Trifluoroacetic acid catalyzed highly regioselective bromocyclization of styrene-type carboxylic acid

Article abstract of DOI:10.1039/c6ob00756b

A trifluoroacetic acid catalyzed highly 6-endo regioselective bromocyclization of styrene-type carboxylic acid has been developed. The resulting 3,4-dihydroisocoumarines are valuable building blocks in organic synthesis.

Full text of DOI:10.1039/c6ob00756b

View Article Online
View Journal
Organic &
Biomolecular
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: T. Chen and Y.
Yeung, Org. Biomol. Chem., 2016, DOI: 10.1039/C6OB00756B.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/obc

Please do not adjust margins
Organic & Biomolecular Chemistry
Page 1 of 5
View Article Online
DOI: 10.1039/C6OB00756B
Organic & Biomolecular Chemistry
COMMUNICATION
Trifluoroacetic Acid Catalyzed Highly Regioselective
Bromocyclization of Styrene-Type Carboxylic Acid†
Tao Chenb and Ying-Yeung Yeung*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
trifluoroacetic acid catalyzed highly 6-endo regioselective halocyclization of 1 suffered from an inherent challenge: the
bromocyclization of styrene-type carboxylic acid has been halocyclization (e.g. bromocyclization) can go through the 5-
developed. The resulting 3,4-dihydroisocoumarines are valuable exo [by the attack of C(1)] and 6-endo [by the attack of C(2)]
building blocks in organic synthesis.
pathways to give a mixture of phthalide and 3,4-
dihydroisocoumarin (Scheme 1). Since both C(1) and C(2) are
benzylic carbons, poor regioselectivity was usually obtained.
The two product isomers have similar solvent mobility on silica
gel and are commonly difficulty to be separated by column
chromatography. These roadblocks obstruct the utility of
Isocoumarine is a class of heterocycle which exhibits various
pharmacological activities such as immunomodulatory,
antiallergic, antifungal and antiangiogenic effects.1 In
particular, 3,4-dihydroisocoumarine structures are ubiquitous
in natural products such as Hydragenol, Phyllodulcin,
macrophyllo and Thunberginol G (Figure 1). Significant efforts
have been devoted to the synthesis of different isocoumarine.
halocyclization of
1 in the synthesis of 3,4-dihydroisocoumarin.
Indeed, it has been reported that treatment of 1a with I2/KI
gave phthalide 3a (X = I) (Scheme 1).5 In order to develop an
X
R1
OH
O
OH
O
R2
O
O
O
6-endo
O
2
R1
1
2
3,4-dihydroisocoumarine
OH
OMe
Hydragenol
OH
Phyllodulcin
OH
R2
OH
OH
OH
O
O
X
R1
5-exo
O
O
O
H
O
1
OMe
OMe
R2
OH
X = halogen
O
3
phthalide
H
Macrophyllo
Thunberginol G
OMe
I
Figure 1 Examples of 3,4-Dihydroisocoumarine-Containing Natural Products.
I2, aq. KI
aq. NaHCO3
OH
O
scaffolds through transition metal-catalyzed reaction.2
O
1a
3a
O
Nonetheless,
synthesis of this kind of compounds is still highly desired.3
Halocyclization of styrene-type olefinic acid offer an
efficient tool towards the synthesis of 3,4-dihydroisocoumarin
.4 The halogen handle in
can easily be manipulated to yield
various functional molecules. However, practically
a versatile and metal-free method to the
Scheme 1 Bromocyclization of Styrene-type Carboxylic Acid.
1
electrophilic halocyclization that is practically useful in the
synthesis of 3,4-dihydrocoumarin, highly regioselective
a
2
2
catalytic protocol is highly desired. To address this problem,
herein we are pleased to report our recent success in using
trifluoroacetic acid (TFA) as the catalyst in the highly 6-endo
selective
bromocyclization
of
1
to
give
3,4-
dihyroisocoumarines
2.
