Synthesis of 2′-C-methyl-branched isonucleosides

Article abstract of DOI:10.1016/j.tet.2008.05.022

Novel regioisomers of 2′-methyl-branched nucleosides were designed and synthesized to mimic potent anti-viral drugs like Valopicitabine. The short and efficient synthesis of the targets involves a one-pot tosylation/cyclization step that leads to an activated furan scaffold on which the isonucleosides were built.

Full text of DOI:10.1016/j.tet.2008.05.022

Tetrahedron 64 (2008) 6657–6661
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of 20-C-methyl-branched isonucleosides
*
Tony Bouisset, Gilles Gosselin, Ludovic Griffe, Jean-Christophe Meillon , Richard Storer
Laboratoire de Chimie Me´dicinale Idenix, Cap Gamma, 1682 rue de la Valsie`re, BP 50001, Montpellier, F-34189 Cedex 4, France
a r t i c l e i n f o
a b s t r a c t
Novel regioisomers of 20-methyl-branched nucleosides were designed and synthesized to mimic potent
anti-viral drugs like Valopicitabine. The short and efficient synthesis of the targets involves a one-pot
tosylation/cyclization step that leads to an activated furan scaffold on which the isonucleosides were
built.
Article history:
Received 8 April 2008
Accepted 7 May 2008
Available online 11 May 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Nucleoside analogues
Isonucleosides
Valopicitabine
Hepatitis C
Anti-virals
1. Introduction
prodrug of 20-C-methyl-cytidine (3) reached phase IIb in clinical
trials.19 Its regioisomer 4 was designed by simple transposition of
In the course of our ongoing research program aiming at the
discovery of potential new anti-viral agents, we became interested
in regioisomeric nucleoside analogues (‘isonucleosides’) in which
the nucleobase is attached to a position other than C-1 of the sugar.
Indeed, these molecules are likely to exhibit better stability and
resistance toward enzymatic degradation than natural nucleosides.
Moreover, they increase the chemical diversity at the disposition
of medicinal chemists involved in the preparation of modified
nucleosides.
In recent years, isonucleosides have attracted the attention of
many groups1–7 and some of the compounds showed significant
activity against human immunodeficiency (HIV) or herpes simplex
viruses.8–13 For instance, iso-ddA (1) displays good anti-HIV activity
and compares favorably to ddA (2), its regioisomer with a nature-
like structure (Fig. 1).14 Hence, it appeared relevant to consider the
preparation of isonucleoside analogues of other anti-virals, aimed
at increasing their activity and specificity or reducing their toxicity.
the aglycon from C-1 of the ribose moiety to the methyl branching
in C-2. Thus, one might expect that the base in 4 could occupy
a position in space similar to that of 3. In this communication, we
present the synthesis of isonucleoside 4 along with its analogues
built from other natural bases. All the synthetic routes proceed via a
common activated furan intermediate, which is the product of
a stereocontrolled ring-closing reaction carried out on a modified
ribitol.
The retrosynthetic plan (Fig. 2) shows that we have chosen
to introduce the nucleobase in the late stages of the synthesis by
an SN2 reaction. Thus, the route was made more convergent and
adaptableto any base. Inthefirstinstance, thehemiacetal functionof
the sugar would have been reduced to a methylene; the preparation
NH2
NH2
N
N
N
N
N
N
HO
HO
HO
HO
N
N
O
O
2. Results and discussion
2.1. General strategy
1
2
NH2
NH2
Our initial targets were 20-
b-methyl-branched nucleosides.
N
N
This class of molecules display potent anti-hepatitis C virus (HCV)
N
O
N
O
properties.15–18 For instance, Valopicitabine the valinyl ester
O
O
OH OH
OH OH
4
3
* Corresponding author. Tel.: þ33 4 67637320; fax: þ33 4 67637326.
E-mail address: meillon.jean-christophe@idenix.com (J.-C. Meillon).
Figure 1.
