NIS-mediated intramolecular oxidative α-functionalization of tertiary amines: Transition metal-free synthesis of 1,2-dihydro-(4H)-3,1-benzoxazin-4-one derivatives

Article abstract of DOI:10.1039/c5ra03882k

A novel method for direct α-functionalization of tertiary amines via NIS-mediated oxidative C-O bond formation, where NIS serves as both an oxidant and an iodination reagent, has been developed. The method provides easy access to either iodinated or non-iodinated 1,2-dihydro-(4H)-3,1-benzoxazin-4-ones, depending on the nature of the reactant, via a regioselective transition metal-free approach. This journal is

Full text of DOI:10.1039/c5ra03882k

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Graphical Abstract
NIS-Mediated Intramolecular Oxidative α-Functionalization of Tertiary Amines:
Transition Metal-free Synthesis of 1,2-Dihydro-(4H)-3,1-benzoxazin-4-one
Derivatives
Abstract: A novel method for direct α-functionalization of tertiary amines via NIS-mediated
oxidative C–O bond formation, where NIS serves both as an oxidant and an iodination
reagent, has been developed. The method provides an easy access to either iodinated or
non-iodinated 1,2-dihydro-(4H)-3,1-benzoxazin-4-ones, depending on the nature of the
reactant, via a regioselective transition metal-free approach.

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ARTICLE TYPE
NISꢀMediated Intramolecular Oxidative αꢀFunctionalization of Tertiary
Amines: Transition Metalꢀfree Synthesis of 1,2ꢀDihydroꢀ(4H)ꢀ3,1ꢀ
benzoxazinꢀ4ꢀone Derivatives
Le Liu, Liang Du, Daisy ZhangꢀNegrerie and Yunfei Du*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A novel method for direct αꢀfunctionalization of tertiary
amines via NISꢀmediated oxidative C–O bond formation,
where NIS serves both as an oxidant and an iodination
10 reagent, has been developed. The method provides an easy
access to either iodinated or nonꢀiodinated 1,2ꢀdihydroꢀ(4H)ꢀ
3,1ꢀbenzoxazinꢀ4ꢀones, depending on the nature of the
reactant, via a regioselective transition metalꢀfree approach.
Introduction
15 Crossꢀdehydrogenative coupling (CDC) reactions, which spares
prefunctinalization of the substrates, boast high synthetic
efficiency and atom economy; and have, hence, attracted much
attention in recent decades.1 Among the CDC reactions, selective
functionalization of sp3 C–H bonds adjacent to a nitrogen atom in
20 simple amines is of fundamental significance in organic
chemistry for the synthesis of nitrogenꢀcontaining compounds.2
Classical strategies to realize such transformations have been
achieved by utilizing transitionꢀmetals such as Ru,3 Fe,4 Cu,5 V,6
etc., with coꢀoxidant to afford an iminium cationic species, which
25 subsequently reacts with various nucleophiles to form a C–C or
C–heteroatom bond for the functionalization of the C–H bond
adjacent to the nitrogen. As one of the most important branches
of CDC reactions, direct C–O bond formation accomplished via
selective sp3 C–H bond functionalization of tertiary amines has
30 been intensively studied for many years.7 In 2006, a novel Pdꢀ
catalyzed highly selective acetoxylation of Bocꢀprotected Nꢀ
methylamines using IOAc as the oxidant and involving a Bocꢀ
directed C–H activation process, was developed by Yu and coꢀ
workers8 (Scheme 1, path a). In 2013, Maycock7a group reported
Scheme 1 Direct C–O bond formation via activation of the C–H bond
adjacent to nitrogen in a tertiary amine
50 there is still much to be improved notably in the reaction
selectivity, the expansion of substrate scope as well as the variety
of the products.
Benzoxazinꢀ4ꢀone derivatives10 are an important class of
heterocyclic compounds, which exhibit a wide range of biological
55 activities including antiꢀinflammatory effects,11 inhibitor of
human leukocyte elastase,12 HSVꢀ1 protase,13 and have been used
as potent inhibitors of chymotrypsin.14 Iodinated 1H–
benzoxazin–4–(H)–one derivatives can not only be readily
functionalized by various coupling reactions for further
60 application, but also be reported as inhibitors of MEK for the
treatment of a variety of proliferative disease.15 The importance
of these benzoxazinones also resides in their occurrence in
natural products and the application as precursors for the
preparation of other heterocyclic compounds.16 Traditionally, the
65 general method for the synthesis of benzoxazinꢀ4ꢀone derivatives
involves the condensation between an aromatic aldehyde and an
anthranilic acid in the presence of an anhydride.17 Kunai reported
a straightforward access to benzoxazinones by CO2 incorporation
with a zwitterion arising from nucleophilic addition of imines to
70 arynes.18 However, efficient approaches to construct these
molecules are still limited in number. On the other hand, Nꢀ
halosuccinimides, aside from serving as halogenation reagents,
have also been widely adopted as an oxidant to realize oxidative
cyclization reactions in recent years.19 We herein disclose a novel
75 Nꢀiodosuccinimide (NIS)ꢀmediated protocol to access 1,2ꢀ
35
a
copperꢀcatalyzed
regioselective
intramolecular
αꢀ
functionalization of tertiary amines to synthesize dihydroꢀ1,3ꢀ
oxazines via C–O bond formation. Similar works have also been
achieved in 2014 by Jana7b and Maiti5e employing stoichiometric
.
Ag2O or catalytic amount of CuCl2 H2O under air, respectively
40 (Scheme 1, path b). Lately, we reported a hypervalent iodineꢀ
mediated synthesis of 1,2ꢀdihydroꢀ(4H)ꢀ3,1ꢀbenzoxazinꢀ4ꢀone
derivatives and 1,2ꢀdihydroꢀ(4H)ꢀ3,1ꢀbenzoxazine derivatives
through intramolecular functionalization of sp3 C–H bond of
diaryl amines,7c an attractive metalꢀfree protocol to functionalize
45 sp3 C–H bond adjacent to the nitrogen (Scheme 1, path c).
