The application of Mitsunobu cyclization for the synthesis of 2′,3′-dideoxy-C-nucleosides designed as didanosine analogues

Article abstract of DOI:10.1055/s-0029-1217364

The synthesis of new 2′,3′-dideoxy-C-nucleosides structurally related to didanosine has been achieved. Their preparation involved condensation of a suitably substituted, lithiated 2- or 4-picoline with 2′,3′- dideoxy-5′-benzylribonolactone, followed by bo

Full text of DOI:10.1055/s-0029-1217364

LETTER
1741
The Application of Mitsunobu Cyclization for the Synthesis of 2¢,3¢-Dideoxy-
C-Nucleosides Designed as Didanosine Analogues
Synthesis of
2
¢
,3
o
¢
-Dideoxy-
C
n
-Nucleoside
y
s
Tite,a Nikolaos Lougiakis,a Alexios-Leandros Skaltsounis,b Panagiotis Marakos,a Nicole Pouli,*a
Roxane Tenta,c Jan Balzarinid
a
Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Athens, Panepistimiopolis Zografou, 15771 Athens,
Greece
Fax +30(210)7274747; E-mail: pouli@pharm.uoa.gr
Department of Pharmacy, Division of Pharmacognosy, University of Athens, Panepistimiopolis Zografou, 15771 Athens, Greece
Department of Science of Nutrition-Dietetics, Harokopio University, 70 El. Venizelou Street, 17671 Athens, Greece
Rega Institute for Medical Research, K.U. Leuven, B-3000 Leuven, Belgium
b
c
d
Received 5 February 2009
studies,4 has inspired investigators to develop new syn-
Abstract: The synthesis of new 2¢,3¢-dideoxy-C-nucleosides struc-
thetic methods for their preparation – an often trouble-
turally related to didanosine has been achieved. Their preparation
involved condensation of a suitably substituted, lithiated 2- or 4-pi-
coline with 2¢,3¢-dideoxy-5¢-benzylribonolactone, followed by
some task.5 In the course of a research program we were
involved in the synthesis of some purine-like C-
borohydride reduction of the resulting hemiacetals, intramolecular nucleosides6 and this research prompted us to prepare
Mitsunobu cyclization of the derived diols, formation of the pyrazo-
some new derivatives that could mimic the drug
lo[3,4-c] or [4,3-b]pyridine ring-system and subsequent removal of
the protecting groups.
didanosine (2¢,3¢-dideoxyinosine; ddI), i.e. modified [8-
aza-1(or 3),9-dideaza]didanosine analogues. This drug
was one of the first to be approved for the treatment of
HIV infected individuals and is still a major component of
Key words: C-nucleosides, heterocycles, pyrazolo[3,4-c]pyridine,
pyrazolo[4,3-b]pyridine, dideoxynucleosides
highly active antiretroviral treatment (HAART) drug
combinations.7
In recent years there has been increased interest in 2¢,3¢-
For the synthesis of the target compounds we used the
dideoxy nucleosides due to their use in the treatment of
substituted picoline 1 (Scheme 1), which was obtained ac-
HIV infections. Antiviral activity is exercised through
cording to published procedures starting from 2-amino-4-
three successive phosphorylations of the 5¢-hydroxyl
picoline,8 and (S)-(+)-g-[(benzyloxy)methyl]-g-butyro-
group, which allows the molecule to interact with target
lactone (2). The latter is a logical precursor of the 2¢,3¢-
enzymes (retroviral reverse transcriptase and DNA poly-
dideoxy-C-nucleoside, and was prepared as described in
merase), either by competing with the natural triphosphate
the literature through Taniguchi deamination of inexpen-
substrate or by causing chain termination, subsequent to
sive L-glutamic acid with nitrous acid, reduction of the re-
incorporation into the growing proviral DNA, as a result
