Synthesis and Structure of Functionalized Homo Heteracalix[2]arene[2]triazines: Effect of All Heteroatom Bridges on Macrocyclic Conformation

Article abstract of DOI:10.1021/acs.joc.8b00284

A number of unprecedented homo heteracalix[2]arene[2]triazines were synthesized by means of a fragment coupling approach. Two directional nucleophilic substitution reactions of N-Boc-protected 1,3-dihydrazobenzene with cyanuric acid chloride and 2-butoxy-

Full text of DOI:10.1021/acs.joc.8b00284

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Article
Synthesis and Structure of Functionalized Homo
Heteracalix[2]arene[2]triazines: Effect of All
Heteroatom Bridges on Macrocyclic Conformation
Dong-Dong Liang, and Mei-Xiang Wang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00284 • Publication Date (Web): 01 Mar 2018
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Synthesis and Structure of Functionalized Homo
Heteracalix[2]arene[2]triazines: Effect of All Heteroatom Bridges on
Macrocyclic Conformation
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Dong-Dong Liang and Mei-Xiang Wang*
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology
(Ministry of Education), Department of Chemistry, Tsinghua University, Beijing
100084, China
wangmx@mail.tsinghua.edu.cn
ABSTRACT: A number of unprecedented homo heteracalix[2]arene[2]triazines were
synthesized by means of a fragment coupling approach. Two directional nucleophilic
substitution reactions of N-Boc-protected 1,3-dihydrazobenzene with cyanuric acid
chloride and 2-butoxy-4,6-dichloro-1,3,5-triazine led to hydrazo-linked trimers which
underwent efficient macrocyclic condensation reaction with functionalized resorcinol
derivatives to afford (NHNBoc)2,O2-calix[2]arene[2]triazine macrocycles which
contain a functional group either on the upper rime or the lower rim. The use of 1,3-
phenylenediamines instead of resorcinol in the reaction produced (NR)2,(NHNBoc)2-
calix[2]arene[2]triazines. Post-macrocyclization modifications such as nucleophilic
substitution reaction of chloro on triazine by amines and removal of Boc from hydrazo
moieties produced homo calix[2]arene[2]triazine derivatives. In the solid state,
(NHNR)2,O2-bridged calix[2]arene[2]triazines with and without a substituent on the
upper rim position, and (NMe)2,(NHNBoc)2-calix[2]arene[2]triazine adopted a typical
partial cone conformation while the heavily twisted 1,3-alternate conformational
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structures were observed for both (NHNBoc)2,O2-calix[2]arene[2]triazines bearing a
functional group on the lower rim position and (NH)2,(NHNBoc)2-
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calix[2]arene[2]triazine.
In
solution,
all
synthesized
homo
9
heteracalix[2]arene[2]triazines existed as the mixture of different macrocyclic
conformers which underwent slow inter-conversions at room temperature relative to
the NMR time scale.
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INTRODUCTION
Heteracalixaromatics1-4 are synthetic macrocycles which are composed of heteroatoms
and meta-(het)arylene in an alternative manner. Being different from conventional
calix[4]arenes, nitrogen and oxygen bridged calix[4]aromatics reported to date adopt
generally 1,3-alternate conformational structures both in crystalline state and in
solution.5 One of the salient structural features is that the bridging heteroatoms
participate in the formation of conjugation with their adjacent aromatic rings to various
degrees leading to the variable bond lengths and angles. As a consequence, the
macrocyclic cavities and, more significantly, the electronic features of 1,3-alternate
heteracalix[4]aromatics are amenable to regulation by means of the interplay between
different heteroatoms and diverse aromatic subunits.6 The unique structural advantages
along with the easy availability render heteracalix[4]aromatics powerful and versatile
macrocyclic host molecules in supramolecular chemistry.1-3 For example,
azacalix[4]pyridines interact selectively with both the electron neutral organic
molecules such as fullerenes7 and the charged species including transition metal ions8
while oxacalix[2]arene[2]triazines form complexes with anions of different geometries,
shapes and volumes owing to non-covalent anion- interactions.9 Applications of
heteracalix[4]aromatics in the fabrication of metal organic frameworks,10 CO2-
absorbents,11 anion responsive vesicles,12 liquid crystals,13 stationary phase14 and
organic catalysts15 have also been demonstrated. Moreover, as a member of
heteracalix[4]aromatics, azacalix[1]arene[3]pyridines are able to form structurally
well-defined high valent arylcopper organometallic complexes, providing unique
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molecular tools to investigate the mechanism of Cu(II)-catalyzed arene C-H bond
activation and functionalization.16
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To fully exploit the applications of heteracalix[4]aromatics in molecular recognition
and self-assembly, endeavor to construct macrocyclic conformers other than 1,3-
alternate one has been reported. One of the strategies is to introduce functional groups
in order to create non-covalent bond interactions intramolecularly. Unfortunately,
fixation of two hydroxy or carboxylic groups at the lower rim positions does not induce
the formation of other conformational oxacalix[2]arene[2]triazines as no effective
intramolecular hydrogen bonds are formed between the hydrogen bond donors and
triazine moieties.17 Steric effect offers another approach to control the conformation.
Installation of very bulky substituents on the aromatic subunits for instance leads indeed
to flattened partial cone oxacalix[2]arene[2]triazines. However, as minor and the
kinetically favored products, the flattened partial cone oxacalix[2]arene[2]triazines are
generated only in very low yields from the crucial macrocyclization reaction step,18
which limits the exploration of their applications in host-guest chemistry. Several years
ago, we designed and prepared -CH2O- and -CH2NR- bridged homo
heteracalix[2]arene[2]triazine host molecules with the purpose of enlarging the
macrocyclic cavity at the expense of regulation of electronic characteristics of
phenylene by heteroatoms. It was discovered that insertion of extra methylene units into
the linking positions of typical heteracalix[2]arene[2]triazines generates new
macrocycles which adopt surprisingly various conformational structures.19 The dipole-
dipole interaction between proximal aromatic rings was rationalized to play a dominant
role in determining the conformational structures. To decrease such dipole-dipole
interaction by increasing the distance of two neighboring aromatic rings, we have very
recently devised all hydrazo-bridged homo calix[2]pyridine[2]triazines, and found all
resulting macrocycles give cone conformers.20 Inspired by the previous
observations,19a,20 we undertook the current study to construct all heteroatom linked
homo X2,(NHNR)2-calix[4]aromatics (X = O or NR’). We envisioned that the
replacement of two of the four single heteroatom linkages in X4-calix[4]aromatics (X
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= O or NR’) with hydrazo moieties would generate different conformers and
particularly the partial cone conformers of homo heteracalix[4]aromatics. We report
herein the synthesis of a number of functionalized O2,(NHNR)2- and (NR’)2,(NHNR)2-
bridged calix[2]arene[2]triazines by means of a convenient fragment coupling method.
