Synthesis, evaluation of anti-HIV-1 and anti-HCV activity of novel 2′,3′-dideoxy-2′,2′-difluoro-4′-azanucleosides

Article abstract of DOI:10.1016/j.bmc.2012.09.026

A series of 2′,3′-dideoxy-2′,2′-difluoro-4′- azanucleosides of both pyrimidine and purine nucleobases were synthesized in an efficient manner starting from commercially available L-pyroglutamic acid via glycosylation of difluorinated pyrrolidine derivative 15. Several 4′-azanucleosides were prepared as a separable mixture of α- and β-anomers. The 6-chloropurine analogue was obtained as a mixture of N ⁷ and N⁹ regioisomers and their structures were identified based on NOESY and HMBC spectral data. Among the 4′-azanucleosides tested as HIV-1 inhibitors in primary human lymphocytes, four compounds showed modest activity and the 5-fluorouracil analogue (18d) was found to be the most active compound (EC₅₀ = 36.9 μM) in this series. None of the compounds synthesized in this study demonstrated anti-HCV activity.

Full text of DOI:10.1016/j.bmc.2012.09.026

Bioorganic & Medicinal Chemistry 20 (2012) 6885–6893
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Synthesis, evaluation of anti-HIV-1 and anti-HCV activity of novel
20,30-dideoxy-20,20-difluoro-40-azanucleosides
Saúl Martínez-Montero a,b, Susana Fernández a, Yogesh S. Sanghvi c, Emmanuel A. Theodorakis b,
,
⇑
Mervi A. Detorio d, Tamara R. Mcbrayer e, Tony Whitaker e, Raymond F. Schinazi d, Vicente Gotor a,
,
⇑
Miguel Ferrero a,
⇑
a Departamento de Química Orgánica e Inorgánica and Instituto Universitario de Biotecnología de Asturias, Universidad de Oviedo, 33006 Oviedo (Asturias), Spain
b Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0358, USA
c Rasayan Inc., 2802 Crystal Ridge Road, Encinitas, CA 92024-6615, USA
d Center for AIDS Research, Department of Pediatrics, Emory University School of Medicine and the Veterans Affairs Medical Center, 1670 Clairmont Road, Decatur, Georgia 30033, USA
e RFS Pharma, LLC, 1860 Montreal Road, Tucker, Georgia 30084, USA
a r t i c l e i n f o
a b s t r a c t
A series of 20,30-dideoxy-20,20-difluoro-40-azanucleosides of both pyrimidine and purine nucleobases were
synthesized in an efficient manner starting from commercially available L-pyroglutamic acid via glyco-
sylation of difluorinated pyrrolidine derivative 15. Several 40-azanucleosides were prepared as a separa-
Article history:
Received 12 July 2012
Revised 11 September 2012
Accepted 13 September 2012
Available online 23 September 2012
ble mixture of a
- and b-anomers. The 6-chloropurine analogue was obtained as a mixture of N7 and N9
regioisomers and their structures were identified based on NOESY and HMBC spectral data. Among the
40-azanucleosides tested as HIV-1 inhibitors in primary human lymphocytes, four compounds showed
modest activity and the 5-fluorouracil analogue (18d) was found to be the most active compound
Keywords:
Azanucleosides
Synthesis
Anti-HIV-1
Anti-HCV
(EC50 = 36.9
lM) in this series. None of the compounds synthesized in this study demonstrated anti-
HCV activity.
Ó 2012 Elsevier Ltd. All rights reserved.
L
-Pyroglutamic acid
1. Introduction
nucleoside analogues is the introduction of fluorine,11 a common
functionality used in drug discovery efforts.12
Considerable research efforts have been concentrated to pre-
pare chemically modified nucleoside derivatives as effective anti-
cancer1 and antiviral agents.2 Modifications in the carbohydrate
moiety of nucleosides have resulted in improved biological proper-
ties.3 In this regard, heteronucleosides, wherein the ring oxygen in
the carbohydrate moiety is replaced by sulfur,4 nitrogen,5 and
more recently selenium,6 have received much attention for their
therapeutic applications. Among the bioactive thionucleosides,
two examples are shown in Figure 1: thiarabine (40-thioaracyto-
sine, 1) is currently in clinical trials as a potent antitumor agent7
and lamivudine [(-)-20,30-dideoxy-30-thiacytidine, 2], in which the
40-oxygen is present but a sulfur atom was placed at the 30-posi-
tion. Lamivudine was approved by the FDA in 1995 for the
treatment of HIV infection.8 More recently, 40-azanucleosides and
40-selenonucleosides have shown significant anti-HCV9 and
anticancer6a activities, respectively. The synthesis of heterocyclic
modified nucleosides has been recently reviewed.10 Another sugar
modification that improves the biological properties of some
The presence of fluorine in the carbohydrate moiety of nucleo-
side offers stabilization of the glycosidic bond. This increases the
resistance to metabolic degradation while improving the lipophil-
icity to cross lipid membranes more effectively. Another important
feature that arises from a structure activity relationship (SAR) anal-
ysis is the lack of a 30-hydroxyl group in many of the nucleoside
analogues that show anti-HIV activity. It is well known that the
incorporation of 30-deoxynucleosides in a viral DNA prevents chain
elongation and terminates cell growth.13
Among fluorinated nucleosides with antiviral activity, represen-
tative examples are FddC (20,30-dideoxy-20-fluorocytosine, 3)14 and
FLT (30-fluoro-30-deoxythymidine, 4)15 which inhibit the HIV
reverse transcriptase. In addition, there are two nucleosides fluori-
nated at the 20-position of the sugar moiety approved by the FDA
for the treatment of cancer: (i) gemcitabine (20-deoxy-20,20-dif-
luorocytidine, 5), a potent drug against ovarian,16 pancreatic,17
and breast18 cancers, and (ii) clofarabine (2-chloro-20-deoxy-20-flu-
oroarabinoadenosine, 6) which is used clinically for the treatment
of leukemia in children.19
Although fluorine substitution in nucleosides and 40-thionucle-
osides11,20 has been extensively studied, few examples of fluori-
nated 40-azanucleosides have been described and even less
⇑
Corresponding authors. Tel.: +34 985 105 013; fax: +34 985 103 448.
E-mail addresses: etheodor@ucsd.edu (E.A. Theodorakis), vgs@uniovi.es
(V. Gotor), mferrero@uniovi.es (M. Ferrero).
0968-0896/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmc.2012.09.026

6886
S. Martínez-Montero et al. / Bioorg. Med. Chem. 20 (2012) 6885–6893
NH2
NH2
N
NH2
pyroglutaminol (8) was accomplished in two steps via formation
of the corresponding methylester from 7, followed by reduction
with NaBH4.23 Protection with 2,2-dimethoxypropane afforded
the bicyclic lactam 9. Electrophilic difluorination was achieved by
the procedure described by Coward and Konas.24 Treatment of
compound 9 with LDA followed by N-fluorodibenzenesulfonimide
(NFSI) at ꢀ78 °C generated a 1.2:1.0 mixture of diastereomers,
which was then subjected to the same reaction conditions again
to furnish difluorinated product 10 in high overall yield. The acid
hydrolysis of hemiaminal ether 10 with AcOH/MeCN/H2O mixture
N
N
N
HO
S
N
O
O
OH
HO
N
O
HO
O
O
S
F
HO
1, Thiarabine
2, Lamivudine
3, F-ddC
NH2
NH2
O
N
N
N
NH
afforded difluorinated L-pyroglutaminol (11) in 80% yield. Protec-
N
F
N
Cl
N
F
O
N
O
HO
F
HO
HO
O
tion of the resulting hydroxyl group of 11 with TBDMSCl yielded
the silylated compound 12, which was then treated with Boc2O un-
der basic conditions to obtain the protected product 13 in high
yield. Reduction of lactam 13 using LiBEt3H in anhydrous THF pro-
vided 14 as a 1.9:1 mixture of anomers. Further reaction of 14 with
Ac2O gave 15 in quantitative yield, which was used for glycosyla-
tion reactions with silylated nucleobases.
