The synthesis of 4-deazaformycin A

Article abstract of DOI:10.1021/jo026715x

The preparation of 4-deazaformycin A has been achieved. The synthesis features the condensation of a suitably substituted, lithiated 4-picoline with 2,3,5-tri-O-benzyl-D-ribonolactone, dehydration of the resulting hemiacetal, and ionic hydrogenation, foll

Full text of DOI:10.1021/jo026715x

Th e Syn th esis of 4-Dea za for m ycin A
Vassilios N. Kourafalos,† Panagiotis Marakos,†
Nicole Pouli,*,† and Leroy B. Townsend*,‡
Department of Pharmacy, Division of Pharmaceutical
Chemistry, University of Athens, Panepistimiopolis Zografou
15771, Athens, Greece, and Department of Medicinal
Chemistry, College of Pharmacy and Department of
Chemistry, College of Literature, Science and The Arts,
The University of Michigan, Ann Arbor, Michigan 48109
F IGURE 1. Structure of the formycins.
pouli@pharm.uoa.gr
sides formycin A (1)5 and formycin B (2)6 (Figure 1),
which closely mimic the isosteric naturally occurring
N-nucleosides adenosine and inosine and substitute for
them in many enzymatic reactions.7
As a continuation of our ongoing efforts toward the
design and synthesis of C-nucleoside antibiotics and
structurally related analogues8 we present here details
on the preparation of 7-amino-3-(â-D-ribofuranosyl)pyra-
zolo[3,4-c]pyridine. This nucleoside can be viewed as a
single modification of formycin A (4-deaza) and thus
provides a very useful probe for chemical and biological
studies on the importance of the 4-nitrogen of formycin
A.
Received November 14, 2002
Abstr a ct: The preparation of 4-deazaformycin A has been
achieved. The synthesis features the condensation of a
suitably substituted, lithiated 4-picoline with 2,3,5-tri-O-
benzyl-D-ribonolactone, dehydration of the resulting hemi-
acetal, and ionic hydrogenation, followed by manipulation
of the protecting groups and subsequent ring closure with
the formation of 7-amino-3-(â-D-ribofuranosyl)pyrazolo[3,4-
c]pyridine.
The biochemical and pharmacological significance of
natural nucleosides and their modified synthetic ana-
logues have attracted wide interest for many years. A
number of nucleoside derivatives have been approved for
the clinical treatment of viral and cancer diseases,1 while
several structurally related agents are under active
investigation by research groups throughout the world.
As a consequence, there is a need for the development of
synthetic approaches that allow access to these novel
potentially bioactive nucleoside derivatives with modified
bases or sugars. C-Nucleosides represent a unique class
of compounds, in which the carbohydrate moiety and the
aglycon are linked via a hydrolytically and enzymatically
stable carbon-carbon bond.2The potential applications
of C-nucleosides in nucleic acid chemistry have recently
attracted wide interest.3In fact, a number of C-nucleo-
sides are important antibiotics, which possess interesting
chemotherapeutic properties.4 Some of the most interest-
ing C-nucleoside antibiotics are the purine-like C-nucleo-
For the synthesis of the target compound (Scheme 1),
we used as starting material the Boc-protected aminopy-
ridine 3,9 which is suitable in terms of its stability to
anionic reaction conditions.10 The lithiation of 3 was
accomplished by using 3.7 equiv of n-butyllithium in dry
THF at -78 °C. The resulting 4-methylene anion attacks
the carbonyl of the easily accessible 2,3,5-tri-O-benzyl-
D-ribonolactone (4)11 to provide a 8:1 R-D/â-D12 anomeric
1
mixture of the hemiacetals 5, as estimated by H NMR.
The major isomer was isolated pure by column chroma-
tography on silica gel and characterized on the basis of
1-D and 2-D NMR data. We observed strong correlation
peaks between one of the 5′-protons and one proton of
the 4-methylene group. The observed stereochemistry at
the anomeric center is in agreement with the expectation
that steric effects would direct the attack of the nucleo-
phile 3 to the less hindered â-face of the sugar lactone
* Towhom correspondenceshouldbeaddressed.Phone: +302107274185.
Fax: +302107274747.
(5) Robins, R. K.; Townsend, L. B.; Cassidy, F.; Gerster, F. G.; Lewis,
A. F.; Miller, R. L. J . Heterocycl. Chem. 1966, 3, 110-114.
(6) Hori, M.; Ito, E.; Takita, T.; Koyama, G.; Takeuchi, T.; Umezawa,
H. J . Antibiot. Ser. A 1964, 17, 96-99.
† University of Athens.