Despite the significant progress made in this area in recent years,9
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Table 1 Optimization of reaction conditionsa
Table 2 Scope of the oxidative annulation and iodination reactiona
Oxidant
(equiv)
Additive
(equiv)
Yieldb(%) Yieldb(%)
Entry
1c NIS (2.0)
Solvent
CH3CN
CH3CN
DCE
EA
THF
of 2a′
of 2a
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
15%
67
2
3
4
5
6
7
8
9
NIS (3.0)
NIS (3.0)
NIS (3.0)
NIS (3.0)
NIS (3.0)
NIS (3.0)
15%
20%
17%
23%
18%
73
68
70
65
DMF
CH3CN
45
NaHCO3 (2.0) <5
Na2CO3 (2.0) ꢀꢀ
NaOAc (2.0) <5
TEA (2.0)
ꢀꢀ
85
93
79
NIS (3.0) CH3CN
NIS (3.0)
CH3CN
CH3CN
CH3CN
CH3CN
10 NIS (3.0)
11 I2 (2.0)
12 I2 (2.0)
trace
ND
trace
ND
10
Na2CO3 (2.0) trace
a All reactions were carried out with 1a (0.4 mmol) and oxidant in the
specific solvent (c = 0.05 M). b Isolated yield. c 15% of the starting
material was recovered. ND = No Desired product.
5
dihydroꢀ(4H)ꢀ3,1ꢀbenzoxazinꢀ4ꢀone
derivatives
via
αꢀ
functionalization of sp3 C–H bond adjacent to the nitrogen of
tertiary amines. To our knowledge, this is an unusual reported
instance to form benzoxazinones or regioseletively iodinated
10 benzoxazinones where NIS serves both as an oxidant and an
iodination reagent (Scheme 1).
Results and Discussion
Initially, we postulated that treating NꢀmethylꢀNꢀ
phenylanthranilic acid 1a with NIS would probably generate an
15 iminum intermediate which could be attacked by the neighboring
carboxylic acid group to afford the aminal 2a′. However, only
15% yield of 2a′ was formed as the minor product, along with the
major product of iodinated 1Hꢀbenzoxazinꢀ4(H)ꢀone 2a formed
in 65% yield. This result suggested a regioselective iodoꢀ
20 aromatic substitution occurred in addition to the expected
oxidative annulation.
Following up with the initial result, we set out to explore the
reaction conditions in hopes to enhance the chemoselectivity of
the reaction, the main research interest of ours in identifying
25 metalꢀfree methods for the preparation of heterocycles.20 The first
study was on the effect of the oxidant amount. The results, with
the amount of oxidant varying between 1.0 and 3.0 equiv, showed
that compound 2a was consistently the major product while 2a′,
the minor product (not shown). The solvent effect study showed
30 that acetonitrile was the most effective in comparison to all the
other solvents including DCE, ethyl acetate, TFE and DMF
(Table 1, entries 1ꢀ6). None of the solvents, however, showed any
significant influence on the chemoselectivity of the reaction.
Further studies revealed that a basic additive could dramatically
35 improve the selectivity as well as the yield (Table 1, entries 7 –
10), as we were able to isolate compound 2a as the single product
with a yield of 93% when Na2CO3 was applied as an additive
(Table 1, entries 8). Additional bases such as K2CO3, Li2CO3 and
pyridine were also examined, but no further enhancement of the
40 yield was observed (not shown). Finally, molecular iodine was
tested as a possible oxidant, yet only 10% yield of 2a was
isolated (Table 1, entries 11ꢀ12). Other Nꢀhalosuccinimides, NBS
45 a All reactions were carried out with 1 (0.4 mmol) in the presence of NIS
(3.0 equiv) and Na2CO3 (2.0 equiv) in CH3CN (c = 0.05 M). b Isolated
yield. c NMR yield employing mesitylene as the internal standard.
and NCS, were also investigated. When NBS was used as the
oxidant, neither the corresponding annulated/brominated product
50 nor 2a′ was observed. In contrast, when NCS was employed, the
reaction proceeded smoothly to give the nonꢀhalogenated 2a′ in
moderate 47% yield (not shown).
To evaluate the efficiency and generality of the newly
established method, various substituted diaryl tertiary amines
55 were examined under the optimized conditions (Table 1, entry 8).
As shown in Table 2, the corresponding 1,2ꢀdihydroꢀ(4H)ꢀ3,1ꢀ
2
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Table 3 Oxidative annulation of tertiary aminesa
disappointment, for Nꢀmethyl amines bearing protecting groups
such as Ac or Ts, no desired reaction took place (not shown).
Attempts to expand the carboxylic acid functional group to a
larger range of nucleophilic moieties for the same transformation
turned out to be unsuccessful. As shown in Scheme 2, subjecting
3, where the carboxylic acid in 1a was replaced by an NHꢀOMe
amide, to the same reaction under optimized conditions, a high
30
35 yield of 1,4ꢀbenzodiazepine derivative 420 was afforded instead.
When the acid moiety was switched to an alcohol, no annulation
took place but only para iodination which yielded amine 6 as the
final product.
40
Scheme 2 Further investigation of scope of the reaction
To gain more insight into the reaction mechanism, some
control experiments were carried out. Treatment of the cyclized
compound 2a′ with NIS in the presence of Na2CO3 under the
optimized conditions led to a completely recovery of the starting
45 material, and this suggested that iodination occurs prior to
cyclization. The hypothesis was supported by the successful
conversion of substrates 1m and 1n to the corresponding
annulated product in 91% and 85% yields, respectively. Another
control experiment was to carry out the experiment in the
50 presence of 2 equiv of a radical scavenger TEMPO. The
unaffected high yield of the expected product excluded a radical
process during the transformation.
a All reactions were carried out with 1 (0.4 mmol) in the presence of NIS
(2.0 equiv) and Na2CO3 (2.0 equiv) in CH3CN (c = 0.05 M). b Isolated
yield. c The reaction mixture was heated to 60 oC.
benzoxazinꢀ4ꢀones bearing a wide range of substitutions could be
obtained in moderate to excellent yields. The substituent effect of
R1 proved to be minimal, with well tolerated both electronꢀ
withdrawing and ꢀdonating R1 groups to give satisfactory yields
(Table 2, entries 1ꢀ6). Larger alkyl groups of various sizes, such
as benzyl, ethyl and hexyl were used to replace the methyl group
5
O
Na
O
OH
Na
O
O
O
O
I
N
N
N
N
Ph
Na
H
I
Na2CO3
Na2CO3
10 as R3. While the yields all remained unwaveringly high (Table 2,
entries 7ꢀ9), the stability of the generated product decreased
dramatically in those containing a longꢀchained alkyl group, as
2h and 2i were found to decompose readily during purification.