O
O
HO
HO
F
4, FLT
5, Gemcitabine
6, Clofarabine
Figure 1. Selected structures of bioactive nucleosides.
biological properties have been reported. Qiu and Qing21 carried
out the preparation of pyrimidine 20- and 30-fluoromethyl-40-aza-
nucleosides, representing the only examples of fluorinated 40-aza-
nucleosides described to-date. To the best of our knowledge there
are no other examples of 40-azanucleosides in which the 20,20-di-
fluoro substituent is directly attached to the pyrrolidine moiety.
Consequently, studies on the synthesis and biological activity of
these nucleoside derivatives are worth pursuing. On the basis of
the above considerations and our ongoing interest in the prepara-
tion and biological evaluation of nucleoside analogues,22 herein,
we report the synthesis and antiviral evaluation of a series of
20,30-dideoxy-20,20-difluoro-40-azanucleosides.
2.1. Synthesis of pyrimidine 40-azanucleosides
Glycosylation of 15 under Vorbrüggen’s conditions25 with
various pyrimidine heterocyclic bases gave
a/b mixtures of
40-azanucleosides in high overall yields (78–85%). The ratio of
a
/b-anomers was determined by HPLC-MS of the crude reaction
mixture (Scheme 2). The poor resolution of the 1H NMR spectra
for the TBDMS protecting 40-azanucleosides hindered the determi-
nation of a/b-ratio. Thus, to assign the stereochemistry of the gly-
cosylation products, TBDMS protecting group was removed and
the configuration of the anomeric carbon was established by
NOESY experiments showing the
a-anomer as the major product.
This was probably due to the steric hindrance of the bulky silyl
protecting group. Similar results were also reported during glyco-
sylation of a Boc-protected proline with pyrimidine base.21 Corre-
lations between H10 and H40 as well as H50 and H6 were clearly
observed in the b-anomers, while correlations between H40 and
2. Results and discussion
The synthesis of difluorinated pyrrolidine 15 as a substrate for
the glycosylation reaction is outlined in Scheme 1. Commercially
available
L-pyroglutamic acid (7) possesses the correct configura-
H6 of the corresponding nucleobase appeared in the a-anomers.
tion to furnish 40-azanucleosides, which mimics the D-configura-
After glycosylation with silylated uracil, the resulting mixture of
anomers 16a/17a had different Rf values, and the products were
easily separable by column chromatography. However the other
tion of naturally occurring nucleosides. Conversion to
L-
nucleosides 16b–d/17b–d were isolated as inseparable
a/b-ano-
O
meric products. Removal of the TBDMS protecting group was
accomplished under standard conditions by treating the separated
pure anomers (16a and 17a) or the anomeric mixtures 16b–c/17b–
c with TBAF in THF to afford 40-azanucleosides 18/19 in excellent
yields (Panel A, Scheme 3). It is noteworthy that after deblocking
the TBDMS, the anomers 18d/19d had different polarity and were
separable by silica gel column chromatography (Panel B, Scheme
HO2C
HO
H
N
H
N
(c)
(a)-(b)
(69%)
O
O
O
O
N
(83%)
7
8
9
(d)-(e)
O
(78%)
Σ= TBDMS
ΣO
HO
H
N
H
N
(g)
(f)
O
O
N
F
(87%)
(80%)
F
F
TBDMSO
TBDMSO
B
F
Boc
N
Boc
N
F
F
F
(a) BH, BSA, MeCN
(b) TMSOTf
12
11
10
F
15
+
B
(h)
(96%)
F
F
16a-d
17a-d
ΣO
ΣO
Boc
Boc
ΣO
Boc
O
N
OH
N
(i)
(84%)
(j)
(100%)
N
OAc
Yield (%)a Ratio (α:β)
F
F
F
16a+17a, B= U
85
78
82
83
2.3:1
2.0:1b
2.0:1
2.7:1
F
F
F
16b+17b, B= T
13
14
15
16c+17c, B= CAc
16d+17d, B= 5FU
Scheme 1. Synthesis of 15. Reagents and conditions: (a) 1 equiv SOCl2, MeOH, rt,
4 h; (b) 2 equiv NaBH4, EtOH, rt, 14 h; (c) 2,2-dimethoxypropane (solvent),
0.03 equiv CSA, reflux, 2 h; (d) 1.4 equiv iPr2NH, 1.2 equiv nBuLi, 1.4 equiv NFSI,
aYield of the glycosylation mixture (16+17)
bRatio calculated by HPLC after TBDMS deprotection
i
THF, ꢀ78 °C; (e) 1.4 equiv Pr2NH, 1.2 equiv nBuLi, 1.4 equiv NFSI, THF, ꢀ78 °C; (f)
AcOH/MeCN/H2O (14:3:3), 90 °C, 14 h; (g) 1.3 equiv TBDMSCl, 1.3 equiv imidazole,
0.1 equiv DMAP, CH2Cl2, rt, 30 min; (h) 2 equiv Boc2O, 1.3 equiv Et3N, 1.1 equiv
DMAP, CH2Cl2, rt, 30 min; (i) 1.2 equiv LiEt3BH, THF, ꢀ78 °C, 1 h; (j) 15 equiv Ac2O,
30 equiv Et3N, CH2Cl2, rt, 30 min.
Scheme 2. Glycosylation of 15 with pyrimidines. Reagents and conditions: (a)
1 equiv 15, 4 equiv base (BH = uracil, thymine, N4-acetylcytosine, and 5-fluoroura-
cil), 6 equiv BSA, MeCN, 80 °C, 1 h; (b) 2.7 equiv TMSOTf, 0–80 °C, 30 min, 78–85%
over two steps.

S. Martínez-Montero et al. / Bioorg. Med. Chem. 20 (2012) 6885–6893
6887
15
A
B
ΣO
HO
Boc
N
Boc
(a)-(b)
B
B
N
TBAF, THF
(91-99%)
Σ= TBDMS
a, B= U
b, B= T
F
F
c, B= CAc
F
F
16a-c/17a-c
18a-c/19a-c
(6%)
(19%)
(8%)
N
(48%)
Cl
N
N
N
N
ΣO
HO
HO
5FU
N
Boc
N
Boc
N
Boc
N
N
F
N
F
ΣO
ΣO
5FU
Boc
N
Boc
N
TBAF, THF
F
F
F
Cl
+
5FU
F
F
F
19d
F
F
16d/17d
18d
22, N9-β-anomer
23, N9-α-anomer
24, N7-β-anomer
25, N7-α-anomer
(65%)
(21%)
Σ= TBDMS
(c) (81%)
(c) (78%)
(c)
(c) (82%)
(75%)
Scheme 3. Removal of TBDMS protecting group. Reagents and conditions: 1.5 equiv
TBAF, THF, 0–25 °C, 1 h.
Cl
N
N
N
N
N
N
N
N
HO
HO
Boc
Boc
N
NHAc
N
NH2
Cl
F
F
N
N
F
F
26, N9-β-anomer
27, N9-α-anomer
28, N7-β-anomer
29, N7-α-anomer
N
O
N
O
HO
HO
Boc
N
Boc
N
Scheme 5. Glycosylation of 15 with 6-chloropurine. Reagents and conditions: (a)
1 equiv 15, 4 equiv 6-chloropurine, 6 equiv BSA, MeCN, 80 °C, 1 h; (b) 2.7 equiv
TMSOTf, 0–80 °C, 30 min; (c) 1.5 equiv TBAF, THF, rt, 1 h.
F
F
F
F
18c/19c
20c/21c
Scheme 4. Synthesis of 20c/21c. Reagents and conditions: NH3 sat-MeOH, rt, 1 h,
82%.
Cl
8.89
N
H2
C6
N
N
N
N
C4
C5
C5
C4
N
9.66 H8
HO
8.84
H8
Boc
N
3). After the two step procedure of glycosylation–deprotection,
four anomers were isolated as pure compounds (18a, 19a, 18d
and 19d) while 18b–c/19b–c were isolated as non separable mix-
N
H2 8.77
HO
Boc
N
N
Cl
F
F
6.88
H1'
H1'
26, N9-β-anomer
6.45
F
F
ture of a/b-anomeric products.
