Regioselective synthesis of 1-arylnaphthalenes from N-tosylaziridine derivatives

Article abstract of DOI:10.1016/j.tetlet.2005.11.142

Regioselective synthesis of 1-arylnaphthalene derivatives was carried out from N-tosylaziridines, which was made from the reaction of N-tosylimines and cinnamyl bromides by using the sulfur ylide chemistry.

Full text of DOI:10.1016/j.tetlet.2005.11.142

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 977–980
Regioselective synthesis of 1-arylnaphthalenes from
N-tosylaziridine derivatives
Ka Young Lee, Seung Chan Kim and Jae Nyoung Kim*
Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, South Korea
Received 17 October 2005; revised 16 November 2005; accepted 25 November 2005
Available online 15 December 2005
Abstract—Regioselective synthesis of 1-arylnaphthalene derivatives was carried out from N-tosylaziridines, which was made from
the reaction of N-tosylimines and cinnamyl bromides by using the sulfur ylide chemistry.
Ó 2005 Elsevier Ltd. All rights reserved.
Regioselective synthesis of naphthalene derivatives has
been and continues to be of great interest in organic
synthesis.1–3 A new synthetic procedure is still highly
desired due to the abundance of the skeleton in many
biologically important natural products. Recently, we
have reported the synthesis of naphthalenes from the
reaction of the Baylis–Hillman acetates derived from
o-halobenzaldehydes and primary nitroalkanes via the
successive SN20–SNAr-elimination strategy.2a Later we
extended the concept to a more general one by using
the Mn(III)-assisted radical cyclization protocol.2b
known ylide chemistry5 from the reaction of N-tosyl-
imine 3 and cinnamyl bromide 2, which could be synthe-
sized from the Baylis–Hillman adduct 1.6 We also
expected that we could prepare the target 1-arylnaph-
thalene 5 by the intramolecular Friedel–Crafts type
ring-opening reaction of 4 and the following elimination
of p-toluenesulfonamide moiety.
As an initial try, we prepared 4a from the reaction of
cinnamyl bromide 2a and N-tosylimine 3a in the pres-
ence of Me2S and K2CO3 in CH3CN at room tempera-
ture according to the previous paper (Scheme 1).5 The
corresponding sulfur ylide reacted with 3a successfully
to give the desired N-tosylaziridine 4a in 62% yield as
a mixture of two diastereomers (trans/cis = 7:5). The
two isomers were difficult to separate in pure states.
However, the stereochemistry will not affect the next
reaction (vide infra), thus we used the mixture for the
next reaction. The other aziridine derivatives 4b–f were
prepared similarly in 60–70% yields (Fig. 2).5–7
The ring-opening reaction of N-tosylaziridines with a
variety of nucleophiles has been used in organic synthe-
sis. The Friedel–Crafts type ring-opening reaction of N-
tosylaziridines with arene compounds was also studied
deeply.4 In these respects, we pursued the synthesis of
1-arylnaphthalene derivatives via the intramolecular
ring-opening reaction of N-tosylaziridines derived from
Baylis–Hillman adducts as depicted in Figure 1.
We thought that the synthesis of requisite aziridine
With these compounds 4 in our hand we examined the
derivatives 4 could be carried out by using the well-
next aziridine-opening and Friedel–Crafts reaction
COOR
Br
OH
COOR
COOR
H
COOR
2
3
N
Ar
N
Ar
Ar
1
Ts
Ts
4
5
Figure 1.
Keywords: 1-Arylnaphthalenes; Baylis–Hillman adducts; N-Tosylaziridines; Sulfur ylide.
*
Corresponding author. Tel.: +82 62 530 3381; fax: +82 62 530 3389; e-mail: kimjn@chonnam.ac.kr
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.11.142

978
K. Y. Lee et al. / Tetrahedron Letters 47 (2006) 977–980
COOR
NTs
Ar
3
Me2S
CH3CN
COOR
- Me2S
K2CO3
4
2
S
S
Br
Scheme 1.
