A general and efficient synthesis of pyridin-2-yl C-ribonucleosides bearing diverse alkyl, aryl, amino, and carbamoyl groups in position 6

Article abstract of DOI:10.1021/jo902313g

(Chemical Equation Presented) An efficient and practical methodology of preparation of 6-substituted pyridin-2-yl C-ribonucleosides was developed. A one-pot two-step addition of 2-lithio-6-bromopyridine to TBS-protected ribonolactone followed by acetylation gave 1β-(6-bromopyridin-2-yl)-1-O- acetyl-2,3,5-tri-O-(tertbutyldimethylsilyl)-D-ribofuranose in high yield. Its reduction with Et₃SiH and BF₃·Et₂O afforded the desired TBS-protected 6-bromopyridine C-ribonucleoside as pure β-anomer in good overall yield of 63%. This intermediate was then subjected to a series of palladium catalyzed cross-coupling reactions, aminations and aminocarbonylations to give a series of protected 1β-(6-alkyl-, 6-aryl-, 6- amino-, and 6-carbamoylpyridin-2-yl)-C-ribonucleosides. Deprotection of silylated nucleosides by Et₃N·3HF gave a series of title free C-ribonucleosides (12 examples).

Full text of DOI:10.1021/jo902313g

pubs.acs.org/joc
A General and Efficient Synthesis of Pyridin-2-yl C-Ribonucleosides
Bearing Diverse Alkyl, Aryl, Amino, and Carbamoyl Groups in Position 6
ꢀ
Martin Stefko, Lenka Slavetınska, Blanka Klepetarova, and Michal Hocek*
ꢀ
ꢁ
ꢁꢀ ꢁ
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Gilead &
IOCB Research Center, Flemingovo naꢁm. 2, CZ-16610 Prague 6, Czech Republic
hocek@uochb.cas.cz
Received October 29, 2009
An efficient and practical methodology of preparation of 6-substituted pyridin-2-yl C-ribonucleo-
sides was developed. A one-pot two-step addition of 2-lithio-6-bromopyridine to TBS-protected
ribonolactone followed by acetylation gave 1β-(6-bromopyridin-2-yl)-1-O-acetyl-2,3,5-tri-O-(tert-
butyldimethylsilyl)-D-ribofuranose in high yield. Its reduction with Et3SiH and BF3 Et2O afforded
3
the desired TBS-protected 6-bromopyridine C-ribonucleoside as pure β-anomer in good overall yield
of 63%. This intermediate was then subjected to a series of palladium catalyzed cross-coupling
reactions, aminations and aminocarbonylations to give a series of protected 1β-(6-alkyl-, 6-aryl-, 6-
amino-, and 6-carbamoylpyridin-2-yl)-C-ribonucleosides. Deprotection of silylated nucleosides by
Et3N 3HF gave a series of title free C-ribonucleosides (12 examples).
3
Introduction
extension of the genetic alphabet.2 Among many (het)aryl
nucleosides forming selective hydrophobic pairs in stable
DNA duplexes due to increased packing and hydrophobic
interactions,3 several promising artificial base-pairs showed
efficient and specific replication by DNA polymerases.4 The
most successful pairs based on 4-substituted 2-methoxyphe-
nyl C-nucleoside in combination with thioisocarbostyril base
has been found5 to be efficiently and selectively replicated
and extended and very recently also the first successful PCR
C-Nucleosides are important stable analogues of natural
N-nucleosides, and their syntheses and many applications in
chemical biology and medicinal chemistry have been subject
to several recent comprehensive reviews.1 Aryl C-20-deoxyri-
bonucleosides usually do not exhibit any pharmaceutically
interesting biological activities but have attracted prominent
attention as candidates for novel base-pairs in the quest for
*To whom correspondence should be addressed. Phone: þ420 220183324.
Fax: þ420 220183559.
(4) (a) Henry, A. A.; Olsen, A. G.; Matsuda, S.; Yu, C.; Geierstanger,
B. H.; Romesberg, F. E. J. Am. Chem. Soc. 2004, 126, 6923–6931. (b) Henry,
A. A.; Yu, C. Z.; Romesberg, F. E. J. Am. Chem. Soc. 2003, 125, 9638–9646.
(c) Hwang, G. T.; Romesberg, F. E. Nucleic Acids Res. 2006, 34, 2037–2045.
(d) Matsuda, S.; Henry, A. A.; Romesberg, F. E. J. Am. Chem. Soc. 2006,
128, 6369–6375. Kim, Y.; Leconte, A. M.; Hari, Y.; Romesberg, F. E. Angew.
Chem., Int. Ed. 2006, 45, 7809–7812. (e) Matsuda, S.; Fillo, J. D.; Henry,
A. A.; Rai, P.; Wilkens, S. J.; Dwyer, T. J.; Geierstanger, B. H.; Wemmer,
D. E.; Schultz, P. G.; Spraggon, G.; Romesberg, F. E. J. Am. Chem. Soc.
2007, 129, 10466–10473. (f) Matsuda, S.; Leconte, A. M.; Romesberg, F. E. J.
Am. Chem. Soc. 2007, 129, 5551–5557. (g) Hari, Y.; Hwang, G. T.; Leconte,
A. M.; Joubert, N.; Hocek, M.; Romesberg, F. E. ChemBioChem 2008, 9,
2796–2799.
