Access to densely functionalized naphthalenes by organobase catalyzed domino reaction of 2-(2-formylaryl)acetophenones with nitroolefins

Article abstract of DOI:10.1016/j.tetlet.2016.06.062

A series of new functionalized naphthalene derivatives having carbonyl and NO₂groups at C-1 and C-3 positions respectively have been prepared in good yields (63–75%) through a one-pot domino reaction of several 2-(2-formylaryl)acetophenone derivatives with a variety of aryl/heteroaryl-substituted 2-nitroolefins in EtOH as a green solvent at 75?°C under air using a catalytic amount of DABCO (30?mol?%) as an inexpensive organocatalyst. This pot-economic process is friendly enough to retain several sensitive functionalities and displays a wide range of substrate scope. Furthermore, the high yielding synthesis of biologically attractive N-(3-naphthyl-substituted)pyrrole frameworks was established through our synthetic procedure.

Full text of DOI:10.1016/j.tetlet.2016.06.062

Tetrahedron Letters 57 (2016) 3326–3329
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Access to densely functionalized naphthalenes by organobase
catalyzed domino reaction of 2-(2-formylaryl)acetophenones with
nitroolefins
⇑
Anuradha Dagar, Soumen Biswas, Shaikh M. Mobin, Sampak Samanta
Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, 452020 Madhya Pradesh, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new functionalized naphthalene derivatives having carbonyl and NO2 groups at C-1 and C-3
positions respectively have been prepared in good yields (63–75%) through a one-pot domino reaction
Received 3 May 2016
Revised 13 June 2016
Accepted 14 June 2016
Available online 16 June 2016
of several 2-(2-formylaryl)acetophenone derivatives with
a variety of aryl/heteroaryl-substituted
2-nitroolefins in EtOH as a green solvent at 75 °C under air using a catalytic amount of DABCO
(30 mol %) as an inexpensive organocatalyst. This pot-economic process is friendly enough to retain
several sensitive functionalities and displays a wide range of substrate scope. Furthermore, the high
yielding synthesis of biologically attractive N-(3-naphthyl-substituted)pyrrole frameworks was
established through our synthetic procedure.
Keywords:
Naphthalene derivatives
Domino
Ó 2016 Elsevier Ltd. All rights reserved.
Organocatalyst
b-Nitrostyrenes
2-(2-Formylaryl)acetophenones
Naphthalene and its derivatives are one of the most important
classes of organic building blocks. They are frequently found in
numerous natural products and bioactive molecules.1 Importantly,
substituted naphthalene derivatives have shown widespread appli-
cations in chemical domains such as pharmaceuticals,1,2 optical and
electronic materials,3 chiral ligands,4 organic dyes3d etc. Therefore,
the development of a one-pot method for the synthesis of densely
functionalized naphthalene derivatives has gained much attention
in the scientific community.1–4,3d Accordingly, many powerful
strategies have been reported toward the efficient syntheses of
naphthalene derivatives5–10 which include the transition metal-salt
mediated benzannulation reaction of enynals with several alke-
nes6a/alkynes6b,c/enols6c,d/secondary amines,6f condensation of
phenylacetaldehydes with alkynes promoted by several Lewis
(AuCl3/AgSbF6,7a TiCl4,7b GaCl3,7c BF37d)/Brønsted acids (HNTf2),7e
Rh-salts/Cu(OAc)2 mediated oxidative coupling of aryl-boronic
acid with alkynes,8 6-endo intramolecular hydroarylation of
toxic metal-salts as catalysts, harmful and volatile solvents
(especially chlorinated solvents), stoichiometric amounts of
external oxidants, harsh reaction conditions, poor atom-economy,
and yields. Therefore, it is necessary to develop an alternative
green protocol for the synthesis of poly-functionalized
naphthalene under metal-free conditions.
In our group, we have been interested to explore the organocat-
alytic domino Michael–Henry reaction for the construction of
six-membered cyclic rings involving nitroolefins as Michael
acceptors.12 Herein, we further disclose a simple, convenient, and
eco-friendly one-pot technique for the access to poly-functional-
ized naphthalenes through a domino Michael-Henry-dehydration-
aromatization reaction involving 2-(2-formylaryl)acetophenones
and several aryl/heteroaryl-substituted 2-nitroolefins in EtOH at
75 °C under air using a catalytic amount of DABCO (30 mol %) as
an inexpensive organic base.