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.05.022

6658
T. Bouisset et al. / Tetrahedron 64 (2008) 6657–6661
HO
R1O
BASE
O
O
TrtO
OH
OH
O
OH
OH
OTs
OTs
TrtO
X
OH OH
OR2 OR3
2 TsCl
O
O
X = Leaving Group
O
R1O
OH
OH
O
7
D-ribose
OR2 OR3
Figure 2. Retrosynthetic analysis.
X
TrtO
OTs
O
OH
O
TrtO
Δ
OTs
O
O
OTs
beginning with a stereoselective
starting material -ribose.
b-hydroxymethylation of the
O
8
D
Scheme 2. Regioselective cyclization of ribitol derivative 7.
2.2. Preparation of key tosylate 8
Following well established procedures,20,21
2.3. Synthesis of the isonucleosides
D
-ribose was pro-
tected by a trityl and an acetonide in two steps to give sugar 5
(Scheme 1). The first important step of the synthesis comprised the
selective hydroxymethylation of the protected sugar.22 When 5 was
reacted with paraformaldehyde in the presence of potassium
carbonate, only the b-hydroxymethylated sugar was produced. The
stereochemical outcome of this thermodynamically controlled
reaction is directed by the acetonide protecting group on the vicinal
With 8 in hand, everything was in place for the introduction of
the bases of our isonucleosides. In the first instance, tosylate 8 was
treated with the sodium salt of adenine in DMF at 110 ꢀC. The
protected isonucleoside 9 was obtained in good yield and eventu-
ally lead to the fully deprotected adenosine analogue 10 upon
treatment with hot acid. The preparation of its guanine analogue
followed a similar pathway: the only difference residing in the use
of 2-amino-6-(methoxyethoxy)-purine25 instead of guanine itself.
Indeed, in a previous work,26 it was observed that this protected
purine gives much better results in nucleophilic substitution re-
actions than free guanine. The condensation step gave slightly
lower yields for 11 than for 9 but the synthesis of 12 was still
completed in two steps since the triple final deprotection was
carried out in one pot using 3 N HCl at 80 ꢀC (Scheme 3).
diols. Indeed, the attack of formaldehyde on the
a-face of the
enolate would result in the formation of a trans junction between
two fused five-membered rings, which is virtually impossible
when a cis junction is available.23 Therefore, the hydroxymethy-
lated sugar 6 was obtained as the sole product of the reaction (as
a mixture of
a/b anomers). Subsequently, 6 was reduced with
sodium borohydride in almost quantitative yield to give ribitol
derivative 7.
Y
TrtO
TrtO
O
OH
O
OH
OH
a
2 steps
Ref. 20
N
N
N
D-ribose
72 %
O
O
O
O
X
N
TrtO
TrtO
O
O
a
5
6
OTs
TrtO
TrtO
OH
O
O
O
O
OH
OH
O
c
b
94 %
OTs
8
9 : X = H, Y = NH2
11 : X = NH2, Y = OCH2CH2OMe
82 %
O
O
O
O
Y
N
7
8
N
N
Scheme 1. Reagents and conditions: (a) (CH2O)n, K2CO3, MeOH, reflux, 24 h; (b)
NaBH4, EtOH, rt, 2 h; (c) TsCl (2.4 equiv), pyridine, rt then 60 ꢀC, 15 h.
N
X
HO
O
10 : X = H, Y = NH2
12 : X = NH2, Y = OH
b
OH OH
The second key step consisted of the preparation of tosylated
furan derivative 8. Hossain and Herdewijn reported a two-step one-
pot cyclization of a protected ribitol using tosyl chloride.24 We
decided to try and adapt their conditions to triol 7. When the latter
was treated with 2.4 equiv of tosyl chloride in pyridine at room
temperature, the two primary alcohols reacted readily to provide
the ditosylated species shown in Scheme 2. This intermediate was
not isolated but instead, forced to cyclize to produce compound 8.