ꢀ
ꢀ
ꢁ
(1) (a) Stambaskꢁy, J.; Hocek, M.; Kocovsky, P. Chem. Rev. 2009, 109,
6729-6764. (b) Adamo, M. F. A.; Pergoli, R. Curr. Org. Chem. 2008, 12, 1544–
1569. (c) Wu, Q. P.; Simons, C. Synthesis 2004, 1533–1553.
(2) Reviews: (a) Wang, L.; Schultz, P. G. Chem. Commun. 2002, 1–11. (b)
Henry, A. A.; Romesberg, F. E. Curr. Opin. Chem. Biol. 2003, 7, 727–733. (c)
Kool, E. T.; Morales, J. C.; Guckian, K. M. Angew. Chem., Int. Ed. 2000, 39,
990–1009. (d) Kool, E. T. Acc. Chem. Res. 2002, 35, 936–943.
(3) (a) Ogawa, A. K.; Abou-Zied, O. K.; Tsui, V.; Jimenez, R.; Case,
D. A.; Romesberg, F. E. J. Am. Chem. Soc. 2000, 122, 9917–9920. Wu, Y. Q.;
Ogawa, A. K.; Berger, M.; Mcminn, D. L.; Schultz, P. G.; Romesberg, F. E.
J. Am. Chem. Soc. 2000, 122, 7621–7632. (c) Guckian, K. M.; Krugh, T. R.;
Kool, E. T. J. Am. Chem. Soc. 2000, 122, 6841–6847. (d) Parsch, J.; Engels, J.
W. J. Am. Chem. Soc. 2002, 124, 5664–5672. (e) Lai, J. S.; Qu, J.; Kool, E. T.
Angew. Chem., Int. Ed. 2003, 42, 5973–5977. (f) Lai, J. S.; Kool, E. T. J. Am.
Chem. Soc. 2004, 126, 3040–3041.
(5) (a) Leconte, A. M.; Hwang, G. T.; Matsuda, S.; Capek, P.; Hari, Y.;
Romesberg, F. E. J. Am. Chem. Soc. 2008, 130, 2336–2343. (b) Seo, Y. J.;
Romesberg, F. E. ChemBioChem 2009, 10, 2394–2400.
442 J. Org. Chem. 2010, 75, 442–449
Published on Web 12/14/2009
DOI: 10.1021/jo902313g
r
2009 American Chemical Society

ꢀ
Stefko et al.
JOCArticle
with a 6-letter genetic alphabet was reported.6 The second
step of the extended genetic code is the transcription to RNA
which was also shown7 to be feasible with the above-
mentioned base-pair and which is now awaiting systematic
exploration. For this purpose, aryl C-ribonucleosides are
needed as building blocks. So far, the C-ribonucleosides
were mainly targets for medicinal chemistry. The most
important classes of cytostatic C-ribonucleosides are car-
bamoylhetaryl C-ribonucleosides8 (tiazofurine and related
analogues of nicotinamine ribonucleoside) functioning as
precursors of inhibitors of inosine 50-monophosphate de-
hydrogenase (IMPDH) and immucilins inhibiting purine
nucleoside phosphorylase.9 Some pyridine C-ribonucleo-
sides have been prepared for the use in modification of
ribozymes10,11 mainly to study the mechanism of catalysis.
The currently most commonly used synthetic app-
roaches1 to C-nucleosides are (i) additions of organometal-
lics to lactones,5,12 (ii) couplings of a halogenoses with
organometallics,13 (iii) electrophilic substitutions of elec-
tron-rich aromatics with sugars under Lewis acid catalysis,14
or (iv) Heck coupling of aryl iodides with glycals.15 Since
none of them is truly general and some of them are non-
selective and inefficient, there is still great need for develop-
ment of efficient synthetic procedures. We are currently
involved in development of modular methodologies suitable
for a generation of functional group diversity in the last one
to two step(s) based on preparation of general halo(het)aryl-
C-nucleoside intermediates followed by displacement of the
halogene for alkyl, aryl or amino substituents by cross-
coupling reactions. We have already developed modular
syntheses of 3- and 4-substituted benzene C-nucleosides,16
6-substituted pyridin-2-yl17 and pyridin-3-yl,18 5-substituted
thiophen-2-yl19 C-20-deoxyribonucleosides and carbamoyl-
phenyl20 C-ribonucleosides. Here we report on the synthesis
of a hitherto missing21 series of 6-substituted pyridin-2-yl
C-ribonucleosides.
(6) Malyshev, D.; Seo, Y. J.; Ordoukhanian, P.; Romesberg, F. E. J. Am.
Chem. Soc. 2009, 131, 14620–14621.
Results and Discussion
(7) Seo, Y. J.; Matsuda, S.; Romesberg, F. E. J. Am. Chem. Soc. 2009,
131, 5726–5727.
