Study on disulfur-backboned nucleic acids: Part IV. Efficient synthesis of 3′,5′-dithio-2′-deoxyguanosine

Article abstract of DOI:10.1016/j.cclet.2009.10.017

An efficient and novel method for synthesizing 3′,5′-dithio-2′-deoxyguanosine was described. In this method normal guanosine was used as the starting material. A very efficient procedure was used to synthesize 2?-O-tosylguanosine 1, which used 0.1 eq. DBT

Full text of DOI:10.1016/j.cclet.2009.10.017

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Chinese Chemical Letters 21 (2010) 131–134
www.elsevier.com/locate/cclet
Study on disulfur-backboned nucleic acids: Part IV. Efficient
synthesis of 30,50-dithio-20-deoxyguanosine
*
Pei Hua Shang, Chang Mei Cheng , Hua Wang, Hong Chao Zheng, Yu Fen Zhao
Key Laboratory of Phosphorus and Chemical Biology, the Ministry of Education, Department of Chemistry, Tsinghua University,
Beijing 10084, China
Received 21 May 2009
Abstract
An efficient and novel method for synthesizing 30,50-dithio-20-deoxyguanosine was described. In this method normal guanosine
´
was used as the starting material. Avery efficient procedure was used to synthesize 2-O-tosylguanosine 1, which used 0.1 eq. DBTO
instead of 2 eq. 1 was treated with LTBH to give 9-(2-deoxy-b-D-threo-pentofuranosyl)guanine 2. 2 could be easily turned to the
target compound.
# 2009 Chang Mei Cheng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
´
Keywords: 3,5-Dithio-2-deoxyguanosine; Guanosine; Synthesis
´
´
The thionucleosides, which have one or more of the sugar hydroxyl replaced by thiol are important monomers [1,2].
They can be used in antisense and RNAi fields. At the same time, the nucleosides with metal binding sites at the 30- and
50-positions would be built into single DNA strand with cooperative participation of metal coordination to replace the
covalent phosphodiester linkage in natural DNA [3]. Recently we started a project to synthesize a series of disulfur-
backbones [4,5] of novel nucleic acids and to apply them to the study of antisense, RNAi, nucleic acids probe and the
original of nucleic acid. Our laboratory had found an efficient procedure for synthesizing 30,50-dithio-20-
deoxyadenosine [6]. 20-Deoxyguanosine and 20-deoxyadenosine have the similar structure. At first we tried to prepare
0
0
0
´
the target compound using the same procedure as 3 ,5 -dithio-2-deoxyadenosine. But 2 -deoxyguanosine was very
expensive, meanwhile the yield of 30-configuration inversion step was very low. Hansske and Robins [7] reported that
20-O-tosyladenosine treated with LTBH then negative ion exchange resin could give 30-configuration inversion-20-
deoxyadenosine. So we synthesized the target compound from guanosine.
In this paper, we would like to report an efficient synthesis of 30,50-dithio-20-deoxyguanosine starting from
guanosine (Scheme 1). Firstly we treated guanosine with DBTO then TsCl to obtain 20-O-tosylguanosine 1,
subsequently 1 was treated with LTBH then negative ion exchange resin to give 9-(2-deoxy-b-D-threo-
pentofuranosyl)guanine 2 in 93% yield. In the above step LTBH should be no less than six times of 1, or else it
would cause a side reaction to give guanine (Fig. 1). Then 2 was converted into N2-benzoyl-9-(2-deoxy-b-D-threo-
pentofuranosyl)guanine (3) and 3 could be smoothly converted into N2-benzoyl-9-(30,50-di-O-methylsulfony-20-
* Corresponding author.
E-mail address: chengcm@mail.tsinghua.edu.cn (C.M. Cheng).
1001-8417/$ – see front matter # 2009 Chang Mei Cheng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.10.017