At the beginning, we chose the model reaction between
compound 1a and 2a in THF using DABCO (30 mol %) under air at
room temperature to explore the optimal reaction conditions. After
b-aryl-a
-alkynylcinnamates9 and [2+2+2] cyclotrimerization of
alkynes.10 Alternatively, 2-(2-oxo-2-arylethyl)benzaldehydes11
have been used as donor–acceptors in the [4+2] cycloaddition
reaction with alkynes or nitroolefins catalyzed by FeCl3 or pyrro-
lidine-DMAP as reported by Zhu11a and Xu11b groups respectively.
Despite the great history on naphthalene syntheses, they suffer
one or more practical problems such as use of expensive and
24 h,
a trace amount of targeted product 3aa was isolated
(5% yield, entry 1). The structure was confirmed by its spectro-
scopic data (1H NMR, 13C NMR, and HRMS). Interestingly, 45% yield
of 3aa was obtained after reaction at 60 °C for 16 h (entry 2).
Hoping better yield of 3aa, we performed this domino reaction in
various common solvents namely 2-MeTHF, EtOH, water (entries
3–5, green solvents), toluene, DMSO, and DMF at 75 °C. Results
showed that the obtained yield (72%, entry 3) of 3aa in ethanol
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.tetlet.2016.06.062
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

A. Dagar et al. / Tetrahedron Letters 57 (2016) 3326–3329
3327
Table 1
Reaction optimizationa
NO2
Ph
CHO
O2N
Conditions
Ph
2a
Ph
O
1a
O
Ph
3aa
Entry
Catalyst
Solvent
Temp (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
Et3N
DBU
DIEPA
Chitosan
K2CO3
NaHCO3
Na2CO3
THF
THF
rt
24
16
12
12
24
12
12
12
20
16
16
16
16
16
16
16
<5
45
72
67
37
63
33
41
15
65
39
46
50
42
47
12
60
75
75
75
75
75
75
75
75
75
75
75
75
75
75
EtOH
2-MeTHF
H2O
Toluene
DMSO
DMF
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
9
10
11
12
13
14
15
16
L-Proline
O
NMe
Me
Me
17
EtOH
75
16
6
Bn
N
H
Me
a
Unless otherwise specified, all reactions were carried out with compound 1a (0.15 mmol), compound 2a (0.18 mmol), and DABCO (0.05 mmol) in the specified solvent
and temperature under air.
b
Isolated yield after column chromatography.
was better than other solvents tested for this reaction (33–67%,
entries 4–8). Next, we tested several commercially available cheap
organic and inorganic bases (Et3N, DBU, DIEPA, chitosan, NaHCO3,
Na2CO3, K2CO3) as catalysts for this domino reaction in EtOH med-
ium. All the above bases were able to promote this annulation
reaction, resulting in moderate to good yields (39–65%, entries
10–15) of 3aa with in 16 h. Among the bases, it is considered that
DABCO was chosen as the best catalyst for this one-pot
Alternatively, the tetrahydronaphthalene 4 may be generated from
same carbanion intermediate 1a0 via [4+2] cycloaddition reaction
of dienolate 1b (equilibrium form of 1a0)11a,c with 2a under the
present conditions.
With the above optimal reaction conditions in hand, we
established the scope and limitations of this reaction by involving
a variety of aryl-substituted 2-nitroolefins and 2-(2-formylaryl)ace-
tophenones as starting materialsin our present catalyticsystem. The
obtained results are summarized in Table 2. It was found that the
annulation reaction of 2-(2- formylphenyl)acetophenone (1a) with
aryl-substituted 2-nitroolefins (2b–2d) proceeded well in the
present catalytic system to provide corresponding naphthalene
derivatives (3ab–3ad) in 64–73% yields. Similarly, several
substituted 2-(2-oxo-2-arylethyl)benzaldehydes (1b–1e) were sub-
jected to react not only with nitrostyrene (2a) but also a variety of
substituted-nitrostyrenes (2b–2g) having electron donating (Me,
OMe) and electron withdrawing functionalities (F, Cl, Br, and CN)
on the aryl rings by this procedure, leading to the satisfactory level
of chemical yields (61–75%) of corresponding anticipated 3-nitron-
aphthalene derivatives (3ba–3fc, ORTEP structure of 3ba as shown
p
-extension process (entry 3).15 It should be noted that a very poor
conversion was observed when L-proline (entry 16)/imidazolidi-
none (entry 17) were employed as catalysts (Table 1).
