One-pot access to pyridocoumarins via Povarov-hydrogen transfer cascade under auto-tandem catalysis of iodine in aqueous micelles

Article abstract of DOI:10.1016/j.tetlet.2014.01.078

A cascade of inverse electron demand Diels-Alder reaction of 6-aminocoumarin-derived aldimines with an excess of styrene and oxidative aromatization of the formal Povarov adducts under autotandem iodine catalysis in aqueous micellar conditions provides direct access to a small library of substituted pyridine annulated coumarins in good to acceptable yields (12 examples). The unusual formation of linearly annulated pyridocoumarins under essentially neutral conditions is a remarkable feature of the protocol.

Full text of DOI:10.1016/j.tetlet.2014.01.078

Tetrahedron Letters 55 (2014) 1564–1568
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
One-pot access to pyridocoumarins via Povarov-hydrogen transfer
cascade under auto-tandem catalysis of iodine in aqueous micelles
⇑
Nemai C. Ganguly , Sumanta Chandra
Department of Chemistry, University of Kalyani, Kalyani 741235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A cascade of inverse electron demand Diels–Alder reaction of 6-aminocoumarin-derived aldimines with
an excess of styrene and oxidative aromatization of the formal Povarov adducts under autotandem iodine
catalysis in aqueous micellar conditions provides direct access to a small library of substituted pyridine
annulated coumarins in good to acceptable yields (12 examples). The unusual formation of linearly annu-
lated pyridocoumarins under essentially neutral conditions is a remarkable feature of the protocol.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 5 August 2013
Revised 15 January 2014
Accepted 18 January 2014
Available online 27 January 2014
Keywords:
Pyridocoumarins
Povarov-hydrogen transfer
Auto-tandem catalysis
Iodine
Aqueous micelles
Quinolines constitute core substructure of a broad spectrum of
biologically relevant molecules including antimalarial,1a antiasth-
matic,1b antihypertensive,1c antiinflammatory1d and several new
generation antibacterial drugs.2 Apart from their substantial pres-
ence in drug and pharmaceutical space, quinoline-based polymers
find diverse utilities in electronics, optoelectronics and nonlinear
optics.3 Tetrahydroquinoline also features in naturally occurring
alkaloids such as flindersine, oricine, vesprine with psychotropic
antiallergic and oestrogenic activities.4a–d Aryl tetrahydroquino-
lines are potential neuroprotective agents4e and act as anti-HIV
agents in vitro.4f Therefore, developing new multicomponent syn-
thetic protocols for these functionalized heterocyclic scaffolds has
evoked major contemporary response.5 The three-component Le-
wis/Bronsted acid-catalysed version of the Povarov reaction
involving aromatic amine, aldehyde and electron-rich dienophile
is a popular approach for entry into this class of compounds.6 It
is an important tool for diversity oriented synthesis (DOS).7 It
allows for functional, skeletal and stereochemical diversity by
installation of different functional elements in these components
and appending other privileged heterocyclic motifs onto quino-
lines for construction of new chemotypes of biological relevance.
3-Aminocoumarin-derived aldimines have been recently em-
ployed as ‘enamine-like’ heterodiene components for inter-, intra-
molecular Povarov reactions.8 Literature survey revealed that
substrate scope of the amine for DOS library construction by Pova-
rov reaction has received limited attention although a host of
aldehydes and dienophiles have been exploited. In continuation
of our interest to develop environmentally benign approaches9
towards biologically important targets we became interested to
evaluate the potential of 6-aminocoumarin (1) as an amine compo-
nent for Povarov reaction with aromatic aldehyde and styrene as
partners. It is easily accessible and the amino group is segregated
from the electron withdrawing influence of the
a-pyrone ring
due to the absence of vinylogous resonance imparting it ‘aniline-
like’ nature. This contention is supported by DFT calculation at
B3LYP level which clearly shows the ambiphilic character of 1 with
C-5 as the seat of highest electron density among ring carbons only
to be followed by C-7 (C-5 and C-7 correspond to 6 and 4 positions
of 6-aminocoumarin in DFT calculations, see Supporting informa-
tion). The in situ 2-azadiene derived from it is anticipated to
serve as a building block for tetrahydropyridocoumarins, either
angularly or linearly fused with its benzenoid ring, via Povarov
cycloaddition. It represents a different paradigm from 3-amino-
coumarin-based annulation of the same motif onto 3, 4 position
of
a-pyrone. Iodine has emerged as an attractive mild Lewis acid
catalyst for a plethora of organic transformations including C–C,
C–O and C–N bond formation reactions.10 It is non-toxic and may
be used in water as well as organic solvents. Herein, we reveal
one-pot three-component imino Diels–Alder reaction of 6-amino-
coumarin (1), aromatic aldehyde and styrene with iodine catalyst
under aqueous micellar conditions to deliver pyridine-annulated
coumarins.
