N(3)-protection of thymidine with Boc for an easy synthetic access to sugar-alkylated nucleoside analogs

Article abstract of DOI:10.1002/cbdv.201100103

The use of Boc as a nucleobase-protecting group in the synthesis of sugar-modified thymidine analogs is reported. Boc was easily inserted at N(3) by a simple and high-yielding reaction and found to be stable to standard treatments for the removal of Ac an

Full text of DOI:10.1002/cbdv.201100103

CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
589
N(3)-Protection of Thymidine with Boc for an Easy Synthetic Access to
Sugar-Alkylated Nucleoside Analogs
by Luca Simeone, Lorenzo De Napoli, and Daniela Montesarchio*
`
Dipartimento di Chimica Organica e Biochimica, Universita di Napoli ꢀFederico IIꢁ, via Cintia, 4,
I-80126 Napoli
(phone: þ39-081-674126; fax: þ39-081-674393; e-mail: daniela.montesarchio@unina.it)
The use of Boc as a nucleobase-protecting group in the synthesis of sugar-modified thymidine
analogs is reported. Boc was easily inserted at N(3) by a simple and high-yielding reaction and found to
t
be stable to standard treatments for the removal of Ac and BuMe2Si (TBDMS) groups, as well as to
ZnBr2-mediated 4,4’-dimethoxytrityl (DMTr) deprotection. Boc Protection proved to be completely
resistant to the strong basic conditions required to regioselectively achieve O-alkylation, therefore,
providing synthetic access to a variety of sugar-alkylated nucleoside analogs. To demonstrate the
feasibility of this approach, two 3’-O-alkylated thymidine analogs have been synthesized in high overall
yields and fully characterized.
Introduction. – Boc is a widely used protecting group [1], first developed for the
peptide synthesis and successively exploited in a wide range of synthetic organic
chemistry procedures. Its application in nucleoside or oligonucleotide chemistry, and
related mimics has been reported only in sporadic cases, as for the protection of
exocyclic amino groups in cytidine, adenosine, and guanosine derivatives [2], and for
the synthesis of modified PNA [3].
To the best of our knowledge, its use for masking the thymine base has been reported
only by Moon and co-workers, who exploited N(3)-Boc-protected thymidine building
blocks to obtain a 3’-deoxy,3’-[18F]fluorinated thymidine analog in high radiochemical
yield [4]. These researchers prepared the Boc-protected nucleoside by reaction with
(Boc)2O in the presence of stoichiometric amounts of 4-(dimethylamino)pyridine
(DMAP) in THF, which required long reaction times (5 h) and led to the target
compound in only 53% yield. A similar protocol was applied by Yu et al. to protect uracil
in the synthesis of 5-fluoro and (E)-5-(2-fluorovinyl)arabinosyluridine (Farall and
FVAU, resp.) via 5-(trimethylstannyl)- and (E)-5-[(2-tributylstannyl)vinyl]arabinosylur-
idine analogs [5]. Also in this case, insertion of the Boc group was obtained in low overall
yields (50%), leading to a mixture of N- and O-derivatized nucleosides.
More efficient procedures to protect thymine and uracil, to attain synthetic access
to a variety of sugar-modified thymidine and uridine analogs, have been explored by
other research groups. Among others, benzyl (Bn; removable by catalytic hydro-
genation) [6], 4-methoxybenzyl (removable by ceric ammonium nitrate treatment) [7],
and 4-nitrophenylethyl (removable by treatment with 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) in pyridine) [8] groups have been adopted to this purpose.
In the course of our studies on nucleoside analogs, we recently developed a novel
protection for the thymine base in nucleoside and oligonucleotide synthesis, based on
ꢂ 2012 Verlag Helvetica Chimica Acta AG, Zꢃrich

590
CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
the 2-(phenylsulfanyl)ethyl group, inserted at N(3) by a Mitsunobu reaction with 2-
(phenylsulfanyl)ethanol [9]. In an efficient ꢀtwo-stageꢁ system, after oxidation of the
thioether to sulfone, this protecting group could be totally removed by a b-elimination
mechanism by using 0.1m aqueous NaOH solution. The use of the 2-(phenylsulfanyl)-
ethyl protecting group, stable to strongly basic conditions, allowed us to selectively
achieve O-alkylation of the ribose moieties in satisfactory yields [10], and, thus, to
5’
efficiently obtain the anti-HIV active Hotodaꢁs TGGGAG3’ sequence [11], alkylated
at the 5’-OH moiety of the thymidine unit with the 3,4-(dibenzyloxy)benzyl group.