28, N7-β-anomer
Next, removal of the acetyl protecting group from the cytidine
derivatives 18c/19c was accomplished by treatment with ammonia
affording an anomeric mixture of 20c/21c (Scheme 4). The depro-
tection of Boc group from the uracil derivative 18a was attempted
with 2 equiv of TFA in CH2Cl2. However, only free uracil was iso-
lated from the reaction mixture showing that despite the stabiliza-
tion of the glycosidic bond by the two fluorine atoms, the
unprotected 40-azanucleosides are still prone to acid-mediated
degradation. Additionally, it has been recently shown that the po-
tent antiviral activity of a series of azanucleoside analogues was
not compromised despite the presence of the Boc protecting
group.9
Figure 2. HMBC correlations of N9 and N7 regioisomers. In red are correlations
mentioned in the text. The 1H NMR values (d ppm) are in blue.
the attachment of 6-chloropurine via N7 or N9 to the pyrrolidine
moiety. In the case of nucleoside 26, H2 and H8 of the 6-chloropu-
rine moiety together with H10 showed correlation (3JCH) with C4 of
the nucleobase (Fig. 2). Correlation of these three hydrogen atoms
with the same carbon can only be possible in the N9 regioisomer. In
the case of compound 28 the expected correlations for an N7 iso-
mer were observed, allowing confirmation of the structure. The
same correlations patterns were also observed in the correspond-
2.2. Synthesis of purine 40-azanucleosides
ing
a-anomers. In addition, the structures were also confirmed
by comparing the UV maxima with previously reported data (see
experimental section).
The ratio of the four products obtained after glycosylation reac-
tion, was determined by HPLC-MS data on the crude reaction mix-
ture of protected nucleosides 22–25 (22:23:24:25, 8:27:12:53). As
The success with the preparation of the pyrimidine nucleosides
together with the few examples of purine 40-azanucleosides de-
scribed in the literature26 encouraged us to try the coupling of 15
with purine nucleobases. It is well know that Vorbrüggen cou-
pling25 of silylated purine nucleobases typically results in N7/N9
isomeric mixtures. In addition, since two anomers may result from
each glycosylated regioisomer, four compounds may be present in
the crude reaction mixture. As expected, after the coupling of 15
with silylated 6-chloropurine, the TLC of the crude reaction mix-
ture showed the presence of four products. Mass spectrometry
analysis indicated the same formula weight for all four products.
Interestingly, for this particular case, four possible products had
sufficient difference in the Rf values to be separated by silica gel
column chromatography (Scheme 5).
seen before for the pyrimidine nucleosides, the
a-anomers were
also the major products with the purine series. Interestingly, de-
26, N9-β-anomer
27, N9-α-anomer
28, N7-β-anomer
29, N7-α-anomer
NH2
(72%)
N
(72%)
(75%)
N
(73%)
N
N
N
N
N
Boc
N
HO
HO
Boc
N
N
NH2
F
F
To identify the structure of glycosylated products, it was neces-
sary to remove the TBDMS protecting group (TBAF in THF). The
structure of 26–29 was established by 2D NMR spectroscopy.
NOESY experiments assisted in the assignment of the stereochem-
ical configuration and HMBC experiments allowed us to determine
F
F
30, N9-β-anomer
31, N9-α-anomer
32, N7-β-anomer
33, N7-α-anomer
Scheme 6. Synthesis of adenine 40-azanucleosides 30–33. Reagents and conditions:
NH3 sat-MeOH, 100 °C, 3 h.

6888
S. Martínez-Montero et al. / Bioorg. Med. Chem. 20 (2012) 6885–6893
spite of higher reaction temperature (80 °C) employed during the
glycosylation step, the N7 product was dominant. This observation,
which is unusual for glycosylation of 6-chloropurine base, is likely
due to the sterically demanding structure of 15.
The treatment of 6-chloropurine analogues 26–29 with ammo-
nia furnished the corresponding adenine derivatives 30–33
(Scheme 6) in good yield.
4. Conclusions
In summary, we have developed an efficient synthesis of novel
20,30-dideoxy-20,20-difluoro-40-azanucleosides both as pyrimidine
and purine analogues. A high yielding sequence of electrophilic
difluorination of L-pyroglutamic acid followed by the coupling of
protected pyrrolidine 15 as the glycosyl donor with four pyrimi-
dine and one purine nucleobase was established. The pyrimidine
40-azanucleosides were obtained as mixtures of
a
- and b-anomeric
products, increasing the breadth of novel nucleoside analogues
available for biological screening. The -anomers were obtained
3. Biological evaluation
3.1. Antiviral assays
a
as major products during glycosylation. After glycosylation and
TBDMS deprotection, the anomeric mixtures of U (18a/19a) and
5-F-U (18d/19d) analogues could be easily separated by silica gel
column chromatography. However, the anomeric mixtures of T
(18b/19b) and C (18c/19c) analogues could not be separated by sil-
ica chromatography. Gratifyingly, glycosylation with 6-chloropu-
rine afforded a separable mixture of four nucleosides arising
from the formation of N7/N9 glycosylated regioisomers and the cor-
All azanucleosides were tested against HIV-1LAI using 30-azido-
30-deoxythymidine (AZT, zidovudine) as a reference in an assay
with human peripheral blood mononuclear (PBM) cells. We opted
for including all products in the screen to maximize the database
despite of the fact that some were isolated as mixture of anomeric
products.
A summary of the data expressed as the effective concentration
required to inhibit viral replication by 50% (EC50) and 90% (EC90) is
shown in the Table 1. The 5-fluorouracil analogues 18d
responding a/b-anomers. These isomers were characterized based
on 2D NMR spectroscopy, and constitute the only examples of fluo-
rinated purine 40-azanucleosides described to date. Further reac-
tion with ammonia of each isomer furnished a direct route for
the purine nucleosides 30–33. All fluorinated 40-azanucleosides
synthesized were tested as inhibitors of HIV-1 in PBM cells. The
(EC50 = 36.9
purine derivatives 27 (EC50 = 64.5
showed modest activity when compared with the AZT as a control.
It is noteworthy that the -anomers demonstrated better activities
l
M), 19d (EC50 = 44.5
l
l
M) together with the 6-chloro
M) and 29 (EC50 = 92.3 M)
l
a
a-5-F-U analogue 18d was found to be the most active compound
in comparison to their b-counterparts. Also, it is of interest to ob-
serve modest activity despite the fact that all products were pro-
tected with the Boc group.
(EC50 = 36.9 M) in this series. These compounds did not exhibit
l
anti-HCV activity in a hepatitis C replicon assay probably due to
the lack of a 30-hydroxyl group or mimic for the moiety. The lim-
ited examples of fluorinated 40-azanucleosides described in the lit-
erature and the interesting activity found in some of the
nucleosides described in this work, warrants further studies with
this new class of compounds.
3.2. Cytotoxicity assays
All compounds were evaluated for their potential cytotoxicity in
uninfected phytohemagglutinin stimulated human PBM cells, in
lymphocytic CEM cells, and Vero (African green monkey Kidney)
cells. The majority of compounds did not show any toxicity except
the b-uracil analogue 19a in Vero cells, 19a, 18d, 19d, 27 and 29 in
PBM cells, and 26, 27 and 29 in CEM cells.
5. Experimental section
All reagents were bought from Aldrich and Acros at highest
commercial quality and used without further purification. All
non-aqueous reactions were carried out under anhydrous condi-
tions in dry, freshly distilled solvents. THF and CH2Cl2 were puri-
fied by passage through a bed of activated alumina. Reactions
were monitored by TLC carried out on 0.25 mm E. Merck silica
gel plates (60F-254) using UV light as visualizing agent and/or
acidic aqueous permanganate. Flash chromatography was
3.3. HCV Replicon assays28
All compounds were tested at 10 lM in an HCV replicon assay
using 20-C-Me-Cytidine as the positive control. No anti-HCV activ-
ity was observed (data not shown).
