Synthesis and structures of pyrimidinophanes containing a nitrogen atom in the bridge

Article abstract of DOI:10.1007/s11172-006-0292-1

The reactions of 1,3-bis(5-bromopentyl)-5-methyl-or 1,3-bis(5-bromopentyl)- 6-methyl-uracil with benzylamine afforded pyrimidinophanes containing the nitrogen atom in the polymethylene bridge. Single-crystal X-ray diffraction study and NMR and UV spectros

Full text of DOI:10.1007/s11172-006-0292-1

Russian Chemical Bulletin, International Edition, Vol. 55, No. 3, pp. 559—568, March, 2006
559
Synthesis and structures of pyrimidinophanes
containing a nitrogen atom in the bridge

V. E. Semenov, A. E. Nikolaev, L. F. Galiullina, O. A. Lodochnikova, I. A. Litvinov, A. P. Timosheva,
V. E. Kataev, Yu. Ya. Efremov, D. R. Sharafutdinova, A. V. Chernova, Sh. K. Latypov, and V. S. Reznik
A. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 73 2253. Eꢀmail: sve@iopc.kcn.ru
The reactions of 1,3ꢀbis(5ꢀbromopentyl)ꢀ5ꢀmethylꢀ or 1,3ꢀbis(5ꢀbromopentyl)ꢀ6ꢀmethylꢀ
uracil with benzylamine afforded pyrimidinophanes containing the nitrogen atom in the
polymethylene bridge. Singleꢀcrystal Xꢀray diffraction study and NMR and UV spectroscopic
study in solution demonstrated that the phenyl and uracil fragments in the macrocycles are in
spatial proximity. Unlike the known macrocycles containing the pyrimidine ring, the
pyrimidinophanes under study are conformationally rigid.
Key words: macrocyclic compounds, pyrimidinophanes, conformation, Xꢀray diffraction
study, NMR spectroscopy, hypochromic effect, dipole moment.
Scheme 1
The chemistry of macrocyclic compounds containing
pyrimidine rings, viz., pyrimidinophanes, including their
synthesis and investigations, is a rapidly developing field
of chemistry of nucleotide bases.1—4 Pyrimidinophanes
are of interest as compounds simulating interactions beꢀ
tween fragments involved in nucleic acids as well as interꢀ
actions between nucleotide bases and proteins. In addiꢀ
tion, macrocycles containing the pyrimidine ring hold
promise as complexꢀforming agents and as compounds
for studying various biological activities.
Earlier,5 with the aim of investigating the complexꢀ
forming properties and biological activities of waterꢀ
soluble pyrimidinophanes, we have synthesized macroꢀ
cycles containing two 6ꢀmethyluracil fragments and niꢀ
trogen atoms in the bridges, viz., 6,20ꢀbis(ethylamiꢀ
no)[9,9](1,3)ꢀ and 7,23ꢀ bis(ethylamino)[11,11](1,3)ꢀ
pyrimidinophanes, by the reactions of 1,3ꢀbis(ethylꢀ
aminoalkyl)ꢀ6ꢀmethylꢀ2,4ꢀdioxoꢀ1,2,3,4ꢀtetrahydroꢀ
pyrimidines (1,3ꢀbis(ethylaminoalkylꢀ6ꢀmethyluracils)
with 1,3ꢀbis(bromoalkyl)ꢀ6ꢀmethyluracils. We syntheꢀ
sized another type of macrocycles, viz., 7ꢀbenzylamiꢀ
no[11](1,3)pyrimidinophanes 4 and 5, by the reactions of
1,3ꢀbis(bromopentyl)ꢀ6ꢀmethyluracil (1) or 1,3ꢀbis(5ꢀ
bromopentyl)ꢀ5ꢀmethyluracil (1,3ꢀbis(5ꢀbromopenꢀ
tyl)thymine) (2), respectively, with a twoꢀtoꢀthreeꢀfold
excess of benzylamine (3) (Scheme 1). The yields of the
macrocycles were at most 20%. We failed to prepare these
pyrimidinophanes in higher yields by varying the reacꢀ
tions conditions (the use of different solvents, dilute soluꢀ
tions, or the addition of tetraalkylammonium salts or tranꢀ
sition metal salts).
R1 = H (1, 4), Me (2, 5); R2 = H (2, 5), Me (1, 4)
Macrocycles 4 and 5 are structural isomers, which
differ in the arrangement of the methyl group at the uracil
fragment, and are characterized by different mass spectra.
In the mass spectrum of compound 4, the intensity of the
molecular ion is 42.8% of the maximum peak, whereas
the intensity of the corresponding molecular ion in the
mass spectrum of compound 5 is 94.1%. It is the arrangeꢀ
ment of the CH3 group that has the most destabilizing
effect on the molecular ion [M]+. In pyrimidinophane 4,
the intensity of the ion corresponding to the loss of the
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 539—547, March, 2006.
1066ꢀ5285/06/5503ꢀ0559 © 2006 Springer Science+Business Media, Inc.

560
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 3, March, 2006
Semenov et al.
methyl group from the molecular ion is 80.7%; in comꢀ
pound 5, only 4.8%. This result can be attributed to the
influence of the N(1)pyr atom, which is responsible for the
cleavage of the βꢀbond with respect to the N(1)—C(16)
bond. In this case, the positive charge is localized on the
nitrogen atom to form a conjugated bond system in the
[M – Me]+ ion.6 The mass spectra of these compounds
differ also in the intensity of the [M – H]+ ion (5.4% for 4
and 41.5% for 5). This difference could be expected from
the aboveꢀconsidered data. A high probability of the loss
of the Me group from [M]+ in pyrimidinophane 4 leads to
a sharp decrease in the probability of the competitive
reaction of proton abstraction from [M]+. The same situꢀ
ation was observed in the case of the loss of the Et group
from [M]+ as a result of the rearrangement in the pentaꢀ
methylene bridges. It is the formation of a stable molecuꢀ
lar ion that makes energetically less favorable processes
(for example, the loss of the ethyl group from ions at
m/z = 340 and 326 with an intensity of 8.7% for comꢀ
pound 4 and 33.1% for compound 5) more probable, all
other factors being the same. The ion at m/z 278 is formed
as a result of the loss of the benzyl radical from [M]+. This
process is predictable,7 and the high intensity of this ion
in the mass spectrum of macrocycle 5 (94.9%) agrees well
with the aboveꢀdiscussed features of the ion fragmentaꢀ
tion for 4 and 5. The formation of the most intense ion
[C6H5CH2]+ (m/z 91) is accompanied by the cleavage of
the βꢀbond with respect to the benzene ring (a weaker
bond). The charge is delocalized with the involvement of
the aromatic ring, which is followed by the rearrangement
into the energetically more favorable tropylium ion.