Aiming at expanding the repertoire of available sugar-alkylated thymidine analogs,
of interest as potential thymidine kinase inhibitors1) and functional building blocks for
the synthesis of sugar-modified oligonucleotides, we were intrigued by fast and simple
synthetic methods to prepare easy-to-handle nucleoside scaffolds. Following the
renovated interest in Boc protection, stimulated by the discovery of novel, very mild
procedures for its removal2), we decided to re-investigate its applicability as a thymine-
protecting group in the chemistry of nucleoside analogs.
Results and Discussion. – As model compounds for this study, we chose 3’-O-acetyl-
5’-O-(4,4-dimethoxytriphenylmethyl)thymidine (1; Scheme 1) and 3’-O-acetyl-5’-O-
[(tert-butyl)dimethylsilyl]thymidine (4; Scheme 2), which allowed us to explore the
compatibility of Boc with the most commonly used protecting groups in nucleoside
manipulations, i.e., the acid-labile 4,4’-dimethoxytrityl (¼ bis(4-methoxyphenyl)phe-
t
nylmethyl; DMTr), the fluoride-labile BuMe2Si (TBDMS) and the base-labile Ac
group.
Insertion of the Boc group was first optimized on 1 (Scheme 1), adopting a very
simple procedure, involving the use of 2 equiv. of (Boc)2O in the presence of Et3N
(3 equiv.) and catalytic amounts of DMAP in different solvents.
At this stage, the solvent was found to dramatically affect the course of the reaction.
When the above mentioned reagents were dissolved in dioxane or MeCN, no
transformation occurred, even forcing the system conditions; in fact, no benefit could
be obtained by prolonging the reaction times, or using high temperatures, or adding to
the mixtures bases stronger than Et3N, as, e.g., EtNiPr2. On the contrary, in a large
variety of other commonly used solvents, the Boc protection was always complete in 1 h
at room temperature, with very good-to-excellent yields (see Table 1)3) [14]. In these
1
)
)
)
For a recent example, see [12].
For recent examples, see [13].
2
3
Solvent-controlled alkylation of 5’-O-protected thymidine derivatives has been described in [14].
Following [14b], when allyl, propargyl, and benzyl bromide were reacted in stoichiometric amounts
with 5’-O-TBDMS-thymidine in the presence of NaH in THF under ultrasound activation, no
reaction was observed. In the same solvent, using 2.5 equiv. of the alkylating agent and of the base,
exclusively the 3’-O-alkylated derivative was found. On the contrary, only the corresponding N(3)-
adduct could be isolated from the reaction conducted in DMF. This behavior was explained in terms
of different dielectric constants of the solvents, with low dielectric constant solvents favoring the O-
alkylation, and high dielectric constant solvents promoting the N-alkylation. In our case, the
dielectric constant of the solvent cannot account for the different course of the Boc insertion on 1,
since the only observed failures occurred in dioxane and MeCN, the first one having a very low
(2.21) and the latter a very high (37.5) dielectric constant.

CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
591
Scheme 1. Synthesis of Boc-Protected 5’-O-DMTr-Thymidine Derivative 2 and Its Conversion into
Detritylated Analog 3a
Table 1. Yields of Insertion of the Boc Group on 1 Dependent of the Used Solvent
Solvent
Yield [%]
Solvent
Yield [%]
MeCN
0
92
92
92
Et2O
DMF
1,4-Dioxane
Pyridine
90
90
0
Benzene
ClCH2CH2Cl
CH2Cl2
99
cases, target compound 2 was isolated as a stable compound after a simple workup, in
yields always higher than 90%.
The best conditions for the introduction of Boc in 1 were then extended to 3’-O-
acetyl-5’-O-TMDS-thymidine (4; Scheme 2). This nucleoside, dissolved in pyridine,
was, therefore, reacted with (Boc)2O (2 equiv.) and Et3N (3 equiv.) in the presence of
0.5 equiv. of DMAP, to afford the target compound 5 in almost quantitative yields.
The 1H- and 13C-NMR spectra of 2 and 5 clearly indicated that the Boc group was
selectively inserted at N(3). In fact, no signal attributable to the NꢀH group could be
1
detected in the H-NMR spectra of these nucleosides recorded in (D6)DMSO; in
parallel, the 13C-NMR spectra of both these compounds showed a unique signal at ca.