sulting carboxylic acid by borane dimethyl sulphide and
of the missing 3¢-hydroxyl group. Due to the fact that HIV
subsequent benzylation of the corresponding alcohol.9
does not encode specific purine and pyrimidine nucleo-
Picoline 1 was lithiated using 2.5 equivalents of n-butyl-
sides or nucleotide-metabolizing enzymes, these nucleo-
lithium in anhydrous THF at –78 °C, and the resulting an-
sides need to be recognized by the host cell kinases and
ion was used to attack the carbonyl of 2 to provide a 1:3
other cellular enzymes in order to become metabolically
a-D/b-D mixture of hemiacetals 3 (50% isolated yield),
bioactivated.1 Potential toxicity to the host and the threat
together with a less polar mixture of compounds 4, which
of antiviral drug resistance has stimulated efforts for the
resulted from the attack of the anion formed on the methyl
synthesis of novel antiviral agents.2 On the other hand, C-
group of the acetamide upon the lactone 2. The anomeric
nucleosides represent a unique class of compounds pos-
mixture of the hemiacetals 3 was then treated with a large
sessing carbohydrate moieties linked to heterocycles via a
excess of boron trifluoride diethyletherate in dichlo-
hydrolytically resistant carbon–carbon bond and, in this
romethane solution to generate a 1:1 E/Z mixture of the
respect, they differ from the more common N-nucleo-
olefin 5 in good yield (80%). Unfortunately, the ionic hy-
sides. The isosteric exchange of nitrogen with carbon is
drogenation of 5, using triethylsilane as the hydride ion
considered to be a minimal structural modification that
donor and trifluoroacetic acid as the proton donor, failed
can lead to compounds with the approximate size and
to furnish the corresponding saturated analogue. There-
shape of natural nucleosides. Indeed, a number of C-nu-
fore, 3 was subjected to sodium borohydride reduction to
cleosides are endowed with interesting antibacterial, anti-
provide the corresponding diastereomeric carbinols 6. In
viral and antitumor properties.3 Their pharmaceutical
order to unambiguously elucidate their structure, the com-
value, together with their potential use in nucleic acid
pounds 4 were also reduced to the diastereomer 7, which
allowed 1-D and 2-D NMR data to be more easily inter-
preted. In the HMBC spectrum of 7, the protons of the
methyl group correlate with the aromatic carbons C-5
SYNLETT 2009, No. 11, pp 1741–1744
Advanced online publication: 12.06.2009
DOI: 10.1055/s-0029-1217364; Art ID: D04209ST
x
x
.x
x
.2
0
0
9
© Georg Thieme Verlag Stuttgart · New York

1742
T. Tite et al.
LETTER
OMe
OMe
NHAc
Me
NHAc
Me
O
a
N
N
OH
O
BnO
N
+
N
H
O
BnO
OMe
OH
1
OMe
N
4
AcHN
3
b
O
N
c
c
BnO
H
5
OMe
OMe
NH2
Me
O
NHAc
N
HO
OH
e
BnO
N
N
H
OH
OH
BnO
BnO
OMe
OH
OH
7
6
9
d
d
OMe
OMe
OMe
Ac
NH2
NHAc
H
N
N
N
N
g
f
N
O
O
O
BnO
RO
BnO
H
H
10
11 R = Bn
12 R = H
8
h
i
O
OMe
H
H
N
H
N
O
HN
HO
N
N
H
N
+
j (from 14)
HO
N
N
O
N
O
HO
H
H
OMe
14
15
13
Scheme 1 Reagents and conditions: (a) (i) n-BuLi, THF, –78 °C, 1 h; (ii) (S)-(+)--[(bezyloxy)methyl]-g-butyrolactone (2), THF, 3 h, 51% for
3, 8% for 4; (b) BF3·Et2O, CH2Cl2, –20 °C, 3 h, 80%; (c) NaBH4, MeOH, r.t., 90 min, 98%; (d) Ph3P, DEAD, THF, 70 °C, 90 min, 40%; (e)
NaOH (2 N), MeOH, 100 °C, 22 h, 76%; (f) Ac2O, CH2Cl2, r.t., 15 h, 70% (2 steps); (g) AcOK, Ac2O, isoamyl nitrite, toluene, 100 °C, 15 h,
84%; (h) H2, Pd/C 10%, EtOH, r.t., 50 psi, 9 h; (i) NH3–MeOH, r.t., 4 h, 79% (2 steps); (j) BCl3, CH2Cl2, r.t., 3 h, 63%.