For initial exploratory studies, an assembly of 6-aminocoumarin
(1, 1 equiv), benzaldehyde (2a, 1.1 equiv) and styrene (3, 1.2 equiv)
⇑
Corresponding author. Tel.: +91 9477864491; fax: +91 33 25828282.
E-mail address: nemai_g@yahoo.co.in (N.C. Ganguly).
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.078

N. C. Ganguly, S. Chandra / Tetrahedron Letters 55 (2014) 1564–1568
1565
was submitted to reaction under aqueous micellar condition [H2O
(10 mL)-SDS (234 mg, 0.8 mmol)] without any additional catalyst
at room temperature as well as 60–70 °C resulting in isolation of
Schiff’s base as the sole product (Table 1 entries 1 and 2). However,
no Povarov adduct was formed in isolable amount. Attempted
reaction with 2 mol % of iodine catalyst without surfactant under
‘on water’ condition11 with a slurry of the reactants at 60–70 °C
did not proceed to completion but, surprisingly, yielded Schiff’s
base reduction product 6a together with minor amounts of two
pyridocoumarins 4a and 5a in a sluggish reaction (entry 3). It
was also revealed that the reaction was considerably accelerated
upon addition of the sodium dodecyl sulfate (234 mg, 0.8 mmol)
to the above reaction mixture under otherwise identical conditions
and provided 4a and 5a in better yields (entry 4). Increment of cat-
alyst loading to 5 mol % further improved the reaction rate and
yield of products. But the major constraint was the substantial
depletion of Schiff’s base due to its concomitant reduction making
it less available for generation of cycloadduct. To address the issue,
we felt that an excess of styrene might be utilized in an additional
role of external sacrificial oxidant to restrict the oxidative role of
the imine particularly because its C@C is more prone to reduction
than C@N bond of imines. It is a cost effective option than earlier
approaches of using an excess of imine to attain higher yields of
targeted quinolines.12
To our gratification, this diversionary strategy worked reason-
ably well and an impressive combined yield of 4a and 5a was
accomplished with an excess of styrene using 5 mol % of iodine cat-
alyst in H2O-SDS at 60–70 °C (entry 7). Nevertheless, the oxidative
capture of putative Povarov adduct by Schiff’s base could not be
eliminated altogether even under this condition. Attempt to cir-
cumvent this problem by using external oxidizers was not very
much of a success either. In separate experiments, DDQ and CAN
(2 millimolar each) were employed as additives and the reactions
were performed in the presence of non-excess styrene (1.2 mmol)
under otherwise similar conditions (entries 8 and 9). Comparable
combined yields of 4a and 5a were scored under these conditions.
However, variable amounts of reduced Schiff’s base were also iso-
lated implying that the internal hydrogen transfer process was not
completely precluded under these conditions. Increasing catalyst
loading to 10 mol % did not further improve the reaction efficiency.
Cationic and neutral amphiphiles viz. CTAB and Triton-X 100 were
also screened but they were less supportive to the reaction (entries
11 and 12). The superior catalytic activity of iodine was presum-
ably linked to its activation by SDS.13 It was also observed that
iodine was a relatively poor catalyst in organic solvents such as
CH3CN and THF (entries 13 and 14). This optimized reaction condi-
tion14 was applied successfully to a diverse range of aromatic
aldehydes substituted with electron-releasing and electron with-
drawing groups (12 cases). These results are summarized in
Table 2.
