Regioselective synthesis of substituted imidate N-[1- methyleneisobenzofuran-3(1H)-ylidene]benzenamines via palladium-catalyzed tandem heteroannulation of o-(1-alkynyl)benzamides with iodobenzene

Article abstract of DOI:10.1055/s-0030-1260973

A variety of substituted imidate N-[1-methyleneisobenzofuran-3(1H)-ylidene] benzenamines have been prepared in good to excellent yields by the palladium-catalyzed tandem heteroannulation of o-(1-alkynyl)benzamides with iodobenzene. The products obtained from this process were unusual substituted N-[isobenzofuran-3(1H)-ylidene]benzenamines. The tandem cyclization of readily available o-(1-alkynyl)benzamides and aryl iodides provided a powerful tool for the preparation of functionally substituted N-[isobenzofuran-3(1H)-ylidene] benzenamine compounds. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1260973

LETTER
1863
Regioselective Synthesis of Substituted Imidate N-[1-Methyleneisobenzo-
furan-3(1H)-ylidene]benzenamines via Palladium-Catalyzed Tandem
Heteroannulation of o-(1-Alkynyl)benzamides with Iodobenzene
P
alladium-Cata
e
d
T
and
-
e
m
Hete
Y
roannulation of
o
-(
i
1-Alkynyl)be
Y
nzamides with Iod
a
obenzene n,*a Cun-Min Tan,a Xue Wang,a Fei Li,a Guo-Lin Gao,b Xi-Meng Chen,a Wang-Suo Wu,a Jian-Jun Wang*a
a
Laboratory of Radiochemistry, School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, P. R. of China
Fax +86(931)8913278; E-mail: yanzeyi@lzu.edu.cn; E-mail: wangjianjun@lzu.edu.cn
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. of China
b
Received 3 April 2011
modic, herbicidal, and insecticidal properties.7 On the oth-
er hand, imidates, which are also called imidoates and
orimido esters, are esters of the hypothetical imidic acids.8
They are very useful synthetic building blocks9 and im-
Abstract: A variety of substituted imidate N-[1-methyleneisoben-
zofuran-3(1H)-ylidene]benzenamines have been prepared in good
to excellent yields by the palladium-catalyzed tandem heteroannu-
lation of o-(1-alkynyl)benzamides with iodobenzene. The products
obtained from this process were unusual substituted N-[isobenzo- portant pharmacophores.10 In view of their potential use-
furan-3(1H)-ylidene]benzenamines. The tandem cyclization of
readily available o-(1-alkynyl)benzamides and aryl iodides provid-
fulness in synthetic and medicinal chemistry, many
efficient methodologies have been developed for the con-
ed a powerful tool for the preparation of functionally substituted
struction of imidate framework by the following reac-
N-[isobenzofuran-3(1H)-ylidene]benzenamine compounds.
tions: 1) the Pinner reaction,11 one of the most popular
Keywords: isobenzofuran, o-(1-alkynyl)benzamide, iodobenzene,
tandem, palladium catalysis
methods in which nitriles and alcohols are condensed to
various imidates in the presence of hydrogen chloride or
hydrogen bromide; 2) the reaction of amines with ortho-
esters;12 3) the modified Staudinger ligation;13 4) some
Carbocycles and heterocycles are basic skeletons of many
other methods including the base-catalyzed reaction of ni-
triles with alcohols,14 the Cu(I)-catalyzed three-compo-
nent coupling of terminal alkyne, sulfonyl azide, and
alcohol,15 the palladium-catalyzed insertion of isonitrile
into ortho-bromoarylalcohol,16 and the three-component
coupling reaction of aryne, isonitrile, and aldehyde.17
biologically active natural products. Because tandem re-
actions allow a considerable increase in the molecular
complexity in a single operation, they have received con-
siderable attention in the design and synthesis of complex
biologically active compounds and natural products.1
During the past decades, palladium-catalyzed domino cy-
clization reactions have emerged as a powerful tool to
construct complex carbocyclic and heterocyclic mole-
cules.2 Among them, the palladium-catalyzed cyclization
of alkynes is particularly effective for the synthesis of a
wide variety of carbocycles and heterocycles.3 Their pop-
ularity stems in part from their tolerance of many func-
tional groups such as carbonyl and hydroxy groups, which
allows them to be employed in the synthesis of highly
complex molecules and industrial chemical processes.4
The key step in these reactions is the addition of arylpal-
ladium halide to the triple bond which affords an interme-
diate that can participate in a wide variety of useful
synthetic processes.
