Visible light photoredox catalysis: Aerobic oxidation of perimidines to perimidinones

Article abstract of DOI:10.1016/j.tet.2014.09.036

Aerobic oxidation of a series of 2,3-dihydro-1H-perimidines to the corresponding 4- and 6-perimidinones via visible light photoredox catalysis using Ru(bpy)₃²⁺ as a catalyst was reported. The scope and limitation of this oxidation were investigated and a possible photochemical mechanism was proposed.

Full text of DOI:10.1016/j.tet.2014.09.036

Tetrahedron 70 (2014) 8219e8225
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Visible light photoredox catalysis: aerobic oxidation of perimidines
to perimidinones
,
Chun-Ku Wu a, Teau-Jiuan Liou b, Hao-Yi Wei a, Pei-Shan Tsai a, Ding-Yah Yang a
*
a Department of Chemistry, Tunghai University, No. 1727, Sec. 4, Taiwan Boulevard, Xitun District, Taichung 40704, Taiwan
b Department of Applied English, Chaoyang University of Technology, No. 168, Jifeng E. Rd., Wufeng District, Taichung 41349, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 May 2014
Received in revised form 7 September 2014
Accepted 12 September 2014
Available online 21 September 2014
Aerobic oxidation of
a series of 2,3-dihydro-1H-perimidines to the corresponding 4- and 6-
perimidinones via visible light photoredox catalysis using Ru(bpy)23þ as a catalyst was reported. The
scope and limitation of this oxidation were investigated and a possible photochemical mechanism was
proposed.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Visible light photoredox catalysis
Perimidine
Perimidinone
Oxidation
1. Introduction
VLPC to replace the use of conventional oxidants such as Fremy’s
salt, which generally suffers from low yields or requires excess
Chemical transformation via photocatalysis with visible light is
one of the emerging strategies to meet the increasing demand for
more sustainable chemical processes.1 Up to date, visible light
photoredox catalysis (VLPC) has been successfully applied to vari-
ous organic transformations, including aza-Henry reaction,2
asymmetric alkylation,3 [2þ2] cycloaddition,4 DielseAlder re-
action,5 reductive dehalogenation,6 oxidations,7 etc. Among the
VLPC-based oxidations, previous reactions involved the use of the
less environmentally benign organic compounds such as aryldia-
zonium ion8 tetrachloromethane9 or bromotrichloromethane10 as
the external oxidants. Recent catalytic oxidation processes, how-
ever, focus primarily on utilizing molecular oxygen as the terminal
oxidant, since the latter is more atom-economical and generates no
waste after reduced. For instance, Zen11 and co-workers have re-
ported the selective and efficient photocatalytic oxidation of sul-
fides to sulfoxides with O2 using tris(bipyridine) ruthenium
complexes (Ru(bpy)23þ) as a photoredox catalyst. Itoh and co-
workers have reported the aerobic oxidation of alcohols to car-
boxylic acids under visible light irradiation of fluorescent lamp
with a catalytic amount of magnesium bromide diethyl etherate12
or 2-chloroanthraquinone.13 These successful examples prompted
us to explore other oxygen-mediated oxidation transformations via
stoichiometric amounts of reagents.14 Here, we describe the oxi-
dation of a series of 2,3-dihydro-1H-perimidines to the corre-
sponding 4- and 6-perimidinones via visible light photocatalysis
under aerobic conditions. The 4- and 6-perimidinones can poten-
tially serve as precursors for further preparation of 1,8-
diaminonaphthalene-based spiroheterocyclic photochromic color-
ants.15 The scope and limitation of this oxidation were investigated
and the photochemical mechanism was proposed on the basis of
the experimental results.
2. Results and discussion
Scheme 1 depicts the preparation of 2,3-dihydro-1H-peri-
midines 1 and 2. It involved the Yb(OTf)3-catalyzed condensation
cyclization16 of naphthalene-1,8-diamine (3) with acetophenones
or benzaldehydes to give 2,3-dihydro-1H-perimidines 1 and 2, re-
spectively. The aerobic oxidation of 1 and 2 via visible light pho-
toredox catalysis using Ru(bpy)23þ as a catalyst in acetonitrile
generated the perimidinones 6, 7 and 8, 9 in moderate to good
yields. The photooxidation was performed in various solvents and
acetonitrile was found to be the optimal one. Tables 1 and 2
summarize the results of the studies. The molecular structures of
the major products 6 and 8 were all elucidated by 1H and 13C NMR
spectroscopy. The perimidinones 6 and 8 could be spectroscopically
identified by the observation of the characteristic downfield
doublet absorption of the 5-H hydrogen at 6.69e6.73 and
* Corresponding author. Tel.: þ886 4 2359 7613; fax: þ886 4 2359 0426; e-mail
address: yang@thu.edu.tw (D.-Y. Yang).
http://dx.doi.org/10.1016/j.tet.2014.09.036
0040-4020/Ó 2014 Elsevier Ltd. All rights reserved.

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C.-K. Wu et al. / Tetrahedron 70 (2014) 8219e8225
Scheme 1. Preparation of perimidines 1e2 and their photooxidation products 6e9.
Table 1
7.00e7.05 ppm, respectively (see Scheme 1 for atom numbering).
Photoxidation of 1aee under VLPC
Selective compounds such as 6d (R¼p-nitrophenyl), 7a (R¼phenyl),
and 8d (R¼p-dimethylaminophenyl) were further characterized by
the X-ray crystallography as shown in Fig. 1.17 Since no reaction was
observed when 1 was irradiated with visible light in the absence of
Ru(bpy)23þ in acetonitrile for 24 h, this result suggests the photo-
oxidation is indeed catalyzed by the ruthenium complex.
In an effort to investigate the scope and limitation of this pho-
toredox catalysis, several aminonaphthalene derivatives (10e13)
were synthesized to explore their structureereactivity relationship
toward VLPC, as shown in Schemes 2 and 3. Compounds 10 and 11
were accessed by sequential methylations of 1a using methyl iodide
in the presence of potassium carbonate as a base at room temper-
ature. The perimidinone 12 was prepared by treating naphthalene-
1,8-diamine (3) with sodium cyanate under acidic conditions via
the literature procedure.18 The amide 13 was readily available by
base-promoted acetylation of naphthalen-1-amine (15) with acetyl
chloride. With these four compounds in hand, their photochemical
reactivity toward VLPC was then evaluated. While aerobic oxidation
of N-monomethylated 10 generated the expected perimidinone
product 14, no product was obtained for photooxidation of N,N-
dimethylated 11 under the same reaction conditions (Scheme 2).