Bromocyclization of 1a was performed using N-
bromosuccinimide (NBS) and CHCl3 as the electrophilic
This journal is © The Royal Society of Chemistry 20xx
Org. Biomol. Chem., 2016, 00, 1-5 | 1
Please do not adjust margins

Please do not adjust margins
Organic & Biomolecular Chemistry
Page 2 of 5
View Article Online
DOI: 10.1039/C6OB00756B
COMMUNICATION
Journal Name
brominating reagent and the solvent, respectively. When no was obtained in exceedingly high 6-endo selectivity (2a/3a =
catalyst was applied, it was found that a mixture of isomers 88:1) (Table 1, entry 11). Ethyl acetate was found to be a
3,4-dihydroisocoumarine 2a and phthalide 3a was obtained suitable solvent to give high regioselectivity but the reaction
with low regioselectivity (2a
/
3a = 5:1) (Table 1, entry 1). To
was sluggish (Table 1, entry 10). A control experiment was
performed using acetonitrile as the solvent in the absence of
catalyst, revealed the crucial role of TFA in controlling the
regioselectivity (Table 1, entry 13). After extensive
experimentation, it was realized that 2a could be
accomplished as the exclusive product when anhydrous
acetonitrile was used (Table 1, entry 12). When the TFA
loading was decreased to 5 mol% or increased to 100 mol%,
the excellent 6-endo selectivity preserved (Table 1, entries 14
and 15). It is noteworthy that TFA can easily be removed in
vacuo, which simplifies the purification process.
Table 1 Reaction optimizationa
Table 2 TFA-Catalyzed Bromocyclization of 1a
Entry
1
Catalyst
None
Solvent
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
THF
Yield b (%)
98
2a:3ac
5:1
2
3d
DMAP
90
1:1.4
1:1.3
30:1
24:1
50:1
3.6:1
5:1
Na2CO3
Ph3PS
82
4
80
5
Ph3PSe
82
6
CF3COOH
AcOH
99
7
98
8
PhCOOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
None
98
9
80
30:1
>99:1
88:1
>99:1
3.8:1
>99:1
>99:1
10
11
12e
13
14 e,f
15 e,g
EtOAc
MeCN
MeCN
MeCN
MeCN
MeCN
36
99
99
99
CF3COOH
CF3COOH
99
99
a
Reactions were carried out with 1a (0.3 mmol), NBS (0.33 mmol), and catalyst
(0.03 mmol, 10 mol%)) in solvent (2 mL) at 25 oC in the absence of light. The
b
Entry Substrate
R1
H
R2
H
Yield b (%)
2:3c
yields were determined by 1H NMR analysis of the mixture with
1
2
1a
1b
1c
1d
1e
1f
99
98
99
98
99
99
95
99
99
99
>99:1 (3.8:1)
>99:1 (4.6:1)
>99:1 (1.1:1)
>99:1 (5:1)
hexamethylbenzene as the internal standard. The ratio was determined by 1H
c
4-F
H
NMR analysis of the mixture. d 1 equivalent of Na2CO3 was used. Anhydrous
e
3
4-Cl
4-Me
4-MeO
-
H
MeCN was used. f 5 mol% CF3COOH was used. g 100 mol% CF3COOH was used.
4
H
5
H
>99:1 (9:1)
efficiently obtain the 3,4-dihydroisocoumarine 2a, a series of
catalysts were subjected to the investigation. Although N,N-
dimethylaminopyridine (DMAP) and Na2CO3 are well-known to
facilitate halolactonization,6 almost no regioselectivity was
observed when these two bases were employed (Table 1,
6
H
>99:1 (8.3:1)
>99:1 (2.5:1)
>99:1 (1.6:1)
>99:1 (8.3:1)
>99:1 (4.8:1)
7
1g
1h
1i
2-Me
3-Me
H
H
8
H
9
4-F
5-F
10
1j
H
a
entries
2
and 3). Triphenylphosphine sulfide and
Reactions were carried out with 1 (0.15 mmol), NBS (0.165 mmol), and TFA
(0.015 mmol, 10 mol%) in MeCN (1 mL) at 25 oC in the absence of light. b Isolated
yield. c The ratio was determined by 1H NMR analysis of the mixture. The ratio of
2:3 in the absence of TFA is indicated in the parentheses.
triphenylphospine selenide, which are effective Lewis bases for
electrophilic bromocyclization reactions,7 offered good 6-endo
selectivity in the bromocyclization of 1a (Table 1, entries 4 and
5). To our delight, the 6-endo selectivity (2a/3a = 50:1) was
With the optimized conditions in hand, a series of carboxylic
dramatically improved when 10 mol% TFA was applied (Table 1,
entry 6). On the other hand, other weaker acids such as acetic
acid or benzoic acid gave much lower selectivity (Table 1,
entries 7 and 8). Next, a brief survey on solvent was conducted
(Table 1, entries 9–11) and acetonitrile was found to be the
suitable reaction media in which 3,4-dihydroisocoumarine 2a
acids
reactions were performed in the absence of TFA, the
respective regioselectivity was poor with the ratio ranging
1 were investigated. As shown in Table 2, when the
2/3
from 1:1 to 9:1. In a stark contrast, the reactions gave the
2 | Org. Biomol. Chem.., 2016, 00, 1-5
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Organic & Biomolecular Chemistry
Page 3 of 5
View Article Online
DOI: 10.1039/C6OB00756B
Journal Name
COMMUNICATION
desired 3,4-dihydroisocoumarines
2
exclusively and reduced pressure; (3) the residue was extracted with Et2O/H2O.
