Syntheses and conformational structures of functionalized tetraoxacalix[2]arene[2]triazines

Article abstract of DOI:10.1016/j.tet.2012.08.069

The syntheses and conformational structures of various functionalized tetraoxacalix[2]arene[2]triazines were studied. Applying the fragment coupling approach and the post-macrocyclization chemical manipulations, a number of tetraoxacalix[2]arene[2]triazines that contain, on the lower rim, one or two aldehyde, ester, carboxylic acid, hydroxymethyl, and aminomethyl functional groups were prepared in moderate to high chemical yields from cheap and commercially available materials. On the basis of X-ray crystallography and NMR spectroscopy, all tetraoxacalix[2]arene[2]triazines containing electron-withdrawing group(s) adopted 1,3-alternate conformation both in solution and in the solid state, while tetraoxacalix[2]arene[2]triazines bearing hydroxymethyl and aminomethyl substituent(s) existed as pinched or distorted partial cone conformers due to the formation of intramolecular hydrogen bond between hydroxyl or amino group and triazine ring.

Full text of DOI:10.1016/j.tet.2012.08.069

Tetrahedron 68 (2012) 9464e9477
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Syntheses and conformational structures of functionalized tetraoxacalix[2]arene
[2]triazines
,
Shuai Pan a, De-Xian Wang a, Liang Zhao b, Mei-Xiang Wang b
*
a Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
b Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2012
Received in revised form 21 August 2012
Accepted 23 August 2012
Available online 4 September 2012
The syntheses and conformational structures of various functionalized tetraoxacalix[2]arene[2]triazines
were studied. Applying the fragment coupling approach and the post-macrocyclization chemical ma-
nipulations, a number of tetraoxacalix[2]arene[2]triazines that contain, on the lower rim, one or two
aldehyde, ester, carboxylic acid, hydroxymethyl, and aminomethyl functional groups were prepared in
moderate to high chemical yields from cheap and commercially available materials. On the basis of
X-ray crystallography and NMR spectroscopy, all tetraoxacalix[2]arene[2]triazines containing electron-
withdrawing group(s) adopted 1,3-alternate conformation both in solution and in the solid state,
while tetraoxacalix[2]arene[2]triazines bearing hydroxymethyl and aminomethyl substituent(s) existed
as pinched or distorted partial cone conformers due to the formation of intramolecular hydrogen bond
between hydroxyl or amino group and triazine ring.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
symmetrical and unsymmetrical heteracalixaromatics of varied
macrocyclic ring sizes. In addition, one-pot reaction strategy under
Heteroatom-bridged calixaromatics, also known as heter-
acalixaromatics, are a new generation of macrocyclic host mole-
cules in supramolecular chemistry.1e5 Being different from the
methylene linkages in conventional calixarenes,6 the bridging
heteroatoms, such as nitrogen can adopt different electronic con-
figurations between sp2 and sp3 and form various conjugation
systems with their adjacent aromatic rings.1,3,7 As a consequence,
heteracalixaromatics exhibit intriguing structural characteristics
and versatile molecular recognition properties.1e5 The self- and
fine-tuning cavities of varied electronic features render heter-
acalixaromatics powerful macrocyclic hosts in the interactions with
neutral organic guests8e10 and with positively11e15 and negatively
charged species.16e19
The parent heteracalixaromatics, the macrocyclic compounds
that contain no functional groups, are easily prepared from simple
and commercially available aromatic compounds. Starting from
aromatic bisnucleophiles and biselectrophiles, the fragment cou-
pling approach1e3 has been established as a powerful methodology
for the construction of a large number of heteracalixarenes,1e3,20
pyridines,9aec pyrazines,15 pyrimidines9d,21 and triazines.22 Re-
markably, this stepwise protocol provides rapid accesses to both
thermodynamically controlled reaction conditions works straight-
forwardly and efficiently for the synthesis of a few symmetrically
substituted heteracalixaromatics.1e3,23
While some parent heteracalixaromatics, such as azacalix[n]
pyridines show binding ability toward guest species ranging from
metal ions,11e13,15 organometallic clusters14 to neutral organic
molecules,8e10 tailor-made functionalized heteracalixaromatics,
which are not widely available, are however much more demanded
for the recognition of specific guests and molecular self-assembly
in supramolecular chemistry study. One of the synthetic methods
is based on post-macrocyclization chemical manipulations.1,3 Using
parent heteracalixaromatics as platforms, selective chemical
modifications on aromatic rings by means of electrophilic21,24 and
nucleophilic aromatic substitution reactions22,25 and arene CeH
bond transformations26 have led to the generation of (multi)func-
tionalized macrocycles. Functional groups have been introduced
readily into the bridging nitrogen atoms through N-alkylation24
and N-arylation,22d yielding uniquely functionalized azacalixar-
omatics. It is noteworthy that post-macrocyclization functionali-
zations in controllable manners enable the fabrication of higher
level and sophisticated molecular architectures including heter-
acalixcrowns and molecular cages.22c,f,g The other route to func-
tionalized heteracalixaromatics comprises the stepwise fragment
coupling reactions employing pre-functionalized aromatic re-
actants.1,3,10 The value of the method has been demonstrated, for
* Corresponding author. E-mail addresses: wangmx@mail.tsinghua.edu.cn,
mxwang@iccas.ac.cn (M.-X. Wang).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.08.069