Entrya Substrate
R
Major
product
Yieldb (%) Minor
product
Yieldb (%)
1
2
3
4
5
1a
1b
1c
1d
1e
Phenyl
p-Methoxyphenyl 6b
o-Methoxyphenyl 6c
p-Nitrophenyl
Methyl
6a
77
82
83
80
82
7a
7b
7c
7d
7e
12
9
10
7
6d
6e
12
a
Subjected to irradiation with
a
23
W household fluorescence lamp
(
lmaxz465 nm).
b
Isolated yield after purification by chromatography on SiO2.
Fig. 1. ORTEP crystal structures of 6d, 7a, and 8d.

C.-K. Wu et al. / Tetrahedron 70 (2014) 8219e8225
8221
Table 2
observations suggest that this oxidation reaction is sensitive to the
substituents on amine moiety. Introduction of an electron-
withdrawing substituent on amine will abolish the resulting com-
pound’s reactivity toward photooxidation.
Photoxidation of 2aee under VLPC
Scheme 4 depicts the proposed mechanism for the formation of
8 from 2 via tandem visible light-mediated catalytic cycle catalyzed
by Ru(bpy)23þ. Other products (6, 7, and 9) are expected to follow the
similar reaction mechanism. First, Ru(bpy)23þ is excited by visible
light to its excited state. It accepts one electron from the amine
donor 2 to yield the Ru(bpy)þ3 and the radical cation 16. The radical
on nitrogen of 16 can delocalize to the para position of the ring to
give 17, which is then trapped by oxygen to generate the peroxide
radical 18. The subsequent dehydration of 18 affords the radical
cation 19. Final back electron transfer from the Ru(bpy)þ3 to 19
furnishes the perimidinone 20 along with the regenerated catalyst.
The perimidinone 20 further serves as the starting material for the
next catalytic cycle by donating an electron on the amine nitrogen
atom to the visible light excited ruthenium complex to give the
radical cation 21 and the Ru(bpy)þ3 . The subsequent electron
transfer from Ru(bpy)þ3 to the molecular oxygen yields the super-
oxide anion radical, along with the regenerated ground-state
Entrya Substrate
R
Major
product (%)
Yieldb Minor
product (%)
Yield
1
2
3
4
5
2a
2b
2c
2d
2e
Phenyl
p-Methoxyphenyl
o-Methoxyphenyl
p-Dimethylaminophenyl 8d
Isopropyl 8e
8a
8b
8c
79
76
81
75
77
9a
9b
9c
9d
9e
<3
<3
<3
<3
<3
a
Subjected to irradiation with
a 23 W household fluorescence lamp
(
lmaxz465 nm).
b
Isolated yield after purification by chromatography on SiO2.
photocatalyst Ru(bpy)32þ 2,19
Final proton and hydrogen transfers
.
from the cation radical 21 to superoxide anion radical furnish the
product perimidinone 8 and the by-product hydrogen peroxide.
The formation of H2O2 at the end of catalytic cycle was confirmed
via commercially available H2O2 detection test strips (see
Supplementary data). Note that no product was obtained when the
photoxidation of 1 or 2 was performed under anaerobic conditions.
These observations suggest that molecular oxygen is essential and
indeed the oxidant for this photoreaction. Compared to the con-
ventional oxidants such as Fremy’s salt or mCPBA,14 this visible
light-mediated oxidation of 2,3-dihydro-1H-perimidines to the
corresponding 4- and 6-perimidinones has the advantage of uti-
lizing atom-economical and environmentally benign molecular
oxygen as the oxygen source.
3. Conclusions
In summary, we report a useful method for ruthenium-catalyzed
aerobic oxidation of the perimidines 1 and 2 to the corresponding 4-
and 6-perimidinones 6e9 under visible light irradiation from
Scheme 2. Preparation of compounds 10 and 11 and their photooxidation.
Scheme 3. Preparation of compounds 12 and 13 and their photooxidation.
This result implies that at least one of the two amino groups on
naphthalene moiety of 1 has to be a secondary amine to initiate the
oxidation. As for 12 and 13, neither of them was found to be sen-
sitive to visible light under VLPC conditions; that is, no corre-
sponding photooxidation product was observed (Scheme 3). These
a household fluorescent lamp with high conversions and good
yields. Further structure-reactivity relationship studies indicated
that this visible light-mediated oxidation of 2,3-dihydro-1H-peri-
midines is sensitive to the substituents on the amine moiety. Either
a tertiaryamine on naphthalene moiety or an electron-withdrawing

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C.-K. Wu et al. / Tetrahedron 70 (2014) 8219e8225
performed with a 23 W household fluorescence lamp (Philips es-
sential 57 lm/W, 6500 K).
4.2. General procedure for the preparation of compounds
1aee and 2aee
To
a solution of naphthalene-1,8-diamine 3 (200.0 mg,
1.3 mmol) in MeCN (6 mL) was added acetophenones (1.3 mmol) or
benzaldehydes (1.3 mmol) and a catalytic amount of Yb(OTf)3
(0.1 equiv) at room temperature. The resulting mixture was stirred
room temperature for 24 h. The solution was then concentrated in
vacuo, and the product was extracted with CH2Cl2/water. The
combined organic layer was dried over MgSO4, concentrated in
vacuo, and the product was purified by column chromatography to
obtain the desired compound.
4.2.1. 2-Methyl-2-phenyl-2,3-dihydro-1H-perimidine
(1a). Pink
solid; yield 72%; Rf¼0.3 (15% EtOAc/hexanes); mp 137e138 ꢀC
(lit.20a 136e138 ꢀC); 1H NMR (CDCl3, 300 MHz)
d 7.59e7.56 (m, 2H),
7.31e7.19 (m, 5H), 7.11 (dd, J¼8.4, 0.9 Hz, 2H), 6.53 (dd, J¼6.9,
0.9 Hz, 2H), 1.82 (s, 3H).
4.2.2. 2-(4-Methoxyphenyl)-2-methyl-2,3-dihydro-1H-perimidine
(1b). Pink solid; yield 75%; Rf¼0.5 (35% EtOAc/hexanes); mp
180e181 ꢀC (lit.20a 178e180 ꢀC); 1H NMR (CDCl3, 300 MHz)
d 7.50
(d, J¼9.0 Hz, 2H), 7.21 (t, J¼9.0 Hz, 2H), 7.11 (dd, J¼8.4, 1.2 Hz, 2H),
6.79 (d, J¼9.0 Hz, 2H), 6.49 (dd, J¼7.5, 1.2 Hz, 2H), 4.62 (br s, 2H),
3.73 (s, 3H), 1.79 (s, 3H).