Our selected approach for the preparation of pyridine C-
ribonucleosides was based on synthesis of 6-bromopyri-
dine-2-yl C-ribonucleoside intermediate and its follow-up
functional group transformation by Pd-catalyzed cross-
couplings, aminations, and aminocarbonylations. For the
preparation of the bromopyridyl nucleoside intermediate,
we have envisaged the use of an analogous approach to the
previously reported17 synthesis of the corresponding 2-pyri-
dyl C-20-deoxyribonucleosides: addition of 2-lithio-6-bro-
mopyridine 2 to easily available TBS-protected ribonolac-
tone 1. The monolitiated 2-bromopyridine 2 was generated
from 2,6-dibromopyridine by a reverse addition technique
according to Cai et al.22 Due to the thermal instability of 2,
the lithiation must have been performed very carefully under
efficient cooling. A solution of 2,6-dibromopyridine in THF
was added to the solution of BuLi within exactly 7 min at
-78 °C, and after the resulting solution was stirred for 15 min
under cooling, a solution of lactone 1 was added and the
reaction proceeded for another 30 min before quenching and
workup. When only a small excess (1.5 equiv) of 2 over
(8) Recent reviews: (a) Chen, L.; Petrelli, R.; Felczak, K.; Gao, G.;
Bonnac, L.; Yu, J. S.; Bennett, E. M.; Pankiewicz, K. W. Curr. Med. Chem.
2008, 15, 650–670. (b) Chen, L. Q.; Pankiewicz, K. W. Curr. Opin. Drug
Discov. Devel. 2007, 10, 403–412. (c) Pankiewicz, K. W.; Patterson, S. E.;
Black, P. L.; Jayaram, H. N.; Risal, D.; Goldstein, B. M.; Stuyver, L. J.;
Schinazi, R. F. Curr. Med. Chem. 2004, 11, 887–900. Examples: (d) Krohn,
K.; Heins, H.; Wielckens, K. J. Med. Chem. 1992, 35, 511–517. (e) Jayaram,
H. N.; Gharehbaghi, K.; Jayaram, N. H.; Rieser, J.; Krohn, K.; Paull, K. D.
Biochem. Biophys. Res. Commun. 1992, 186, 1600–1606. (f) Srivastava, P. C.;
Pickering, M. V.; Allen, L. B.; Streeter, D. G.; Campbell, M. T.; Witkowski,
J. T.; Sidwell, R. W.; Robins, R. K. J. Med. Chem. 1977, 20, 256–262. (g)
Robins, R. K.; Srivastava, P. C.; Narayanan, V. L.; Plowman, J.; Paull, K. D.
J. Med. Chem. 1982, 25, 107–108. (h) Gebeyehu, G.; Marquez, V.; Vancott,
A.; Cooney, D. A.; Kelley, J. A.; Jayaram, H. N.; Ahluwalia, G. S.; Dion, R.
L.; Wilson, Y. A.; Johns, D. G. J. Med. Chem. 1985, 28, 107–108. (i)
Franchetti, P.; Cappellacci, L.; Perlini, P.; Jayaram, H. N.; Butler, A.;
Schneider, B. P.; Collart, F. R.; Huberman, E.; Grifantini, M. J. Med. Chem.
1998, 41, 107–108. (j) Franchetti, P.; Marchetti, S.; Cappellacci, L.; Jayaram,
H. N.; Yalowitz, J. A.; Goldstein, B. M.; Barascut, J. L.; Dukhan, D.;
Imbach, J. L.; Grifantini, M. J. Med. Chem. 2000, 43, 1264–1270. (k)
Zatorski, A.; Watanabe, K. A; Carr, S. F.; Goldstein, B. M.; Pankiewicz,
K. W. J. Med. Chem. 1996, 39, 2422–2426.
(9) (a) Miles, R. W.; Tyler, P. C.; Furneaux, R. H.; Bagdassarian, C. K.;
Schramm, V. L. Biochemistry 1998, 37, 8615–8621. (b) Kicska, G. A.; Long,
€
L.; Horig, H.; Fairchild, C.; Tyler, P. C.; Furneaux, R. H.; Schramm, V. L.;
Kaufman, H. L. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 4593–4598. (c) Evans,
G. B.; Furneaux, R. H.; Gainsford, G. J.; Hanson, J. C.; Kicska, G. A.;
Sauve, A. A.; Schramm, V. L.; Tyler, P. C. J. Med. Chem. 2003, 46, 155–160.
(10) (a) Matulic-Adamic, J.; Biegelman, L. Tetrahedron Lett. 1996, 37,
6973–6976. (b) Matulic-Adamic, J.; Biegelman, L.; Portmann, S.; Egli, M.;
Usman, N. J. Org. Chem. 1996, 61, 3909–3911. (c) Matulic-Adamic, J.;
Biegelman, L. Tetrahedron Lett. 1997, 38, 203–206. (d) Matulic-Adamic, J.;
Biegelman, L. Tetrahedron Lett. 1997, 38, 1669–1672. (e) Burgin, A. B.;
Ginzales, C.; Matulic-Adamic, J.; Karpiesky, A. M.; Usman, N.; McSwiggen,
J. A.; Beigelman, L. Biochemistry 1996, 35, 14090–14097.
(15) (a) Wang, Z. X.; Wiebe, L. I.; De Clercq, E.; Balzarini, J.; Knaus, E.
€
E. Can. J. Chem. 2000, 78, 1081–1088. (b) Haberli, A.; Leumann, C. J. Org.
Lett. 2001, 3, 489–492. (c) Sun, Z.; Ahmed, S.; McLaughlin, L. W. J. Org.
Chem. 2006, 71, 2922–2926.
ꢁꢀ
ꢁ
ꢁꢀ
(16) (a) Hocek, M.; Pohl, R.; Klepetarova, B. Eur. J. Org. Chem. 2005,
ꢀ
ꢁ
4525–4528. (b) Stefko, M.; Pohl, R.; Klepetarova, B.; Hocek, M. Eur. J. Org.