The influence of the bridging heteroatoms and the substituents on the macrocyclic
conformations was examined. X-ray crystallography showed the all acquired
compounds did not adopt typical 1,3-alternate conformation. Gratifyingly, most of
homo calix[2]arene[2]triazines gave predominantly partial cone conformers in which
hydrazo formed conjugation with both benzene and triazine rings.
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RESULTS AND DISCUSSION
We commenced our study with the synthesis of O2,(N2H2)2-bridged
calix[2]arene[2]triazine 5 (R = H). Retro-synthetically, macrocycle 5 could be
constructed via two distinct fragment coupling approaches. One involves the
macrocyclic condensation between 1,3-dihydrazobenzene 1a (R = H) and 1,3-bis((4,6-
dichloro-1,3,5-triazin-2-yl)oxy)benzene 2, which is readily obtained from the two
directional nucleophilic aromatic substitution of resorcinol with two equivalents of
cyanuric acid chloride. The other synthetic route comprises the reaction of a di-tert-
butyl 1,1'-(1,3-phenylene)bis(2-(4,6-dichloro-1,3,5-triazin-2-yl)hydrazine intermediate
3a with resorcinol 4 (Scheme 1). For convenience, compounds 1a and 3a and their
analogs are referred roughly to as the trimers or trimeric intermediates throughout this
article.
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Scheme 1. Two distinct fragment coupling approaches to (NHNR)2,O2-
calix[2]arene[2]triazines
Since 1,3-dihydrazobenzene 1a (R = H) was found to undergo spontaneous
decomposition under ambient conditions, N-Boc-protected 1,3-dihydrazobenzene 1b
(R = Boc), which was prepared from copper-mediated cross coupling reaction between
1,3-diiodobenzene and tert-butyl hydrazinecarboxylate21 (see Supporting Information),
was employed in the synthesis. Under the various conditions tested, the reaction of 1b
and 2 did not produce desired macrocyclic compound (Method A). Instead, in all cases,
the reaction gave a mixture of polar and inseparable oligomers. We then turned our
attention to the synthesis of 6a from the reaction of 3a with resorcinol 4a. The trimeric
intermediate 3a was obtained straightforwardly by treating N-Boc-protected 1,3-
dihydrazobenzene 1b with an excess amount of cyanuric acid chloride 7a (3 equiv) at
o
0 C in the presence of DIPEA as an acid scavenger. Under the identical conditions,
trimer 3b was prepared analogously in 75% when 2-butoxy-4,6-dichloro-1,3,5-triazine
7b was applied (Scheme 2).
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Scheme 2. Preparation of hydrazo-linked trimers 3a and 3b
In contrast to the efficient construction of heteracalix[2]arene[2]triazines reported
previously,6a the synthesis of homo heteracalix[2]arene[2]triazines 6a was not trivial.
Trimer 3a exhibited great reactivity toward resorcinol 4a in most common organic
solvents when tertiary amines such as Et3N and DIPEA were used as an acid scavenger.
Disappointingly, however, no macrocyclic products were obtained under the optimal
conditions which worked nicely for the synthesis of heteracalix[2]arene[2]triazines.6a
In all cases, after reactant 3a was consumed within 6 h, a mixture of highly polar and
inseparable oligomers was yielded. Only when an inorganic base such as Cs2CO3 or
K2CO3 was used, the reaction between 3a and 4a in acetone at room temperature
afforded target macrocycle 6a in 8% or 19%, respectively, in addition to a large amount
of oligomers (Table S1 in Supporting Information). Using K2CO3 as a base, other
reaction parameters were further investigated. Notably, the reaction outcomes were
strongly dependent on the solvent system used. The results compiled in Table S2
showed clearly that nonpolar and less polar solvents including CCl4, toluene, ether and
THF appeared not beneficial to the reaction as more than 50% to nearly a quantitative
yield of the starting trimer 3a were recovered after reaction. It should also be pointed
out that the reaction was inhibited completely in water whereas oligemerization took
place exclusively in acetonitrile. Comparable chemical yields (13% - 19%) were
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obtained for 6a when the reaction was conducted in 1,4-dioxane, DMF and DMSO.
Accidently, we observed the slight increase of chemical yield of 6a from 19% to 25%
when a mixture of acetone and water (v : v = 9 : 1) was used as the reaction media
9
o
(entries 1 and 2 in Table 1). Slight increase of reaction temperature to 40 C led to a
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further improvement to 39% yield (entry 4, Table 1). Either a higher or a lower
temperature resulted in no satisfactory results (entries 3 and 5, Table 1). Finally, after
optimization of the mixed solvent systems (Table S3 and entries 6 - 8, Table 1), we were
pleased to discover that, in the presence of K2CO3, macrocyclic condensation between
3a and 4a proceeded smoothly at 40 oC in a mixture of acetonitrile and water (v : v =
5 : 1 – 9 : 1) to afford product 6a in 46% yield (entry 7, Table 1). Although the exact
reason why the reaction of 3a with 4a differed from that of 1b and 2 remained unknown
at this stage, the linear tetramer precursors derived from two reactions may probably
adopt different conformations which were amenable to macrocyclization and
oligomerization, respectively.
Table 1. Development of the synthesis of 6a from the reaction of 3a with 4a
entry
solvent
temp (oC)
yield of 6a (%)
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acetone
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-15
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acetone / H2O (9 / 1)
acetone / H2O (9 / 1)
acetone / H2O (9 / 1)
acetone / H2O (9 / 1)
acetonitrile / H2O (9 / 1)
acetonitrile / H2O (7 / 1)
acetonitrile / H2O (5 / 1)
Under the optimized conditions, the hydrazo-linked trimer 3a underwent effective
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macrocyclic condensation reaction with resorcinol derivatives 4b-g to produce
functionalized (NHNBoc)2,O2-calix[2]arene[2]triazine macrocycles. As summarized in
Table 2, the reaction of 3,5-dihydroxybenzaldehyde 4b, ethyl 3,5-dihydroxybenzoate
4c22 and 5-(4-nitrophenoxy)benzene-1,3-diol 4d23 with 3a afforded the corresponding
macrocyclic products 6b-c in good yields, permitting therefore the functionalization of
(NHNBoc)2,O2-calix[2]arene[2]triazine with an aldehyde, ester and 4-nitrophenoxy
group at the upper rim position (entries 2-4, Table 2). The lower-rim-functionalized
(NHNBoc)2,O2-calix[2]arene[2]triazine macrocycles 6e-g were readily accessible by
the same fragment coupling method. The employment of 2,6-dihydroxybenzonitrile
4e24 and 2-nitrobenzene-1,3-diol 4f in the reaction with 3a thus furnished the formation
of cyanided and nitrated homo calix[2]arene[2]triazines 6e and 6f, respectively (entries
5 and 6, Table 2). When 3a was treated with 2,6-dihydroxybenzaldehyde 4g,25 the
lower-rim-formylated (NHNBoc)2,O2-calix[2]arene[2]triazine 6g, a positional isomer
of 6b was also generated albeit in a low yield due to the formation of oligomers under
the identical macrocyclization reaction conditions. The hydrazo-connected trimer 3b,
which was derived from 2-butoxy-4,6-dichloro-1,3,5-triazine (supra vide), was also
able to undergo macrocyclic condensation with resorcinol. Because of the presence of
an electron-donating n-butoxy on the terminal triazine ring, trimer 3b was less
electrophilic than trimer 3a. Nevertheless, under more forcing conditions such to use
Cs2CO3 as a base and boiling acetonitrile as media, the reaction proceeded efficiently
to give a high yield of (NHNBoc)2,O2-calix[2]arene[2]triazine 6h (entry 8, Table 2).