4.2.3. 2-(2-Methoxyphenyl)-2-methyl-2,3-dihydro-1H-perimidine
(1c). Pink solid; yield 85%; Rf¼0.5 (20% EtOAc/hexanes); mp
142e143 ꢀC; 1H NMR (CDCl3, 300 MHz)
d
7.30 (dd, J¼7.8, 1.8 Hz, 1H),
7.21e7.03 (m, 5H), 6.80 (d, J¼7.8 Hz, 1H), 6.69 (t, J¼7.8 Hz, 1H), 6.49
(d, J¼7.2 Hz, 2H), 5.48 (br s, 2H), 3.89 (s, 3H), 1.82 (s, 3H); 13C NMR
(CDCl3, 75 MHz)
d 156.8, 140.6, 134.5, 133.4, 128.4, 127.1, 126.9,
Scheme 4. Proposed catalytic cycle catalyzed by Ru(bpy)32þ
.
120.6, 117.0, 113.2, 111.6, 105.4, 68.3, 55.5, 29.2; IR
n (KBr) 3356,
2929, 1596, 1229, 1020, 808 cm-1; HRMS (EI) calcd for C19H18N2O
[Mþ] 290.1419, found 290.1417.
substituent on amine will exclude the resulting compounds from
photooxidation. Efforts toward the extension of this photooxidation
to other functional groups are currently underway.
4.2.4. 2-(4-Nitrophenyl)-2-methyl-2,3-dihydro-1H-perimidine
(1d). Yellow solid; yield 91%; Rf¼0.4 (30% EtOAc/hexanes); mp
191e192 ꢀC (lit.20b 192e193 ꢀC); 1H NMR (CDCl3, 300 MHz)
d 8.04
4. Experimental section
4.1. General
(d, J¼8.4 Hz, 2H), 7.67 (d, J¼8.4 Hz, 2H), 7.23 (td, J¼8.1, 0.9 Hz, 2H),
7.15 (dd, J¼8.1, 0.9 Hz, 2H), 6.60 (dd, J¼7.2, 0.9 Hz, 2H), 4.86 (br s,
2H), 1.81 (s, 3H).
Melting points were determined on a Mel-Temp melting point
apparatus in open capillaries and are uncorrected. MS were per-
formed on JEOL JMS-SX/SX 102A spectrometer. IR spectra were
obtained using a 1725XFT-IR spectrophotometer. Single crystal
structures were determined by a Bruker AXS SMART-1000 X-ray
single-crystal diffractometer. 1H and 13C NMR spectra were recor-
ded at 300 and 75 MHz on a Varian VXR300 spectrometer as well as
600 and 150 MHz on a Varian Unity Inova 600 spectrometer.
4.2.5. 2,2-Dimethyl-2,3-dihydro-1H-perimidine (1e). Brown solid;
yield 89%; Rf¼0.3 (15% EtOAc/hexanes); mp 116e117 ꢀC (lit.20a
115 ꢀC); 1H NMR (CDCl3, 300 MHz)
d
7.23 (t, J¼7.5 Hz, 2H), 7.14
(dd, J¼8.1, 1.2 Hz, 2H), 6.47 (dd, J¼7.5, 1.2 Hz, 2H), 4.03 (br s, 2H),
1.50 (s, 6H).
4.2.6. 2-Phenyl-2,3-dihydro-1H-perimidine (2a). Yellow solid; yield
88%; Rf¼0.4 (10% EtOAc/hexanes); mp 103e104 ꢀC (lit.20b 104 ꢀC);
Chemical shifts were reported in parts per million on the
d
scale
1H NMR (CDCl3, 300 MHz)
d 7.66e7.61 (m, 2H), 7.46e7.44 (m, 2H),
relative to an internal standard (tetramethylsilane, or appropriate
solvent peaks) with coupling constants given in hertz. 1H NMR
multiplicity data are denoted by s (singlet), d (doublet), t (triplet), q
(quartet), and m (multiplet). Analytical thin-layer chromatography
(TLC) was carried out on Merck silica gel 60G-254 plates (25 mm)
and developed with the solvents mentioned. Flash chromatography
was performed in columns of various diameters with Merck silica
gel (230e400 mesh ASTM 9385 kieselgel 60H) by elution with the
solvent systems. The peroxide detection was carried out using
Macherey Nagel 91319 Quantofix Peroxide 25 test strips (Apps
Laboratories, Germany). The visible light irradiation reaction was
7.29e7.21 (m, 5H), 7.53 (dd, J¼6.6,1.8 Hz, 2H), 5.48 (s,1H), 4.67 (br s,
2H).
4.2.7. 2-(4-Methoxyphenyl)-2,3-dihydro-1H-perimidine (2b). Light
purple solid; yield 89%; Rf¼0.4 (10% EtOAc/hexanes); mp
152e153 ꢀC (lit.20b 152 ꢀC); 1H NMR (CDCl3, 300 MHz)
d 7.57 (d,
J¼8.4 Hz, 2H), 7.28e7.19 (m, 4H), 6.97 (d, J¼8.4 Hz, 2H), 6.52 (dd,
J¼6.9, 1.5 Hz, 2H), 5.43 (s, 1H), 4.50 (br s, 2H), 3.85 (s, 3H).
4.2.8. 2-(2-Methoxyphenyl)-2,3-dihydro-1H-perimidine
(2c). Yellow solid; yield 85%; Rf¼0.4 (10% EtOAc/hexanes); mp

C.-K. Wu et al. / Tetrahedron 70 (2014) 8219e8225
8223
148e149 ꢀC; 1H NMR (CDCl3, 300 MHz)
d
7.67 (dd, J¼7.5,1.8 Hz,1H),
111.6, 110.7, 74.3, 55.4, 30.2; IR
n
(KBr) 3408, 2936, 1592, 1297, 1230,
7.33 (td, J¼8.7, 2.1 Hz, 1H), 7.25 (t, J¼9.0 Hz, 2H), 7.19 (dd, J¼9.0,
1.2 Hz, 2H), 7.00 (td, J¼7.5, 0.9 Hz, 1H), 6.94 (dd, J¼8.7, 0.9 Hz, 1H),
6.55 (dd, J¼7.5, 1.2 Hz, 2H), 5.92 (s, 1H), 4.65 (br s, 2H), 3.88 (s, 3H);
1025 cmꢁ1; HRMS (EI) calcd for C19H16N2O2 [Mþ] 304.1212, found
304.1210.