Single crystals of pyrimidinophanes 4 and 5 were grown
from DMSO. Xꢀray diffraction study demonstrated that
the crystals contain no solvent molecules. There are two
crystallographically independent molecules (A and B) in
the crystal structure of pyrimidinophane 4. The geometric
parameters of these molecules are equal within experiꢀ
mental error, except for the signs of the corresponding
torsion angles. (One of two independent molecules is
shown in Fig. 1. The average geometric parameters are
given in Table 1 and the further discussion.)
C(6)
C(21)
C(22)
C(5)
N(7)
C(20)
C(25)
C(8)
C(9)
C(19)
C(23)
C(4)
C(3)
C(2)
C(24)
C(10)
C(11)
2.8
C(17)
C(18)
2.9
C(16)
O(17)
C(12)
N(1)
N(13)
C(14)
O(14)
C(15)
Fig. 1. Geometry of molecule B of pyrimidinophane 4 in the
crystal. The shortest distances (Å) between the phenyl and uracil
fragments are indicated by dashed lines.
C(6)
C(21)
C(5)
C(8)
C(20)
C(22)
C(23)
C(19)
N(7)
C(3)
C(9)
C(4)
C(2)
C(24)
C(25)
C(10)
4.1
C(17)
O(17)
N(1)
2.7
C(11)
C(12)
C(16)
C(14)
N(13)
O(14)
C(15)
C(18)
Fig. 2. Geometry of molecule 5 in the crystal. The disorder is
omitted for simplicity. The shortest distances (Å) between the
phenyl and uracil fragments are indicated by dashed lines.
There is one molecule per asymmetric unit in the crysꢀ
tal structure of pyrimidinophane 5. One of the atoms in
the bridge, viz., C(10), is disordered over two positions
with approximately equal occupancies.
listed in Table 2. It can be seen that the plane of the
pyrimidine ring (1) is virtually orthogonal to both the
plane of the benzene ring (3) and the mean plane of the
14ꢀmembered heterocycle (2). However, edgeꢀtoꢀface
π—π interactions or C—H...π interactions cannot, apparꢀ
ently, occur between the pyrimidine and benzene rings,
because these fragments are shifted with respect to each
other. The distances between the centers of the rings are
5.8 and 6.0 Å in macrocycles 4 and 5, respectively.8,9
The polymethylene bridge together with the nitrogen
and carbon atoms of the pyrimidine ring and the nitrogen
atom in the bridge form a 14ꢀmembered heterocycle. In
In the crystals, the molecules of pyrimidinophanes 4
and 5 (Figs 1 and 2; Table 1) adopt a folded Cꢀshaped
conformation with the phenyl and uracil fragments in
close proximity. The conformations of pyrimidinophanes
4 and 5 can be quantitatively characterized by the mutual
arrangement of the following three planes: the plane of
the pyrimidine ring (1), the mean plane of the 14ꢀmemꢀ
bered heterocycle (2), and the plane of the benzene
ring (3). The mutual arrangement of these fragments is
shown in Fig. 3. The corresponding dihedral angles are

Pyrimidinophanes with nitrogen in the bridge
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 3, March, 2006
561
Table 1. Selected geometric parameters of pyrimidinophanes 4
and 5 (Xꢀray diffraction data)
(3)
Parameter
4
5
Bond length
d/Å
O(14)—C(14)
O(17)—C(17)
N(1)—C(16)
N(1)—C(2)
N(1)—C(17)
N(7)—C(8)
N(7)—C(6)
N(7)—C(19)
N(13)—C(14)
N(13)—C(17)
N(13)—C(12)
C(2)—C(3)
C(3)—C(4)
C(4)—C(5)
C(5)—C(6)
C(8)—C(9)
1.220(5)
1.222(5)
1.384(5)
1.473(5)
1.395(5)
1.465(5)
1.465(6)
1.451(6)
1.402(5)
1.375(5)
1.464(5)
1.515(6)
1.520(5)
1.527(5)
1.520(6)
1.528(6)
1.509(5)
1.428(6)
1.344(5)
1.493(6)
—
1.228(4)
1.215(3)
1.372(3)
1.476(4)
1.370(3)
1.464(4)
1.462(4)
1.464(4)
1.404(4)
1.390(3)
1.485(4)
1.518(4)
1.506(4)
1.514(4)
1.504(4)
1.524(5)
1.446(6)
1.426(4)
1.339(4)
—
(2)
(1)
Fig. 3. Mutual arrangement of the fragments in pyrimidinophanes
4 and 5.
C(11)—C(12)
C(14)—C(15)
C(15)—C(16)
C(16)—C(18)
C(15)—C(18)
C(19)—C(20)
Table 2. Interplanar angles in macrocycles 4 and 5 (Xꢀray difꢀ
fraction data)
Planes
Dihedral angle/deg
1.501(4)
1.506(4)
4
5
1.503(6)
Bond angle
ω/deg
(1), (2)
(1), (3)
(2), (3)
88.5
86.7
70.9
87.6
88.7
71.0
C(2)—N(1)—C(16)
C(2)—N(1)—C(17)
C(16)—N(1)—C(17)
C(6)—N(7)—C(8)
C(6)—N(7)—C(19)
C(8)—N(7)—C(19)
C(12)—N(13)—C(14)
C(12)—N(13)—C(17)
C(14)—N(13)—C(17)
O(14)—C(14)—N(13)
O(14)—C(14)—C(15)
N(13)—C(14)—C(15)
C(14)—C(15)—C(16)
N(1)—C(16)—C(15)
C(15)—C(16)—C(18)
C(16)—C(15)—C(18)
N(1)—C(16)—C(18)
C(14)—C(15)—C(18)
O(17)—C1(7)—N(13)
O(17)—C1(7)—N(1)
N(7)—C(19)—C(20)
N(1)—C(2)—C(3)
C(2)—C(3)—C(4)
C(3)—C(4)—C(5)
C(4)—C(5)—C(6)
N(7)—C(6)—C(5)
N(7)—C(8)—C(9)
N(13)—C(12)—C(11)
123.6(3)
115.0(3)
121.0(3)
111.8(3)
111.6(3)
111.0(3)
118.5(3)
116.7(3)
124.3(3)
120.2(3)
125.3(4)
114.5(3)
122.5(4)
120.1(3)
121.0(4)
—
120.1(2)
117.2(2)
121.8(2)
112.6(2)
110.3(2)
110.3(2)
118.7(2)
116.3(2)
124.8(2)
119.5(3)
124.6(3)
115.9(2)
119.0(2)
123.0(2)
—
transannular C(4)...C(10) distance is 3.97 and 3.95 Å in
pyrimidinophanes 4 and 5, respectively (Fig. 4). This shape
of the bridge is characterized by four C—H...N interacꢀ
tions between the intraannular hydrogen atoms at the
C(4) and C(10) atoms, on the one hand, and the N(1)
and N(13) atoms of the pyrimidine rings and the N(7)
atom in the bridge, on the other hand. The parameters of
the C—H...N interactions are given in Table 3.