148 ppm, attributed to the sp2-C-atom of the Boc CO group linked to an N-atom, with
the concomitant absence of signals at ca. 155 ppm, expected in case of its attachment to
an O-atom. In contrast to previous findings [5], we never observed migration of the Boc

592
CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
Scheme 2. Synthesis of Boc-Protected 5’-O-TBDMS-Thymidine Derivative 5 and Its Conversion into 3a
group from N(3) to O(4) or O(2), both in this and all of the successive manipulations.
In addition, no transposition of the carbamates to the corresponding O-tBu derivatives
[2b] was observed, as confirmed by MS analysis of the reaction mixtures and of the
isolated reaction products.
With two Boc-protected thymidine derivatives, 2 and 5, in our hands, we searched,
first, for efficient conditions allowing the selective removal of the DMTr group from 2.
Having a priori eliminated the use of even diluted, weak protic acids – not able to
guarantee the requested selectivity between the two acid-labile protecting groups Boc
and DMTr – anhydrous Lewis acids, particularly ZnBr2 [15] and CuSO4 [16], were
investigated in a variety of reaction systems. The use of the latter always caused partial
cleavage of the Boc group. On the contrary, satisfactory results were obtained with
anhydrous ZnBr2, which led to 3a in 84% yields. The isolation of only traces of 3b from
the mixture essentially confirmed that this treatment did not markedly affect the Boc
group. The best conditions for the preparation of 3a involved the use of 2 equiv. of
ZnBr2 in anhydrous CH2Cl2 under stirring at room temperature for 1.5 h. Prolonging
the reaction times, even using a lower excess of the Lewis acid, was detrimental for the
success of the reaction, causing also partial Boc removal; similar results were obtained
when anhydrous MeOH was used in lieu of CH2Cl2 as the solvent (Table 2).
Removal of the TBDMS groups from 5 was carried out first by treatment with
standard 1m Bu4NF (TBAF) solution in THF, which, quite unexpectedly, was found to
partially cleave the Boc group. On the contrary, full selectivity was obtained with the

CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
593
Table 2. Conditions Tested for the DMTr Removal from Nucleoside 2 and Relative Yields
Equiv. of ZnBr2
Solvent
Reaction time [h]
Yield after isolation [%]
3a
3b
2.5
3.0
1.5
2.5a)
CH2Cl2
CH2Cl2
CH2Cl2
MeOH
1.5
2.5
2.5
1.2
84
60
60
55
9
32
32
0
a) ZnBr2 was added as a sat. MeOH soln.
milder desilylating reagent Et3N·3 HF [17], which gave the desired Boc-protected
nucleoside 3a in quantitative yields.
To prepare 3’-O-alkylated thymidine analogs, the Ac group at the 3’-OH moiety had
to be removed first. For this step, a standard treatment with concentrated aqueous NH3
was tested on 2, which, after 2.5 h at room temperature, fully converted the starting
material to desired 6 (Scheme 3). A model reaction for the 3’-O-alkylation was carried
out first by reacting 6 with BnBr and NaH (1.2 equiv. each) in DMF, giving the target
adduct 7 in 85% yields. Then, the alkylation reaction on 6 was carried out with
Scheme 3. Synthesis of 3’-O-Alkylated Thymidine Analogs 7–9

594
CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
ClCH2COOMe. Due to the lower reactivity of alkyl chlorides compared to bromides, a
higher excess of the alkylating agent and the base (2.5 equiv.) was here required to lead
the reaction to completion, which, after chromatography, gave methyl ester 8 in 82%
yields.
Hydrolysis of 8 with 1m NaOH (30 min, room temperature) led to target 3’-O-
carboxymethyl sodium salt 9, obtained in 82% yields as a pure compound (TLC and
NMR) after a simple extraction (66% overall yield for four steps starting from 1). This
thymidine analog, as a convenient precursor of 3’-O-(carboxymethyl)thymidine [18], is
of interest as a versatile building block for backbone-modified oligonucleotides. Its use
as a useful monomer linker to be attached at the NH2 end of a PNA chain in the
synthesis of DNA-PNA chimera4) is currently under way in our laboratory and will be
reported elsewhere.
Conclusions. – In this study, we have described the use of Boc to conveniently
protect the thymine nucleobase for a simple and general synthetic access to a variety of
sugar-alkylated thymidine analogs. Conditions for its quantitative installation on sugar-
protected thymidine derivatives were found, as well as for the selective removal of the
commonly used protecting groups DMTr, TBDMS, and Ac from the corresponding
Boc-protected nucleosides. These high-yielding procedures, in principle, allow us to
easily obtain useful and versatile nucleoside building blocks for several base-promoted
sugar modifications. To demonstrate the feasibility of this approach, two sugar-
alkylated thymidine analogs, 7 and 9, have been synthesized and characterized.