S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
9465
example, in the synthesis of functionalized inherently chiral
heteracalixaromatics.22b
One of the salient structural features of heteracalix[4]aromatics
is their conformational preference for
a 1,3-alternate con-
formation.1e3 Only in the case of synthesis under kinetic controlled
conditions, products of a pinched partial core conformation are
obtained in low yields.22e The predominant formation of 1,3-
alternate conformation of heteracalix[4]aromatics is mainly at-
tributable to the dipoleedipole repulsions among aromatic rings,27
which is in stark contrast to conventional calix[4]arenes, which
adopt core conformation mainly because of intramolecular circular
hydrogen bond interactions.6 To construct other conformational
structures, we attempted previously the replacement of benzene
rings in tetraoxacalix[2]arene[2]triazines with phenol moieties.10
Although two phenolic hydroxyl groups are present in the lower
rim positions, the resulting dihydroxylated tetraoxacalix[2]arene
[2]triazines still give 1,3-alternate conformation. It is most probably
that no effective intramolecular hydrogen bonds are formed be-
tween phenolic hydroxyl groups and triazine rings because of un-
favorable hydrogen bond angles, yielding therefore no regulating
effect on conformations of macrocycles. Nevertheless, we envi-
sioned that the effective intramolecular hydrogen bonding be-
tween approximal aromatic rings or between groups attached on
two approximal aromatic rings would enable the generation of
conformational structures other than a 1,3-alternate conformer. To
test our hypothesis of tuning conformational structures by means
of intramolecular hydrogen bonding, and also to construct novel
and functional heteracalixaromatics, we undertook the current
study. We report herein the synthesis of tetraoxacalix[2]arene[2]
triazines in which benzene ring contains a hydrogen bond donor on
the lower rim position. We demonstrate for the first time the reg-
ulation of 1,3-alternate conformation to designed partial cone
structure of heteracalix[2]arene[2]triazines through the formation
of intramolecular hydrogen bond of the triazine moiety with its
neighboring benzylic alcohol.
Scheme 1. Synthesis of formyl-substituted tetraoxacalix[2]arene[2]triazine 4a.
1,3,5-triazine 5 at 0 ꢀC resulted in the formation of a linear trimer 6a
in 75% yield of 6b in 67% yield. Being slightly less reactive than 3,
higher temperature was necessary to promote the reaction of 6
with 1b. To our delight, refluxing a mixture of 6 and 1b with Cs2CO3
in acetonitrile gave rise to the formation of macrocyclic products.
The monoformyl-substituted tetraoxacalix[2]arene[2]triazine 7a
was isolated in 70% yield while a yield of 45% was obtained for bis-
aldehyde product 7b (Scheme 2). The symmetrically diformylated
macrocycle 7b was also prepared from a one-pot reaction between
1b and 5, and the chemical yield was around 30%. As we expected,
both products 7a and 7b were soluble in common organic solvents.
Having had aldehyde compounds 7 in hand, the synthesis of
hydroxymethyl-substituted tetraoxacalix[2]arene[2]triazines was
conducted. The reduction of aldehyde 7a into alcohol was not trivial
at all. When NaBH4 was used as a reducing agent in methanol,
macrocyclic ring opening along with the reduction of aldehyde was
always observed even when the reaction was performed at ꢁ10 ꢀC.
At room temperature, treatment of 7a with NaBH4 in methanol
afforded linear product 8 in <15% yield (Scheme 3). To avoid
macrocyclic ring opening reaction due to the nucleophilic attack of
methanol at triazine ring, tetrahydrofuran (THF) was employed as
solvent. Unfortunately, reaction gave a mixture of very polar and
not separable compounds. A small amount of products 10 and 11
were isolated after work-up and silica gel column chromatography.
The structures of the products 10 and 11, which were confirmed
unambiguously by single crystal X-ray crystallography (see
Supplementary data), indicated a hydrolytic macrocyclic ring
opening process followed by reductive amination and further hy-
drolysis (Scheme 3).
2. Results and discussion
Because of the unfavorable intramolecular hydrogen bond in-
teractions, 1,3-alternate dihydroxy-substituted tetraoxacalix[2]
arene[2]triazines form intermolecular hydrogen bonds between
phenolic hydroxyl and triazine nitrogen moieties, yielding a tetra-
meric aggregator in the solid state. To facilitate the formation of
intramolecular hydrogen bonds between hydroxyl and its adjacent
triazine ring, we decided to replace the phenol segment by
a
hydroxymethylbenzene (benzylic alcohol) and an amino-
methylbenzene (benzylamine) unit.10 To obtain the desired prod-
ucts, we started with the synthesis of tetraoxacalix[2]arene[2]
triazines 4 that bear versatile formyl group(s) at the lower rim
position of the benzene ring(s). The fragment coupling strategy,1
which works nicely for the synthesis of unsymmetrically
substituted heteracalixaromatics, was employed. As depicted in
Scheme 1, a linear trimer 3a,22a obtained conveniently from the
reaction between resorcinol 1a and cyanuric chloride 2, underwent
macrocyclization reaction with 2,6-dihydroxybenzaldehyde 1b at
ambient temperature in acetone to afford target macrocycle 4a in
50% yield. Surprisingly, the reaction between 3b and 1b under
identical conditions led to oligomers rather than macrocyclic
compound 4b. The aldehyde 4a was found not soluble in most of
the organic solvents, prohibiting its chemical transformations into
other functional groups.
To circumvent solubility problems, we then introduced dipro-
pylamino group into the triazine ring. In the presence of diisopro-
pylethylamine (DIPEA) as an acid scavenger, two aromatic
nucleophilic substitution reaction of resorcinol 1a or 2,6-dihy-
droxybenzaldehyde 1b with 4,6-dichloro-2-dipropylamino-
Fortunately, reduction of 7 using borane as a reducing agent in
dry THF gave satisfactory results. For example, in the presence of
2 equiv of BH3$THF, aldehyde 7a was converted quantitatively into
12a within 0.5 h at ꢁ20 ꢀC. Pure product 12a was obtained in 25%