It is of interest to analyze intermolecular interactions
in the crystals of pyrimidinophanes 4 and 5. The indepenꢀ
dent molecules A and B in macrocycle 4 are involved in
different interactions with the adjacent molecules. The
pyrimidine rings of molecules A related by the symmetry
operation [–x, 1 – y, –z] are involved in rather strong9,10
122.0(3)
118.9(3)
—
118.9(3)
122.4(2)
122.2(2)
113.3(3)
112.3(2)
114.5(3)
113.6(3)
114.2(2)
114.7(2)
113.8(3)
113.4(3)
122.5(3)
120.3(4)
114.3(3)
113.6(3)
114.0(3)
113.3(4)
113.1(4)
112.4(3)
112.7(4)
112.1(3)
Table 3. Parameters of intramolecular C—H...N interactions in
pyrimidinophanes 4 and 5
Fragment
d(H...N)/Å
∠С—H...N/deg
4
5
4
5
С(4)—H...N(1)
C(10)—H...N(7)
C(4)—H...N(7)
C(10)—H...N(13)
2.58
2.64
2.67
2.52
2.51
2.48
2.67
2.61
105
102
105
101
104
108
105
105
the most developed projection, the conformation of the
heterocycle has a figureꢀofꢀeight shape, and the shortest

562
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 3, March, 2006
Semenov et al.
of two adjacent molecules of pyrimidinophane 5 overlap
to a greater degree than the aromatic systems of the
6ꢀmethyluracil fragments of molecules A in the crystal
structure of pyrimidinophane 4. The parameters of the
π—π interactions of two molecules of macrocycle 5 reꢀ
lated by a center of symmetry (the symmetry transformaꢀ
tion [1 – x, –y, 1 – z]) are as follows: the distances
between the centers of the rings are 3.4 Å, the dihedral
angle between the rings is 0°, and the distance between
the planes of the rings is 3.34 Å. On the whole, the molꢀ
ecules in the crystals of pyrimidinophanes 4 and 5 are
packed in layers. The layers are linked to each other by
π—π and C—H...O interactions.
N(7)
C(4)
C(10)
The structures of the pyrimidinophanes in solution
was studied by NMR spectroscopy. The full assignment of
the signals in the 1H and 13C NMR spectra of compounds
4 and 5 in CDCl3 was made based on the 2D COSY,
2D TOCSY, 2D HSQC, 2D HMBC,11,12 and 1D ROESY
experiments13 at 303 and 263 K. The low temperature was
used to increase the intensities of crossꢀpeaks in the
2D HMBC spectra.
N(1)
N(13)
Fig. 4. Intramolecular C—H...N interactions in pyrimidinoꢀ
phanes 4 and 5.
Let us consider the assignment of the signals in the
1H and 13C NMR spectra in detail using pyrimidinophane
4 as an example (Fig. 6). In the first step, the structures of
the fragments starting from C(5)H for the 6ꢀmethyluracil
fragment, from H(2) and H(12) for the aliphatic bridges,
and from the CH2 group for the benzyl fragment were
determined using both 1D and 2D NMR experiments
aimed at revealing correlations due to homoꢀ and heteroꢀ
nuclear spinꢀspin coupling constants. In the next step,
the fragments are combined based on 2D HMBC correꢀ
lations.
π—π interactions (the distance between the centers of the
rings is 3.8 Å, the dihedral angle between the rings is 0°,
and the distance between the planes of the rings is 3.39 Å).
In the adjacent molecules B related by the symmetry opꢀ
eration [1 – x, –y, –z], the pyrimidine rings are shifted
with respect to each other, and the distance between their
centers is 5.06 Å. There are C—H...O interactions beꢀ
tween these rings involving the proton of the C(18)H3
group and the O(17) atom (H(18B)...O(17B), 2.56 Å;
C(18)—H(18B)...O(17B), 145.8°; Fig. 5). In the crystal
structure, the aromatic systems of the thymine fragments
Information on the presence of through twoꢀ or moreꢀ
bond spinꢀspin coupling constants between the protons of
a
b
c
O(17A)
C(17A)
N(13A)
C(2)
N(1)
O(17)
C(18B)
N(1A)
C(14A)
C(16A)
C(17)
N(13)
C(16)
C(15)
N(1B)
C(16B)
O(14A) C(15A)
C(14)
O(14)
C(15B)
C(14B)
O(17B)
N(13B)
O(14B)
Fig. 5. Diagram of overlap of the pyrimidine rings in the adjacent independent molecules A of macrocycle 4 (a), independent
molecules B of macrocycle 4 (b), and molecules of macrocycle 5 (c).

Pyrimidinophanes with nitrogen in the bridge
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 3, March, 2006
563
CHCl3
*
H2O
CHCl3
Me(16)
C(15)H
Ho
Hm
Hp
Ph
H(3)
7.3
7.2
δ, м.д.
H(5),H(9)
H(6,8)
H(10)
H(4)
CH2Ph
H(11)
H(2)
H(12)
H(12)
H(2)
H(3)
7
6
5
4
3
2
1
δ
1
Fig. 6. H NMR spectrum of macrocycle 4 in CDCl3 at T = 303 K (the signals of an unidentified impurity in the solvent are marked
with asterisks).
the macrocycle can be obtained from 2D TOCSY spectra.
The protonꢀproton correlations found in the 2D COSY
and 2D TOCSY spectra are shown in Fig. 7, a. The
2D HMBC correlations and the observed nuclear
Overhauser effects (NOE) for macrocycle 4 are presented
in Fig. 7, b.