O
O
O
I2 or ICl
NHR1
R2
NR1
R2
NR1
+
R2
I
I
Scheme 1 Larock developed nitrogen-participated cyclization
Very recently, Larock reported a method of regio- and ste-
reoselective synthesis of isoindolin-1-ones by the electro-
philic cyclization of o-(1-alkynyl)benzamides with ICl, I2,
and NBS18 (Scheme 1). Compared to Larock’s nitrogen-
participated result, when similar alkynyl carboxamides
were employed as substrates, electrophilic oxygen-partic-
ipated cyclization gave exclusively pyran derivatives in
our experiments (Scheme 2).19 Thus, it indicates that the
relative nucleophilicity of nitrogen and oxygen atoms of
the amide group is a crucial factor in determining the re-
action result in these processes. In addition, it has been re-
ported that vinylic, aryl, and alkynylpalladium complexes
promoted tandem cyclizations of alkynes possessing nu-
cleophilic center to form various carbo- and heterocycles
by in situ coupling/cyclization reactions.2,3 Pursuing our
interest in constructing carbocycles and heterocycles,19
we investigated the palladium-catalyzed annulation of
o-(1-alkynyl)benzamides initiated by the addition of
The substituted isobenzofurans (IBF) are integral parts of
numerous natural products and are extremely important in
medicinal chemistry.5 Among these isobenzofuran deriv-
atives, 3-substituted isobenzofuran-1(3H)-ones are vital
heterocyclic motifs in many bioactive compounds such as
isocoumarins, anthraquinones, anthracyclines, and sever-
al alkaloids.6 For example, 3-alkylidene isobenzofuran-
1(3H)-one derivatives are reported to possess antispas-
SYNLETT 2011, No. 13, pp 1863–1870
x
x
.x
x
.2
0
1
1
Advanced online publication: 25.07.2011
DOI: 10.1055/s-0030-1260973; Art ID: W08611ST
© Georg Thieme Verlag Stuttgart · New York

1864
Z.-Y. Yan et al.
LETTER
arylpalladium halide to the triple bond. Unexpectedly, this in addition to isoindolinone 3aa, was isolated as the minor
chemistry produces five-membered cyclic imidate N- product. By NMR and mass spectroscopic analyses, the
[isobenzofuran-3(1H)-ylidene]benzenamine in moderate new product was identified as the unusual five-membered
to excellent yields with good regio- and stereoselectivity. ring imidate N-[isobenzofuran-3(1H)-ylidene]benzen-
Herein, we wish to report the successful synthesis of amine 4aa. Although Larock reported the electrophilic cy-
substituted [isobenzofuran-3(1H)-ylidene]benzenamine clization of o-(1-alkynyl)benzamides to synthesize
compounds by the palladium-catalyzed tandem reactions. isoindolinones by nucleophilic attack of the nitrogen of
the amide group on the activated carbon–carbon triple
bond.18 However, the present reaction gave isobenzofuran
OH
OH
O
I
derivative by nucleophilic attack of the oxygen of the
amide group, which was apparently different with
Larock’s work. Stimulated by the unexpected result, we
further investigated the effects of solvents, bases, and cat-
alysts in this reaction. The use of alternative bases in the
reaction gave greatly altered selectivity. For example,
stronger bases such as K2CO3, Cs2CO3, and Et3N mainly
gave isoindolinone (Table 1, entries 1–5); weak inorganic
bases NaHCO3, NaOAc, and NaOAc·3H2O also resulted
in a mixture of the above two products in prolong time
