The synthesis of 6-deazaformycin A

Article abstract of DOI:10.1055/s-0029-1218002

The synthesis of the new C-nucleoside 6-deazaformycin A was achieved through the condensation of a suitably substituted lithiated 2-picoline with 2,3,5-tri-O-benzyl-d-ribonolactone, borohydride reduction of the resulting hemiacetals, followed by intramolecular Mitsunobu cyclization of the carbinols, manipulation of the protecting groups, and subsequent ring closure to result in the formation of 7-amino-3-(β-d-ribofuranosyl)pyrazolo[4,3-b]pyridine. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1218002

LETTER
2927
The Synthesis of 6-Deazaformycin A
6
-Deazaformyc
o
in
A
ny Tite, Nikolaos Lougiakis, Panagiotis Marakos, Nicole Pouli*
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
Received 21 July 2009
catabolic enzyme adenosine deaminase (ADA) to the less
Abstract: The synthesis of the new C-nucleoside 6-deazaformycin
active inosine analogue, formycin B, another naturally oc-
A was achieved through the condensation of a suitably substituted
lithiated 2-picoline with 2,3,5-tri-O-benzyl-D-ribonolactone, boro-
hydride reduction of the resulting hemiacetals, followed by in-
curing C-nucleoside.5 The high toxicity associated with
the formycins has restricted their use for clinical develop-
tramolecular Mitsunobu cyclization of the carbinols, manipulation ment, but on the other hand has stimulated interest on the
of the protecting groups, and subsequent ring closure to result in the
formation of 7-amino-3-(b-D-ribofuranosyl)pyrazolo[4,3-b]pyri-
dine.
synthesis of structural analogues and their study from both
chemical and biological points of view. A considerable
number of base-modified analogues has been prepared
and tested, including 5-chloro,6 5-fluoro,7 5-aminoformy-
Key words: C-nucleosides, heterocycles, formycin A, pyrazo-
lo[4,3-b]pyridine
cin A,7 5,7-disubstituted deazaformycin A,8 and 2- or 3-
deoxyformycin A.8 Some triazolo-C-nucleosides related
to formycin A have been also prepared and have demon-
The limited use of currently approved nucleosides due to
the emergence of poor pharmacokinetic properties, meta-
bolic degradation, cross resistance, and inherent toxicity
has stimulated the development of more efficient and sta-
ble analogues. The subclass of C-nucleosides embodies
bioactive natural compounds with an increased hydrolytic
and enzymatic stability when compared to the more labile
N-nucleosides.1 Among purine-like C-nucleosides formy-
cin A (7-amino-3-b-D-ribofuranosyl)-1H-pyrazolo[4,3-
d]pyrimidine), a naturally occurring bioisostere of ade-
nosine isolated from Nocardia interforma2 possesses a
broad spectrum of antibiotic activity (antimetabolic, anti-
viral, antineoplastic, and antiparasitic activities)3 mainly
attributed to its ability to replace adenosine in a variety of
enzymatic reactions and affect purine nucleoside and nu-
cleic acid metabolism via triphosphorylation by adeno-
sine kinase.4 Formycin A is readily deaminated by the
strated significant cytotoxicity against L1210 and WIL2
leukemia cells in culture.9
During the last few years we have been involved in the
study of the deazaformycins, which can provide useful in-
formations on the importance of the heteroatoms present
on the formycin scaffold, concerning the ability of these
molecules to behave as antimetabolites and to possess
cytotoxic activity thereof.10 As a continuation of this on-
going medicinal chemistry effort, we present herein the
synthesis of the new isomeric C-nucleoside 6-deazafor-
mycin A {7-amino-3-(b-D-ribofuranosyl)-pyrazolo[4,3-
b]pyridine}.
The synthesis of the target compound was effected
through the lithium-mediated coupling of tert-butyl-N-[3-
trifluoroacetamido-2-methylpyridin-4-yl]carbamate (10)
with the easily accessible 2,3,5-tri-O-benzyl-D-ribonolac-
R
R
NH2
NH2
NH2
NO2 O2N
+
O2N
NO2
Me
b
d
+
N
O
Me
N
Me
N
Me
N
Me
N
1 R = H
2 R = NO2
3 R = NO2
4 R = NH2
5
6
7
c
a
e
NHBoc
NHBoc
NO2
NHR
c
N
Me
N
Me
9
R = H
8
f
10 R = NHCOCF3
Scheme 1 Reagents and conditions: (a) H2SO4, HNO3 (65%), 160 °C, 3 h, 96%; (b) PCl3, CHCl3, r.t., 18 h, 83%; (c) H2, 10% Pd/C, EtOH,
r.t., 5 h, yield almost quantitative; (d) H2SO4, HNO3 (65%), 65 °C, 3 h, 38% for 5; (e) THF, NaH (2.5 equiv), 80 min, r.t, then (Boc)2O (1.1
equiv), 3 h, 88%; (f) (CF3CO)2O, CH2Cl2, r.t., 3 h, 95%.