13C NMR (CDCl3, 75 MHz)
d
156.9, 142.1, 134.8, 129.7, 128.5, 127.4,
4.3.4. 2-(4-Nitrophenyl)-2-methyl-1H-perimidin-6(2H)-one
126.8, 121.0, 117.5, 113.4, 110.5, 105.9, 61.3, 55.4; IR
n
(KBr) 3360,
(6d). Red solid; yield 80%; Rf¼0.3 (40% EtOAc/hexanes); mp
2837, 1597, 1239, 1023, 810 cmꢁ1; HRMS (EI) calcd for C18H16N2O
[Mþ] 276.1263, found 276.1260.
159e160 ꢀC; 1H NMR (CDCl3, 300 MHz)
d
8.21 (d, J¼9.0 Hz, 2H), 7.77
(d, J¼9.0 Hz, 2H), 7.41 (t, J¼7.5 Hz, 1H), 7.35 (dd, J¼7.5, 1,2 1H), 7.19
(d, J¼10.2 Hz, 1H), 6.89 (dd, J¼7.5, 1.2 Hz, 1H), 6.72 (d, J¼10.2 Hz,
4.2.9. 2-(4-(Dimethylamino)phenyl)-2,3-dihydro-1H-perimidine
1H), 4.50 (br s, 1H), 1.98 (s, 3H); 13C NMR (CDCl3, 75 MHz)
d 185.3,
(2d). Brown solid; yield 85%; Rf¼0.4 (10% EtOAc/hexanes); mp
154.4,153.6,147.2,141.9, 140.7,134.9,133.7,129.4,126.5,123.7,118.4,
168e169 ꢀC (lit.20b 168 ꢀC); 1H NMR (CDCl3, 300 MHz)
d
7.50 (d,
116.1, 111.3, 74.3, 31.5; IR n (KBr) 3298, 2929, 1594, 1518, 1301,
J¼9.0 Hz, 2H), 7.27e7.18 (m, 4H), 6.77 (d, J¼9.0 Hz, 2H), 6.50 (dd,
755 cmꢁ1; HRMS (EI) calcd for C18H13N3O3 [Mþ] 319.0957, found
319.0950.
J¼6.9, 1.2 Hz, 2H), 5.39 (s, 1H), 4.50 (br s, 2H), 3.00 (s, 6H).
4.2.10. 2-Isopropyl-2,3-dihydro-1H-perimidine (2e). White solid;
yield 88%; Rf¼0.4 (15% EtOAc/hexanes); mp 69e70 ꢀC; 1H NMR
4.3.5. 2,2-Dimethyl-1H-perimidin-6(2H)-one (6e). Purple solid;
yield 82%; Rf¼0.4 (40% EtOAc/hexanes); mp 197e198 ꢀC (lit.14
185e190 ꢀC); 7.35e7.26 (m, 2H), 7.13 (d, J¼10.2 Hz, 1H), 6.71e6.68
(m, 1H), 6.70 (d, J¼10.2 Hz, 1H), 4.03 (br s, 1H), 1.60 (s, 6H).
(CDCl3, 300 MHz)
d
7.22 (dd, J¼7.8, 6.9 Hz, 2H), 7.15 (dd, J¼7.8,
0.9 Hz, 2H), 6.51 (dd, J¼6.9, 0.9 Hz, 2H), 4.29 (d, J¼4.8 Hz, 2H), 1.95
(sd, J¼6.9, 4.8 Hz, 1H), 1.11 (d, J¼6.9 Hz, 6H); 13C NMR (CDCl3,
150 MHz)
n
d
142.0, 134.8, 126.8, 117.3, 113.6, 105.6, 69.3, 32.3, 17.2; IR
4.3.6. 2-Methyl-2-phenyl-1H-perimidin-4(2H)-one
(7a). Purple
(KBr) 3356, 2964, 1599, 1415, 811, 745 cmꢁ1; HRMS (EI) calcd for
14H16N2 [Mþ] 212.1313, found 212.1316.
solid; yield 12%; Rf¼0.3 (40% EtOAc/hexanes); mp 89e90 ꢀC; 1H
C
NMR (CDCl3, 300 MHz) d 7.60e7.57 (m, 2H), 7.36e7.28 (m, 3H),
7.25 (d, J¼9.9 Hz, 1H), 7.20 (dd, J¼8.1, 7.2 Hz, 1H), 6.62 (dd, J¼8.4,
4.3. General procedure for the preparation of compounds
6aee, 7aee, 8aee, 9aee, and 14
0.9 Hz, 1H), 6.57 (dd, J¼8.4, 0.9 Hz, 1H), 6.33 (d, J¼9.9 Hz, 1H), 4.32
(br s, 1H), 2.00 (s, 3H); 13C NMR (CDCl3, 150 MHz)
d 185.1, 152.1,
146.8, 144.5, 143.5, 134.3, 130.0, 128.3, 128.0, 127.7, 125.3, 118.8,
A solution of the 2,3-dihydro-1H-perimidines 1aee or 2aee
(0.5 mmol) and a catalytic amount of Ru(bpy)23þ (0.05 equiv) in
MeCN (200 mL, 2.5ꢂ10ꢁ3 M, without degassing) was placed in
a 250 mL round bottom flask. The mixture was irradiated at room
temperature in front of (approximately 5 cm) a 23 W fluorescent
light bulb (Philips essential 57 lm/W, 6500 K). Upon complete
consumption of the 2,3-dihydro-1H-perimidines (monitored by
TLC), the solution was concentrated in vacuo, and the residue was
purified by column chromatography to obtain the desired
product.
116.5, 110.5, 74.9, 31.7; IR n (KBr) 3341, 1655, 1620, 1578, 1272,
757 cmꢁ1; HRMS (EI) calcd for C18H14N2O [Mþ] 274.1106, found
274.1112.
4.3.7. 2-(4-Methoxyphenyl)-2-methyl-1H-perimidin-4(2H)-one
(7b). Purple solid; yield 9%; Rf¼0.3 (50% EtOAc/hexanes); mp
76e77 ꢀC; 1H NMR (CDCl3, 300 MHz)
d
7.49 (d, J¼9.0 Hz, 2H), 7.25
(d, J¼9.9 Hz, 1H), 7.19 (dd, J¼8.4, 7.5 Hz, 1H), 6.86 (d, J¼9.0 Hz, 2H),
6.58 (t, J¼8.4 Hz, 2H), 6.33 (d, J¼9.9 Hz, 1H), 4.25 (br s, 1H), 3.77 (s,
3H), 1.97 (s, 3H); 13C NMR (CDCl3, 150 MHz)
d 185.1, 159.0, 151.8,
144.5, 143.5, 139.3, 134.3, 130.0, 128.1, 126.6, 118.7, 116.4, 113.6,
4.3.1. 2-Methyl-2-phenyl-1H-perimidin-6(2H)-one
(6a). Purple
110.4, 74.7, 55.3, 31.6; IR n (KBr) 3297, 1578, 1248, 1175, 830,
solid; yield 77%; Rf¼0.5 (40% EtOAc/hexanes); mp 80e81 ꢀC; 1H
813 cmꢁ1; HRMS (EI) calcd for C19H16N2O2 [Mþ] 304.1212, found
304.1210.