The resonances for the geminal protons at the C(2)
atom in the bridge appear as two broadened multiplets at
δ 4.43 and 3.53. The presence of crossꢀpeaks between the
signals for these protons and resonances for other protons
of the bridge in the 2D TOCSY spectrum allows the deꢀ
termination of the resonances for all protons of the
C(2)—C(6) chain. Crossꢀpeaks between the signals for
the vicinal CH2 groups were distinguished among the sigꢀ
nals observed in the 2D TOCSY spectrum based on the
analysis of the crossꢀpeaks in the 2D COSY spectrum,
thus making possible the assignment of all signals. For
example, based on the presence of crossꢀpeaks between
the signals for the C(2)H protons and broadened multiꢀ
plets at δ 1.91 and 1.42 in the 2D COSY spectrum, these
signals were assigned to the resonance of the protons at
the C(3) atom of the bridge. The positions of the resoꢀ
nances for other protons of the C(2)—C(6) chain were
determined analogously (δ 1.30 for C(4)H, 1.40 for C(5)H,
and 2.33 for C(6)H). The crossꢀpeaks between the signals
for the protons at δ 4.43 and 3.53 and the 13C signal at
δ 152.6 (C(17)) and the correlations between C(15),
C(2)H, and the CH3 group and the signal at δ 151.7 in the
2D HMBC spectrum of pyrimidinophane 4 additionally
confirmed the assignment of the signals for the C(2)H
group and made it possible to assign the signal at δ 151.7
to the C(16) atom.
a
2D COSY correlation
2D TOCSY correlation
b
In the 1H NMR spectrum of macrocycle 4, the assignꢀ
ment of the signals for the C(12)—C(8) chain, in which
two broadened multiplets at δ 4.28 and 3.96 belong to the
protons at C(12), was made analogously. This assignment
was confirmed by the presence of crossꢀpeaks between
the signals at δ 4.28 and 3.96 and the resonance for C(14)
of the 6ꢀmethyluracil fragment in the 2D HMBC specꢀ
trum of pyrimidinophane 4. In addition, the 1D ROESY
spectrum (Fig. 8, b) shows NOE between the signals
2D HMBC correlation
selected NOE
Fig. 7. Observed correlations based on the spinꢀspin coupling
constants (a) and 2D HMBC and 1D ROESY experiments (b).

564
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 3, March, 2006
Semenov et al.
Me(16)
H(11)
H(3)
H(5),H(9)
H(4),H(10)
Ph
C(15)H
a
b
c
H(6),H(8)
CH2Ph
H(12)
4
H(2)
H(3)
H(2)
7
6
5
3
2
1
δ
Me(16)
Ph
H(6),H(8)
H(5),H(9)
H(4),H(10)
C(15)H
7
6
5
4
3
2
1
δ
C(15)H
H(2)
H(2)
CH2Ph
Ph
{Me(16)}
2
7
7
6
6
5
5
4
4
3
3
1
1
δ
δ
C(15)H
CH2Ph
H(6)
H(6),H(8)
d
{Ph}
2
Fig. 8. 1H NMR (a) and 1D ROESY (b—d) spectra of pyrimidinophane 4.
for the protons at the C(2) atom and the methyl group
The 1H NMR spectrum of pyrimidinophane 5 is simiꢀ
at C(16) of the 6ꢀmethyluracil fragment, which adꢀ
ditionally confirms the validity of the differentiation
and assignment of the signals for the C(2)H and C(12)H
protons.
The chemical shifts of the carbon atoms of the
C(2)—C(6) and C(12)—C(8) chains were determined
based on the 2D HSQC spectrum taking into account the
known positions of the resonances for the protons in the
bridge.
The signals for the geminal protons of the methylene
group of the benzyl fragment at 263 K appear as an AB
system. The resonances for the phenyl protons, which
appear as two triplets and one doublet, were identified
from the crossꢀpeaks between the signals for these proꢀ
tons and the lowꢀfield signal in the 2D TOCSY spectrum.
In addition, the 2D COSY spectrum shows crossꢀpeaks
between the signals for the Hm and Ho protons and beꢀ
tween the Hm and Hp protons. Based on the crossꢀpeak
between the signals for the CH2Ph and Ho protons and
the signal at δ 140.5 in the 2D HMBC spectrum, the latter
was assigned to the C(1´) atom. The chemical shifts of
other carbon atoms involved in the benzyl fragment were
determined based on the corresponding crossꢀpeaks in
the 2D HSQC spectrum.
lar to that described above. Analysis for the macrocycle
was performed analogously. The 2D HMBC spectrum
of 5, unlike the spectrum of pyrimidinophane 4, shows
additional crossꢀpeaks between the CH3 group and C(14),
C(16), and C(2)H and NOE between C(2)H and C(16)H.
Therefore, the geminal protons of the CH2 groups at
N(13) and N(1) of pyrimidinophanes 4 and 5 are essenꢀ
tially nonequivalent. The magnetic nonequivalence of
these protons is a consequence of conformational rigidity
of the pyrimidinophane molecules, on the one hand, and
the anisotropy of the pyrimidine ring, on the other hand.
In this case, the conformational rigidity implies that the
inversion of the macrocycle between two or several symꢀ
metrical forms (intramolecular transitions) is slow on the
NMR time scale, resulting in the spectrum nonaveraged
due to exchange.
Due to larger conformational flexibility of pyrimidinoꢀ
phanes containing two or more pyrimidine fragments, the
1H NMR spectra measured under fast exchange condiꢀ
tions are virtually firstꢀorder spectra of an AX system.14—17
The flexibility (the inversion rate) of pyrimidinophanes
increases with increasing length of the bridge. In the
1H NMR spectra of [n](1,3)pyrimidinophanes described
earlier,14 the signals for the geminal protons first become

Pyrimidinophanes with nitrogen in the bridge
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 3, March, 2006
565
broader and then become narrower as the length of the
bridge increases in going from macrocycles containing
10 methylene groups in the bridge to macrocycles conꢀ
taining 11 or more methylene groups, and finally the specꢀ
tral pattern becomes similar to a firstꢀorder spectrum.
The flexibility of the bridge in macrocycles containing a
larger number of CH2 groups in the bridge between the
N(1) and N(13) atoms is also higher than that in pyrimiꢀ
dinophanes 4 and 5, which is accompanied by the correꢀ
sponding changes in the 1H NMR spectra. These data will
be discussed elsewhere.