quantitatively regardless of the electronic property of the Pure product 2a (analyzed by NMR) was obtained (1.50 g, 99%
substituent. For instance, electron-deficient 1b–c and yield) from the Et2O fraction. Succinimide, the Br carrier, was
electron-rich aryls (1d–e) substrates worked well to give the recovered quantitatively from the aqueous extract.
desired 6-endo cyclization products 2b–e (Table 2, entries 2 5).
(
)
–
Sterically hindered substituents such as 2-naphthyl (1f), 2-
methylphenyl (1g) and 3-methylphenyl (1h) substrates were
also examined and 2f–h were obtained smoothly as the
exclusive products (Table 2, entries
6–8). In addition,
substrates such as 1i and 1j with the substituent on benzoic
acid moiety, the corresponding 6-endo selective products were
also obtained as the sole products in quantitative yield (Table
2, entries 9 and 10). A group of substrates 1k–1n were also
studied (Table 3). These substrates contain electron-
withdrawing R1 groups, which lead to the 5-exo selective
products predominately in the uncatalyzed electrophilic
bromocyclization reaction. Nonetheless, the inherent
preference of 5-exo was overridden in the presence of 10
mol% of TFA, furnishing 2k–2n in high 6-endo selectivity and
reaction yields (Table 3, entries 1–5). For the case using
Scheme 2 Chromatography-Free Synthesis of 2a in Gram-Scale.
We applied this type of catalysis to the one-pot synthesis
of cis-3-aryl-4-(phenylthio)dihydroisocoumarines (Scheme 3).
After the bromocyclization of
1, TFA was removed under
reduced pressure. Thiophenol and potassium carbonate were
then added and the resultant solution was heated to 75 oC for
1 h. The desired cis-3-aryl-4-(phenylthio)dihydroisocoumarines
substrate 1n, the 2n
/3n selectivity could be further enhanced
6
were obtained in high yields with excellent
when 1 equivalent of TFA was used (Table 3, entry 5).
diastereoselectivities. This transformation is complementary to
Table 3 Studies on Highly Electron-Deficient Substrates 1k–
1na
the electrophilic cyclization towards the construction of trans-
3-aryl-4-(phenylthio)dihydroisocoumarines
disulfide as the reagent.9
7
using the diaryl
Br
R1
R1
TFA
(10 mol%)
NBS, MeCN
25 oC, 24 h
O
COOH
remove TFA
1
O
2
in vacuo
ref. 8
PhSH, K2CO3
MeCN, 75 oC, 1 h
Ph
Ph
S
S
R1
R1
O
O
Entry Substrate
R1
Yieldb (%) 2:3 with TFAc 2:3 without TFAc
O
O
7
6
1
2
3
4
5
1k
1l
2-Cl
98
91
99
98
99
11:1
2:1
1:3
1:12
1:2
dr > 99:1
3-NO2
4-Br
1m
1n
1n
>99:1
4:1
5:1d
substrate
product 6
yield (%)
4-CF3
4-CF3
1:3.3
1:3.3
1a
1c
1d
1f
6a
6c
6d
6f
80
74
81
75
a
Reactions were carried out with 1a (0.3 mmol), NBS (0.33 mmol), and catalyst
(0.03 mmol, 10 mol%)) in solvent (2 mL) at 25 oC in the absence of light. The
b
yields were determined by 1H NMR analysis of the mixture with
Scheme 3 One-pot Synthesis of 6.
hexamethylbenzene as the internal standard. The ratio was determined by 1H
c
NMR analysis of the mixture. d 1 equivalent of Na2CO3 was used. Anhydrous
e
MeCN was used.
Although the role of TFA in inducing high regioselectivity of
the cyclization of remains unclear, it is interesting to see that
1
The reaction was found to be readily scalable without loss
of reaction yield and regioselectivity. Since TFA has a relatively
low boiling point, it could be removed under reduced pressure.