‡ The University of Michigan.
(1) (a) Elion, G. B.; Furman, P. A.; Fyfe, G. A.; de Miranda, P.;
Beauchamp, L.; Schaeffer, H. J . Proc. Natl. Acad. Sci. U.S.A. 1977,
74, 5716-5720. (b) Moore, R. D.; Hidalgo, J .; Sugland, B. W.; Chaisson,
R. E. N. Engl. J . Med. 1991, 324, 1412-1416. (c) Yarchoan, R.; Pluda,
J . M.; Thomas, R. V.; Mitsuya, H.; Brouwers, P.; Wyvill, K. M.;
Hartman, N.; J ohns, D. G.; Broder, S. Lancet 1990, 336, 526-529.
(2) For a review of the chemistry and biology of C-nucleosides, see:
Watanabe, K. A. The Chemistry of C-Nucleosides. In Chemistry of
Nucleosides and Nucleotides; Townsend, L. B., Ed; Plenum Press: New
York, 1994; Vol. 3, pp 421-535.
(3) (a) Von Krosigk, U.; Benner, S. A. J . Am. Chem. Soc. 1995, 117,
5261-5362. (b) Matulic-Adamic, J .; Beigelman, L.; Portmann, S.; Egli,
M.; Usman, N. J . Org. Chem. 1996, 61, 3909-3911. (c) Hilbrand, S.;
Blaser, A.; Parel, P. S.; Leumann, C. J . J . Am. Chem. Soc. 1997, 119,
5499-5511.
(7) Buchanan, J . G. Fortschr. Chem. Org. Natr. 1983, 44, 243-312.
(8) (a) Gudmundsson, K. S.; Drach, J . C.; Townsend, L. B. J . Org.
Chem 1997, 62, 3453-3459. (b) Walker, J . A., II; Liu, W.; Wise, D. S.;
Drach, J . C.; Townsend, L. B. J . Med. Chem. 1998, 41, 1236-1241. (c)
Liu, W.; Wise, D. S.; Townsend, L. B. J . Org. Chem 2001, 66, 4783-
4786. (d) Kourafalos, V. N.; Marakos, P.; Pouli, N.; Townsend, L. B.
Synlett 2002, 1479-1482.
(9) Kourafalos, V. N.; Marakos, P.; Pouli, N.; Terzis, A.; Townsend,
L. B. Heterocycles 2002, 57, 2335-2343.
(10) Ihle, N. C.; Krause, A. J . Org. Chem. 1996, 61, 4810-4811.
(11) (a) Cousineau, T. J .; Secrist, J . A., III J . Org. Chem. 1979, 44,
4, 4351-4358. (b) Timpe, W.; Dax, K.; Wolf, N.; Weidmann, H.
Carbohydr. Res. 1975, 69, 51-60.
(12) It should be pointed out that for hemiacetals 5 the prefix R
refers to the position of the glycosidic OH group relative to the
configuration at the reference C-atom (C-4′ in 5; i.e., the methylpy-
ridinyl moiety is in the â-position). For C-glycosides (no glycosidic OH
present), the prefix R-D refers to the alkyl (or aryl) position relative to
the reference C-atom.
(4) (a) Srivastava, P. C.; Pickering, M. V.; Allen, L. B.; Streeter, D.
G.; Campbell, M. T.; Witowski, J . T.; Sidwell, R. W.; Robins, R. K. J .
Med. Chem. 1977, 20, 256-262. (b) Orozco, M.; Canela, E. I.; Franco,
R. Mol. Pharmacol. 1989, 35, 257-264.
10.1021/jo026715x CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/15/2003
6466
J . Org. Chem. 2003, 68, 6466-6469

SCHEME 1a
a
Reagents and conditions: (a) (i) n-BuLi, THF, -78 °C, (ii) 2,3,5-tri-O-benzyl-D-ribonolactone (4), THF, -78 °C up to room temperature;
(b) Ac2O, Et3N, CH2Cl2, rt; (c) BF3‚Et2O, CH2Cl2, -20 °C; (d) CF3CO2H, Et3SiH, CH2Cl2, rt; (e) phthalic anhydride, toluene, 50 °C; (f)
AcOK, Ac2O, isoamyl nitrite, C6H6, reflux; (g) CH3ONa/ CH3OH, rt; (h) BCl3, CH2Cl2, -78 °C.
