Chemistry Letters Vol.34, No.3 (2005)
433
We found that, thioacetic acid only attacked the 30-postion,
not the 50-position of the intermediate 4. But when we elongated
the reaction time to 3 days, the 30,50-S-acteylthymidine 6 was al-
so obtained, which were identified by mass spectra. However,
the 3 days reaction led the mixture to a black-viscous syrup,
whose TLC was so complex, from which 6 could not be easily
separated. Thus, we had to synthesize compound 6 step by step.
We employed AcSK as a nucleophile to attack the 50-tosyl
group. Compound 4 reacted with AcSK readily in the solution
of dioxane at 50 ꢁC for 90 h, afforded S,S0-diacetyl-30,50-dithio-
thymidine 6 in 73% yield.8
When we used NaN3 as the nucleophilic reagent to react
with compound 4, the 50-azido derivate 7 was unexpectedly
formed as a sole product at 60 ꢁC, without the founding of 30-azi-
do or the 30,50-diazido isomers in the solution.9 However, a re-
cent study reported10 that when 50-O-mesyl-2,30-anhydrothymi-
dine was heated with NaN3 in DMF solution at 130 ꢁC, the
30,50-diazidothymidine was formed in a high yield. Presumably,
under basic conditions, the 50-position has a higher activity than
the 30-position and gives 50-substituted derivative as the main
product at a low temperature; while in thioacetic acid solution,
the acidic surrounding enhances the activity of 30-position, so
the nucleophile selectively attacks the 30-position. Such regiose-
lective reaction of 4 attracted much attention from us, and the
further studies had been carried on.
Two new methods were employed to synthesize the target
compound 8. And we believed that both of these methods are ef-
ficiently and conveniently. Firstly, we successfully reduced the
acetyl groups of 6 with LiAlH4 to afford 30,50-dithiothymidine
in 91% yield.11 Secondly, in our previous report, a more conven-
ient method was developed to remove the acetyl groups of 30,50-
dithioadenosine,12 and we found that this method was also suit-
able for thymidine. So we treated compound 6 with EtSNa in the
solution of EtSH under N2 atmosphere. The reaction was com-
pleted only in 5 min, and then the mixture was neutralized with
acetic acid and was extracted with chloroform. After evaporating
the solvent, the residue was saturated with ethyl ether to precip-
itate compound 8 in 83% yield. We found the solid state of 30,50-
dithiothymidine can effectively prevent the free thiol groups
from being oxidized to disulfide bonds and can be preserved
safely under N2 for long.13
In conclusion, we presented a novel and efficient route to
synthesize the 30,50-dithiothymidine starting from thymidine.
We believed that the methods that we have described in this ar-
ticle should have more general applications in the preparation of
other 20-deoxyribonucleoside analogs, which containing one or
more thiol groups at the 30- and 50-positions of the deoxyribose
moieties. The further work to develop antisense drugs and to
study the origin of nucleic acid is now in progress.
4
J. Chiba, K. Tanaka, Y. Ohshiro, R. Miyake, S. Hiraoka, M. Shiro,
and M. Shionoya, J. Org. Chem., 68, 331 (2003).
A. M. Michelson and A. R. Todd, J. Chem. Soc., 1955, 816.
M. Imazawa, T. Ueda, and T. Ukita, Chem. Pharm. Bull., 23, 604
(1975).
5
6
7
S-acetyl-50-O-tosyl-30-thiothymidine 5 A solution of 200 mg
(0.44 mmole) of compound 4 in 10 mL of thioacetic acid was
heated under N2 atmosphere to reflux for 12 h. The solvent was
evaporated in vacuo and the residue was chromatographed on sili-
ca gel (eluent firstly with CH2Cl2:Petroleum ether = 5:1; second-
ly with CH2Cl2:Methanol = 20:1) to give 0.22 g of compound 5
yielding 91.7%. 1H NMR (500 MHz, CDCl3) ꢁ 9.40 (1H, s, NH), ꢁ
7.81 (2H, d, J ¼ 8:3 Hz, Ar–H), ꢁ 7.47 (1H, s, 6-H), ꢁ 7.38 (2H, d,
J ¼ 8:0 Hz, Ar–H), ꢁ 6.23 (H, dd, J ¼ 6:3, 6.2 Hz, 10-H), ꢁ 4.36
(1H, dd, J ¼ 1:8, 11.0 Hz, 50-H), ꢁ 4.26 (1H, dd, J ¼ 3:1,
10.9 Hz, 500-H), ꢁ 4.12 (1H, ddd, J ¼ 2:2, 3.1, 2.8 Hz, 40-H), ꢁ
4.00 (1H, ddd, J ¼ 6:5, 6.1, 9.0 Hz, 30-H), ꢁ 2.46 (3H, s,
Ts-Me), ꢁ 2.37 (2H, m, 20-H), ꢁ 2.36 (3H, s, Ac–Me), ꢁ 1.95
(3H, s, 5-Me); ESIMS m=z 477 ðM þ NaÞþ.