Drastically different from the cases of R1 and R3, the substituent
15 effect of R2 was more pronounced. For substrates bearing one or
more electronꢀdonating groups on the B ring, diiodination
occurred and diiodinated/annulated products were formed in
moderate to good yields (Table 2, entries 10ꢀ12).
O
1a
A
B
O
O
Na
Na
H
I
O
O
O
O
O
O
I
N
N
N
N
Na2CO3
I
2a
C
D
E
I
I
Scheme 3 Proposed mechanism
55
On the basis of all the experiment results, we postulate a
mechanistic pathway. Illustrated in Scheme 3, deprotonation of
the acid resulted in the benzoate A, which underwent a typical
electrophilic substitution reaction of aromatic ring to form C.
Oxidation with the second equiv of NIS resulted in the formation
For substrates bearing an electronꢀwithdrawing R2 group, no
20 iodination occurred but oxidative annulation proceeded smoothly
(Table 3, entries 1ꢀ5). Electronꢀwithdrawing R2 groups such as
halogens, NO2 and CF3 were all tolerated and the nonꢀiodinated
products were all obtained in great yields (Table 3, entries 1ꢀ4),
including one case where there was an electronꢀdonating –CH3
25 group on the A ring (Table 3, entry 5). The dibenzylic substituted
tertiary amine 1r was also accessible and afforded the annulated
60 ammonium D which was subsequently transformed into the
corresponding iminum intermediate E. Finally, intramolecular
annulation led to the ultimate formation of product 2a. The
regioselectivity of preferred paraꢀsubstitution can be
straightforwardly explained by the structure of the cationic B,
65 which carries the electronic advantage over the metaꢀsubstituted
counterparts and the stereo advantage over the orthoꢀsubstituted
product 2r in
a relatively low yield of 43%. To our
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one. Finally, the electrophilic substitution mechanism for the 55 anhydrous Na2SO4, filtered, concentrated and purified by flash
iodination effectively explains the negative effect of an electronꢀ
withdrawing R2 group on the formation of the iodinated product,
as well as the observation of multiple substitutions with an
electronꢀdonating R2 group.
column chromatography on silica gel to give the product.
Known substrates 1a,22 le,7c 1f, 7c 1g,22 1h,21 1k,20 1mꢀq,7c 1r,7c
320 and 57c were prepared according to the literature procedures.
5
5ꢀMethylꢀ2ꢀ(methyl(phenyl)amino)benzoic
acid
(1b).
60 Yellowish solid (1.08 g), yield: 45% (over three steps), mp. 84 –
86 oC. 1H NMR (600 MHz, CDCl3) δ 14.10 (s, 1H), 8.18 (s, 1H),
7.38 (d, J = 7.7 Hz, 1H), 7.33 – 7.25 (m, 2H), 7.10 – 7.04 (m,
1H), 7.00 (d, J = 8.1 Hz, 1H), 6.91 (d, J = 7.5 Hz, 2H), 3.19 (s,
3H), 2.42 (s, 3H). 13C NMR (150 MHz, CDCl3) δ 166.4, 148.3,
65 148.2, 138.1, 135.8, 132.6, 129.3, 126.7, 125.6, 123.1, 118.6,
Conclusions
In conclusion, we have developed
intramolecular αꢀfunctionalization reaction of tertiary amines via
C–O bond formation to access synthetically and biologically
10 interesting iodinated or nonꢀiodinated 1,2ꢀdihydroꢀ(4H)ꢀ3,1ꢀ
benzoxazinꢀ4ꢀone derivatives. Besides its novelty as a CDC
reaction, the oneꢀpot procedure, mild reaction conditions, and
transition metalꢀfree feature render it a useful alternative to the
existing methods for the preparation of benzoxazinꢀ4ꢀone
15 derivatives.
a novel NISꢀmediated
+
42.0, 21.0. HRMS (ESI) calcd for C15H16NO2 [M + H+]
242.1176, found 242.1171.
5ꢀMethoxyꢀ2ꢀ(methyl(phenyl)amino)benzoic acid (1c).
Yellow solid (1.36 g), yield: 53% (over three steps), mp. 123 –
70 125 oC. 1H NMR (600 MHz, CDCl3) δ 14.55 (brs, 1H), 7.84 (d, J
= 2.9 Hz, 1H), 7.29 (t, J = 7.9 Hz, 2H), 7.11 (dd, J = 8.8, 3.0 Hz,
1H), 7.06 (t, J = 7.3 Hz, 1H), 7.02 (d, J = 8.8 Hz, 1H), 6.91 (d, J
= 8.1 Hz, 2H), 3.88 (s, 3H), 3.20 (s, 3H). 13C NMR (150 MHz,
CDCl3) δ 166.2, 158.6, 148.4, 143.4, 129.3, 128.0, 127.1, 123.1,
75 122.5, 118.5, 114.5, 55.9, 42.0. HRMS (ESI) calcd for
C15H16NO3+ [M + H+] 258.1125, found 258.1123.
Experimental Section
All reactions were carried out at room temperature under air
unless otherwise noted. H (600 MHz or 400 MHz) and 13C (150
1
o
MHz or 100 MHz) NMR spectra were recorded at 25 C. The
3,4,5ꢀTrimethoxyꢀ2ꢀ(methyl(phenyl)amino)benzoic
acid
20 chemical shifts (δ) are reported in ppm with reference to TMS
(0 ppm) and coupling constants (J) are given in Hz. High
resolution mass spectrometry (HRMS) was obtained on a QꢀTOF
micro spectroMeter. Melting points were determined with a
MicroMelting point apparatus without corrections. Organic
25 solutions were concentrated by rotary evaporation below 40 °C in
vacuum.
(1d). Brown solid (0.98 g), yield: 31% (over three steps), mp. 117
– 119 oC. 1H NMR (600 MHz, CDCl3) δ 14.58 (brs, 1H), 7.67 (s,
80 1H), 7.32 – 7.26 (m, 2H), 7.03 (t, J = 7.1 Hz, 1H), 6.90 (d, J = 8.4
Hz, 2H), 3.96 (s, 3H), 3.90 (s, 3H), 3.26 (s, 3H), 3.23 (s, 3H). 13
C
NMR (150 MHz, CDCl3) δ 166.1, 153.1, 150.9, 148.8, 147.7,
136.1, 129.1, 122.5, 121.8, 117.2, 108.7, 60.8, 60.1, 56.2, 39.5.