We propose the following possible mechanism for the
formation of compound 3aa as depicted in Scheme 1. At the first
step, carbanion intermediate 1a0 (or enolate 1a00) is generated via
an abstraction of an active methylene proton from 1a by a base.
The former may undergo Michael addition to b-nitrostyrene (2a),
followed by intramolecular Henry reaction to generate tetrahy-
dronaphthalene derivative 4 under the basic conditions. Finally,
the naphthalene derivative 3aa is formed from intermediate 4
via dehydration, followed by aerial oxidation of intermediate 5.
NO2
H
O
OH
CHO
base
NO2
NO2
2a
O
O
NO2
Henry
Ph
base
Ph
Michael
Ph -H2O
Ph
Ph
O
Ph
1a'
Ph
O
Ph
O
O
Ph
4a
1a
4
5
n
o
O
i
H
t
i
d
d
a
aerial
oxidation
o
l
c
y
c
]
2
+
NO2
[o]
4
[
O
NO2
2a
Ph
Ph
Ph
O
Ph
O
Ph
1a"
1b
O
3aa
Scheme 1. Possible mechanism for the domino reaction.

3328
A. Dagar et al. / Tetrahedron Letters 57 (2016) 3326–3329
Table 2
Generality of the reaction
DABCO (30 mol%)
EtOH, 75 C, 12-18h, air
R
R
NO2
R2
R
R
CHO
O2N
°
R2
O
O
1a-e
2a-i
3aa-3fc
R1
R1
2
1
2a
2b
2c
2f
2g
2h
: R = Ph;
: R2 = 4-BrC6H4
2
1a:
1b:
1c
1d
1e
R = H, R = H
1
: R2 = 4-MeC6H4
: R = 4-CNC6H4
R = OMe, R = H
: R = OMe, R1 = Me
2
: R = 4-MeOC6H4
: R2 = 2-furyl
2
: R = OMe, R1 = OMe
: R = OMe, R1 = F
: R2 = 4-FC6H4
: R
= 2-thiophenyl
2d
2i
2e: R2 = 4-ClC6H4
NO2
NO2
NO2
NO2
O
OMe
MeO
O
O
F
O
Me
Me
3aa, 72%
3ab, 70%
NO2
3ad, 64%
NO2
3ac, 73%
NO2
MeO
MeO
MeO
NO2
MeO
MeO
MeO
OMe
MeO
O
O
O
F
O
3bb, 69%
3bd, 66%
NO2
3bc, 73%
NO2
3ba,71%
MeO
MeO
MeO
MeO
MeO
NO2
MeO
MeO
NO2
MeO
CN
O
O
O
O
Br
O
Cl
3bh, 67%
3bg, 61%
NO2
3be, 63%
3bf, 66%
MeO
MeO
NO2
MeO
MeO
MeO
MeO
NO2
MeO
MeO
NO2
O
O
O
OMe
Me
S
O
O
Me
3bi, 65%
3ca, 74%
3cc, 71%
NO2
3ch, 65%
Me
MeO
MeO
NO2
MeO
MeO
MeO
NO2
MeO
NO2
MeO
OMe
F
MeO
MeO
S
O
O
O
OMe
O
Me
3ci,64%
3da, 73%
3dc, 75%
3fc, 66%
MeO
in Fig. 1 and ESI). It should be noted that hetero-aryl-substituted
2-nitroolefins such as 2h and 2i are found to be good Michael
acceptors toward the one-pot annulation reaction with 1b–c. As a
result, good yields of highly functionalized 2-heteroaryl-substi-
tuted-3-nitronaphthalene derivatives (3bh–3ci) were obtained in
64–67% yields. Furthermore, several functionalities namely Me,
OMe, F, Cl, Br, CN, NO2, C@O, furan, thiophene etc are well-tolerated
in our optimal reaction conditions (Scheme 2).