SYNLETT 2009, No. 18, pp 2927–2930
x
x
.x
x
.2
0
0
9
Advanced online publication: 02.10.2009
DOI: 10.1055/s-0029-1218002; Art ID: D19309ST
© Georg Thieme Verlag Stuttgart · New York

2928
T. Tite et al.
LETTER
tone.11 The use of the trifluoroacetamide 10 in the lithia- acetamide 10 upon treatment with Boc anhydride, catalyt-
tion reaction, instead of the corresponding acetamide, was ic reduction, and trifluoroacetylation.
based on our previous observation10c concerning the pre-
dominance of anion formation on the acetamide methyl
rather than on the methyl group of 2-picoline, due to its
The lithiation of the trifluoroacetamide 10 was accom-
plished by using 4 equivalents of n-butyllithium in dry
THF at –78 °C. The resulting 2-methylene anion attacks
low acidity. The carbamate 10 was synthesized in seven
the carbonyl of 2,3,5-tri-O-benzyl-D-ribonolactone and
steps from the commercially available 2-picoline N-oxide
results into the anomeric mixture of hemiacetals 11
(1, Scheme 1). Compound 1 was nitrated and the resulting
(Scheme 2). The crude mixture 11 was reduced with sodi-
nitro-derivative 2 was treated with phosphorus trichloride
um borohydride in dry methanol to provide the diastereo-
meric carbinols 12 (2 steps, 43%). The trifluoroacetyl
group of 12 was then selectively cleaved using a saturated
potassium carbonate solution and the resulting aminodiols
to provide the picoline 3 which was reduced by catalytic
hydrogenation into the previously reported 4-amino-2-
picoline 4.12 Compound 4 was then nitrated,13 and the
desired nitro isomer 5 was converted into the trifluoro-
13 were submitted to Mitsunobu cyclization.14 The yield
NHBoc
NHCOCF3
NHBoc
NHR
NHBoc
NHCOCF3
N
O
N
OH
a
b
BnO
BnO
N
Me
OH
OBn
OH
OBn
10
BnO
BnO
c
12 R = COCF3
13 R = H
11
NHR
NHBoc
NH2
d
NHAc
H
N
e
O
BnO
from 14
+
BnO
N
O
N
O
BnO
OBn
NHBoc
BnO
H2N
15
H
H
BnO
OBn
16 R = Boc
17 R = H
BnO
OBn
14
f
g
Me
NH
N
N
N
O
O
N
O
O
N
N
N
N
Ac
N
NR
h
NHAc
N
from 18
+
+
N
O
O
BnO
O
O
BnO
BnO
BnO
H
H
H
H
BnO
OBn
BnO
OBn
BnO
OBn
BnO
OBn
18
19
20
21 R = Ac
22 R = H
i
i
j
NH2
H
N
N
N
NH
N
N
O
N
O
RO
HO
H
H
RO
OR
HO
OH
31
29 R = Bn
30 R = H
j
Scheme 2 Reagents and conditions (a) n-BuLi (4 equiv), 2,3,5-tri-O-benzyl-D-ribonolactone, THF, –78 °C, 3 h; (b) NaBH4 (4 equiv), MeOH,
r.t., 90 min, 43% (2 steps); (c) K2CO3, MeOH, H2O, 65 °C, 12 h, 64%; (d) Ph3P (1.7 equiv), THF, DEAD (2.5 equiv), 60 °C, 90 min, 73% for
14, 14% for 15; (e) Ac2O, CH2Cl2, r.t., 2 h, 93%; (f) CH2Cl2, TFA, r.t., 12 h, 70%; (g) phthalic anhydride (3.5 equiv), Et3N, THF–toluene, reflux,
36 h, 69% for 18, 7% for 19; (h) KOAc (1.5 equiv), Ac2O (3 equiv), isoamyl nitrite (1.5 equiv), toluene, 100 °C, 15 h, 30% for 20, 20% for 21,
43% for 22; (i) MeOH, NH3, r.t., 15 min, 96%; (j) CH2Cl2, BCl3, –78 °C, 2 h, 93%.