Trimer 3b also reacted smoothly with 2-nitrobenzene-1,3-diol 4f to produce the
corresponding nitro-bearing macrocycle 6i (entry 9, Table 2).
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Table 2. Synthesis of functionalized (NHNBoc)2,O2-calix[2]arene[2]triazines 6a-h
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a
entry
%
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6
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9
3a (R1 = Cl)
3a (R1 = Cl)
3a (R1 = Cl)
3a (R1 = Cl)
3a (R1 = Cl)
3a (R1 = Cl)
3a (R1 = Cl)
3b (R1 = n-BuO)
3b (R1 = n-BuO)
4a (R2 = R3 = H)
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15
77b
47b
6a
6b
6c
6d
6e
6f
4b (R2 = CHO, R3 = H)
4c (R2 = CO2Et, R3 = H)
4d (R2 = 4-NO2-C6H4O, R3 = H)
4e (R2 = H, R3 = CN)
4f (R2 = H, R3 = NO2)
4g (R2 = H, R3 = CHO)
4a (R2 = R3 = H)
6g
6h
6i
4f (R2 = H, R3 = NO2)
a Isolated yield. b Cs2CO3 (2.2 equiv) was used as a base. The reaction was performed
in refluxing CH3CN for 6 h.
It is worth addressing that the resulting (NHNBoc)2,O2-calix[2]arene[2]triazine
compounds 6 would provide a useful and versatile platform for the fabrication of high-
ordered and sophisticated molecular architectures based on a wide variety of well-
known functional group transformations. In addition, the chlorotriazine components
would constitute invaluable handles allowing direct functionalization on the upper rim
of triazine through nucleophilic aromatic substitution reaction. As a demonstration, the
displacement of chloro substituents on triazine rings by dimethylamine occurred easily
at 80 oC in DMSO to afford N,N-dimethyl-substituted macrocycle 6j in 81% (Scheme
3). Moreover, the chemical manipulations on the hydrazo linkages would further enrich
the molecular diversity of homo heteracalix[2]arene[2]triazines. Scheme 3 illustrates
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for example the synthesis of (N2H2)2,O2-calix[2]arene[2]triazines 8a and 8b which
were not accessible by the fragment coupling synthesis starting with 1,3-
dihydrazobenzene (vide supra). In the presence of TFA, the Boc groups on hydrazo
moieties were removed nicely under very mild conditions, affording product 8 in good
yields.
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Scheme 3. Post-macrocyclization functionalizations
The established fragment coupling strategy was applied successfully in the construction
of (NR)2,(NHNBoc)2-calix[2]arene[2]triazines. This has been exemplified in the
synthesis of macrocycles 10 and 12. As depicted in Scheme 4, in the presence of DIPEA,
the reaction of trimer 3a with N,N’-dimethyl-1,3-phenylenediamine 9a in warm
acetonitrile produced all nitrogen atom bridged homo calix[2]arene[2]triazine
compound 10. The reaction between 3a and 1,3-phenylenediamine 9b took place
similarly. Being sparingly soluble in common organic solvents, purification of 11 was
difficult. To circumvent the solubility problem, the compound was transformed in situ
into macrocycle 12 taking the advantage of the reactivity of chlorotriazine toward
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nucleophile diethylamine. The one-pot two-step synthesis therefore yielded the desired
macrocycle in 23% yield. Deprotection of Boc groups resulted in the formation of homo
(NH)2,(NHNH)2-calix[2]arene[2]triazine 13 (Scheme 4).
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Scheme 4. Synthesis of (NR)2,(NHNBoc)2-calix[2]arene[2]triazines 10, 12 and 13.
The structure of all homo heteracalix[2]arene[2]triazine products was in agreement
with the spectroscopic data. The constitution of products for example was supported by
both mass spectra and microanalysis while the variable temperature NMR spectra
revealed macrocyclic ring structures (see Supporting Information). To put the structure
beyond ambiguity, and also to shed light on the conformation of these novel
macrocycles, high quality single crystals of products 6a, 6c, 6e, 6f, 8a, 8b, 10, 12 and
13 were obtained and their structures were determined by X-ray diffraction analysis.
As we anticipated, the structures depicted in Figure 1 – 6 show that most of the homo
(NHNR)2,O2-calix[2]arene[2]triazine macrocycles adopt the similar partial cone
conformations in the solid state. The presence of both a substituent at the low-rim
position of benzene ring and two Boc groups on hydrazo moieties, however, caused the
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deformation of the partial cone structure due to probably the steric effect. For example,
in the case of compounds 6a and 6c, the phenylene connected to hydrazo linkers and
two triazine rings orientate toward the same direction whereas the other phenylene in
between two oxygen atoms positions to the opposite direction, yielding the typical
partial cone conformation with a mirror plane that bisects the phenylene moieties.
Introduction of a cyano and a nitro group into the lower-rim position resulted in the
twisted 1,3-alternate conformations of macrocycles 6e and 6f, respectively. In
comparison to 6a and 6c, it was evident that two N-Boc groups in 6e and 6f are almost
anti-parallelly aligned. After removal of two Boc groups from hydrazo bridges, the
resulting (N2H2)2,O2-calix[2]arene[2]triazines 8a and 8b, with and without a substituent
on the lower-rim, resumed typical partial cone structures again. The bond lengths and
bond angles (Figures S5, S6, S9, and S10 in Supporting Information) indicated that in
partial cone conformers such as 6a, 6c, 8a and 8b all bridging oxygen atoms form
stronger conjugation with their adjacent triazine rings. Two pairs neighboring lone-pair
electrons on a hydrazo moiety appeared orthogonal to each other, forming conjugation
with triazine and benzene rings, respectively. Careful analysis of the bond lengths and
angles (Figures S7 and S8 in Supporting Information) of twisted 1,3-alternate
conformers of 6e and 6f revealed however a strong conjugational system of carbamate.