Experimental Part
General. TLC: SiO2 plates from Merck (60, F254); visualization with UV light and then by treatment
with a 10% Ce(SO4)2/H2SO4 aq. soln. Column chromatography (CC): silica gel (SiO2; Kieselgel 40,
0.063–0.200 mm; Merck). NMR Spectra: Bruker WM-400 or Varian XR-200 spectrometers; chemical
shifts in ppm with respect to the residual solvent signal; J values in Hz; peak assignments on the basis of
1
standard H,1H-COSY and HSQC experiments. ESI-MS: Waters Micromass ZQ instrument, equipped
with an electrospray source; in the positive-ion and/or negative-ion mode.
Procedure for the Insertion of Boc on 1. Synthesis of 2. 3’-O-Acetyl-5’-O-[bis(4-methoxyphenyl)-
(phenyl)methyl]thymidine (1; 53 mg, 0.090 mmol), dissolved in the appropriate solvent, was reacted with
(Boc)2O (40 mg, 0.18 mmol), Et3N (38 ml, 0.27 mmol), and 4-(dimethylamino)pyridine (DMAP; 5.5 mg,
0.045 mmol). After 1 h under stirring at r.t., the solvent was removed under reduced pressure, and the
residue was purified by CC (SiO2; hexane/AcOEt 7:3 (v/v)). The yields obtained for each reaction are
compiled in Table 1.
3’-O-Acetyl-5’-O-[bis(4-methoxyphenyl)(phenyl)methyl]-3-[(tert-butoxy)carbonyl]-2’-deoxy-3,4-di-
1
hydrothymidine (2). Foam. Rf (hexane/AcOEt 1:1) 0.6. H-NMR (CDCl3, 200 MHz): 7.61 (d, J¼1.2,
HꢀC(6)); 7.39–6.81 (m, 13 arom H of DMTr); 6.40 (dd, J¼7.4, 6.0, HꢀC(1’)); 5.45–5.41 (m, HꢀC(3’));
4.14–4.12 (m, HꢀC(4’)); 3.82 (s, 2 MeO); 3.46 (ABX, J¼2.8, 10.4, CH2(5’)); 2.46–2.40 (m, CH2(2’)); 2.10
(s, MeCO); 1.60 (s, tBu); 1.35 (d, J¼1.0, Me of Thy). 13C-NMR (CDCl3, 50 MHz): 170.2 (MeCO); 161.1
(C(4)); 158.7, 144.2, 135.0, 130.0, 128.1, 128.0, 127.1, 113.2 (arom. C of DMTr); 148.5 (C(2)); 148.0
(NꢀCOꢀN); 135.3 (C(6)); 111.3 (C(5)); 87.2 (C or DMTr); 86.6 (Me3C); 84.6 (C(1’)); 84.1 (C(4’)); 75.3
(C(3’)); 63.6 (C(5’)); 55.2 (MeO); 38.1 (C(2’)); 27.4 (Me3C); 21.0 (MeCO); 11.7 (Me of Thy). ESI-MS
(pos.): 686.1 ([MþH]þ ).
4
)
For an example of linker monomers in the synthesis of DNA-PNA, see [19].

CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
595
Procedure for the Removal of the DMTr Group from 2. Synthesis of 3a. Compound 2 (30 mg,
0.044 mmol) was dissolved in 1 ml of anh. CH2Cl2, then dry ZnBr2 (25 mg, 0.110 mmol) was added. The
mixture was stirred at r.t. for 1.5 h, then the reaction was quenched by addition of few drops of MeOH.
The crude product was diluted with CH2Cl2 and washed twice with H2O. The org. phase was dried
(Na2SO4), concentrated under reduced pressure, and purified by CC (SiO2; hexane/AcOEt 2 :3).
Products 3a and 3b were isolated in 84 and 9% yield, resp.
Other reaction conditions were tested (see Table 2), but only lower yields of 3a were obtained under
all other conditions.