The threeꢀdimensional structures of pyrimidinophanes
4 and 5 in solutions in CDCl3 were determined based on
oneꢀdimensional NOE experiments using gradients to reꢀ
focus the signals, which made it possible to substantially
increase the sensitivity threshold of the method for these
effects. The spectrum of macrocycle 4 shows nontrivial
crossꢀpeaks between C(16)Me and C(15)H and the sigꢀ
nals for the phenyl protons (Figs 8 and 9). The spectrum
of macrocycle 5 shows nontrivial crossꢀpeaks between
C(15)Me, C(16)H, and ArH. These data indicate that the
protons of the uracil and phenyl fragments are in close
proximity in CDCl3, i.e., the molecule adopts a folded
conformation analogous to that shown in Figs 1, 2, and 9.
These threeꢀdimensional structures of the macrocycles
are, apparently, responsible for the deshielding effect of
the benzene ring on C(15)H in pyrimidinophane 4 and on
C(15)Me in pyrimidinophane 5. In the spectra of comꢀ
pounds 1 and 2, the signals for these protons are observed
at higher field.
UV spectra of pyrimidinophanes are determined by abꢀ
sorption of the uracil fragment at 272 nm and are characꢀ
terized by the molar extinction coefficients ε = 8056
and 7735 for macrocycles 4 and 5, respectively. We
used acyclic 1,3ꢀbis(bromobutyl)ꢀ6ꢀmethyluracil (6) and
1,3ꢀbis(butyl)thymine (7) with ε = 10157 (267 nm) and
ε = 8507 (272 nm), respectively, as the reference comꢀ
pounds. As can be seen from the above data, the intensity
of absorption of the uracil fragment in cyclic structures is
lower than that in acyclic structures, and this decrease
can be numerically expressed by the hypochromic effect
(in percent).18 This effect is (1 – 8056/10157)•100 =
20.7% and (1 –7735/8507)•100 = 9.1% for pyrimidinoꢀ
phanes 4 and 5, respectively. Therefore, macrocycles 4
and 5 exhibit the hypochromic effect with respect to the
corresponding 1,3ꢀbisalkylꢀsubstituted uracils. According
to the theory of the hypochromic effect,19,20 a decrease in
the intensity of light absorbed by a nucleotide base can be
attributed to the involvement of the latter in stacking
interactins with other aromatic systems. Apparently, the
pyrimidinophanes adopt conformations, in which the
plane of the benzene ring is parallel to the plane of the
pyrimidine ring, in a CHCl3 solution on the UV time
scale. According to the theory of π—π interactions,21 the
distance between the rings should be no larger than 3.4 Å,
which is consistent with the NMR spectroscopic data, in
particular, with the presence of NOE between the protons
of the uracil and phenyl fragments.
The uracil fragments have large dipole moments (for
example, the dipole moment for 1,3,6ꢀtrimethyluracil
is 4.69 D).22 Hence, the dipole moment method23 can be
used as an independent method for the analysis of the
structures of pyrimidinophanes 4 and 5 in solution. The
experimental dipole moments for macrocycles 4 and 5 in
benzene are 4.85 0.05 and 3.90 0.04 D, respectively. In
subsequent calculations of the theoretical molecular diꢀ
pole moments of compounds 4 and 5, the polarity of the
pyrimidine fragments was described as follows. Using the
approach to estimations of the polarities of pyrimidines
proposed earlier,22 we represented the dipole moment of
1,3,6ꢀtrimethyluracil (4.69 D)22 as the following three
vectors: C(17)→O (1.13 D), N(1)→C(2) (2.63 D), and
C(14)→O (2.63 D). In turn, the dipole moment of
1,3,5ꢀtrimethyluracil (3.98 D)22 was represented as the
following three vectors: C(17)→O (1.33 D), N(1)→C(2)
(2.30 D), and C(14)→O (2.30 D). The dipole moments of
other polar bonds of pyrimidinophanes were estimated
based on the published data:23,24 0.53 D for C—N, 0.28 D
for H—Csp3, and 0.36 D for CH2—Ph. The theoretical
molecular dipole moments of compounds 4 and 5, which
were calculated according to the vectorꢀadditive scheme24
with the use of the aboveꢀmentioned dipole moments of
the bonds and the atomic coordinates determined by Xꢀray
diffraction, are 4.85 and 3.98 D, respectively. A good
agreement between the experimental and theoretical moꢀ
The fact that the uracil and phenyl fragments are in
close proximity (the molecules adopt a folded conformaꢀ
tion in CHCl3) was confirmed by comparing the UV specꢀ
tra of pyrimidinophanes 4 and 5 with those of acyclic
1,3ꢀbisalkylꢀsubstituted 6ꢀmethyluracil and thymine. The
Fig. 9. Assumed threeꢀdimensional structure of pyrimidinoꢀ
phanes 4 in CDCl3 and observed nontrivial NOE.

566
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 3, March, 2006
Semenov et al.
Table 4. Crystallographic parameters of compounds 4 and 5 and
details of Xꢀray diffraction study
lecular dipole moments indicates that macrocycles 4 and 5
in benzene solutions adopt conformations very similar to
those observed in the crystals (see Figs 1 and 2).
Parameter
4
5
To summarize, we synthesized pyrimidinophanes, in
which the N(1) and N(3) atoms of the 6ꢀmethyluracil or
thymine fragments are linked to each other by a bridge
containing 10 methylene groups and the N atom bearing
the benzyl substituent. Compared to macrocycles conꢀ
taining two or more pyrimidine rings, the pyrimidinoꢀ
phanes under consideration are more rigid, which is reꢀ
flected, in particular, in the spectroscopic features of
pyrimidinophanes. In the crystals, the geometry of the
pyrimidinophane molecules is characterized by the mutuꢀ
ally orthogonal arrangement of the benzene and pyrimiꢀ
dine rings with the shortest distances between the protons
of these aromatic fragments varying from 2.7 to 4.1 Å. In
solution, the uracil and phenyl fragments are in close
proximity (NMR and UV spectroscopic data and the reꢀ
sults obtained by the dipole moment method), although it
is difficult to unambiguously determine their mutual arꢀ
rangement. The factors responsible for the close arrangeꢀ
ment of the components involved in macrocycles 4 and 5
remain unclear. Nevertheless, interactions responsible for
the aboveꢀdescribed structural features can be determined
by varying the distances between the uracil fragment and
the substituent at the N atom in the bridge.