In other word, one of the lone-pair electrons on hydrazo conjugate with the Boc group
rather than the phenylene ring. It seemed that, in addition to steric effect, competition
of the Boc substituents with the phenylene ring in conjugating with nitrogen lone-pair
electrons imposed perturbation on the macrocyclic conformation.
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Figure 1. X-ray molecular structure of 6a (25% probability) with side (left) and top
(right) views. Alkyl substituents and hydrogen atoms were omitted for clarity.
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Figure 2. X-ray molecular structure of 6c (25% probability) with side (left) and top
(right) views. Alkyl substituents and hydrogen atoms were omitted for clarity.
Figure 3. X-ray molecular structure of 6e (25% probability) with side (left) and top
(right) views. Alkyl substituents and hydrogen atoms were omitted for clarity.
Figure 4. X-ray molecular structure of 6f (25% probability) with side (left) and top
(right) views. Alkyl substituents and hydrogen atoms were omitted for clarity.
Figure 5. X-ray molecular structure of 8a (25% probability) with side (left) and top
(right) views. Alkyl substituents and hydrogen atoms were omitted for clarity.
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Figure 6. X-ray molecular structure of 8b (25% probability) with side (left) and top
(right) views. Alkyl substituents and hydrogen atoms were omitted for clarity.
Being very similar to (NHNBoc)2,O2-calix[2]arene[2]triazine 6a, an all nitrogen atom-
linked
homo
calix[2]arene[2]triazine,
namely,
(NHNBoc)2,(NMe)2-
calix[2]arene[2]triazine 10, adopted a symmetric partial cone conformation in the solid
state (Figure 7). Evidenced by the bond lengths and angles (Figure S11), each triazine
ring formed conjugation with its two linking nitrogen atoms while two planar tert-
butoxycarbonylamino moieties were not coplanar with the meta-phenylene in between.
As illustrated in Figure 8, replacement of two NMe bridges and chloro substituents in
10 with NH and diethylamino units, respectively, led macrocycle 12 to give
unexpectedly a heavily distorted conformation. After further removal of Boc groups
from the linking hydrazine moieties, (NH)2,(NHNH)2-calix[2]arene[2]triazine 13
afforded a twisted partial conformation (Figure 9). It seems that the presence of N,N-
diethylamino group on the upper-rim position of the triazine ring had a marked effect
on the conformational structure of all nitrogen atom-linked homo
calix[2]arene[2]triazines. Revealed by the bond lengths (Figures S12 and S13 in
Supporting Information) of bridging nitrogen atoms to the carbons of aromatic rings
and by the bond angles (Figures S12 and S13 in Supporting Information) around linking
units, both benzene and triazine rings formed varied conjugation systems with their
neighboring nitrogen atoms.
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Figure 7. X-ray molecular structure of 10 (25% probability) with side (left) and top
(right) views. Alkyl substituents and hydrogen atoms were omitted for clarity.
Figure 8. X-ray molecular structure of 12 (25% probability) with side (left) and top
(right) views. Alkyl substituents and hydrogen atoms were omitted for clarity.
Figure 9. X-ray molecular structure of 13 (25% probability) with side (left) and top
(right) views. Alkyl substituents and hydrogen atoms were omitted for clarity.
It should be noted that the partial cone and the distorted 1,3-alternate conformers
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observed in the crystalline state may not retain in solution. As shown by H and C
NMR spectra of all aforementioned products at and below room temperature (Figures
S2-S4), which gave very broad resonance proton and carbon signals, homo
calix[2]arene[2]triazines existed most likely as a mixture of conformers in solution. The
signals became sharpened with the increase of measuring temperature (Figure 10). As
shown in Supporting Information, for macrocycles without N-Boc substituents, decent
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1H and 13C NMR spectra with well-resolved signals were obtained at 80 oC - 100 oC. In
the case of products 10 and 12, a probe temperature as high as to 146 oC was required
to assure the quality of spectra. The outcomes of variable temperature 1H NMR spectra
(Figures 10 and S2-S4), suggested clearly that different conformers in equilibria in
solution were able to undergo very fast inter-conversion at a high temperature relative
to the NMR time scale.
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Figure 10. Partial variable temperature H and 13C NMR spectra of 6a in d2-
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CDCl2CDCl2.
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CONCLUSION
We have presented the efficient and convenient synthesis of a series of functionalized
homo heteracalixaromatics, namely, (NHNR)2,O2-calix[2]arene[2]triazines and
(NR)2,(NHNR’)2-calix[2]arene[2]triazines by means of a fragment coupling strategy.
The unprecedented homo heteracalix[4]arenes adopt predominantly partial cone
conformation in the solid state. In solution, the novel macrocycles exist as a mixture of
conformers which undergo very fast inter-conversion at high temperatures relative to
the NMR time scale. Our study has demonstrated an alternative approach to engineer
the macrocyclic conformation and cavity structures. Through the formation of varied
conjugations with the neighboring aromatic components, hydrazo moiety was able to
tune the electronic feature of heteracalixaromatics. Studies on the construction of stable
macrocyclic conformers and their applications in molecular recognition and self-
assembly are being actively pursued in this laboratory and results will be disclosed in
due course.
Experimental Section
General Information. Unless otherwise noted, all reactions were carried out in oven-
dried glassware. Anhydrous solvents were purified and dried following standard
procedures. All commercially available reagents were used as received. TLC analysis
was performed on pre-coated, glass-backed silica gel plates and visualized with UV
light. Unless otherwise noted, flash column chromatography was performed on silica
gel (200 - 300 mesh). Melting points were uncorrected. The 1H NMR, 13C NMR spectra
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were recorded on a 400 MHz NMR spectrometer. H and C NMR chemical shifts
were reported relative to the signals of residual of chloroform (7.26 ppm, 77.16 ppm),
DMSO (2.50 ppm, 39.52 ppm), CDCl2CDCl2 (5.98 ppm, 73.80 ppm) and that of
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internal standard of TMS (0.0 ppm), respectively. Abbreviations are used in the
description of NMR data as follows: chemical shift (, ppm), multiplicity (s = singlet,
d = doublet, t = triplet, q = quartet, dt = doublet of triplets, m = multiplet), coupling
constant (J, Hz). The electron spray ionization mass spectra (ESI-MS) were recorded
on a MASS spectrometer. Infrared spectra were recorded with KBr pellets in the 4000-
400 cm−1 region. Compounds 1b,21 4c,22 4d,23 4e,24 4g25 and 7b26 were prepared
following the reported procedures.