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S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
from solution of 7b or 12b in a mixture of acetone and petroleum
ether and from solution of 17d or 18b in acetonitrile gave X-ray
quality single crystals. Molecular structures of symmetrically bis-
substituted macrocyclic products 7b, 12b, 17d and 18b were then
determined by X-ray diffraction analysis. As shown in Figs. 1e3,
tetraoxacalix[2]arene[2]triazines containing two aldehyde (7b),
ester (17d) or carboxylic acid (18b) groups adopt 1,3-alternate
conformation. In all cases, two benzene rings are nearly face-to-
face paralleled with the distances between upper rim carbons
ꢀ
and between lower rim carbons being around 4.2e4.4 A and
ꢀ
5.1e5.8 A, respectively. Two triazine rings, on the other hand, tend
to be procumbent relative to the plain defined by four bridging
oxygen atoms, giving the upper-rim and lower rim distances
ꢀ
ꢀ
around 9.0e9.4 A and 4.6 A, respectively. Careful scrutiny of the
bond lengths and angles of bridging oxygen atoms (see captions of
Figs. 1e3) revealed the formation of conjugation of bridging oxygen
atom with its neighboring triazine ring rather than benzene ring.
Bishydroxymethyl-substituted macrocycle 12b, however, gives the
flattened pinched cone conformation. Two triazine rings are edge-
to-edge aligned on the same plain as four bridging oxygen atoms
while two benzene rings, which are anti-paralleled each other, are
almost perpendicular to the plain (Fig. 4). Distances of a hydroxyl
oxygen atom to each lower rim nitrogen atom of two triazine rings
ꢀ
ꢀ
are 3.306 A and 3.156 A, respectively, indicating weak OeH/N
interactions.
To get deep insight into the conformational structures of func-
tional tetraoxacalix[4]aromatics in solution phase, 1H NMR and 13
C
NMR spectra were carefully examined. In agreement to parent
tetraoxacalix[2]arene[2]triazines,1,22a each symmetrically bis-
substituted tetraoxacalixarene, such as 7b, 17b, 17d or 18b gave
a simple and single set of proton and carbon resonance signals in 1H
and 13C NMR spectra, respectively (Figs. 5 and 6, see Supplementary
data). Since the substituents on both benzene and triazine rings
prohibit the rotation of each aromatic ring around the annulus, the
outcomes of NMR study suggested the predominant adoption of
symmetric 1,3-alternate conformation of these macrocycles. While
functional tetraoxacalix[2]arene[2]triazines substituted by only
one formyl (7a), benzyl ester (17a), methyl ester (17c) or carboxylic
acid (18a) group exhibit analogously the single set of proton and
carbon signals as compounds 7b,17b,17d and 18b (Figs. 5 and 6, see
Supplementary data), mono-hydroxymethylated and mono-
aminomethylated tetraoxacalix[2]arene[2]triazines 12a and 13
give NMR spectra in which two different sets of signals corre-
sponding to two different triazine rings within a macrocycle were
observed (Figs. 5 and 6). In different solvents, such as in CDCl3,
CD3COCD3, even in the presence of a small amount of D2O, similar
1H NMR spectra were observed for compound 12a. Desymmetri-
zation of molecular structures or the broken of symmetric re-
lationship of two triazine rings, which were evidenced by NMR
spectroscopic data, was best explained by the formation of intra-
molecular hydrogen bond between hydroxyl or amino group and
one of the triazine rings of macrocyclic compounds. Gratifyingly,
the intramolecular hydrogen bonding was validated by the variable
temperature 1H NMR spectra of 12a and 13. As illustrated in Fig. 7,
the proton signal of benzylic alcohol of 12a shifted gradually from
7.2 ppm to 8.3 ppm when the temperature decreased from 25 ꢀC
to ꢁ60 ꢀC. Similar downfield shift of the signal of hydroxyl proton of
Scheme 2. Synthesis of soluble formyl-functionalized tetraoxacalix[2]arene[2]tri-
azines 7.
isolated yield after silica gel column chromatography, since it un-
derwent decomposition during purification process. Similar re-
duction of 7b with BH3$THF at room temperature afforded diol
product 12b in 90% isolated yield (Scheme 4).
To synthesize aminomethyl-substituted tetraoxacalix[2]arene
[2]triazine 13, reductive amination of 7a was investigated. Under
various reaction conditions, such as using NH4Cl and NH2OH$HCl as
nitrogen sources and NaBH4, NaBH(OAc)3, borane and H2/Pd/C as
reducing agents at different temperatures, no desired product was
obtained (see Supplementary data). Finally, interaction of 7a with
a mixture of NH3 solution in 2-propanol (2 M), acetic acid (2 equiv)
and NaBH(OAc)3 (4 equiv) in dry THF gave product 13, albeit in 20%
yield (Scheme 5).
In addition to hydroxyl and amino groups that may function as
intramolecular hydrogen bond donors, carboxylic acid was also
considered in our study. Scheme 6 illustrates the synthesis of tet-
raoxacalix[2]arene[2]triazines that contain one and two carboxylic
acid groups. Thus, macrocyclic condensation reaction between
a linear trimer 14 and benzyl or methyl 2,6-dihydroxybenzoate 15
in hot acetonitrile with the aid of K2CO3 yielded macrocycle 17a or
17c efficiently. Good yields were also obtained for the synthesis of
bis-ester-bearing tetraoxacalix[2]arene[2]triazines 17b and 17d
from 15 and 4,6-dichloro-2-methoxy-1,3,5-triazine 16 in one-pot
reaction fashion. Debenzylation reaction of 17a and 17b, which
was achieved efficiently by means of catalytic hydrogenolysis, fur-
nished mono- and bis-carboxylic acid products 18a and 18b in 90%
and 78% yields, respectively (Scheme 6).
12b was also observed albeit with
a lower degree (see
Supplementary data). It should be noted that the chemical shift of
the hydroxyl proton signal of bis-hydroxymethylated compound
12b varied dramatically from that of mono-hydroxymethylated
analog 12a. While the former compound gave a hydroxyl proton
signal at 2.41 ppm, the hydroxyl proton of the later compound
resonated at 8.30 ppm at 213 K. The huge difference in chemical
shift between two compounds is the reflection of different con-
formational structures (vide infra).
The structures of all functionalized tetraoxacalix[2]arene[2]tri-
azine products were established on the basis of spectroscopic data
and microanalyses. To assign the structures without ambiguity, and
also to understand the conformational behaviors in the solid state,
single crystals of products were grown. Slow evaporation of solvent

S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
9467
Scheme 3. Reduction of 7a with NaBH4 in methanol.
Scheme 5. Synthesis of aminomethylated tetraoxacalix[2]arene[2]triazine 13.
To shed further light on the three dimensional structures of
formyl-substituted compounds 7a and 7b, and hydroxymethyl-
substituted compounds 12a and 12b in solution, nuclear Over-
hauser effect spectroscopy (NOESY) was studied. At room temper-
ature, no nuclear Overhauser effect (NOE) was found for all
compounds (see Supplementary data). When the probe tempera-
ture was decreased to 193 K, the NOE was clearly evidenced
Scheme 4. Synthesis of hydroxymethylated tetraoxacalix[2]arene[2]triazines 12.