Table 1
Effect of analogues against HIV-1LAI in human peripheral blood mononuclear (PBM) cells
Analogue
Base
Anti-HIV-1 activity in PBM cellsa
EC90
Cytotoxicity (IC50
CEM cells
, l
M)b
EC50
,
lM
,
lM
PBM cells
VERO cells
AZT
18a
19a
18b/19b
18c/19c
18d
19d
26
27
28
29
30
b-T
0.0017
>100
>100
>100
>100
36.9
44.5
>100
64.5
>100
92.3
>100
>100
>100
>100
0.0027
>100
>100
>100
>100
75.6
>100
>100
81.4
>100
>100
30.5
28.2
>100
58.1
>100
59.5
>100
>100
>100
>100
14.3
56.0
>100
44.4
a-U
>100
>100
>100
>100
>100
>100
57.1
38.1
>100
55.4
b-U
a
a
a
/b-T
/b-C
-5-F-U
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
b-5-F-U
N9-b-6-Cl-Pu
90.8
>100
>100
>100
>100
>100
>100
>100
>100
N9-
N7-b-6-Cl-Pu
N7- -6-Cl-Pu
N9-b-A
N9- -A
N7-b-A
N7- -A
a
-6-Cl-Pu
a
>100
>100
>100
>100
31
32
33
a
a
a
HIV drug susceptibility assay was done as previously describe in Ref. 27.
Cytotoxicity assays in PBM, CEM and Vero cells were done as previously described in Ref. 20d.
b

S. Martínez-Montero et al. / Bioorg. Med. Chem. 20 (2012) 6885–6893
6889
performed using silica gel 60 (230–400 mesh). LC-ESI–MS analyses
were carried out in a chromatogram with UV detector at 254 nm
using Agilent Poroshell column 120 SB, C18, or Mediterranea col-
umn (250 ꢁ 45 mm) flow 1 mL minꢀ1 rt gradient MeCN-H2O as
eluent. Melting points were taken on samples in open capillary
tubes and are uncorrected. 1H, 13C NMR, and DEPT were obtained
using Varian Mercury and/or Bruker 300.13, 400.13 or
600.13 MHz for 1H, and 75.5, 100.61 MHz or 150.92 for 13C. The
same spectrometers were used for the acquisition of 1H–1H homo-
nuclear (COSY and NOESY) and 1H–13C heteronuclear (HSQC and
HMBC) correlations. Optical rotations were recorded on a Jasco P-
5.4. Synthesis of (5S)-3,3-difluoro-5-hydroxymethyl-2-pyrrolid
inone (11)
Compound 10 (1.65 g, 8.37 mmol) was stirred in a mixture of
acetic acid, acetonitrile and water (14:3:3, v/v) (20 mL). The solu-
tion was heated at 90 °C for 14 h. After evaporation of solvents,
the residue was purified by flash chromatography (10% MeOH/
CH2Cl2) to yield pure 11 as a white solid (1.04 g, 80%). The synthe-
sis of 11 has been previously described.24
5.5. Synthesis of (5S)-5-(tert-butyldimethylsilyloxymethyl)-3,3-
difluoro-2-pyrrolidinone (12)
1010 polarimeter and values are reported as follows: ½a kT
ꢂ
(c: g/
100 mL, solvent). High resolution mass spectra (HRMS) were re-
corded on a VG 7070 HS mass spectrometer under electron spray
ionization (ESI) conditions. The tert-butyldimethyl silyl protecting
group is abbreviated below as TBDMS.
To a solution of 11 (960 mg, 6.39 mmol) in dry CH2Cl2 (40 mL)
at 0 °C was added imidazole (566 mg, 8.31 mmol), DMAP
(0.63 mmol, 78 mg) and TBDMSCl (1.25 g, 8.31 mmol). The reac-
tion was stirred at room temperature for 30 min, quenched by
5.1. Synthesis of (S)-pyroglutaminol (8)
the addition of water (300 lL) and extracted with CH2Cl2. The com-
bined organic layers were dried over Na2SO4, solvents were evap-
orated and the residue subjected to flash chromatography (15%
AcOEt/hexane) to afford 12 (1.8 g, 87%) as viscous oil. Rf: (20%
To
a
cooled solution of
L-pyroglutamic acid (7) (6.0 g,
41.9 mmol) in dry MeOH (80 mL), was added SOCl2 (4.9 g,
41.9 mmol) dropwise with magnetic stirring at room temperature
for 2 h. The mixture was concentrated under vacuum to give the
methyl ester as clear oil (5.2 g, 80%). This oil (33.2 mmol) was
poured in a flask, dissolved in dry EtOH (80 mL) and NaBH4
(2.54 g, 67.0 mmol) was added portionwise. After stirring at room
temperature for 2 h, the mixture was acidified with concentrated
HCl to pH 1. Solvents were removed under vacuum and the residue
subjected to flash chromatography (15% MeOH, CH2Cl2) to afford 8
AcOEt/hexane):0.23. ½a D20
ꢂ
+35 (c 0.5, CH2Cl2). 1H NMR (CDCl3,
t
400.13 MHz): d 0.05 (s, 6H, Me2Si), 0.87 (s, 9H, Bu), 2.29 (m, 1H,
H3), 2.55 (m, 1H, H-3), 3.54 (dd, 1H, CH2O, JHH 6.2 Hz, JHH
10.3 Hz), 3.67 (dd, 1H, CH2O, JHH 4.4 Hz, JHH 10.4 Hz), 3.80 (m,
1H, H-5), 7.67 (br s, 1H, NH). 13C NMR (CDCl3, 100.61 MHz): d 5.4
(2CH3, tBu), 18.4 (1C, tBu), 26.0 (3CH3, tBu), 33.1 (t, C-4, JCF
22.1 Hz), 50.7 (C-5), 65.1 (CH2O), 118.0 (t, C-3, JCF 249.5 Hz),
166.7 (t, C@O, JCF 31.2 Hz). HRMS (ESI+) Calcd for C11H21F2NO2SiNa
[M+Na]+ 288.1202, found 288.1203.
(3.3 g 28.5 mmol). The synthesis of
8 has been previously
described.23
5.6. Synthesis of (5S)-5-(tert-butyldimethylsilyloxymethyl)-N-
tert-butyloxycarbonyl-3,3-difluoro-2-pyrrolidinone (13)
5.2. Synthesis of (5S)-2,2-dimethyl-8-oxo-1-aza-3-oxa-bicyclo
[3.3.0]octane (9)
To a solution of 12 (854 mg, 3.21 mmol), in dry CH2Cl2 (28 mL),
was added Et3N, (0.6 mL, 4.17 mmol), DMAP (427 mg, 3.53 mmol)
and Boc2O (1.41, 6.42 mmol). The reaction was stirred at room tem-
perature for 30 min. Solvents were concentrated to dryness and the
residue purified by column chromatography (10% AcOEt/hexane) to
A mixture of compound 8 (3.3 g, 28.7 mmol), CSA (0.68 mmol,
158 mg) and 2,2-dimethoxypropane (DMP; 12 mL) was refluxed
for 2 h. The volatiles components (DMP, MeOH) were removed in
vacuo. Fresh DMP was added, and the mixture again refluxed for
2 h. This process was repeated a total of three times. After the final
evaporation the residue was subjected to flash chromatography
(50% AcOEt/hexane) and then distilled under vacuo to afford 9 as
a colorless oil (3.67 g, 83%). The synthesis of 9 has been previously
described.24
give 13 (1.13 g, 96%). Rf (20% AcOEt/hexane): 0.53. ½a D20
ꢀ51 (c 0.5,
ꢂ
CH2Cl2). 1H NMR (CDCl3, 400.13 MHz): d 0.02 (s, 6H, Me2Si), 0.85 (s,
9H, tBu), 1.53 (s, 9H, tBu), 2.48 (m, 2H, H-4), 3.70 (dd, 1H, CH2O, JHH
2.5 Hz, JHH 10.3 Hz), 3.83 (dd, 1H, CH2O, JHH 5.1 Hz, JHH 10.3 Hz),
4.22 (m, 1H, H-5). 13C NMR (CDCl3, 100.61 MHz): d 5.7 (tBu), 18.4
(tBu), 26.0 + 27.8 (tBu + Boc), 31.0 (t, C-4, JCF 22.1 Hz), 53.4 (C-5),
62.3 (CH2O), 84.7 (Boc), 116.6 (t, C-3, JCF 251.0 Hz), 149.2 (C@O),
166.7 (t, C@O, JCF 32.2 Hz). MS (ESI+, m/z) 382 [(M+NH4)+ 100%];
388 [(M+Na)+ 10%]. HRMS (ESI+) Calcd for C16H29F2NO4SiNa
[M+Na]+ 388.1726, found 388.1727.