Under the reaction conditions, the secondary alcohol remained
available and could react with the tosylates. By just raising up the
temperature to 60 ꢀC, the tosylate-promoted cyclization occurred
cleanly to give 8 in high yield. Yet again, the isopropylidene pro-
tection allowed a perfect regiochemical control of the process for
the reasons disclosed earlier.23 One of the tosylates remained out of
the reach of the secondary alcohol during the reaction that only
produced the cis-[3.3.0] bicyclic 8. The quaternary carbon formed in
the reaction recovered its chirality that was lost after the reduction
step. The key tosylated furan derivative 8 was synthesized in 5 steps
Scheme 3. Reagents and conditions: (a) for 9, adenine, NaH, DMF, 110 ꢀC, 15 h, 75%; for
11, 2-amino-6-(methoxyethoxy)-purine, NaH, DMF, 120 ꢀC, 15 h, 63%; (b) for 10, 2 N
HCl, 80 ꢀC, 2 h, 95%; for 12, 3 N HCl, 80 ꢀC, 3 h, 70%.
The same approach was applied to pyrimidines. Although uracil
is known as a somewhat weaker nucleophile than adenine or other
protected purines,27 we managed to get satisfying results with
conditions similar to those previously described for purines
(Scheme 4). Tosylate 8 was substituted with the sodium salt of uracil
in 24 h in refluxing DMF. Isonucleoside 13, which was obtained in
good yield, was the key intermediate for the preparation of our two
pyrimidine targets. When the classical double deprotection condi-
tions in aqueous HCl were applied to 13, uridine analogue 14 was
isolated in almost quantitative yield. On the other hand, 13 could be
used in the cytosine base elaboration process described by Miah
et al.28 After activation with trifluoroacetic anhydride and p-nitro-
phenol,13 was treated with ammonia to give the protected cytidine
analogue 15 in moderate yield. The straightforward final
and 42% yield from D-ribose.

T. Bouisset et al. / Tetrahedron 64 (2008) 6657–6661
6659
O
N
and 2-amino-6-(methoxyethoxy)-purine was synthesized accord-
ing to Ref. 25. All other chemicals were purchased from Sigma-
Aldrich or Acros.
O
N
NH
NH
TrtO
TrtO
O
HO
O
O
O
a
b
O
4.1.1. (2R,3R)-1-Bis(hydroxymethyl)-1,2-O-isopropylidene-4-
triphenylmethyloxy-butane-1,2,3-triol (7)
61%
93%
OTs
O
O
O
O
OH OH
To a solution of 2-C-hydroxymethyl-2,3-O-isopropylidene-5-O-
14
13
8
triphenylmethyl-D-ribofuranose (6) (17.0 g, 36.8 mmol) in EtOH
58%
(2 steps)
(200 mL) was slowly added a solution of NaBH4 (2.7 g, 70.8 mmol)
in EtOH (300 mL). The mixture was stirred for 2 h at room tem-
perature. Then, solid NH4Cl (17 g) was added and the suspension
was stirred for 15 min. After filtration, EtOH was evaporated and
the crude material was purified by chromatography (silica gel,
DCM/MeOH 97:3) to afford 7 (16.0 g, 94%) as a white foam: TLC
c, d
NH2
NH2
N
N
TrtO
N
O
HO
N
O
b
O
Rf¼0.46 (C); 1H NMR (400 MHz, CDCl3)
d
1.37 (s, 3H), 1.38 (s, 3H),
O
92%
2.54 (br, 1H), 2.69 (t, 1H, J¼6.6 Hz), 3.08 (d, 1H, J¼3.9 Hz), 3.33 (dd,
1H, J¼6.4, 9.6 Hz), 3.50 (dd, 1H, J¼3.0, 9.6 Hz), 3.64 (dd, 1H, J¼6.6,
11.7 Hz), 3.76–3.90 (m, 4H), 4.01 (d, 1H, J¼9.3 Hz), 7.25–7.38 (m,
O
O
OH OH
15
4
9H), 7.42–7.50 (m, 6H); 13C NMR (100.5 MHz, CDCl3)
d 26.5, 28.4,
Scheme 4. Reagents and conditions: (a) uracil, NaH, DMF, 140 ꢀC, 24 h; (b) 2 N HCl,
80 ꢀC, 2 h; (c) N-methylpyrrolidine, trifluoroacetic anhydride then, p-nitrophenol,
MeCN, 0 ꢀC, 3.5 h; (d) NH3-saturated MeOH, 100 ꢀC, 5 h.
