NMR studies on 4-thio-5-furan-modified and 4-thio-5-thiophene-modified nucleosides

Article abstract of DOI:10.1002/mrc.4501

Systematic NMR characterization of 4-thio-5-furan-pyrimidine nucleosides or 4-thio-5-thiophene-pyrimidine nucleosides (ribonucleosides and 2′-deoxynucleosides) was performed. All proton and carbon signals of 4-thio-5-thiophene-ribouridine and related analogues were unambiguously assigned. The orientations of the base (4-thiouridine or its deoxy analogue) relative to the ring (furan or thiophene) are explored by a NMR approach and further supported by X-ray crystallographic studies. The procedures presented here would be applicable to other modified nucleosides and nucleotides. Copyright

Full text of DOI:10.1002/mrc.4501

NMR studies on 4-Thio-5-furan- and
4-thio-5-thiophene-modified Nucleosides
Xiao-Hui Zhanga and Yao-Zhong Xub,
aCollege of Environment and Chemical Engineering, Dalian University Dalian 116622, China
b Department of Life, Health and Chemical Sciences, the Open University, Walton Hall, Milton Keynes,
MK7 6AA, UK
Abstract: Systematic NMR characterization of 4-thio-5-furan- or 4-thio-5-thiophene-
pyrimidine nucleosides (ribonucleosides and 2'-deoxynucleosides) was performed. All proton
and carbon signals of 4-thio-5-thiophene-ribouridine and related analogues were
unambiguously assigned. The orientations of the base (4-thiouridine or its deoxy analogue)
relative to the ring (furan or thiophene) are explored by a NMR approach and further
supported by x-ray crystallographic studies. The procedures presented here would be
applicable to other modified nucleosides and nucleotides.
1. Introduction
Nucleosides are important molecules, playing some fundamental roles in many biological
processes, including cellular survival and growth. A large number of modified nucleosides
have been designed to understand and to elucidate these processes and consequently used as
potential drugs[1] [2] due to their structural similarity to natural ones. We have been working
,
on designing and developing 4-thiothymidines as effective anti-cancer agents in combination
with UVA irradiation. Previously we reported 4-thiothymidine[3],[4] and its 5-bromo-[5] and
5-iodo-deoxyuridine[6] as potential anti-tumor drugs.[7],[8],[9] More recently we worked on such
thio-analogues by conjugating 4-thiothymine with an aromatic ring, such as furan and
thiophene, to shift their UVA absorption towards longer wavelengths to enhance the
sensitivity to UVA irradiation. These ring-attached nucleosides are expected to have unique
physical and chemical properties and are currently being exploited for their biological
applications and therapeutic activities
In this paper, we report our findings from a systematic NMR study of these rarely
available modified nucleosides using various NMR techniques (including COSY, NOESY)
and also compare these findings with those acquired by using x-ray crystallography to offer a
 Corresponding author. Tel.: +86 411 8740 2378; e-mail: zhangxiaohui@dlu.edu.cn
 Corresponding author. Tel.: +44 1908 654 064; e-mail: y.z.xu@open.ac.uk.
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process which may
lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1002/mrc.4501
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better understanding of the structural information of these modified nucleosides in solution
and in solid state. Such structural knowledge can help to appreciate how these thio-analogues
would work with UVA light and ultimately to develop modified nucleosides as anti-cancer
therapeutics.[2]
2. Results and Discussion
2.1. Synthesis and characterization
Chemical synthesis: The target products, 4-thio-5-(furyl-2-yl/ thiophen-2-yl)-pyrimidine
deoxyribonucleosides (2a and 3a) and 4-thio-5-(furyl-2-yl/ thiophen-2-yl)-pyrimidine
ribonucleosides (2b and 3b) were prepared from their corresponding deoxy-uridine (1a) and
uridine (1b) as shown in Scheme 1 (below). The preparation involved a 5-step synthesis
(protecting the sugar hydroxyl groups; iodination at the 5-position of the base; replacing the
iodo group with a ring (furan/thiophene); thioation at the 4-position of the base and finally
releasing the sugar hydroxyls from the protecting groups). A detailed synthesis will be
published in elsewhere.