5.3. Synthesis of (5S)-2,2-dimethyl-7,7-difluoro-8-oxo-1-aza-3-
oxa-bicyclo[3.3.0]octane (10)
Diisopropyl amine (2.3 mL, 16.5 mmol) was added to dry THF
with magnetic stirring and the solution was cooled to ꢀ78 °C.
N-buthylitium (3.7 mL, 13.9 mmol) was added slowly and the
mixture was stirred for 1 h. A solution of 9 (1.8 g, 11.6 mmol)
in THF (9 mL) was added slowly. The mixture was stirred for
1 h at ꢀ78 °C before the addition of a solution of N-fluorodiben-
zenesulfonimide (NFSI; 5.19 g, 16.5 mmol) in THF (18 mL), the
solution was again stirred for 45 min and then quenched by
the addition of saturated NH4Cl. THF was removed under vacuo
and the residue extracted with AcOEt and water. The combined
organic layers were dried over Na2SO4, filtered and evaporated.
The residue was purified by flash chromatography (20–50%
AcOEt-hexane) to give the monofluorinated lactam (1.84 g,
92%). The same fluorination procedure was carried out using
the previous monofluorinated lactam (1.84 g, 10.6 mmol) as a
substrate to get the difluorinated compound 10 (1.72 g, 85%) as
a pale yellow oil. The synthesis of 10 has been previously
described.24
5.7. Synthesis of (5S)-5-(tert-butyldimethylsilyloxymethyl)-N-
tert-butyloxycarbonyl-3,3-difluoro-2-hydroxy-pyrrolidine (14)
(mixture of anomers)
To a solution of 13 (368 mg, 1.0 mmol) in anhydrous THF
(9.3 mL) was slowly added dropwise LiEt3BH (504
at ꢀ78 °C under argon atmosphere. The reaction mixture was stir-
red 1 h, then additional LiEt3BH (706 L, 0.70 mmol) was added.
lL, 0.5 mmol)
l
After 1 h, the reaction was quenched with water (4 mL) and the or-
ganic solvent was evaporated. The aqueous phase was extracted
with CH2Cl2 dried over Na2SO4, filtered and concentrated under
vacuum. The resulting residue was purified by flash chromography
(30% AcOEt/hexane) to afford a inseparable mixture of anomers 14
(311 mg, 84%) in 4:1 ratio. Rf (20% AcOEt/hexane): 0.51. Major iso-
mer 1H NMR (CDCl3, 400.13 MHz): d 0.11 (s, 6H, Me2Si), 0.91 (9H,
tBu), 1.48 (s, 9H, Boc), 2.45 (m, 2H), 3.39 (d, 1H, CH2O, JHH 10.2 Hz),

6890
S. Martínez-Montero et al. / Bioorg. Med. Chem. 20 (2012) 6885–6893
3.83 (d, 1H, CH2O, JHH 10.0 Hz), 4.07 (d, 1H, H-5, JHH 10.0 Hz), 5.16
(t, 1H, H-2, JHF 10.2 Hz). Minor isomer 1H NMR (CDCl3,
JHH 2.7 Hz, JHH 8.1 Hz), 4.31 (m, 1H, CH2O), 4.64 (t, 1H, OH, JHH
4.4 Hz), 5.63 (d, 1H, H-5B, JHH 8.1 Hz), 6.36 (d, 1H, H-2, JHH
13.8 Hz), 8.33 (d, 1H, H-6B, JHH 8.1 Hz), 10.22 (br s, 1H, NH). 13C
t
t
400.13 MHz): d 0.11 (s, 6H, Bu), 0.91 (9H, Bu), 1.48 (s, 9H, Boc),
2.45 (m, 2H), 3.51 (d, 1H, CH2O, JHH 9.8 Hz), 3.69 (m, 1H, CH2O),
4.21 (m, 1H, H-5), 5.16 (m, 1H, H-2). MS (ESI+, m/z) 368 [(M+H)+
10%]; 390 [(M+Na)+ 100%]. HRMS (ESI+) Calcd for C16H31F2NO4SiNa
[M+Na]+ 390.1883, found 388.1885.
NMR (acetone-d6, 75.5 MHz):
d 27.3 (Boc), 33.1 (t, C-4, JCF
22.6 Hz), 54.1 (C-5), 57.1 (CH2O), 71.8 (m, C-2), 81.5 (Boc), 101.6
(C-5B), 124.5 (t, C-3, JCF 251.8 Hz), 140.1 (C-6B), 150.7 (C-2B),
153.6 (C@O), 162.6 (C-4B). MS (ESI+, m/z) 348 [(M+H)+ 100%];
370 [(M+Na)+ 35%]. HRMS (ESI+) Calcd for C14H19F2N3O5Na
[M+Na]+ 370.1185, found 370.1190.
5.8. Synthesis of (5S)-2-acetyloxy-5-(tert-butyldimethylsilyl
oxymethyl)-N-tert-butyloxycarbonyl-3,3-difluoro-pyrrolidine
(15)
5.12. 1-[(2S,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-thymine (18b) and 1-[(2R,5S)-N-tert-
butyloxycarbonyl-3,3-difluoro-5-(hydroxymethyl)pyrrolidin-2-
yl]-thymine (19b)
(mixture of anomers). To a solution of anomers 14 (300 mg) in
CH2Cl2 (10 mL) was added Et3N (3.4 mL, 2.5 mmol), Ac2O (1.2 mL,
12.0 mmol), and DMAP (catalytic). The solution was allowed to stir
for 30 min. Solvents were evaporated and the residue purified by
flash column chromatography (20% AcOEt/hexane) to afford 15
(332 mg, 100%). The presence of TBDMS protecting complicates
the analysis of the NMR spectra. Rf (20% AcOEt/hexane): 0.64. 1H
Seventy-eight percentage of yield after glycosylation and 98%
yield for TBDMS deprotection. Rf (5% MeOH/CH2Cl2): 0.22.
a-iso-
mer (18b) 1H NMR (CDCl3, 400.13 MHz): d 1.38 (s, 9H, Boc), 1.92
(s, 3H, CH3), 2.54 (m, 2H, H-4), 3.23 (s, 1H, OH), 3.71 (s, 1H,
CH2O), 4.07 (s, 1H, CH2O), 4.35 (s, 1H, H-5), 6.35 (br s, H-2), 6.83
(1H, H-6B), 9.32 (br s, 1H, NH). b-isomer (19b) d 1.42 (s, 9H,
Boc), 1.90 (s, CH3), 2.54 (m, 2H, H-4), 3.15 (br s, 1H, OH), 3.77
(dd, 1H, CH2O, JHH 2.8 Hz, JHH 11.2 Hz), 4.10 (q, 1H, H-5, JHH
4.0 Hz), 4.26 (d, 1H, CH2O, JHH 11.2 Hz), 6.18 (d, 1H, H-2, JHF
13.2 Hz), 7.66 (s, 1H, H-6B), 9.24 (br s, 1H, NH). MS (ESI+, m/z)
362 [(M+H)+ 25%]; 384 [(M+Na)+ 100%]. HRMS (ESI+) Calcd for
t
NMR (CDCl3, 400.13 MHz): d 0.11 (s, 6H, Me2Si), 0.95 (9H, Bu),
1.47 (s, 9H, Boc), 2.09 (m, 2H), 3.77 (br s, 1H), 4.01 (br s, 2H),
6.49 (br s, 1H). MS (ESI+, m/z) 409 [(M+H)+ 10%]; 432 [(M+Na)+
100%]. HRMS (ESI+) Calcd for C18H33F2NO5SiNa [M+Na]+
432.1988, found 432.2003.