3’-O-Acetyl-3-[(tert-butoxy)carbonyl]-2’-deoxy-3,4-dihydrothymidine (3a). Oil. Rf (hexane/AcOEt
1
3 :7) 0.4. H-NMR (CDCl3, 200 MHz): 7.55 (d, J¼1.2, HꢀC(6)); 6.25 (dd, J¼7.4, 6.8, HꢀC(1’)); 5.34–
5.32 (m, HꢀC(3’)); 4.11–4.09 (m, HꢀC(4’)); 3.93–3.91 (m, CH2(5’)); 2.42–2.36 (m, CH2(2’)); 2.09 (s,
t
MeCO); 1.93 (d, J¼1.4, Me of Thy); 1.60 (s, Bu). 13C-NMR (CDCl3, 100 MHz): 170.6 (MeCO); 161.1
(C(4)); 148.5 (C(2)); 147.7 (C of carbamate); 135.3 (C(6)); 110.9 (C(5)); 86.7 (Me3C); 86.0 (C(1’)); 85.0
(C(4’)); 74.5 (C(3’)); 62.4 (C(5’)); 37.3 (C(2’)); 27.3 (Me3C); 20.8 (MeCO); 12.5 (Me of Thy). ESI-MS
(pos.): 791.2 ([2MþNa]þ ), 423.0 ([M þ K]þ ), 407.1 ([MþNa]þ ).
Compound 3b was identified by TLC and NMR comparison with an authentic sample.
Procedure for the Insertion of Boc in 4. Synthesis of 5. 3’-O-Acetyl-5’-O-[(tert-butyl)dimethylsil-
yl]thymidine (4; 53 mg, 0.090 mmol) was dissolved in 1 ml of dry pyridine, then (Boc)2O (40 mg,
0.180 mmol), Et3N (38 ml, 0.270 mmol), and DMAP (5.5 mg, 0.045 mmol) were added in this order. The
mixture was stirred at r.t. for 1 h, the solvent was removed under reduced pressure, and the residue was
purified by CC (SiO2; hexane/AcOEt 7:3). The desired compound 5 was obtained in 98% isolated yield
(60 mg, 0.088 mmol).
3’-O-Acetyl-3-[(tert-butoxy)carbonyl]-5’-O-[(tert-butyl)dimethylsilyl]-2’-deoxy-3,4-dihydrothymi-
1
dine (5). Oil. Rf (hexane/AcOEt 4 :6) 0.8. H-NMR (CDCl3, 200 MHz): 7.54 (d, J¼1.2, HꢀC(6)); 6.33
(dd, J¼5.2, HꢀC(1’)); 5.25–5.21 (m, HꢀC(3’)); 4.11–4.09 (m, HꢀC(4’)); 3.91–3.89 (m, CH2(5’)); 2.47–
t
t
2.38 (m, CH2(2’)); 2.08 (s, MeCO); 1.93 (d, J¼1.2, Me); 1.61 (s, Bu); 0.93 (s, BuSi); 0.13 (s, Me2Si).
13C-NMR (CDCl3, 50 MHz): 170.6 (MeCO); 161.2 (C(4)); 148.5 (C(2)); 147.9 (C of carbamate); 134.3
(C(6)); 110.9 (C(5)); 86.7 (Me3C); 85.4 (C(1’)); 85.0 (C(4’)); 75.3 (C(3’)); 63.5 (C(5’)); 37.3 (C(2’)); 27.4
(Me3C); 25.9 (Me3CSi); 20.9 (MeCO); 18.3 (Me3CSi); 12.5 (Me of Thy); ꢀ5.6 (Me2Si). ESI-MS (pos.):
536.8 ([MþK]þ ), 520.9 ([MþNa]þ ), 499.1 ([MþH]þ ).
Procedure for the Removal of TBDMS Group from 5. Compound 5 (20 mg, 0.040 mmol), dissolved
in 200 ml of anh. THF, was reacted with Et3N·3 HF (20 ml, 0.120 mmol). The mixture was stirred at r.t. for
1 h, then the soln., diluted with CH2Cl2, was washed twice with H2O; the org. phase was dried (Na2SO4),
concentrated under reduced pressure, and purified by CC (SiO2; hexane/AcOEt 2 :3). The desired
product was obtained in quant. yield, and identified as identical to nucleoside 3a, isolated from
detritylation of 2.
Procedure for the Removal of the Ac Group from 2. Synthesis of 6. Compound 2 (61 mg, 0.089 mmol)
was dissolved in 800 ml of MeOH, and 200 ml of a NH4OH soln. (29.1% p.p.) were added. After stirring
for 2.5 h at r.t., the mixture was concentrated under reduced pressure. The crude product was then
washed with CH2Cl2/H2O, and the org. phase was dried (Na2SO4) and concentrated under reduced
pressure. The desired product 6 was obtained in almost quant. yield without any observable side-reaction.