62.1, 64.8, 65.4, 68.7, 78.6, 84.5, 87.1, 108.7, 127.2, 128.0, 128.6, 143.6;
MS(ES) calcd [MþNa]þ (C28H32NaO6): 487, found: 487.
deprotection of the latter gave 4, the regioisomer of the anti-hepa-
titis C drug 3.
4.1.2. (1R,2R,3R)-1-(Triphenylmethyloxymethyl)-2,3-O-
isopropylidene-3-C-(p-toluenesulfonyloxymethyl)-furan-
2,3-diol (8)
3. Conclusion
To a solution of 7 (12.7 g, 27.3 mmol) in dry pyridine (125 mL)
was added tosyl chloride (12.5 g, 65.8 mmol). The solution was
stirred at room temperature for 3 h and then, at 60 ꢀC for 15 h. The
solvent was evaporated and the crude material was purified by
chromatography (silica gel, hexanes/EtOAc 8:2) to afford 8 (13.4 g,
82%) as a white foam: TLC Rf¼0.27 (A); 1H NMR (400 MHz, CDCl3)
We have described a short and efficient synthesis of novel
isonucleosides that could possibly mimic known anti-viral drugs.
This approach could be easily applied to the preparation of mole-
cules bearing non-natural nucleobases or targetting specific viruses
by further simple modifications made to the furan ring.
Compounds 4, 10, 12, and 14 were evaluated for anti-viral ac-
tivity in cell culture experiments against the following viruses:
Hepatitis C (HCV replicon system), Bovine Viral Diarrhea, Human
Immunodeficiency, Dengue and West Nile. Neither activity nor
cytotoxicity was found for any of these molecules at concentrations
d
1.32 (s, 3H), 1.50 (s, 3H), 2.44 (s, 3H), 3.17 (dd, 1H, J¼5.0, 10.2 Hz),
3.24 (dd, 1H, J¼4.7, 10.2 Hz), 3.91 (s, 2H), 4.07–4.23 (m, 3H), 4.42 (d,
1H, J¼1.9 Hz), 7.25–7.36 (m, 11H), 7.38–7.49 (m, 6H), 7.70 (d, 2H,
J¼8.1 Hz); 13C NMR (100.5 MHz, CDCl3)
d 21.7, 27.5, 27.8, 63.5, 70.1,
75.0, 84.8, 84.9, 87.3, 90.5, 114.7, 127.2, 128.0, 128.7, 129.8, 132.6,
143.5, 144.9, 158.4; MS(ES) calcd [MþNa]þ (C35H36NaO7S): 623,
found: 623.
of up to 100 mM.
Two hypotheses one could advance to explain this lack of
activity are the price paid in terms of entropy by the addition of
rotational flexibility between the sugar and the nucleobase or the
change in spatial distance between the base and the primary
alcohol of the isonucleosides.
4.1.3. iso-20-C-Methyl-20,30-O-isopropylidene-50-O-
triphenylmethyl-adenosine (9)
To a stirred suspension of adenine (1.35 g, 9.92 mmol) in DMF
(18 mL) was added NaH (60% oil dispersion) (0.33 g, 8.26 mmol) at
room temperature. After the evolution of hydrogen had ceased, the
creamy mixture was stirred at 80 ꢀC for 20 min. Then, tosylate 8
(2.00 g, 3.33 mmol) dissolved in DMF (18 mL) was added via a sy-
ringe. The resulting suspension was stirred at 110 ꢀC for 15 h and
then, cooled down to room temperature. The solid was removed by
filtration over Celite and rinsed with DCM. The filtrate was evap-
orated and the brown residue was taken up in DCM (50 mL). The
resulting solution was extracted with 0.5 N HCl (50 mL), satd
NaHCO3 solution (50 mL), and H2O (50 mL). After drying over
Na2SO4, the organic phase was evaporated. The crude material was
purified by chromatography (silica gel, DCM/MeOH 96:4) to afford
9 (1.41 g, 75%) as a white foam: TLC Rf¼0.47 (C); 1H NMR (400 MHz,
4. Experimental section
4.1. General
All 1H chemical shifts are reported in
or DMSO (
2.55). All 13C chemical shifts are reported in
CDCl3 (center of triplet, 77.2) or DMSO-d6 (center of septet,
d
relative to CHCl3 (
relative to
39.5).