Scheme 1 and Legend Here
Characterization: The structures of 4-thio-5-(furyl-2-yl/thiophen-2-yl)-pyrimidine
nucleosides (2a-b and 3a-b) have been thoroughly characterized and confirmed by various
spectrometric studies, including FTIR and HR-MS.
2.2. NMR study of compounds 2 and 3.
2.2.1. 1H NMR
For the brevity of this paper, a modified nucleoside, 4-thio-5-thiophene-ribouridine (3b), is
1
illustrated as a typical example. A section of 1D H-NMR spectrum of 3b is shown in
electronic supplementary info (ESI) (Fig. S1). The protons on the nucleoside can be classed
into three types: sugar, base and ring (furan/thiophene). In general chemical shifts for the
sugar protons are below 6 ppm, the protons from the ring (thiophene) are located around
7ppm while the proton (6-H) of the base is above 8ppm. Their respective assignments will be
presented and discussed below.
a) Assignment of the sugar protons
The chemical shift of 1’-H of the sugar has been long established and generally found to
locate at around 6 ppm.[10] In the case of 4-thio-5-thiophene-ribouridine (3b), it is found to be
at 5.76 ppm, from which we can readily locate its neighbouring proton (i.e. 2’-H) via their
cross peak (Peak A, in the Left panel of Figure 1). By following the arrow, the 2’-H would
lead us to its cross peak (Peak B) with 2’-OH.
The carbon at 5’-position has two bonded protons. As this set of protons is unique in the
whole molecule and can be easily identified as the pair of the signals at the region below 4
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ppm (more accurately at 3.56 and 3.72 ppm, see the right end of Fig. S1 in ESI and the top
right corner in the left panel of Figure 1). So we can start from here to trace its neighbouring
protons via peak C (crossing with 4’-H) and peak D (crossing with 5’-OH). The remaining
signals for 3’-OH and 3’-H can be identified via their cross peak E. These assignments are
further supported by the following lines of evidence: a) all three OH signals disappeared
during D2O exchange, b) the cross peaks between 2’-H and 3’-H and between 3’-H and 4’-H
are identifiable in the range of 4 ppm.
Figure 1 and Legend Here
For related deoxy analogues, the chemical shifts for the protons of
4-thio-5-thiophene-deoxyribouridine (3a) have an identical pattern except 2’-H has a much
lower value since its attached carbon-2 is not bonded directly to the electron-withdrawing O
atom.
b) Assignment of the base protons (N3-H and 6-H)
There are only two protons on the base of these 5-substituted nucleosides (compounds 2
and 3). (See Scheme 1 for the chemical structures). One is an imino proton (N3-H, at
N3-position) and can be found at the far left end (lowest field) of the 1H NMR spectrum. The
chemical shift of N3-H in 3b is found to be at 12.89 ppm and noticeably higher than those of
the equivalent imino protons of un-thiolated thymine bases (usually between 11 and 12
ppm).[11],[12],[13] This increased  value can be ascribed to the presence of sulfur atom at the
4-position of the molecule. The imino proton is exchangeable, so this type of NMR signal has
been verified by D2O exchange. The other proton on the base is 6-H. As it is bonded with the
carbon within the electron-rich and conjugated base, its chemical shift can be found at the
relatively lower field (8.51 ppm). This assignment has been also confirmed by 2D (1H-13C)
spectrum (data not shown).
c) Assignment of the protons in the 5-membered rings (thiophene and furan).
The chemical structure of C-5-linked thiophene is shown in the left panel of Figure 2. In
the ring there are three protons, of which 4”-H is the only proton neighboured (thus closely
coupled) with two protons (i.e. 3”-H and 5”-H). So the signal at 7 ppm, which has cross peaks
with both 3” and 5”-protons, can be assigned as 4”-H (shown in the right panel of Figure 2).