To show the potential synthetic utility of the prepared 3-nitron-
aphthalene derivatives, the chemoselective reduction of the NO2
group of 3ba to 3-aminonaphthalene 6 (71% yield) has been
successfully performed by using Fe/AcOH/H2O as a reducing agent
under refluxing conditions. Furthermore, the novel synthesis of a
pharmacologically important class of poly-functionalized
N-(3-naphthyl)pyrroles [7, 8a (ORTEP data of 8a, ESI) and 8b]
has been achieved in 77%, 86%, and 88% yields respectively through
a one-pot reaction of 3-aminonaphthalene derivative 6 with
2,5-dimethoxytetrahydrofuran or
a-phenylacetylenyl-b-aryl-sub-
stituted nitrostyrenes (2a and 2b) as electrophiles using method
A13 and B14 respectively.
In conclusion, we have developed a DABCO catalyzed one-pot
domino Michael–Henry (or 4+2 cycloaddition)–dehydration–
aromatization reaction of 2-(2-formylaryl)acetophenones with a
Figure 1. ORTEP diagram of compound 3ba, thermal ellipsoids drawn at the 50%
probability level.

A. Dagar et al. / Tetrahedron Letters 57 (2016) 3326–3329
3329
MeO
AcOH, reflux, 2h
OMe
O
MeO
MeO
NO2
Ph
MeO
MeO
NH2
Ph
MeO
MeO
N
Fe/AcOH:H2O
Method A
110 °C, 12h
Ph
Ph
O
Ph
6:
O
Ph
7:
77% yield
O
3ba
71% yield
Ph
NO2
Method B
R
2a
Cu(OTf)2 (10.0 mol%)
THF, rt, 5h
Ph
N
NO2
MeO
MeO
R
Ph
Ph
8a:
O
,
R = Ph 86% yield
8b: R = 4-MeC6H4, 88% yield
Scheme 2. Synthesis of N-(3-naphthyl-substituted)pyrrole scaffolds.
3. (a) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452; (b) Reddy, R. A.;
Baumeister, U.; Keith, C.; Tschierske, C. J. Mater. Chem. 2007, 17, 62; (c)
Svoboda, J.; Novotná, V.; Kozmik, V.; Glogarová, M.; Weissflog, W.; Diele, S.;
Pelzl, G. J. Mater. Chem. 2003, 13, 2104; (d) Watson, M. D.; Fechtenkötter, A.;
Müllen, K. Chem. Rev. 2001, 101, 1267.
4. For a recent review of BINOL-based catalysts, see: Shibasaki, M.; Matsunaga, S.
BINOL. In Privileged Chiral Ligands and Catalysts; Zhou, Q.-L., Ed.; Wiley: New
York, 2011; pp 295–332.
wide range of aryl-substituted 2-nitroolefins in EtOH as a green
solvent under aerobic conditions. This green protocol provides
good yields of poly-functionalized naphthalene derivatives
possessing synthetically valuable ketone and nitro functionalities
at C-1 and C-3 positions respectively and excels several sensitive
functionalities. In addition, this one-pot p-extension process does
5. (a) Zhang, K.; Cai, L.; Jiang, X.; Garcia-Garibay, M. A.; Kwon, O. J. Am. Chem. Soc.
2015, 137, 11258; (b) Wagner, F.; Harms, K.; Koert, U. Org. Lett. 2015, 17, 5670;
(c) Gulda, V.; Balmamurugan, R. J. Org. Chem. 2011, 76, 9919; (d) Manojveer, S.;
Balamurugan, R. Org. Lett. 2014, 16, 1712.