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S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
and 13C NMR spectra (Figs. 5 and 6), the deshielding effect of hy-
droxyl proton and the NOE between hydroxyl proton and Hc of the
other benzene ring into account, it was proposed that tetraoxacalix
[2]arene[2]triazine 12a adopts a heavily distorted partial cone
conformation in solution (Fig. 10, inset). It should be noted that, in
the case of tetraoxacalix[2]arene[2]triazines 18a and 18b that were
substituted by one and two carboxylic acid groups, respectively,1,3-
alternate conformational structure was found exclusively in solu-
tion and in the crystalline state. This was probably attributable to
the unfavorable geometry of the rigid carboxylic acid group at the
lower rim position to form hydrogen bond with the nitrogen atom
of the adjacent triazine ring.
3. Conclusion
Utilizing the fragment coupling approach and the post-
macrocyclization chemical manipulation protocol, a number of
tetraoxacalix[2]arene[2]triazines functionalized on the lower rim
position with one or two aldehyde, ester, carboxylic acid, hydrox-
ymethyl and aminomethyl substituents have been synthesized
from cheap and commercially available starting materials. While all
tetraoxacalix[2]arene[2]triazines containing electron-withdrawing
group(s) adopt 1,3-alternate conformation both in solution and in
the solid state, bis-hydroxymethyl-, mono-hydroxymethyl- and
mono-aminomethyl-substituted tetraoxacalix[2]arene[2]triazines
exist as pinched or distorted partial cone conformers due to the
intramolecular hydrogen bond interactions between hydroxyl or
amino group and triazine ring. Our study has provided the first
example to regulate the conformational structures of heteracalix-
aromatics through the intramolecular hydrogen bond interactions.
It is anticipated that enhancement of rationally designed intra-
molecular non-covalent bond interactions on the skeleton of
heteracalixaromatics would lead to the generation of desired con-
formers other than 1,3-alternate conformational structures.
4. Experimental part
4.1. Synthesis of 4a
Scheme 6. Synthesis of ester and acid-functionalized tetraoxacalix[2]arene[2]triazines
17 and 18.
To a stirred solution of DIPEA (2.4 mmol) in acetone (120 mL) at
room temperature was added simultaneously both solutions of
3a22a (1 mmol) in acetone (50 mL) and 1b (1 mmol) in acetone
(50 mL) through two dropping funnels at the same rate. After ad-
dition (ca. 10 h), the mixture was kept stirring for another 14 h. The
solvent was removed using a rotary evaporator, and the residue was
chromatographed on a silica gel column eluted with a mixture of
acetone and petroleum ether (1:2) to give product 4a. Re-
crystallization in ethanol gave pure product as a pale yellow solid.
Compound 4a (236 mg, 50%): mp 285e286 ꢀC; 1H NMR (300 MHz,
between aldehyde proton and the lower rim proton of the distal
benzene ring for monoformyl-substituted tetraoxacalix[2]arene[2]
triazine 7a (Fig. 8 and Supplementary data), while bisformyl-
substituted analog 7b did not show spacial correlation between
formyl and benzene ring (see Supplementary data). The outcomes
are in agreement with their 1,3-alternate conformational struc-
tures. In other words, tetraoxacalix[2]arene[2]triazines that are
substituted by one and two electron-withdrawing groups, such as
aldehyde (7a and 7b), ester (17aed) and carboxylic acid (18a and
18b) at the lower rim position adopt in solution 1,3-alternate
conformation, the structure that is similar to the X-ray crystal
structures observed for 7b, 17d and 18b (Figs. 1e3). In the case of
acetone-d6)
1H), 7.24 (d, J¼8.2 Hz, 2H), 7.04 (dd, J¼8.2, 1.9 Hz, 2H), 6.94 (s, 1H);
13C NMR (75 MHz, DMSO-d6)
186.7, 172.7, 171.9, 171.7, 151.6, 151.1,
1699,
d
9.90 (s, 1H), 7.75 (t, J¼8.3 Hz, 1H), 7.42 (t, J¼8.2 Hz,
d
bis-hydroxymethylated
tetraoxacalix[2]arene[2]triazine
12b,
136.8, 130.9, 121.5, 119.8, 119.6, 115.9; IR (KBr) IR (KBr) n
NOESY spectrum at 193 K shows correlation between all protons of
hydroxymethyl substituent and aromatic protons of the distal
benzene ring (Fig. 9), reflecting the proximity of two moieties. In
solution, the macrocycle 12b adopts therefore most likely the
pinched partial cone conformation that is similar to its structure in
the solid state (Fig. 4). The pinched partial cone also best explains
the strong shielding effect of hydroxyl proton experienced, as hy-
droxyl proton is positioned above the centroid of the other benzene
ring (see Fig. 4 and Supplementary data). Interestingly, as indicated
by low temperature NOESY spectrum (Fig.10), only hydroxyl proton
1550 cmꢁ1; MS (MALDI-TOF) m/z 471.1 [MþH]þ (100%), 473.1
[MþHþ2]þ (40), 493.1 [MþNa]þ (52), 495.1 [MþNaþ2]þ (20). Anal.
Calcd for C19H8Cl2N6O5$C2H5OH: C, 48.76; H, 2.73; N, 16.25. Found:
C, 48.62; H, 2.36; N, 16.36.
4.2. General procedure for the synthesis of 6
After a mixture of 528 (4 mmol) and Cs2CO3 (5 mmol) in ace-
tonitrile (40 mL) was heated to 60 ꢀC, compound 1 (2 mmol) dis-
solved in acetonitrile (20 mL) was added drop-wise. The resulting
mixture was kept stirring at 60 ꢀC for another 6 h. After removal of
precipitate through filtration and solvent using a rotary evaporator,
the residue was chromatographed on a silica gel column eluted
(d
¼8.30 ppm), which is hydrogen bonded to one of the triazine ring
within macrocycle 12a, was found to interact with Hc of the other
benzene ring. Taking the unsymmetric structure reflected by 1H

S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
9469
ꢀ
Fig. 1. X-ray molecular structure of 7b with top view (A, C) and side view (B). Selected bond lengths [A]: C2eO1 1.354, O1eC4 1.396, C3eO2 1.348, O2eC9 1.399. Selected angles:
:C5eC4eO1 121.5ꢀ, :C4eO1eC2 119.6ꢀ, :O1eC2eN2 117.8ꢀ. The probability is 25%. Some hydrogen atoms were omitted for clarity.