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P.H. Shang et al. / Chinese Chemical Letters 21 (2010) 131–134
Scheme 1. (a) Bu2SnO, MeCN, EtN3, TsCl, (b) LiEt3BH, DMSO, THF, (c) Me3SiCl, C5H5N; BzCl, MeOH, NH3, MeOH, (d) MsCl, C5H5N, (e)
AcSK, Dioxane, N2, 50 8C, 40 h, and (f) EtSH, EtSNa.
Fig. 1. The side reaction to give guanine.
deoxy-b-D-threo-pentofuranosyl)guanine 4. We treated 4 with AcSK in anhydrous 1,4-dioxane to give N2-benzoyl-
´
´
´
´
´
3,5-dithio-3,5-di-S-acetyl-2-deoxyguanosine 5, then 5 was deprotected with EtSNa in EtSH to obtain the target
compound 6, with the total yield of 10%.
1. Experimental
All solvents were freshly distilled. DMSO, MeCN, dioxane and pyridine were dried by standard procedures. All
solvents and reagents used can be obtained by commercial sources. TLC analyses were carried out on silica gel F254
and the spots were examined with UV light or by exposure to vaporised iodine. Column chromatography was carried
1
out by using silica gel (100–200 mesh). H NMR spectra were obtained on JOEL JNM-ECA600 and JOEL JNM-
ECA300 spectrometers. Mass spectra were recorded with a Bruker ESQUIRE-LC ion trap spectrometer equipped with
a gas nebulizer probe, capable of analyzing ions up to m/z 6000.
Synthesis of compound 1 [8–11]: A suspension of 28.3 g (100 mmol) of guanosine (dried overnight in vacuo at
80 8C on P2O5) and 49.8 g (200 mmol) of dibutyltin oxide in a mixture of 750 mL of anhydrous MeOH and 600 mL of
dimethylformamide was stirred for 4 days at room temperature and refluxed for 3 h. The reaction mixture was cooled
in an ice bath, and 210 mL of triethylamine and 286 g (1.5 mol) of tosyl chloride were added. After stirring for 15 min,
the clear solution was evaporated, coevaporated with pyridine, and dissolved in a mixture of pyridine–acetic anhydride
(10:1, l L). The reaction mixture was kept overnight at room temperature and 100 mL of MeOH was added. The
reaction was stirred for 1 h, evaporated, and diluted with CHCl3 (1 L). The suspension was filtered, and the filtrate was
washed with H2O (3 ꢀ 200 mL), dried, and evaporated. The residue was treated with ammonia in methanol (800 mL)
overnight at room temperature and evaporated. When the residue was diluted with CHC13–MeOH (9:1), a crystalline
precipitate occurred, which was isolated and washed with MeOH and Et2O. The mother liquor was further purified by
column chromatography (CHC13–MeOH 85:15 with a little concd. ammonium hydroxide); total yield 26.8 g
1
(61.4 mmol, 61%). m.p. 260–261 8C. H NMR (DMSO-d6): d 10.66 (s, 1H, NH), 7.70 (s, 1H, 8-CH), 7.46 (d, 2H,
J = 7.6 Hz, phenyl), 7.13 (d, 2H, J = 8.3 Hz, phenyl), 6.64 (s, 2H, NH2), 5.97 (d, 1H, J = 4.8 Hz, OH), 5.85 (d, 1H,
J = 7.6 Hz, 10-H), 5.37 (dd, 1H, J = 5.2, 7.2 Hz, 20-H), 5.23 (t, 1H, OH), 4.29–4.28 (m, 1H, 30-H), 3.99–3.98 (m, 1H, 40-
H), 3.61–3.43 (m, 2H, 50-CH2), 2.31 (s, 3H, CH3); 13C NMR (DMSO-d6): d 156.9 (6-C), 153.8 (8-C), 150.7 (5-C),