5.9. General procedure for glycosylation of fluorinated
pyrrolidine 15 with pyrimidine bases followed by deprotection.
Synthesis of nucleosides 18a–d/19a–d
C
15H21F2N3O5Na [M+Na]+ 384.1341, found 384.1349.
5.13. N4-acety1-1-[(2S,5S)-N-tert-butyloxycarbonyl-3,3-
difluoro-5-(hydroxymethyl)pyrrolidin-2-yl]cytosine (18c) and
N4-acety1-1-[(2R,5S)-N-tert-butyloxycarbonyl-3,3-difluoro-5-
(hydroxymethyl)pyrrolidin-2-yl]-cytosine (19c)
To a stirred solution of 15 (0.3 mmol, 122 mg) and the different
bases (1.2 mmol) in dry MeCN was added BSA (0.59 mL, 1.8 mmol).
The reaction mixture was stirred at 80 °C for 1 h. This solution was
cooled to 0 °C and TMSOTf (0.17 mL, 0.83 mmol) was added drop-
wise. The solution was heated at 80 °C for 1 h. The reaction was
quenched by the addition of Et3N (0.2 mL). Solvents were evapo-
rated and the resulting residue purified by column chromatogra-
phy (20–50% AcOEt/hexane) to afford the protected 40-
azanucleosides (16a–d/17a–d) (78–85%). Then stirred solutions of
the nucleoside derivatives in THF (4 mL) were treated with 1.0 M
solutions of TBAF in THF (1.5 equiv). Reactions were quenched
with water, and after evaporation of solvents the residues purified
by flash chromatography (2–5% MeOH/CH2Cl2) to afford the pure
40-azanucleosides (18a–d/19a–d) as white solids (90–99%).
Eighty-two percentage of yield after glycosylation and 91% yield
for TBDMS deprotection. Rf (70% AcOEt/hexane): 0.11.
a-isomer
(18c): 1H NMR (MeOH-d4, 300.13 MHz): d 1.33 (s, 9H, Boc), 2.21
(s, 3H, CH3), 2.61 (m, 2H, H-4), 3.58 (t, 1H, CH2O, JHH 8.7 Hz),
3.91 (s, 1H, CH2O), 4.33 (m, 1H, H-5), 6.45 (d, 1H, H-2, JHF
12.9 Hz), 7.46 (s, 1H, H-5), 7.92 (s, 1H, H-5). b-isomer (19c): d
1.41 (s, 9H, Boc), 2.20 (s, 3H, CH3), 2.62 (m, 2H, H-4), 3.66 (d, 1H,
CH2O, JHH 11.4 Hz), 4.07 (s, 1H, H-5), 4.30 (s, 1H, CH2O), 6.47 (d,
1H, H-2, JHF 12.9 Hz), 7.41 (d, 1H, H-5B, JHF 7.5 Hz), 8.76 (d, 1H,
H-6B, JHF 7.5 Hz). HRMS (ESI+) Calcd for C15H22F2N4O5Na [M+Na]+
411.1450, found 411.1467.
5.10. 1-[(2S,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-uracil (18a)
5.14. 1-[(2S,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-5-fluorouracil (18d)
Sixty-one percentage of yield after glycosylation and 99% yield
for TBDMS deprotection. Rf (5% MeOH/CH2Cl2): 0.19. mp: 82–
Eighty-three percentage of overall yield (
a
+ b) after glycosyla-
-anomer). Rf
+57 (c 0.5, CH2Cl2).
85 °C. ½a 2D0
ꢂ
+18 (c 1.0, CH2Cl2). 1H NMR (CDCl3. 600.15 MHz,
tion and 65% yield for TBDMS deprotection (pure a
328 K): d 1.41 (s, 9H, Boc), 2.55 (m, 2H, H-4), 3.71 (dd, 1H, CH2O,
JHH 6.2 Hz, JHH 11.1 Hz), 3.91 (s, 1H, CH2O), 4.36 (s, 1H, H-5), 5.75
(s, 1H, H-5B), 6.35 (br s, H-2), 7.09 (1H, H-6B), 9.55 (br s, 1H, NH).
13C NMR (CDCl3. 150.92 MHz, 328 K): d 28.1 (Boc), 35.0 (C-4), 58.1
(C-5), 63.7 (CH2O), 70.6 (m, C-2), 83.4 (Boc), 101.6 (C-5B), 124.6
(C-3), 138.1 (C-6B), 150.2 (C-2B), 153.0 (C@O), 162.9 (C-4B). HRMS
(ESI+) Calcd for C14H19F2N3O5Na [M+Na]+ 370.1185, found 370.1186.
(5% MeOH/CH2Cl2): 0.24. mp: 61-63 °C. ½a D20
ꢂ
1H NMR (acetone-d6, 300.13 MHz): d 1.39 (rotamers, 9H, Boc),
2.75 (m, 2H, H-4), 3.55 (t, 1H, CH2O, JHH 6.2 Hz), 3.98 (d, 1H,
CH2O, JHH 9.6 Hz), 4.43 (s, 1H, H-4), 6.30 (d, 1H, JHF 9.6 Hz), 7.84
(s, 1H, H-6). 13C NMR (acetone-d6, 75.5 MHz): d 27.3 (Boc), 33.1
(t, C-4, JCF 23.4 Hz), 54.1 (C-5), 60.7 (CH2O), 70.9 (m, C-2), 81.5
(Boc), 123.2 (d, C-6B, JCF 36.2 Hz), 124.5 (t, C-3, JCF 249.8 Hz),
142.0 (d, C-5B, JCF 249.1 Hz), 149.2 (C-2B), 156.2, 156.6 (C-
4 + C@O). HRMS (ESI+) Calcd for C15H18F3N3O5Na [M+Na]+
388.1091, found 388.1108.
5.11. 1-[(2R,5S)-N-tert-butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-uracil (19a)
Twenty-four percentage of yield after glycosylation and 98%
5.15. 1-[(2R,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
yield for TBDMS deprotection. Rf (5% MeOH/CH2Cl2): 0.21. mp:
xymethyl)pyrrolidin-2-yl]-5-fluorouracil (19d)
73–75 °C.
