Synthesis and self-assembled helical supramolecular polymer of ethyl 7,8-dihydro-3-hydroxy-9-methyl-7-(4′-nitrophenyl)-6H-dibenzo[c]- pyran-6-one-8-carboxylate

Article abstract of DOI:10.1002/hc.21133

A structurally novel compound was isolated as the main product of tandem Pechmann-dehydration between diethyl 4-hydroxy-4-methyl-2-(4′-nitrophenyl) -6-oxocyclohexane-1,3-dicarboxylate (1) and resorcinol in the presence of trifluoroacetic acid. The structu

Full text of DOI:10.1002/hc.21133

Heteroatom Chemistry
Volume 25, Number 1, 2014
Synthesis and Self-Assembled Helical
Supramolecular Polymer of Ethyl
7,8-dihydro-3-hydroxy-9-methyl-7-
(4ꢀ-nitrophenyl)-6H-dibenzo[c]-
pyran-6-one-8-carboxylate
Chaoyue Chen, Jinsong Hu, Xiaomei Zhang, Jianjun Shi, and Jie He
School of Chemical Engineering, Anhui University of Science and Technology, No.168, Shungeng
Middle Road, Huainan 232001, Anhui, People’s Republic of China
Received 18 August 2013; revised 23 October 2013
then arranged alternatively along the b-axis direction
ABSTRACT: A structurally novel compound was
˚
ꢁC
with a pitch length of 7.894 A. 2013 Wiley Periodi-
cals, Inc. Heteroatom Chem. 25:35–42, 2014; View
this article online at wileyonlinelibrary.com. DOI
10.1002/hc.21133
isolated as the main product of tandem Pechmann–
dehydration between diethyl 4-hydroxy-4-methyl-2-
(4ꢀ-nitrophenyl)-6-oxocyclohexane-1,3-dicarboxylate
(1) and resorcinol in the presence of trifluoroacetic
acid. The structure of the product was determined as a
racemate of (7R,8R)- and (7S,8S)-ethyl 7,8-dihydro-3-
hydroxy-9-methyl-7-(4ꢀ-nitrophenyl)-6H-dibenzo[c]-
pyran-6-one-8-carboxylate (3a) enantiomers by single
crystal X-ray diffraction analysis. The X-ray crystal
structure revealed that 3a possesses an extended
and more stable conjugated aromatic system as a
consequence of the stereoselectivity of intramolecular
dehydration behavior of Pechmann condensation
product 2. In the crystal superstructure, the (7R,8R)-
and (7S,8S)-isomers of 3a respectively self-assembled
into left- and right-handed supramolecular helical
INTRODUCTION
As an important moiety of coumarin derivatives,
3:4-fused six-member carbocyclic ring coumarins
(3:4-carbocyclic fused ring system) are receiving
growing interests due to their biological activities
[1–23]. Typically, there are five kinds of 3:4-fused six-
member carbocyclic ring coumarins (Fig. 1). Among
them, derivatives of 7,8,9,10-tetrahydro-6H-benzo[c]
chromen-6-ones (a) [1–11] and 6H-dibenzo[c]pyran-
6-ones (e) [12–23] are well developed and docu-
mented, while other kinds of them, such as deriva-
tives of 9,10-dihydro-6H-benzo[c]chromen-6-ones
(b) [21, 22], 7,10-dihydro-6H-benzo[c]chromen-6-
ones (c) [21,23], especially 7,8-dihydro-6H-benzo[c]
chromen- 6-ones (d), are relatively seldom discussed
in the literature, to the best of our knowledge.
˚
˚
chains with a channel size of 3.70 A × 10.20 A in
virtue of intermolecular hydrogen bonding together
with dramatic twisting between carboxylate group
at C8 and tricyclic ring framework of 3a, which are
Correspondence to: Chaoyue Chen; e-mail: njuchaoyuechen@
163.com.
Contract grant sponsor: Provincial Key Project of Natural Sci-
ence Research for Colleges and Universities of Anhui Province of
China and the Doctoral Fund of Anhui University of Science and
Technology.
