A convenient method for the conversion of β-thymidine to α-thymidine based on TMSOTf-mediated C1′-epimerization

Article abstract of DOI:10.1016/S0040-4039(02)00411-2

A practically useful method for the synthesis of α-thymidine from β-thymidine was developed by trimethylsilyl triflate-mediated C1′-epimerization. When 5′-O-diphenylacetyl-3′-O-toluoylthymidine was trimethylsilylated and successively treated with TMSOTf in acetonitrile, the resulting α-thymidine derivative was crystallized and isolated without silica-gel column chromatography. Deprotection gave unprotected α-thymidine in an overall yield of 50% from β-thymidine.

Full text of DOI:10.1016/S0040-4039(02)00411-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3251–3254
A convenient method for the conversion of b-thymidine to
a-thymidine based on TMSOTf-mediated C1%-epimerization
Yuichi Sato, Gohsuke Tateno, Kohji Seio and Mitsuo Sekine*
Department of Life Science, Tokyo Institute of Technology, Nagatsuta, Midoriku, Yokohama 227-8501, Japan
Received 23 January 2002; revised 20 February 2002; accepted 22 February 2002
Abstract—A practically useful method for the synthesis of a-thymidine from b-thymidine was developed by trimethylsilyl
triflate-mediated C1%-epimerization. When 5%-O-diphenylacetyl-3%-O-toluoylthymidine was trimethylsilylated and successively
treated with TMSOTf in acetonitrile, the resulting a-thymidine derivative was crystallized and isolated without silica-gel column
chromatography. Deprotection gave unprotected a-thymidine in an overall yield of 50% from b-thymidine. © 2002 Published by
Elsevier Science Ltd.
Recently, much attention has been paid to the antisense
and antigene strategies which enable us to regulate
selectively malignant genes and mRNAs, respectively,
for gene therapy.1 Particularly, Imbach and other
researchers have reported that a-oligodeoxynucleotide
derivatives, which have a-configuration at the anomeric
center of nucleotide components, possess promising
hybridization affinity toward RNAs as antisense
molecules.2,3 Their studies overturned the long-held
stereotype that a-deoxynucleosides are useless byprod-
ucts produced by glycosylation of deoxyriboside deriva-
tives with silylated or metallated bases.4 Development
of highly efficient methods for the synthesis of a-
deoxynucleosides is now required. Although several
methods have been reported for the predominant syn-
thesis of a-deoxynucleosides to date,5,6 these methods
involve multi-step reactions and do not attain ideal
selectivity. Compared with these de novo syntheses of
a-deoxynucleosides, simple TMSOTf-mediated C1%-
epimerization7 of commercially available b-deoxy-
nucleosides, which was reported by Saneyoshi,8 would
provide a more straightforward route to a-deoxy-
nucleosides on a large scale.
our hands. Quite recently, we have reported the
TMSOTf-mediated b–a epimerization of 5%-O-car-
bamoylated or 5%-O-thiocarbamoylated b-thymidine
derivatives.9 It turned out that these C1%-epimerizations
are essentially associated with not the neighboring
group participation of the carbonyl oxygen or sulfur of
the 5%-O-carbamoyl or thiocarbamoyl group but the
steric hindrance of the 5%-O-acyl group. However, the
reactions required for introduction of the carbamoyl or
thiocarbamoyl group into the 5%-hydroxyl group are
sluggish and the reagents are expensive. In the same
paper,9 we also reported a procedure for isolation of
5%-O-pixyl-a-thymidine from a 75:25 mixture of 3%,5%-O-
ditoluoylated a- and b-thymidines, which involves
tedious steps for elimination of the 5%-O-pixylated b-
isomer by crystallization and chromatograpy. However,
5%-O-DMTr-a-thymidine as a more generally accessible
starting material for the synthesis of a-DNA derivatives
could not be isolated by a similar method, since 5%-O-
DMTr-b-thymidine could not be crystallized in any
organic solvents tested. In this paper, we wish to report
a more practical and general method for the conversion
of b-thymidine to a-thymidine.
Based on the previous results,9 we chose various acyl
groups, i.e. acetyl (Ac), phenylacetyl (PA), diphenyl-
acetyl (DPA), and triphenylacetyl (TPA) as the 5%-O-
protectors of b-thymidine to see if the steric bulkiness
of the 5%-O-protector affects the a/b ratio in the epimer-
ization. The 3%,5%-O-diacylated thymidine derivatives
2a–j(b) were synthesized according to Method A or
Method B, as shown in Scheme 1. Method A involves
a two-step acylation of b-thymidine. Method B was
used for the synthesis of 3%,5%-O-protected thymidine
However, it is extremely difficult to separate the desired
3%,5%-O-diacylated a-thymidine derivatives from the still
remaining 3%,5%-O-diacylated b-thymidine derivatives
when the conventional toluoyl group was chosen as the
5%- and 3%-hydroxyl protecting groups.8 This previous
method required a tedious procedure for purification of
a-thymidine so that its overall yield is low (ca. 20%) in
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Published by Elsevier Science Ltd.
PII: S0040-4039(02)00411-2