d 7.26)
d
d
d
d
The spin multiplicities are indicated by the symbols s (singlet),
d (doublet), t (triplet), m (multiplet), and br (broad). Reactions were
monitored by thin-layer chromatography (TLC) using Merck silica
gel 60-F254 precoated plates with visualization by irradiation with
an UV lamp or by charring after immersion in a solution of
(NH4)2SO4 and H2SO4 in aqueous EtOH. TLC plates were developed
with solvent systems (A) EtOAc/hexanes 1:3, (B) EtOAc/hexanes 1:1,
(C) DCM/MeOH 9:1 or (D) DCM/MeOH 8:2. Column chromatogra-
CDCl3)
d
1.07 (s, 3H), 1.50 (s, 3H), 3.29 (dd, 1H, J¼5.2, 10.2 Hz), 3.38
(dd, 1H, J¼4.7, 10.2 Hz), 3.96 (d, 1H, J¼10.1 Hz), 4.13 (d, 1H,
J¼10.1 Hz), 4.24 (m, 1H), 4.43 (s, 2H), 4.65 (s, 1H), 5.87 (br, 2H),
7.25–7.38 (m, 9H), 7.42–7.50 (m, 6H), 7.85 (s, 1H), 8.25 (s, 1H); 13C
phies were performed on either silica gel 60 (normal phase) or C18
-
NMR (100.5 MHz, CDCl3) d 27.6, 27.7, 46.5, 64.1, 76.0, 85.1, 85.5, 87.5,
branched silica gel 40–63 m (reverse phase) and eluted with the
m
91.7, 114.2, 119.0, 127.2, 127.9, 128.8, 141.9, 143.6, 150.4, 153.1, 155.4;
indicated solvent system. Yields refer to chromatographically and
spectroscopically (NMR) homogeneous materials. The reactions
were generally carried out in an argon atmosphere using Fluka dry
MS(ES) calcd [MþH]þ (C33H34N5O4): 564, found: 564.
4.1.4. iso-20-C-Methyl-adenosine (10)
solvents.
nylmethyl-
2-C-Hydroxymethyl-2,3-O-isopropylidene-5-O-triphe-
D-ribofuranose (6) was synthesized according to Ref. 22
A suspension of 9 (1.20 g, 2.13 mmol) in 2 N HCl (50 mL) was
heated at 80 ꢀC for 2 h. The mixture was cooled down to room

6660
T. Bouisset et al. / Tetrahedron 64 (2008) 6657–6661
temperature and extracted with DCM (2ꢁ50 mL). The aqueous phase
was evaporated and the residue was taken up in H2O (30 mL). The
resulting solutionwas neutralized with DowexÒ 1X8-200 resin (OHꢂ
form) andfiltered. The filtratewas evaporated andthe crude material
was purified by chromatography (reverse phase, H2O/MeCN 98:2) to
afford 10 (0.57 g, 95%) as a white solid: TLC Rf¼0.10 (D); 1H NMR
brown residue was taken up in DCM (200 mL). The resulting so-
lution was extracted with 0.5 N HCl (200 mL), satd NaHCO3 solution
(200 mL), and H2O (200 mL). After drying over Na2SO4, the organic
phase was evaporated. The crude material was purified by chro-
matography (silica gel, DCM/MeOH 98:2) to afford 13 (4.39 g, 61%)
as a white foam: TLC Rf¼0.64 (C); 1H NMR (400 MHz, CDCl3)
d 1.34
(400 MHz, DMSO)
d
3.27–3.35 (d, 1H, J¼9.6 Hz, under H2O peak),
(s, 3H), 1.52 (s, 3H), 3.21 (dd, 1H, J¼5.1, 10.2 Hz), 3.34 (dd, 1H, J¼4.9,
10.2 Hz), 3.96 (d,1H, J¼10.2 Hz), 3.97 (s, 2H), 4.04 (d,1H, J¼10.2 Hz),
4.22 (td,1H, J¼1.6, 4.9 Hz), 4.55 (d,1H, J¼1.6 Hz), 5.66 (dd,1H, J¼2.2,