As a part of our current research, we em-
ployed multisubstituted cyclic β-keto esters that
underwent Pechmann reaction with phenols to
construct 3:4-fused six-member carbocyclic ring
coumarins (a). However, when diethyl 4-hydroxy-
4-methyl-2-(4ꢀ -nitrophenyl)-6-oxocyclohexane-1,
Contract grant number: 11117.
2013 Wiley Periodicals, Inc.
ꢁC
35

36 Chen et al.
FIGURE 1 Structures of 3:4-fused six-member ring coumarins.
SCHEME 1 Construction of 3:4-fused six-member ring coumarins.
1H NMR and 13C NMR spectra were recorded on
a Bruker ARX-300 (300 MHz) NMR spectrometer
(Bruker, Karlsruhe, Germany). Mass spectrometry
electrospray ionization (MS-ESI) spectra were ob-
tained on a Finnigan-Mat LCQ mass spectrometer
(Thermo Finnigan, San Jose, CA). Elemental analy-
ses were carried out on an Elementar Vario MICRO
CUBE (German). Melting points were determined on
an electrically heated RK-Z melting point apparatus
(Analytical Instrument Factory in Tianjin, People’s
Republic of China) and were uncorrected.
3-dicarboxylate (1) and resorcinol were chosen
as representative model reactants and subjected
to reflux conditions in trifluoroacetic acid (TFA)
(Scheme 1), we obtained a small amount of desired
coumarin-fused 3-hydroxy-2-cyclohexene derivative
2 (Pechmann condensation product), together with
a large amount of its intramolecular dehydrated
product. Because there are three different types of
alpha-hydrogen atoms relative to hydroxyl group
in 2 that can be eliminated with the hydroxyl
group, and the exact structure of product 3 was
1
hard to elucidate by H and 13C nuclear magnetic
Crystals were obtained by slow evaporation from
mixture of EtOAc:hexane = 1:1 solution of the pure
compound 3a. A yellow single crystal of suitable size
was selected for X-ray diffraction analysis. Measure-
ments were made on an Enraf–Nonius CAD4 diffrac-
tometer equipped with graphite crystal monochrom-
resonance (NMR) determination due to its unusual
coupling effect between the adjacent protons. So,
we felt that it would be desirable to extensively
investigate the intramolecular dehydration behavior
of 2 and determine the exact structure of the
intramolecular dehydrated product of 3. In this
context, we further examined and presented the
structure of a representative example, namely ethyl
7,8-dihydro-3-hydroxy-9-methyl-7-(4ꢀ-nitrophenyl)-
6H-dibenzo[c]pyran-6-one-8-carboxylate 3a, by
single crystal X-ray diffraction studies.
˚
atized Mo Kα radiation (λ = 0.71073 A) at 293(2) K.
Synthesis of Intermediates and the Target
Compounds
Diethyl
4-hydroxy-4-methyl-2-(4ꢀ-nitrophenyl)-6-
oxocyclohexane-1,3-dicarboxylate (1) was syn-
thesized according to the method reported by
Pandiarajan et al. [24] except that piperidine was
used instead of methylamine. A mixture of ethyl ace-
toacetate (2.66 g, 20.4 mmol), 4-nitrobenzaldehyde
(1.51 g, 10 mmol), and piperidine (0.4 mL) in
EXPERIMENTAL
Materials and Methods
All reagents were obtained from commercial sup-
pliers and used without further purification. The
Heteroatom Chemistry DOI 10.1002/hc

Ethyl 7,8-dihydro-3-hydroxy-9-methyl-7-(4ꢀ-nitrophenyl)-6H-dibenzo[c]- pyran-6-one-8-carboxylate 37
SCHEME 2 Possible dehydrated structures of compound 2.
ethanol (20 mL) was stirred at room temperature
for about 12 h. The precipitate formed was filtered
and purified by recrystallization from ethanol. Pure
compound 1 was white solid, melting point (mp):
187–189◦C (literature [24]: 188◦C).