8
S,S0-diacetyl-30,50-dithiothymidine 6 To a solution of 95 mg
(0.21 mmole) of 5 in 20 mL of Dioxane, 47 mg (0.42 mmole) po-
tassium salt of thioacetate was added to form a suspension. The
mixture was stirring at 50 ꢁC under N2 atmosphere for 90 h, and
then it was filtrated and evaporated. The residue was chromato-
graphed on silica gel (eluent CH2Cl2:Methanol = 20:1) to afford
55 mg of compound 6 in 73% yield. 1H NMR (500 MHz, CDCl3)
ꢁ 9.91 (1H, s, NH), ꢁ 7.34 (1H, s, 6-H), ꢁ 6.13 (1H, dd, J ¼ 5:4,
6.7 Hz, 10-H), ꢁ 4.03 (1H, ddd, J ¼ 3:6, 7.5, 7.1 Hz, 40-H), ꢁ 3.81
(1H, ddd, J ¼ 8:2, 8.2, 8.4 Hz, 30-H), ꢁ 3.38 (1H, dd, J ¼ 3:6,
14.3 Hz, 50-H), ꢁ 3.27 (1H, dd, J ¼ 6:6, 14.5 Hz, 500-H), ꢁ 2.48
(2H, m, 20-H), ꢁ 2.40 (3H, s, Ac–Me), ꢁ 2.39 (3H, s, Ac–Me), ꢁ
1.98 (3H, s, 5-Me); ESIMS m=z 381 ðM þ NaÞþ.
9
50-Azido-2,30-anhydothymidine 7 A solution of 20 mg (0.05
mmole) of compound 4 and 13-mg sodium azide in 10-mL
DMF was heated at 60 ꢁC under stirring for 2 days. The solvent
was taken away by forming azeotrope with toluene in vacuo.
The residue was washed with ethyl ether, to give 11.8 mg com-
pound 7 (91%). 1H NMR (300 MHz, DMSO-d6) ꢁ 7.60 (1H, s,
6-H), ꢁ 5.88 (1H, d, J ¼ 3:8 Hz, 10-H), ꢁ 5.27 (1H, s, 30-H), ꢁ
4.37 (1H, ddd, J ¼ 2:4, 5.2, 7.2 Hz, 40-H), ꢁ 3.52 (2H, ddd,
J ¼ 5:2, 7.2, 13.0 Hz, 50-H), ꢁ 2.59 (1H, d, J ¼ 12:7 Hz, 20-H),
ꢁ 2.44 (1H, d, J ¼ 13:4 Hz, 200-H), ꢁ 1.76 (3H, s, 5-Me); ESIMS
m=z 272 ðM þ NaÞþ.
10 I. Lavandera, S. Fernandez, M. Ferrero, and V. Gotor, J. Org.
Chem., 66, 4079 (2001).
11 C. Cheng, C. X. Ling, and Z. W. Miao, Presented at The 2nd
National Conference on Organic Chemistry of Chinese (2001),
p 700.
12 H. Zheng and C. Cheng, Synlett, 2004, 2585.
13 30,50-dithiothymidine 8, Method A: A suspension of 100 mg
(2.6 mmole) of LiAlH4 in 10 mL anhydrous THF was cooled to
0 ꢁC and a solution of 80 mg (0.22 mmole) of compound 6 in
10-mL anhydrous THF was added drop wise under N2. The
reaction mixture was stirring for 4 h at 0 ꢁC, then the 1 N HCl
was added to adjust the pH of the mixture to 3. Extracted with
20 mL of chloroform, and the organic layers were dried over
anhydrous MgSO4. Evaporated the chloroform, the residue was
chromatographed on silica gel (eluent CH2Cl2:Methanol = 20:1)
to give 55.7 mg of compound 8 in 91% yield; Method B: To a
solution of 50 mg (0.14 mmole) compound 6 in 5-mL ethyl thiol,
was added 13-mg (0.16 mmole) sodium salt of ethyl thiol under
Nitrogen. The suspension was stirring for 5 min at rt, then was
neutralized with acetic acid. Then extracted with 20 mL of
ethyl acetate, dried over anhydrous MgSO4 and evaporated the
organic layer. The residual oil was saturated with 50-mL ethyl
ether and the precipitate was collected by filtration, washed with
cold ethyl ether (4 ꢂ 1 mL), to give 32 mg of the compound 8 in
83% yield.
This work was supported by the National Natural Science
Foundation of China (No. 20272031) and the Basic Science
Research Foundation of Tsinghua University (JC 2001046).
References and Notes
1
a) A. A. El-Barbary, A. I. Khodair, E. B. Pedersen, and C. Nielsen,
Monatsh. Chem., 125, 1017 (1994). b) E. J. Reist, A. Benitez, and
L. Goodman, J. Org. Chem., 29, 554 (1964).
2
3
C. M. Niemeyer, Angew. Chem., Int. Ed., 40, 4128 (2001).
A. Eleuteri, C. B. Reese, and Q. Song, J. Chem. Soc., Perkin
Trans. 1, 1996, 2237.
Published on the web (Advance View) February 23, 2005; DOI 10.1246/cl.2005.432