+
HRMS (ESI) calcd for C17H20NO5 [M + H+] 318.1336, found
General procedure for the synthesis of substrate 1
85 318.1335.
2ꢀ(Hexyl(phenyl)amino)benzoic acid (1i). Yellow oil (1.1 g),
yield: 37% (over three steps). 1H NMR (600 MHz, CDCl3) δ
14.82 (s, 1H), 8.38 (d, J = 7.8 Hz, 1H), 7.57 (t, J = 7.5 Hz, 1H),
7.44 (t, J = 7.5 Hz, 1H), 7.30 (t, J = 7.4 Hz, 2H), 7.13 (d, J = 7.9
90 Hz, 1H), 7.08 (t, J = 7.1 Hz, 1H), 6.97 (d, J = 7.8 Hz, 2H), 3.52
(s, 2H), 1.65 – 1.52 (m, 2H), 1.36 – 1.21 (m, 6H), 0.91 – 0.79 (d,
3H). 13C NMR (150 MHz, CDCl3) δ 166.4, 149.2, 147.6, 134.7,
132.3, 129.4, 127.8, 127.5, 127.1, 123.7, 119.9, 54.6, 31.4, 26.9,
To a solution of 2ꢀiodobenzoic acid or 2ꢀbromobenzoic acid
derivative (10 mmol) in 1,4ꢀdioxane (c = 0.3 M) was added Cu2O
30 (0.5 equiv), Nꢀmethylmorpholine (1.5 equiv), and aniline (1.5
equiv) or its derivative. The resulting mixture was refluxed under
N2 until the completion of the reaction. The mixture was filtered,
and aqueous KOH solution (0.5 N, 2.5 equiv) was added to the
residue and filtered. The filtrate was extracted with DCM to
35 remove the impurities and the pH value of the water phase was
adjusted to 4 using conc. HCl. The solid generated was filtered
and dried. And the crude product was used directly without
further purification.
+
26.8, 22.5, 14.0. HRMS (ESI) calcd for C19H24NO2 [M + H+]
95 298.1802, found 298.1805.
2ꢀ(Methyl(oꢀtolyl)amino)benzoic acid (1j). Pale white solid
o
1
(1.04 g), yield: 43% (over three steps), mp. 108 – 110 C. H
NMR (600 MHz, CDCl3) δ 14.53 (brs, 1H), 8.30 (dd, J = 7.8, 1.3
Hz, 1H), 7.45 (td, J = 7.9, 1.5 Hz, 1H), 7.39 (d, J = 8.0 Hz, 1H),
100 7.36 – 7.30 (m, 2H), 7.18 – 7.13 (m, 2H), 6.89 (d, J = 8.0 Hz,
1H), 3.20 (s, 3H), 1.93 (s, 3H). 13C NMR (150 MHz, CDCl3) δ
166.6, 151.5, 146.4, 134.5, 132.7, 132.5, 132.0, 127.0, 126.4,
125.5, 124.7, 124.5, 120.4, 45.1, 19.0. HRMS (ESI) calcd for
C15H16NO2+ [M + H+] 242.1176, found 242.1172.
To a solution of the crude product in DMF (c = 0.3 M) was
40 slowly added NaH (2.5 equiv) at 0 oC. And the mixture was
stirred for 30 min, followed by the addition of methyl iodide (4.0
equiv) or the corresponding haloalkane. The reaction was allowed
to stir at room temperature overnight. The mixture was poured
into water and extracted with EA (3 × 80 mL). The organic phase
45 was dried over anhydrous Na2SO4, filtered, and concentrated. The
residue was purified by flash column chromatography on silica
gel to give the corresponding esters.
105
2ꢀ((3,5ꢀDimethoxyphenyl)(methyl)amino)benzoic acid (1l).
Yellow solid (1.32 g), yield: 46% (over three steps), mp. 88 – 90
To a solution of the ester in MeOH/H2O (5:1, c = 0.6 M) was
added KOH (2.0 equiv), and the resulting mixture was allowed to
50 heat to reflux. Upon completion monitored by TLC, the mixture
was concentrated and poured into water. The solution was
acidified with conc. HCl, and the solid was filtered to give the
desired product. If no precipitation, the reaction mixture was
extracted with EA (3 × 60 mL). The organic phase was dried over
1
oC. H NMR (600 MHz, CDCl3) δ 14.15 (brs, 1H), 8.35 (d, J =
6.8 Hz, 1H), 7.59 (t, J = 7.6 Hz, 1H), 7.45 (t, J = 7.2 Hz, 1H),
7.16 (d, J = 8.0 Hz, 1H), 6.18 (s, 1H), 6.04 (s, 2H), 3.72 (s, 6H),
110 3.18 (s, 3H). 13C NMR (150 MHz, CDCl3) δ 166.1, 161.4, 150.4,
150.2, 135.0, 132.5, 128.0, 127.1, 126.0, 100.0, 97.8, 94.4, 55.4,
+
42.1. HRMS (ESI) calcd for C16H18NO4 [M + H+] 288.1230,
4
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found 288.1231.
(d, J = 8.5 Hz, 2H), 7.23 (td, J = 8.4, 3.0 Hz, 1H), 7.04 (dd, J =
60 9.0, 4.3 Hz, 1H), 6.86 (d, J = 8.5 Hz, 2H), 5.53 (s, 2H). 13C NMR
(150 MHz, CDCl3) δ 162.6, 158.4 (d, JCꢀF = 243.7 Hz), 144.2,
142.2 (d, JCꢀF = 2.2 Hz) 138.9, 124.4, 122.8 (d, JCꢀF = 23.9 Hz),
General procedure for the synthesis of compounds 2, 4 and 6
To a solution of NꢀalkylꢀNꢀaryl anthranilic acids 1 (0.4 mmol)
in CH3CN (8 mL) was added NIS (1.2 mmol) followed by the
addition of Na2CO3 (0.8 mmol) at ambient temperature. The
reaction mixture was maintained at room temperature and
monitored by TLC. Upon completion, the mixture was poured
into sat. aqueous solution of Na2S2O3 (30 mL) and extracted by
ethyl acetate (3 × 50 mL). The combined organic phase was
10 washed with brine (1 × 80 mL). Dried over anhydrous Na2SO4,
evaporation of the solvent under reduced pressure and
purification of the crude residue by flash column chromatography
on silica gel (EA/PE with 5‰TEA) afforded the desired products.