δ = 7.45 (d, J = 1.8 Hz)
H
under a variety of acidic conditions. To our delight we
could obtain the final compound 5a directly in 85% yield
when we subjected 4a under benzene/H2SO4 (3 equiv)/
refluxing conditions within 2 h. By using the same con-
ditions we prepared 5b–f successfully in 79–84% yields
(Table 1). As shown in Scheme 2, the reaction might
occur via the aziridine ring-opening and Friedel–Crafts
COOMe
δ = 4.19 (dd, J = 6.9 and 1.8 Hz)
H
NOE
δ= 3.96 (d, J = 6.9 Hz)
H
Cl
N
Ts
Figure 2. NOE of cis-4e.
Table 1. Synthesis of N-tosyl aziridines and 1-arylnaphthalenes
Entry
Substrate 2
Substrate 3
Aziridine 4 (%)a
COOMe
Naphthalene 5 (%)b
COOMe
Br
COOMe
NTs
1
H
N
3a
2a
Ts
4a (62)c
5a (85)
COOEt
COOEt
COOEt
Br
H
2
3
4
3a
N
Ts
2b
5b (84)
4b (60)c
COOMe
COOMe
Cl
H
NTs
2a
N
Cl
Ts
Cl
3b
4c (70)c
5c (81)
COOMe
COOMe
COOMe
Br
H
3a
3a
3a
N
Ts
2c
5d (80)
4d (63)d
COOMe
COOMe
COOMe
Cl
H
Cl
5
N
Br
Cl
Ts
2d
5e (83)
4e (63)c
OMe
OMe
COOMe
OMe
COOMe
H
COOMe
Br
6
N
Ts
4f (68)d
2e
5f (79)
a CH3CN, Me2S (1.5 equiv), K2CO3 (1.0 equiv), rt, 5 h.
b Benzene, H2SO4 (3 equiv), reflux, 2 h.
c Mixtures of cis/trans.6
d Pure cis-isomer.6

K. Y. Lee et al. / Tetrahedron Letters 47 (2006) 977–980
979
COOR
NHTs
COOR
NHTs
H2SO4 (3 equiv)
PhH, reflux, 2 h
- TsNH2
5
4
Ar
Ar
Scheme 2.
Y. L.; Zhu, G.-D.; Freeman, J. C. Synlett 1998, 754; (c)
Dubois, L.; Mehta, A.; Tourette, E.; Dodd, R. H. J. Org.
Chem. 1994, 59, 434; (d) Stamm, H.; Onistschenko, A.;
Buchholz, B.; Mall, T. J. Org. Chem. 1989, 54, 193; (e)
Vedejs, E.; Klapars, A.; Warner, D. L.; Weiss, A. H. J. Org.
Chem. 2001, 66, 7542.
type reaction and the concomitant elimination of
p-toluenesulfonamide to furnish the naphthalene
compound.8
In summary, we developed an efficient synthetic method
of 1-arylnaphthalenes starting from the Baylis–Hillman
adducts.
5. For the synthesis and stereochemistry of aziridines, see: (a)
Li, A.-H.; Dai, L.-X.; Hou, X.-L.; Chen, M.-B. J. Org.
Chem. 1996, 61, 4641; (b) Li, A.-H.; Dai, L.-X.; Hou, X.-L.
J. Chem. Soc., Perkin Trans. 1 1996, 2725; (c) Zhou, Y.-G.;
Li, A.-H.; Hou, X.-L.; Dai, L.-X. Tetrahedron Lett. 1997,
38, 7225; (d) Li, A.-H.; Dai, L.-X.; Hou, X.-L. J. Chem.
Soc., Perkin Trans. 1 1996, 867; (e) Matano, Y.; Yoshi-
mune, M.; Suzuki, H. J. Org. Chem. 1995, 60, 4663.
6. Typical procedure for the synthesis of aziridine 4a and 1-
arylnaphthalene 5a: The cinnamyl bromide derivative 2a
was prepared by the reaction of the Baylis–Hillman adduct
1 and aq HBr (rt, 5 h) in 92% yield. Synthesis of 2b–e was
also carried out similarly with HBr at room temperature (5–
16 h, 85–95%). To a stirred mixture of cinnamyl bromide 2a
(254 mg, 1.0 mmol) and N-tosylimine 3a (273 mg,
1.0 mmol) in CH3CN (3 mL) was added Me2S (93 mg,
1.5 mmol) and K2CO3 (138 mg, 1.0 mmol) at room tem-
perature and stirred for 5 h. After usual workup and
column chromatographic purification process (hexanes/
ether, 4:1) we obtained 4a, 278 mg (62%) as a mixture of
Acknowledgements
This work was supported by the grant (R-05-2003-000-
10042-0) from the Basic Research Program of the Korea
Science and Engineering Foundation (now controlled
under the authority of Korea Research Foundation).