Chem. 2008, 1689–1704. (c) Joubert, N.; Urban, M.; Pohl, R.; Hocek, M.
Synthesis 2008, 1918–1932.
ꢁꢀ
ꢁ
(11) Lu, J.; Li, N.-S.; Koo, S. C.; Piccirilli, J. A. J. Org. Chem. 2009, 74,
8021–8030.
(17) Urban, M.; Pohl, R.; Klepetarova, B.; Hocek, M. J. Org. Chem.
2006, 71, 7322–7328.
ꢁꢀ
ꢁ
(12) (a) Matsuda, S.; Romesberg, F. E. J. Am. Chem. Soc. 2004, 126,
14419–14427. (b) Mathis, G.; Hunziker, J. Angew. Chem., Int. Ed. 2002, 41,
(18) Joubert, N.; Pohl, R.; Klepetarova, B.; Hocek, M. J. Org. Chem.
2007, 72, 6797–6805.
€
ꢁ
ꢁꢀ ꢁ
(19) Barta, J.; Pohl, R.; Klepetarova, B.; Ernsting, N. P.; Hocek, M. J.
3203–3205. (c) Brotschi, C.; Haberli, A.; Leumann, C. J. Angew. Chem., Int.
Ed. 2001, 40, 3012–3014. (d) Brotschi, C.; Mathis, G.; Leumann, C. J.
Chem.;Eur. J. 2005, 11, 1911–1923. (e) Zahn, A.; Brotschi, C.; Leumann,
C. J. Chem.;Eur. J. 2005, 11, 2125–2129. (f) Reese, C. B.; Wu, Q. Org.
Biomol. Chem. 2003, 1, 3160–3172.
(13) (a) Chaudhuri, N. C.; Kool, E. T. Tetrahedron Lett. 1995, 36, 1795–
1798. (b) Ren, R. X.-F.; Chaudhuri, N. C.; Paris, P. L.; Rumney, S. IV; Kool,
E. T. J. Am. Chem. Soc. 1996, 118, 7671–7678. (c) Griesang, N.; Richert,
C. Tetrahedron Lett. 2002, 43, 8755–8758. (d) Aketani, S.; Tanaka, K.;
Yamamoto, K.; Ishihama, A.; Cao, H.; Tengeiji, A.; Hiraoka, S.; Shiro, M.;
Shinoya, M. J. Med. Chem. 2002, 45, 5594–5603. (e) Hainke, S.; Singh, I.;
Hemmings, J.; Seitz, O. J. Org. Chem. 2007, 72, 881–8819.
Org. Chem. 2008, 73, 3798–3806.
ꢀ
(20) Stefko, M.; Pohl, R.; Hocek, M. Tetrahedron 2009, 65, 4471–4483.
(21) The only known examples prepared in the past by low-yielding
multistep syntheses are 13a and 13j. Syntheses of 13a: (a) Belmans, M.;
Vrijens, I.; Esmans, E.; Dommisse, R.; Lepoivre, J.; Alderweireldt, F.;
Townsend, L.; Wotring, L.; Balzarini, J.; De Clercq, E. Nucleosides Nucleo-
tides 1986, 5, 441–455. (b) De Vos, E.; Esmans, E. L.; Lepoivre, J. A.;
Alderweireldt, F. C.; Dommisse, R. A.; Franc-ois, P.; Touillaux, R.; Balzarini,
J.; De Clercq, E. Nucleosides Nucleotides 1991, 10, 1573–1598. Syntheses of
13j:. (c) Kabat, M. M.; Pankiewicz, K. W.; Sochacka, E.; Watanabe, K. A.
Chem. Pharm. Bull. 1988, 36, 634–640. (d) Pankiewicz, K. W.; Zeidler, J.;
Ciszewski, L. A.; Bell, J. E.; Goldstein, B. M.; Jayaram, H. N.; Watanabe, K.
A. J. Med. Chem. 1993, 36, 1855–1859.
(14) (a) Yokoyama, M.; Nomura, M.; Togo, H.; Seki, H. J. Chem. Soc.,
Perkin Trans. 1 1996, 2145–2149. (b) He, W.; Togo, H.; Yokoyama, M.
Tetrahedron Lett. 1997, 38, 5541–5544. (c) Hainke, S.; Arndt, S.; Seitz, O.
Org. Biomol. Chem. 2005, 3, 4233–4238.
(22) Cai, D.; Hugnes, D. L.; Verhoeven, T. R. Tetrahedron Lett. 1996, 37,
2537–2540.
J. Org. Chem. Vol. 75, No. 2, 2010 443

ꢀ
Stefko et al.
JOCArticle
SCHEME 1a
aConditions: (a) 2 (4 equiv), THF, -78 °C, 15 min (75%); (b) Et3SiH (3 equiv), BF3 Et2O (1.2 equiv), DCM, -20 °C (0%); (c) AcCl/DMAP, pyridine,
3
50 °C, 12 h, 20% for details see the Supporting Information; (d) 2 (4 equiv), THF, -78 °C, 15 min, then Ac2O, -78 °Cf rt (70%); (e) Et3SiH (3 equiv),
BF3 Et2O (1.2 equiv), hexanes, -20 °C (91%).