For the synthesis of compounds 2mꢀr, 2.0 equiv of NIS was used
15 following the similar procedure mentioned above.
121.3 (d, JC-F = 7.5 Hz), 119.0 (d, JCꢀF = 7.7 Hz), 116.5 (d, JCꢀF
=
23.5 Hz), 88.9, 81.0. HRMS (ESI) calcd for C14H10F127INO2+ [M
65 +H+] 369.9735, found 369.9731.
5
1ꢀ(4ꢀIodophenyl)ꢀ2ꢀphenylꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀ
o
one (2g). White solid (162.3 mg), yield: 95%, mp. 141 – 143 C.
1H NMR (600 MHz, CDCl3) δ 7.92 (dd, J = 7.9, 1.0 Hz, 1H),
7.66 (d, J = 8.7 Hz, 2H), 7.49 – 7.46 (m, 2H), 7.46 – 7.41 (m,
70 1H), 7.34 – 7.28 (m, 3H), 7.09 (d, J = 8.3 Hz, 1H), 7.02 (t, J = 7.6
Hz, 1H), 6.93 (d, J = 8.7 Hz, 2H), 6.72 (s, 1H). 13C NMR (150
MHz, CDCl3) δ 162.6, 144.6, 144.4, 138.8, 137.1, 135.1, 130.7,
129.2, 128.8, 127.2, 124.9, 122.6, 119.2, 117.9, 90.9, 88.8.
HRMS (ESI) calcd for C20H15127INO2+ [M + H+] 428.0142, found
75 428.0145.
4ꢀ(4ꢀIodophenyl)isochromanꢀ1ꢀone (2a). White solid (131
mg), yield: 93%, mp. 129 – 131 oC. 1H NMR (600 MHz, CDCl3)
δ 8.09 (dd, J = 7.9, 1.2 Hz, 1H), 7.68 (d, J = 8.7 Hz, 2H), 7.50 –
7.44 (m, 1H), 7.17 – 7.11 (m, 1H), 7.02 (d, J = 8.3 Hz, 1H), 6.90
20 (d, J = 8.7 Hz, 2H), 5.52 (s, 2H). 13C NMR (150 MHz, CDCl3) δ
163.5, 146.0, 143.8, 138.8, 134.8, 131.1, 124.7, 123.1, 118.5,
1ꢀ(4ꢀIodophenyl)ꢀ2ꢀmethylꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀ
one (2h). 88% yield based on crude NMR within mesitylene as
1
internal standard. H NMR (600 MHz, CDCl3) δ 7.97 (d, J = 7.8
Hz, 1H), 7.64 (d, J = 8.4 Hz, 2H), 7.33 (t, J = 7.8 Hz, 1H), 6.98
80 (t, J = 7.5 Hz, 1H), 6.84 (d, J = 8.4 Hz, 2H), 6.64 (d, J = 8.3 Hz,
117.3, 88.9, 80.5. HRMS (ESI) calcd for C14H11127INO2 [M +
+
1H), 5.69 (dd, J = 12.0, 6.0 Hz, 1H), 1.49 (d, J = 6.0 Hz, 3H). 13
C
H+] 351.9829, found 351.9823.
NMR (150 MHz, CDCl3) δ 163.7, 146.6, 142.4, 139.0, 135.1,
130.7, 128.5, 121.9, 118.4, 116.1, 91.2, 87.4, 19.9. HRMS (ESI)
calcd for C15H13127INO2+ [M + H+] 365.9985, found 365.9981.
1ꢀ(4ꢀIodophenyl)ꢀ2ꢀpentylꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀ
one (2i). 87% yield based on crude NMR within mesitylene as
1ꢀ(4ꢀIodophenyl)ꢀ6ꢀmethylꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀ
25 one (2b). White solid (109.5 mg), yield: 75%, mp. 150 – 152 C.
o
1H NMR (600 MHz, CDCl3) δ 7.89 (s, 1H), 7.65 (d, J = 8.5 Hz,
2H), 7.30 (dd, J = 8.3, 1.8 Hz, 1H), 6.96 (d, J = 8.3 Hz, 1H), 6.86
(d, J = 8.7 Hz, 2H), 5.51 (s, 2H), 2.37 (s, 3H). 13C NMR (150
MHz, CDCl3) δ 163.8, 144.4, 143.5, 138.7, 136.0, 133.4, 130.8,
30 124.2, 119.2, 117.7, 88.3, 80.7, 20.7. HRMS (ESI) calcd for
C15H13127INO2+ [M + H+] 365.9985, found 365.9985.
85
1
internal standard. H NMR (600 MHz, CDCl3) δ 8.06 (d, J = 7.8
Hz, 1H), 7.68 (d, J = 8.3 Hz, 2H), 7.44 (t, J = 7.8 Hz, 1H), 7.10
(t, J = 7.5 Hz, 1H), 6.89 (d, J = 8.3 Hz, 2H), 6.86 (d, J = 8.2 Hz,
90 1H), 5.60 (t, J = 6.7 Hz, 1H), 2.0 – 1.9 (m, 1H), 1.86 – 1.79 (m,
1H), 1.60 – 1.54 (m, 1H), 1.51 – 1.45 (m, 1H), 1.32 – 1.26 (m,
3H), 0.83 – 0.89 (m, 5H). 13C NMR (150 MHz, CDCl3) δ 163.2,
145.3, 144.3, 138.8, 135.0, 130.5, 126.7, 122.6, 119.8, 117.4,
91.7, 89.8, 33.3, 31.2, 24.6, 22.5, 13.9. HRMS (ESI) calcd for
95 C19H21127INO2+ [M + H+] 422.0611, found 422.0610.
1ꢀ(4ꢀIodophenyl)ꢀ6ꢀmethoxyꢀ1Hꢀbenzo[d][1,3]oxazinꢀ
4(2H)ꢀone (2c). White solid (137.1 mg), yield: 90%, mp. 141 –
o
1
143 C. H NMR (600 MHz, CDCl3) δ 7.64 (d, J = 8.4 Hz, 2H),
35 7.54 (d, J = 2.9 Hz, 1H), 7.11 (dd, J = 8.9, 2.9 Hz, 1H), 7.02 (d, J
= 8.9 Hz, 1H), 6.83 (d, J = 8.6 Hz, 2H), 5.54 (s, 2H), 3.86 (s, 3H).