5’-O-[Bis(4-methoxyphenyl)(phenyl)methyl]-3-[(tert-butoxy)carbonyl]-2’-deoxy-3,4-dihydrothymi-
dine (6). Oil. Rf (hexane/AcOEt 1:1) 0.4. 1H-NMR (CDCl3, 200 MHz): 7.65 (d, J¼1.2, HꢀC(6)); 7.44–
6.84 (m, 13 arom. H of DMTr); 6.40 (dd, J¼7.4, 6.0, HꢀC(1’)); 4.62–4.60 (m, HꢀC(3’)); 4.10–4.07 (m,
HꢀC(4’)); 3.82 (s, 2 MeO); 3.45 (ABX, J¼2.8, 10.4, CH2(5’)); 2.46–2.38 (m, CH2(2’)); 1.63 (s, tBu); 1.45
(d, J¼1.2, Me of Thy). 13C-NMR (CDCl3, 50 MHz): 161.1 (C(4)); 158.7, 144.2, 135.0, 130.0, 128.1, 128.0,
127.1, 113.2 (arom. C of DMTr); 148.5 (C(2)); 148.0 (C of carbamate); 135.3 (C(6)); 110.8 (C(5)); 86.9 (C
or DMTr); 86.6 (Me3C); 86.2 (C(1’)); 85.0 (C(4’)); 72.1 (C(3’)); 63.4 (C(5’)); 55.2 (MeO); 41.1 (C(2’));
27.4 (Me3C); 11.7 (Me of Thy). ESI-MS (pos.): 673.0 ([MþK]þ ), 666.9 ([MþNa]þ ), 645.0 ([MþH]þ ).
Procedure for the Synthesis of 7. Compound 6 (50 mg, 0.078 mmol) was dissolved in 400 ml of anh.
DMF and cooled at 08. After 5 min, BnBr (11 ml, 0.094 mmol) and NaH (60% p.p., 4.0 mg, 0.094 mmol)
were added in this order. The mixture was stirred at r.t. for 4 h, then a few drops of MeOH were added at
08 to quench the reaction. The suspension was concentrated under reduced pressure, and the crude was

596
CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
then washed with CH2Cl2/H2O; the org. phase was dried (Na2SO4), concentrated under reduced pressure,
and purified by CC (SiO2; hexane/AcOEt 2 :3). The desired product 7 was obtained in 85% yield (49 mg,
0.066 mmol).
3’-O-Benzyl-5’-O-[bis(4-methoxyphenyl)(phenyl)methyl]-3-[(tert-butoxy)carbonyl]-2’-deoxy-3,4-
1
dihydrothymidine (7). Oil. Rf (hexane/AcOEt 1:1) 0.8. H-NMR (CDCl3, 200 MHz): 7.63 (d, J¼1.2,
HꢀC(6)); 7.36–6.79 (m, 18 arom. H of DMTr and Bn); 6.34 (dd, J¼7.6, 5.8, HꢀC(1’)); 4.58–4.40 (AB, J ¼
11.8, PhCH2O); 4.33–4.31 (m, HꢀC(3’)); 4.19–4.17 (m, HꢀC(4’)); 3.79 (s, 2 MeO); 3.53–3.26 (ABX, J¼
2.9, 10.6, CH2(5’)); 2.61–2.17 (m, CH2(2’)); 1.61 (s, tBu); 1.42 (d, J¼1.0, Me of Thy). 13C-NMR (CDCl3,
50 MHz): 161.3 (C(4)); 158.7, 144.2, 137.3, 130.0, 128.5, 128.1, 128.0, 127.9, 127.6, 127.1, 113.2 (arom. C of
DMTr and Bn); 148.5 (C(2)); 148.0 (C of carbamate); 135.3 (C(6)); 110.9 (C(5)); 86.9 (C or DMTr); 86.2
(C(1’)); 85.2 ((Me3C); 84.2 (C(4’)); 78.3 (PhCH2); 71.3 (C(3’)); 63.4 (C(5’)); 55.2 (MeO); 46.2 (C(2’));
27.4 (Me3C); 11.4 (Me). ESI-MS (pos.): 772.67 ([MþK]þ ), 756.77 [MþNa]þ ), 735.91 ([MþH]þ ).
Procedure for Synthesis of 8. Compound 6 (40 mg, 0.062 mmol) was dissolved in 0.5 ml of dry DMF,
then ClCH2COOMe (14 ml, 0.15 mmol) and NaH (60% p.p., 8.0 mg, 0.19 mmol) were added. The mixture
was stirred at r.t. for 24 h, then a few drops of MeOH were added at 08 to quench the reaction. The
suspension was taken to dryness, then the crude was redissolved in CH2Cl2 and washed twice with H2O;
the org. phase was dried (Na2SO4), concentrated under reduced pressure, and purified by CC (SiO2;
hexane/AcOEt 3 :7). The desired ester 8 was obtained in 82% yield (37 mg, 0.051 mmol).