To this end, we conducted a gram-scale reaction and purified
the product without using column chromatography8 with the
following sequence: (1) 1.12 g of 1a was subjected to the
bromocyclization under the optimized conditions; (2) the
solvent MeCN and the catalyst TFA were removed under
the regioselectivity of the cyclization can be overridden
particularly for the electron-deficient substrates (Table 3).
Several experiments were conducted in order to shed light on
the mechanistic picture. We had suspected that
2 and 3 might
be interchangeable in the presence of TFA.10 However, there
was no observable change in the constitution when either 2a
or 3a was stirred in acetonitrile and 10 mol% of TFA,
This journal is © The Royal Society of Chemistry 20xx
Org. Biomol. Chem., 2016, 00, 1-5 | 3
Please do not adjust margins

Please do not adjust margins
Organic & Biomolecular Chemistry
Page 4 of 5
View Article Online
DOI: 10.1039/C6OB00756B
COMMUNICATION
and
Journal Name
suggesting that
(Scheme 4, eq. 1). We have also conducted the cyclization that water or MeOH might interrupt the hydrogen-bond
using anhydrous MeOH as the solvent and it was realized that interaction between and TFA, resulting in lower
the effect of TFA on the regioselectivity was negligible (Scheme regioselectivity. Even though the formation of phthalide
2
3
were formed independently from
1
dihydroisocoumarin 2 through species A. We also suspected
1
a
3
4, eq. 2). Together with the observation that the moisture has through 5-exo-cyclization was favored when the aryl group is
a deteriorated effect on the regioselectivity (Table 1, entries highly electron-deficient [which can destabilize the partial
11 and 12), one of the possible explanations is that water (or carbocation on C(2)], the effect of TFA appears to be strong
MeOH) might interrupt the hydrogen-bond interaction which can override the inherent preference (see Table 3).
between the substrate and TFA.
In summary, we have developed an efficient methodology
to synthesize 3,4-dihydroisocoumarine derivatives through
highly 6-endo selective bromocyclization of styrene-type
carboxylic acids by simply using TFA as the catalyst. Catalytic
amount of TFA could improve the ratios of 6-endo/5-exo
products to > 99:1 in most cases. The reaction could be well
carried out in gram-scale synthesis without loss of the
selectivity. The bromocyclized product could be obtained
without using of chromatography and succinimide could be
easily recovered. This methodology has been applied to the
Br
Br
H
(1)
(2)
O
O
2a
3a
O
O
( )-2a
catalyst
none
10 mol% TFA 11:1
2a:3a
15:1
NBS, MeOH
+
25 oC, 3 h
one-pot
synthesis
of
cis-3-aryl-4-
( )-3a
COOH
(phenylthio)dihydroisocoumarine.
study is underway.
A detailed mechanistic
1a
TFA
H
We thank the financial supports from The Chinese
University of Hong Kong Direct Grant (project code: 4053157)
and Tier 1 funding from National University of Singapore (grant
no. 143-000-605-112).
O
N
O
O
Br
1
Ar
2
1
O
H
O
H
Notes and references
O
CF3
minor
major
1
(a) H. Matsuda, H. Shimoda, J. Yamahara, M. Yoshikawa,
Bioorg. Med. Chem. Lett., 1998, , 215 220; (b) H. Matsuda,
H. Shimoda, M. Yoshikawa, Bioorg. Med. Chem., 1999,
1445 1450; (c) K. Nozawa, M. Yamada, Y. Tsuda, K. Kawai, S.
Nakajima, Chem. Pharm. Bull., 1981, 29, 2491 2495; (d) K.
Nozawa, M. Yamada, Y. Tsuda, K. Kawai, S. Nakajima, Chem.
Pharm. Bull., 1981, 29, 2689 2691; (e) J. H. Lee, Y. J. Park, H.
S. Kim, Y. S. Hong, K.-W. Kim, J. J. Lee, J. Antibiot., 2001, 54
463 466.
8
–
7,
–
Br
Br
Ar
2
1
1
2
–
Ar
+
+
O
O
H
B
A
H
–
O
+ O
O
,
+ O
H
H
–
O
CF3
CF3
O
2
Selected examples for tranisition metal catalyzed reactions in
the synthesis of isocoumarines, see: (a) A.-Y. Peng, Y.-X.