13C NMR (150 MHz, CDCl3) δ 163.7, 156.1, 145.0, 139.2, 138.6,
124.0, 123.8, 121.9, 119.3, 111.8, 87.9, 81.2, 55.9. HRMS (ESI)
calcd for C15H13127INO3+ [M + H+] 381.9935, found 381.9937.
1ꢀ(2,4ꢀDiiodoꢀ6ꢀmethylphenyl)ꢀ1Hꢀbenzo[d][1,3]oxazinꢀ
4(2H)ꢀone (2j). White solid (123.7 mg), yield: 63%, mp. 147 –
o
1
149 C. H NMR (600 MHz, CDCl3) δ 8.36 (d, J = 1.9 Hz, 1H),
7.71 (s, 1H), 7.63 (dd, J = 8.7, 2.0 Hz, 1H), 7.59 (d, J = 8.2 Hz,
100 1H), 6.81 (d, J = 8.2 Hz, 1H), 6.25 (d, J = 8.6 Hz, 1H), 5.37 (s,
2H), 2.19 (s, 3H). 13C NMR (150 MHz, CDCl3) δ 162.4, 147.1,
143.6, 140.8, 140.2, 139.6, 138.2, 136.9, 128.9, 117.8, 116.3,
93.3, 82.9, 79.8, 17.9. HRMS (ESI) calcd for C15H12127I2NO2+ [M
+ H+] 491.8952, found 491.8956.
40
1ꢀ(4ꢀIodophenyl)ꢀ6,7,8ꢀtrimethoxyꢀ1Hꢀbenzo[d][1,3]oxazinꢀ
4(2H)ꢀone (2d). Brown solid (135.8 mg), yield: 77%, mp. 57 –
o
1
59 C. H NMR (600 MHz, CDCl3) δ 7.60 (d, J = 8.7 Hz, 2H),
7.38 (s, 1H), 6.75 (d, J = 8.7 Hz, 2H), 5.54 (s, 2H), 3.94 (s, 3H),
3.93 (s, 3H), 3.63 (s, 3H). 13C NMR (150 MHz, CDCl3) δ 163.6,
45 151.5, 148.3, 145.8, 145.5, 138.2, 133.2, 123.5, 115.7, 107.0,
88.0, 82.9, 61.2, 60.5, 56.3. HRMS (ESI) calcd for
C17H17127INO5+ [M + H+] 442.0146, found 442.0140.
7ꢀChloroꢀ1ꢀ(4ꢀiodophenyl)ꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀ
one (2e). White solid (124.7 mg), yield: 81%, mp. 152 – 154 C.
105
1ꢀ(2,4ꢀDiiodoꢀ5ꢀmethoxyphenyl)ꢀ1Hꢀbenzo[d][1,3]oxazinꢀ
4(2H)ꢀone (2k). White solid (137.9 mg), yield: 68%, mp. 166 –
168 C. H NMR (600 MHz, CDCl3) δ 8.29 (s, 1H), 8.09 (d, J =
7.8 Hz, 1H), 7.48 – 7.40 (m, 1H), 7.12 (t, J = 7.6 Hz, 1H), 6.65
(d, J = 8.2 Hz, 1H), 6.59 (s, 1H), 5.55 (s, 1H), 5.38 (s, 1H), 3.76
o
1
o
50 1H NMR (600 MHz, CDCl3) δ 8.03 – 7.98 (m, 1H), 7.74 (d, J =
7.6 Hz, 2H), 7.08 (d, J = 8.5 Hz, 1H), 6.96 (s, 1H), 6.92 (d, J =
8.5 Hz, 2H), 5.50 (s, 2H). 13C NMR (150 MHz, CDCl3) δ 162.8,
147.1, 142.7, 141.5, 139.1, 132.5, 125.3, 123.3, 117.5, 114.8,
110 (s, 3H). 13C NMR (150 MHz, CDCl3) δ 163.9, 159.8, 148.5,
147.2, 146.8, 135.2, 131.1, 122.4, 117.7, 116.0, 111.2, 87.5, 86.2,
+
80.3, 56.8. HRMS (ESI) calcd for C15H12127I2NO3 [M + H+]
90.1, 80.3. HRMS (ESI) calcd for C14H1035Cl127INO2 [M + H+]
+
507.8901, found 507.8906.
55 385.9439, found 385.9435.
1ꢀ(2,4ꢀDiiodoꢀ3,5ꢀdimethoxyphenyl)ꢀ1Hꢀbenzo[d][1,3]oxaꢀ
115 zinꢀ4(2H)ꢀone (2l). Brown oil (88 mg), yield: 41%. 1H NMR
(600 MHz, CDCl3) δ 8.10 (d, J = 7.8 Hz, 1H), 7.45 (t, J = 7.3 Hz,
6ꢀFluoroꢀ1ꢀ(4ꢀiodophenyl)ꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀ
one (2f). Pale solid (112.1 mg), yield: 76%, mp. 126 – 128 oC. 1H
NMR (600 MHz, CDCl3) δ 7.76 (dd, J = 7.9, 2.3 Hz, 1H), 7.68
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1H), 7.12 (t, J = 7.5 Hz, 1H), 6.66 (d, J = 8.2 Hz, 1H), 6.45 (s,
1H), 5.59 (s, 1H), 5.39 (d, J = 8.2 Hz, 1H), 3.91 (s, 3H), 3.77 (s, 60 132.8, 132.1, 126.4, 125.6, 124.4, 123.1, 121.1, 118.5, 116.7,
3H). 13C NMR (150 MHz, CDCl3) δ 165.0, 153.0, 145.8, 133.9,
3H). 13C NMR (150 MHz, CDCl3) δ 164.0, 161.5, 160.7, 146.9,
135.2, 131.1, 122.4, 117.9, 116.0, 107.2, 85.2, 82.7, 80.5, 60.8,
57.1. HRMS (ESI) calcd for C16H14127I2NO4+ [M + H+] 537.9007,
found 537.9009.
62.3, 37.7.