The procedure proved successful for all aromatic aldehydes
investigated (12 examples). The presence of electron-releasing
p-allyloxy and -methoxy did not significantly influence either the
combined yield of quinolines (73–66%) or the product distribution
(entries 2 and 3). However, electronically similar 3, 4-methylenedi-
oxybenzaldehyde delivered relatively poor overall yield of 54%
(entry 4). On the other hand, those substituted with electron-
withdrawing Br, F and CHO scored lower overall yields (30–48%),
with the exception of p-chlorobenzaldehyde (76%). The angularly
annulated products were always preferred to their linear counter-
parts. It was strikingly demonstrated for 4-fluorobenzaldehyde and
benzene 1, 3-dialdehyde where isolable amount of the linear
products was not formed (entries 8 and 9). The hydrophobic envi-
ronment of the micellar cavity seems to favour the more compact
angularly annulated product.15 Another notable feature was the
absence of formal tetrahydroquinoline precursors of all these
products. 6-Amino-1-methyl quinoline-2-(1H)-one (1a) also
participated in the reaction to provide two new skeletal structures,
8-(4-chlorophenyl)-4-methyl-10-phenylbenzo[f]quinolin-3(4H)-
one (4f) and 7-(4-chlorophenyl)-1-methyl-9-phenylbenzo[g]quin-
olin-2(1H)-one (5f). Furyl-2-aldehyde (2i) was also tolerated and
gave impressive yield of pyridocoumarins. Interestingly, the
elusive tetrahydroquinoline intermediate was isolated as the
major product (40%) with p-iodobenzaldehyde as the aldehyde
Table 1
Optimization of iodine-catalysed three-component reaction of 6-aminocoumarin, benzaldehyde and styrene
Ph
Ph
Ph
NH
Ph
+
PhCHO,
Ph
N
2a
N
3
NH2
+
O
O
I2, solvent
additive
O
O
O
O
Ph
O
O
5a
4a
6a
1
Entry
I2 (mol %)
Solvent
Additives (mmol)
3 (mmol)
Reaction time (h)a
Productsb (%)
4a
5a
6a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
—
—
2
2
5
5
5
5
5
10
5
5
5
5
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
THF (5 mL)
CH3CN (5 mL)
SDS
SDS
—
SDS
1.2
1.2
1.2
1.2
1.2
1.2
4
1.2
1.2
4
8c
8c
8
5
5
5
2
2
2
2
2
2
4.5
3
—
—
—
—
5
7
8
7
20
10
15
20
18
13
12
13
—
—
10
16
20
17
55
54
48
53
35
44
40
42
29
44
52
47
10
15
17
14
20
24
22
20
SDS
SDSd
SDS
SDS, DDQe
SDS, CANe
SDS
CTAB
Triton-X 100
—
4
4
4
4
—
a
Reactions were conducted on one milimolar scale of 1 with 2a and 3 in 1.1 and 1.2/4 milimolar ratios respectively at 60–70 °C, solvent (10 mL), surfactant (0.8 mmol),
unless otherwise stated. For entry 2, the reaction was performed at rt.
b
Isolated yield upon column chromatography.
Schiff’s base was isolated in 42 and 58 %yields at rt and 60–70 °C respectively.
SDS (0.4 mmol).
2 milimoles of DDQ and CAN were used.
c
d
e

1566
N. C. Ganguly, S. Chandra / Tetrahedron Letters 55 (2014) 1564–1568
Table 2
NO
ESY
Y
S
E
O
Cascade Povarov-hydrogen-transfer reaction catalysed by I2
N
H
H
5
H
6
Y
4
S
9
3
H
O
N
E
Ph
O
H
O
7
10
5
N
Ar
NH
Ph
Ar
1
R
ArCHO,
(1.1 eq)
Ar
+
2
N
H
O
N
7
O
3 (4 eq)
NH2
N
10
9
+
3
H
X
O
O
X
I2 (10 mol%),
O
X
O
X
(1 eq)
H
Ph
Y
H
H2O (8 mL),
SDS (10 cmc),
70-80 oC, stir
4
S
6
5
E
O
1: X = O, R = H
5a
N
4a
1a: X = NCH3, R = H
1b
: X = O, R = Br
Figure 1. Single-crystal X-ray analysis of 7 confirmed its skeletal structure (Fig. 2).