Crystal system
Space group
а/Å
b/Å
c/Å
Monoclinic
P21/с
Monoclinic
Р21/a
9.544(5)
12.358(8)
34.99(3)
91.52(6)
4125(5)
8
369.50
1.19
6.1
12.426(7)
10.098(8)
16.61(1)
92.27(7)
2083(3)
4
369.50
1.18
6.0
β/deg
V/Å3
Z
M
dcalc/g cm–3
Absorption
coefficient, µ/cm–1
Radiation (λ/Å)
θꢀScan range/deg
Scanning technique
Number of measured
reflections
CuKα, 1.54184
4.63 ≤ θ ≤ 70.02 5.13≤ θ ≤ 69.95
ω
7705
ω
3989
Number of observed
reflections with I > 2σ(I )
Final R factors:
R
3618
2167
0.078
0.204
0.999
7231
0.061
0.173
1.003
3804
Rw
Fitting parameter
Number of reflections
used in the refinement
Number of parameters
in the refinement
Number of independent
490
249
Experimental
The 1H, 13C, and 2D NMR spectra, including COSY,
HSQC, HMBC, and ROESY experiments, for compounds 4
and 5 were recorded on a Bruker AVANCEꢀ600 Fourierꢀ
transform spectrometer operating at 600.000 (1H) and
150.864 (13C) MHz in CDCl3 at 30 °C with the use of the
residual signal of CDCl3 (δ 7.26 and δ 77.0) as the internal
7622(0.053)
3950(0.015)
reflections (Rint
)
Note. Check reflections: two check reflections distributed in
orientation and three check reflections distributed in intensity
after every 200 reflections. The positions of the hydrogen atoms
were revealed from difference Fourier maps and refined with
fixed positional and thermal displacement parameters in the
final steps.
H
C
standard. The electron impact mass spectra were obtained on a
MATꢀ212 Finnigan mass spectrometer at 1000 resolution with
the use of the MSS MASPEC II data system. The experimental
conditions: a direct inlet system, the ion source temperature
was varied from 20 to 300 °C, the ionizing electron energy
was 70 eV, and the electron emission current was 1.0 mA. The
IR spectra were measured on a Bruker Vectorꢀ22 Fourierꢀtransꢀ
form spectrometer under standard conditions in KBr pellets.
The UV spectra were recorded on a Specord UVꢀVis spectroꢀ
photometer (Carl Zeiss Jena). The experimental dipole moꢀ
ments were determined in dilute benzene solutions at 20 °C
using the second Debye method.23,24
The melting points were measured on a Boetius hotꢀstage
apparatus and are uncorrected. Thinꢀlayer chromatography was
performed on Silufolꢀ254 plates. The spots in plates were visualꢀ
ized with UV light.
All solvents and reagents were dried.
1,3ꢀBis(5ꢀbromopentyl)ꢀ6ꢀmethyluracil30 (1), 1,3ꢀbis(bromoꢀ
butyl)ꢀ6ꢀmethyluracil30 (6), and 1,3ꢀbis(4ꢀbromobutyl)thymine16
(7) were prepared according to known procedures.
Xꢀray diffraction data sets for compounds 4 and 5 were
collected on automated fourꢀcircle EnrafꢀNonius CADꢀ4
diffractometers at room temperature (20 °C). Crystals of comꢀ
pounds 4 and 5 were grown from DMSO. The crystallographic
parameters and details of Xꢀray data study are given in Table 4.
The calculations were carried out with the use of the MoIEN
program25 on an Alpha Station 200. The structures were solved
by direct methods using the SIR program26 and refined using the
SHELXL9727 and WinGX programs.28
1,3ꢀBis(5ꢀbromopentyl)thymine (2). A solution of 1,5ꢀdiꢀ
bromopentane (155.9 g, 677.8 mmol) in DMF (90 mL) was
added dropwise with stirring to a solution of the disodium salt of
thymine (14.4 g, 84.7 mmol) in DMF (150 mL). The reaction
mixture was stirred at 50—60 °C for 5 h. The solvent was reꢀ
moved in vacuo, benzene (100 mL) was added, and the mixture
was filtered. The filtrate was concentrated to 10—15 mL and
chromatographed through an alumina column. The column was
successively washed with petroleum ether and a 2 : 1 diethyl
ether—petroleum ether mixture. Compound 2 was obtained as
an oil from the first fractions in a yield of 15.9 g (45%), Rf 0.74
The figures were drawn and the intermolecular contacts were
analyzed using the PLATON program.29

Pyrimidinophanes with nitrogen in the bridge
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 3, March, 2006
567
(diethyl ether as the eluent). Found (%): C, 42.38; H, 5.71;
N, 6.53; Br, 37.77. C15H24Br2N2O2. Calculated (%): C, 42.47;
H, 5.70; N, 6.60; Br, 37.67. 1H NMR (CDCl3), δ: 7.01 (s, 1 H,
C(16)H); 3.95 (t, 2 H, NpyrCH2, J = 7.2 Hz); 3.73 (t, 2 H,
NpyrCH2, J = 7.2 Hz); 3.42 (m, 4 H, 2 CH2Br); 1.93 (s, 3 H,
C(15)CH3); 1.89 (m, 4 H, 2 NpyrCCH2); 1.76—1.58 (m, 4 H,
2 CH2CBr); 1.50 (m, 4 H, 2 CCCH2CC). MS (EI, 70 eV), m/z:
426 (9), 424 (22), 422 (10), 346 (28), 345 (88), 344 (28), 343
(88), 275 (75), 195 (86), 140 (100).
1635 (C=O, uracil fragment). 1H NMR (CDCl3), δ: 7.27 (t,
2 H, Hm, J = 7.3 Hz); 7.25 (t, 1 H, Hp, J = 7.3); 7.15 (d, 2 H, Ho,
J = 7.3 Hz); 6.97 (s, 1 H, C(16)H); 4.48 (m, 1 H, C(2)H); 4.28
(m, 1 H, C(12)H); 4.03 (m, 1 H, C(12)H); 3.44 (d, 1 H, CHPh,
J = 12.2 Hz); 3.35 (d, 1 H, CHPh, J = 12.2 Hz); 3.18 (m, 1 H,
C(2)H); 2.32 (m, 4 H, C(6)H2, C(8)H(2)); 2.04 (s, 3 H,
C(15)CH3); 1.90 (m, 1 H, C(3)H); 1.80 (m, 1 H, C(11)H); 1.64
(m, 1 H, C(11)H); 1.48 (m, 1 H, C(3)H); 1.31 (m, 4 H, C(5)H2,
C(9)H2); 1.25 (m, 2 H, C(4)H2); 1.21 (m, 2 H, C(10)H2).