9470
S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
ꢀ
Fig. 2. X-ray molecular structure of 17d with top view (A, C) and side view (B). Selected bond lengths [A]: C3BeO2B 1.334, O2BeC4B 1.402, C8BeO5B 1.400, O5BeC10B 1.338,
C12BeO7B 1.347, O7BeC13B 1.406, C17BeO10B 1.403, O10BeC1B 1.349. Selected angles: :C9BeC4BeO2B 121.4ꢀ, :C4BeO2BeC3B 118.4ꢀ, :O2BeC3BeN3B 118.8ꢀ. The probability
is 25%. Hydrogen atoms were omitted for clarity.
with a mixture of ethyl acetate and petroleum ether (1:8) to give
product 6.
3.49 (t, J¼7.5 Hz, 4H), 3.26 (t, J¼7.5 Hz, 4H), 1.61 (h, J¼7.5 Hz, 4H),
1.45 (h, J¼7.5 Hz, 4H), 0.91 (t, J¼7.4 Hz, 6H), 0.73 (t, J¼7.4 Hz, 6H);
Compound 6a (800 mg, 75%): white solid: mp 115e116 ꢀC; 1H
13C NMR (75 MHz, CDCl3)
d 171.0, 170.2, 165.5, 152.4, 129.2, 119.1,
NMR (300 MHz, CDCl3)
d
7.38 (t, J¼8.1 Hz, 1H), 7.08e7.05 (m, 3H),
115.9, 49.6, 49.5, 20.8, 20.5, 11.2, 11.0; IR (KBr) n ;
1578, 1507 cmꢁ1

S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
9471
ꢀ
Fig. 3. X-ray molecular structure of 18b with top view (A, C) and side view (B). Selected bond lengths [A]: C1eO3 1.346, O3eC5 1.407, C7eO2 1.407, O2eC3 1.342. Selected angles:
:C8eC7eO2 116.2ꢀ, :C7eO2eC3 117.4ꢀ, :O2eC3eN3 118.5ꢀ. The probability is 25%. Some hydrogen atoms were omitted for clarity.

9472
S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
ꢀ
Fig. 4. X-ray molecular structure of 12b with top view (A, C) and side view (B). Selected bond lengths [A]: C4eO1 1.407, O1eC3 1.358, C2eO2 1.362, O2eC6 1.409. Selected angles:
:C9eC4eO1 117.0ꢀ, :C4eO1eC3 115.9ꢀ, :O1eC3eN2 117.3ꢀ. The probability is 25%. Some hydrogen atoms were omitted for clarity.
MS (MALDI-TOF) m/z 535.1 [MþH]þ (50), 537.1 [MþHþ2]þ (20),
557.0 [MþNa]þ (100%), 559.0 [MþNaþ2]þ (50). Anal. Calcd for
C24H32Cl2N8O2: C, 53.83; H, 6.02; N, 20.93. Found: C, 53.94; H, 6.00;
N, 20.66.
11.3; IR (KBr) n
3431, 2965, 2874, 1698, 1589, 1502, 1362 cmꢁ1; MS
(MALDI-TOF) m/z 585.0 [MþH]þ (100%), 586.0 [Mþ2]þ (20), 587.0
[MþHþ2]þ (60), 588.0 [Mþ4]þ (10). Anal. Calcd for C25H32Cl2N8O3:
C, 53.29; H, 5.72; N, 19.89. Found: C, 53.39; H, 5.81; N, 19.75.
Compound 6b (783 mg, 67%): pale yellow solid: mp 130e131 ꢀC;
1H NMR (300 MHz, acetone-d6)
d
10.19 (s, 1H), 7.87 (t, J¼8.3 Hz, 1H),
4.3. Synthesis of 7a
7.38 (d, J¼8.3 Hz, 2H), 3.54 (t, J¼7.5 Hz, 4H), 3.33 (t, J¼7.5 Hz, 4H),
1.64 (h, J¼7.5 Hz, 4H), 1.49 (h, J¼7.5 Hz, 4H), 0.90 (t, J¼7.4 Hz, 6H),
To a boiling solution of Cs2CO3 (2.5 mmol) in acetonitrile
(250 mL) was added simultaneously both solutions of 6a22a
(1 mmol) in acetonitrile (50 mL) and 1b (1 mmol) in acetonitrile
0.73 (t, J¼7.4 Hz, 6H); 13C NMR (75 MHz, acetone)
d 187.2, 171.6,
171.5, 166.5, 154.1, 136.1, 122.7, 122.4, 50.4, 50.2, 21.4, 21.1, 11.4,

S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
9473
Fig. 6. Partial 13C NMR spectra of compounds 7a, 12a, 13, 18a, 7b, 12b and 18b (from
top to bottom).
Fig. 5. Partial 1H NMR spectra of compounds 7a, 12a, 13, 18a, 7b, 12b and 18b (from top
to bottom).
(50 mL) through two dropping funnels at the same rate. After ad-
dition (ca. 1 h), the mixture was kept stirring for another 1 h. The
solvent was removed using a rotary evaporator, and the residue was
chromatographed on a silica gel column eluted with a mixture of
acetone and petroleum ether (1:4) to give product 7a (421 mg, 70%)
as a white solid. Compound 7a: mp 120e121 ꢀC; 1H NMR (300 MHz,
CDCl3)
d
9.90 (s, 1H), 7.39 (t, J¼8.2 Hz,1H), 7.12 (t, J¼8.1 Hz, 1H), 6.90
(d, J¼8.2 Hz, 2H), 6.76 (dd, J¼8.1, 2.2 Hz, 2H), 6.52 (t, J¼2.2 Hz, 1H),
3.70e3.52 (m, 8H), 1.77e1.66 (m, 8H), 0.98 (t, J¼7.4 Hz, 12H); 13C
NMR (75 MHz, CDCl3) d 186.2, 172.0, 171.8, 167.7, 153.3, 152.3, 134.9,
129.3, 121.9, 120.8, 119.0, 117.3, 49.3, 20.8, 11.3; IR (KBr)
n 1699,
1605 cmꢁ1; MS (MALDI-TOF) m/z 601.4 [MþH]þ (100%). Anal. Calcd
for C31H36N8O5: C, 61.99; H, 6.04; N, 18.65. Found: C, 62.29; H, 6.32;
N, 18.90.
4.4. Synthesis of 7b
To a refluxing solution of 6b (1 mmol) and Cs2CO3 (2.5 mmol) in
acetonitrile (120 mL) was added drop-wise a solution of 1b
(1 mmol) in acetonitrile (50 mL). After addition (ca. 0.5 h), the re-
action was finished. The solvent was removed using a rotary
evaporator, and the residue was chromatographed on a silica gel
column eluted with a mixture of acetone and petroleum ether (1:2)
to give product 7b (283 mg, 45%) as a white solid. Compound 7b:
mp 215e216 ꢀC; 1H NMR (300 MHz, acetone-d6)
d 9.72 (s, 2H), 7.57
(t, J¼8.2 Hz, 2H), 7.01 (d, J¼8.2 Hz, 4H), 3.73e3.54 (m, 8H),
1.82e1.70 (m, 8H), 0.98 (t, J¼7.4 Hz, 12H); 13C NMR (75 MHz, ace-
tone-d6)
11.5; IR (KBr)
d 186.3, 172.7, 168.8, 154.1, 136.2, 122.9, 121.8, 50.1, 21.5,
n
1699, 1606 cmꢁ1; MS (MALDI-TOF) m/z 629.3
Fig. 7. Partial variable temperature 1H NMR spectra of 12a.