Synlett 2009, No. 18, 2927–2930 © Thieme Stuttgart · New York

LETTER
6-Deazaformycin A
2929
of this reaction is highly improved by the use of the ami- It seems that the low acidity of the 2-picoline methyl
nopyridine 13 rather than the trifluoroacetamide 12. Chro- group enables this alternative cyclization; intramolecular
matographic purification of the reaction mixture provided nucleophilic attack of the intermediate diazonium
the desired less polar b-epimer 14 in 73% yield,15 along species18 on the phthalimide carbonyl results in amino
with a minor amount (14%) of the a-epimer 15. The stereo- group deprotection and, under these conditions, the for-
chemistry was unambiguously determined on the basis of mation of the 1,2,3-triazolo[4,5-c]pyridine ring system is
NOE spectroscopic data, where we observed a clear cross highly favorable.19
peak between H-1¢ and H-4¢ for 14; whereas a clear cross
Both the N-phthaloyl and the acetyl groups of compounds
peak between H-1¢, H-2¢, and H-3¢ was characteristic in
20 and 21 were easily cleaved upon reaction with metha-
the case of the a-anomer 15. In order to proceed to the
nolic ammonia and the resulting tri-O-benzyl derivatives
pyrazolopyridine ring formation, the b-anomer 14 was
29 and 22 were converted, respectively, to the target 6-
converted to the corresponding acetamide 16, and the N-
deazaformycin A (30)20 and the substituted triazolopyri-
Boc protecting group was replaced with N-phthaloyl,
dine 31.21
since we had previously found that the carbamate group is
In conclusion, we have developed an efficient method to
prepare the novel C-nucleoside 6-deazaformycin A. The
synthesis was accomplished via the attachment of a pro-
tected ribonolactone to a substituted picoline followed by
reduction of the resulting hemiacetals. Mitsunobu cycliza-
tion of the derived diols furnished stereoselectively the
desired b-glycoside which upon ring closure provided the
pyrazolo[4,3-b]pyridine C-nucleoside, along with a con-
siderable amount of a triazolopyridine derivative.
not stable under the cyclization reaction conditions.10b,16
The N-phthaloyl derivative 18 was prepared upon reflux-
ing a solution of 17 using triethylamine and excess phthal-
ic anhydride in order to minimize (7%) the inevitable
formation of 2-methylimidazolo[4,5-c]pyridine 19,10b
which is obtained as a byproduct, due to intermolecular
nucleophilic attack of the 4-amino group of 17 to the 3-
acetamide carbonyl. Treatment of 18 with isoamyl nitrite
in the presence of potassium acetate and acetic anhydride
in refluxing toluene17 provided, through rearrangement of
the intermediate N-nitroso compound, the 1-acetylpyrazo-
lo[4,3-b]pyridine 20, but in rather poor yield (30%). This
reaction resulted in the formation of a considerable
amount of compound 21, which was partly deacetylated
during the silica gel column purification into 22. 1H NMR
analysis of 22 revealed the presence of the methylene
linker as a broad signal which was ascertained through 13C
NMR and DEPT experiments (d = 40.74 ppm correspond-
ing to the methylene group). Mass spectrometric analysis
of 22 [MS + 1 = 537; MS – 28 (N2) = 508], permitted the
characterization of the compound as a triazolopyridine C-
nucleoside (Scheme 2).
Acknowledgment
This work was supported by a Research Grant provided by the
Greek Ministry of Industry, Energy, and Technology (Program
ENTER).
References and Notes
(1) Watanabe, K. A. In Chemistry of Nucleosides and
Nucleotides, Vol. 3; Townsend, L. B., Ed.; Plenum Press:
New York, 1994, 421–535.
(2) (a) Hori, M.; Ito, E.; Takita, T.; Umezawa, H. J. Antibiot.,
Ser. A 1964, 17, 96. (b) Koyama, G.; Umezawa, H.
J. Antibiot. Ser. A 1965, 18, 175.
In order to confirm this observation we have studied the
cyclization on the phthalimide 25 which was easily pre-
pared from 9 (Scheme 3). Heating 25 in toluene provided
the 7-phthalimidopyrazolo[4,3-b]pyridine (26) in poor
yield (24%), whereas the acetate of the triazolo[4,5-c]-
pyridine 27 as well as the corresponding deacetylated de-
rivative 28 were the major products isolated upon purifi-
cation by column chromatography. These results provided
evidence on the difference of reactivity between the 3-ac-
etamido-4-phthalimido-2-picoline and the previously re-
ported isomeric 3-acetamido-2-phtalimido-4-picoline.10b
(3) (a) Bzowska, A. In Modified Nucleosides in Biochemistry,
Biotechnology and Medicine; Herdewijn, P., Ed.; Wiley:
New York, 2008, 473–510. (b) Hidaka, T.; Katayama, K.;
Yamashita, K.; Yamashita, T.; Watanabe, K.; Shimazaki,
M.; Ohno, M.; Takeuchi, T.; Umezawa, H. J. Antibiot. 1980,
33, 303. (c) Rosowsky, A.; Solan, V. C.; Gudas, L. J. J. Med.