4.13 To isolate and characterize the â-anomer as well we
have acetylated the hemiacetal 5 after treatment with
acetic anhydride in triethylamine. This reaction did not
provide the hemiacetal acetates but resulted in the open-
chain enol ester 6.
Reaction of the hemiacetal 5 with boron trifluoride
diethyl etherate in dichloromethane solution gave excel-
lent yield of the olefin 7, with exclusive Z-stereoselectiv-
ity. The stereochemistry was unambiguously determined
on the basis of NOE spectral data, where we observed a
strong correlation peak between the olefinic proton and
the 2′-H. Compound 7 was submitted to catalytic hydro-
genation over Pd/C to afford a complex mixture of
products. We subsequently found that the application of
ionic hydrogenation to 7, using triethylsilane as a hydride
ion donor and trifluoroacetic acid as the proton donor,14
furnished, after chromatographic separation, N-[2-amino-
4-(2,3,5-tri-O-benzyl-â-D-ribofuranosyl)methylpyridin-3-
yl]acetamide (8) as the major product (48%) and the
R-anomer 9 as the minor product (12%). The configura-
tion at C-1′ was assigned on the basis of NOE experi-
ments and for compound 8 we observed a clear cross-
peak between the 1′ and 4′ hydrogens. The concomitant
deprotection of the 2-amino group of 7 is in favor of the
reaction sequence, since according to our previous obser-
vations the direct cyclization of the N-Boc protected
derivative would result in the formation of the corre-
sponding 7-(ribofuranosylmethyl)imidazolo[4,5-b]pyridin-
2-one.9 This annulation occurs through an intermolecular
nucleophilic attack of the acetamide nitrogen to the
carbamate carbonyl, followed by ring closure and elimi-
nation of tert-butyl alcohol. Therefore, we have converted
(13) (a) Dondoni, A.; Scherrmann, M. C. J . Org. Chem. 1994, 59,
6404-6412. (b) Piccirilli, J . A.; Krauch, T.; MacPherson, L. J .; Benner,
S. A. Helv. Chim. Acta 1991, 74, 397-406.
(14) (a) Posner, G. H.; Switzer, C. J . Am. Chem. Soc. 1986, 108,
1239-1244. (b) Kurzanov, D. N.; Parnes, Z. N.; Loim, N. M. Synthesis
1974, 633-651.
J . Org. Chem, Vol. 68, No. 16, 2003 6467

mL). The organic extracts were dried (Na2SO4) and concentrated
to dryness to give an oil that was purified by flash chromatog-
raphy (silica gel 40 × 3 cm), using a mixture of CH2Cl2/AcOEt
7/3 as the eluent. This resulted in 700 mg (27%) of pure (R)-5 as
a colorless syrup and 500 mg (19%) of a 7/3 mixture of (R)- and
(â)-5, as determined by 1H NMR. Data for 5, R-anomer: 1H NMR
(CDCl3) δ 1.51 (s, 9H, t-Bu), 2.04 (s, 3H, CH3), 2.92 (d, 1H, 4-CH2,
J ) 14.08 Hz), 3.14 (d, 1H, 4-CH2, J ) 14.08 Hz), 3.19 (dd, 1H,
H-5′, J 5′,4′ ) 5.09 Hz, J 5′,5′ ) 10.17 Hz), 3.33 (dd, 1H, H-5′, J 5′,4′
) 3.52 Hz, J 5′,5′ ) 10.17 Hz), 3.63 (d, 1H, H-2′, J 2′,3′ ) 5.08 Hz),
3.83 (m, 1H, H-3′), 4.32-4.62 (m, 7H, H-4′, 6 × benzylic
methylene H), 4.71 (br s, 1H, OH, D2O exchangeable), 6.59 (d,
1H, H-5, J 5,6 ) 5.08 Hz), 7.23-7.38 (m, 15H, 3 × C6H5), 7.78 (br
s, 1H, NHBoc, D2O exchangeable), 8.19 (d, 1H, H-6, J 6,5 ) 5.08
Hz), 9.06 (br s, 1H, -NHAc, D2O exchangeable). 13C NMR
(CDCl3) δ 23.2, 28.2, 39.2, 69.8, 72.4, 73.3, 73.4, 76.2, 77.6, 80.3,
80.5, 104.7, 122.7, 124.7, 127.9, 128.1, 128.3, 128.4, 128.5, 136.5,
136.7, 137.3, 141.4, 146.0, 147.0, 152.3, 169.8. Anal. Calcd for
the aminopyridine 8 to the corresponding N-phthaloyl
derivative 10, by treatment of 8 with phthalic anhydride
in THF at 50 °C. This protecting group is stable under
the conditions of the cyclization reaction that follows and
at the same time the formation of the above mentioned
imidazolo[4,5-b]pyridinone is avoided. Our first attempt
to prepare the N-phthaloyl derivative involved heating
a solution of 8 in THF at reflux temperature, in the
presence of phthalic anhydride. However, we isolated 10
together with a significant amount (35%) of the 7-sub-
stituted 2-methylimidazolo[4,5-b]pyridine 11. This com-
pound (11) was obviously obtained through an intermo-
lecular nucleophilic attack of the 2-amino group to the
3-acetamide carbonyl of 8. We subsequently found that
the formation of this byproduct (11) is minimal (less than
5%) if the reaction is conducted at lower temperature (50
°C).