MHz, DMSO-d6) δ (ppm): 171.5, 161.0, 159.7, 154.3,
152.7, 148.4, 146.4, 129.8, 126.5, 123.7, 118.8, 113.5,
112.2, 102.4, 68.1, 60.5, 57.9, 43.0, 40.5, 28.2, 14.5.
MS-ESI m/z [M+H]+ Calcd. for C23H22NO8: 440.13;
found: 440.25. Anal. Calcd for C23H21NO8: C, 62.87;
H, 4.82. Found: C, 62.93; H, 4.91.
Ethyl 3,9-dihydroxy-9-methyl-7-(4ꢀ-nitrophenyl)-
6-oxo-7,8,10-trihydro-6H-benzo[c]-chromene-8-car-
boxylate (2) and ethyl 7,8-dihydro-3-hydroxy-9-
methyl-7-(4ꢀ-nitro-phenyl)-6H-dibenzo[c]pyran-6-
one-8-carboxylate (3a). A mixture consisting of
4.2 mmol (0.46 g) of resorcinol, 4.0 mmol (0.79 g)
of diethyl 4-hydroxy-4-methyl-2-(4ꢀ-nitrophenyl)-6-
oxocyclohexane-1,3-dicarboxylate (1), and 15 mL
of TFA was refluxed for 12 h. After completion
of the reaction, TFA in the reaction mixture was
evaporated in vacuum with a rotary evaporator.
The residue was diluted with ethyl acetate (30 mL),
and then washed with saturated NaHCO3 solution.
The organic layer was separated, and the aqueous
layer was extracted with ethyl acetate twice (2 ×
30 mL). The combined organic layers were washed
with brine, dried over anhydrous Na2SO4, and
concentrated. The residue was purified by column
chromatography (silica gel, EtOAc–hexane = 1:2 as
eluent) to afford pure 2 and 3a.
Ethyl 7,8-dihydro-3-hydroxy-9-methyl-7-(4ꢀ-nitro-
phenyl)-6H-dibenzo[c]pyran-6-one-8-carboxylate(3a).
Yield: 61%, yellow solid; mp 184–187◦C. 1H NMR
(300 MHz, DMSO-d6) δ (ppm): 10.61 (s, 1H), 8.12
(d, J = 8.7 Hz, 2H), 7.91 (d, J = 8.7 Hz, 1H), 7.49 (d,
J = 8.7 Hz, 2H), 7.09 (s, 1H), 6.85 (dd, J = 9.0, 2.4 Hz,
1H), 6.76 (d, J = 2.7 Hz, 1H), 4.76 (s, 1H), 4.14–4.02
(m, 2H), 3.58 (s, 1H), 2.05 (s, 3H), 1.15 (t, J = 7.5 Hz,
3H). 13C NMR (75 MHz, DMSO-d6) δ (ppm): 170.4,
161.6, 160.9, 155.3, 149.0, 147.1, 144.1, 142.9, 129.1,
126.4, 124.2, 118.5, 113.7, 112.3, 109.5, 103.0, 61.6,
51.2, 39.0, 24.6, 14.4. MS-ESI m/z [M+H]+ Calcd.
for C23H20NO7: 422.12; found: 422.10. Anal. Calcd
for C23H19NO7: C, 65.55; H, 4.54. Found: C, 65.65;
H, 4.59.