The signal located at 7.5 ppm is in the lowest field among these three protons and so can be
assigned as 5”-H as it is close to the sulfur atom. From 5”-H, we can find its cross peak with
4”-H (F) and then track to 4”-H. From there, we can identify another cross peak (G), leading
1
to the remaining proton (i.e. 3”-H). In addition H-13C COSY spectrum was also used to
13
provide further supports to the assignment of these three ring protons (see the section of C
NMR assignment below).
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Figure 2 and Legend Here
The chemical shifts (shown in Table 1) for the protons at the furan ring of
4-thio-5-furan-ribouridine (2b) have a very similar pattern to those of the thiophene analogue
(3b) except its 5”-H has a higher value as the proton at 5”-position of 2b is close to the
electron-withdrawing O atom in 2b. For the related deoxynucleoside analogues (2a and 3a),
1
the NMR data on these 5-membered rings (furan and thiophene) are very similar, and all H
NMR peaks for the target nucleosides (2 and 3) have been identified and the data are listed in
Table 1.
Table 1 Here
For comparison, Table 1 also includes 1H NMR data[14],[15] of 5-(furyl-2-yl)-
deoxyribouridine (here named as 4a) and 5-(furyl-2-yl)-ribouridine (4b). These compounds
(4a and 4b) are the 4-oxy analogues to 4-thio-5-(furyl-2-yl)- deoxyribouridine (2a) and
4-thio-5-(furyl-2-yl)-ribouridine (2b). By comparison of the data of 4a/4b with those of
2a/2b, it is clear these assignments agree very well with each other. The only difference is on
the N3-H proton (see the highlighted values in Table 1). This is because 4a/4b have an
oxygen atom at the 4-position while 2a/2b contain a sulfur atom at the same position. The
presence of sulfur atom at the 4-position shifts its neighbouring N3-H to a lower field (i.e. a
higher  value), consistent with our early observations[12], [13]
2.2.2. 13C NMR
13
1
As C NMR would provide complimentary and additional structural information to H
NMR, thus we also carried out a systemic study of these modified nucleosides. Our approach
was first to generate a complete 1D 13C spectrum of one of our modified nucleosides and then
assign each of the signals with the aid of other techniques. Again we use
13
4-thio-5-thiophene-ribouridine (3b) as an example. A complete 1D C spectrum (Fig. S2) is
shown in ESI. The assignments of these peaks are discussed individually in each of the three
individual parts (sugar, base and the ring) as illustrated below.
a) Assignment of the sugar carbons
1
The above-described H-NMR data can be used as a starting point for assigning 13C NMR
signals, thus it is feasible to trace their corresponding carbons from the identified protons.
This was exampled by 1H-13C 2D NMR of 3b. The sugar section is shown in Figure 3 (for the
full spectrum, see Fig. S3 in ESI).
1
It is straightforward to find out from the section of H-13C 2D NMR (Figure 3) that 1’-H
has its corresponding 13C signal at near 90 ppm that is the signal for the carbon at 1’-position
(ie. C-1’), see the dotted line in Figure 3. Similarly for other sugar protons (i.e. 2’-H, 3’-H,
4’-H and 5’-H), the chemical shifts of the related carbons can be clearly identified from
Figure 3 and also listed in Table 2. It is worth noting that the chemical shifts for C-1’ carbons
in the ribo-sugar (eg. 2b and 3b) are always higher than those of C-4’ carbons while in the
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deoxyribose sugar (eg. 2a and 3a) the chemical shifts for C-1’ are lower than those of C-4’,
see Table 2. This generality is in good agreement with what reported in our early work.[13]
This can be ascribed to the fact the presence of oxygen atom at the 2’-position (of ribo-sugar)
has a stronger effect on its directly neighbouring C-1’ carbon than its slightly distant C-4’
carbon.
Figure 3 and Legend Here
b) Assignment of the base carbons
The pyrimidine base has four carbons (C-2, C4, C-5 and C-6), of which only C-6 has a
bonded H atom. Thus the signal for C-6 can be readily identified from the known 6-H via
1H-13C 2D NMR (HMQC technique). Taking 3b again as an example, since the singlet peak
(δ 8.51 ppm) has been previously assigned as 6-H (cf. Table 1), the 6-H peak can be traced in
the HMQC spectrum to identify a cross-peak leading to its coupled carbon at 135.77ppm.