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S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
Fig. 8. NOESY spectra of 7a at 213 K.
Fig. 9. NOESY spectra of 12b at 193 K.

S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
9475
Fig. 10. NOESY spectra of 12a at 213 K.
[MþH]þ (100%), 651.2 [MþNa]þ (80), 667.2 [MþK]þ (20). Anal.
Calcd for C32H36N8O6: C, 61.13; H, 5.77; N, 17.82. Found: C, 60.97; H,
5.78; N, 17.58.
added, and the resulting mixture was extracted with ethyl acetate
(3ꢂ10 mL). Organic layers were combined and dried with anhy-
drous MgSO4. After filtration, the filtrate was concentrated under
reduced pressure, and the residue was chromatographed on a silica
gel column eluted with a mixture of acetone and petroleum ether
(1:4) to give very small amount of products 10 and 11. Slow evap-
oration of the solvent from different fractions that contained 10 and
11 gave single crystals of 10 and 11, respectively.
4.5. Reduction of 7a with NaBH4 in methanol
To a solution of 7a (1 mmol) in methanol (10 mL) at room
temperature was added portion-wise NaBH4 (1 mmol). After stir-
ring for additional 2 h, a saturated aqueous solution of NH4Cl
(10 mL) was added, and the resulting mixture was extracted with
ethyl acetate (3ꢂ10 mL). Organic layers were combined and dried
with anhydrous MgSO4. After filtration, the filtrate was concen-
trated under reduced pressure, and the residue was chromato-
graphed on a silica gel column eluted with a mixture of acetone and
petroleum ether (1:2) to give product 8 (ca. 98 mg, <15%) as a white
4.7. General procedure for the synthesis of 12
To an argon protected solution of 7 (0.5 mmol) in dry THF
(10 mL) at ꢁ20 ꢀC (7a) or at room temperature (7b) was added
BH3$THF solution in THF (1 M, 1 mL). The resulting mixture was
stirred at ꢁ20 ꢀC (7a) or at ambient temperature (7b) for another
0.5 h. The reaction was quenched by adding a saturated NH4Cl
solution in water (20 mL), and the mixture was extracted with
ethyl acetate (3ꢂ20 mL). The organic layers were combined and
dried with anhydrous MgSO4. After removal of drying agent by
filtration and of solvent by evaporation under a reduced pressure,
the residue was chromatographed on a silica gel column eluted
with a mixture of ethyl acetate and petroleum ether (1:6) to afford
product 12.
solid: mp 150e151 ꢀC; 1H NMR (300 MHz, acetone-d6)
d 11.40 (s,
1H), 8.63 (s, 1H), 7.48 (t, J¼8.1 Hz, 1H), 7.26e7.10 (m, 4H), 6.81 (d,
J¼8.1 Hz, 1H), 6.65 (d, J¼8.0 Hz, 1H), 4.51 (s, 2H), 3.50e3.31 (m, 8H),
3.27 (s, 3H), 1.64e1.43 (m, 8H), 0.93e0.63 (m, 12H); 13C NMR
(75 MHz, acetone-d6)
d 173.4, 172.9, 167.7, 158.4, 153.8, 153.0, 152.9,
130.1,129.7, 120.1,119.7,117.7,117.0,114.5,113.8, 65.6, 58.1, 50.7, 50.1,
50.0, 49.7, 21.8, 21.5, 21.4, 11.4, 11.3, 11.2; IR (KBr) n 3429, 1591, 1559,
1530 cmꢁ1; MS (MALDI-TOF) m/z 657.4 [MþNa]þ (100). Anal. Calcd
for C32H42N8O6: C, 60.55; H, 6.67; N, 17.65. Found: C, 60.41; H, 6.70;
N, 17.54.
Compound 12a (75 mg, 25%): oil; 1H NMR (600 MHz, CD2Cl2)
d
7.45 (t, J¼8.2 Hz, 1H), 7.22 (t, J¼8.1 Hz, 2H), 7.09 (t, J¼8.2 Hz, 2H),
7.03 (s,1H), 6.93 (d, J¼8.1 Hz,1H), 6.78 (d, J¼8.3 Hz,1H), 5.52 (s, 2H),
3.58e3.37 (m, 8H), 1.72e1.52 (m, 6H), 0.96 (t, J¼7.4 Hz, 9H), 0.82 (t,
4.6. Reduction of 7a with NaBH4 in THF
J¼7.4 Hz, 3H); 13C NMR (75 MHz, CD2Cl2)
d 173.7, 172.1, 171.2, 171.1,
To a solution of 7a (1 mmol) in THF (10 mL) at room temperature
was added portion-wise NaBH4 (1 mmol). After stirring for addi-
tional 2 h, a saturated aqueous solution of NH4Cl (10 mL) was
168.0, 167.9, 156.4, 153.6, 152.8, 151.3, 130.3, 128.9, 118.4, 118.1, 117.3,
115.9, 115.0, 114.5, 64.7, 50.1, 49.8, 49.6, 49.5, 21.2, 21.0, 11.4, 11.3; IR
(KBr)
n
3432, 1592, 1523 cmꢁ1; MS (MALDI-TOF) m/z 603.3 [MþH]þ