(2ꢀ((4ꢀIodophenyl)(methyl)amino)phenyl)methanol
Yellow oil (96.3 mg), yield: 71%. H NMR (600 MHz, CDCl3) δ
(6).
1
5
7.53 (d, J = 7.5 Hz, 1H), 7.39 (d, J = 9.0 Hz, 2H), 7.35 (td, J =
1ꢀ(4ꢀFluorophenyl)ꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀone (2m). 65 7.5, 1.7 Hz, 1H), 7.31 (td, J = 7.4, 1.3 Hz, 1H), 7.12 – 7.09 (m,
7c
o
1
Yellow solid (88.5 mg), yield: 91%, mp. 76 – 78 C. H NMR
(600 MHz, CDCl3) δ 8.08 (d, J = 7.9 Hz, 1H), 7.46 (t, J = 7.1 Hz,
10 1H), 7.17 – 7.03 (m, 5H), 6.93 (d, J = 8.3 Hz, 1H), 5.51 (s, 2H).
13C NMR (150 MHz, CDCl3) δ 163.9, 160.3 (d, JC-F = 244.4 Hz),
147.0, 139.9, 135.0, 131.1, 125.5 (d, JC-F = 8.4 Hz), 122.5, 117.9,
116.7 (d, JC-F = 22.6 Hz), 116.5, 80.9.
1H), 6.32 (t, J = 6.0 Hz, 2H), 4.54 (s, 2H), 3.18 (s, 3H), 2.03 (s,
1H). 13C NMR (150 MHz, CDCl3) δ 149.1, 145.6, 139.0, 137.7,
129.5, 129.1, 128.1, 127.3, 115.7, 79.0, 61.8, 39.9. HRMS (ESI)
calcd for C14H15127INO+ [M + H+] 340.0193, found 340.0195.
70 Acknowledgements
1ꢀ(4ꢀBromophenyl)ꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀone (2n).
15 7c Yellow solid (103 mg), yield: 85%, mp. 110 – 112 oC. 1H NMR
(600 MHz, CDCl3) δ 8.09 (d, J = 7.7 Hz, 1H), 7.54 – 7.44 (m,
3H), 7.15 (t, J = 7.5 Hz, 1H), 7.03 (d, J = 8.0 Hz, 3H), 5.54 (s,
2H). 13C NMR (150 MHz, CDCl3) δ 163.7, 146.1, 143.0, 135.0,
132.9, 131.1, 124.5, 123.1, 118.5, 118.3, 117.2, 80.6.
We acknowledge the National Science Foundation of China
(#21472136) and the National Basic Research Project
(2014CB932201) for financial support.
Notes and references
75 a School of Pharmaceutical Science and Technology, Tianjin University,
Tianjin 300072, China. E-mail: duyunfeier@tju.edu.cn
Fax: +86-22-27404031; Tel: +86-22-27404031
† Electronic Supplementary Information (ESI) available: NMR spectra
for all new compounds, See DOI:10.1039/b000000x/
80
20
1ꢀ(3ꢀ(Trifluoromethyl)phenyl)ꢀ1Hꢀbenzo[d][1,3]oxazinꢀ
4(2H)ꢀone (2o).7c Following the general procedure, the reaction
o
mixture was heated to 60 C. 2o was obtained as pale solid (99.6
o
1
mg), yield: 85%, mp. 68 – 69 C. H NMR (400 MHz, CDCl3) δ
8.14 (d, J = 7.1 Hz, 1H), 7.53 (t, J = 8.4 Hz, 2H), 7.47 (d, J = 7.6
25 Hz, 1H), 7.40 (s, 1H), 7.34 (d, J = 7.8 Hz, 1H), 7.22 (t, J = 7.6
Hz, 1H), 7.08 (d, J = 8.3 Hz, 1H), 5.62 (s, 2H). 13C NMR (100
MHz, CDCl3) δ 163.5, 145.7, 144.7, 135.0, 132.4 (q, JC-F = 32.4
Hz), 131.2, 130.4, 125.7, 123.6, 123.5 (q, JC-F = 270.9 Hz), 121.8
(q, JC-F = 3.7 Hz), 119.2 (q, JC-F = 3.7 Hz), 118.7, 117.7, 80.6.
1
For selected examples, see: (a) C. S. Yeung and V. M. Dong, Chem.
Rev., 2011, 111, 1215; (b) W. Yoo and C. Li, Top. Curr. Chem., 2010,
292, 281; (c) Z. Shi and F. Glorious, Angew. Chem., Int. Ed., 2012,
51, 9220; (d) C. Zhang, C. Tang and N. Jiao, Chem. Soc. Rev., 2012,
41, 3464; (e) R. D. Baxter, D. Sale, M. K. Engle and J. Yu, J. Am.
Chem. Soc., 2012, 134, 4600; (f) A. M. R. Smith and K. K. Hii, Chem.
Rev., 2011, 111, 1637; (g) C. Sun, B. Li and Z. Shi, Chem. Rev., 2011,
111, 1293; (h) L. Ackermann, Chem. Rev., 2011, 111, 1315. (i) A. J.
Nell, J. P. Hehn, P. F. Tripet and F. D. Toste, J. Am. Chem. Soc.,
2013, 133, 14044; (j) J. L. Jeffrey and R. Sarpong, Chem. Sci., 2013,
4, 4092.
For selected examples, see (a) S. Doye, Angew. Chem., Int. Ed., 2001,
40, 3351; (b) N. Chatani, T. Asaumi, T. Ikeda, S. Yorimitsu, Y. Ishii,
F. Kakiuchi and S. Murai, J. Am. Chem. Soc., 2000, 122, 12882; (c) Z.
Li, and C. Li, J. Am. Chem. Soc., 2004, 126, 11810; (d) Y. Takehiko,
A. Yoshimasa and N. Hiroto, J. Am. Chem. Soc., 2005, 127, 11610;
(e) J. Xie, H. Li, J. Zhou, Y. Cheng and C. Zhu, Angew. Chem., Int.
Ed., 2012, 51, 1252; (f) Y. Ma, G. Zhang, J. Zhang, D. Yang and R.
Wang, Org. Lett., 2014, 16, 5358; (g) H. Ueda, K. Yoshida and H.
Tokuyama, Org. Lett., 2014, 16, 4194.
For selected examples, see: (a) D. A. DiRocco and T. Rovis, J. Am.
Chem. Soc., 2012, 134, 8094; (b) M. Rueping, C. Vila, R. M.
