Proton NMR investigations on 6-alkylamino-2-alkylthioadenosine derivatives

Full text of DOI:10.1002/mrc.4151

Letter - spectral assignment
Received: 29 April 2014
Revised: 26 August 2014
Accepted: 29 August 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.4151
Proton NMR investigations on
6-alkylamino-2-alkylthioadenosine derivatives
Hongguang Du,a Qiwen He,a Ning Chen,a Jiaxi Xu,a Fei Chenb
and Guocheng Liub*
Chemical shifts are reported in ppm on δ scale, and signals
are expressed as s (singlet), d (doublet), t (triplet), q (quartet),
Introduction
Purine nucleosides have taken up an important role in living
m (multiplet) and br s (broad).
systems. Not only are purines the basic subunits of the nucleic
acids, but they also interact with enzymes and other proteins as
components of cofactors and signal molecules.[1–3] Their modified
nucleosides and nucleotides are very important classes of
compounds used in the therapy of a wide variety of diseases,
because they can act as antiviral,[4] antitumor[5] and antimicrobial
agents.[6] Purine nucleosides in which the alkylthio group at C-2
position and the alkylamine substituent at C-6 position typically
excel in antiplatelet activities[7–13] (Fig. 1). So the synthesis and
characterization of the modified purine nucleosides have attracted
the attention of our and other many scientific teams.[14–18]
The distribution of electrons around the purine skeleton affects
not only its chemical properties and reactivity but also the nuclear
magnetic resonance (NMR) parameters. The nature of the
substituent is reflected in the NMR chemical shifts and nuclear
spin–spin coupling constants, which makes NMR spectroscopy an
excellent tool for investigating and interpreting the structure,
reactivity and intermolecular interactions in terms of the electron
distribution.[19,20] The use of proton NMR spectroscopy could be a
helpful tool to achieve it. Recently, we have published a complete
analysis of proton NMR data for 6-alkylamino-2-alkylthioadenosine
derivatives, and we observed that two protons of SCH2 at C-2 of pu-
rine skeleton are split into one/two ‘dt’ or ‘dq’ coupling peaks,
rather than canonical ‘t’ or ‘q’ peaks,[9] but the reason is not clear.
To reveal the general behaviors of the chemical shifts signals of
SCH2 group depending on the configuration for SCH2-containing
adenosine derivatives, two series of 6-alkyl(aryl)amino-2-alkyl(aryl)
thioadenosines and 6-alkyl(aryl)amino-2-alkylthiopurines were
synthesized, and their proton NMR spectroscopic properties
were described.
Results and discussion
The 6-alkyl(aryl)amino-2-alkyl(aryl)thioadenosines (5a–l) and 6-alkyl
(aryl)amino-2-alkylthiopurines (5m–o) were synthesized according
to our previous reported procedures,[9,10] which were outlined in
Scheme 1. 6-Alkyl(aryl)amino-2-alkyl(aryl)thioadenosines (5a–l)
was synthesized from the starting material guanosine (1)
via acetylation, chlorination, diazotization–alkanethiolation and
ammonolysis. 6-Alkyl(aryl)amino-2-alkylthiopurines (5m–o) were
prepared by treating with 2-bromoethyl acetate, diazotization–
alkanethiolation and ammonolysis from 2-amino-6-chloropurine (6).
All of the 6-alkyl(aryl)amino-2-alkyl(aryl)thioadenosines and
6-alkyl(aryl)amino-2-alkylthiopurines derivatives, except 5a,
5c, 5e, 5 k and 5 m, are new compounds (Table 1).
Analysis of the proton NMR data of 6-alkyl(aryl)amino-2-alkyl
(aryl)thioadenosines (Table 2), we found that NH at C-6 of purine
skeleton is a broad peak around 8.20 ppm, C-8 hydrogen is close
to 8.50–7.90 ppm as a single peak; and then the chemical shifts
of the nine protons in ribose are assigned in the region of
5.9–3.4ppm. The SCH2 at C-2 is assigned in the region of
3.10–2.90 ppm. However, if the substituent is SCH3, the chemical
shift is assigned at 2.5ppm, which is often buried in the signals
(two methyl peaks) of deuterated dimethylsulfoxide (DMSO-d6)
when it was used as the solvent (see ref. 9, compounds 5b1, 5b7,
5b10, 5b16 and 5b 20).