(3J coupling) and C-4 (2J coupling), whereas the methyl- could be considered relevant concern the preparation of
ene protons possess a strong correlation with both the ano- 4(5)-tetrahydrofuranylimidazoles.12 In order to improve
meric carbon and the carbonyl. On the other hand, in the the yield, the Mitsunobu reaction was carried out on the
HMBC spectrum of 6, the protons of the acetamide meth- aminopyridine 9, and the resulting glycoside 10 was
yl group correlate with the carbonyl and, at the same time, acetylated to give the desired compound 8 in very good
the methylene group possesses a strong correlation with overall yield (70%). This compound was then refluxed in
both the anomeric carbon and the aromatic carbons C-4 toluene with isoamyl nitrite, in the presence of acetic
(2J coupling) and C-5 (3J coupling).
The application of the Mitsunobu reaction10 to the free di-
ols 6 provided the C-glycoside 8 in a 1:1 diastereomeric a/
anhydride and potassium acetate, to give a mixture of the
a- and b-1-acetylpyrazolopyridines 11 through the rear-
rangement of the intermediate N-nitroso-compound.13
b configuration, although in low yield (optimized yield The 5¢-benzyl group was removed by catalytic hydrogena-
40% when the Mitsunobu reaction was carried out at re- tion over palladium on carbon, and the acetyl group was
flux and terminated after 90 min). During the cyclisation easily cleaved by treatment with methanolic ammonia to
of 6, a less polar and complex mixture was isolated as the give a 1:1 mixture of both C-nucleoside anomers 13 and
major product of the reaction. This methodology has been 14, which were separated by column chromatography and
successfully employed in some cases to produce C-nucleo- identified. The configuration at C-1¢ was assigned on the
sides.11 However, to our knowledge, this is the first time basis of NOE experiments, which allowed us to observe
that the intramolecular Mitsunobu cyclization has been clear correlation peaks between H-1¢ and H-4¢ for 14 (b-
applied for the synthesis of 2¢,3¢-dideoxy-C-nucleosides anomer) and between H-1¢, H-2¢, H-3¢ and H-5¢ for 13, in
mimicking the purine skeleton. The only examples that accordance with the a-configuration (cis-relationship).
Synlett 2009, No. 11, 1741–1744 © Thieme Stuttgart · New York

LETTER
Synthesis of 2¢,3¢-Dideoxy-C-Nucleosides
1743
OMe
N
OMe
OMe
R
NHCOCF3
R
c
d
CH3
N
O
N
OH
BnO
BnO
OH
OH
16 R = NH2
a
b
17 R = NHCOMe
19
20 R = NHCOCF3
21 R = NH2
e
18 R = NHCOCF3
f
H
O
OMe
OMe
R
Ac
BnO
N
N
N
g
N
N
+
N
O
N
O
Ac
OMe
BnO
BnO
24
H
H
25
22 R = NH2
23 R = NHAc
a
h
h
O
OMe
H
O
H
N
H
N
RO
j
N
N
N
N
NH
N
O
N
H
O
HO
RO
i
H
H
OMe
26 R = Bn
28 R = H
27 R = Bn
29 R = H
30
i
Scheme 2 Reagents and conditions: (a) Ac2O, CH2Cl2, r.t., 10 h, 92%; (b) (CF3CO)2O, CH2Cl2, r.t., 1 h, 95%; (c) (i) n-BuLi, THF, –78 °C,
90 min; (ii) (S)-(+)-g-[(benzyloxy)methyl]-g-butyrolactone (2), THF, 2 h; (d) NaBH4, MeOH, r.t., 90 min, 44% (2 steps); (e) K2CO3, H2O–
MeOH, 60 °C, 20 h, 87%; (f) Ph3P, DEAD, THF, 70 °C, 1 h, 75%; (g) AcOK, Ac2O, isoamyl nitrite, toluene, 100 °C, 15 h, 30% for 24, 40%
for 25; (h) NH3–MeOH, r.t., 4 h, 95%; (i) BCl3, CH2Cl2, r.t., 2 h, 80%; (j) NaI, Me3SiCl, MeCN, reflux, 48 h, 58%.