(see Fig. S3 in ESI). However the other three carbons (C-2, C-4 and C-5) are all quaternary
carbons (without bonded hydrogens). Thus, HMQC is no use in this instance. HMBC
(heteronuclear multiple bond correlation) is a possible means; the only non-exchangeable
proton in the base is at the 6-position and is of three bond distance to both C-2 and C-4 and of
two bond distance to C-5. So HMBC could be used to identify C-5 among them, but is
unlikely to provide a conclusive solution to distinguish between 2-C and 4-C. Distortionless
Enhancement by Polarization Transfer (DEPT) is a useful technique, which can allows us to
determine multiplicity of carbon atoms substitution with hydrogens. As those carbon atoms
without hydrogen attached, namely quaternary carbons, would become invisible in a DEPT
spectrum. By comparing 1D 13C NMR of 3b and its DEPT-135 (see Fig. S4a and S4b in ESI),
it is easy to see that the peaks at 187 ppm (C-4), 147 ppm (C-2), 137 ppm (C-2”) and 118
ppm (C-5) are missing in the DEPT-135 (Figure 4) and thus they can be identified as
quaternary carbons. Three of them are from the base (i.e. C-2, C4 and C-5) and the other is
from C-2” of the thiophene.
Figure 4 and Legend Here
In our early work[13] we have demonstrated that presence of sulfur atom at the 4-position
makes the chemical shift of C-4 move toward extremely low field, thus the peak with the
highest  value (187 ppm) should be assigned for C-4. C-2 is a type of urea and its  value
should be relatively higher, thus we can assign the peak at 147 ppm as C-2, and the peak at
118 ppm can be assigned for C-5 since its chemical shift is affected by the electronegativity
of the substituent linked to C-5 position.[13] The remaining quaternary carton (137pp) can thus
be assigned for C-2” of the thiophene, see below for further discussion.
3) Assignment of the ring (thiophene) carbons
The chemical structure of linked thiophene and its labelling are shown in the left top corner
of Fig. S5. There are four carbon atoms (C-2”, C3”, C-4” and C-5”) in the ring of thiophene.
As each of the three carbons (C-3”, C-4” and C-5”) has a bonded hydrogen, so they can be
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1
readily identified via H-13C COSY spectrum (Fig. S5). Although C-3” (=126.2) and C-4”
13
1
have an identical value (=126.2) in C NMR, however their respective values in H NMR
are noticeably different (see Fig. S5) and thus can be assigned confidently. We have applied
the same approach to other target nucleosides and their 13C data are listed in Table 2.
Table 2 Here
Table 2 also includes 13C NMR data[14] of 4a [5-(furyl-2-yl)-deoxyribouridine] for
comparison. The assignments of 4a are in good agreement with the assignments of 2a (its
4-thio-analogue). Again the sole difference is on the carbon at the 4-position, see the values in
13
bold format. It is worth mentioning that the C data for 4b [5-(furyl-2yl)-uridine] are listed,
have not been assigned.[14]
2.2.3. Relative orientation of the moieties
Nuclear Overhauser Effect spectroscopy (NOESY) is a useful tool to establish the
correlations between nuclei that are spatially close and can provide valuable information
about molecular structures. Thus, we used NOSEY technique to study the structural
orientation among nucleoside’s moieties: the base, the ring and the sugar. Again take 3b as an
example. Its NOESY spectrum is shown in Figure 5.
Figure 5 and Legend Here
The cross peaks in NOESY indicate the concerned nuclei are spatially close. In this case,
peak K suggests that the proton at 6-position of the base (i.e. 6-H) is spatially close to 3”-H
under the experimental conditions (room temperature). Cross peaks L and M indicate 6-H is
also spatially close to the protons at 2’-position (i.e 2’-H) and 3’-position (i.e. 3’-H) of the
sugar. Therefore, we can propose the orientation of the thiophene is upward (termed as
parallel orientation), namely the sulfur atom is located at upper part of the ring as shown in
Figure 7 insert. This model is consistent with our findings from x-ray structural studies
shown at in Figure 6.