5’-O-[Bis(4-methoxyphenyl)(phenyl)methyl]-3-[(tert-butoxy)carbonyl]-2’-deoxy-3,4-dihydro-3’-O-
1
(2-methoxy-2-oxoethyl)thymidine (8). Oil. Rf (hexane/AcOEt 1:1) 0.5. H-NMR (CDCl3, 200 MHz):
7.63 (s, HꢀC(6)); 7.44–6.84 (m, 13 arom. H of DMTr); 6.33 (dd, J¼5.6, 6.0, HꢀC(1’)); 4.33–4.10 (m,
HꢀC(3’), HꢀC(4’), CH2COOMe); 3.81 (s, 2 MeO); 3.75 (s, COOMe); 3.41 (ABX, J¼2.8, 10.4, CH2(5’));
2.58–2.26 (m, CH2(2’)); 1.61 (s, tBu); 1.44 (s, Me of Thy). 13C-NMR (CDCl3, 50 MHz): 170.7 (COOMe);
161.1 (C(4)); 158.7, 144.2, 135.0, 130.0, 128.1, 128.0, 127.1, 113.2 (arom. C of DMTr); 148.5 (C(2)); 147.3 (C
of carbamate); 135.3 (C(6)); 110.6 (C(5)); 86.8 (C or DMTr); 84.9 (Me3C); 84.2 (C(1’)); 80.2 (C(4’)); 79.2
(C(3’)); 63.4 (C(5’)); 62.2 (CH2COOMe); 55.2 (MeO); 52.2 (COOMe); 37.1 (C(2’)); 27.4 (Me3C); 11.8
(Me of Thy). ESI-MS (pos.):754.75 ([MþK]þ ), 738.73 ([MþNa]þ , 716.94 ([MþH]þ ).
Procedure for the Synthesis of the Sodium Salt 9. Compound 8 (12 mg, 0.017 mmol) was dissolved in
100 ml of dry THF, then NaOH (0.7 mg, 0.019 mmol) in 20 ml of H2O was added. The mixture was kept
under stirring at r.t. for 30 min, then the mixture was taken to dryness, redissolved in CH2Cl2, and washed
twice with H2O; the org. phase was dried (Na2SO4) and concentrated under reduced pressure. The
desired salt 9 was obtained in 82% yield (10 mg, 0.014 mmol).
Sodium 5’-O-[Bis(4-methoxyphenyl)(phenyl)methyl]-3-[(tert-butoxy)carbonyl]-3’-O-(carboxylato-
methyl)-2’-deoxy-3,4-dihydrothymidine (9). Oil. Rf (AcOEt) 0.1. 1H-NMR (CDCl3, 400 MHz): 7.61 (s,
HꢀC(6)); 7.31–6.76 (m, 13 arom. H of DMTr); 6.34 (br., HꢀC(1’)); 4.18–4.16 (m, HꢀC(3’), HꢀC(4’));
3.79 (s, CH2CO); 3.73 (s, 2 MeO); 3.37–3.35 (m, CH2(5’)); 2.55–2.14 (m, CH2(2’)); 1.49 (s, tBu); 1.25 (s,
Me of Thy). 13C-NMR (CDCl3, 50 MHz): 175.6 (COO); 161.1 (C(4)); 158.6, 144.0, 129.9, 127.9, 127.0,
113.2 (arom. C of DMTr); 148.6 (C(2)); 147.8 (C of carbamate); 135.1 (C(6)); 111.1 (C(5)); 86.9 (C or
DMTr); 86.5 (Me3C); 85.2 (C(1’)); 83.9 (C(4’)); 80.9 (C(3’)); 68.6 (CH2COO); 63.8 (C(5’)); 55.1 (MeO);
37.3 (C(2’)); 27.2 (Me3C); 11.5 (Me of Thy). ESI-MS (neg.): 700.88 (Mꢀ ).
We acknowledge MIUR (PRIN) for grants in support of this investigation. We also thank
`
C. I. M. C. F., Universita degli Studi di Napoli ꢀFederico IIꢁ, for the NMR and MS facilities.
REFERENCES
[1] P. G. M. Wuts, T. W. Greene, in ꢀProtective Groups in Organic Synthesisꢁ, 4th edn., Wiley-
´
Interscience, New York, 2006; P. J. Kocienski, in ꢀProtecting Groupsꢁ, 3rd edn., Thieme, Stuttgart,
2005; C. Agami, F. Couty, Tetrahedron 2002, 58, 2701.