3
lactone 1 was used, the desired hemiketal 3 was isolated in
moderate 20% yield. Increasing the excess of 2 to 4 equiv led
to complete consumption of lactone 1 within 10 min to give
the target hemiketal 3 in very good 75% yield (Scheme 1, step
a). The structure of 3 in CDCl3 solution was proved by NMR
(NOE) as a pure β-anomeric hemiketal (as opposed to
mixture of open hydroxy ketone and both anomeric hemi-
ketals reported previously17 for solution of the analogous
20-deoxyribo derivative).
yield accompanied by hemiketal 3 (21%) and another side
product 5 (10%). Compound 5 has been characterized by
NMR and X-ray diffraction (Figure 1) as product of lithia-
tion of 2,6-dibromopyridine to position 4 followed by addi-
tion to lactone and acetylation. To avoid its formation, the
lithiation protocol was reoptimized. Finally, we used a
diluted (0.3 M) solution of n-BuLi in hexanes for the lithia-
tion of 2,6-dibromopyridine in THF at -78 °C followed by
addition to 1, slow quenching with Ac2O, and warming to
ambient temperature to prepare acetate 4 in good yield of
70% accompanied by only a trace amount of 5 (<2%). The
acetylated hemiketal 4 was isolated and characterized as a
pure β-anomer.
The second step was the reduction of hemiketal intermedi-
ate 3 to ether (C-ribonucleoside) 6. This reaction is usually
performed by using Et3SiH in presence of BF3 Et2O.
3
Although on pyridine 20-deoxyribonucleosides17 and on
benzene ribonucleosides16b these reactions worked perfectly
under standard conditions, the reduction of 3 did not
proceed even after extensive optimization (for details see
the Supporting Information). Since there were some litera-
ture examples10d,23 of efficient reductive 10-deacetoxylations
as an alternative 10-deoxygenation in these types of hemi-
ketals, we have pursued acetylation of hemiketal 3. It may be
converted into a kinetic (β-anomer 4) or a thermodynamic
(R-anomer) by acetylation either in situ or after isolation of
the hemiketal.23b Unfortunately, acylation of 3 with Ac2O,
AcCl, or AcCl/DMAP in pyridine was inefficient to give the
target 10-OAc derivative 4 only in a maximum 21% yield
(Scheme 1, step c, for details see the Supporting Information).
In order to increase the nucleophilicity of the 10-OH group,
we turned it into an alkoxide by treatment with LiHMDS at
-78 °C prior to addition of Ac2O to give the acetate 4 in only
slightly improved yield of 34%. Further optimizations of the
base, reaction time and acylating agent did not bring any
improvement (see the Supporting Information).
TABLE 1. Optimization of Reduction of Acetate 4 to C-Nucleosides 6
and 7 (Step e) in Scheme 1a
entry
solvent
temp (°C) time (min) yield (%) β/R ratio 6/7
1
2
3
4
5
6
DCM
DCM
DCM
CH3NO2
toluene
hexanes
-78
-40
0
-40
-20
-20
10
5
5
10
5
5
71
85
87
88
90
91
85:15
83:17
77:23
83:17
95:5
99:1
aGeneral conditions: Et3SiH (3 equiv), BF3 Et2O (1.2 equiv).
3
Reduction of the hemiketal ester 4 was then performed
with Et3SiH/BF3 Et2O under different conditions (Table 1).
3
The stereoselectivity (β- and R-anomeric products 6 and 7)
was strongly dependent on a solvent and temperature. In
DCM at -78 °C, a mixture of 6 and 7 (85:15 ratio) was
obtained in 71% yield (entry 1), while at -40 °C the yield was
increased to 85% without a dramatic change of stereoselec-
tivity (entry 2). Increasing the temperature to 0 °C caused a
decrease of selectivity to 77:23 (entry 3). Switching to more
polar solvent (CH3NO2) at -40 °C gave the same yield and
selectivity as in DCM (entry 4). On the other hand, the use of
the less polar toluene at -20 °C increased the selectivity to
95:5 with 90% yield (entry 5). Finally, the optimized proce-
dure was performed in hexanes at -20 °C to achieve an
excellent 91% yield of reduction with superior β/R selectivity
99:1 (entry 6). By the optimized sequence consisting of
Therefore, we focused on a one-pot addition of 2 to
lactone 1 directly followed by acetylation of the in situ
generated hemiketal alkoxide by Ac2O (Scheme 1, step d).
This protocol gave the desired acetyl hemiketal 4 in 38%
(23) (a) Dondoni, A.; Scherrmann, M.-C. J. Org. Chem. 1994, 59, 6404–
6412. (b) Gudmundsson, K. S.; Drach, J. D.; Townsend, L. B. Tetrahedron
Lett. 1996, 37, 2365–2368.
444 J. Org. Chem. Vol. 75, No. 2, 2010

ꢀ
Stefko et al.
JOCArticle
FIGURE 1. Structural formula (a) and ORTEP drawing of crystal structure (b) of side product 5. Thermal ellipsoids are drawn at the 30%
probability level, and hydrogens on TBS-groups were omitted for clarity. CCDC 752141.