C(2) attack
C(1) attack
5-exo
6-endo
Ding, J. Am. Chem. Soc., 2003, 125, 15006
Liao, C.-H. Cheng, J. Org. Chem., 1995, 60, 3711
Subramanian, V. R. Batchu, D. Barange, M. Pal. J. Org. Chem.,
2005, 70, 4778 4783; (d) S. Roy, B. Neuenswander, D. Hill, R.
C. Larock, J. Comb. Chem., 2009, 11, 1128 1135; (e) R.
Prakash, K. Shekarrao, S. Gogoi, R. C. Boruah, Chem.
Commun., 2015, 51, 9972 9974; (f) R. K. Chinnagolla, M.
Jeganmohan, Chem. Commun., 2012, 48, 2030 2032; (g) A.
C. Tadd, M. R. Fielding, M. C. Willis, Chem. Commun., 2009,
6744 6746; (h) S. J. Cai, F. Wang, C. J. Xi, J. Org. Chem., 2012,
77, 2331 2336; (i) K. Ueura, T. Satoh, M. Miura, Org. Lett.,
2007, , 1407 1409; (j) K. Ueura, T. Satoh, M. Miura, J. Org.
Chem., 2007, 72, 5362 5367; (k) B. Chen, S. M. Ma, Org.
Lett., 2013, 15, 3884 3887; (l) W.-J. Yoo, T. V. Q. Nguyen, S.
Kobayashi, Angew. Chem. Int. Ed., 2014, 53, 10213 10217;
(m) S. Warratz, C. Kornhaaß, A. Cajaraville, B. NiepÖtter, D.
–
15007; (b) H.-Y.
–
3716; (c) V.
Br
Br
Ar
Ar
H
–
O
–
O
2
3
O
O
–
–
phthalide
Scheme 4 A Plausible Reaction Mechanism.
3,4-dihydroisocoumarin
–
–
A plausible mechanism is illustrated in Scheme 4. While TFA
could interact with NBS, we speculated that TFA could also
9
–
–
hydrogen-bond to the carboxylic moiety in 1. Since both C(1)
–
–
and C(2) are benzylic carbons, the partial positive charge in the
bromiranium ion species could be stabilized on both carbons
Stalke, L. Ackermann, Angew. Chem. Int. Ed., 2015, 54, 5513
5517.
–
(i.e. species
is usually obtained in the bromocyclization of
protonation of the carboxylic acid in by TFA would lead to
A
or
B
); this could explain why a mixture of
2 and 3
1
. However,
3
(a) S. K. Gadakh, A. Sudalai, RSC Adv., 2014,
(b) W. E. Bauta, D. P. Lovett, Jr. W. R. Cantrell, B. D. Burke, J.
Org. Chem., 2003, 68, 5967 5973; (c) R. S. Mali, K. N. Babu, J.
Org. Chem., 1998, 63, 2488 2492; (d) M. Uchiyama, H.
4, 57658–57661;
1
–
the destabilization of the partial positive charge on C(1)
(through conjugation), which would disfavor the formation of
–
Ozawa, K. Takuma, Y. Matsumoto, M. Yonehara, K. Hiroya, T.
species
B and result in the selective formation of 3,4-
4 | Org. Biomol. Chem.., 2016, 00, 1-5
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Organic & Biomolecular Chemistry
Page 5 of 5
View Article Online
DOI: 10.1039/C6OB00756B
Journal Name
COMMUNICATION
Sakamoto, Org. Lett., 2006,
8
, 5517
–
5520; (e) G.-H. Ma, B.
Jiang, X.-J. Tu, Y. Ning, S.-J. Tu, G. Li, G. Org. Lett., 2014, 16
4504 4507; (f) G. C. A. Bellinger, W. E. Campbell, R. G. F.
Giles, J. D. Tobias, J. Chem. Soc. Perkin Trans. 1, 1982, 2819
,
–
–
2825; (g) M. E. Botha, R. G. F. Giles, C. M. Moorhoff, L. M.
Engelhardt, A. H. White, A. Jardine, S. C. Yorkea, J. Chem. Soc.
Perkin Trans. 1, 1991, 89
Synlett, 2004, 2736 2738; (i) G. L. Bras, A. Hamze, S.
Messaoudi, O. Provot, P.-B. L. Calvez, J. D. Brion, M. Alami,
Synthesis, 2008, 1607 1611; (j) K. Sudarshan, M. K. Manna, I.
S. Aidhen, Eur. J. Org. Chem., 2015, 1797 1803.
(a) A. K. Verma, V. Rustagi, T. Aggarwal, A. P. Singh. J. Org.