The target derivative 15 resulted from compound 14 upon contrast to the corresponding isomer 14, the methoxy
treatment with boron trichloride in dichloromethane solu- group of compound 29 proved to be totally stable under
tion at –78 °C (63% yield).14
Lewis acid demethylation conditions. Consequently, 29
was converted into the target didanosine analogue 3015 by
reaction with trimethylsilyl chloride in the presence of so-
dium iodide.16
Our initial approach towards the synthesis of the 6-deaza-
didanosine analogue, was based on lithiation of the pi-
coline 17, which was easily obtained from 16,6c and react-
ed with the lactone 2 (Scheme 2). However, this method The antiviral activity of the compounds against HIV-1 and
provided only one coupling product, analogous to 4, in HIV-2 induced cytopathicity in CEM cell cultures was
43% isolated yield. Consequently, for the preparation of evaluated. The inhibitory effects of the compounds on the
the corresponding inosine isomer, we used the trifluoro- proliferation of murine leukemia cells (L1210) and human
acetamide 18 which, through lithiation and coupling with T-lymphocyte cells (Molt4/C8, CEM) were also deter-
2, provided access to the anomeric mixture of the hemiac- mined according to published procedures.17 Antiviral
etals 19. This mixture was subjected to borohydride re- evaluation of the compounds 14, 15, 29 and 30 revealed
duction and resulted in the diastereomeric carbinol 20 that these dideoxy C-nucleosides were antivirally inactive
(44% yield for 2 steps). Upon deprotection of the trifluo- at subtoxic concentrations (EC50: >250 mM). These deriv-
roacetyl group under standard conditions, the resulting atives were neither cytotoxic against L1210 cells (IC50
aminodiols 21 were subjected to intramolecular Mitsuno- values: 382, 356, >500 and 232 mM, respectively), nor
bu cyclisation and yielded a 1:1 diastereomeric mixture of against T-lymphocyte cells (IC50 values: 495, >500, >500,
the a/b-anomers 22 in good yield (75%). The amino group 296 mM, respectively).
was then acetylated and the resulting acetamide 23 was re-
In conclusion, we have prepared two novel dideoxy C-
fluxed as described before, in toluene with isoamyl nitrite,
nucleosides which can be considered as direct analogues
in the presence of acetic anhydride and potassium acetate,
of the drug didanosine (ddI). The synthetic methodology
to give the a- and b-1-acetylpyrazolopyridines 24 and 25,
involves the attachment of a suitable g-lactone to substi-
respectively, which were separated by column chroma-
tuted picolines, reduction of the derived hemiacetals fol-
tography and characterized. The acetyl groups were easily
lowed by Mitsunobu cyclization and transformation of the
pyridine ring into the purine-like heterocyclic base.
Cleavage of the protective groups provided the target nu-
cleaved upon treatment of 24 or 25 with methanolic am-
monia and the 5¢-O-benzyl group was then removed with
boron trichloride in a dichloromethane solution to yield
cleosides.
the a- and b-C-nucleosides 28 and 29, respectively. In
Synlett 2009, No. 11, 1741–1744 © Thieme Stuttgart · New York

1744
T. Tite et al.
LETTER
(10) (a) Mitsunobu, O. Synthesis 1981, 1. (b) Hughes, D. L. Org.
Prep. Proced. Int. 1996, 28, 127.
(11) (a) Weizman, H.; Tor, Y. J. Am. Chem. Soc. 2001, 123,
3375. (b) Guianvarc’h, D.; Fourrey, J.-L.; Tran Huu Dau,
M.-E.; Guerineau, V.; Benhida, R. J. Org. Chem. 2002, 67,
3724.