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Preparation of 3a. To an ice-bath cooled solution of cyanuric chloride 7a (552 mg, 3
mmol) in tetrahydrofuran (20 mL) was added dropwise a mixture of substituted N-Boc-
protected 1,3-dihydrazobenzene 1b (338 mg, 1.0 mmol) and diisopropylethylamine
(324 mg, 2.5 mmol) in tetrahydrofuran (10 mL) during 1 h. The reaction mixture was
stirred for 3h. After removal of diisopropylethylamine hydrochloride salt through
filtration, the filtrate was mixed with saline (100 mL) and extracted with ethyl acetate
(3 x 50 mL). After drying (Na2SO4) and then removing the solvent, the residue was
chromatographed on a silica gel column with a mixture of petroleum ether and ethyl
acetate (v / v = 8 / 1) as the mobile phase to give pure 3a (336 mg, 53%): white solid,
mp 198-199 oC; 1H NMR (400 MHz, CDCl2CDCl2, 100oC) δ 8.02 (s, 2H), 7.59 (s, 1H),
7.33 (s, 3H), 1.47 (s, 18H); 13C NMR (100 MHz, CDCl2CDCl2, 100oC) δ 171.3, 168.2,
152.08, 141.5, 128.7, 122.0, 120.1, 83.7, 27.9; IR (KBr, cm–1) ν 3283, 2974, 1700,
1561,1511. HRMS (ESI-ion trap) calcd. for C22H24Cl4N10O4Na: [M+Na]+ 657.0604.
Found: 657.0594.
Preparation of 3b. Following the procedure for the preparation of 3a, 3b was
synthesized from the reaction of 2-butoxy-4,6-dichloro-1,3,5-triazine 7b (666 mg, 3
mmol) with 1b (338 mg, 1.0 mmol) at room temperature in the presence of
diisopropylethylamine (324 mg, 2.5 mmol). Pure 3b was obtained from flash column
chromatographed on a silica gel column (petroleum ether and ethyl acetate = 5 / 1) (532
mg, 75% yield): white solid, mp 95-97 oC; 1H NMR (400 MHz, CDCl2CDCl2, 120oC)
δ 7.69 (br, 3H), 7.47-7.15 (m, 3H), 4.39 (t, J = 6.6 Hz, 4H), 1.79-1.70 (m, 4H), 1.47 (s,
13
22H), 0.98 (t, J = 7.6 Hz, 6H); C NMR (100 MHz, CDCl2CDCl2, 120oC) δ 171.5,
171.2, 169.2, 152.4, 141.9, 128.1, 120.9, 118.5, 82.8, 68.7, 30.4, 27.9, 18.7, 13.2; IR
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(KBr, cm–1) ν 3233, 2966, 1730, 1566,1447. HRMS (ESI-ion trap) calcd. for
C30H42Cl2N10O6Na: [M+Na]+ 731.2558. Found: 731.2554.
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General procedure for the synthesis of homo heteracalix[2]arene[2]triazines 6a-g.
At 40 oC, both solutions of resorcinol or its derivatives (1 mmol) in acetonitrile (25 mL)
and 3a (1 mmol) in acetonitrile (25 mL) were added dropwise at the same time and the
same rate to a solution of K2CO3 (276 mg, 2.0 mmol) in a mixture of acetonitrile (175
mL) and water (25 mL). After addition of two reactants, which took about 1 h, the
resulting mixture was stirred for another 6 h. The solvents were removed, and the
residue was mixed with saline (100 mL) and extracted with ethyl acetate (3 x 50 mL)
and then dried with anhydrous Na2SO4. After removal of solvent, the residue was
chromatographed on a silica gel column with a mixture of petroleum ether and ethyl
acetate as the mobile phase to give products.
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6a was obtained from flash column chromatographed on a silica gel column (petroleum
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ether and ethyl acetate = 8 / 1) (309 mg, 46% yield): white solid, mp 220 C
(decomposed); 1H NMR (400 MHz, CDCl2CDCl2, 90oC) δ 7.82 (s, 2H), 7.41-7.35 (m,
2H), 7.20 (t, J = 8.0 Hz, 1H), 7.05-6.93 (m, 5H), 1.48 (s, 18H); 13C NMR (100 MHz,
CDCl2CDCl2, 90oC) δ 175.4, 174.8, 173.0, 155.4, 155.3, 144.3, 132.5, 131.3, 124.5,
123.5, 123.1, 118.2, 86.5, 31.3; IR (KBr, cm–1) ν 3232, 2981, 1726, 1563. HRMS (ESI-
ion trap) calcd. for C28H28Cl2N10O6Na: [M+Na]+ 693.1468. Found: 693.1458.
Elemental analysis calcd. (%) for C28H28Cl2N10O6: C, 50.08; H, 4.20; N, 20.86. Found:
C, 49.85; H, 4.21; N, 20.62. A high quality single crystal for X-ray diffraction analysis
was obtained by diffusing n-hexane vapor into the solution of 6a in ethyl acetate.
6b was obtained from flash column chromatographed on a silica gel column (petroleum
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ether and ethyl acetate = 3 / 1) (238 mg, 34% yield): white solid, mp 226 C
(decomposed); 1H NMR (400 MHz, CDCl2CDCl2, 90oC) δ 9.95 (s, 1H), 7.96 (s, 2H),
7.54 (d, J = 2.4Hz, 2H), 7.33 (s, 1H) 7.21-7.16 (m, 2H), 6.96 (d, J = 8.4 Hz, 2H) 1.46
(s, 18H); 13C NMR (100 MHz, CDCl2CDCl2, 90oC) δ 191.9, 175.6, 174.5, 172.8, 155.9,
155.2, 144.2, 141.6, 131.4, 124.3, 123.8, 123.6, 122.3, 86.9, 31.4; IR (KBr, cm–1) ν
3236, 3101, 2979, 1729, 1566. HRMS (ESI-ion trap) calcd. for C29H28Cl2N10O7Na:
[M+Na]+ 721.1412. Found: 721.1408. Elemental analysis calcd. (%) for
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C29H28Cl2N10O7: C, 49.79; H, 4.03; N, 20.02. Found: C, 49.65; H, 4.06; N, 19.67.
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6c was obtained from flash column chromatographed on a silica gel column (petroleum
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ether and ethyl acetate = 3 / 1) (424 mg, 57% yield): white solid, mp 218-219 C; H
NMR (400 MHz, CD3CN, 80oC) δ 8.85 (s, 2H), 7.84 (s, 2H), 7.37-7.27 (m, 3H), 7.14
(d, J = 7.6 Hz, 2H), 4.45 (q, J = 6.8 Hz, 2H) 1.54-1.45 (m, 21H); 13C NMR (100 MHz,
CD3CN, 80oC) δ 173.2, 173.0, 171.2, 165.7, 153.8, 153.5, 142.8, 134.8, 129.9, 123.5,
122.7, 121.3, 120.7, 84.0, 63.0, 28.8, 14.9; IR (KBr, cm–1) ν 3226, 2982, 1729, 1562.