Koenigs, K. Poscharny and D. C. Fabry, Chem. Commun., 2011, 47,
2360; (c) D. B. Freeman, L. Furst, A. G. Condie and C. R. J.
Stephenson, Org. Lett., 2012, 14, 94; (d) J. Xuan, Y. Cheng, J. An, L.
Lu, X. Zhang and W. Xiao, Chem. Commun., 2011, 47, 8337; (e) M.
Rueping, C. Vila, R. M. Koenigs, K. Poscharny and D. C. Fabry,
Chem. Commun., 2011, 47, 2360; (f) S. Kumar, P. Kumar and S. L.
Jain, RSC Adv., 2013, 3, 24013.
85
90
30
1ꢀ(3ꢀNitrophenyl)ꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀone (2p).
7c
Following the general procedure, the reaction mixture was
heated to 60 oC. 2p was obtained as a yellow solid (79 mg), yield:
73%, mp. 131 – 133 oC. 1H NMR (600 MHz, CDCl3) δ 8.14 (d, J
= 7.8 Hz, 1H), 8.03 (d, J = 8.0 Hz, 1H), 7.99 (s, 1H), 7.57 (t, J =
2
35 8.0 Hz, 2H), 7.48 (d, J = 7.5 Hz, 1H), 7.29 – 7.26 (m, 1H), 7.13
(d, J = 8.2 Hz, 1H), 5.66 (s, 2H). 13C NMR (150 MHz, CDCl3) δ
163.3, 149.2, 145.5, 144.9, 135.2, 131.4, 130.7, 127.6, 124.4,
119.4, 119.2, 118.2, 116.6, 80.5.
95
1ꢀ(4ꢀChlorophenyl)ꢀ6ꢀmethylꢀ1Hꢀbenzo[d][1,3]oxazinꢀ
40 4(2H)ꢀone (2q).7c White solid (82 mg), yield: 75%, mp. 109 –
o
1
100
105
111 C. H NMR (600 MHz, CDCl3) δ 7.89 (s, 1H), 7.35 – 7.29
(m, 3H), 7.04 (d, J = 8.6 Hz, 2H), 6.95 (d, J = 8.3 Hz, 1H), 5.52
(s, 2H), 2.37 (s, 3H). 13C NMR (150 MHz, CDCl3) δ 164.0,
143.7, 143.2, 136.1, 133.3, 130.7, 130.2, 129.8, 123.8, 119.1,
45 117.5, 80.9, 20.8.
3
1ꢀBenzylꢀ2ꢀphenylꢀ1Hꢀbenzo[d][1,3]oxazinꢀ4(2H)ꢀone
(2r).7c Yellow oil (54 mg), yield: 43%. 1H NMR (600 MHz,
CDCl3) δ 7.97 (dd, J = 7.8, 1.3 Hz, 1H), 7.46 – 7.40 (m, 3H), 7.34
– 7.29 (m, 5H), 7.28 – 7.24 (m, 3H), 6.95 (t, J = 7.5 Hz, 1H), 6.90 110
50 (d, J = 8.3 Hz, 1H), 6.33 (s, 1H), 4.63 (d, J = 16.1 Hz, 1H), 4.41
(d, J = 16.1 Hz, 1H). 13C NMR (150 MHz, CDCl3) δ 163.9,
148.3, 136.9, 136.0, 135.4, 130.9, 129.4, 128.8, 128.9, 127.9,
4
5
For selected examples, see: (a) W. Han and A. R. Ofial, Chem.
Commun., 2009, 45, 5024; (b) H. Li, Z. He, X. Guo, W. Li, X. Zhao
and Z. Li, Org. Lett., 2009, 11, 4176; (c) P. Liu, Y. Liu, E. L. Wong,
S. Xiang and C. Che, Chem. Sci., 2011, 2, 2187; (d) M. Brzozowski,
J. A. Forni, G. P. Savage and A. Polyzos, Chem. Commun., 2015, 51,
334; (e) C. M. R. Volla, and P. Vogel, Org. Lett., 2009, 11, 1701; (f)
W. Han, P. Mayer, A. and R. Ofial, Adv. Synth. Catal., 2010, 352,
1667.
(a) For selected examples, see: (a) S. A. Girard, T. Knuauber and C.
Li, Angew. Chem., Int. Ed, 2014, 53, 74; (b) Z. Li and C. Li, J. Am.
Chem. Soc., 2005, 127, 3672; (c) G. Zhang, Y. Zhang and R. Wang,
Angew. Chem., Int. Ed, 2011, 50, 10429; (d) W. R. Evans, J. R. Zbieg,
S. Zhu, W. Li and D. W. C. MacMillan, J. Am. Chem. Soc., 2013,
127.5, 127.2, 120.8, 116.4, 116.0, 90.5, 53.3.
115
10ꢀMethoxyꢀ5ꢀmethylꢀ5Hꢀdibenzo[b,e][1,4]diazepinꢀ
55 11(10H)ꢀone (4).21 White solid (89 mg), yield: 88%, mp. 125 –
1
127 oC. H NMR (600 MHz, CDCl3) δ 7.90 (dd, J = 7.6, 1.1 Hz,
1H), 7.49 (dd, J = 8.3, 1.4 Hz, 1H), 7.41 (td, J = 8.1, 1.6 Hz, 1H),
120
7.20 – 7.16 (m, 1H), 7.15 – 7.03 (m, 4H), 3.87 (s, 3H), 3.35 (s,
6
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135, 16074; (e) A. Modak, U. Dutta, R. Kancherla, S. Maity, M.
Bhadra, S. M. Mobin and D. Maiti, Org. Lett., 2014, 16, 2602; (f) O.
Basle and C. Li, Green Chem., 2008, 10, 249; (g) Y. Yan, Y. Yuan
and N. Jiao, Org. Chem. Front., 2014, 1, 1176.
For selected examples, see: (a) A. Sud, D. Sureshkumar and M.
Klussmann, Chem. Commun., 2009, 45, 3169; (b) L. Rout and T.
Punniyamurthy, Adv. Synth. Catal., 2005, 347, 1958; (c) C. Zhu, J.
Xia and C. Chen, Tetrahedron Lett., 2014, 55, 232; (d) S. Singhal, S.
L. Jain and B. Sain, Chem. Commun., 2009, 45, 2371.
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