13C NMR (CDCl3), δ: 163.9 (C(14)), 152.0 (C(17)), 138.6
(C(16)), 109.4 (C(15)), 140.2 (C(1´)), 128.7 (C(2´)), 127.9
((C(3´)), 126.7 (C(4´)), 59.0 ((C(1´)), 53.5 (C(6), C(8)), 49.0
(C(12)), 40.5 (C(2)), 27.9 (C(11)), 27.1 (C(9)), 26.3 (C(3)),
22.9 (C(5)), 22.6 (C(4), C(10)), 13.2 (C(15)C). MS (EI, 70 eV),
m/z (Irel (%)): 371 (3.4), 370 (26.4), 369 [M]+ (94.1), 368
[M – 1]+ (41.5), 354 [M – 15]+ (4.8), 340 [M – 29]+ (33.1), 327
(4.8), 314 (6.2), 313 (19.0), 292 (7.1), 279 (24.0), 278 [M – 91]+
(94.9), 250 (29.3), 134 (29.9), 127 (5.3), 120 (16.6), 91 (100.0),
42 (17.8), 41 (18.4).
7ꢀBenzylꢀ16ꢀmethylꢀ1,7,13ꢀtriazabicyclo[11.3.1]heptadecaꢀ
15ꢀeneꢀ14,17ꢀdione (4). Potassium carbonate (4.90 g, 35.5 mmol)
was added to a solution of benzylamine 3 (1.90 g, 17.8 mmol) in
nꢀbutanol (250 mL), and then a solution of compound 1 (2.5 g,
5.9 mmol) in nꢀbutanol (100 mL) was added with stirring at
90 °C. The reaction mixture was stirred at 100—110 °C for 7 h
and then cooled. The solvent was distilled off, chloroform
(150 mL) was added to the residue, the mixture was filtered, and
the filtrate was concentrated to 10—20 mL and chromatographed
through an alumina column. The column was washed with a 1 : 3
petroleum ether—diethyl ether mixture to prepare pyrimidinoꢀ
phane 4 in a yield of 0.35 g (16%), Rf 0.29 (diethyl ether—hexane,
5 : 1, as the eluent), m.p. 114 °C. Found (%): C, 71.40; H, 8.57;
N, 11.39. C22H31N3O2. Calculated (%): C, 71.51; H, 8.46;
N, 11.37. Found: m/z 369.242 [M]+. C22H31N3O2. Calculated:
M = 369.2416. UV (CHCl3), λmax/nm (logε): 271 (3.91).
IR (KBr), ν/cm–1: 3085, 3021, 1614, 1493, 765, 718/699
(C(16)H, CaromH, benzene ring), 2954, 2929, 2862, 1469, 1429,
1378, 1341, 744 (CH3, CH2), 2794 (CH2(N)), 1687, 1652 (C=O,
uracil fragment). 1H NMR (CDCl3), δ: 7.25 (t, 2 H, Hm, J =
7.3 Hz); 7.19 (t, 1 H, Hp, J = 7.3 Hz); 7.16 (d, 2 H, Ho, J =
7.3 Hz); 5.69 (s, 1 H, C(15)H); 4.42 (m, 1 H, C(2)H); 4.28 (m,
1 H, C(12)H); 3.95 (m, 1 H, C(12)H); 3.53 (m, 1 H, C(2)H);
3.43 (d, 1 H, CHPh, J = 12.2 Hz); 3.37 (d, 1 H, CHPh, J =
12.2 Hz); 2.33 (m, 2 H, C(8)H2); 2.32 (m, 2 H, C(6)H2); 2.22
(s, 3 H, C(16)CH3); 1.91 (m, 1 H, C(3)H); 1.81 (m, 1 H,
C(11)H); 1.61 (m, 1 H, C(11)H); 1.42 (m, 1 H, C(3)H), 1.35
(m, 2 H, C(4)H2); 1.40 (m, 4 H, C(5)H2, C(9)H2); 1.30 (m,
2 H, C(10)H2). 13C NMR (CDCl3), δ: 162.6 (C(14)), 152.6
(C(17)), 151.7 (C(16)), 101.7 (C(15)), 140.5 (C(1´)), 128.7
(C(2´)), 127.9 (C(3´), 126.5 ((C(4´)), 58.6 (C(C(1´)), 53.9
(C(8)), 53.5 (C(6)), 44.3 (C(12)), 40.3 (C(2)), 28.2 (C(11)),
27.6 (C(10)), 27.5 (C(4)), 26.6 (C(3)), 22.8 (C(5), C(9)), 20.1
(C(16)C). MS (EI, 70 eV), m/z (Irel (%)): 371 (1.7), 370 (10.2),
369 [M]+ (42.8), 368 [M – 1]+ (5.4), 354 [M – 15]+ (80.7), 340
[M – 29]+ (8.7), 327 (7.4), 314 (7.3), 313 (31.4), 292 (27.6), 279
(10.1), 278 [M – 91]+ (46.6), 134 (13.0), 127 (7.8), 120 (7.3), 91
(100.0), 42 (15.5), 41 (21.4).
This study was financially supported by the Russian
Foundation for Basic Research (Project Nos 05ꢀ03ꢀ
32497ꢀa, 05ꢀ03ꢀ32558ꢀa, and 04ꢀ03ꢀ32156).
References
1. V. M. Cherkasov and I. V. Boldyrev, Khim. Geterotsikl.
Soedin., 1998, 867 [Chem. Heteroc. Compd., 1998, 34 (Engl.
Transl.)].
2. R. E. Cramer, K. A. Mitchell, A. Y. Hirazumi, and S. L.
Smith, J. Chem. Soc., Dalton Trans., Inor. Chem., 1994, 563.
3. S. Kumar, G. Hundal, D. Paul, M. S. Hundal, and H. Singh,
J. Org. Chem., 1999, 64, 7717.
4. V. P. Krivonogov, G. A Tolstikov, G. G. Kozlova, G. A.
Sivkova, D. N. Lazareva, Yu. I. Murinov, F. S. Zarudii,
V. A. Myshkin, A. F. Ismagilova, L. V. Spirikhin, and I. B.
Abdrakhmanov, Proc. I Int. Conf. "The Chemistry and Bioꢀ
logical Activity of NitrogenꢀContaining Heterocycles and Alkaꢀ
loids," Eds V. G. Kartsev and G. A. Tolstikov, IridiumꢀPress,
Moscow, 2001, 1, 348.