HRMS (ESI-ion trap) calcd. for C31H32Cl2N10O8Na: [M+Na]+ 765.1674. Found:
765.1670. Elemental analysis calcd. (%) for C31H32Cl2N10O8: C, 50.08; H, 4.34; N,
18.84. Found: C, 50.03; H, 4.35; N, 18.97. A high quality single crystal for X-ray
diffraction analysis was obtained by diffusing n-hexane vapor into the solution of 6c in
ethyl acetate.
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6d was obtained from flash column chromatographed on a silica gel column (petroleum
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ether and ethyl acetate = 5 / 1) (348 mg, 43% yield): white solid, mp 219-220 C; H
NMR (400 MHz, CD3CN, 80oC) δ 8.71 (s, 2H), 8.19 (d, J = 8.4 Hz, 2H), 7.42 (s, 1H),
7.26 (t, J = 8.4 Hz, 1H), 7.09-7.04 (m, 4H), 6.91 (s, 2H), 6.82 (s, 1H) 1.37 (s, 18H); 13C
NMR (100 MHz, CD3CN, 80oC) δ 173.2, 172.9, 171.3, 163.5, 157.3, 154.7,153.5, 145.1,
142.9, 129.9, 127.5, 123.1, 121.4, 119.3, 114.6, 113.4, 84.0, 28.8; IR (KBr, cm–1) ν 3234,
2979, 1730, 1563. HRMS (ESI-ion trap) calcd. for C34H31Cl2N11O9Na: [M+Na]+
830.1573. Found: 830.1568. Elemental analysis calcd. (%) for C34H31Cl2N11O9: C,
50.50; H, 3.86; N, 19.06. Found: C, 50.17; H, 3.76; N, 18.75.
6e was obtained from flash column chromatographed on a silica gel column (petroleum
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ether and ethyl acetate = 3 / 1) (209 mg, 30% yield): white solid, mp 197-198 C; H
NMR (400 MHz, CDCl2CDCl2, 90oC) δ 7.82 (s, 2H) , 7.59 (t, J = 8.0 Hz, 1H), 7.24-
7.15 (m, 4H), 6.97 (dd, J = 8.4 Hz, J = 2.4 Hz, 2H), 1.48 (s, 18H); 13C NMR (100 MHz,
CDCl2CDCl2, 90oC) δ 175.9, 174.2, 173.1, 156.8, 155.2, 144.2, 136.3, 131.5, 124.5,
123.2, 122.4, 113.5, 106.2, 86.8, 31.4; IR (KBr, cm–1) ν 3235, 2979, 1734, 1691, 1572.
HRMS (ESI-ion trap) calcd. for C29H27Cl2N11O6Na: [M+Na]+ 718.1412. Found:
718.1408. Elemental analysis calcd. (%) for C29H27Cl2N11O6: C, 50.01; H, 3.91; N,
22.12. Found: C, 49.88; H, 3.99; N, 21.81. A high quality single crystal for X-ray
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diffraction analysis was obtained by diffusing n-hexane vapor into the solution of 6e in
ethyl acetate.
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ether and ethyl acetate = 2 / 1) (294 mg, 41% yield): white solid, mp 232 C
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(decomposed); 1H NMR (400 MHz, CDCl2CDCl2, 120oC) δ 7.93 (s, 2H) , 7.59 (t, J =
8.0 Hz, 1H), 7.28-7.15 (m, 4H), 7.00-6.98 (m, 2H), 1.49 (s, 18H); 13C NMR (100 MHz,
CDCl2CDCl2, 120oC) δ 175.7, 174.0, 173.0, 155.2, 147.4, 144.3, 140.6, 134.5, 131.1,
125.7, 122.9, 122.8, 86.8, 31.4; IR (KBr, cm–1) ν 3220, 2976, 1720, 1701, 1573. HRMS
(ESI-ion trap) calcd. for C28H27Cl2N11O8Na: [M+Na]+ 738.1311. Found: 738.1306. A
high quality single crystal for X-ray diffraction analysis was obtained by diffusing n-
hexane vapor into the solution of 6f in ethyl acetate.
6g was obtained from flash column chromatographed on a silica gel column (petroleum
o
ether and ethyl acetate = 3 / 1) (105 mg, 15% yield): white solid, mp 192 C
(decomposed); 1H NMR (400 MHz, CDCl2CDCl2, 120oC) δ 10.13 (s, 1H), 7.79 (s, 2H),
7.56 (t, J = 8.4 Hz, 1H), 7.25 (s, 1H) 7.18 (t, J = 8.0 Hz, 1H), 7.11 (d, J = 8.4 Hz, 2H),
6.97 (d, J = 7.2 Hz, 2H), 1.48 (s, 18H); 13C NMR (100 MHz, CDCl2CDCl2, 120oC) δ
188.3, 175.6, 174.7, 173.2, 155.5, 155.2, 144.3, 137.0, 131.2, 125.1, 125.0, 123.3, 122.6,
86.7, 31.3; IR (KBr, cm–1) ν 3349, 3229, 2982, 1717, 1700 1569. HRMS (ESI-ion trap)
calcd. for C29H28Cl2N10O7Na: [M+Na]+ 721.1412. Found: 721.1410.
General procedure for the synthesis of homo heteracalix[2]arene[2]triazines 6h-i.
Both solutions of resorcinol 4, or its derivative 4f (1 mmol), in acetonitrile (25 mL) and
3b (1 mmol) in acetonitrile (25 mL) were added dropwise at the same time and the same
rate to a solution of Cs2CO3 (717 mg, 2.2 mmol) in boiling acetonitrile (200 mL). After
addition of two reactants, which took about 1 h, the resulting mixture was stirred for
another 6 h. The solvents were then removed, and the residue was mixed with saline
(100 mL) and extracted with ethyl acetate(3 x 50 mL) and dried with anhydrous Na2SO4.
After removal of solvent, the residue was chromatographed on a silica gel column with
a mixture of petroleum ether and ethyl acetate as the mobile phase to give products.