SCHEME 2a
revealed that it is a pure R-anomer (as opposed to β-anomer
of the analogous pyridine derivative, vide supra). Unfortu-
nately, also in this case the reduction of 9 with Et3SiH/
BF3 Et2O did not proceed. In analogy to previous experi-
3
ments, acylated hemiketal 10 was prepared by addition of 8
to lactone 1 followed by Ac2O quenching. Surprisingly, this
reaction proceeded with opposite stereoselectivity to give
acetate 10 as pure β-anomer in 45% yield. It was then
reduced with Et3SiH/BF3 Et2O in DCM at 0 °C. The
3
product 11 isolated in 66% yield was characterized as a pure
undesired R-anomer. It seems that the presence of the
N-oxide reverted the stereoselectivity of each step from
retention to inversion of the configuration probably due to
different kinetic stability or thermodynamic equilibria of the
R- and β-anomeric hemiketal intermediates. The reactions
are interesting, but apparently, this approach was not
suitable for the preparation of 6.
With key intermediate 6 in hand, we turned our attention
to cross-coupling,17 amination,17 and aminocarbonylation20
reactions (Scheme 3, Table 2). Cross-coupling reactions with
organoaluminum or organozinc reagents were used for in-
troduction of sp3 (alkyl and benzyl) substituents. The reac-
tions of 6 with trimetylaluminum, trietylaluminum, and
benzylzinc bromide were performed under standard condi-
tions in the presence of Pd(PPh3)4 in THF at 65 °C without
any optimization. In all cases, desired nucleosides 12a-c
were obtained in excellent yields (90-92%) (entries 1-3). 6-
Phenylpyridine C-ribonucleoside 12d was prepared by the
Suzuki-Miyaura cross-coupling of 6 with phenylboronic
acid in toluene in the presence of K2CO3 and Pd(PPh3)4 at
100 °C in 92% yield (entry 4).
In order to introduce hetaryl substituents, the Stille cross-
coupling reactions in DMF using PdCl2(PPh3)2 were used.
Reactions of 6 with 2-thienyl(tributyl)stannane (entry 5) and
2-furyl(tributyl)stannane (entry 6) proceeded very smoothly
within 1.5 h at 100 °C to give the desired 4-(2-thienyl)-
pyridine 12e and 4-(2-furyl)pyridine C-ribonucleosides 12f
in 90% and 92% yield, respectively. On the other hand,
reaction of 6 with 2-pyridyl(tributyl)stannane (entry 7) re-
quired a longer time to reach complete conversion, and
moreover, formation of some byproduct was observed.
The extraction and column chromatography isolation of
aConditions: (a) 8 (2 equiv), THF, -78 °C, 30 min, (60%); (b) 8 (2 equiv),
THF, -78 °C, 15 min, then Ac2O, -78 °C f rt (45%) 8; (c) Et3SiH (3
equiv), BF3 Et2O (1.2 equiv), 20 min, DCM, 0 °C.
3
one-pot lithiation, addition, and acetylation followed by
reduction in hexanes, the target TBS-protected 6-bromopyr-
idine-2-yl C-ribonucleoside intermediate 6 was prepared (as
pure β-anomer determined by NOE) in 63% isolated overall
yield based on lactone 1.
In parallel, we were also interested in studying of an
alternative approach for the preparation of pyridine-C-
ribonucleosides based on addition of 2-lithio-6-bromopyri-
dine N-oxide 8 to lactone 1 (Scheme 2). An initial idea was to
increase electron density in position 2 of the pyridine moiety
(“umpolung” of the molecule), which may facilitate reduc-
tion of the free hemiketal without the need of acetylation. To
prove this hypothesis, an addition of 2-lithio-6-bromopyridine
N-oxide 8 to lactone 1 was performed resulting in pyridine
N-oxide hemiketal 9 in 60% yield. The NOESY spectrum
J. Org. Chem. Vol. 75, No. 2, 2010 445

ꢀ
Stefko et al.
JOCArticle
SCHEME 3. Synthesis of a Series of Pyridine C-Ribonucleosides
TABLE 2. Cross-Couplings, Aminations and Aminocarbonylations of Bromopyridyl C-Ribonucleoside 6 Followed by Deprotection
reaction
(yield, %)
deprotection
(yield, %)
entry
reagent
catalyst
ligand/base
solvent
conditions
1
2
3
4
5
6
7
8
9
10
Me3Al
Et3Al
BnZnBr
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd2dba3
THF
THF
THF
toluene
DMF
DMF
72 h, 65 °C
48 h, 65 °C
24 h, 65 °C
12 h, 100 °C
1.5 h, 100 °C
1.5 h, 100 °C
24 h, 100 °C
3.5 h, 60 °C
4.5 h, 40 °C
3 h, 80 °C
12a (92)
12b (91)
12c (90)
12d (92)
12e (90)
12f (92)
12g (61)
12h (86)
12i (86)
12j (78)
13a (87)
13b (89)
13c (90)
13d (89)
13e (89)
13f (90)
13g (88)
13h (90)
13i (89)
13j (90)
PhB(OH)2
K2CO3
2-Bu3Sn-thiophene
2-Bu3Sn-furane
2-Bu3Sn-pyridine
LiN(SiMe3)2
DMF
THF
cHex JohnPhosa,
JohnPhos,b tBuONa
Xantphosc/K3PO4
Me2NH HCl
3
Pd2dba3
Pd(OAc)2
toluene
toluene/
DMSO
toluene/
DMSO
toluene
NH4Cl/CO(1 atm)
11
12
MeNH2 HCl/CO(1 atm)
3
Pd(OAc)2
Pd(OAc)2
Xantphosc/K3PO4
Xantphosc/K3PO4
3 h, 80 °C
12k (82)
12l (83)
13k (93)
13l (88)
Me2NH HCl/CO(1 atm)
3
3 h, 80 °C
acHex JohnPhos = (2-biphenyl)dicyclohexylphosphane. bJohnPhos = (2-biphenyl)di-tert-butylphosphane. Xantphos = 4,5-Bis(diphenylphos-
phino)-9,9-dimethylxanthene.
c
the resulting nucleoside 12g from lipophilic tin-containing
side products was more complicated, reflected in the lower
isolated yield of 61%.