Chem., 2010, 75, 7691 7703; (b) J. Chen, L. Zhou, C. K. Tan,
Y.-Y. Yeung, J. Org. Chem., 2012, 77, 999 1009.
R. S. Mali, A. P. Massey, M. I. Talele, J. Chem. Research (S),
1998, 68 69.
(a) J. L. Huang, H. Y. Wang, A.-Y. Peng, Eur. J. Org. Chem.,
2014, 8126 8132; (b) W. Zhang, H. D. Xu, H. Xu, W. P. Tang,
J. Am. Chem. Soc., 2009, 131, 3832 3833; (c) R. Antonioletti,
S. Malancona, P. Bovicelli, Tetrahedron, 2002, 58, 8825
8831; (d) R. Antonioletti, S. Malancona, F. Cattaruzza, P.
Bovicelli, Tetrahedron, 2003, 59, 1673 1678.
–95; (h) M. Brasholz, H.-U. Reissig,
–
–
–
4
–
–
5
6
–
–
–
–
–
7
(a) Y. Kawato, A. Kubota, H. Ono, H. Egami, Y. Hamashima,
Org. Lett., 2015, 17, 1244; (b) Y.-C. Wong, Z. Ke, Y.-Y. Yeung,
Org. Lett., 2015, 17, 4944; (c) Z. Ke, C. K. Tan, F. Chen, Y.-Y.
Yeung, J. Am. Chem. Soc., 2014, 136, 562; (d) Y. Sawamura,
H. Nakatsuji, A. Sakakura, K. Ishihara, Chem. Sci., 2013, 4,
4181; (e) F. Chen, C. K. Tan, Y.-Y. Yeung, J. Am. Chem. Soc.,
2013, 135, 1232; (f) S. J. Balkrishna, C. D. Prasad, P. Panini,
M. R. Detty, D. Chopra, S. Kumar, J. Org. Chem., 2012, 77
,
9541; (g) X. Jiang, C. K. Tan, L. Zhou, Y.-Y. Yeung, Angew.
Chem. Int. Ed., 2012, 51, 7771; (h) C. K. Tan, C. Le, Y.-Y.
Yeung, Chem. Commun., 2012, 48, 5793; (i) L. Zhou, J. Chen,
C. K. Tan, Y.-Y. Yeung, J. Am. Chem. Soc., 2011, 133, 9164; (j)
C. K. Tan, L. Zhou, Y.-Y. Yeung, Org. Lett., 2011, 13, 2738; (k)
S. Snyder, A. D. S. Treitler, A. P. Brucks, J. Am. Chem. Soc.,
2010, 132, 14303; (l) S. E. Denmark, M. T. Burk, Proc. Natl.
Acad. Sci. U.S.A., 2010, 107, 20655; (m) S. A. Snyder, D. S.
Treitler, Angew. Chem. Int. Ed., 2009, 48, 7899; (n) S. R.
Mellegaard, J. A. Tunge, J. Org. Chem., 2004, 69, 8979.
8
(a) G. An, C. Seifert, G. Li, Org. Biomol. Chem., 2015, 13
1600 1617; (b) C. W. Seifert, S. Pindi, G. Li, J. Org. Chem.,
2015, 80, 447−452; (c) B. Yang, M. Shen, X. Ji, Z. Xu, H. Sun,
B. Jiang, G. Li, J. Org. Chem., 2016, 81, 2488 2493.
S. A. Shahzad, C. Venin, T. Wirth, Eur. J. Org. Chem., 2010,
3465 3472.
10 (a) W. P. Jackson, S .V. Ley, J. A. Morton, Chem. Commun.,
1980, 1028 1029; (b) W. P. Jackson, S. V. Ley, J. A. Morton,
Chem. Commun., 1980, 1173 1174; (c) W. P. Jackson, S. V.
Ley, J. A. Morton, Tetrahedron Lett., 1981, 22, 2601 2604;
(d) S. V. Ley, B. Lygo, H. Molines, J. A. Morton, Chem.
Commun., 1982, 1251 1252; (e) A. C. Cunat, D. Diez-Martin,
S. V. Ley, F. J. Montgomery, J. Chem. Soc. Perkin Trans. 1,
1996, 611 620; (f) M. Tiecco, L. Testaferri, M. Tingoli, F.
Marini, Chem. Commun., 1994, 221 222.
,
–
–
9
–
–
–
–
–
–
–
This journal is © The Royal Society of Chemistry 20xx
Org. Biomol. Chem., 2016, 00, 1-5 | 5
Please do not adjust margins