Acknowledgment
This work was supported by a Research Grant provided by the
Greek Ministry of Industry, Energy, and Technology (Program
ENTER).
(12) (a) Harusawa, S.; Araki, L.; Imazu, T.; Ohishi, H.;
Sakamoto, Y.; Kurihara, T. Chem. Pharm. Bull. 2003, 51,
325. (b) Harusawa, S.; Araki, L.; Terashima, H.; Kawamura,
M.; Takashima, S.; Sakamoto, Y.; Hashimoto, T.;
Yamamoto, Y.; Yamatodani, A.; Kurihara, T. Chem. Pharm.
Bull. 2003, 51, 832.
References and Notes
(1) (a) Balzarini, J.; De Clercq, E. Biochemical pharmacology.
Nucleoside and nonnucleoside reverse transcriptase
inhibitors active against HIV, In Textbook of AIDS
Medicine; Merigan, T. C.; Bartlett, J. G.; Bolognesi, D.,
Eds.; Williams and Wilkins: Philadelphia, 1998, Chap. 47,
815–847. (b) Balzarini, J. Pharm. World Sci. 1994, 16, 113.
(2) De Clercq, E. Antiviral Res. 2005, 67, 56.
(3) (a) Wu, O.; Simons, C. Synthesis 2004, 1533.
(b) Watanabe, K. A. In Chemistry of Nucleosides and
Nucleotides, Vol. 3; Townsend, L. B., Ed.; Plenum Press:
New York, 1994, 421–535.
(4) (a) Guianvarc’h, D.; Fourrey, J.-L.; Maurisse, R.; Sun, J.-S.;
Benhida, R. Org. Lett. 2002, 4, 4209. (b) Haberli, A.;
Leumann, C. J. Org. Lett. 2002, 4, 3275. (c) Heinrich, D.;
Wagner, T.; Diederichsen, U. Org. Lett. 2007, 9, 5311.
(d) Matulic-Adamic, J.; Beigelman, L.; Portmann, S.; Egli,
M.; Usman, N. J. Org. Chem. 1996, 61, 3909.
(5) (a) Redpath, P.; Macdonald, S.; Migaud, M. E. Org. Lett.
2008, 10, 3323. (b) Wellington, K. W.; Benner, S. A.
Nucleosides, Nucleotides Nucleic Acids 2006, 25, 1309.
(c) Simons, C. Nucleoside Mimetics: Their Chemistry and
Biological Properties; Gordon and Breach: Amsterdam,
2001. (d) Togo, H.; He, W.; Waki, Y.; Yokoyama, M.
Synlett 1998, 700. (e) Levy, D. E.; Tang, C. The Chemistry
of C-Glycosides; Pergamon: Oxford, 1995. (f) Postema,
M. H. D. C-Glycoside Synthesis; CRC: Boca Raton FL,
1995. (g) Jaramillo, C.; Knapp, S. Synthesis 1994, 1.
(6) (a) Kourafalos, V. N.; Marakos, P.; Pouli, N.; Townsend,
L. B. Synlett 2002, 1479. (b) Kourafalos, V. N.; Marakos,
P.; Pouli, N.; Townsend, L. B. J. Org Chem. 2003, 68, 6466.
(c) Korouli, S.; Lougiakis, N.; Marakos, P.; Pouli, N. Synlett
2008, 181.
(13) Marakos, P.; Pouli, N.; Wise, D.; Townsend, L. B. Synlett
1997, 561.