Chem. 1986, 28, 1096. (d) Giziewicz, J.; De Clercq, E.;
Luczak, M.; Shugar, D. Biochem. Pharmacol. 1975, 24,
1813. (e) Orozco, M.; Canela, E. I.; Franco, R. Mol.
Pharmacol. 1989, 35, 257.
(4) Malaisse, W. J.; Sener, A.; Gruber, H. E.; Erion, M. D.
Biochem. Med. Metabolic Biol. 1994, 53, 22.
N
N
N
NHBoc
NHR
NH2
NR
NHAc
Me
O
O
O
O
N
d
c
b
N
N
+
H
N
NHAc
Me
N
Me
R = H
N
N
9
24
a
27 R = Ac
28 R = H
N
Me
23 R = Ac
e
25
26
Scheme 3 Reagents and conditions: (a) Ac2O, CH2Cl2, r.t., 4 h, 95%; (b) CH2Cl2, TFA, r.t, 15 h, 97%; (c) phthalic anhydride (3.5 equiv),
toluene–THF, Et3N, reflux, 12 h, 63%; (d) KOAc (1.5 equiv), Ac2O (3 equiv), isoamylnitrite (1.5 equiv), toluene, 100 °C, 15 h, 24% for 26,
21% for 27, 37% for 28; (e) MeOH, NH3, r.t., 15 min, yield almost quantitative.
Synlett 2009, No. 18, 2927–2930 © Thieme Stuttgart · New York

2930
T. Tite et al.
LETTER
(5) Ishizuka, M.; Sawa, T.; Koyama, G.; Takeuchi, T.;
Umezawa, H. J. Antibiot. 1968, 21, 1.
(6) Upadhya, K. G.; Sanghvi, Y. S.; Robins, R. K.; Revankar,
G. R.; Ugarkar, B. G. Nucleic Acids Res. 1986, 25, 1747.
(7) Secrist, J. A. III.; Shortnacy, A. T.; Montgomery, J. A.
J. Med. Chem. 1985, 28, 1740.
(8) Liu, M.-C.; Luo, M.-Z.; Mozdziesz, D. E.; Sartorelli, A. C.
Nucleosides, Nucleotides Nucleic Acids 2005, 24, 45.
(9) Ramasamy, K.; Ugarkar, B. G.; McKernan, P. A.; Robins,
R. K.; Revankar, G. R. J. Med. Chem. 1986, 29, 2231.
(10) (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.
(11) (a) Cousineau, T. J.; Secrist, J. A. III. J. Org. Chem. 1979,
44, 4351. (b) Timpe, W.; Dax, K.; Wolf, N.; Weidmann, H.
Carbohydr. Res. 1975, 69, 51.
(12) Reich, M. F.; Fabio, P. F.; Lee, V. J.; Kuck, N. A.; Testa,
R. T. J. Med. Chem. 1989, 32, 2474.
(13) (a) Deady, L. W.; Korytsky, O. L.; Rowe, J. E. Aust. J.
Chem. 1982, 35, 2025. (b) Indumathy, M.; Malcolm, R.
J. Heterocycl. Chem. 1992, 29, 359.
(C-4¢), 81.29 [(CH3)3CCONH], 82.60 (C-1¢), 112.73 (C-5),
127.80, 127.88, 128.14, 128.45, 128.54 (tertiary benzylic
CH), 130.80 (C-3), 136.62 (C-4), 137.93, 138.09, 138.16
(quaternary benzylic C), 142.27 (C-6), 148.53 (C-2),152.58
(CO).
(16) Kourafalos, V. N.; Marakos, P.; Pouli, N.; Terzis, A.;
Townsend, L. B. Heterocycles 2002, 57, 2335.
(17) Marakos, P.; Pouli, N.; Wise, D.; Townsend, L. B. Synlett
1997, 561.
(18) Chapman, D.; Hurst, J. J. Chem. Soc., Perkin Trans. 1 1980,
2398.
(19) Jones, G. Adv. Heterocycl. Chem. 2002, 83, 1.