Compound 10 was then heated at reflux in benzene
(90 °C) with isoamyl nitrite, in the presence of acetic
anhydride.15 This furnished a mixture of the 1- and
2-acetylpyrazolo[3,4-c]pyridines 12 through a rearrange-
ment of the intermediate N-nitroso compound.
Both the N-phthaloyl and acetyl groups were easily
cleaved upon treatment of 12 with methanolic ammonia.
This provided the tri-O-benzyl derivative 13 which was
then converted to the target compound 7-amino-3-(â-D-
ribofuranosyl)pyrazolo[3,4-c]pyridine (14) by a reaction
of 13 with boron trichloride in a dichloromethane solu-
tion.
C
39H45N3O8: C, 68.50; H, 6.63; N, 6.15. Found: C, 68.23; H, 6.51;
N, 6.06.
(1R,2S,3S)-4-Acetoxy-5-[[3-a ceta m id o-2-(N-a cetyl-N-ter t-
bu toxyca r bon yl)a m in o]p yr id in -4-yl]-2,3-bis-ben zyloxy-1-
ben zyloxym eth ylp en t-4-en yl Aceta te (6). To a solution of 5
(46 mg, 0.067 mmol) in dry CH2Cl2 (2 mL) was added Ac2O (0.06
mL) and triethylamine (0.06 mL) and the reaction was stirred
at room temperature overnight. The solvents were vacuum
evaporated and the residue was purified by flash chromatogra-
phy (silica gel 12 × 1 cm), using a mixture of cyclohexane/AcOEt
7/3 as the eluent, to give pure 6, as an oil (44 mg, 80%). 1H NMR
(CDCl3) δ 1.39 (s, 9H, t-Bu), 1.97 (s, 3H, CH3), 2.06 (s, 3H, CH3),
2.27 (s, 3H, CH3), 2.53(s, 3H, CH3), 3.62 (dd, 1H, H-1a, J 1a,1
3.91 Hz, J 1a,1a ) 10.95 Hz), 3.68 (dd, 1H, H-1a, J 1a,1 ) 5.86 Hz,
J 1a,1a ) 10.95 Hz), 3.93 (m, 1H, H-2), 4.11 (d, 1H, H-3, J 3,4
)
)
5.47 Hz), 4.40-4.49 (m, 3H, 3 × benzylic methylene H), 4.61
(br s, 2H, 2 × benzylic methylene H), 4.69 (d, 1H, 1 × benzylic
methylene H, J ) 11.74 Hz), 5.38 (m, 1H, H-1), 6.24 (s, 1H, H-5),
7.19-7.33 (m, 15H, 3 × C6H5), 7.57 (d, 1H, H-5′, J 5′,6′ ) 5.09
Hz), 8.51 (d, 1H, H-6′, J 6′,5′ ) 5.09 Hz). 13C NMR (CDCl3) δ 21.0,
21.1, 25.7, 26.0, 27.5, 68.3, 71.7, 71.8, 73.0, 73.8, 78.5, 78.6, 84.4,
112.6, 123.1, 127.6, 127.8, 127.9, 128.1, 128.2, 128.3, 128.4, 131.5,
137.0, 137.6, 137.9, 143.5, 148.7, 150.5, 151.1, 151.9, 167.8, 170.0,
172.6, 172.9. Anal. Calcd for C45H51N3O11: C, 66.73; H, 6.35; N,
5.19. Found: C, 66.50; H, 6.51; N, 5.24.
In conclusion, we have developed an efficient method
to prepare 4-deazaformycin A, through the attachment
of a protected ribonolactone to a suitably substituted
picoline, followed by an elaboration of the pyrazolo[3,4-
c]pyridine ring system, with the appropriate exocyclic
functional groups.