As can be seen from the proton NMR spectrum of 6-chloro-2-
propylthio- 9-(2′,3′,5′-tri-O-acetyl-β-D-ribofuranosyl)-9H-purine (4b,
Fig. 2) in DMSO-d6 solution, the two proton signals of SCH2 were
not shown as canonical ‘t’ peak, but as ‘dt’ coupling peak, and the
*
Correspondence to: Guocheng Liu, The CAS Key Laboratory of Genome Sciences
and Information, Beijing Institute of Genomics, Chinese Academy of Sciences,
Beijing 100101, China. E-mail: liuguocheng@big.ac.cn
Experiment
The proton and carbon NMR spectra were recorded with
Varian Mercury plus 200 (200 MHz), Varian Mercury plus 300
(300 MHz), Bruker 400 AMX (400 MHz) or Bruker 600 AMX
(600 MHz) spectrometer in CDCl 3 or DMSO-d 6 as the solvent
with tetramethylsilane as an internal standard at 298 K.
a State Key Laboratory of Chemical Resource Engineering, Department of Organic
Chemistry, College of Science, Beijing University of Chemical Technology, Beijing
100029, China
b The CAS Key Laboratory of Genome Sciences and Information, Beijing Institute of
Genomics, Chinese Academy of Sciences, Beijing 100101, China
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H. Du et al.
Table 1. Chemical structures of 6-alkyl(aryl)amino-2-alkyl(aryl)thioadenosines
(5a–l) and 6-alkyl(aryl)amino-2-alkylthiopurines (5m–o)
Entry Compound 5
R
R1
R2
R3
Figure 1. Chemical structures and numberings of 6-alkylamino-2-alkylthio-
9-substitutedpurines.
1
5a[9]
5b
n-Pr
i-Pr
CH 2(CH2)4CH3
CH2(CH2)4CH3
CH 2(CH2)4CH3
CH 3OCH2CH2
c-C6H11
H
R4
R4
R4
R4
R4
R4
R4
R4
R4
R4
R4
R4
R5
R5
R5
2
H
assignment of the J(SCH2) values was 13.1 and 7.3 Hz. To our sur-
prise, the multiplicity of CH2 contiguous to sulfur (S) of
compounds 5a, 5c , 5e, 5 k, 5j and 5 l in DMSO-d6 is not the ‘t’ or
‘q’ peaks, either, but the ‘dt’ or ‘dq’ coupling peaks, and the
assignment of the J(SCH2) values was still 13.1 and 7.3Hz (Fig. 2).
The results demonstrate that two protons of SCH2 are
diastereomerism and their chemical shifts non-equivalence. Each
of protons signal is split twice, which are first coupled each other
as doublet peaks, and then is split by their adjacent substituent
3
5c[9]
5d
n-Bu
n-Bu
Et
H
4
H
5
5e[9]
5f
H
6
n-Pr
n-Bu
n-Bu
CH2Ph
n-Pr
Et
p-MePhCH2
p-MePhCH2
p-MeOPhCH2
p-MeOPhCH2CH 2
CH(CH3)Ph
n-Bu
H
7
5g
H
8
5h
H
9
5i
H
10
11
12
13
14
15
5j
H
5k[9]
5l
n-Bu
n-Bu
H
1
CH2 or CH3 into triplet or quartet peaks. However, when the H
n-Pr
Et
n-Bu
NMR spectrum was recorded in CDCl3, the two protons of SCH2
were split into two separated ‘dt’ or ‘dq’ coupling peaks and
the J(SCH 2) values were still 13.1 and 7.3 Hz (Fig. 3). It indicates
5m[10]
5n
n-Pr
n-Bu
n-Pr
PhCH2CH2
n-Bu
H
5o
n-Bu
Scheme 1. Synthesis of 6-alkyl(aryl)amino-2-alkyl(aryl)thioadenosines and 6-alkyl(aryl)amino-2-alkylthiopurines.