Spectroscopic data was obtained from the Korea Basic
Science Institute, Gwangju branch.
References and notes
1. For some of recent synthesis and usefulness of naphtha-
lenes, see: (a) Nishii, Y.; Yoshida, T.; Asano, H.; Wakasugi,
K.; Morita, J.-i.; Aso, Y.; Yoshida, E.; Motoyoshiya, J.;
Aoyama, H.; Tanabe, Y. J. Org. Chem. 2005, 70, 2667; (b)
Nishii, Y.; Wakasugi, K.; Koga, K.; Tanabe, Y. J. Am.
Chem. Soc. 2004, 126, 5358; (c) Martinez, A. D.; Deville, J.
P.; Stevens, J. L.; Behar, V. J. Org. Chem. 2004, 69, 991; (d)
Xie, X.; Kozlowski, M. C. Org. Lett. 2001, 3, 2661; (e)
Barluenga, J.; Vazquez-Villa, H.; Ballesteros, A.; Gonzalez,
J. M. Org. Lett. 2003, 5, 4121; (f) Kawasaki, S.; Satoh, T.;
Mirua, M.; Nomura, M. J. Org. Chem. 2003, 68, 6836; (g)
Huang, Q.; Larock, R. C. Org. Lett. 2002, 4, 2505; (h)
Rucker, M.; Bruckner, R. Synlett 1997, 1187; (i) Seong, M.
R.; Song, H. N.; Kim, J. N. Tetrahedron Lett. 1998, 39,
7101; (j) Zhang, J.; Ho, D. M.; Pascal, R. A., Jr. J. Am.
Chem. Soc. 2001, 123, 10919; (k) Huang, Q.; Larock, R. C.
J. Org. Chem. 2003, 68, 7342; (l) Katritzky, A. R.; Zhang,
G.; Xie, L. J. Org. Chem. 1997, 62, 721; (m) Kabalka, G.
W.; Ju, Y.; Wu, Z. J. Org. Chem. 2003, 68, 7915, and
references cited therein.
2. For the synthesis of naphthalene derivatives from Baylis–
Hillman adducts, see: (a) Kim, J. N.; Im, Y. J.; Gong, J. H.;
Lee, K. Y. Tetrahedron Lett. 2001, 42, 4195; (b) Im, Y. J.;
Lee, K. Y.; Kim, T. H.; Kim, J. N. Tetrahedron Lett. 2002,
43, 4675; (c) Im, Y. J.; Chung, Y. M.; Gong, J. H.; Kim, J.
N. Bull. Korean Chem. Soc. 2002, 23, 787.
3. For the synthesis and usefulness of 1-arylnaphthalene
derivatives, see: (a) Hamura, T.; Miyamoto, M.; Matsu-
moto, T.; Suzuki, K. Org. Lett. 2002, 4, 229; (b) Sarkar, T.
K.; Panda, N.; Basak, S. J. Org. Chem. 2003, 68, 6919; (c)
Ukita, T.; Nakamura, Y.; Kubo, A.; Yamamoto, Y.;
Takahashi, M.; Kotera, J.; Ikeo, T. J. Med. Chem. 1999,
42, 1293; (d) Elakkari, E.; Floris, B.; Galloni, P.; Taglia-
testa, P. Eur. J. Org. Chem. 2005, 889; (e) Nishii, Y.;
Yoshida, T.; Tanabe, Y. Tetrahedron Lett. 1997, 38, 7195;
(f) Nishii, Y.; Tanabe, Y. Tetrahedron Lett. 1995, 36, 8803.
4. For the Friedel–Crafts type reaction involving aziridine
moiety, see: (a) Yadav, J. S.; Reddy, B. V. S.; Abraham, S.;
Sabitha, G. Tetrahedron Lett. 2002, 43, 1565; (b) Bennani,
1
two diastereoisomers (trans/cis = 7:5 based on H NMR).