Exp er im en ta l Section
(Z)-t er t -Bu t yl-N -[3-a ce t a m id o-4-(2,3,5-t r i-O-b e n zyl-D-
r ibofu r a n osylid en em eth yl)p yr id in -2-yl] Ca r ba m a te (7).
Boron trifluoride diethyl etherate (0.45 mL, 3.6 mmol) was added
dropwise under argon at -20 °C to a solution of 5 (R/â) (0.82 g,
1.2 mmol) in dry CH2Cl2 (20 mL). The resulting solution was
stirred at -20 °C for 6 h and then neutralized (pH 7) with a
saturated NaHCO3 solution. The mixture was extracted with
CH2Cl2 (3 × 50 mL), and the combined organic extracts were
dried (Na2SO4) and then concentrated to dryness. The residue
was purified by flash chromatography (silica gel, 18 × 1 cm),
using a mixture of CH2Cl2/AcOEt 7/3 as the eluent to give 0.80
Tetrahydrofuran (THF) was distilled from sodium/benzophe-
none ketyl immediately prior to use, while dichloromethane was
freshly distilled from calcium hydride. Melting points are
uncorrected. 1H NMR spectra and 2-D spectra (1H-1H COSY,
NOESY, HMQC, and HMBC) were recorded at 400 MHz and
13C NMR spectra were recorded at 50 MHz in deuterated
solvents and were referenced to TMS (δ scale). Flash column
chromatography was accomplished with silica gel 60 (0.040-
0.063 mm). Analytical thin-layer chromatography (TLC) was
carried out on precoated (0.25 mm) silica gel F-254 plates.
Elemental analyses were performed at the Microanalytical
Sections of the National Hellenic Research Foundation and are
within (0.4% of the theoretical values.
ter t-Bu tyl-N-[3-acetam ido-4-(2,3,5-tr i-O-ben zyl-1-h ydr oxy-
r,â-D-r ibofu r a n osyl)m eth ylp yr id in -2-yl] Ca r ba m a te (5). To
a solution of picoline 39 (1 g, 3.77 mmol) in dry THF (100 mL)
at -78 °C was added under argon n-BuLi (9.4 mL, 15.04 mmol,
1.6 M solution in hexanes). The resulting light-yellow solution
was stirred at -78 °C for 15 min and the temperature then
raised to 5 °C for 20 min. The orange solution was cooled to -78
°C and a solution of the D-ribonolactone 411 (1.81 g, 4.33 mmol)
in dry THF (10 mL) was added dropwise. The resulting mixture
was stirred at -78 °C for 15 min and then at room temperature
for an additional 5 h. A saturated ammonium chloride solution
was then added to the reaction mixture to quench the excess
n-BuLi. The solvent was vacuum evaporatedand water was
added to the residue and then extracted with CH2Cl2 (3 × 50
1
g (99%) of compound 7. Mp 155-156 °C (AcOEt-n-hexane). H
NMR (CDCl3) δ 1.51 (s, 9H, t-Bu), 2.12 (s, 3H, CH3), 3.59 (dd,
1H, H-5′, J 5′,4′ ) 4.40 Hz, J 5′,5′ ) 11.24 Hz), 3.77 (dd, 1H, H-5′,
J 5′,4′ ) 2.69 Hz, J 5′,5′ ) 11.24 Hz), 4.03 (dd, 1H, H-3′, J 3′,2′ ) 4.89
Hz, J 3′,4′ ) 6.60 Hz), 4.36 (d, 1H, H-2′, J 2′,3′ ) 4.89 Hz), 4.48-
4.79 (m, 7H, H-4′, 6 × benzylic methylene H), 5.48 (s, 1H, Cd
CH), 7.12 (br s, 1H, NHAc, D2O exchangeable), 7.22-7.41 (m,
15H, 3 × C6H5), 7.83 (d, 1H, H-5, J 5,6 ) 5.38 Hz), 8.16 (d, 1H,
H-6, J 6,5 ) 5.38 Hz), 8.47 (br s, 1H, NHBoc, D2O exchangeable).
13C NMR (CDCl3) δ 23.5, 28.3, 68.8, 70.3, 72.2, 73.5, 76.0, 76.6,
81.7, 84.1, 96.5, 120.8, 121.7, 127.7, 127.9, 128.0, 128.2, 128.6,
137.4, 137.6, 137.9, 143.0, 146.1, 146.7, 154.3, 158.4, 168.8. Anal.