(Table 1, entries 6–8); using organic bases such as pyri-
dine and 2,6-lutidine finally suppressed isoindolinone
product and slowly gave the desired compound 4aa after
20 hours (Table 1, entries 9–13). With regard to the reac-
tion solvent, generally, more polar solvents tended to re-
duce product selectivity, and less polar solvents generally
required longer reaction time. None of those tested, how-
ever, were efficient or produced better results than in
MeCN. In addition, Pd(PPh3)4 was further proven to be
the best catalyst for the present conversion. Thus, the pal-
ladium-catalyzed tandem heteroannulation of o-(1-alky-
nyl)benzamides 1 with organic halides 2 was conducted
under the optimum conditions of Pd(PPh3)4 as the cata-
lyst, 2,6-lutidine as the base in refluxing MeCN.20
NIS
N
N
H
O
Scheme 2 We reported oxygen-participated cyclization
In the preliminary study, we started our investigations of
reaction conditions by using o-(1-alkynyl)benzamide 1a
(0.5 mmol), 1.2 equivalents of iodobenzene, 5 mol% of
Pd2(dba)3 as the catalyst, and 2.0 equivalents of K2CO3 in
DMF as the solvent at 80 °C under argon atmosphere. The
intramolecular addition product isoindolinone 3aa was
isolated as the only product (Table 1, entry 1). Further
study revealed that if K2CO3 or Cs2CO3 was used as base,
and DMSO, DMF, or THF was used as the solvent, and
the same product isoindolinone 3aa was still isolated ex-
clusively (Table 1, entries 1–4). Iodobenzene failed to
participate in the above process, and the expected tandem
reaction of o-(1-alkynyl)benzamide 1a with iodobenzene
was proved to be not successful. Based on the result ob-
tained, we envisioned that the base might play undesirable
role in the present reaction. Thus, we attempted to intro-
duce weaker base to the reaction system. To our delight,
when Et3N was employed as the base, a new compound,
Table 1 Optimization of the Pd-Catalyzed Cyclization of o-(1-Alkynyl)benzamide 1a with Iodobenzene 2aa
O
O
N
N
H
N
Pd(0)
+
+
I
O
base, solvent
H
1a
2a
3aa
4aa
Entry
Catalysts
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Base
Solvent
Yield (%)b
0 (65)
1
2
3
4
5
6
K2CO3
K2CO3
Cs2CO3
K2CO3
Et3N
DMF
DMSO
DMSO
THF
0 (67)
0 (71)
0 (68)
THF
27 (56)
15 (68)
NaHCO3
THF
Synlett 2011, No. 13, 1863–1870 © Thieme Stuttgart · New York

LETTER
Palladium-Catalyzed Tandem Heteroannulation of o-(1-Alkynyl)benzamides with Iodobenzene
1865
Table 1 Optimization of the Pd-Catalyzed Cyclization of o-(1-Alkynyl)benzamide 1a with Iodobenzene 2aa (continued)
O
O
N
N
H
N
Pd(0)
+
+
I
O
base, solvent
H
1a
2a
3aa
4aa
Entry
7
Catalysts
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Base
Solvent
Yield (%)b
25 (60)
23 (58)
56 (0)
NaOAc
THF
8
NaOAc×3H2O
pyridine
THF
9
THF
10
11
12
13
14
15
16
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
2,6-lutidine
THF
63 (0)
MeCN
MeCN
MeCN
MeCN
MeCN
toluene
70 (0)
Pd2(dba)3×CHCl3
Pd/C
68 (0)
49 (0)
Pd(OAc)2/Ph3P
Pd(PPh3)4
no reaction
73 (0)
Pd(PPh3)4
34 (0)
a Reactions were carried out on a 0.5 mmol scale in 3.0 mL of solvent under argon atmosphere with 1.0 equiv of 1a, 1.2 equiv of iodobenzene,
2.0 equiv of base, and 0.05 equiv of catalyst at 80 °C for the appropriate time (TLC analysis).
b The numbers in parentheses are the isolated yields of isoindolinone 3aa.