not involve any expensive and toxic metal-salts, reduces the use
of hazardous and volatile organic solvents (such as chlorinated
solvents), obviates the need for external oxidants and
inert-atmosphere, does not form toxic by-products (solely H2O)
etc which have been common problem found in the existing
methods. Furthermore, the synthesized naphthalene scaffold has
been successfully transformed into the biologically interesting
6. Transition metal catalyzed (a) Zhao, X.; Zhang, X.-G.; Tang, R.-Y.; Deng, C.-L.; Li,
J.-H. Eur. J. Org. Chem. 2010, 4211; (b) Asao, N.; Nogami, T.; Lee, S.; Yamamoto,
Y. J. Am. Chem. Soc. 2003, 125, 10921; (c) Asao, N.; Takahashi, K.; Lee, S.;
Kasahara, T.; Yamamoto, Y. J. Am. Chem. Soc. 2002, 124, 12650; (d) Barluenga, J.;
Vázquez-Villa, H.; Merino, I.; Ballesteros, A.; González, J. M. Chem. Eur. J. 2006,
12, 5790; (e) Asao, N.; Aikawa, H. J. Org. Chem. 2006, 71, 5249; (f) Gou, F.-R.;
Huo, P.-F.; Bi, H.-P.; Guan, Z.-H.; Liang, Y.-M. Org. Lett. 2009, 11, 3418.
7. (a) Balamurugan, R.; Gudla, V. Org. Lett. 2009, 11, 3116; (b) Kabalka, G. W.; Ju, Y.;
Wu, Z. J. Org. Chem. 2003, 68, 7915; (c) Viswanathan, G. S.; Wang, M.; Li, C.-J.
Angew. Chem., Int. Ed. 2002, 41, 2138; (d) Xiang, S.; Hu, H.; Ma, J.; Li, Y.; Wang, B.;
Feng, C.; Zhao, K.; Hu, P.; Chen, X. Sci. China Chem. 2013, 56, 945; (e) Ponra, S.;
Vitale, M. R.; Michelet, V.; Ratovelomanana-Vidal, V. J. Org. Chem. 2015, 80, 3250.
8. Fukutani, T.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett. 2009, 11, 5198.
9. Kang, D.; Kim, J.; Oh, S.; Lee, P. H. Org. Lett. 2012, 14, 5636.
10. (a) Reppe, W.; Schweckendieck, W. J. Liebigs Ann. Chem. 1948, 560, 104; (b)
Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1984, 23, 539.
11. (a) Zhu, S.; Xiao, Y.; Guo, Z.; Jiang, H. Org. Lett. 2013, 15, 898; (b) Li, S.-G.; Hu, X.-
Q.; Jia, Z.-X.; Xu, P.-F. Tetrahedron 2010, 66, 8557; (c) Zhu, S.; Liang, R.; Jiang, H.;
Wu, W. Angew. Chem., Int. Ed. 2012, 51, 10861.
12. Organocatalytic Michael–Henry reaction, see: (a) Jaiswal, P. K.; Biswas, S.;
Singh, S.; Pathak, B.; Mobin, S. M.; Samanta, S. RSC Adv. 2013, 3, 10644; (b)
Jaiswal, P. K.; Biswas, S.; Singh, S.; Samanta, S. Org. Biomol. Chem. 2013, 11,
8410; (c) Biswas, S.; Jaiswal, P. K.; Singh, S.; Mobin, S. M.; Samanta, S. Org.
Biomol. Chem. 2013, 11, 7084; (d) Biswas, S.; Dagar, A.; Srivastava, A.; Samanta,
S. Eur. J. Org. Chem. 2015, 4493; (e) Singh, S.; Srivastava, A.; Samanta, S.
Tetrahedron Lett. 2012, 53, 6087.
poly-functionalized N-(3-naphthyl)pyrrole frameworks in
a
productive manner. Therefore, we believe this protocol will
deserve much attention in synthetic organic chemistry as an alter-
native powerful technique for the synthesis of poly-functionalized
naphthalenes in an economical and environment friendly manner.
Examination of more substrates scope and their synthetic
applications are underway in our laboratory.
Acknowledgments
The authors thank DST (Project No. SB/S1/OC-19/2013) research
grants, Govt. of India for the generous financial support and SIC
facility, IIT Indore. S. Biswas is also thankful to UGC for his
fellowship.
Supplementary data
13. Bharathiraja, G.; Sengoden, M.; Kannan, M.; Punniyamurthy, T. Org. Biomol.
Chem. 2015, 13, 2786.