A mixture of aziridine 4a (224 mg, 0.5 mmol) and H2SO4
(147 mg, 1.5 mmol) in benzene (5 mL) was heated to reflux
for 2 h. After usual workup and column chromatographic
purification process (hexanes/ether, 20:1) we obtained 5a,
117 mg (85%). The other aziridines 4b–f and naphthalenes
5b–f were synthesized analogously and the spectroscopic
data of prepared compounds are as follows.
Compound 4a: 62% (trans/cis = 7:5); clear oil; IR (KBr)
1705, 1331, 1161 cmꢀ1; 1H NMR (CDCl3, 500 MHz, trans-
isomer) d 2.30 (s, 3H), 2.34 (s, 3H), 3.72 (dd, J = 5.0 and
1.0 Hz, 1H), 3.84 (s, 3H), 4.34 (d, J = 5.0 Hz, 1H), 7.05–
7.34 (m, 9H), 7.42 (d, J = 8.5 Hz, 2H), 7.79 (d, J = 8.5 Hz,
1
2H), 7.95 (s. 1H); H NMR (CDCl3, 500 MHz, cis-isomer)
d 2.17 (s, 3H), 2.40 (s, 3H), 3.49 (s, 3H), 3.97 (d, J = 7.0 Hz,
1H), 4.23 (dd, J = 7.0 and 2.0 Hz, 1H), 6.52 (d, J = 8.0 Hz,
2H), 6.71 (d, J = 8.0 Hz, 2H), 7.05–7.34 (m, 7H), 7.48 (d,
J = 2.0 Hz, 1H), 7.81 (d, J = 8.5 Hz, 2H); 13C NMR
(CDCl3, 125 MHz, cis+trans mixture) d 21.03, 21.17,
21.54, 21.61, 44.66, 46.54, 48.39, 49.52, 51.76, 52.31,
123.72, 124.49, 126.86, 127.07, 127.84, 127.91, 127.98,
128.12, 128.41, 128.56, 128.41, 129.06, 129.38, 129.49,
129.52, 130.19, 131.62, 133.88, 133.98, 135.48, 136.56,
137.00, 138.13, 143.69, 144.07, 144.33, 145.55, 167.29,
167.49.
Compound 4b: 60% (trans/cis = 2:1); clear oil; 1H NMR
(CDCl3, 300 MHz, trans-isomer) d 1.39 (t, J = 7.2 Hz, 3H),
2.29 (s, 3H), 2.33 (s, 3H), 3.69 (dd, J = 4.8 and 1.2 Hz, 1H),
4.25–4.41 (m, 2H), 4.42 (d, J = 4.8 Hz, 1H), 7.06–7.44 (m,
11H), 7.79 (d, J = 8.4 Hz, 2H), 7.94 (s, 1H); 1H NMR
(CDCl3, 300 MHz, cis-isomer) d 1.26 (t, J = 7.2 Hz, 3H),
2.17 (s, 3H), 2.37 (s, 3H), 3.93 (d, J = 6.6 Hz, 1H), 3.98–
4.41 (m, 3H), 6.52 (d, J = 8.1 Hz, 2H), 6.73 (d, J = 8.1 Hz,
2H), 7.06–7.44 (m, 7H), 7.50 (d, J = 1.5 Hz, 1H), 7.75 (d,
J = 8.4 Hz, 2H).

980
K. Y. Lee et al. / Tetrahedron Letters 47 (2006) 977–980
Compound 4c: 70% (trans/cis = 3:1); clear oil; 1H NMR
1H); 13C NMR (CDCl3, 75 MHz) d 14.36, 21.19, 61.07,
126.03, 126.08, 126.37, 127.22, 128.14, 129.03, 129.68,
129.85, 130.24, 133.02, 133.84, 137.06, 137.24, 140.58,
166.73; ESIMS m/z 291.1 (M++H).
(CDCl3, 300 MHz, trans-isomer) d 2.32 (s, 3H), 3.66 (dd,
J = 4.8 and 1.2 Hz, 1H), 3.89 (s, 3H), 5.08 (d, J = 4.8 Hz,
1H), 6.92–7.42 (m, 11H), 7.83 (d, J = 8.1 Hz, 2H), 8.00 (s,
1H); 1H NMR (CDCl3, 300 MHz, cis-isomer) d 2.38 (s,
3H), 3.56 (s, 3H), 4.35 (dd, J = 6.6 and 1.5 Hz, 1H), 4.44 (d,
J = 6.6 Hz, 1H), 6.92–7.42 (m, 11H), 7.54 (d, J = 1.5 Hz,
1H), 7.77 (d, J = 8.4 Hz, 2H).