Figure 6 and Legend Here
However the orientation for 5-furan-analogue is quite different. We were able to observe
NOE effects for 6-H of the base with 2’-H and 3’-H of the sugar, but no NOE effect was
observed between 6-H and 3”-H (see Fig. S6 in ESI), implying the furan ring is downward
(termed as antiparallel orientation), namely the oxygen atom in the furan is away from the
sulfur at the 4-position of the base. Again this finding is also consistent with our result from
x-ray structural study of 2b shown in Figure 6.
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It is reassuring to see the same result obtained by NMR (from a solution state) and by x-ray
crystallography (from a solid state) regarding the orientation of the base relative to the ring
(furan/thiophene). This finding would be useful when exploring biological activities of these
modified nucleosides. However, still lacking is a suitable explanation of why 3b has a
parallel orientation while 2b has an antiparallel structure. Although we could attribute these
orientations to the fact that the large-sized sulfur atom in 3b has to avoid the stereo hindrance
from both the base and the sugar moieties, thus keeping its parallel orientation while the
smaller oxygen atom in 2b does not need to do so. However, it should also be pointed out that
the NOESY experiment was carried out at ambient temperature. So if the temperature is
increased, the rotation of the ring around the C5-C2” becomes feasible, then the orientation
could be changed. More work is definitely required to answer these questions conclusively.
3. Conclusions
4-thio-5-furan- and 5-thiophene-modified ribouridines and their 2'-deoxy analogues have
1
13
been successfully prepared. In addition to standard analyses, their H and C NMR have
been systemically investigated and various NMR spectroscopic approaches are used to
unambiguously assign all protons and all carbons in these novel modified nucleosides. The
orientations of the base (in 4-thiouridine or its deoxy analogue) relative to the ring (furan or
thiophene) are confirmed by both NMR and x-ray crystallographic studies.
4. Experimental Section
General procedures for the synthesis of modified nucleosides (2 and 3)
Melting point was determined on a XR-4-type micro-melting point detector without
correction. The compounds synthesized were purified by column chromatography using silica
gel (200–300mesh) except for recrystallization and thin-layer chromatography(TLC) using
silica gel 60 F254 plates (250 mm; Qingdao Ocean Chemical Company, China). IR spectra
were recorded using a Nicolet 550 Spectrophotometer (4000～400 cm-1) with a crystalline
sample spread on KBr pellets. UV spectra were recorded using UV-VIS spectrophotometer
(JASCO, Japan); 1H NMR and 13C NMR spectra were obtained by a 500 MHz Bruker AV-400
spectrometer with TMS as an internal standard. The mass spectrum was obtained on
Hewlett-Packard 1100 LC/MSD spectrometer.
Protecting the sugar hydroxyl groups: The nucleoside (deoxyribouridine, 1a or uridine, 1b)
(4.09 mmol) dissolved in anhydrous pyridine (12 mL), was treated with dry acetic anhydride
(2.75 mL, 29 mmol) for 5 h. The yields were at 97-99%.
Iodination at the 5-psotion of the base: A mixture of acetyl protected nucleosides
(0.5mmol), iodine (76 mg, 0.3 mmol), Ceric ammonium nitrate (137 mg, 0.25 mmol) and
MeCN (8ml) was stirred at 80℃ for 1h. The yields were around 45% -70%.
Replacing the iodo group with a ring (furan/thiophene): Bis (triphenylphosphine)
palladium (II) chloride (0.016 g, 0.023 mmol）and 2-(trimethyltannyl)-furan /-thiophene (3.42
mmol）was added to the solution of 5-iodo-acetyl protected nucleosides (1.14 mmol. The
mixture was heated at 90℃ and refluxing for 3 h. The yields were around 75%.