[2] a) H.-R. Jung, T. K. Vu, S.-K. Choi, S.-M. Park, B.-H. Kim, Bull. Kor. Chem. Soc. 2010, 31, 549;
b) R.-W. Wang, B. Gold, Org. Lett. 2009, 11, 2465; c) B. Y. Michel, P. Strazewski, Tetrahedron 2007,

CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
597
63, 9836; d) S. A. Sikchi, P. G. Hultin, J. Org. Chem. 2006, 71, 5888; e) Y. Huang, S. Dey, P. Garner,
Tetrahedron Lett. 2003, 44, 1441.
[3] B. G. de la Torre, R. Eritja, Bioorg. Med. Chem. Lett. 2003, 13, 391.
[4] M. Yun, S. J. Oh, H.-J. Ha, J. S. Ryu, D. H. Moon, Nucl. Med. Biol. 2003, 30, 151.
[5] C.-S. Yu, L.-W. Chiang, C.-H. Wu, Z.-K. Hsu, M.-H. Lee, S.-D. Pan, K. Pei, Synthesis 2006, 22, 3835.
[6] F. A. Luzzio, M. E. Menes, J. Org. Chem. 1994, 59, 7267.
[7] A. Van Aeroschot, L. Jie, P. Herdewijn, Tetrahedron Lett. 1991, 32, 1905.
[8] B. S. Schultz, W. Pfleiderer, Tetrahedron Lett. 1983, 24, 3587.
[9] J. DꢁOnofrio, L. De Napoli, G. Di Fabio, D. Montesarchio, Synlett 2006, 6, 845.
[10] J. DꢁOnofrio, L. Petraccone, E. Erra, L. Martino, G. Di Fabio, L. De Napoli, C. Giancola, D.
Montesarchio, Bioconjugate Chem. 2007, 18, 1194.
[11] H. Hotoda, M. Koizumi, R. Koga, M. Kaneko, K. Momota, T. Ohmine, H. Furukawa, T. Agatsuma,
T. Nishigaki, J. Sone, S. Tsutsumi, T. Kosaka, K. Abe, S. Kimura, K. Shimada, J. Med. Chem. 1998,
41, 3655.
[12] S. Van Poecke, A. Negri, J. Janssens, N. Solaroli, A. Karlsson, F. Gago, J. Balzarini, S.
Van Calenbergh, Org. Biomol. Chem. 2011, 9, 892.
[13] N. S. Shaikh, A. S. Gajare, V. H. Deshpande, A. V. Bedekar, Tetrahedron Lett. 2000, 41, 385; E.
Marcantoni, M. Massaccesi, E. Torregiani, G. Bartoli, M. Bosco, L. Sambri, J. Org. Chem. 2001, 66,
4430; S. N. Raghavendra, B. P. Kumar, H. Longqin, Tetrahedron Lett. 2006, 47, 389; A. Thaqi, A.
McCluskey, J. L. Scott, Tetrahedron Lett. 2008, 49, 6962; J. Wang, Y.-L. Liang, J. Qu, Chem.
Commun. 2009, 5144; G. Wang, C. Li, J. Li, X. Jia, Tetrahedron Lett. 2009, 50, 1438.
[14] a) K. Teste, L. Colombeau, A. Hadj-Bouazza, R. Lucas, R. Zerrouki, P. Krausz, Y. Champavier,
Carbohydr. Res. 2008, 343, 1490; b) J.-C. Wu, Z. Xi, C. Gioeli, J. Chattopadhyaya, Tetrahedron 1991,
47, 2237.
[15] V. Kohli, H. Blçcker, H. Kçster, Tetrahedron Lett. 1980, 21, 2683.
[16] G. Randazzo, R. Capasso, M. R. Cicala, A. Evidente, Carbohydr. Res. 1980, 85, 298.
[17] E. Westmanu, R. Stromberg, Nucleic Acids Res. 1994, 22, 2430.
[18] M. D. Edge, A. S. Jones, J. Chem. Soc. C 1971, 1933; M. H. Halford, A. S. Jones, J. Chem. Soc. C
1968, 2667.
[19] D. Musumeci, G. N. Roviello, M. Valente, R. Sapio, C. Pedone, E. M. Bucci, Biopolymers 2004, 76,
535.
Received March 28, 2011