Compound 5c: 81%; yellow solid, mp 139–140 °C; IR (KBr)
1720, 1242 cmꢀ1 1H NMR (CDCl3, 300 MHz) d 3.97 (s,
;
3H), 7.34–7.43 (m, 3H), 7.50–7.58 (m, 4H), 7.98–8.03 (m,
2H), 8.67 (d, J = 1.8 Hz, 1H); 13C NMR (CDCl3, 75 MHz)
d 52.25, 125.91, 126.37, 126.63, 128.71, 126.80, 128.51,
129.26, 129.58, 129.73, 131.22, 131.97, 132.62, 133.80,
133.97, 137.85, 138.50, 167.04; ESIMS m/z 297.1 (M++H).
Compound 5d: 80%; yellow solid, mp 89–90 °C; IR (KBr)
Compound 4d: 63% (cis only); clear oil; IR (KBr) 1716,
1331, 1265, 1161 cmꢀ1 1H NMR (CDCl3, 300 MHz, cis-
;
isomer) d 2.16 (s, 3H), 2.34 (s, 3H), 2.38 (s, 3H), 3.46 (s,
3H), 3.99 (d, J = 6.6 Hz, 1H), 4.22 (dd, J = 6.6 and 1.8 Hz,
1H), 6.55 (d, J = 8.1 Hz, 2H), 6.71 (d, J = 8.1 Hz, 2H), 6.99
(d, J = 8.7 Hz, 2H), 7.02 (d, J = 8.7 Hz, 2H), 7.27 (d,
J = 8.1 Hz, 2H), 7.44 (d, J = 1.8 Hz, 1H), 7.82 (d,
J = 8.1 Hz, 2H).
1720, 1242 cmꢀ1 1H NMR (CDCl3, 300 MHz) d 2.45 (s,
;
3H), 2.46 (s, 3H), 3.96 (s, 3H), 7.29–7.40 (m, 5H), 7.69 (s,
1H), 7.89 (d, J = 8.4 Hz, 1H), 7.96 (d, J = 1.8 Hz, 1H), 8.54
(s, 1H); 13C NMR (CDCl3, 75 MHz) d 21.23, 22.11, 52.13,
125.02, 125.98, 126.23, 128.70, 129.04, 129.56, 129.83,
130.14, 131.24, 134.06, 137.15, 137.19, 138.48, 139.90,
167.35; ESIMS m/z 291.1 (M++H).
Compound 4e: 63% (trans/cis = 1:5); clear oil; 1H NMR
(CDCl3, 300 MHz, trans-isomer) d 2.31 (s, 3H), 2.36 (s,
3H), 3.62 (dd, J = 5.1 and 1.2 Hz, 1H), 3.85 (s, 3H), 4.43 (d,
J = 5.1 Hz, 1H), 7.00–7.43 (m, 10H), 7.78 (d, J = 8.4 Hz,
Compound 5e: 83%; yellow solid, mp 94–95 °C; IR (KBr)
1
2H), 7.88 (s, 1H); H NMR (CDCl3, 300 MHz, cis-isomer)
1724, 1246 cmꢀ1 1H NMR (CDCl3, 300 MHz) d 2.47 (s,
;
d 2.20 (s, 3H), 2.40 (s, 3H), 3.52 (s, 3H), 3.96 (d, J = 6.9 Hz,
1H), 4.19 (dd, J = 6.9 and 1.8 Hz, 1H), 6.55 (d, J = 8.1 Hz,
2H), 6.77 (d, J = 8.1 Hz, 2H), 7.00–7.32 (m, 6H), 7.45 (d,
J = 1.8 Hz, 1H), 7.77 (d, J = 8.4 Hz, 2H); 13C NMR
(CDCl3, 75 MHz, cis-isomer) d 21.03, 21.60, 44.29, 46.61,
51.87, 124.30, 126.36, 127.05, 127.99, 128.02, 128.13,
129.54, 130.68, 132.39, 134.83, 135.28, 137.28, 142.32,
144.43, 167.12.