14. (a) Clauson-Kass, N.; Tyle, Z. Acta Chem. Scand. 1952, 6, 667; (b) Paillet-Loilier,
M.; Fabis, F.; Lepailleur, A.; Bureau, R.; Butt-Gueulle, S.; Dauphin, F.; Delarue,
C.; Vaudry, H.; Rault, S. Bioorg. Med. Chem. Lett. 2005, 15, 3753.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.06.
062.
15. Synthesis of phenyl(3-nitro-2-phenylnaphthylen-1-yl)methanone: To
a stirred
solution of 2-(2-formylphenyl)acetophenone (1a, 33.6 mg, 0.15 mmol) and b-
nitrostyrene (2a, 26.8 mg, 0.18 mmol) in EtOH (1.0 mL) was added DABCO
(7.0 mg, 0.03 mmol) at 75 °C under air for 12 h. The progress of the reaction
was monitored by TLC. Upon completion of the reaction, it was extracted with
EtOAc (3 ꢀ 10 mL), washed with water and brine respectively, and dried over
Na2SO4. The combined organic phases were collected and evaporated by a
rotary evaporator under reduced pressure to give the crude product. The crude
mass was purified by column chromatography over silica-gel to furnish the
pure product 3aa (38.1 mg, 72% yield). The product was characterized by its
corresponding spectroscopic data (1H and 13C NMR, HRMS).
References and notes
1. (a) de Koning, C. B.; Rousseau, A. L.; van Otterlo, W. A. L. Tetrahedron 2003, 59,
7; (b) Bringmann, G.; Ochse, M.; Schupp, O.; Tasler, S. In Progress in the
Chemistry of Organic Natural Products; Herz, W., Falk, H., Kirby, G. W., Moore, R.
E., Tamm, C., Eds.; Springer: New York, 2001; Vol. 82, p 1; (c) Zhang, H.; Guo, Z.;
Wu, N.; Xu, W.; Han, L.; Li, N.; Han, Y. Molecules 2012, 17, 843; (d) Krohn, K.;
Kounam, S. F.; Cludius-Brandt, S.; Draeger, S.; Schulz, B. Eur. J. Org. Chem. 2008,
3615; (e) Dai, J.; Liu, Y.; Zhou, Y.-D.; Nagle, D. G. J. Nat. Prod. 2007, 70, 1824; (f)
Lee, J.-C.; Lee, C.-H.; Su, C.-L.; Huang, C.-W.; Liu, H.-S.; Lin, C.-N.; Won, S.-J.
Carcinogenesis 2005, 26, 1716; (g) Brasholz, M.; Sörgel, S.; Azap, C.; Reissig, H.-
U. Eur. J. Org. Chem. 2007, 3801; (h) Lowell, A. N.; Fennie, M. W.; Kozlowski, M.
C. J. Org. Chem. 2008, 73, 1911.
Phenyl(3-nitro-2-phenylnaphthylen-1-yl)methanone (3aa): Yellow solid, mp
140–142 °C, yield 72%; 1H NMR (400 MHz, CDCl3)
d 8.53 (s, 1H), 8.08
(d, J = 8.0 Hz, 1H), 7.60–7.71 (m, 4H), 7.50–7.52 (d, J = 7.3 Hz, 2H), 7.42–7.46
(m, 1H), 7.24–7.34 (m, 3H), 6.89–7.17 (m, 3H); 13C NMR (100 MHz, CDCl3) d
197.0, 147.6, 139.9, 137.2, 134.3, 133.8, 131.6, 131.4, 130.2, 130.1, 129.5, 129.4,
128.7, 128.5, 128.4, 128.2, 128.1, 125.8, 125.0; HRMS (ESI) m/z calculated for
2. (a) Xie, X.; Kozlowski, M. C. Org. Lett. 2001, 3, 2661; (b) Ukita, T.; Nakamura, Y.;
Kubo, A.; Yamamoto, Y.; Takahashi, M.; Kotera, J.; Ikeo, T. J. Med. Chem. 1999,
42, 1293.
C
23H16NO3 [M+H]+: 354.1125, found 354.1132.