3252
Y. Sato et al. / Tetrahedron Letters 43 (2002) 3251–3254
derivatives which could not be obtained by Method A.
These results are summarized in Table 1.
methyldisilazane (HMDS) in CH3CN under reflux for 1
h. After 2 equiv. of TMSOTf was added to the resulting
4-O-(trimethylsilyl)thymidine derivatives 3a–j(b), the
mixture was stirred at 30°C for 20 h, as shown in
Scheme 2. At appropriate times, aliquots were taken
In the b–a epimerization, thymidine derivatives 2a–j(b)
were silylated by treatment with 5 equiv. of hexa-
R1Cl
R2Cl
Method A
O
N
O
NH
O
NH
O
N
R1O
HO
O
O
O
N
Method B
1) DMTrCl
NH
O
R2O
2b-j(β)
HO
1(β)
R1Cl
HO
O
2) TolCl
3) H+
TolO
2a(β)
Scheme 1.
Table 1. Synthesis of 3%,5%-O-diacylated thymidine derivatives 2a–j(b)
Compd
R1
R2
Method
Solvent
First acylation
Second acylation
Yield from 1(b) Yield from 2a(b)
(%)
(%)
2a(b)
2b(b)
H
Ac
Tol
Tol
–
B
Py
Py
–
–
–
68a
–
–
92
Ac2O (1.5 equiv., rt,
2 h)
2c(b)
2d(b)
2e(b)
2f(b)
2g(b)
2h(b)
2i(b)
2j(b)
PA
Tol
Tol
Tol
B
Py
Py
MPACl (1.5 equiv., rt,
0.5 h)
–
–
–
73
–
A
A
A
A
A
A
A
TolCl (2.2 equiv., rt,
1 h)
96 [82b]
DPA Tol
Py:CH2Cl2 (9:1,
v/v)
Py:CH2Cl2 (9:1,
v/v)
Py:CH2Cl2 (9:1,
v/v)
Py:CH2Cl2 (9:1,
v/v)
DPACl (1.2 equiv., rt, TolCl (1.5 equiv., rt, 78 [55b]
–
1 h)
3 h)
TPA
DPA
Tol
H
TPACl (1.5 equiv.,
60°C, 2 h)
DPACl (1.2 equiv., rt,
1 h)
DPACl (1.2 equiv., rt, Ac2O (1.5 equiv., rt, 74
1 h) 2 h)
DPACl (1.2 equiv., rt, BzCl (1.5 equiv., rt, 81
TolCl (1.5 equiv., rt, 46
3 h)
–
–
77
–
DPA Ac
DPA Bz
DPA DPA
–
Py:CH2Cl2 (9:1,
v/v)
Py
–
1 h)
1 h)
–
DPACl (2.2 equiv., rt,
1 h)
86
–
a The yield was that for conversion of 1(b) to 2a(b) via a three-step reaction.
b Isolated yield by crystallization without using silica-gel column chromatography.
OTMS
N
N O
R1O
R2O
O
TMSOTf
HMDS
O
N
N
2a-j(β)
R1O
R2O
O
CH3CN
OTMS
3a-j(α)
3a-j(β)
O
NH
HO
R1O
O
O
O
N
R1O
O
+
HO
R2O
N
NH
O
N
O
O
NH
R2O
2a-j(β)
O
2a-j(α)
1(α)
Scheme 2.