Thioation at the 4-position of the base: 5-(furyl-2-yl/thiophen-2-yl)-acetyl protected
nucleosides (2.32 mmol) were dissolved in peroxide-free 1,4-dioxane (50 mL) and P2S5 (1.0 g,
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4.50 mmol) was added. The mixture was refluxed for 3-4 h. The yields were around 49-64%.
Releasing the sugar hydroxyls from the protecting groups:
4-thio-5-(furyl-2-yl/thiophen-2-yl)-acetyl protected nucleosides (2.14 mmol) were suspended
in absolute MeOH (60 mL, 1.5 mmol) and saturated with dry ammonia gas. The mixture was
stirred at room temperature for 4.5 h. The yields for the target products
(4-thio-5-(furyl-2-yl/thiophen-2-yl) nucleosides (2a-b and 3a-b) were around 45%-70%.
NMR instruments and techniques
500 MHz from Bruker (AV-500, FT NMR) were used in all NMR measurements.
DMSO-d6 was used as the NMR solvent unless indicated otherwise. General parameters for
1
1D H NMR: Spectrometer frequency [sfrq]: 500.1500095 MHz; Pulse width [pw] 9.5 s;
Acquisition time [at] 3.2767999 s; Sweep width [sw]: 19.9935 ppm; Digital resolution:
0.305176 Hz.
The COSY spectra (DMSO-d6) were obtained in the magnitude mode with 1024 points
in the F2 dimension and 256 increments in the F1 dimension. Each increment FID was
obtained with 12 scans with a relaxation delay of 2 s. HMQC spectra (DMSO-d6) were
obtained in the magnitude mode with 1024 points in the F2 dimension and 256 increments in
the F1 dimension. Each increment FID was obtained with 16 scans with a relaxation delay of
2 s. NOESY spectra experiments were obtained with a relaxation delay of 2 s and mixing
time of 0.8 s. All experiments were carried out at room temperature (25°C).
5. Acknowledgements
The authors are grateful to Dr. Peter Horton of UK National Crystallography Service for
his expertise with x-ray determination of structures. The work was supported by the Dalian
government's science-technology plan projects (Grant No. 2014E12SF074), the public
welfare foundation project of science enterprise investigation in Liaoning province of China
(2015), and the Dalian Jinzhou New Area government's science-technology plan projects
(Grant No. KJCX-ZTPY-2014-0009).
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6. References
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[2] M. P. Burke, K. M. Borland, V. A. Litosh Curr. Topics in Med. Chem. 2016, 16, 1231-1241.
[3] A. Massey, Y. Z. Xu, P. Karran Curr. Biol. 2001, 11, 1142-1146.
[4] X. H. Zhang, G. Jeffs, X. L. Ren, P. O'Donovan, B. Montaner, C. M. Perrett, P. Karran, Y. Z. Xu DNA Repair.
2007, 6, 344-354.
[5] Y. Z. Xu, X. H. Zhang, H. C. Wu, A. Massey, P. Karran Bioorg. & Med. Chem. Lett. 2004, 14, 995-997.
[6] X. H. Zhang, H. Y. Yin, G. Trigiante, R. Brem, P. Karran, M. B. Pitak, S. J. Coles, Y. Z. Xu Chem. Lett. 2015,
44, 147-149.
[7] E. Gemenetzidis, O. Shavorskaya, Y. Z. Xu, G. Trigiante J. Dermatological Treatment. 2013, 24, 209-214.
[8] S. W. Pridgeon, R. Heer, G. A. Taylor, D. R. Newell, K. O'Toole, M. Robinson, Y. Z. Xu, P. Karran, A. V.
Boddy British J. Cancer. 2011, 104, 1869-1876.
[9] R. Brem, X. H. Zhang, Y. Z. Xu, P. Karran J. Photochem. and Photobiol. B-Biology. 2015, 145, 1-10.
[10] C. B. Reese Tetrahedron. 1978, 34, 3143-3179.
[11] E. M. B. Janke, A. Dunger, H. H. Limbach, K. Weisz Magn. Reson. Chem. 2001, 39, S177-S182.
[12] X. H. Zhang, Y. Z. Xu Molecules. 2011, 16, 5655-5664.