8.0 Hz), 7.21–7.35 (m, 10H), 7.45–7.50 (m, 6H), 8.92 (br, 1H); 13C
3.38–3.48 (m, 1H), 3.54–3.64 (m, 2H), 3.65–3.73 (m, 1H), 3.87 (d, 1H,
J¼9.6 Hz), 4.16 (d, 1H, J¼14.1 Hz), 4.26 (d, 1H, J¼14.1 Hz), 4.70 (t, 1H,
J¼5.5 Hz), 4.98 (s,1H), 5.25 (d,1H, J¼6.5 Hz), 7.19 (br, 2H), 8.05 (s,1H),
8.12 (s,1H); 13C NMR (100.5 MHz, DMSO)
d
48.5, 61.8, 73.0, 74.8, 78.2,
NMR (100.5 MHz, CDCl3) d 27.6, 28.1, 50.5, 63.8, 75.8, 85.0, 85.7,
84.1, 118.7, 142.3, 150.3, 152.8, 156.4; MS(ES) calcd [MþH]þ
(C11H16N5O4): 282, found: 282, calcd [MꢂH]ꢂ (C11H14N5O4): 280,
found: 280; HRMS(FAB) calcd [MþH]þ (C11H16N5O4): 282.1202,
found: 282.1205.
87.4, 92.0, 101.6, 114.2, 127.1, 127.9, 128.8, 143.5, 146.1, 150.8, 163.5;
MS(ES) calcd [MþNa]þ (C32H32NaN2O6): 563, found: 563, calcd
[MꢂH]ꢂ (C32H31N2O6): 539, found: 539.
4.1.8. iso-20-C-Methyl-uridine (14)
4.1.5. iso-20-C-Methyl-20,30-O-isopropylidene-50-O-
triphenylmethyl-6-O-(methoxyethyl)-guanosine (11)
A suspension of 13 (1.20 g, 2.22 mmol) in 2 N HCl (50 mL) was
heated at 80 ꢀC for 2 h. The mixture was cooled down to room
temperature and extracted with DCM (2ꢁ50 mL). The aqueous
phase was evaporated and the residue was taken up in H2O (30 mL).
The resulting solution was neutralized with DowexÒ 1X8-200 resin
(OHꢂ form) and filtered. The filtrate was evaporated and the crude
material was purified by chromatography (reverse phase, H2O/
MeCN 95:5) to afford 14 (0.53 g, 93%) as a white solid: TLC Rf¼0.26
To a stirred suspension of 2-amino-6-(methoxyethoxy)-purine
(2.08 g, 9.92 mmol) in DMF (18 mL) was added NaH (60% oil dis-
persion) (0.33 g, 8.26 mmol) at room temperature. After the evo-
lution of hydrogen had ceased, the solution was stirred at 80 ꢀC for
20 min. Then, tosylate 8 (2.00 g, 3.33 mmol) dissolved in DMF
(18 mL) was added via a syringe. The solution was stirred at 120 ꢀC
for 15 h and then, cooled down to room temperature. The solvent
was evaporated and the brown residue was taken up in DCM
(50 mL). The resulting solution was extracted with 0.5 N HCl
(50 mL), satd NaHCO3 solution (50 mL), and H2O (50 mL). After
drying over Na2SO4, the organic phase was evaporated. The crude
material was purified by chromatography (silica gel, DCM/MeOH
97:3) to afford 11 (1.34 g, 63%) as a white foam: TLC Rf¼0.69 (C); 1H
(D); 1H NMR (400 MHz, DMSO)
d 3.35–3.48 (m, 2H), 3.52–3.72 (m,
4H), 3.79 (d, 1H, J¼9.6 Hz), 3.86 (d, 1H, J¼14.1 Hz), 4.63 (t, 1H,
J¼5.6 Hz), 4.78 (s, 1H), 5.16 (d, 1H, J¼6.1 Hz), 5.51 (d, 1H, J¼7.9 Hz),
7.55 (d, 1H, J¼7.9 Hz), 11.19 (br, 1H); 13C NMR (100.5 MHz, DMSO)
d
51.9, 61.8, 73.1, 74.8, 78.6, 83.4, 100.6, 147.4, 152.0, 164.2; MS(ES)
calcd [MꢂH]ꢂ (C10H13N2O6): 257, found: 257; HRMS(FAB) calcd
[MþH]þ (C10H15N2O6): 259.0930, found: 259.0940.