Y. Sato et al. / Tetrahedron Letters 43 (2002) 3251–3254
3253
and quenched by addition of CHCl3 and water. The
protecting groups are more thermodynamically unfa-
vorable than compound 2a(b) having the acetyl group,
as shown in Fig. 1.
ratio of 2a–j(a) and 2a–j(b) in the organic phase was
1
estimated by H NMR.
In all cases, the epimerization in CH3CN reached an
equilibrated state within 20 h, and the equilibrium
constants K of these a–b epimerizations were calcu-
lated. These results are shown in Table 2. The mixtures
of 2a–j(a) and 2a–j(b) obtained after 20 h were purified
by silica gel column chromatography without further
separation of a pair of anomers. As shown in Table 2,
the mixtures of 2b–f(a) and 2b–f(b) could be recovered
in 80–93% yields. When 3%-O-toluoylthymidine 2a(b) or
5%-O-diphenylacetylthymidine 2g(b) was used in the
epimerization, unfavorable decomposition predomi-
nantly occurred over the epimerization, presumably due
to much faster dehydration.10 It was also found that,
compared with the a-anomer selectivity of compound
2b (a:b=82:12) having the acetyl group, those of com-
pounds 2e and 2f having the Ph2CHC(O)- and
Ph3CC(O)- groups were increased up to 88:12 and 91:9,
respectively.
On the other hand, when 5%-O-diphenylacetylthymidine
derivatives 2h–j having an Ac, Bz, or DPA group at the
3%-position were similarly treated with TMSOTf, the
a/b ratio varied from 82:18–88:12. In these cases, how-
ever, there is no clear-cut relationship between the
a-selectivity and the steric hindrance of the 3%-protect-
ing group. It seems to us that the a/b ratio is dependent
on other factors such as the electronic effect of the
3%-protecting and the stacking effect of the 3%-protecting
group with the silylated thymine base residue group
which can stabilize the whole conformation of a-
thymidine derivatives preferencially over b-thymidine
derivatives.
Since the epimerization is a reversible process, the a/b
ratio essentially depends on the thermodynamical sta-
bility of both isomers. It seems to us that 91:9 is the
ultimate a/b ratio in this kind of epimerization.
These results imply that the ultimate a/b ratio in these
epimerizations highly depends on the steric factor on
the 5%-O-protectors. It is likely that the Ph2CHC(O)-,
and Ph3CC(O)-groups interact more sterically with the
silylated thymine base than the acetyl group so that the
b-thymidine derivatives 2e(b) and 2f(b) having these
As the starting material, compound 2e(b) was prepared
in a higher yield (78%) than 2f(b) (46%). It was also
found that compound 2e(a) could be easily crystallized
in ethanol from the 88:12 mixture of 2e(a) and 2e(b)
obtained by the C1%-epimerization without column
chromatography. This finding eliminates the need for
Table 2. C1%-epimerization of 3%,5%-O-diacylated thymidine derivatives 2a–j(a/b)
Compd
R1
R2
Recovery of 2(a)/2(b) (%)
a:ba
Rf (TLC)b of a- and
b-epimers
Equilibrium constant K
a
b
2a(a/b)
2b(a/b)
2c(a/b)
2d(a/b)
2e(a/b)
2f(a/b)
2g(a/b)
2h(a/b)
2i(a/b)
2j(a/b)
H
Ac
PA
Tol
DPA
TPA
DPA
DPA
DPA
DPA
Tol
Tol
Tol
Tol
Tol
Tol
H
Ac
Bz
DPA
Decomp.
86
90
93
93
80
Decomp.
87
89
88
–
–
–
–
82:18
83:17
85:15
88:12
91:9
0.29
0.55
0.56
0.63
0.84
–
0.42
0.64
0.69
0.37
0.62
0.63
0.68
0.89
–
0.44
0.71
0.77
4.59
4.81
5.54
7.62
9.00
–
4.43
7.47
6.14
–
82:18
88:12
86:14
a The ratio measured after 20 h.
b TLC was preformed three times by use of CHCl3–AcOEt–toluene (1:1:1, v/v/v).
OTMS
OR1
N
O
OR1
TMSOTf
CH3CN
O
O
N
N
O
TolO
3b-f(α)
N
TolO
OTMS
3b-f(β)
5'-O-Steric effect
Ac < MPA < Tol < DPA < TPA
Figure 1.

3254
Y. Sato et al. / Tetrahedron Letters 43 (2002) 3251–3254
column chromatographic separation which is virtually
impossible since both epimers have very similar Rf
values, as shown in Table 2. Thus, the a-isomer 2e(a)
could be isolated as pure crystals (mp 201°C) in 76%
yield. From the mother solution, the 2e(b)-enriched
mixture could be recovered and reused for the further
epimerization. As far as the difference in the Rf value
between the two epimers is concerned, 2d and 2j
showed significant differences, but it turned out that
multi-time separation was required to purify the a-iso-
mers. This bothersome process is apparently unsuitable
for the large-scale synthesis of a-thymidine.
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In consideration of the recovery of 2e(b) after crystal-
lization of 2e(a), the optimized overall yield is expected
to be ca. 65%. a-Thymidine thus obtained was found to
1
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The present method would provide a large-scale synthe-
sis of a-thymidine which is a key synthetic material for
the synthesis of a-DNA derivatives. Further studies to
apply of our method to the synthesis of other a-
deoxyribonucleosides are under way.
Acknowledgements
This work was supported by a Grant from ‘Research
for the Future’ Program of the Japan Society for the
Promotion of Science (JSPS-RFTF97I00301).
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