X-Ray Crystallography
A single crystal of compound 3a suitable for X-ray
diffraction analysis was obtained by slow evapora-
tion from EtOAc–hexane (1:1) solutions. A yellow
crystal having dimensions of 0.30 × 0.20 × 0.10 mm3
was mounted on the top of the glass fibers. The
data were collected at 293(2) K on an Enraf–Nonius
CAD4 diffractometer (Delft, the Netherlands) us-
ing graphite monochromatic Mo Kα radiation (λ =
Ethyl 3,9-dihydroxy-9-methyl-7-(4ꢀ-nitrophenyl)-
6-oxo-7,8,10-trihydro-6H-benzo[c]-chromene-8-car-
boxylate (2). Yield: 21%, white solid; mp 252–
254◦C. 1H NMR (300 MHz, dimethyl sulfoxide
(DMSO)-d6) δ (ppm): 10.47 (s, 1H), 8.08 (d, J =
8.7 Hz, 2H), 7.64 (d, J = 8.7 Hz, 1H), 7.39 (d, J =
8.7 Hz, 2H), 6.82 (dd, J = 8.4, 2.4 Hz, 1H), 6.67 (d,
J = 2.4 Hz, 1H), 4.84 (s, 1H), 4.39 (d, J = 10.8 Hz,
1H), 4.05–3.90 (m, 2H), 3.15 (d, J = 18.6 Hz, 1H),
2.98 (d, J = 18.6 Hz, 1H), 2.82 (d, J = 10.8 Hz, 1H),
1.34 (s, 3H), 1.03 (t, J = 7.2 Hz, 3H). 13C NMR (75
˚
0.71073 A). The intensity data were corrected for Lp
factors and empirical. The structure was solved by
direct methods with the SHELXTL-97 program [25].
The final refinement was made by full-matrix least-
squares techniques with anisotropic thermal param-
eters for the nonhydrogen atoms on F2. All the H
Heteroatom Chemistry DOI 10.1002/hc

38 Chen et al.
FIGURE 2 (a) and (b) ORTEP view of the crystal structure of (7R,8R)- and (7S,8S)-isomers of 3a; (c) and (d) the illustration
of left-handed helix (M helix) and right-handed helix (P helix) by H-bonding interaction in 3a; (e) view of 1D helical chains in 3a
along the b-axis direction (hydrogen atoms are omitted for clarity).
atoms were added according to the theoretical mod-
els. Multiscan absorption correction was applied by
use of the SADABS program [26].
posit@ccdc.cam.ac.uk) and is also available as sup-
porting information.
CCDC-951554 contain the supplementary crys-
tallographic data for this paper (excluding the struc-
ture factors), which can be obtained free of charge
from the Cambridge Crystallographic Data Cen-
tre (www.ccdc.cam.ac.uk/data request/cif; or a copy
of the cif file of compound 3a can be obtained
free of charge by e-mail inquiry to deposition: de-
RESULTS AND DISCUSSION
To initiate our study, diethyl 4-hydroxy-4-methyl-
2-(4ꢀ-nitrophenyl)-6-oxocyclo-hexane-1,3-dicarboxy-
late (1) was prepared in excellent yield by condens-
ing 4-nitrobenzaldehyde with ethyl acetoacetate
under the catalysis of piperidine. As shown in
Heteroatom Chemistry DOI 10.1002/hc

Ethyl 7,8-dihydro-3-hydroxy-9-methyl-7-(4ꢀ-nitrophenyl)-6H-dibenzo[c]- pyran-6-one-8-carboxylate 39
TABLE 1 Crystal Data and Structure Refinement for Com-
pound 3a
Scheme 1, condensation of diethyl 4-hydroxy-4-
methyl-2-(4ꢀ-nitrophenyl)-6-oxocyclohexane-1,3-
di-carboxylate (1) with resorcinol gave 3-hydroxy-
2-cyclohexene derivative 2 in 21% isolated yield
(Pechmann condensation product), together with
an unexpected product 3 in 61% yield, which
probably was formed by dehydration of compound
2. Molecular ion peak in MS-ESI of the unexpected
product also supported this supposition. When
compound 2 was subjected to reaction condi-
tions normally used for dehydration, for example,
p-Toluenesulfonic acid in toluene, the same product
3 can be obtained in 92% yield (Scheme 1). So
it can be safely concluded that the Pechmann
Empirical Formula
C23H19NO7
Formula weight
Temperature (K)
Crystal system
Space group
421.39
293(2)
Monoclinic
P21/c
Unit cell dimensions
˚
a (A)
16.069(3)
7.8940(16)
15.980(3)
˚
b (A)
˚
c (A)
α (°)
β (°)
γ (°)
90.00
90.15(3)
90.00
2027.0(7)
3
˚
Volume (A )
Z
4
condensation was accompanied by
a tandem
Density (calcd.) (g/cm3)
Absolute coefficient (mm−1
F(000)
1.381
0.103
880
Pechmann–dehydration reaction. Theoretically,
three different alpha-hydrogen atoms related to
hydroxyl group in 2 can be eliminated with the
hydroxyl group (Scheme 2). The exact structure of
the product of the tandem Pechmann–dehydration
reaction needs to be further elucidated.