3H), 3.97 (s, 3H), 7.32 (d, J = 8.4 Hz, 2H), 7.36 (d,
J = 8.4 Hz, 2H), 7.48 (dd, J = 8.7 and 2.1 Hz, 1H), 7.91
(s, 1H), 7.93 (d, J = 8.7 Hz, 1H), 8.02 (d, J = 2.1 Hz, 1H),
8.55 (s, 1H); 13C NMR (CDCl3, 75 MHz) d 21.24, 52.31,
125.10, 127.09, 127.18, 127.49, 129.27, 129.72, 129.99,
131.19, 131.28, 134.52, 134.56, 136.28, 137.65, 140.03,
166.90; ESIMS m/z 311.1 (M++H).
Compound 5f: 79%; yellow solid, mp 123–124 °C; IR (KBr)
Compound 4f: 68% (cis-only); clear oil; IR (KBr) 1716,
1720, 1254 cmꢀ1 1H NMR (CDCl3, 300 MHz) d 2.44 (s,
;
1331, 1250, 1161 cmꢀ1
;
1H NMR (CDCl3, 300 MHz, cis-
3H), 3.97 (s, 3H), 4.03 (s, 3H), 6.85 (dd, J = 7.2 and 1.2 Hz,
1H), 7.27–7.50 (m, 6H), 8.01 (d, J = 1.2 Hz, 1H), 9.04 (s,
1H); 13C NMR (CDCl3, 75 MHz) d 21.20, 52.12, 55.64,
104.26, 118.25, 124.49, 125.27, 126.02, 126.74, 128.41,
128.95, 129.85, 134.96, 137.13, 137.35, 140.21, 156.58,
167.39; ESIMS m/z 307.1 (M++H).
isomer) d 2.18 (s, 3H), 2.36 (s, 3H), 3.47 (s, 3H), 3.51 (s, 3H),
3.86 (d, J = 6.9 Hz, 1H), 4.18 (dd, J = 6.9 and 1.8 Hz, 1H),
6.50–7.26 (m, 10H), 7.68 (d, J = 1.8 Hz, 1H), 7.80 (d,
J = 8.1 Hz, 2H).
Compound 5a: 85%; yellow solid, mp 95–96 °C; IR (KBr)
1720, 1246 cmꢀ1
;
1H NMR (CDCl3, 300 MHz) d 2.45 (s,
7. As reported the cis/trans ratios of the diastereoisomers were
much different depending upon the structure of the
substrates.5 We obtained cis-isomers exclusively for the
cases of 4d and 4f. However, cis/trans mixtures were
obtained for 4a–c and 4e in a variable ratios.6 In Figure 2
we show the NOE data of pure cis-4e as an example.
8. During the evaluation process of this paper, one of the
reviewers suggested the synthesis of naphthalenes from the
reaction of N-aryl(or N-alkyl)aziridine of Baylis–Hillman
adducts instead of 4a–f. However, the preparation of N-
phenylaziridine derivative from the reaction of 2a and
benzaldehyde N-phenylimine failed as reported in Ref. 5a.
3H), 3.97 (s, 3H), 7.29 (d, J = 8.1 Hz, 2H), 7.38 (d,
J = 8.1 Hz, 2H), 7.50–7.54 (m, 2H), 7.92–7.99 (m, 2H),
8.00 (d, J = 1.8 Hz, 1H), 8.59 (s, 1H); 13C NMR (CDCl3,
75 MHz) d 21.21, 52.20, 126.03, 126.11, 126.43, 126.87,
128.23, 129.04, 129.71, 129.85, 130.33, 133.02, 133.88,
136.99, 137.27, 140.67, 167.23; ESIMS m/z 277.1 (M++H).
Compound 5b: 84%; yellow solid, mp 96–97 °C; IR (KBr)
1716, 1242 cmꢀ1 1H NMR (CDCl3, 300 MHz) d 1.43 (t,
;
J = 6.9 Hz, 3H), 2.45 (s, 3H), 4.43 (q, J = 6.9 Hz, 2H), 7.30
(d, J = 8.4 Hz, 2H), 7.39 (d, J = 8.4 Hz, 2H), 7.47–7.55 (m,
2H), 7.91–8.00 (m, 2H), 8.01 (d, J = 1.8 Hz, 1H), 8.60 (s,