(14) 1,6-Dihydro-3-(2,3-dideoxy-b-D-ribofuranosyl)-7H-
pyrazolo[3,4-c]pyridin-7-one (15): mp 270–272 °C
(CH2Cl2–MeOH); 1H NMR (400 MHz, CD3OD): d = 1.88–
2.02 (m, 1 H, H-3¢), 2.08–2.25 (m, 2 H, H-2¢, H-3¢), 2.28–
2.40 (m, 1 H, H-2¢), 3.63 (dd, 1 H, H-5¢, J5¢,4¢ = 5.48 Hz,
J
5¢,5¢ = 11.35 Hz), 3.70 (dd, 1 H, H-5¢, J5¢,4¢ = 4.30 Hz,
J5¢,5¢ = 11.35 Hz), 4.13–4.22 (m, 1 H, H-4¢), 5.22 (t, 1 H, H-
1¢, J1¢,2¢ = 7.04 Hz), 6.87 (d, 1 H, H-4, J4,5 = 6.65 Hz), 6.95 (d,
1 H, H-5, J5,4 = 6.65 Hz). 13C NMR (50 MHz, CD3OD): d =
28.54 (C-3¢), 32.97 (C-2¢), 65.59 (C-5¢), 77.27 (C-1¢), 81.94
(C-4¢), 101.72 (C-4), 125.69 (C-3a), 125.93 (C-5), 134.94
(C-7a), 148.07 (C-3), 156.55 (C-7). Anal. Calcd for
C11H13N3O3: C, 56.16; H, 5.57; N, 17.86. Found: C, 56.01;
H, 5.69; N, 17.68
(15) 1,4-Dihydro-3-(2,3-dideoxy-b-D-ribofuranosyl)-7H-
pyrazolo[4,3-b]pyridin-7-one (30): Oil. 1H NMR (400 MHz,
CD3OD): d = 2.00–2.23 (m, 3 H, 2 × H-3¢, 1 × H-2¢), 2.35–
2.46 (m, 1 H, H-2¢), 3.71 (dd, 1 H, H-5¢, J5¢,4¢ = 3.13 Hz,
J
5¢,5¢ = 11.74 Hz), 3.89 (dd, 1 H, H-5¢, J5¢,4¢ = 2.74 Hz,
J5¢,5¢ = 11.74 Hz), 4.22–4.29 (m, 1 H, H-4¢), 5.29 (t, 1 H, H-
1¢, J1¢,2¢ = 7.04 Hz), 6.26 (d, 1 H, H-6, J6,5 = 7.04 Hz), 7.77 (d,
1 H, H-5, J5,6 = 7.04 Hz). 13C NMR (50 MHz, CD3OD): d =
28.46 (C-3¢), 34.19 (C-2¢), 69.95 (C-5¢), 77.95 (C-1¢), 81.76
(C-4¢), 110.23 (C-6), 128.49 (C-3a), 134.58 (C-7a), 138.77
(C-5), 142.86 (C-3), 171.12 (C-7). Anal. Calcd for
C11H13N3O3: C, 56.16; H, 5.57; N, 17.86. Found: C, 56.38;
H, 5.39; N, 17.99
(7) (a) Faulds, D.; Drogden, D. Drugs 1992, 44, 94. (b) De
(16) (a) Olah, G. A.; Narang, S. C.; Gupta, B. G. B.; Malhotra, R.
J. Org. Chem. 1979, 44, 1247. (b) Ramasany, K.; Imamura,
N.; Robins, R. K.; Revankar, G. R. J. Heterocycl. Chem.
1988, 25, 1893.
(17) Lougiakis, N.; Marakos, P.; Pouli, N.; Balzarini, J. Chem.
Pharm. Bull. 2008, 56, 775.
Clercq, E. J. Clin. Virol. 2004, 30, 115.
(8) Chapman, D.; Hurst, J. J. Chem. Soc., Perkin Trans. 1 1980,
2398.
(9) (a) Taniguchi, M.; Koga, K.; Yamada, S. Tetrahedron 1974,
30, 3547. (b) Kang, S.-K.; Jeon, J.-H.; Yamaguehi, T.;
Hong, R.-K.; Ko, B.-S. Tetrahedron: Asymmetry 1995, 6,
97. (c) Okabe, M.; Sun, R.-C.; Tam, S. Y.-K.; Todaro, L. J.;
Coffen, D. L. J. Org. Chem. 1988, 53, 4780.
Synlett 2009, No. 11, 1741–1744 © Thieme Stuttgart · New York