NMR (CDCl3, 300 MHz) d 7.59e7.57 (m, 2H), 7.40e7.29 (m, 5H), 7.20
(d, J¼10.2 Hz, 1H), 6.79 (dd, J¼7.2, 1.5 Hz, 1H), 6.70 (d, J¼10.2 Hz,
1H), 4.37 (br s, 1H), 1.96 (s, 3H); 13C NMR (CDCl3, 75 MHz)
185.7,
153.5,146.7,142.4,141.0,134.4,133.3,129.2,128.4,127.7,125.2,117.9,
d
4.3.8. 2-(2-Methoxyphenyl)-2-methyl-1H-perimidin-4(2H)-one
(7c). Purple solid; yield 10%; Rf¼0.3 (30% EtOAc/hexanes); mp
115.0, 110.8, 74.6, 31.1; IR
n
(KBr) 3340, 2924, 1625, 1299, 761,
187e188 ꢀC; 1H NMR (CDCl3, 300 MHz)
7.27e7.22 (m, 1H), 7.267 (d, J¼9.9, 1H), 7.10 (dd, J¼8.4, 7.5 Hz, 1H),
6.97 (t, J¼7.5 Hz, 2H), 6.48 (dd, J¼9.0, 7.5 Hz, 1H), 6.38 (d, J¼9.9 Hz,
1H), 5.67 (br s, 1H), 3.96 (s, 3H), 1.98 (s, 3H); 13C NMR (CDCl3,
d
7.57 (dd, J¼7.8, 1.5 Hz, 1H),
697 cmꢁ1; HRMS (EI) calcd for C18H14N2O [Mþ] 274.1106, found
274.1114.
4.3.2. 2-(4-Methoxyphenyl)-2-methyl-1H-perimidin-6(2H)-one
150 MHz)
115.2, 113.2, 111.4, 108.4, 73.1, 66.8, 55.6, 55.5, 55.3, 25.3; IR
3398, 1638, 1597, 1420, 1238, 1014 cmꢁ1; HRMS (EI) calcd for
19H16N2O2 [Mþ] 304.1212, found 304.1211.
d
155.7,155.5,144.0,135.9,133.6,133.5,128.6,128.0,121.3,
(6b). Purple solid; yield 82%; Rf¼0.5 (50% EtOAc/hexanes); mp
n
(KBr)
68e69 ꢀC; 1H NMR (CDCl3, 300 MHz)
J¼7.5 Hz, 1H), 7.30 (d, J¼7.5, 1.5 Hz, 1H), 7.18 (d, J¼9.9 Hz, 1H), 6.89
(d, J¼9.0 Hz, 2H), 6.78 (dd, J¼7.5, 1.5 Hz, 1H), 6.69 (d, J¼9.9 Hz, 1H),
4.39 (br s, 1H), 3.79 (s, 3H), 1.94 (s, 3H); 13C NMR (CDCl3, 75 MHz)
d
7.50 (d, J¼9.0 Hz, 2H), 7.35 (t,
C
4.3.9. 2-Methyl-2-(4-nitrophenyl)-1H-perimidin-4(2H)-one
d
186.2, 159.2, 153.6, 143.4, 141.4, 139.8, 134.5, 133.7, 129.4, 127.0,
(7d). Purple solid; yield 7%; Rf¼0.1 (40% EtOAc/hexanes); mp
118.5, 114.8, 114.0, 110.7, 74.7, 55.4, 31.5; IR
n
(KBr) 3408, 2927, 1592,
198e199 ꢀC; 1H NMR (CDCl3, 300 MHz)
d
8.17 (d, J¼9.0 Hz, 2H),
1297, 1231, 1025 cmꢁ1; HRMS (EI) calcd for C19H16N2O2 [Mþ]
304.1212, found 304.1211.
7.78 (d, J¼9.0 Hz, 2H), 7.28 (d, J¼9.9 Hz, 1H), 7.25 (t, J¼7.8 Hz, 1H),
6.71 (d, J¼7.8 Hz, 1H), 6.64 (d, J¼7.8 Hz, 1H), 6.34 (d, J¼9.9 Hz, 1H),
4.51 (br s, 1H), 2.02 (s, 3H); 13C NMR (CD3OD, 150 MHz)
d 157.8,
4.3.3. 2-(2-Methoxyphenyl)-2-methyl-1H-perimidin-6(2H)-one
152.8, 147.1, 143.0, 136.5, 134.1, 127.3, 123.3, 116.1, 113.2, 108.0, 72.6,
(6c). Purple solid; yield 83%; Rf¼0.5 (30% EtOAc/hexanes); mp
66.0, 55.2, 54.2, 24.5; IR n (KBr) 3368, 1628, 1510, 1345, 1064,
131e132 ꢀC; 1H NMR (CDCl3, 300 MHz)
d
7.57 (dd, J¼8.1, 1.8 Hz, 1H),
754 cmꢁ1; HRMS (EI) calcd for C18H13N3O3 [Mþ] 319.0957, found
319.0953.
7.29 (d, J¼10.2 Hz, 1H), 7.28e7.22 (m, 3H), 6.99e6.94 (m, 2H), 6.73
(d, J¼10.2 Hz,1H), 6.67 (dd, J¼6.6, 2.7 Hz,1H), 5.81 (br s,1H), 3.95 (s,
3H), 1.92 (s, 3H); 13C NMR (CDCl3, 75 MHz)
d
185.7, 155.5, 153.9,
4.3.10. 2,2-Dimethyl-1H-perimidin-4(2H)-one (7e). Purple solid;
yield 12%; Rf¼0.3 (40% EtOAc/hexanes); mp 113e114 ꢀC (lit.14 oil);
143.1, 140.9, 134.6, 133.7, 133.0, 129.1, 128.7, 127.3, 120.9, 117.8, 114.4,

8224
C.-K. Wu et al. / Tetrahedron 70 (2014) 8219e8225
1H NMR (CDCl3, 300 MHz)
d
7.28 (d, J¼10.2 Hz, 1H), 7.16 (t, J¼7.8 Hz,
NMR(CDCl3, 300 MHz)
d
8.70 (d, J¼9.3 Hz, 2H), 8.07 (d, J¼8.4 Hz,
1H), 6.56 (d, J¼7.5 Hz, 1H), 6.50 (d, J¼8.4 Hz, 1H), 6.34 (d, J¼10.2 Hz,
1H), 7.82 (dd, J¼8.4, 6.9 Hz, 1H), 7.67 (d, J¼9.9 Hz, 1H), 7.64
(d, J¼6.9 Hz, 1H), 6.81 (d, J¼9.3, 2H), 6.75 (d, J¼9.9 Hz, 1H), 3.91
(s, 3H).