The Hartwig-Buchwald24 reactions were used for intro-
duction of N-substituents. Unsubstituted aminopyridine
derivate 12h was prepared by the reaction of 6 with lithium
bis(trimetylsilyl)amide in the presence of Pd2dba3 and
Buchwald-type ligand (2-biphenyl)dicyclohexylphosphane
(cHex JohnPhos)25 in 86% yield (Table 2, entry 8). The
Buchwald reaction was also used for the introduction of the
dimethylamino group. The reaction of 6 with dimethylamine
hydrochloride (Me2NH HCl) was performed under stan-
dard conditions, using Pd2dba3, 2-(di-tert-butylphosphanyl)-
biphenyl (JohnPhos),25 and t-BuONa as a base at 40 °C for
4.5 h to give the target dimethylaminopyridine C-ribonucleoside
12i in 86% yield (Table 2, entry 9) .
Amides 12j-l were prepared by Pd-catalyzed aminocarbo-
nylation reactions20 of 6 with corresponding amine hydrochlor-
ides under atmospheric CO pressure in presence of 5 mol % of
Pd(OAc)2, 10 mol % of Xanthphos, and K3PO4. All reac-
tions were finished within 3 h at 80 °C yielding pyridine-6-
carboxamide C-ribonucleosides 12j (78%), 12k (82%), and
12l (83%).
FIGURE 2. Structural formula (a) and ORTEP drawing of crystal
structure (b) of C-nucleoside 13i. Thermal ellipsoids are drawn at
the 50% probability level. CCDC 752142.
3
For final deprotection of the silylated nucleosides 12a-l,
treatment with Et3N 3HF26 was used. Heating at 40 °C for 2
3
days followed by treatment with NaHCO3 resulted in forma-
tion of the desired free nucleosides 13a-l. Chromatographic
purification on reversed-phase flash chromatography and
subsequent lyophylization gave free nucleosides 13a-l in
excellent yields (87-93%, Table 2, entries 1-12).
Crystal structure of free C-ribonucleoside 13i (Figure 2)
reveals the C20-endo (S) conformation typical for 20-deoxyr-
ibonucleosides. On the other hand, in DMSO solution the
series of C-ribonucleosides 13a-l shows a preference for the
expected C30-endo (N) conformation as shown by analysis of
interaction constants in 1H NMR (see the Supporting
Information). However, remeasuring 13i in chloroform
(24) Yang, B. H.; Buchwald, S. L. J. Organomet. Chem. 1999, 576, 125–
146.
(25) (a) Wolfe, J. P.; Tomori, H.; Sadighi, J. P.; Yin, J.; Buchwald, S. L. J.
Org. Chem. 2000, 65, 1158–1174. (b) Lee, D.-Y.; Hartwig, J. F. Org. Lett.
2005, 7, 1169–1172.
ꢀ
(26) Capek, P.; Pohl, R.; Hocek, M. J. Org. Chem. 2005, 70, 8001–8008.
446 J. Org. Chem. Vol. 75, No. 2, 2010

ꢀ
Stefko et al.
JOCArticle
showed again preference for C20-endo (S) conformation. It
seems that the intramolecular H-bond of 50-OH to pyridine
nitrogen (which is not present in DMSO solution) might be
the reason for the preference of the C20-endo (S) conforma-
tion in the solid state and chloroform solution.
All final C-ribonucleosides 13a-l were tested on cytostatic
and anti-HCV activities, and none of them showed any
significant effect at 10 μM concentrations.
In conclusion, we have developed an efficient methodol-
ogy for the synthesis of diverse 6-substituted pyridine-2-yl C-
ribonucleosides. The key intermediate, TBS-protected 6-
bromopyridine 6, was prepared in three steps: one-pot addi-
tion of bromopyridyllithium 2 to protected ribonolactone 1
with in situ acetylation followed by reduction of the acety-
1H, J5,4=7.8 Hz, J5,3=0.9 Hz, H-5); 7.47 (t, 1H, J4,5=J4,3=7.8
Hz, H-4); 7.62 (dd, 1H, J3,4=7.8 Hz, J3,5=0.9 Hz, H-3). 13C
NMR (125.7 MHz, CDCl3): -5.8, -5.7, -5.5, -4.9, -4.6, and
-4.4 (CH3Si); 18.0 ((CH3)3C); 21.9 (CH3CO); 25.7, 25.8, and
25.9 ((CH3)3C); 62.7 (CH2-50); 72.4 (CH-30); 79.5 (CH-20); 87.6
(CH-40); 105.1 (C-10); 119.4 (CH-3); 127.1 (CH-5); 138.6 (CH-4);
140.8 (C-6); 159.8 (C-2); 170.0 (CO). IR spectrum (CCl4): 3052,
2954, 2897, 1756, 1581, 1559, 1472, 1463, 1436, 1402, 1402, 1390,
1365, 1365, 1263, 1253, 1160, 1129, 1073, 1023, 1005, 989, 969,
939, 878, 839, 682, 672, 614 cm-1. [R]20D = þ21.8 (c 4.18,
CHCl3).