Entry Amines Ar
Time (h)
Productsa (yield in %)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1a
1
1
2a, C6H5
2b, 4-OMe C6H4
2c, 4-O-allyl C6H4
2d,-OCH2O-C6H4
2e, 4-Cl C6H4
2e, 4-Cl C6H4
2f, 4-Br C6H4
2g, 4-F C6H4
2h, 3-CHO C6H4
2
2.5
2
4a (55) 5a (20) 6a (10)
4b (53) 5b (20) 6b (20)
4c (48) 5c (18) 6c (25)
4d (44) 5d (10) 6d (28)
4e (54) 5e (22) 6e (15)
4f (40) 5f (15) 6f (30)
4g (40) 5g (10) 6g (28)
3
2.5
3.5
3.5
3
4h (48)
4i (30)
—
—
6h (30)
6i (40)
1
3
2i,
10
1
2.5
4j (50)
4k (25) 7b (40) 6k (7)
8b (72)
5j (30)
6j (10)
O
11
12
1
1b
2j, 4-I C6H4
2a, C6H5
3
5
a
Refers to isolated yield after chromatographic separation. All products were
characterized by FTIR, 1H, 13C NMR, MS and elemental (C, H, N) analysis.
b
Compounds 7 and 8 are tetrahydroquinolines.
I
Br
H
N
NH
O
O
O
O
Figure 2. ORTEP diagram of 7.
8
7
(Fig. 3). Interestingly, noncovalent IÁ Á Á
p interaction is also evident
component (entry 11). It was reasoned that installation of bromine
substituent at C-5 of 6-aminocoumarin would necessarily preclude
angular annulation thereby directing the cycloaddition to other-
wise less favourable C-7 position. Treating a mixture of 6-amino-
5-bromocoumarin (1b), benzaldehyde (2a) and styrene under the
optimized reaction conditions for 5 h led to exclusive formation
of the corresponding linearly fused pyaranotetrahydroquinolone
8 in 72% yield (entry 12).
(Fig. 4) (supporting information).
This catalytic protocol basically relies upon orchestrated events
of the formation of 6-aminocoumarin derived aldimine, its partic-
ipation as a 2-azadiene in inverse electron demand Diels–Alder
cycloaddition with styrene to give formal Povarov adducts and
finally its aromatization by hydrogen transfer from either the
N-coumarinyl aldimine or styrene delivering pyridocoumarins. A
tentative catalytic cycle for the auto-tandem catalysis of iodine is
depicted in Scheme 1.
The 1H NMR spectra 4a and 5a allowed complete assignment of
their structures. The product 4a exhibited, in addition to typical
To ascertain the influence of micellar environment and iodine in
the initial event of Schiff’s base formation, a vigorously stirred slur-
ry of equimolar amounts of 6-aminocoumarin (1) and 4-bromo-
benzaldehyde (2f) was allowed to react in water at 60–70 °C for
6 h. The reaction did not proceed at all (TLC monitoring). In sepa-
rate experiments, 1 and 2f were reacted in H2O-SDS, without and
with iodine catalyst (5 mol %) under otherwise same conditions.