1H), 3.91 (br s, 1H), 1.64 (s, 6H).
4.3.11. 2-Phenyl-6H-perimidin-6-one (8a). Yellow solid; yield 79%;
Rf¼0.5 (40% EtOAc/hexanes); mp 190e191 ꢀC; 1H NMR (CDCl3,
4.3.18. 2-(2-Methoxyphenyl)-4H-perimidin-4-one (9c). Green solid;
yield<3%; Rf¼0.3 (40% EtOAc/hexanes); mp 147e148 ꢀC; 1H
300 MHz)
d
8.68e8.64 (m, 2H), 8.40 (d, J¼7.2 Hz, 1H), 8.39 (d,
J¼8.4 Hz, 1H), 8.05 (dd, J¼8.4, 7.2 Hz, 1H), 7.87 (d, J¼10.2 Hz, 1H),
NMR(CDCl3, 300 MHz)
d
8.21 (d, J¼8.7 Hz, 1H), 7.93 (dd, J¼9.0,
7.59e7.56 (m, 3H), 7.05 (d, J¼10.2 Hz,1H); 13C NMR (CDCl3, 75 MHz)
6.9 Hz,1H), 7.82 (dd, J¼7.5,1.8 Hz,1H), 7.72 (d, J¼6.9 Hz,1H), 7.71 (d,
J¼9.9 Hz, 1H), 7.46 (td, J¼8.7, 7.8 Hz, 1H), 7.11 (t, J¼7.5 Hz, 1H), 7.06
(d, J¼8.7 Hz, 1H), 6.77 (d, J¼9.9 Hz, 1H), 3.87 (s, 3H).
d
184.0, 162.7, 157.5, 149.5, 141.5, 137.5, 137.4, 135.6, 133.9, 131.1,
129.1, 128.74, 128.67, 128.5, 118.3; IR
n (KBr) 1661, 1619, 1541, 1353,
1116, 693 cmꢁ1; HRMS (EI) calcd for C17H10N2O [Mþ] 258.0793,
found 258.0795.
4.3.19. 2-(4-(Dimethylamino)phenyl)-4H-perimidin-4-one
(9d). Purple solid; yield<3%; Rf¼0.2 (50% EtOAc/hexanes); mp
4.3.12. 2-(4-Methoxyphenyl)-6H-perimidin-6-one
(8b). Yellow
208e209 ꢀC; 1H NMR (CDCl3, 300 MHz)
d
8.61 (d, J¼9.3 Hz, 2H), 8.13
solid; yield 76%; Rf¼0.3 (50% EtOAc/hexanes); mp 225e226 ꢀC; 1H
(d, J¼8.1 Hz, 1H), 7.88 (dd, J¼8.1, 6.9 Hz, 1H), 7.63 (d, J¼9.9 Hz, 1H),
7.56 (d, J¼7.2 Hz, 1H), 7.04 (d, J¼9.3 Hz, 2H), 6.73 (d, J¼9.9 Hz, 1H),
3.09 (s, 6H).
NMR (CDCl3, 300 MHz)
d
8.62 (d, J¼9.0 Hz, 2H), 8.35 (dd, J¼7.2,
0.9 Hz, 1H), 8.33 (dd, J¼8.4, 0.9 Hz, 1H), 8.01 (dd, J¼8.4, 7.2 Hz, 1H),
7.83 (d, J¼10.2 Hz, 1H), 7.07 (d, J¼9.0 Hz, 2H), 7.03 (d, J¼10.2 Hz,
1H), 3.92 (s, 3H); 13C NMR (CDCl3, 75 MHz)
d
184.1, 162.4, 162.2,
4.3.20. 2-Isopropyl-4H-perimidin-4-one
(9e). Purple
solid;
157.3, 149.6, 141.5, 137.2, 135.4, 133.8, 130.4, 130.1, 128.5, 128.4,
yield<3%; Rf¼0.3 (40% EtOAc/hexanes); mp 168e169 ꢀC; 1H NMR
118.0, 114.1, 55.4; IR
n
(KBr) 3072, 2939, 1538, 1354, 1236,
(CDCl3, 300 MHz)
1H), 7.669 (d, J¼9.9 Hz, 1H), 7.667 (d, J¼8.4 Hz, 1H), 6.74 (d,
d
8.11 (d, J¼8.4 Hz, 1H), 7.89 (dd, J¼8.4, 7.2 Hz,
841 cmꢁ1; HRMS (EI) calcd for C18H12N2O2 [Mþ] 288.0899, found
288.0891.
J¼9.9 Hz, 1H), 3.65 (septet, J¼6.9 Hz, 1H), 1.50 (d, J¼6.9 Hz, 6H).
4.3.13. 2-(2-Methoxyphenyl)-6H-perimidin-6-one (8c). Green solid;
yield 81%; Rf¼0.3 (30% EtOAc/hexanes); mp 193e194 ꢀC; 1H NMR
4.3.21. 1,2-Dimethyl-2-phenyl-1H-perimidin-6(2H)-one (14). Purple
solid; yield 82%; Rf¼0.2 (15% EtOAc/hexanes); mp 145e146 ꢀC; 1H
(CDCl3, 300 MHz)
d
8.46e8.41 (m, 2H), 8.07 (dd, J¼8.4, 7.2 Hz, 1H),
NMR (CDCl3, 300 MHz) d 7.58e7.55 (m, 2H), 7.46e7.29 (m, 5H), 7.09
7.87 (d, J¼9.2 Hz, 1H), 7.79 (dd, J¼7.2, 1.8 Hz, 1H), 7.52e7.46 (m, 1H),
(d, J¼9.9 Hz, 1H), 6.76 (d, J¼8.4 Hz, 1H), 6.70 (d, J¼9.9 Hz, 1H), 2.62
7.15 (dd, J¼7.5, 0.9 Hz, 1H), 7.10 (d, J¼7.5 Hz, 1H), 7.05 (d, J¼9.2 Hz,
(s, 3H), 1.98 (s, 3H); 13C NMR (CDCl3, 75 MHz)
d
185.7, 151.8, 145.3,
1H), 3.89 (s, 3H); 13C NMR (CDCl3, 75 MHz)
d
183.9, 164.1, 157.6,
144.3, 141.1, 134.2, 133.8, 129.2, 128.6, 128.0, 126.5, 114.4, 113.4,
110.6, 80.6, 32.9, 24.1; IR v (KBr) 2925, 1738, 1651, 1583, 1306,
814 cmꢁ1; HRMS (EI) m/z calcd for C19H16N2O [Mþ] 288.1263, found
288.1270.