6h was obtained from flash column chromatographed on a silica gel column (petroleum
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ether and ethyl acetate = 4 / 1) (575 mg, 77% yield): white solid, mp 236-237 C; H
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NMR (400 MHz, CDCl2CDCl2, 90oC) δ 7.72 (s, 2H), 7.41 (s, 1H), 7.31 (t, J = 8.4 Hz,
1H), 7.17-7.12 (m, 1H),7.00-6.92 (m, 5H), 4.44 (t, J = 6.4 Hz, 4H), 1.79-1.75 (m, 4H),
1.52-1.45 (m, 22H), 0.99 (t, J = 7.6 Hz, 6H); 13C NMR (100 MHz, CDCl2CDCl2, 90oC)
δ 176.02, 175.97, 174.1, 155.8, 155.7, 144.9, 132.0, 130.7, 124.9, 123.3, 123.1, 118.6,
85.7, 71.4, 33.9, 31.4, 22.2, 16.8; IR (KBr, cm–1) ν 3228, 2960, 1719, 1597, 1570.
HRMS (ESI-ion trap) calcd. for C36H46N10O8Na: [M+Na]+ 769.3392. Found: 769.3389.
Elemental analysis calcd. (%) for C36H46N10O8: C, 57.90; H, 6.21; N, 18.76. Found: C,
57.74; H, 6.25; N, 18.53.
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6i was obtained from flash column chromatographed on a silica gel column (petroleum
ether and ethyl acetate = 5 / 1) (372 mg, 47% yield): white solid, mp 234-235 oC; 1H-
NMR (400 MHz, CDCl2CDCl2, 100oC) δ 7.61 (s, 2H), 7.41 (t, J = 8.2 Hz, 2H), 7.23-
7.05 (m, 3H), 6.98 (d, J = 7.8 Hz, 2H), 4.57-4.35 (m, 4H), 1.92-1.70 (m, 4H), 1.64-1.28
(m, 22H), 1.14-0.92 (m, 6H); 13C NMR (100 MHz, CDCl2CDCl2, 100oC) δ 172.7, 171.8,
170.6, 152.4, 144.3, 141.4, 137.7, 130.6, 127.2, 122.2, 120.2, 119.8, 82.8, 68.4, 30.6,
28.0, 18.8, 13.4; IR (KBr, cm–1) ν 3230, 2965, 1722, 1610, 1599, 1419. HRMS (ESI-
ion trap) calcd. for C36H46N11O10: [M+H]+ 792.3424. Found: 792.3409.
Synthesis of 6j. To a solution of 6a (201 mg, 0.3 mmol) in DMSO (6 mL) was added
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dimethylamine (2 M in THF, 0.6 mL, 1.2 mmol) at 80 C. The mixture was stirred at
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80 C for another 0.5 h. After 6a was consumed, the mixture was mixed with saline
(100 mL) and extracted with ethyl acetate (3 x 50 mL). After drying (Na2SO4) and then
removing the solvent, the residue was chromatographed on a silica gel column with a
mixture of petroleum ether and ethyl acetate (v /v = 3 /1) as the mobile phase to give 6j
(168 mg, 81% yield): white solid, mp 202-204 oC; 1H-NMR (400 MHz, CDCl2CDCl2,
90oC) δ 7.44 (s, 1H), 7.35 (s, 2H), 7.23 (t, J = 7.8 Hz, 1H), 7.12 (t, J = 7.8 Hz, 1H),
13
7.05-6.74 (m, 5H), 3.21 (s, 12H), 1.44 (s, 18H); C NMR (100 MHz, CDCl2CDCl2,
90oC) δ 171.2, 169.2, 166.5, 152.9, 152.6, 142.0, 128.2, 126.9, 121.8, 120.2, 119.8,
115.5, 82.0, 36.5, 28.0; IR (KBr, cm–1) ν 3236, 2961, 1586, 1415. HRMS (ESI-ion trap)
calcd. for C32H40N12O6Na: [M+Na]+ 711.3086. Found: 711.3085.
General procedure for the synthesis of homo heteracalix[2]arene[2]triazines 8a
and 8b. To a solution of 6h or 6i (0.3 mmol) in boiling DCM (6 mL) was added
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trifluoroacetic acid (268 μL, 3.6 mmol). The mixture was refluxed for 6 h. The mixture
was cooled to room temperature, and water was added. The aqueous layer was adjusted
to pH = 7 - 8 with a saturated aqueous solution of NaHCO3, then the mixture was
extracted with EtOAc (10 mL × 3), the combined organic phase was dried over
anhydrous Na2SO4. After removal of solvent, the residue was chromatographed on a
silica gel column with a mixture of chloroform and ethyl acetate as the mobile phase to
afford products.
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8a was obtained from flash column chromatographed on a silica gel column
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(chloroform and ethyl acetate = 4 / 1) (87 mg, 53% yield): white solid, mp >300 C;
1H-NMR (400 MHz, CD3CN-DMSO, 80 oC) δ 8.91 (s, 2H), 7.43 (t, J = 8.0 Hz, 1H),
7.07 (d, J = 8.2 Hz, 2H), 6.84 (t, J = 8.0 Hz, 1H), 6.67 (s, 1H), 6.17 (br, 4H), 5.75 (s,
1H), 4.35 (t, J = 6.6 Hz, 4H), 1.84-1.63 (m, 4H), 1.57-1.32 (m, 4H), 0.97 (t, J = 7.6 Hz,
13
6H); C NMR (100 MHz, CDCl3, 60oC) δ 173.0, 172.7, 170.2, 153.0, 149.5, 130.2,
128.4, 120.4, 115.4, 106.3, 100.1, 68.3, 31.0, 19.2, 13.8; IR (KBr, cm–1) ν 3237, 2961,
1591. HRMS (ESI-ion trap) calcd. for C26H31N10O4: [M+H]+ 547.2524. Found:
547.2510. Elemental analysis calcd. (%) for C26H30N10O4: C, 57.13; H, 5.53; N, 25.63.
Found: C, 57.39; H, 5.79; N, 25.76. A high quality single crystal for X-ray diffraction
analysis was obtained by diffusing n-hexane vapor into the solution of 8a in DCM.
8b was obtained from flash column chromatographed on a silica gel column
(chloroform and ethyl acetate = 20 / 1) (145 mg, 82% yield): white solid, mp 273-274
oC; 1H-NMR (400 MHz, CDCl2CDCl2, 100oC) δ 7.43 (s, 1H), 7.23 (s, 2H), 7.12 (d, J =
7.3 Hz, 2H), 6.91 (t, J = 7.8 Hz, 1H), 6.20 (d, J = 7.3 Hz, 2H), 5.84 (s, 1H), 5.05-3.96
(br, 6H), 1.96-1.67 (m, 4H), 1.60-1.45 (m, 4H), 1.02 (t, J = 7.3 Hz, 6H); 13C NMR (100
MHz, CDCl2CDCl2, 100oC) δ 172.4, 171.5, 169.7, 144.2, 137.6, 130.2, 129.5, 121.7,
120.2, 105.5, 98.9, 68.4, 30.6, 18.8, 13.4; IR (KBr, cm–1) ν 3242, 2961, 1594, 1422.