Whereas the imine 9b was obtained in 50% yield in water-SDS
without iodine, marked improvement in yield and reaction rate
was observed upon addition of iodine catalyst to the reaction mix-
ture (Scheme 2). Nevertheless, the formation of Schiff’s base, albeit
slowly, in modest yield suggested the facilitatory role of aqueous
micelles. The encapsulation of reactants in a compact hydrophobic
interior of the micelle helps expulsion of polar water arising out of
condensation from the reaction zone.16 The control experiments
also demonstrate the catalytic influence of iodine in this step pre-
sumably by way of electrophilic activation of aldehyde carbonyl. A
similar facilitatory role of iodine catalyst has ample literature
precedents.9b,d,17 The major catalytic role of iodine in subsequent
generation of 4a and 5a has been already underscored during opti-
mization process (Table 1 entries 4–7). Significantly, the yield of
the reduced Schiff’s base 6a was nearly twice the combined yield
of 4a and 5a when non-excess amount of styrene was used (Table 1
entries 3–6). Inasmuch as elevation of tetrahydroquinoline to the
oxidation level of the quinoline entails reduction of two C@N
bonds, this observation confirms the aldimine as dehydrogenating
doublet signals of vinyl protons of the
a-pyrone ring at 6.06 (1H,
J = 10.2 Hz) and 7.33 (1H, J = 10.2 Hz), a diagnostic pair of one-pro-
ton doublets at d 7.22 and 8.39 (each J = 9.2 Hz) corresponding to
H-5 and H-6 respectively. This is consistent with angular annula-
tions of the pyridine ring to the existing benzenoid ring of couma-
rin. A notable feature was the appearance of a sharp one-proton
signal at d 7.85 attributable to pyridine ring proton H-9 flanked
by two phenyl groups. In contrast, 1H NMR of 5a significantly
exhibited two sharp singlet resonance signals at 7.80 and 8.38
due to benzenoid para protons H-10 and H-5 respectively. The
highly downfield nature of the signal was attributed to its peri nat-
ure with respect to pyridine nitrogen. The a-pyrone doublets also
appeared relatively downfield at d 6.53 and 7.93 (each J = 9.6 Hz)
compared to those of 4a. Their structures were further validated
by NOESY studies. The cross peaks between H-2-H-1 and H-5-H-
6 are compatible with angularly fused pyridocoumarin 4a. Simi-
larly, the presence of cross peaks between H-3-H-4 and H-4-H-5
confirms the formation of linear fusion in 5a (Fig. 1). The exhibition
of strong molecular ion base peaks at m/z = 350 [M+1]+ for both 4a
and 5a supports the assigned structures as 8,10-diphenyl-3H-pyr-
ano[3,2-f]quinolin-3-one and 7,9-diphenyl-2H-pyrano[2,3-g]quin-
olin-2-one respectively.
It shows supramolecular dimeric nature through both intramo-
lecular bifurcated hydrogen bonding between coumarin carbonyl
and H-18 (Fig. 3, C@OÁ Á ÁH, 2.56 Å) and
pÁ Á Á
p
interactions. These
interactions
dimeric units are further associated by C–HÁ Á Á
p

N. C. Ganguly, S. Chandra / Tetrahedron Letters 55 (2014) 1564–1568
1567
Figure 3. Non-covalent interactions in compound 7.
substituted styrenes as dienophiles.12b,c The implication of iodine
catalyst in this step was also demonstrated by a control experi-
ment. Attempted oxidation by exposing 7 to 2 mol equiv of the cor-
responding Schiff’s base in H2O-SDS without iodine at 60–70 °C for
3 h proved abortive. The desired oxidation was smoothly achieved
upon addition of 5 mol % of iodine to the above reaction mixture
(65%, 3 h). These experiments are presented in Scheme 2.
In conclusion, direct expeditious entry to pyrano[3.2-f]quinolin-
3-ones and pyrano[2.3-g]quinolin-2-ones was accomplished by
three-component coupling of 6-aminocoumarin, aromatic alde-
hyde and an excess of styrene by Povarov-hydrogen transfer
auto-tandem catalytic route with iodine (5 mol %) in water in the
presence of sodium dodecyl sulfate. The unusual linear fusion of
the pyridine ring to the existing benzenoid ring of the coumarin
and unprecedented autocatalytic role of iodine under essentially
neutral micellar conditions are the key features of the protocol.
Figure 4. IodineÁ Á Á
p interaction.
Ph
Ph
N
Ph
+
N
O
O
O
O
5a
O
Ph
O
4a
Ph
N
Ph
Acknowledgements
NH
I2-H2O-SDS
Ph
3
O
O
6
a
One of the authors (S.C.) thanks the Council of Scientific and
Industrial Research, New Delhi, for financial assistance by way of
a research fellowship (SRF-NET). Facilities provided by the DST-
FIST program and UGC-SAP program, Government of India, are also
acknowledged.
1
Povarov reaction
Ph
Ph
Hydrogen transfer
H
N
NH
Ph
+
O
O
O
O
4a/
5a/
Ph
Scheme 1.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.01.
078.