157.2,149.2,141.4,137.4,135.5,133.9,131.6,131.2,129.4,128.4,128.3,
120.9, 117.9, 112.1, 56.0; IR
n (KBr) 2919, 2850, 1649, 1468, 1247,
1019 cmꢁ1; HRMS (EI) calcd for C18H12N2O2 [Mþ] 288.0899, found
288.0891.
4.4. General procedure for the preparation of compounds 10
and 11
4.3.14. 2-(4-(Dimethylamino)phenyl)-6H-perimidin-6-one
(8d). Purple solid; yield 75%; Rf¼0.3 (40% EtOAc/hexanes);
mp 231e232 ꢀC; 1H NMR (CDCl3, 300 MHz)
d
8.54 (d, J¼9.3 Hz,
To a solution of 1a (260 mg, 1.0 mmol) and triethylamine
(202 mg, 2.0 mmol) in CH2Cl2 (10 mL) was added dropwise methyl
iodine (312 mg, 2.2 mmol) in a course of 15 min. The resulting
mixture was further stirred at room temperature for 30 min. Upon
complete consumption of the starting material (monitored by TLC),
the solution was extracted with CH2Cl2/water. The combined or-
ganic layer was dried over MgSO4, concentrated in vacuo, and the
crude product was purified by column chromatography to give the
desired compound.
2H), 8.28 (d, J¼7.8 Hz, 2H), 7.96 (dd, J¼8.1, 7.8 Hz, 1H), 7.81
(d, J¼10.2 Hz, 1H), 7.00 (d, J¼10.2 Hz, 1H), 6.83 (d, J¼9.3 Hz, 2H),
3.10 (s, 6H); 13C NMR (CDCl3, 75 MHz)
d 184.4, 163.0, 157.1,
152.4, 149.8, 141.7, 137.0, 135.1, 133.5, 130.1, 128.5, 127.7,
125.0, 117.7, 111.7, 40.2; IR
n (KBr) 1607, 1536, 1392, 1362, 1176,
823 cmꢁ1; HRMS (EI) calcd for C19H15N3O [Mþ] 301.1215, found
301.1223.
4.3.15. 2-Isopropyl-6H-perimidin-6-one (8e). Green solid; yield
77%; Rf¼0.4 (40% EtOAc/hexanes); mp 181e182 ꢀC; 1H NMR (CDCl3,
4.4.1. 1,2-Dimethyl-2-phenyl-2,3-dihydro-1H-perimidine
300 MHz)
d
8.37 (d, J¼7.5 Hz, 1H), 8.30 (d, J¼8.4 Hz, 1H), 8.01 (dd,
(10). White solid; yield 75%; Rf¼0.4 (15% EtOAc/hexanes); mp
162e163 ꢀC; 1H NMR (CDCl3, 300 MHz)
d 7.71e7.67 (m, 2H),
J¼8.4, 7.5 Hz, 1H), 7.76 (d, J¼10.2 Hz, 1H), 7.00 (d, J¼10.2 Hz, 1H),
3.45 (septet, J¼6.9 Hz, 1H), 1.48 (d, J¼6.9 Hz, 6H); 13C NMR (CDCl3,
7.42e7.31 (m, 4H), 7.25e7.14 (m, 3H), 6.49 (d, J¼7.5 Hz, 1H), 6.38
(dd, J¼7.5, 1.2 Hz, 1H), 4.38 (br s, 1H), 2.61 (s, 3H), 1.71 (s, 3H); 13C
150 MHz)
d 184.1, 173.7, 157.3, 149.1, 141.4, 137.2, 135.1, 133.6, 128.8,
128.3, 118.0, 38.2, 21.8; IR
n (KBr) 2966, 1654, 1546, 1297, 1064,
NMR (CDCl3, 75 MHz)
d 144.4, 143.0, 139.7, 134.3, 128.44, 128.43,
848 cmꢁ1; HRMS (EI) calcd for C14H16N2 [Mþ] 224.0950, found
224.0953.
127.6, 127.3, 126.8, 117.0, 116.6, 113.0, 105.2, 103.7, 73.0, 33.5, 19.6; IR
n
(KBr) 3398, 2964, 1587, 1407, 810, 700 cmꢁ1; HRMS (EI) calcd for
19H18N2 [Mþ] 274.1470, found 274.1477.
C
4.3.16. 2-Phenyl-4H-perimidin-4-one (9a). Green solid; yield<3%;
Rf¼0.2 (50% EtOAc/hexanes); mp 173e174 ꢀC; 1H NMR(CDCl3,
4.4.2. 1,2,3-Trimethyl-2-phenyl-2,3-dihydro-1H-perimidine
300 MHz)
d
8.75e8.72 (m, 2H), 8.19 (d, J¼8.7 Hz, 1H), 7.22 (dd,
(11). White solid; yield 62%; Rf¼0.6 (15% EtOAc/hexanes); mp
119e120 ꢀC; 1H NMR (CDCl3, 300 MHz)
d 7.72e7.69 (m, 2H),
J¼8.7, 6.9 Hz, 1H), 7.693 (d, J¼9.9 Hz, 1H), 7.687 (d, J¼6.9 Hz, 1H),
7.57e7.54 (m, 3H), 6.77 (d, J¼9.9 Hz, 1H).
7.46e7.37 (m, 3H), 7.35 (t, J¼7.8 Hz, 2H), 7.20 (dd, J¼8.4, 0.9 Hz, 2H),
6.49 (d, J¼7.8 Hz, 2H), 2.51 (s, 6H), 1.53 (s, 3H); 13C NMR (CDCl3,
4.3.17. 2-(4-Methoxyphenyl)-4H-perimidin-4-one
(9b). Yellow
75 MHz)
d 144.4, 142.5, 133.9, 128.5, 128.31, 128.27, 127.0, 116.7,
solid; yield<3%; Rf¼0.2 (45% EtOAc/hexanes); mp 187e188 ꢀC; 1H
113.0, 103.5, 78.7, 34.4, 12.3; IR n (KBr) 3053, 2971, 1575, 1409, 808,

C.-K. Wu et al. / Tetrahedron 70 (2014) 8219e8225
8225
705 cm-1; HRMS (EI) calcd for C20H20N2 [Mþ] 288.1626, found
References and notes
288.1631.