9476
S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
(100%), 625.3 [MþNa]þ (50). Anal. Calcd for C31H39N8O5: m/z:
603.3038. FT-ICRMS: 603.3025 [MþH]þ.
acetone-d6) d 175.8,174.5,174.0,163.3,153.3,151.9,133.9,131.1,121.8,
119.9, 119.7, 117.5, 56.2, 52.7; IR (KBr)
n
1728, 1590, 1564 cmꢁ1; MS
Compound 12b (285 mg, 90%): white solid; mp 235e236 ꢀC; 1H
(ESI) m/z 514.9 [MþNa]þ (100%). Anal. Calcd for C22H16N6O8$CH3OH:
NMR (300 MHz, CD2Cl2)
d
7.32 (t, J¼8.1 Hz, 2H), 6.95 (d, J¼8.1 Hz,
C, 52.67; H, 3.84; N, 16.02. Found: C, 52.98; H, 3.55; N, 15.94.
4H), 4.27 (d, J¼5.8 Hz, 4H), 3.74e3.54 (m, 8H), 1.84e1.70 (m, 10H),
1.03 (t, J¼7.4 Hz, 12H); 13C NMR (75 MHz, CD2Cl2)
d
173.0, 167.9,
4.10. General procedure for the synthesis of 17b and 17d
153.5, 130.3, 129.0, 120.6, 50.0, 21.1, 11.5; IR (KBr)
n 3484, 1609, 1579,
1518 cmꢁ1; MS (MALDI-TOF) m/z 633.4 [MþH]þ (30), 655.4
[MþNa]þ (100%), 671.4 [MþK]þ (22). Anal. Calcd for C32H40N8O6: C,
60.75; H, 6.37; N, 17.71. Found: C, 60.62; H, 6.45; N, 17.44.
A mixture of 15a (1 mmol) or 15b (1 mmol), 16 (1 mmol) and
K2CO3 (2 mmol) in acetonitrile (100 mL) was refluxed for 1 h. After
removal of solvent under reduced pressure, the residue was chro-
matographed on a silica gel column eluted with a mixture of ace-
tone and petroleum ether (1:3) to give crude product.
Recrystallization from ethanol gave pure compound 17b or 17d.
Compound 17b (456 mg, 65%): white solid; mp 160e161 ꢀC; 1H
4.8. Synthesis of 13
NH3 solution in a mixture of 2-propanol and THF was prepared
by mixing commercially available NH3 solution in 2-propanol (2 M,
0.5 mL) with dry THF (1.5 mL), while acetic acid solution was ob-
tained by dissolving glacial acetic acid (60 mg, 1 mmol) in dry THF
(2 mL). Under argon protection and at room temperature, both
solutions were added simultaneously through two syringes at the
same rate using a syringe pump to the solution of 7a (0.5 mmol) in
dry THF (10 mL). After stirring for 0.5 h, NaBH(OAc)3 (2 mmol) was
added and the resulting mixture was kept stirring for another 1.5 h.
The reaction was quenched by adding hydrochloric acid (1 M, 5 mL)
and water (20 mL). The aqueous mixture was extracted with ethyl
acetate (3ꢂ20 mL), and organic layers were combined and dried
with anhydrous MgSO4. After filtration, the filtrate was concen-
trated under reduced pressure and residue was chromatographed
on a silica gel column eluted with a mixture of ethyl acetate and
petroleum ether (1:8) to give pure compound 13. 13 (60 mg, 20%):
NMR (300 MHz, acetone-d6)
d
7.52 (t, J¼8.2 Hz, 2H), 7.35 (s, 10H),
7.05 (d, J¼8.2 Hz, 4H), 5.08 (s, 4H), 3.94 (s, 6H); 13C NMR (75 MHz,
acetone-d6)
d 174.6, 173.4, 161.2, 151.9, 135.6, 133.3, 128.5, 128.3,
128.1, 121.6, 118.5, 67.0, 55.1; IR (KBr)
n
1723, 1598, 1563 cmꢁ1; MS
(MALDI-TOF) m/z 725.0 [MþNa]þ (100%). Anal. Calcd for
C36H26N6O10: C, 61.54; H, 3.73; N, 11.96. Found: C, 61.69; H, 3.87; N,
11.87.
Compound 17d (413 mg, 75%): white solid; mp 135e136 ꢀC;
1H NMR (300 MHz, acetone-d6)
d
7.53 (t, J¼8.2 Hz, 2H), 7.06 (d,
J¼8.2 Hz, 4H), 4.10 (s, 6H), 3.56 (s, 6H); 13C NMR (75 MHz,
DMSO-d6)
52.1; IR (KBr)
d 174.1, 172.9, 161.0, 151.3, 133.9, 121.7, 116.8, 55.8,
n
1723, 1597, 1572, 1425, 1359 cmꢁ1; MS (MALDI-
TOF) m/z 573.2 [MþNa]þ (100%). Anal. Calcd for
C19H8Cl2N6O8$C2H5OH: C, 52.35; H, 4.06; N, 14.09. Found: C,
52.57; H, 3.82; N, 13.90.
oil; 1H NMR (300 MHz, acetone-d6)
d
7.42 (t, J¼8.2 Hz, 1H), 7.24 (s,
1H), 7.18 (t, J¼8.2 Hz, 1H), 7.01 (dt, J¼8.0, 2.1 Hz, 1H), 6.94 (d,
J¼8.4 Hz, 1H), 6.82 (d, J¼7.6 Hz, 1H), 6.82 (d, J¼7.6 Hz, 1H), 4.60 (d,
J¼4.8 Hz, 2H), 3.47e3.24 (m, 8H), 1.62e1.37 (m, 8H), 0.86 (t,
J¼7.4 Hz, 9H), 0.68 (t, J¼7.4 Hz, 3H); 13C NMR (75 MHz, CDCl3)
4.11. General procedure for the synthesis of 18a and 18b
Under H2 atmosphere with
a balloon, a mixture of 17a
(0.5 mmol) or 17b (0.5 mmol) and Pd(OH)2/C (20%, 200 mg) in
cyclohexane (20 mL) was stirred at room temperature for 12 h. The
catalyst was removed by filtration through a Celite pad, and the
filtrate was concentrated under reduced pressure. The residue was