NMR (400 MHz, CDCl3)
d 1.09 (s, 3H), 1.49 (s, 3H), 3.35 (d, 2H,
J¼4.5 Hz), 3.44 (s, 3H), 3.81 (t, 2H, J¼4.9 Hz), 3.94 (d, 1H, J¼10.0 Hz),
4.06 (d, 1H, J¼10.0 Hz), 4.21–4.38 (m, 3H), 4.44 (s, 2H), 4.61–4.69
(m, 3H), 7.22–7.39 (m, 9H), 7.51–7.60 (m, 6H), 7.66 (s, 1H); 13C NMR
4.1.9. iso-20-C-Methyl-20,30-O-isopropylidene-50-O-
triphenylmethyl-cytidine (15)
To a solution of 13 (1.62 g, 3.00 mmol) in MeCN (30 mL) at 0 ꢀC
were added N-methylpyrrolidine (3.40 mL) and trifluoroacetic an-
hydride (1.28 mL, 9.13 mmol). The yellow solution was stirred for
30 min then, p-nitrophenol (1.68 g, 12.16 mmol) was added and the
resulting solution was stirred at 0 ꢀC for 3 h. The reaction was
quenched by addition of H2O (5 mL) and DCM (100 mL) was added.
The solution was extracted with satd NaHCO3 solution (3ꢁ100 mL)
and H2O (100 mL). After drying over Na2SO4, the organic phase was
evaporated. The yellow residue was taken up in NH3-saturated
MeOH (50 mL) and heated up at 100 ꢀC under pressure for 5 h. The
reaction was cooled down to room temperature and MeOH was
evaporated. The crude material was purified by chromatography
(silica gel, DCM/MeOH 95:5) to afford 15 (0.94 g, 58%) as a yellow
(100.5 MHz, CDCl3) d 27.6, 27.7, 46.1, 59.1, 64.0, 65.7, 70.5, 76.0, 85.0,
85.2, 87.4, 91.8, 114.3, 114.8, 127.2, 128.0, 128.8, 140.8, 143.8, 154.3,
159.2, 161.1; MS(ES) calcd [MþH]þ (C36H40N5O6): 638, found: 638.
4.1.6. iso-20-C-Methyl-guanosine (12)
A suspension of 11 (1.10 g, 1.73 mmol) in 3 N HCl (50 mL) was
heated at 80 ꢀC for 3 h. The mixture was cooled down to room
temperature and extracted with DCM (2ꢁ50 mL). The aqueous
phase was evaporated and the residue was taken up in H2O (30 mL).