1. For recent reviews on visible light photoredox catalysis, see: (a) Zeitler, K. An-
gew. Chem., Int. Ed. 2009, 48, 9785e9789; (b) Yoon, T. P.; Ischay, M. A.; Du, J. Nat.
Chem. 2010, 2, 527e532; (c) Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc.
Rev. 2011, 40, 102e113; (d) Tucker, J. W.; Stephenson, C. R. J. J. Org. Chem. 2012,
77, 1617e1622; (e) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev.
2013, 113, 5322e5363.
2. Condie, A. G.; Gonzalez-Gomez, J. C.; Stephenson, C. R. J. J. Am. Chem. Soc. 2010,
132, 1464e1465.
3. Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77e80.
4. Ischay, M. A.; Anzovino, M. E.; Du, J.; Yoon, T. P. J. Am. Chem. Soc. 2008, 130,
12886e12887.
5. (a) Hurtley, A. E.; Cismesia, M. A.; Ischay, M. A.; Yoon, T. P. Tetrahedron 2011, 67,
4442e4448; (b) Lin, S.; Ischay, M. A.; Fry, C. G.; Yoon, T. P. J. Am. Chem. Soc. 2011,
133, 19350e19353.
6. (a) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. J. Phys. Chem. 1990, 94, 722e726; (b)
Narayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. J. J. Am. Chem. Soc. 2009, 131,
8756e8757.
7. (a) Cano-Yelo, H.; Deronzier, A. J. Chem. Soc., Faraday Trans. 1984, 80, 3011e3019;
(b) Su, Y.; Zhang, L.; Jiao, N. Org. Lett. 2011, 13, 2168e2171.
8. Cano-Yelo, H.; Deronzier, A. Tetrahedron Lett. 1984, 25, 5517e5520.
9. Liu, Q.; Li, Y. N.; Zhang, H. H.; Chen, B.; Tung, C. H.; Wu, L. Z. J. Org. Chem. 2011,
76, 1444e1447.
4.5. Preparation of compound 12
To a solution of naphthalene-1,8-diamine (3, 15.8 g, 0.1 mol) in
hot dilute HCl (300 mL, 1.0 M) was added sodium cyanate (6.5 g,
0.1 mol). The resulting mixture was heated at 80 ꢀC for 3 h. After
cooled down to room temperature, the white precipitate was fil-
tered and washed with water and hexanes to give a white solid;
yield 90%; Rf¼0.2 (80% EtOAc/hexanes); mp 304e305 ꢀC (lit.17
ꢀ
ꢀ
304e305 ꢀC); 1H NMR (DMSO-d6, 300 MHz)
d
7.73 (d, J¼8.4 Hz,
2H), 7.45 (t, J¼7.5 Hz, 2H), 7.36 (d, J¼7.5 Hz, 2H).
4.6. Preparation of compound 13
To a solution of the 1-aminonaphthalene (15, 286 mg, 2.0 mmol)
in CH2Cl2 (15 mL) was added triethylamine (222 mg, 2.2 mmol) at
0
ꢀC. After stirred for 10 min, acetyl chloride (173 mg, 2.2 mmol)
10. Tucker, J. W.; Narayanam, J. M. R.; Shah, P. S.; Stephenson, C. R. J. Chem. Com-
was added dropwise, and the resulting mixture was further stirred
at room temperature for 45 min. Upon complete consumption of 1-
aminonaphthalene (monitored by TLC), the reaction was quenched
with water and the product was extracted by CH2Cl2 (50 mLꢂ2).
The combined organic layer was dried over MgSO4 and concen-
trated in vacuo. The residue was then purified by column chro-
matography to give a white solid; yield 92%; Rf¼0.3 (15% EtOAc/
hexanes); mp 157e158 ꢀC (lit.20c 159 ꢀC); 1H NMR (CDCl3, 300 MHz)
mun. 2011, 5040e5042.
11. Zen, J. M.; Liou, S. L.; Kumar, A. S.; Hsia, M. S. Angew. Chem., Int. Ed. 2003, 42,
577e579.
12. Hirashima, S.; Itoh, A. Green Chem. 2007, 9, 318e320.
13. Hirashima, S.; Hattori, K.; Tada, N.; Miura, T.; Itoh, A. Synthesis 2013, 45,
2684e2688.
14. Herbert, J. M.; Woodgate, P. D.; Denny, W. A. Heterocycles 1987, 26, 1043e1050.
15. Davis, R.; Tamaoki, N. Org. Lett. 2005, 7, 1461e1464.
16. Prakash, G. K. S.; Mathew, T.; Panja, C.; Vaghoo, H.; Venkataraman, K.; Olah, G.
A. Org. Lett. 2007, 9, 179e182.
17. Crystallographic data (excluding structure factors) for 6d, 7a, and 8d have been
deposited with the Cambridge Crystallographic Data Centre as supplementary
publication number CCDC-928661, -993697, and -928660, respectively. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif,
by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: þ44
1223 336033.
18. Herbert, J. M.; Woodgate, P. D.; Denny, W. A. J. Med. Chem. 1987, 30, 2081e2086.
19. Zhu, S.; Das, A.; Bui, L.; Zhou, H.; Curran, D. P.; Rueping, M. J. Am. Chem. Soc.
2013, 135, 1823e1829.
d
7.87e7.85 (m, 3H), 7.70 (d, J¼8.1 Hz, 1H), 7.56 (br s, 1H), 7.52e7.44
(m, 3H), 2.31 (s, 3H).
Acknowledgements
The authors thank the Ministry of Science and Technology of the
Republic of China, Taiwan, for financially supporting this research
under Contract No. NSC 102-2113-M-029-004-MY2.
20. (a) Zhang, J. M.; Zhang, S.; Zhang, S. L. Synth. Commun. 2007, 37, 2615e2624; (b)
Pihlaja, K.; Zelenin, K. N.; Ovcharenko, V. V.; Sinkkonen, J.; Maloshitskaya, O.
Tetrahedron 2004, 60, 6913e6921; (c) Leete, E.; Gregory, H.; Gros, E. G. J. Am.
Chem. Soc. 1965, 87, 3475e3479.
Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2014.09.036.