With the optimized reaction conditions in hand, the reac- amides bearing n-butyl and benzyl groups on the nitrogen
tions of o-(1-alkynyl)benzamide 1 with a wide variety of were employed as substrates, the tandem reaction afford-
aryl halids 2 were examined and afforded the cyclic im- ed the best result, and the desired cyclic imidates 4ha and
idates 4 in moderate to excellent yields. As shown in 4ia was obtained 91% and 94% yields, respectively
Table 2, the nature of the substituents attached to aromatic (Table 2, entries 8 and 9). The effect of substitution on the
halide, and the amide nitrogen had a major impact on the aromatic ring of iodobenzene was also examined based on
success of the reaction. Difference in the rates of reaction the reactions of 1a with a variety of substituted iodoben-
or the yields was observed for substrates bearing different zenes under the optimal reaction conditions. All the tested
functional groups. First, we proceeded to elucidate the reactions afforded the corresponding substituted N-[1-
scope of the reaction by examining the effect of various (diphenylmethylene)isobenzofuran-3(1H)-ylidene]benzen-
substituents on the aniline ring. The tandem reaction of N- amine 4 in moderate to excellent yields. In contrast, weakly
phenylcarboxamide 1a generated the corresponding cy- electron-donating 1-iodo-4-methylbenzene gave exclu-
clic imidate in 73% yield for 20 hours (Table 2, entry 1). sively the desired product 4ab in good yield and in good
Generally, introduction of electron-donating group such regioselectivity and stereoselectivity (Table 2, entry 10).
as methyl or methoxy groups on the aniline ring signifi- However, the cyclization of N-phenyl-2-(2-phenylethy-
cantly enhanced the reactivity. Higher yields were ob- nyl)benzamide 1a with the stronger electron-donating 1-
tained in shorter reaction time (Table 2, entries 2, 3, 6). In iodo-4-methoxybenzene exhibited poor reactivity, and the
contrast, lower yields were observed but in good regio- corresponding product 4ac was isolated in 57% yield after
selectivity for weak electron-withdrawing Cl and Br prolonged reaction time. In addition to 1H NMR and 13C
groups (Table 2, entries 4, 5, 7). However, the electron- NMR spectroscopic data, the formation of N-{(E)-1-[(4-
withdrawing NO2 group attached to the aniline nitrogen methoxyphenyl)(phenyl)methylene]isobenzofuran-3(1H)-
afforded intramolecular cyclization product, and isoin- ylidene}benzenamine (E)-4ac was also confirmed by the
dolinone 3 was obtained as the exclusive product. The corresponding X-ray crystallographic data (Figure 1).21
isoindolinone derivative 3 was formed by nucleophilic ni- On the whole, introducing an electron-withdrawing group
trogen attack of the carbon–carbon triple bond activated on the aromatic ring of iodobenzene substantially im-
by Pd(0) catalyst prior to tandem cyclization. When benz- proved the yield of the reaction. For example, 1-chloro-4-
Synlett 2011, No. 13, 1863–1870 © Thieme Stuttgart · New York

1866
Z.-Y. Yan et al.
LETTER
iodobenzene bearing an electron-withdrawing Cl group in uct, and the expected product was isolated in only 27%
the para position usually produced better results (Table 2, yield (Table 2, entry 13). However, when chlorobenzene
entry 12). When bromobenzene was employed as the sub- was used as the reactant, no desired product was observed.
strate, the reaction gave isoindolinone 3 as the major prod-
Table 2 Pd-Catalyzed Tandem Cyclization of o-(1-Alkynyl)benzamide 1 with Aryl Halides 2a
O
R1
Pd(PPh3)4
N
R1
XR3
R3
R2
+
H
2,6-lutidine,
refluxing MeCN
N
O
R2
1
2
X = I, Br
4
Entry
Substrate 1
Substrate 2
Product
Time (h)
Yield (%)b
O
I
N
H
N
1
20
73
O
2a
4aa
1a
Me
O
I
N
H
N
2
3
4
5
16
16
24
24
78
86
69
66
O
Me
OMe
Br
2a
4ba
1b
OMe
O
I
N
H
N
N
N
O
O
O
2a
4ca
4da
4ea
1c
Br
O
I
N
H
2a
1d
O
I
N
H
Cl
2a
Cl
1e
Synlett 2011, No. 13, 1863–1870 © Thieme Stuttgart · New York

LETTER
Palladium-Catalyzed Tandem Heteroannulation of o-(1-Alkynyl)benzamides with Iodobenzene
1867
Table 2 Pd-Catalyzed Tandem Cyclization of o-(1-Alkynyl)benzamide 1 with Aryl Halides 2a (continued)
O
R1
Pd(PPh3)4
N
R1
XR3
R3
R2
+
H
2,6-lutidine,
refluxing MeCN
N
O
R2
1
2
X = I, Br
4
Entry
Substrate 1
Substrate 2
Product
Time (h)
Yield (%)b
O
I
N
H
OMe
N
6
16
80
61
91
O
O
O
O
MeO
2a
4fa
4ga
4ha
1f
O
I
N
H
Cl
N
7
24
Cl
2a
1g
O
Bu
I
N
H
N
8
16
Bu
2a
1h
O
I
N
H
N
9
16
94
2a
1i
4ia
Me
O
I
N
H
10
N
20
71
O
Me
2b
4ab
1a
MeO
O
I
N
H
11
30
57
N
O
OMe
2c
1a
4ac
Synlett 2011, No. 13, 1863–1870 © Thieme Stuttgart · New York

1868
Z.-Y. Yan et al.