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1H-NMR, adenosine, chiral carbons, chemical shift non-equivalence
Table 2. 1H NMR chemical shifts (ppm) for 6-alkyl(aryl)amino-2-alkyl(aryl)thioadenosines (5)
Compound 5
H-8
NH-6
H-1′
OH-2′
OH-3′
OH-5′
H-2′
H-3′
H-4′
H-5′a
H-5′b
SCH2-2
5a[9]
5c[9]
5d
8.19
8.19
8.23
8.21
8.23
8.24
8.23
8.24
7.92
7.90
7.88
7.73
8.47
8.49
8.47
8.42
5.80
5.80
5.81
5.81
5.81
5.81
5.81
5.80
5.37
5.37
5.43
5.41
5.40
5.43
5.42
5.39
5.13
5.12
5.18
5.18
5.15
5.18
5.17
5.14
5.06
5.04
5.08
5.07
5.05
5.08
5.07
5.04
4.58
4.58
4.58
4.58
4.59
4.60
4.58
4.56
4.13
4.12
4.12
4.13
4.12
4.13
4.13
4.12
3.92
3.92
3.92
3.92
3.92
3.92
3.92
3.92
3.64
3.64
3.63
3.64
3.64
3.65
3.53
3.53
3.56
3.53
3.53
3.53
3.07
3.05
3.08
3.08
3.06
3.09
3.07
3.07
5e[9]
5f
2.99
3.02
3.03
5g
5h
3.60-3.43
5j
3.63
3.52
2.98
2.95
Figure 2. The SCH2 splitting in 1H-NMR spectra of compounds 4b, 5a, 5c, 5j, 5k and 5l in DMSO-d6.
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H. Du et al.
Figure 3. The SCH2 splitting in 1H-NMR spectra of compounds 5e, 5k, 5j and 5l in CDCl3.
Figure 4. The SCH2 splitting in 1H-NMR spectra of compounds 5m, 5n and 5o.
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1H-NMR, adenosine, chiral carbons, chemical shift non-equivalence
that the SCH 2 was also split into ‘dt’ or ‘dq’ coupling peaks, but
as two separate parts.
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The four chiral carbons in ribose, which connected with the 9-N
of the purine skeleton, especially the 1′-carbon, the nearest chiral
center, although there are seven chemical bonds between the
SCH2 and 1′-carbon, generate a long-range chiral effect resulting
the two protons of SCH2 chemical shift non-equivalence. However,
when the non-chiral carbon-substituent at the 9-N of the purine
skeleton, the two protons in SCH2 were normally split into a ‘t’ or
‘q’ peaks, rather than two ‘dt’ or ‘dq’ peaks (5m–o, Fig. 4). This
proves that the four chiral carbons at the 9-N of purine skeleton
result the two protons of SCH 2 diastereomerism, their chemical
shift non-equivalence and split into two ‘dt’ or ‘dq’ coupling peaks.
1
Further analysis of the H-NMR of the target compounds 5a–l,
suggesting that there is existence of solvent effect between
CDCl3 and DMSO-d6. Such as the chemical shifts of NH and
SCH2, the NH chemical shift of compound 5j–l in CDCl3 was
around 7.53 ppm but moved to 8.41ppm in DMSO-d6 (Figs. 2
and 3). It was assumed that a hydrogen bond was generated
between solvent (CDCl3) and the sample. The SCH2 was split
into two separated moieties in CDCl3 as ‘dt’ coupling peaks
but in DMSO-d6 occurs in an integral for the coupling peaks
(Fig. 2 and Fig. 3). In addition to the proton NMR data of
SCH2 and NH, the peaks of other groups also have a little
change, but not very obviously.
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In summary, the preparation and proton NMR spectroscopic
characterization of several novel SCH2-containing adenosine
1
derivatives were described. The H-NMR spectra of 6-alkylamino-2-
alkylthioadenosine derivatives suggest that the two protons in the
SCH2 are diastereomerism, their chemical shifts non-equivalence
and split into two ‘dt’ or ‘dq’ coupling peaks. However, the two pro-
tons of SCH2 of 9-acetoxyethyl-6-alkylamino-2-alkylthiopurine deriv-
atives are normally split into a canonical ‘t’ or ‘q’ peak. This proves
that the chiral carbons at the 9-N of purine skeleton have a long-
range chiral effect on the two diastereotopic protons of SCH2.
Supporting information
Acknowledgement
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s website.
This work was supported in part by the National Natural Science
Foundation of China (project no. 21272022).
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