Calcd for C39H43N3O7: C, 70.36; H, 6.51; N, 6.31. Found: C,
70.02; H, 6.65; N, 6.07.
N -[2-Am in o-4-(2,3,5-t r i-O-b e n zyl-â-D -r ib ofu r a n osyl)-
m eth ylp yr id in -3-yl]a ceta m id e (8). Trifluoracetic acid (1.16
mL, 15 mmol) and Et3SiH (1.20 mL, 7.5 mmol) were added under
argon at 0 °C to a solution of 7 (500 mg, 0.75 mmol) in dry CH2-
Cl2 (10 mL). The resulting solution was stirred overnight at room
temperature and then neutralized with a saturated NaHCO3
(15) Marakos, P.; Pouli, N.; Wise, D. S.; Townsend, L. B. Synlett
1997, 561-562.
6468 J . Org. Chem., Vol. 68, No. 16, 2003

solution. The mixture was extracted with CH2Cl2 (3 × 30 mL)
and the combined organic extracts were dried (Na2SO4) and
concentrated to dryness. The residue was purified by flash
chromatography (silica gel, 18 × 1 cm), using a mixture of CH2-
Cl2/MeOH 98/2 as the eluent to give 8 (217 mg, 51%), together
with the corresponding R-anomer 9 (80 mg, 19%). Data for 8:
1H NMR (CDCl3) δ 1.97 (s, 3H, CH3), 2.62 (dd, 1H, 4-CH2, J )
13.89, 6.58 Hz), 2.96 (dd, 1H, 4-CH2, J ) 13.89, 2.93 Hz), 3.19
(dd, 1H, H-5′, J 5′,4′ ) 5.12 Hz, J 5′,5′ ) 10.24 Hz), 3.38 (dd, 1H,
H-5′, J 5′,4′ ) 3.65 Hz, J 5′,5′ ) 10.24 Hz), 3.49 (dd, 1H, H-2′, J 2′,1′
) 7.67 Hz, J 2′,3′ ) 5.11 Hz), 3.64 (dd, 1H, H-3′, J 3′,2′ ) 5.11 Hz,
J 3′,4′ ) 2.93 Hz), 4.14 (m, 1H, H-4′), 4.25 (m, 1H, H-1′), 4.34-
4.64 (m, 6H, 6 × benzylic methylene H), 4.75 (br s, 2H, NH2,
D2O exchangeable), 6.27 (d, 1H, H-5, J 5,6 ) 5.12 Hz), 7.18-7.38
(m, 15H, 3 × C6H5), 7.81 (d, 1H, H-6, J 6,5 ) 5.12 Hz), 8.82 (br s,
1H, NHAc, D2O exchangeable). 13C NMR (CDCl3) δ 23.2, 34.2,
70.1, 71.9, 72.3, 73.4, 76.2, 78.8, 81.5, 81.7, 116.7, 119.3, 127.8,
127.9, 128.0, 128.1, 128.2, 128.3, 128.4, 128.5, 128.6, 137.3, 141.9,
145.4, 154.9, 168.9. Anal. Calcd for C34H37N3O5: C, 71.94; H,
6.57; N, 7.40. Found: C, 71.76; H, 6.45; N, 7.67. Data for the
R-anomer (9): 1H NMR (CDCl3) δ 2.07 (s, 3H, CH3), 2.52 (d, 1H,
4-CH2, J ) 13.54 Hz), 2.89 (d, 1H, 4-CH2, J ) 13.54 Hz), 3.71
(m, 3H, H-4′, 2 × H-5′), 4.02 (m, 1H, H-2′), 4.24 (m, 1H, H-3′),
4.50-4.69 (m, 6H, H-1′, 5 × benzylic methylene H), 4.85-4.92
(m, 3H, 1 × benzylic methylene H, NH2, D2O exchangeable), 6.31
(d, 1H, H-5, J 5,6 ) 5.12 Hz), 7.24-7.38 (m, 15H, 3 × C6H5), 7.77
(d, 1H, H-6, J 6,5 ) 5.12 Hz), 9.82 (br s, 1H, NHAc, D2O
exchangeable). 13C NMR (CDCl3) δ 23.1, 39.0, 70.0, 71.4, 73.5,
73.6, 74.1, 79.3, 80.2, 82.7, 117.6, 120.2, 127.8, 127.9, 128.0,
128.2, 128.3, 128.4, 128.5, 128.6, 128.7, 137.5, 137.9, 138.0, 141.7,
144.6, 154.7, 169.0. Anal. Calcd for C34H37N3O5: C, 71.94; H,
6.57; N, 7.40. Found: C, 72.08; H, 6.51; N, 7.15.