chromatographed on a silica gel column eluted with a mixture of
acetone and petroleum ether (2:1) to give crude product. Pure
compound 18a or 18b was obtained from recrystallization from
ethanol.
d
173.3, 171.6, 170.3, 167.6, 167.5, 166.3, 156.3, 153.2, 152.9, 151.0,
129.4, 127.3, 117.7, 117.0, 116.7, 116.6, 114.6, 113.0, 48.7, 37.5, 20.8,
20.7, 11.2, 11.1; IR (KBr)
n
3424, 2963, 1589, 1527, 1379 cmꢁ1; MS
(MALDI-TOF) m/z 603.3 [MþH]þ (100%), 625.3 [MþNa]þ (50). Anal.
Calcd for C31H39N9O4: m/z: 602.3198. FT-ICRMS: 602.3191 [MþH]þ.
4.9. General procedure for the synthesis of 17a and 17c
Compound 18a (215 mg, 90%): white solid; mp 196e197 ꢀC; 1H
To a boiling solution of K2CO3 (2 mmol) in acetonitrile (120 mL)
was added simultaneously both solutions 14 (1 mmol) in acetoni-
trile (50 mL) and of 15a or 15b29,30 (1 mmol) in acetonitrile (50 mL)
through two dropping funnels at the same rate. After addition (ca.
2 h), the resulting mixture was refluxed for another 4 h. The solvent
was removed using a rotary evaporator, and the residue was
chromatographed on a silica gel column eluted with a mixture of
acetone and petroleum ether (1:5) to give crude product. Pure
compound 17a or 17c was obtained from recrystallization from
methanol.
NMR (300 MHz, acetone-d6)
d
7.52 (t, J¼8.2 Hz, 1H), 7.35 (t,
J¼8.2 Hz, 1H), 7.09 (d, J¼8.2 Hz, 2H), 6.95 (dd, J¼8.2, 2.2 Hz, 2H),
6.71 (t, J¼2.2 Hz, 1H), 4.08 (s, 6H); 13C NMR (75 MHz, acetone-d6)
d
174.9, 173.6, 173.1, 162.6, 152.4, 151.0, 132.6, 130.2, 120.8, 120.1,
119.0, 116.6, 55.2; IR (KBr) n
3429, 1725, 1593, 1574, 1362 cmꢁ1; MS
(ESI) m/z 479.1 [MþH]þ (100%). Anal. Calcd for C21H14N6O8$-
CH3OH: C, 51.77; H, 3.55; N, 16.47. Found: C, 52.00; H, 3.41; N,
16.18.
Compound 18b (204 mg, 78%): white solid; mp 227e228 ꢀC;
1H NMR (400 MHz, acetone-d6)
d
7.48 (t, J¼8.2 Hz, 2H), 7.03 (d,
Compound 17a (455 mg, 80%): white solid; mp 158e159 ꢀC; 1H
J¼8.2 Hz, 4H), 4.05 (s, 6H); 13C NMR (100 MHz, acetone-d6)
NMR (300 MHz, acetone-d6)
d
7.51 (d, J¼8.2 Hz, 1H), 7.34 (dd, J¼4.9,
d
174.8, 173.6, 161.7, 151.8, 132.5, 121.3, 119.3, 55.1; IR (KBr) n
1.7 Hz, 3H), 7.28 (d, J¼8.2 Hz, 1H), 7.19 (dd, J¼6.4, 3.2 Hz, 2H), 7.06
3428, 1727, 1598, 1571 cmꢁ1
; MS (MALDI-TOF) m/z 544.9
(d, J¼8.2 Hz, 2H), 6.88 (dd, J¼8.2, 2.2 Hz, 2H), 6.29 (t, J¼2.2 Hz, 1H),
[MþNa]þ (100%), 558.9 [MþK]þ (20). Anal. Calcd for
C28H20N6O8$C2H5OH: C, 50.82; H, 4.26; N, 3.68. Found: C, 51.34;
H, 4.30; N, 13.59.
5.03 (s, 2H), 4.06 (s, 6H); 13C NMR (75 MHz, acetone-d6)
d 174.8,
173.4, 173.0, 161.8, 152.3, 150.9, 135.2, 133.0, 130.1, 129.0, 128.6,
128.6, 121.0, 119.0, 118.9, 116.5, 67.1, 55.3; IR (KBr)
n 1725, 1570,
1427 cmꢁ1; MS (ESI) m/z 569.5 [MþH]þ (100%). Anal. Calcd for
C28H20N6O8: C, 59.16; H, 3.55; N, 14.78. Found: C, 59.22; H, 3.73;
N, 14.71.
Acknowledgements
Compound 17c (418 mg, 85%): white solid; mp 141e142 ꢀC; 1H
We thank the National Natural Science Foundation of China
(21132005, 21121004, 20972161), Ministry of Science and Tech-
nology (2011CB932501), Tsinghua University and the Chinese
Academy of Sciences for financial support.
NMR (300 MHz, acetone-d6)
d
7.53 (t, J¼8.2 Hz,1H), 7.34 (t, J¼8.2 Hz,
1H), 7.09 (d, J¼8.2 Hz, 2H), 6.94 (dd, J¼8.2, 2.2 Hz, 2H),
6.73 (t, J¼2.2 Hz, 1H), 4.08 (s, 6H), 3.56 (s, 3H); 13C NMR (75 MHz,

S. Pan et al. / Tetrahedron 68 (2012) 9464e9477
9477
18. Li, S.; Fa, S.-X.; Wang, Q.-Q.; Wang, D.-X.; Wang, M.-X. J. Org. Chem. 2012,
77, 1860.
Supplementary data
19. Chen, Y.; Wang, D.-X.; Huang, Z.-T.; Wang, M.-X. Chem. Commun. 2011, 8112.
20. (a) Tsue, H.; Ono, K.; Tokita, S.; Ishibashi, K.; Matsui, K.; Takahashi, H.; Miyata,
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Ishibashi, K.; Inoue, R.; Shimizu, S.; Takahashi, D.; Tamura, R. CrystEngComm
2012, 14, 1021.
1H and 13C NMR spectra of products, X-ray structures of 7b, 10,
11, 12b, 17d, 18b (CIFs). Supplementary data related to this article
can be found at http://dx.doi.org/10.1016/j.tet.2012.08.069.
21. Wang, L.-X.; Wang, D.-X.; Huang, Z.-T.; Wang, M.-X. J. Org. Chem. 2010, 75, 741.
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