LETTER
Table 2 Pd-Catalyzed Tandem Cyclization of o-(1-Alkynyl)benzamide 1 with Aryl Halides 2a (continued)
O
R1
Pd(PPh3)4
N
R1
XR3
R3
R2
+
H
2,6-lutidine,
refluxing MeCN
N
O
R2
1
2
X = I, Br
4
Entry
12
Substrate 1
Substrate 2
Product
Time (h)
Yield (%)b
Cl
O
I
N
H
N
16
91
O
Cl
2d
4ad
1a
O
Br
N
H
N
13
30
37
O
2e
4ae
1a
O
I
N
H
N
O
14
20
79
2a
Me
Me
1j
4ja
O
I
N
H
N
O
15
20
83
2a
Cl
Cl
1k
4ka
O
I
N
H
N
16
20
75
O
2a
4la
1l
a Reactions were carried out in 3.0 mL of refluxing MeCN under argon atmosphere with 0.50 mmol 1, 0.60 mmol aryl halides, 2.0 equiv of 2,6-
lutidine, and 0.05 equiv of Pd(PPh3)4 for an appropriate time.
b Isolated yields.
Finally, using N-phenylcarboxamides and iodobenzene as ing group or an electron-donating group on the benzene
the model substrates, we also briefly investigated the ef- ring were employed in the reaction, and the corresponding
fect of various substituents on the remote end of the products were isolated in good yields (Table 2, entries 14
alkyne moiety in the present cyclization process. o-(1- and 15). Cyclohexenyl- and phenyl-substituted alkynes
Alkynyl)benzamides bearing with an electron-withdraw- exhibited the similar reactivity (Table 2, entry 1 vs. 16).
Synlett 2011, No. 13, 1863–1870 © Thieme Stuttgart · New York

LETTER
Palladium-Catalyzed Tandem Heteroannulation of o-(1-Alkynyl)benzamides with Iodobenzene
1869
picted in Scheme 3. The process was assumed to consist
of the following key steps: 1) oxidative addition of the or-
ganic halide to Pd(0) catalyst to form palladium(II) inter-
mediate 5; 2) coordination of the resulting palladium
intermediate 5 to triple bond of alkyne to generate impor-
tant complex 6;22 3) intramolecular nucleophilic attack of
the oxygen of the carboxamides group on the vinylic pal-
ladium intermediate to afford a six-membered metallo-
cycle intermediate 7 including the carbocation; 4)
deprotonation of intermediate 7 promoted by the weak
base nucleophile in the reaction mixture to form an enam-
ine intermediate 8; 5) reductive elimination of the inter-
mediate 8 to afford the expected product 4 and
regeneration of the Pd(0) catalyst. In the presence of a
stronger base, we assumed that o-(1-alkynyl)benzamide 1
underwent intramolecular nucleophilic attack of the nitro-
gen of the amide group on the carbon–carbon triple bond,
followed by deprotonation to directly afford the cyclized
products. As we observed, stronger base favored the in-
tramolecular reaction. In this case, the organic halide can-
not participate in the above reaction. Therefore, the
isoindolinone (phthalimidine) 3 as was isolated as the ma-
jor product.
Figure 1 Structure of cyclic imidate (E)-4ac
Disappointingly, TMS-substituted alkyne afforded none
of the desired product, possibly due to steric hindrance to
cyclization by the bulky TMS group that blocked the in-
coming oxygen nucleophile. Unfortunately, when the re-
action of terminal alkyne was investigated, a mixture of
unidentified products was obtained.