)
Crystal size (mm)
Temperature (K)
Radiation (A)
0.3 × 0.2 × 0.1
293(2)
˚
Mo Kα 0.71073
1.3, 25.4
θ Min, max (°)
Data set
limiting indices –19 < h < 19;
–9 < k < 0; –19 < l < 0
3876, 3725
0.032
The postulated structures 3b and 3c (Scheme 2)
1
were excluded by the analysis of H and 13C NMR
Total Unique Data
R (int)
spectroscopy of the Pechmann–dehydration prod-
uct, and 3a was left as the only possible structure.
Quite unexpectedly, the adjacent protons Ha and Hb
Nref, Npar
R, wR2, S
CCDC number
3725, 280
0.0589, 0.1760, 1.01
951554
1
in H NMR spectrum (in DMSO-d6) of structure 3a
are both represented as singlets and no coupling ef-
fect is seen between them, while the same protons Ha
and Hb in the 1H NMR spectrum of 2 (in DMSO-d6)
are split into two doublets with a coupling constant
atoms C7/C12/C11/C10. The value of the total puck-
ering amplitude, QT, is 0.693 A. The C9-C8-C7-C12
˚
3
of JHa–Hb = 10.8 Hz, an observation of an AX-type
torsion angle was determined as –44.13◦. It is wor-
thy to note that the bond C9 C10 is somewhat out of
the plane of the coumarin ring, making the torsion
angle of C9-C10-C11-C12 being –13.00◦. Careful in-
spection shows that the distance between the two hy-
drogen atoms respectively attached to C10 and C16
spin system.
To reveal the stereochemical information and es-
pecially the special orientation of the two adjacent
protons Ha and Hb, pure 3a was left to stand for 1
week, at which time a single crystal was obtained and
its structure solved by X-ray crystallography (Fig. 2).
The relevant crystallographic data are presented in
Table 1, the selected bond lengths, bond angles, and
torsion angles are given in Tables 2, 3, and 4, re-
spectively. The H-bonding data are listed in Table 5.
X-ray analysis reveals that the structure of 3a crystal-
lized in the monoclinic crystal system, space group
of P21/c.
˚
is 2.182 A, which is shorter than the sum of the van
˚
der Waals radii of two hydrogen atoms (2.4 A) [29],
indicating a steric interaction between them. Hence,
this kind of arrangement can ease the steric hin-
drance caused by the two hydrogen atoms. Overall,
the tricyclic framework of the 1,3-cyclohexadiene-
fused coumarin ring system displays a nearly copla-
nar configuration.
In the crystal structures of (7R,8R)- and (7S,8S)-
3a (Figs. 2a and 2b), a 1,3-cyclohexadiene-fused
coumarin ring system represents the central core of
the molecule, with two side chains attached at C7
and C8 atoms, respectively. The 1,3-cyclohexadiene
ring (C7-C8-C9-C10-C11-C12) which annulated at
the 3,4-positions of the coumarin scaffold adopts
a flattened half-chair conformation [27, 28] with
4-Nitrophenyl attached at C7 and carboxylate
group attached at C8 are on the opposite side of
the tricyclic framework of the compound 3a. The
benzene ring of 4-nitrophenyl group is almost per-
pendicular to the 1,3-cyclohexadiene ring (dihedral
angle of 88.797◦), this can be attributed to the flat-
tened half-chair conformation of 1,3-cyclohexadiene
ring and the steric interaction of aryl group with
carboxylate group on coumarin ring. A dramatic
twisting is also observed between carboxylate group
˚
atoms C8 and C9 deviating by 0.693 and 0.280 A,
respectively, from the mean plane defined by other
Heteroatom Chemistry DOI 10.1002/hc