P.H. Shang et al. / Chinese Chemical Letters 21 (2010) 131–134
133
145.5 (2-C), 136.3 (phenyl), 132.2 (phenyl), 130.1 (phenyl), 127.4 (phenyl), 117.4 (4-C), 87.1 (10-C), 84.1 (20-C), 80.0
(40-C), 70.2 (30-C), 61.7 (50-C), 21.6 (CH3); ESI-MS: m/z 438 (M+H)+, 460 (M+Na)+, 152 (Base+H)+.
Synthesis of compound 2: A solution of 4.37 g (10 mmol) of 1 in 70 mL of anhydrous Me2SO under dry nitrogen
was treated with 100 mL of 1 mmol/L LTBH/THF and the resulting solution was stirred at room temperature
overnight. Careful quenching with 5 mL of H2O was followed by concentration of the solution to syrup in vacuo. The
residue was dispersed in 100 mL H2O and treated with some Dowex 1 ꢀ 2(OHꢁ) resin at room temperature for 1 h and
resulting suspension was filtrated. The filtrate was concentrated in vacuo. The residue was purified by column
chromatography (CH2Cl2–MeOH 5:1 with some ammonia) to give 2.65 g 9-(2-deoxy-b-D-threo-pentofuranosyl)-
guanine with yield of 93%. Decompose at 195–198 8C. 1H NMR (DMSO-d6): d 7.95 (s, 1H, 8-CH), 6.53 (s, 2H, NH2),
6.02 (dd, 1H, J = 2.0, 8.1 Hz, 10-H), 4.33–4.30 (m, 1H, 30-H), 3.90–3.85 (m, 1H, 40-H), 3.75–3.56 (m, 2H, 50-H, 500-H),
2.74–2.65 (m, 1H, 200-H), 2.18–2.13 (m, 1H, 20-H); ESI-MS: m/z 268 (M+H)+, 290 (M+Na)+, 152 (Base+H)+.
Synthesis of compound 3: A solution of 231 mg (0.8 mmol) of 2 in 5 mL anhydrous pyridine was treated with
0.38 mL (2.94 mmol) trimethylchlorosilane (TMSCl 98%) and the resulting solution was stirred at room temperature
for 1 h, cooled with ice bath to 0 8C. Then 0.12 mL benzoyl chloride (0.99 mmol, 99%) was added dropwise, and after
removing the ice bath, the solution was stirred at room temperature for 6 h. Careful quenching with 1 mL H2O was
followed by treating with NH3–MeOH (2 mL) for 15 min, concentrated the solution to syrup in vacuo. The residue was
dispersed in 4 mL ice water to give some solid, then filtrated, washed with ice water and Et2O to give 291 mg 3 with
yield of 98%. m.p. 276–278. 1H NMR (DMSO-d6): d 8.27 (s, 1H, 8-CH), 8.04 (d, 2H, J = 7.2 Hz, phenyl), 7.69 (t, 1H,
J = 7.5 Hz, phenyl), 7.57 (t, 2H, J = 7.2, 7.5 Hz, phenyl), 6.22 (dd, 1H, J = 8.3, 1.7 Hz, 10-H), 4.39–4.37 (m, 1H, 30-H),
3.96–3.94 (m, 1H, 40-H), 3.77–3.59 (m, 2H, 50-H, 500-H), 2.80–2.71 (m, 1H, 200-H), 2.30–2.25 (m, 1H, 20-H); ESI-MS:
m/z 394 (M+Na)+, 256 (Base+H)+.
Synthesis of compound 4: 278 mg (0.75 mmol) of 3 in 5 mL anhydrous pyridine was added dropwise to a solution
of 0.15 mL (1.95 mmol) menthane sulfonyl chloride in 1 mL anhydrous pyridine at 0 8C in 30 min. After removing the
ice bath the solution was stirred at room temperature for 14 h. Then 3 mL ice water and 10 mL CHCl3 was added, and
the organic layer was separated, dried with anhydrous MgSO4 for 30 min, evaporated, and then coevaporated with
toluene. The residue was purified by column chromatography (EtOAc) to give 296 mg 4 with yield of 75%. 1H NMR
(CDCl3): d 8.00 (d, 2H, J = 7.2 Hz, phenyl), 7.86 (s, 1H, 8-CH), 7.63 (t, 1H, J = 7.6 Hz, phenyl), 7.52 (dd, 2H, J = 7.2,
7.6 Hz, phenyl), 6.13 (dd, 1H, J = 4.1, 7.9, 10-H), 5.42–5.41 (m, 1H, 30-H), 4.56–4.50 (m, 2H, 500-H, 50-H), 4.47–4.46
(m, 1H, 40-H), 3.29–3.26 (m, 1H, 200-H), 3.14 (s, 3H, CH3), 3.06 (s, 3H, CH3), 2.95–2.89 (m, 1H, 20-H); ESI-MS: m/z
528 (M+H)+, 550 (M+Na)+, 256 (Base+H)+.
Synthesis of compound 5: A solution of 264 mg (0.5 mmol) of 4 in 10 mL anhydrous 1,4-dioxane was added to
228 mg (2 mmol) AcSK. The suspension was stirred at 60 8C for 40 h under dry nitrogen, and then evaporated. The
residue was distributed between CHCl3 and H2O. The organic layer was dried with anhydrous for 30 min, and then
evaporated to give a syrup. The residue was purified by column chromatography (EtOAc) to give 73 mg 5 with yield of
30%. ESI-MS: m/z 488 (M+H)+, 510 (M+Na)+, 256 (Base+H)+.
Synthesisofcompound6: A solutionof49 mg(0.1 mmol)of5 in5 mLEtSH wasaddedto10 mg(0.12 mmol)EtSNa,
the suspension was stirred at room temperature for 5 min under dry nitrogen, then neutralized with HOAc. 20 mL EtOAc
was added and the organic layer was separated, dried, and evaporated. The residue was saturated with 5 mL Et2O and the
precipitatewas collected by filtration andwashed with cold Et2O (4 ꢀ 1 mL)to give 24 mg 6 with yield of 80%. 1H NMR
(DMSO-d6 containing one drop of D2O): d 7.96 (s, 1H, 8-CH), 6.16 (t, 1H, J = 6.5 Hz, 10-H), 4.40 (ddd, 1H, J = 2.8, 4.8,
4.8 Hz, 40-H), 3.89–3.88 (m, 1H, 30-H), 3.62 (dd, 1H, J = 4.5, 12.0 Hz, 50-H), 3.55 (dd, 1H, J = 4.8, 12.0 Hz, 500-H), 2.61–
2.52 (m, 1H, 30-H), 2.28 (ddd, 1H, J = 3.1, 5.9, 13.1 Hz, 200-H); ESI-Ms: m/z 300 (M+H)+, 322 (M+Na)+.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 20872078) and the Basic
Science Research Foundation of Tsinghua University (No. JC 2001046).
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[11] (Method b): To stirred a solution of Et3N (139 mg, 0.75 mmol), guanosine (134 mg, 0.5 mmol) and DBTO (30 mg, 0.1 mmol) in MeCN (dried
with P2O5, 12 mL) at room temperature for 10 min, then TsCl (115 mg, 0.6 mmol) was added, and the suspension was stirred for about 28 h at
the same temperature. The solid materials were collected, then washed with MeCN (dried with P2O5, 5 mL) and CH2Cl2 (5 mL). The crude
product was purified by column chromatography (CHC13–MeOH 85:15 with a little concd. ammonium hydroxide).