(20) Data for 7-Amino-(3-b-D-ribofuranosyl)pyrazolo[4,3-
b]pyridine (30)
Pale yellow foam. 1H NMR (400 MHz, CD3OD): d = 3.82
(dd, 1 H, H-5¢, J5¢,4¢ = 12.13 Hz, J4¢,5¢¢ = 2.35 Hz), 3.94 (dd, 1
H, H-5¢, J5¢,4¢ = 12.13 Hz, J4¢,5¢¢ = 2.74 Hz), 4.15–4.19 (m, 1
H, H-4¢), 4.28 (dd, 1 H, H-3¢, J3¢,2¢ = 5.08 Hz, J3¢,4¢¢ = 3.52
Hz), 4.48 (dd, 1 H, H-2¢, J2¢,3¢ = 5.08 Hz, J2¢,1¢¢ = 7.04 Hz),
5.22 (d, 1 H, H-1¢, J1¢,2¢¢ = 7.04 Hz), 6.65 (d, 1 H, H-6,
J6,5 = 5.86 Hz), 8.07 (d, 1 H, H-5, J5,6 = 5.86 Hz). 13C NMR
(50 MHz, CD3OD): d = 63.28 (C-5¢), 73.74 (C-3¢), 77.50 (C-
2¢), 80.80 (C-1¢), 87.16 (C-4¢), 102.89 (C-6), 129.15 (C-7a),
132.90 (C-3a), 143.15 (C-5), 143.22 (C-3), 146.97 (C-7).
Anal. Calcd for C11H14N4O4: C, 49.62; H, 5.30; N, 21.04.
Found: C, 49.81; H, 5.22; N, 20.87.
(14) Guianvarc’h, D.; Fourrey, J.-L.; Tran Huu Dau, M.-E.;
Guerineau, V.; Benhida, R. J. Org. Chem. 2002, 67, 3724.
(15) Data for tert-Butyl-N-[3-amino-2-(2,3,5-tri-O-benzyl-b-
D-ribofuranosylmethyl)pyridin-4-yl]carbamate (14)
Oil. 1H NMR (400 MHz, CDCl3): d = 1.55 (s, 9 H, t-Bu),
3.00 (dd, 1 H, 2-CH2, J = 7.33, 13.69 Hz), 3.10 (dd, 1 H, 2-
CH2, J = 3.91, 13.69 Hz), 3.42 (dd, 1 H, H-5¢, J5¢,4¢ = 4.40 Hz,
(21) Data for 4-[(b-D-Ribofuranosyl)methyl]-3H-
[1,2,3]triazolo[4,5-c]pyridine (31)
Oil. 1H NMR (400 MHz, CDCl3): d = 3.52–3.63 (m, 2 H,
CH2, H-5¢), 3.65–3.74 (m, 2 H, CH2, H-5¢), 3.80–3.86 (m, 1
H, H-4¢), 4.02 (t, 1 H, H-3¢), 4.17 (t, 1 H, H-2¢), 4.30–4.37
(m, 1 H, H-1¢), 7.81 (d, 1 H, H-7, J7,6 = 6.26 Hz), 8.30 (d,
1 H, H-6, J6,7 = 6.26 Hz). 13C NMR (50 MHz CDCl3):
d = 38.49 (CH2), 62.64 (C-5¢), 72.21 (C-3¢), 76.64 (C-2¢),
83.41 (C-4¢), 85.95 (C-1¢), 109.07 (C-7), 139.48 (C-6),
142.29 (C-3a), 144.67 (C-7a), 153.13 (C-4). Anal. Calcd for
C11H14N4O4: C, 49.62; H, 5.30; N, 21.04. Found: C, 49.55;
H, 5.09; N, 21.18.
J5¢,5¢ = 10.27 Hz), 3.45–3.56 (br s, 2 H, D2O exchange, NH2),
3.56 (dd, 1 H, H-5¢, J5¢,4¢ = 2.94 Hz, J5¢,5¢ = 10.27 Hz), 3.72 (t,
1 H, H-3¢, J = 5.87 Hz), 3.92 (t, 1 H, H-2¢, J = 4.40 Hz),
4.16–4.21 (m, 1 H, H-4¢), 4.38–4.68 (m, 7 H, H-1¢,
6 × CH2Ph), 6.91 (s, 1 H, D2O exchange, NHBoc), 7.23–
7.41 (m, 15 H, 3 × C6H5), 7.63 (d, 1 H, H-5, J5,6 = 5.38 Hz),
8.05 (d, 1 H, H-6, J6,5 = 5.38 Hz). 13C NMR (50 MHz
CDCl3): d = 28.40 [(CH3)3CCONH], 39.34 (CH2), 69.64 (C-
5¢), 72.08, 73.41 (CH2Ph), 77.16 (C-3¢), 79.61 (C-2¢), 80.52
Synlett 2009, No. 18, 2927–2930 © Thieme Stuttgart · New York

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