40 Chen et al.
TABLE 2 Selected Bond Lengths for Compound 3a
TABLE 5 Geometry for Hydrogen Bonds in the Crystal
Structure of 3a
˚
˚
Bond Lengths
(A)
Bond Lengths
(A)
D
H . . . A
D
H
H
A
D
A
D
H
A
C7-C8
1.533(3)
1.507(4)
1.505(4)
1.519(3)
1.498(4)
1.335(4)
1.460(4)
1.460(4)
C11-C13
C11-C12
C7-H7A
C8-H8A
C10-H10A
C12-C15
C6-C7
1.445(4)
1.354(3)
0.9800
0.9800
0.9300
1.445(4)
1.519(4)
C7-C12
C8-C21
C8-C9
C9-C20
C9-C10
C10-C11
C10-C11
O4 H4B . . . O7
0.8200 1.8800 2.700(4)
174.00
162.00
118.00
149.00
C10 H10A . . . O5 0.9300 2.4300 3.327(3)
C20 H20C . . . O6 0.9600 2.5500 3.120(4)
C22 H22A . . . O1 0.9700 2.4900 3.354(5)
(Fig. 2e). As the left- and right-handed helical chains
coexist in the crystal structure, the whole crystal
is racemic and does not exhibit optical activity. It
is noteworthy that molecular structures with he-
lical polymer chain morphology, as shown in 3a,
have practical implication in multidisciplinary areas
on account of their structure similarities with DNA
[30–35].
attached at C8 and the 1,3-cyclohexadiene ring with
the dihedral angle between them being 86.263◦. Fur-
ther inspection shows that the hydroxyl group at C18
and carboxylate group at C8 of adjacent molecules
of 3a are linked by strong H bonding. In addition,
several intralayer C
H O interactions are also ob-
served for the title compound, despite their weak-
ness. However, no obvious π–π interaction is found
in the crystal structure.
A much more detailed analysis of X-ray crystal-
lography of 3a shown that the H7-C7-C8-H8 torsion
angle is 78.862◦, and therefore explained the reason
of the unusual coupling effect between C Ha and
Interestingly, the cooperation of the dramatic
twisting between the carboxylate group attached
at C8 and the 1,3-cyclohexadiene ring as well
as the continuous H bonding induces the forma-
tion of attractive left- and right-handed 1D helical
supramolecular polymer chains as shown in Figs. 2c
and 2d. The (7S,8S)-isomer of 3a induced a left-
handed helical channel, while a right-handed heli-
cal channel was obtained from its enantiomer, the
(7R,8R)-isomer of 3a. Both the channel sizes of the
left- and right-handed helical channels were deter-
C
Hb (H7 and H8 in Fig. 2) mentioned above. It is
well known that the maximum coupling constant 3J
will occur when the dihedral angle is 0◦ or 180◦ and
the minimum coupling constant (often ≈0 Hz) oc-
curs when the dihedral angle is near 90◦ (actually oc-
curring around 85◦), according to the Karplus rela-
tionship [36–38]. Therefore, this phenomenon could
be explained by the Karplus relationship so that the
Ha–Hb coupling constant 3J is about 0 Hz. The X-ray
crystallography and 1H NMR spectrum of 3a are also
indicative of the intramolecular dehydration taking
place only between the hydroxyl group and the Hc/Hd
˚
mined as ∼3.70 × 10.20 A, and they arranged alter-
˚
natively along the b axis with the pitch of 7.894 A
TABLE 3 Selected Bond Angles for Compound 3a
Bond Angles
(°)
Bond Angles
(°)
Bond Angles
(°)
C10-C11-C12
C7-C12-C15
C11-C12-C15
C7-C12-C11
C8-C7-C12
C6-C7-C12
C7-C8-C9
119.0(2)