5. R. Kh. Giniyatullin, A. S. Mikhailov, V. E. Semenov, V. D.
Akamsin, V. S. Reznik, A. V. Chernova, G. M. Dorozhkina,
A. A. Nafikova, Sh. K. Latypov, Yu. Ya. Efremov, D. R.
Sharafutdinova, A. T. Gubaidullin, and I. A. Litvinov, Izv.
Akad. Nauk, Ser. Khim., 2003, 1511 [Russ. Chem. Bull.,
Int. Ed., 2003, 52, 1595 (Engl. Transl.)].
6. N. S. Vul´fson, V. G. Zaikin, and A. I. Mikaya, Massꢀ
spektrometriya organicheskikh soedinenii [Mass Spectrometry
of Organic Compounds], Khimiya, Moscow, 1986, S. 50 (in
Russian).
7. V. G. Zaikin, A. V. Varlamov, A. I. Mikaya, and N. S.
Prostakov, Osnovy massꢀspektrometrii organicheskikh
soedinenii [Principles of Mass Spectrometry of Organic Comꢀ
pounds], Nauka, Moscow, 2001, 98 (in Russian).
8. M. Umezawa and M. Nishio, Nucleic Acids Res., 2002,
30, 2183.
7ꢀBenzylꢀ15ꢀmethylꢀ1,7,13ꢀtriazabicyclo[11.3.1]heptadecaꢀ
15ꢀeneꢀ14, 17ꢀdione (5) was prepared under conditions of the
synthesis of macrocycle 4 from compound 3 (2.14 g, 20 mmol),
K2CO3 (2.95 g, 21 mmol), and compound 2 (4.1 g, 9.7 mmol).
An alumina column was washed with diethyl ether, and pyrimiꢀ
dinophane 5 was isolated from the fractions in a yield of 0.6 g
(17%), Rf 0.50 (diethyl ether—hexane, 5 : 1, as the eluent),
m.p. 130—131 °C. Found (%): C, 71.49; H, 8.41; N, 11.40.
C22H31N3O2. Calculated (%): C, 71.51; H, 8.46; N, 11.37.
Found: m/z 369.242 [M]+. C22H31N3O2. Calculated: M =
369.2416. UV (CHCl3), λmax/nm (logε): 272 (3.89). IR (KBr),
ν/cm–1: 3064, 3027, 3003, 1600, 1492, 765/758, 718/699
(C(16)H, CaromH, benzene ring), 2950, 2927, 2863, 1467, 1443,
1379, 1345, 738 (CH3, CH2), 2808, 2789 (CH2(N)), 1697, 1663,
9. M. Nishio, Cryst. Eng. Comm., 2004, 6, 130.
10. S. K. Burley and G. A. Petsko, J. Am. Chem. Soc., 1988,
108, 7995.
11. A. E. Derome, Modern NMR Techniques for Chemistry Reꢀ
search, Pergamon, New York, 1987.

568
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 3, March, 2006
Semenov et al.
12. AttaꢀurꢀRahman, One and Two Dimensional NMR Spectroꢀ
scopy, Elsevier, Amsterdam, 1989, 579 pp.
13. K. Stott, J. Stonehouse, J. Keeler, T. L. Hwang, and A. J.
Shaka, J. Am. Chem. Soc., 1995, 117, 4199.
14. T. Itahara, Bull. Chem. Soc. Jpn., 1996, 69, 3239.
15. V. E. Semenov, V. D. Akamsin, V. S. Reznik, Yu. Ya.
Efremov, D. R. Sharafutdinova, A. A. Nafikova, and N. M.
Azancheev, Mendeleev Commun., 2001, 96.
16. V. E. Semenov, V. D. Akamsin, V. S. Reznik, A. V. Chernova,
G. M. Dorozhkina, Y. Y. Efremov, and A. A. Nafikova,
Tetrahedron Lett., 2002, 52, 9683.
23. L. McClellan, Tables of experimental dipole moments, Vol. 1,
W. H. Freeman and Co., SanꢀFrancisco—London, 1963,
713 pp.
24. V. I. Minkin, O. A. Osipov, and Yu. A. Zhdanov, Dipol´nye
momenty v organicheskoi khimii [Dipole Moments in Organic
Chemistrty], Khimiya, Leningrad, 1968, 248 pp. (in Russian).
25. A. Altomare, G. Cascarano, C. Giacovazzo, and D. Viterbo,
Acta Crystallogr., Sec. A. Fund. Crystallogr., 1991, 47, 744.
26. L. H. Straver and A. J. Schierbeek, MolEN. Structure Deterꢀ
mination System. Program Description, Nonius B. V, Delft,
1994, 1,180 p.
17. A. S. Mikhailov, R. Kh. Giniyatullin, V. E. Semenov, V. S.
Reznik, A. A. Nafikova, Sh. K. Latypov, Yu. Ya. Efremov,
and D. R. Sharafutdinova, Izv. Akad. Nauk, Ser. Khim.,
2003, 1324 [Russ. Chem. Bull., Ent. Ed., 2003, 52, 1399].
18. H. M. Warshaw and J. Jr. Tinoco, J. Mol. Biol., 1966, 20, 29.
19. J. Jr. Tinoco, J. Am. Chem. Soc., 1960, 82, 4785.
20. W. J. J. Rhodes, J. Am. Chem. Soc., 1961, 83, 3609.
21. C. A. Hunter, Chem. Soc. Rev. 1994, 23, 101.
27. G. M. Sheldrick, SHELXL97, Computer Program for Crystal
Structure Determination, University Göttingen, 1997.
28. L. J. Farrugia, J. Appl. Crystal., 1999, 32, 837.
29. A. L. Spek, Acta Crystallogr., Sec. A. Fund. Crystallogr.,
1990, 46, 34.
30. V. S. Reznik, I. Sh. Salikhov, Yu. S. Shvetsov, and B. E.
Ivanov, Izv. Akad. Nauk SSSR, Ser. Khim., 1980, 2568 [Bull.
Acad. Sci. USSR, Div. Chem. Sci.,1980, 29 (Engl. Transl.)].
22. A. P. Timosheva, L. V. Ermolaeva, S. G. Vulfson, N. V.
Utyaganov, and V. E. Kataev, Izv. Akad. Nauk, Ser. Khim.,
1993, 866 [Russ. Chem. Bull., 1993, 42, 818 (Engl. Transl.)].
Received July 11, 2005;
in revised form December 14, 2005