N -[4-(2,3,5-Tr i-O-b e n zyl-â-D-r ib ofu r a n osyl)m e t h yl-2-
p h th a lim id op yr id in -3-yl]a ceta m id e (10). Phthalic anhydride
(157 mg, 1.06 mmol) was added to a solution of 8 (600 mg, 1.06
mmol) in dry toluene (60 mL) and the reaction mixture was
heated at 50 °C, under argon, overnight. The suspension was
filtered, the solvent was vacuum evaporated, and the residue
was purified by flash chromatography (silica gel, 23 × 2 cm),
using a mixture of CH2Cl2/AcOEt 8/2 as the eluent to give pure
10 (642 mg, 87%) and 11 (30 mg, 5%). Data for 10: Oil. 1H NMR
(CDCl3) δ 1.78 (s, 3H, CH3CO), 2.77 (dd, 1H, 4-CH2, J ) 13.90,
5.48 Hz), 3.12 (dd, 1H, 4-CH2, J ) 13.90, 3.29 Hz), 3.43 (dd, 1H,
H-5′, J 5′,5′ ) 10.35 Hz, J 5′,4′ ) 4.26 Hz), 3.51 (m, 2H, H-5′, H-2′),
3.79 (dd, 1H, H-3′, J ) 2.56, 5.12 Hz), 4.25 (m, 1H, H-4′), 4.34
(m, 2H, H-1′, 1 × benzylic methylene H), 4.43 (br s, 2H, 2 ×
benzylic methylene H), 4.50-4.68 (m, 3H, 3 × benzylic meth-
ylene H), 6.81 (d, 1H, H-5, J 5,6 ) 4.76 Hz), 7.21-7.38 (m, 15H,
3 × C6H5), 7.78 (m, 2H, H-4′′, H5′′), 7.92 (m, 2H, H-3′′, H-6′′),
8.29 (d, 1H, H-6, J 6,5 ) 4.76 Hz), 8.42 (br s, 1H, NHAc, D2O
exchangeable). 13C NMR (CDCl3) δ 22.9, 34.0, 70.2, 72.0, 72.3,
73.2, 76.4, 78.5, 80.3, 82.2, 123.7, 124.0, 126.6, 127.9, 128.0,
128.1, 128.4, 128.5, 128.6, 128.7, 128.8, 131.5, 132.0, 132.4, 134.3,
134.4, 137.4, 137.5, 137.9, 143.1, 145.2, 146.9, 166.1, 166.2, 168.0.
Anal. Calcd for C42H39N3O7: C, 72.29; H, 5.63; N, 6.02. Found:
C, 72.53; H, 5.52; N, 5.95. Data for 2-methyl-7-[(2,3,5-tri-O-
benzyl-â-D-ribofuranosyl)methyl]imidazolo[4,5-b]pyridine (11):
Oil. 1H NMR (CDCl3) δ 2.61 (s, 3H, CH3), 3.21 (dd, 1H, 4-CH2,
J ) 14.19, 7.83 Hz), 3.31 (dd, 1H, 4-CH2, J ) 14.19, 4.41 Hz),
3.48 (dd, 1H, H-5′, J 5′,5′ ) 10.76 Hz, J 5′,4′ ) 4.89 Hz), 3.55 (dd,
1H, H-5′, J 5′,5′ ) 10.76 Hz, J 5′,4′ ) 4.40 Hz), 3.78 (m, 1H, H-2′),
3.89 (m, 1H, H-3′), 4.25 (m, 1H, H-4′), 4.34-4.55 (m, 7H, H-1′,
6 × benzylic methylene H), 7.04 (d, 1H, H-6, J 6,5 ) 4.89 Hz),
7.14-7.33 (m, 15H, 3 × C6H5), 8.19 (d, 1H, H-5, J 5,6 ) 4.89 Hz).
13C NMR (CDCl3) δ 15.4, 35.1, 70.1, 71.8, 71.9, 73.4, 77.1, 80.1,
80.8, 81.1, 118.6, 127.7, 127.8, 127.9, 128.0, 128.2, 128.3, 128.4,
129.2, 130.7, 130.8, 137.6, 137.7, 137.9, 141.8, 153.1. Anal. Calcd
for C34H35N3O4: C, 74.29; H, 6.42; N, 7.64. Found: C, 73.97; H,
6.31; N, 7.78.