½
a 2D0
ꢂ
ꢀ54 (c 0.5, CH2Cl2). 1H NMR (acetone-d6,
300.13 MHz): d 1.42 (s, 9H, Boc), 2.63 (m, 2H, H-4), 3.74 (ddd,
Eighty-three percentage of overall yield (
a + b) after glycosyla-
1H, CH2O, JHH 2.4 Hz, JHH 4.2 Hz, JHH 11.4 Hz), 4.10 (tt, 1H, H-5,
tion and 27% yield for TBDMS deprotection (pure b-anomer). Rf

S. Martínez-Montero et al. / Bioorg. Med. Chem. 20 (2012) 6885–6893
6891
(5% MeOH/CH2Cl2): 0.29. mp: 82–84 °C. ½a D20
ꢂ
ꢀ57 (c 0.5, CH2Cl2). 1H
½
a 2D0
ꢂ
ꢀ12 (c 0.5, CH2Cl2). UV kmax (MeOH) 265 nm (6866 Mꢀ1
NMR (acetone-d6, 300.13 MHz): d 1.44 (s, 9H, Boc), 2.75 (m, 2H, H-
4), 3.72 (dd, 1H, CH2O, JHH 1.5 Hz, JHH 11.4 Hz), 4.12 (tt, 1H, H-5, JHH
2.1 Hz, JHH 8.4 Hz), 4.43 (tt, 1H, CH2O, JHH 2.7 Hz, JHH 11.4 Hz), 6.33
(d, 1H, H-2, JHF 13.8 Hz), 8.79 (d, 1H, H-6, JHF 7.5 Hz). 13C NMR (ace-
tone-d6, 125.61 MHz): d 27.3 (Boc), 33.1 (t, C-4, JCF 23.4 Hz), 54.1
(C-5), 57.1 (CH2O), 71.8 (m, C-2), 81.5 (Boc), 124.4 (d, C-6B, JCF
36.2 Hz), 124.5 (t, C-3, JCF 251.8 Hz), 142.0 (d, C-5B, JCF 231.8 Hz),
149.3 (C-2B), 156.3, 156.7 (C-4 + C@O). HRMS (ESI+) Calcd for
cmꢀ1). 1H NMR (CDCl3, 300.13 MHz): d 1.22 (rotamers, 9H, Boc),
2.67 (m, 1H, H-4), 3.17 (m, 1H, H-4), 3.79 (dd, 1H, CH2O, JHH
5.7 Hz, JHH 10.5 Hz), 4.05 (dd, 1H, CH2O, JHH 5.7 Hz, JHH 11.1 Hz),
4.65 (s, 1H, H-4), 6.18 (d, 1H, H-2, JHF 10.5 Hz), 8.15 (s, 1H, H-2),
8.77 (s, 1H, H-8). 13C NMR (CDCl3, 75.5 MHz): d 27.9 (Boc), 33.5
(t, C-4, JCF 22.7 Hz), 58.9 (C-5), 64.5 (CH2O), 72.5 (m, C-2), 83.2
(Boc), 124.6 (t, C-3, JCF 254.4 Hz), 132.0 (C-5B), 144.4 (C-8B),
150.9 (C-4B), 151.7 (C-6B), 152.3 (C-2B), 153.2 (C@O). HRMS
(ESI+) Calcd for C15H18ClF2N5O3Na [M+Na]+ 412.0958, found
412.0963.
C
15H18F3N3O5Na [M+Na]+ 388.1091, found 388.1085.
5.16. 1-[(2S,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-cytosine (20c) and 1-[(2R,5S)-N-tert-
butyloxycarbonyl-3,3-difluoro-5-(hydroxymethyl)pyrrolidin-2-
yl]-cytosine (21c)
5.20. 7-[(2R,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-6-chloropurine (28)
Eight percentage of yield after glycosylation and 81% yield for
TBDMS deprotection. Rf (10% MeOH/CH2Cl2): 0.50. 1H NMR (CDCl3,
400.13 MHz): d 1.31 (9H, Boc), 2.55 (td, 1H, H-4, J 7.2, J 15.2), 2.89
(m, 1H, H-4), 3.85 (dd, 1H, CH2O, JHH 2.0 Hz, JHH 11.6 Hz), 4.16 (t,
1H, H-5, JHH 6.2 Hz), 4.69 (d, 1H, JHH 11.6 Hz), 6.88 (d, 1H, H-2,
JHF 12.0 Hz), 8.77 (s, 1H, H-2B), 8.84 (s, 1H, H-8B). 13C NMR (CDCl3,
100.61 MHz): d 27.9 (Boc), 33.5 (t, C-4, JCF 23.6 Hz), 57.4 (C-5), 60.2
(CH2O), 72.2 (m, C-2), 83.3 (Boc), 122.5 (C-5B), 123.3 (t, C-3, JCF
254.5 Hz), 143.1 (C-4B), 147.4 (C-8B), 152.7 (C-2B), 153.5 (C@O),
161.8 (C-6B). HRMS (ESI+) Calcd for C15H18ClF2N5O3Na [M+Na]+
412.0958, found 412.0943.
The mixture of anomers 18c/19c (44 mg, 0.11 mmol) was dis-
solved in a saturated solution of ammonia in MeOH (3 mL). The
reaction was stirred for 2 h at room temperature for 2 h. MeOH
was evaporated and the residue subjected to flash chromatogra-
phy 1–5% MeOH/CH2Cl2 to afford the mixture of anomers 20c/
21c (32 mg, 82%) as a white solid. Rf (5% MeOH/CH2Cl2): 0.38.
a
-isomer (20c) 1H NMR (MeOH-d4, 300.13 MHz): d 1.39 (rota-
mers, 9H, Boc), 2.59 (m, 2H, H-4), 3.52 (t, 1H, CH2O, JHH 8.7 Hz),
3.92 (s, 1H, CH2O), 4.32 (m, 1H, H-5), 5.96 (s, 1H, H-5), 6.45 (d,
1H, H-2, JHF 13.5 Hz), 7.45 (s, 1H, H-6). b-isomer (21c) d 1.39
(rotamers, 9H, Boc), 2.59 (m, 2H, H-4), 3.67 (dd, 1H, CH2O, JHH
2.1 Hz, JHH 11.4 Hz), 4.05 (tt, 1H, H-5, JHH 2.6 Hz, JHH 7.4), 4.26
(ddd, 1H, CH2O, JHH 2.3 Hz, JHH 11.4, JHH 11.6), 5.91 (d, 1H, H-
5B, JHH 7.5 Hz), 6.40 (d, H-2, JHF 13.5 Hz), 8.29 (d, 1H, H-6B, JHH
9.1 Hz). HRMS (ESI+) Calcd for C14H20F2N4O4Na [M+Na]+
369.1345, found 369.1348.
5.21. 7-[(2S,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-6-chloropurine (29)
Forty-eight percentage of yield after glycosylation and 78% yield
for TBDMS deprotection. Rf (10% MeOH/CH2Cl2): 0.47. mp: 76-
78 °C. ½a 2D0
ꢂ
+9 (c 0.5, CH2Cl2). UV kmax (MeOH) 270 nm (6616 Mꢀ1
5.17. General procedure for the synthesis of purine nucleosides
(26–29)
cmꢀ1). 1H NMR (CDCl3, 400.13 MHz): d 1.23 (rotamers, 9H, Boc),
2.67 (m, 2H, H-4), 3.28 (br s, 1H, OH), 3.79 (m, 1H, CH2O), 4.11
(m, 1H, CH2O), 451 (s, 1H, H-4), 6.83 (d, 1H, H-2, JHF 9.2 Hz), 8.30
(s, 1H, H-8), 8.91 (s, 1H, H-2). 13C NMR (CDCl3, 75.5 MHz): d 27.7
(Boc), 33.7 (t, C-4, JCF 22.7 Hz), 57.6 (C-5), 63.0 (CH2O), 72.4 (m,
C-2), 83.7 (Boc), 122.7 (C-5B), 124.2 (t, C-3, JCF 254.4 Hz), 143.2
(C-4B), 144.8 (C-8B), 152.5 (C@O), 152.8 (C-2B), 161.8 (C-6B).
HRMS (ESI+) Calcd for C15H18ClF2N5O3Na [M+Na]+ 412.0958, found
412.0970.
Similar procedure as the described for the synthesis of nucleo-
sides 18a–d/19a–d. After glycosylation and solvents evaporation
the residue was purified by column chromatography (20% AcOEt/
hexane) to afford the silyl protected isomeric nucleosides 22–25.
Each separated 40-azanucleoside was treated with 1.0 M solutions
of TBAF in THF (1.5 equiv) to give pure 26–29 as white solids
(75–82%).
5.22. General procedure for the synthesis of adenine nucleo
sides (30–33)
5.18. 9-[(2R,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-
(hydroxymethyl)pyrrolidin-2-yl]-6-chloropurine (26)
40-Azanucleosides 26–29 (40 mg, 0.11 mmol) were treated with
a saturated solution of ammonia in MeOH (3 mL) and stirred for
1 h at 100 °C in a sealed tube. The reaction is cooled to room
temperature, MeOH evaporated and the residue purified by col-
umn chromatography (5–10% MeOH/CH2Cl2) to afford nucleosides
30–33 as white solids (72–75%).