[13] X. H. Zhang, J. Wang, Y. Z. Xu Magne. Reson. Chem. 2013, 51, 523-529.
[14] N. J. Greco, Y. Tor Tetrahedron. 2007, 63, 3515-3527.
[15] S. G. Srivatsan, Y. Tor J. Amer. Chem. Soc. 2007, 129, 2044-2053.
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Table 1 1H NMR data for all protons in furan- and thiophene-modified nucleosides
Cpds
Sugar
3’-H
4.21
4.03
4.29
4.06
Base
Ring
4″-H
6.46
6.52
7.03
7.03
Exchangeable
1’-H
6.08
5.82
6.14
5.79
2’-H
2.18
4.14
2.27
4.16
4’-H
3.81
3.94
3.85
3.93
5’-H 6-H
3″-H
7.26
7.29
7.32
7.31
5″-H
7.56
7.61
7.51
7.52
NH
2’-OH
3’-OH
5’-OH
5.29
5.81
5.29
5.34
2a
2b
3a
3b
3.55
3.67
3.63
3.67
8.43
8.61
8.44
8.55
12.83
12.91
12.91
12.92
-
5.10
5.81
5.23
5.09
5.81
-
5.53
4a*
4b*
6.22
5.86
2.18
4.13
4.28
4.00
3.84
3.90
3.61
3.68
8.33
8.42
6.86
6.86
6.52
6.53
7.62
7.61
11.63
11.66
-
5.27
5.11
5.08
5.20
5.43
*) Data[14] for 4a [5-(furyl-2yl)-2’-deoxyribouridine] and data[15] for 4b
[5-(furyl-2yl)-ribouridine] are from the literature.
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Table 2 13C NMR data for all carbons in furan- and thiophene-modified nucleosides
Cpd
Sugar
3′
Base
Ring
3″
1′
2′
4′
5′
2
4
5
6
2″
147
147
137
137
4″
114
114
126
126
5″
86.2
89.6
86.1
89.6
40.9
74.8
40.9
74.8
70.5
69.9
70.2
69.9
88.4
85.4
88.4
85.4
61.3
60.6
61.1
60.6
148
148
147
148
186
186
187
188
110
110
118
118
135
135
135
136
112
112
126
126
142
142
127
127
2a
2b
3a
3b
84.7
40.1
70.4
87.6
61.1
149
160
106
135
146
108
112
141
4a*
13
*) The C data and assignments of 4a [5-(furyl-2yl)-2’-deoxyribouridine] is from literature
[14]
.
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(5'-H)
H
(5'-H)
H
(5'-OH)
Base
H
HO
O
(1'-H)
(4'-H)
H
H
H
(2'-H)
(3'-H)
HO
OH
(3'-H)
(2'-OH)
Figure 1
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(1")
S
(Base)
C5
H
(5"-H)
(2")
H
H
(4"-H)
(3"-H)
Figure 2
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H
H
(5'-H)
C5'
(5'-H)
Base
H
HO
O
H
C1'
C2'
C4'
C3'
(4'-H)
(1'-H)
H
H
(2'-H)
(3'-H)
HO
OH
Figure 3
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5"
3"
S
S
4"
2"
4
HN
5
6
2
O
N
Sugar
Figure 4
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Figure 5
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Figure 6
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5"
X
S
O
N
4"
4
HN
2"
HN
5
3"
i-v
O
N
O
O
2a R=H X=O
2b R=OH X=O
O
HO
HO
2'
R
2'
R
3a R=H X=S
3b R=OH X=S
1a R=H
1b R=OH
HO
HO
X
SnBu3
i: Ac2O/pyrdiine; ii: I2, CAN/MeCN; iii:
iv: P2S5 v: NH3/CH3OH
/ PdCl2(PPh3)2
Scheme 1. Synthetic routes to 4-thio-5-(furyl-2-yl/thiophen-2-yl)-pyrimidine nucleosides.
The conversional numberings are used for the sugar and base. The numberings of the ring
(furan or thiophene) are shown in the top right of the structures 2 and 3.
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