7-Am in o-3-(2,3,5-tr i-O-ben zyl-â-D-r ibofu r an osyl)pyr azolo-
[3,4-c]p yr id in e (13). To a suspension of 10 (237 mg, 0.34 mmol)
in dry benzene (50 mL) were added under argon AcOK (33 mg,
0.34 mmol) and Ac2O (0.1 mL, 1.02 mmol). This reaction mixture
was heated at 80° C and isoamyl nitrite (0.07 mL, 0.51 mmol)
was then added dropwise. The resulting mixture was heated at
reflux for 6 h. The insoluble material was filtered off and washed
with hot toluene (10 mL), and the combined filtrates were
evaporated to dryness to afford a yellowish solid (215 mg),
corresponding to a mixture of the acetamides 12. A saturated
methanolic ammonia solution (50 mL) was added to the above
mixture and the resulting solution was stirred at room temper-
ature overnight. The solvent was removed in vacuo and the
residue was purified by flash chromatorgaphy (silica gel, 25 ×
1 cm), using a mixture of CH2Cl2/MeOH 95/5 as the eluent, to
give pure 13 (153 mg, 84%) as a white foam. 1H NMR (CDCl3)
δ 3.63 (dd, 1H, H-5′, J 5′,5′ ) 10.27 Hz, J 5′,4′ ) 3.43 Hz), 3.79 (dd,
1H, H-5′, J 5′,5′ ) 10.27 Hz, J 5′,4′ ) 2.94 Hz), 4.17 (m, 1H, H-3′),
4.33 (m, 1H, H-2′), 4.40 (m, 1H, H-4′), 4.48-4.66 (m, 6H, 6 ×
benzylic methylene H), 5.50 (d, 1H, H-1′, J 1′,2′ ) 5.38 Hz), 6.90
(d, H-4, J 4,5 ) 5.86 Hz), 7.16-7.35 (m, 15H, 3 × C6H5), 7.47 (d,
1H, H-5, J 5,4 ) 5.86 Hz). 13C NMR (CDCl3) δ 69.4, 72.6, 72.6,
73.7, 77.3, 77.8, 81.1, 81.2, 104.8, 122.8, 128.1, 128.6, 128.7,
132.9, 136.2, 137.4, 137.6, 137.8, 140.0, 148.3. Anal. Calcd for
C
32H32N4O4: C, 71.62; H, 6.01; N, 10.44. Found: C, 71.81; H,
6.12; N, 10.09.
7-Am in o-3-(â-D-r ibofu r an osyl)pyr azolo[3,4-c]pyr idin e (14).
A solution of BCl3 (1 M in hexane, 2.46 mL) was added dropwise
under argon to a solution of 13 (110 mg, 0.205 mmol) in dry
CH2Cl2 (10 mL) at -78 °C. The reaction mixture was stirred at
-78 °C for 1 h and then a cold solution of CH2Cl2/MeOH (1/1.8
mL) was added. The solvents were evaporated and the residue
was purified by flash chromatography (silica gel, 4.5 × 1.2 cm),
using a mixture of CH2Cl2/MeOH 8/2 as the eluent to give pure
14 (44 mg, 81%) as an oil. 1H NMR (CD3OD) δ 3.77 (dd, 1H,
H-5′, J 5′,5′ ) 12.36 Hz, J 5′,4′ ) 4.30 Hz), 3.86 (dd, 1H, H-5′, J 5′,5′
) 12.36 Hz, J 5′,4′ ) 3.23 Hz), 4.08 (m, 1H, H-4′), 4.17 (m, 2H,
H-2′, H-3′), 5.16 (d, 1H, H-1′, J 1′,2′ ) 6.45 Hz), 7.21 (d, H-4, J 4,5
) 6.72 Hz), 7.29 (d, 1H, H-5, J 5,4 ) 6.72 Hz). 13C NMR (CD3OD)
δ 62.9, 72.7, 76.9, 78.2, 87.6, 106.3, 121.5, 123.9, 136.4, 138.8,
151.4. Anal. Calcd for C11H14N4O4: C, 49.62; H, 5.30; N, 21.04.
Found: C, 49.72; H, 5.22; N, 20.88.
Ack n ow led gm en t. The present study was sup-
ported by a grant from the National Scholarship Foun-
dation of Greece.
J O026715X
J . Org. Chem, Vol. 68, No. 16, 2003 6469