R
N
H2O (10 mL)
6h
Br
60-7
0
oC
I
2 (5 mol %),
CHO
H2O (10 mL),
SDS (0.8 mmol),
60-70 oC
30 min
NH2
+
H2O (10 mL),
References and notes
N
9b
(82 %)
SDS(0.8 mmol),
60-70 oC
2h
O
O
O
O
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2f
Br
9b (50 %)
I
I
I2 (5 mol %),
H2O (10 mL),
SDS (0.8 mmol),
60-70 oC, 3h
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N
NH
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P-conju-
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10. For a very recent review on iodine-catalyzed organic reactions, see: (a) Ren, Y.-
M.; Cai, C.; Yang, R.-C. RSC Adv. 2013, 3, 7182; (b) Yadav, J. S.; Reddy, B. V. S.;
Rao, T. S.; Bhavani, K.; Raju, A. Tetrahedron Lett. 2010, 51, 2622; (c) Theerthagiri,
P.; Lalitha, A.; Arunachalam, P. N. Tetrahedron Lett. 2010, 51, 2813; (d) Lee, S. J.;
Lee, J. J.; Kim, C.-H.; Jun, Y. M.; Lee, M. M.; Kim, B. H. Tetrahedron Lett. 2009, 50,
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14. Representative procedure for synthesis of 4a and 5a: To a vigorously stirring
white emulsion of SDS (234 mg, ꢀ0.8 mmol, 10 cmc) in water (10 mL) were
sequentially added 6-aminocoumarin (160 mg, 1 mmol) and benzaldehyde
(118 mg, 1.1 mmol), iodine (13 mg, ꢀ5 mol %) and styrene (416 mg, 4 mmol)
and the stirring was stirred for 2 h at 60–70 °C. The reaction was quenched by
addition of 5 mol % of sodium thiosulfate solution (5 mL) and the cooled
reaction mixture was extracted with ethyl acetate (3 Â 5 mL). The combined
organic extract was washed with water (2 Â 3 mL) and dried (Na2SO4). It was
concentrated under reduced pressure to yield a yellow residue which was
chromatographed over silica gel (60–120 mesh). Elution with EtOAc–light
petrol (1:12) yielded a mixture of 4a and 5a and the amine 6a in later fractions
as a pure yellow solid (25 mg, ꢀ10%). The mixture of 4a and 5a were separated
by flash chromatography over silica gel (230–400 mesh) with EtOAc–light
petrol (1:20) as eluent to give 4a (192 mg, 55%) and 5a (70 mg, 20%).
Compound 4a: White solid; mp: 210–212 °C; IR (KBr): 1744, 1592, 1549, 1484,
1360, 1215, 1177, 1115, 982, 904, 825, 796, 716 cm; 1H NMR (300 MHz,
CDCl3): d 8.39 (d, J = 9.3 Hz, 1H), 8.21 (m, 2H), 7.85 (s, 1H), 7.72 (d, J = 9.3 Hz,
1H), 7.58–7.43 (m, 8H), 7.33 (d, J = 10.2 Hz, 1H), 6.06 (d, J = 10.2 Hz, 1H); 13C
NMR (100 MHz, CDCl3): d 160.2, 156.0, 154.9, 148.0, 146.8, 141.7, 141.0, 138.3,
135.3, 129.8, 129.5, 129.0, 128.4, 127.4, 122.8, 122.3, 121.2, 113.8, 113.4,
105.2; MS (ESI): m/z = 350 [M+H]; Anal. Calcd for C24H15NO2: C, 82.50; H, 4.33;
N, 4.01. Found: C, 82.34; H, 4.09; N, 4.15.
Compound 5a: White solid; mp: 216–218 °C; IR (KBr): 1733, 1634, 1592, 1538,
1450, 1354, 1234, 1182, 1121, 1089, 926, 825, 745 cm; 1H NMR (300 MHz,
CDCl3): d 8.38 (s, 1H), 8.20–8.18 (m, 2H), 7.93 (d, J = 9.6 Hz, 1H), 7.90 (s, 1H),
7.80 (s, 1H), 7.58–7.49 (m, 8H), 6.53 (d, J = 9.6 Hz, 1H); 13C NMR (100 MHz,
CDCl3): d 160.2, 157.3, 150.5, 148.9, 145.2, 143.1, 138.9, 137.4, 129.8, 129.7,
129.4, 129.0, 128.9, 127.7, 127.5, 121.5, 120.8, 118.4, 111.4, 104.9; HRMS calcd
for C24H15NO2 [M+H]: 350.1176; found: 350.1179.
ˇ ˇ
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