In conclusion, an efficient, regioselective, palladium-cat-
alyzed synthesis of substituted N-[isobenzofuran-3(1H)-
ylidene]benzenamine (cyclic imidate) using o-(1-alky-
nyl)benzamides 1 with iodobenzene has been developed.
A variety of iodobenzenes undergo this process to give the
unexpected products 4 in good yields and in good regio-
and stereoselectivity. The tandem cyclization of readily
available o-(1-alkynyl)benzamides afforded a rapid in-
crease in molecular complexity and provided a powerful
tool for the preparation of a wide range of functionally
N
O
R1
ArX
4
2
R2
Ar
R1
Pd(0)
substituted
N-[isobenzofuran-3(1H)-ylidene]benzen-
N
O
amines. Our further studies will focus on the development
of more tandem reaction of o-(1-alkynyl)benzamide to
form other heterocyclic compounds.
O
N
R1
R2
X
Pd
Ar
Pd
5
8
O
R2
R1
H
Ar
3
N
H
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
H+
weak base
R1
strong base
R1
R2
O
1
HN
+
Acknowledgment
N
O
H
We thank National Natural Science Foundation of China (NSFC)
(J1030932, 21001060) and China Postdoctoral Science Foundation
(CPSF-20090450843) for the financial support of this work.
Pd
R1
O
7
R2
HN
Ar
X–
R2
6
PdX
References and Notes
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R2
Scheme 3
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mechanism accounting for the formation of the substitut-
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Synlett 2011, No. 13, 1863–1870 © Thieme Stuttgart · New York

1870
Z.-Y. Yan et al.
LETTER
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(20) General Procedure for the Preparation of N-[Isobenzo-
furan-1 (3H)-ylidene]benzenamine Compounds
To a solution of o-(1-alkynyl)benzamides 1 (0.50 mmol) in
MeCN (3.0 mL) was added 2,6-lutidine (1.00 mmol). The
mixture was stirred for 10 min and Pd(PPh3)4 (5 mol%) and
organic halides (0.60 mmol) were added. The resulting
mixture was then heated under an argon atmosphere at
refluxing temperature. When the reaction was considered
complete, as determined by TLC analysis, the reaction
mixture was cooled to r.t., quenched with a sat. aq solution
of NH4Cl, and extracted with EtOAc. The combined organic
extracts were washed with H2O and sat. brine. The organic
layers were dried over Na2SO4 and filtered. The solvents
were evaporated under reduced pressure. The residue was
purified by chromatography on silica gel to afford the
corresponding N-[1-(diphenylmethylene)isobenzofuran-3
(1H)-ylidene]benzenamine. Thus, starting with 1a (149 mg,
0.5 mmol) and iodobenzene 2a (122 mg, 0.6mmol), a
yellowish solid product 4aa (273 mg, 73%) was isolated; mp
173–175 °C. 1H NMR (300 MHz, CDCl3): d = 6.24 (d,
J = 8.0 Hz, 1 H), 7.14–7.29 (m, 5 H), 7.35–7.41 (m, 5 H),
7.48–7.52 (m, 7 H), 8.00 (d, J = 7.6 Hz, 1 H). 13C NMR (75
MHz, CDCl3): d = 120.77, 123.27, 123.42, 124.02, 124.76,
127.50, 127.86, 128.38, 128.59, 129.24, 129.26, 129.86,
130.66, 130.82, 131.68, 136.67, 137.80, 137.90, 145.64,
145.77, 153.98. IR (neat): 3052, 2923, 1681, 1013, 755, 693
cm–1. Anal. Calcd for C27H19NO: C, 86.84; H, 5.13; N, 3.75;
O, 4.28. Found: C, 86.70; H, 5.06; N, 3.73; O, 4.35.
(21) The atomic coordinates for 4ac have been deposited at the
Cambridge Crystallographic Data Centre (deposition
number: CCDC 800275). The coordinates can be obtained
on request from the Director Cambridge Crystallographic
Data Centre. Postal Address: 12 Union Road, Cambridge
CB2 1EZ, UK; Email: deposit@ccdc.cam.ac.uk; fax: +44
(1223) 336033; tel.: +44 (1223)762910.
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Synlett 2011, No. 13, 1863–1870 © Thieme Stuttgart · New York