The resulting solution was neutralized with DowexÒ 1X8-200 resin
(OHꢂ form) and filtered. The filtrate was evaporated and the crude
material was purified by chromatography (reverse phase, H2O/
MeCN 98:2) to afford 12 (0.36 g, 70%) as a white solid: 1H NMR
foam: TLC Rf¼0.34 (C); 1H NMR (400 MHz, CDCl3)
d 1.28 (s, 3H), 1.50
(s, 3H), 3.27 (d, 2H, J¼5.5 Hz), 3.87–4.08 (m, 4H), 4.20 (m, 1H), 4.58
(400 MHz, DMSO)
d
3.26–3.36 (m, 1H, under H2O peak), 3.37–3.46
(m, 1H), 3.54–3.67 (m, 3H), 3.87 (d, 1H, J¼9.7 Hz), 4.00 (m, 2H), 4.66
(d, 1H, J¼1.6 Hz), 5.59 (d, 1H, J¼7.2 Hz), 7.18–7.38 (m, 10H), 7.44–
(t,1H, J¼5.6 Hz), 4.93 (s,1H), 5.21 (d,1H, J¼6.0 Hz), 6.45 (br, 2H), 7.62
7.54 (m, 6H); 13C NMR (100.5 MHz, CDCl3)
d 27.7, 28.0, 51.5, 63.4,
(s,1H),10.59 (br,1H); 13C NMR (100.5 MHz, DMSO)
d
48.3, 61.6, 73.0,
75.7, 85.0, 85.5, 87.1, 92.3, 93.7, 113.8, 127.0, 127.9, 128.8, 143.7, 147.2,
74.9, 78.1, 83.6,116.5,138.9,151.9,154.0,157.4; MS(ES) calcd [MþH]þ
(C11H16N5O5): 298, found: 298, calcd [MꢂH]ꢂ (C11H14N5O5): 296,
found: 296; HRMS(FAB) calcd [MþH]þ (C11H16N5O5): 298.1151,
found: 298.1146.
156.6, 165.8; MS(ES) calcd [MꢂH]ꢂ (C32H32N3O5): 538, found: 538.
4.1.10. iso-20-C-Methyl-cytidine (4)
A suspension of 15 (900 mg, 1.67 mmol) in 2 N HCl (40 mL) was
heated at 80 ꢀC for 2 h. The mixture was cooled down to room
temperature and extracted with DCM (2ꢁ50 mL). The aqueous
phase was evaporated and the residue was taken up in H2O (30 mL).
The resulting solution was neutralized with DowexÒ 1X8-200 resin
(OHꢂ form) and filtered. The filtrate was evaporated and the crude
material was purified by chromatography (reverse phase, H2O/
MeCN 98:2) to afford 4 (395 mg, 92%) as a white solid: 1H NMR
4.1.7. iso-20-C-Methyl-20,30-O-isopropylidene-50-O-
triphenylmethyl-uridine (13)
To a stirred suspension of uracil (4.46 g, 39.87 mmol) in DMF
(65 mL) was added NaH (60% oil dispersion) (1.32 g, 33.19 mmol) at
room temperature. After the evolution of hydrogen had ceased, the
mixture was stirred at 80 ꢀC for 20 min. Then, tosylate 8 (8.00 g,
13.29 mmol) dissolved in DMF (65 mL) was added via a syringe. The
resulting suspension was stirred at 140 ꢀC for 24 h and then, cooled
down to room temperature. The solvent was evaporated and the
(400 MHz, DMSO)
d 3.33–3.44 (m, 2H), 3.52–3.68 (m, 4H), 3.82 (d,
1H, J¼9.7 Hz), 3.91 (d, 1H, J¼13.8 Hz), 4.63 (t, 1H, J¼5.6 Hz), 4.93 (s,
1H), 5.05 (d, 1H, J¼6.0 Hz), 5.62 (d, 1H, J¼7.2 Hz), 6.88–7.17 (br, 2H),

T. Bouisset et al. / Tetrahedron 64 (2008) 6657–6661
6661
7.53 (d, 1H, J¼7.2 Hz); 13C NMR (100.5 MHz, DMSO)
d 53.2, 61.8,
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found: 256; HRMS(FAB) calcd [MþH]þ (C10H16N3O5): 258.1090,
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Supplementary data
1H and 13C NMR spectra for compounds 4 and 7–15. Supple-
mentary data associated with this article can be found in the online
version, at doi:10.1016/j.tet.2008.05.022.
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