116.3(2)
121.8(2)
121.8(2)
110.5(2)
111.2(2)
112.4(2)
107.6(2)
C7-C8-C21
C8-C9-C20
C10-C9-C20
C8-C9-C10
C9-C10-C11
C10-C11-C13
C12-C11-C13
C6-C7-H7A
110.3(2)
118.8(2)
122.4(2)
118.7(2)
122.4(2)
121.3(2)
119.7(2)
107.00
C8-C7-H7A
C12-C7-H7A
C7-C8-H8A
107.00
107.00
109.00
109.00
109.00
119.00
119.00
C9-C8-H8A
C21-C8-H8A
C9-C10-H10A
C11-C10-H10A
C9-C8-C21
TABLE 4 Selected Torsion Angles for Compound 3a
Torsion Angles
(°)
Torsion Angles
(°)
Torsion Angles
(°)
C6-C7-C8-C9
81.3(3)
-44.1(3)
30.1(3)
C12-C7-C8-C21
C7-C8-C9-C10
C7 -C8-C9-C20
C21-C8-C9-C20
C8-C9-C10-C11
C20-C9-C10-C11
C9-C10-C11-C13
76.0(3)
33.6(3)
–148.7(2)
89.7(3)
–4.4(4)
178.0(3)
170.0(3)
C9-C10-C11-C12
C13-C11-C12-C7
C10-C11-C12-C7
C13-C11-C12-C15
C10-C11-C12-C15
–13.0(4)
175.4(2)
–1.7(4)
–0.1(4)
–177.1(3)
C12-C7-C8-C9
C8-C7-C12-C11
C6-C7-C12-C15
C6-C7 -C8-C21
C8-C7-C12-C15
C6-C7-C12-C11
79.3(3)
–158.6(2)
–154.2(2)
–96.4(3)
Heteroatom Chemistry DOI 10.1002/hc

Ethyl 7,8-dihydro-3-hydroxy-9-methyl-7-(4ꢀ-nitrophenyl)-6H-dibenzo[c]- pyran-6-one-8-carboxylate 41
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to form 3a, but not between the hydroxyl group and
the Hb to form 3c, or methyl to form 3b (Scheme 2).
This is probably because the former can form an ex-
tended and more stable conjugated aromatic system
(3a) when compared with 3b and 3c.
CONCLUSIONS
In an attempt to construct 3:4-fused six-member
carbocyclic ring coumarin, an unknown Pechmann–
dehydration product was isolated as the main
product when condensation of diethyl 4-hydroxy-4-
methyl-2-(4ꢀ-nitrophenyl)-6-oxocyclohexane-1,3-di-
carboxylate (1) with resorcinol in the presence
of TFA. The exact structure of the Pechmann–
dehydration product was determined as a race-
mate of enantiomers (7R,8R)- and (7S,8S)-ethyl
7,8-dihydro-3-hydroxy-9-methyl-7-(4-nitro-phenyl)-
6H-dibenzo-[c]pyran-6-one-8-carboxylate (3a) by
single crystal X-ray diffraction studies together with
1
MS-ESI, H, and 13C NMR spectroscopy. The X-ray
crystal structure also revealed that 3a possesses
an extended and more stable conjugated aromatic
system as a consequence of the selectivity of in-
tramolecular dehydration behavior of Pechmann
condensation product 2. An attractive feature of
the crystal structure of compound 3a is that the
(7S,8S)- and (7R,8R)-isomers respectively form
left- and right-handed helical polymer chains via
supramolecular self-assembly with the channel size
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˚
of 3.70 × 10.20 A, which are arranged alternatively
along the b-axis direction with pitch length of
˚
7.894 A. Further inspection shows that the continu-
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between carboxylate group attached at C8 and the
1,3-cyclohexadiene ring favor the formation of the
helical supramolecular chains.
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SUPPORTING INFORMATION
Supporting information related to the crystal-
lographic data of compound 3a is available
from Chaoyue Chen njuchaoyuechen@163.com on
request.
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