Six percentage of yield after glycosylation and 75% yield for
TBDMS deprotection. Rf (10% MeOH/CH2Cl2): 0.52. 1H NMR (CDCl3,
400.13 MHz): d 1.29 (9H, Boc), 2.64 (m, 1H, H-4), 2.99 (m, 1H, H-4),
3.84 (dd, 1H, CH2O, JHH 2.8 Hz, JHH 11.6 Hz), 4.24 (t, 1H, H-5, JHH
8.0 Hz), 4.46 (d, 1H, JHH 11.6 Hz), 6.45 (d, 1H, H-2, JHF 12.8 Hz),
8.77 (s, 1H, H-2B), 8.84 (s, 1H, H-8B). 13C NMR (CDCl3,
100.61 MHz): d 27.9 (Boc), 33.5 (t, C-4, JCF 23.2 Hz), 57.7 (C-5),
61.6 (CH2O), 72.2 (m, C-2), 83.3 (Boc), 124.5 (t, C-3, JCF 225.9 Hz),
131.71 (C-5B), 144.2 (C-8B), 151.4 (C-4B), 151.6 (C-6B), 152.1 (C-
2B), 153.8 (C@O). HRMS (ESI+) Calcd for C15H18ClF2N5O3Na
[M+Na]+ 412.0958, found 412.0969.
5.23. 9-[(2R,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-adenine (30)
Seventy-two percentage of yield. Rf (10% MeOH/CH2Cl2): 0.31.
1H NMR (THF-d8, 400.13 MHz): d 1.25 (s, 9H, Boc), 2.60 (m, 1H,
H-4), 3.01 (m, 1H, H-4), 3.70 (d, 1H, CH2O, JHH 10.8 Hz), 4.07 (m,
1H, H-5), 4.28 (d, 1H, CH2O, JHH 10.4 Hz), 4.67 (br s, 1H, OH), 6.40
(d, 1H, H-2, JHF 13.6 Hz), 6.47 (br s, 2H, NH2), 8.14 (s, 1H, H-2 or
H-8), 8.39 (s, 1H, H-8 or H-2). 13C NMR (THF-d8, 100.61 MHz): d
25.3 (Boc), 31.4 (t, C-4, JCF 11.3 Hz), 57.7 (C-5), 58.9 (CH2O), 69.0
5.19. 9-[(2S,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-
(hydroxymethyl)pyrrolidin-2-yl]-6-chloropurine (27)
Ninteen percentage of yield after glycosylation and 82% yield for
TBDMS deprotection. Rf (10% MeOH/CH2Cl2): 0.52. mp: 64-66 °C.

6892
S. Martínez-Montero et al. / Bioorg. Med. Chem. 20 (2012) 6885–6893
(m, C-2), 79.2 (Boc), 117.7 (C-5B), 122.5 (t, C-3, JCF 252.3 Hz), 136.8
(C-8B), 148.3 (C-4B), 150.8 (C-2), 151.7 (C-6B), 161.8 (C@O). HRMS
(ESI+) Calcd for C15H21F2N6O3 [M+H]+ 371.1638, found 371.1642.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmc.2012.09.026.
5.24. 9-[(2S,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-adenine (31)
References and notes
1. Parker, W. B. Chem. Rev. 2009, 109, 2880.
Seventy-two percentage of yield. Rf (10% MeOH/CH2Cl2): 0.29.
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mp: 105–107 °C. ½a D20
ꢀ2 (c 0.5, MeOH). UV kmax (MeOH) 260 nm
ꢂ
(8975 Mꢀ1 cmꢀ1). 1H NMR (CDCl3, 400.13 MHz): d 1.16 (rotamers,
9H, Boc), 2.58 (t, 1H, H-4, JHH 15.2 Hz), 3.15 (m, 1H, H-4), 3.75 (dd,
1H, CH2O, JHH 6.4 Hz, JHH 10.9 Hz), 4.05 (dd, 1H, CH2O, JHH 5.2 Hz,
JHH 10.9 Hz), 4.63 (s, 1H, H-5), 6.07 (d, 1H, H-2, JHF 9.6 Hz), 6.13
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13C NMR (CDCl3, 75.5 MHz):
d 27.7 (Boc), 34.3 (t, C-4, JCF
22.5 Hz), 58.5 (C-5), 63.9 (CH2O), 71.8 (m, C-2), 82.6 (Boc), 119.5
(C-5B), 124.8 (t, C-3, JCF 252.3 Hz), 139.4 (C-8B), 149.3 (C-4B),
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HRMS (ESI+) Calcd for C15H21F2N6O3 [M+H]+ 371.1638, found
371.1628.
5.25. 7-[(2R,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-(hydro
xymethyl)pyrrolidin-2-yl]-adenine (32)
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Seventy-five percentage of yield. Rf (10% MeOH/CH2Cl2): 0.29.
mp: 225–227 °C. 1H NMR (THF-d8, 300.13 MHz): d 1.29 (s, 9H,
Boc), 2.68 (m, 2H, H-4), 3.63 (d, 1H, CH2O, JHH 11.1 Hz), 4.06 (m,
2H, CH2O + H-5), 5.38 (t, 1H, OH, JHH 9.6 Hz), 6.75 (d, 1H, H-2, JHF
10.5 Hz), 6.87 (s, 2H, NH2), 8.24 (s, 1H, H-2 or H-8), 8.86 (s, 1H,
H-8 or H-2). 13C NMR (MeOH-d4, 75.5 MHz): d 28.0 (Boc), 31.4 (t,
C-4, JCF 22.3 Hz), 57.4 (C-5), 60.2 (CH2O), 73.7 (m, C-2), 81.9
(Boc), 111.4 (C-5B), 124.6 (t, C-3, JCF 252.9 Hz), 143.5 (C-8B),
151.6 (C-4B), 152.9 (C-2B), 153.8, 160.4 (C-6B + C@O). HRMS
(ESI+) Calcd for C15H21F2N6O3 [M+H]+ 371.1638, found 371.1648.
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5.26. 7-[(2S,5S)-N-tert-Butyloxycarbonyl-3,3-difluoro-5-
(hydroxymethyl)pyrrolidin-2-yl]-adenine (33)
Seventy-three percentage of yield. Rf (10% MeOH/CH2Cl2): 0.27.
mp: 206–208 °C. ½a D20
ꢂ
+29 (c 0.5, MeOH). UV kmax (MeOH) 275 nm
(4644 Mꢀ1 cmꢀ1). 1H NMR (MeOH-d4, 300.13 MHz) d 1.56 (rota-
mers, 9H, Boc), 2.80 (m, 2H, H-4), 3.61 (t, 1H, CH2O, JHH 9.9 Hz),
4.03 (d, 1H, CH2O, JHH 9.0 Hz), 4.52 (s, 1H, H-5), 6.68 (d, 1H, H-2,
JHF 9.3 Hz), 8.33 (s, 1H, H-2 or H-8), 8.54 (s, 1H, H-8 or H-2). 13C
NMR (MeOH-d4, 75.5 MHz): d 26.6 (Boc), 33.9 (m, C-4), 56.9
(C-5), 60.3 (CH2O), 72.1 (m, C-2), 82.1 (Boc), 111.4 (C-5B), 124.6
(t, C-3, JCF 256.7 Hz), 142.6 (C-8B), 151.8, 152.1 (C-4B or C@O or
C-6B), 152.6 (C-2B), 158.9 (C-4B or C@O or C-6B). HRMS (ESI+)
Calcd for C15H21F2N6O3 [M+H]+ 371.1638, found 371.1628.
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Acknowledgments
Financial support of this work by the Spanish Ministerio de
Educación y Ciencia (MEC) (Project MEC-CTQ-2007-61126) and
PCTI (2006–2009)/FEDER (2007–2013) Asturias (Project EQUIP09-
07) are gratefully acknowledged. S.M.-M. thanks MEC for a predoc-
toral fellowship. We also acknowledge the National Institutes of
Health (NIH) for financial support of this work through Grant Num-
ber R01 GM081484. We thank the National Science Foundation for
instrumentation grants CHE9709183 and CHE0741968. We also
thank Dr. Y. Su (UCSD Mass Spectrometry) for mass analysis. This
work was also supported in part by NIH grant 2P30-AI-050409
and the Department of Veterans Affairs.
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