HRMS (ESI-ion trap) calcd. for C26H30N11O6: [M+H]+ 592.2375. Found: 592.2366. A
high quality single crystal for X-ray diffraction analysis was obtained by diffusing n-
hexane vapor into the solution of 8b in THF.
Preparation of 10. Both solutions of N,N′-dimethyl-m-phenylenediamine 9a (136 mg,
1 mmol) in acetonitrile (25 mL) and 3a (634 mg, 1 mmol) in acetonitrile (25 mL) were
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diisopropylethylamine (284 mg, 2.2 mmol) in acetonitrile (200 mL) at 40 C. After
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addition of two reactants, which took about 1 h, the resulting mixture was stirred for
another 6 h. The solvents were removed, and the residue was mixed with saline (100
mL) and extracted with ethyl acetate(3 x 50 mL) and then dried with anhydrous Na2SO4.
After removal of solvent, the residue was chromatographed on a silica gel column with
a mixture of petroleum ether and ethyl acetate (v /v = 5 / 1) as the mobile phase to give
products 10 (405 mg, 58% yield): white solid, mp 260 oC decomposed; 1H NMR (400
MHz, CDCl2CDCl2, 146oC) δ 7.49 (s, 1H), 7.33 (br, 3H), 7.16-6.94 (m, 6H), 3.51 (s,
6H), 1.51 (s, 18H); 13C NMR (100 MHz, CDCl2CDCl2, 146oC) δ 170.2, 168.1, 166.4,
152.8, 144.7, 142.0, 129.0, 127.4, 126.7, 123.6, 121.0, 119.3, 82.6, 38.2, 28.4; IR (KBr,
cm–1) ν 3228, 2977, 1725, 1578. HRMS (ESI-ion trap) calcd. for C30H34Cl2N12O4Na:
[M+Na]+ 719.2095. Found: 719.2082. Ahigh quality single crystal for X-ray diffraction
analysis was obtained by diffusing n-hexane vapor into the solution of 10 in THF.
Preparation of 12. Both solutions of m-phenylenediamine 9b (108 mg, 1 mmol) in
acetonitrile (25 mL) and 3a (634 mg, 1 mmol) in acetonitrile (25 mL) were added
dropwise at the same time and the same rate to a solution of diisopropylethylamine (284
mg, 2.2 mmol) in acetonitrile (200 mL) at 40 oC. After addition of two reactants, which
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took about 1 h, the resulting mixture was kept stirring at 40 C for another 6 h. After
the mixture was heated to reflux, diethylamine (438 mg, 6 mmol) was added dropwise,
and the resulting reaction mixture was refluxed for another 3 h. The solvents were then
removed, and the residue was mixed with saline (100 mL) and extracted with ethyl
acetate (3 x 50 mL), and dried with anhydrous Na2SO4. After removal of solvent, the
residue was chromatographed on a silica gel column with a mixture of petroleum ether
and ethyl acetate (v /v = 10 / 1) as the mobile phase to give products 12 (171 mg, 23%
yield): white solid, mp 220-221 oC; 1H NMR (400 MHz, CDCl2CDCl2, 146oC) δ 8.23
(br, 2H), 7.18-7.14 (m, 4H), 6.89 (s, 2H), 6.74 (s, 2H), 6.60 (s, 2H), 3.62-3.58 (m, 8H),
1.41 (s, 18H), 1.21 (s, 12H); 13C NMR (100 MHz, CDCl2CDCl2, 146oC) δ 167.9, 166.1,
165.3, 153.9, 143.5, 140.0, 128.4, 127.5, 121.2, 119.0, 81.8, 41.3, 28.3, 13.3; IR (KBr,
cm–1) ν 3270, 2976, 1725, 1587, 1572. HRMS (ESI-ion trap) calcd. for C36H51N14O4:
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[M+H]+ 743.4194. Found: 743.4212. It should be noted that the product 12 did not give
satisfactory 13C NMR spectra even at very high temperatures (146 oC) probably because
of the high energy barrier for inter-conversion of various conformers. A high quality
single crystal for X-ray diffraction analysis was obtained by diffusing n-hexane vapor
into the solution of 12 in DCM at 0 oC.
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Preparation of 13. To a solution of 12 (0.3 mmol) in boiling DCM (6 mL) was added
trifluoroacetic acid (268 μL, 3.6 mmol). The mixture was refluxed for 6 h. The mixture
was cooled to room temperature, and water was added. The aqueous layer was adjusted
to pH = 7 - 8 with a saturated aqueous solution of NaHCO3, then the mixture was
extracted with CHCl3 (10 mL x 3), the combined organic phase was dried over
anhydrous Na2SO4. After removal of solvent, the residue was chromatographed on a
silica gel column with a mixture of chloroform and ethyl acetate (v /v = 10 / 1)as the
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mobile phase to afford products 13 (119 mg, 73% yield): white solid, mp 290 C
decomposed; 1H NMR (400 MHz, CDCl2CDCl2, 100oC) δ 7.20 (t, J = 8.0 Hz, 1H), 7.05
(t, J = 7.8 Hz, 1H), 6.61 (br, 4H), 6.41-6.19 (m, 5H), 6.05 (s, 2H), 5.97 (s, 1H), 3.57 (q,
J = 7.0 Hz, 8H), 1.15 (t, J = 6.8 Hz 12H); 13C NMR (100 MHz, CDCl2CDCl2, 100oC)
δ 168.3, 165.2, 164.9, 150.6, 139.6, 129.5, 128.3, 105.6, 41.1, 13.0; IR (KBr, cm–1) ν
3320, 3272, 2974, 1526, 1497. HRMS (ESI-ion trap) calcd. for C26H35N14: [M+H]+
543.3164. Found: 543.3158. It should be noted that the product 13 did not give
satisfactory 13C NMR spectra even at very high temperatures (146 oC) probably because
of the high energy barrier for inter-conversion of various conformers. A high quality
single crystal for X-ray diffraction analysis was obtained by diffusing n-hexane vapor
into the solution of 13 in 1,4-dioxane at 0 oC.
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
wangmx@mail.tsinghua.edu.cn
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Notes
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The authors declare no competing financial interests.
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ACKNOWLEDGMENT
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We thank the National Natural Science Foundation of China (21732004, 201572111,
91427301) for financial Support.
1
Supporting Information. H and 13C NMR spectra of all products, and X-ray
structures of 6a, 6c, 6e, 6f, 8a, 8b, 10, 12 and 13 (CIFs). This material is available free
of charge via the Internet at http://pubs.acs.org.
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