectrum of 8b, the acetyl
1H NMR and 13C NMR spectra of the adducts. In this methyl protons appear as a singlet at 2.36 δ. The two
case, also, the formation of the C(9)-C(10) bridge in carboxylate methyl protons appear as singlets almost
the Diels-Alder adducts results in the hindered rotation in the same range at 3.754 δ and 3.757 δ. In the case
about the aryl C(1)-N bond. Since the groups attached of 8b, none of the methyl protons experience any kind

61 Page 6 of 7
J. Chem. Sci. (2018) 130:61
of shielding effect as that of 8a and hence appear at Acknowledgements
1
the same range in the H NMR spectrum. This is fur-
The authors are grateful to University Grant Commission
ther confirmed from the 13C NMR spectrum. Here the
peaks at 175.2 δ and 172.3 δ correspond to the acetyl and
benzoyl carbonyl groups. The two carboxylate carbonyl
groups are shown by the peaks at 166.8 δ and 165.6
δ. In the isomer 8b, as the carboxylate methyl groups
do not fall in the shielding zone, they appear almost in
(UGC), New Delhi, India for the financial support under the
scheme of UGC-BSR Research Fellowship in Science. The
authors are also thankful to the Head of the Department of
Chemistry, Manipur University, Manipur, India for providing
the necessary facilities.
the same range at 51.9 δ and 51.6 δ. This is because References
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at C(9) and C(10) positions appear as singlets at 5.47 δ
and 5.60 δ. The aromatic protons appear as multiplet in
the range 7.01–7.95 δ. The overall result of our studies
are summarised in Table 1.
To confirm the structures of 6a, 6b and 8a, 8b, we
would have liked the single crystal X-ray study of at
least one of the compounds. However, in spite of several
attempts, unfortunately, we could not get good crys-
tals suitable for X-ray crystallography. Nevertheless, the
NMRdataarequitereasonabletoestablishthestructures
of the compounds.
From our studies, it has been observed that the forma-
tion of C(9)-C(10) bridge due to Diels-Alder reaction
can bring about the restricted rotation about the aryl
C(1)-N single bond. This results in the formation of
atropisomers when the groups attached to the C(1)-N
position are non-equivalent. In both the cases of 6 and
8, the isomers 6a and 8a were observed as the major
π
adducts. This may be because of the possible –electron
interaction between the ethylenic bridge and the aryl
system making them more stable. The Diels-Alder reac-
tion gave good yield in all the three cases.
4. Conclusions
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5577
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12. De Kok P M T, Bastiaansen L A M, Van Lier P
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to the steric interaction between the C(9)- C(10) bridge
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1313
π
N position are non-equivalent. The -electron system of
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obtained as the major isomer in very good overall yield.
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Supplementary Information (SI)
Supplementary Information is available at www.ias.ac.in/
chemsci.

J. Chem. Sci. (2018) 130:61
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