1β-(6-Bromopyridin-2-yl)-1-deoxy-2,3,5-tri-O-(tert-butyldime-
thylsilyl)-D-ribofuranose (6). Et3SiH (1.3 mL, 8.16 mmol, 3
equiv) was added in one portion to a stirred solution of acylated
hemiketal 4 (1.88 g, 2.72 mmol) in dry hexanes (14 mL) in an
ice-brine bath (-20 °C) under argon. After 5 min, BF3 Et2O
3
lated hemiacetal 4 by Et3SiH/BF3 Et2O, in excellent overall
3
(0.39 mL, 3.26 mmol, 1.2 equiv) was added in one portion. The
resulting mixture was stirred for an additional 5 min, followed
by addition of Et3N (10 mL) and evaporation of solvents. Crude
product was directly chromatographed on silica gel in gradient
hexanes to 1.6% EtOAc in hexanes to give 6 (1.58 g, 90%) as a
colorless oil. HRMS (ESI) C28H55BrNO4Si3: [M þ H] calcd
632.2617, found 632.2619. 1H NMR (499.8 MHz, CDCl3):
-0.18, -0.04, 0.070, 0.072, 0.10, and 0.11 (6 ꢀ s, 6 ꢀ 3H,
yield of 63%. It must be noted that the protocol is extremely
sensitive to efficient cooling and precise adherence of reac-
tion times in order to get reproducibly high yields. Interest-
ingly, when the same sequence was performed for the
corresponding pyridine N-oxide derivatives, each step pro-
ceeded with inversion of configuration at C10 giving unde-
sired R-anomeric bromopyridine N-oxide C-ribonucleoside
11. The bromopyridine C-ribonucleoside intermediate 6
readily undergoes cross-coupling reactions with alkylalumi-
num or -zinc reagents and arylboronic acids or hetarylstan-
nanes to give 6-alkyl or 6-hetaryl derivatives. Hartwig-
Buchwald aminations and Pd-catalyzed aminocarbonyla-
tions of 6 gave 6-amino- and 6-carbamoylpyridine C-ribo-
nucleosides in high yields. Free nucleosides 13a-l were
CH3Si); 0.85, 0.91, and 0.93 (3 ꢀ s, 3 ꢀ 9H, (CH3)3C); 3.75 (dd,
0
0
1H, Jgem=11.2, J5 b,4 =3.1, H-5 b); 3.86 (dd, 1H, Jgem=11.2,
0
0
0
0
J5 a,4 =3.8, H-5 a); 4.06 (td, 1H, J4 ,5 =3.8, 3.1, J4 ,3 =3.8, H-4 );
4.11 (dd, 1H, J3 ,2 =4.3, J3 ,4 =3.8, H-3 ); 4.15 (dd, 1H, J2 ,1
0
0
0
0
0
0
0
0
0
0
0
0
=
0
0
0
0
0
0
5.7, J2 ,3 =4.3, H-2 ); 4.88 (d, 1H, J1 ,2 =5.7, H-1 ); 7.35 (dd, 1H,
J5,4=7.8, J5,3=1.1, H-5); 7.48 (dd, 1H, J4,5=7.8, J4,3=7.6, H-4);
7.55 (dd, 1H, J3,4=7.6, J3,5=1.1, H-3). 13C NMR (125.7 MHz,
CDCl3): -5.5, -5.4, -5.0, -4.61, -4.58, and -4.4 (CH3Si);
18.0, 18.1, and 18.4 (C(CH3)3); 25.86, 25.88, and 26.0 ((CH3)3C);
63.0 (CH2-50); 72.8 (CH-30); 78.6 (CH-20); 84.2 (CH-10); 85.2
(CH-40); 120.3 (CH-3); 126.8 (CH-5); 138.5 (CH-4); 141.3 (C-6);
162.2 (C-2). IR spectrum (CCl4): 3051, 2956, 2796, 1582, 1557,
1463, 1435, 1408, 1390, 1362, 1255, 1154, 1124, 1044, 1005, 996,
989, 967, 940, 878, 838, 682, 571 cm-1. [R]20D =þ3.8 (c 6.01,
CHCl3).
prepared by desilylation of the intermediates by Et3N 3HF.
3
They did not exert any considerable cytotoxicity, and there-
fore, they are good candidates for conversion to nucleoside
triphosphates as model compounds for studying specificity
and fidelity of RNA polymerases or primases.
Experimental Section
1β-(6-Methylpyridin-2-yl)-1-deoxy-2,3,5-tri-O-(tert-butyldime-
thylsilyl)-D-ribofuranose (12a). Me3Al (0.48 mL, 0.951 mmol, 2
equiv, 2 M in toluene) was added to a vigorously stirred solution
of 6 (301 mg, 0.476 mmol) and Pd(PPh3)4 (28 mg, 0.024 mmol,
5 mol %) in THF (10 mL) under argon. The mixture was stirred
at 65 °C for 72 h and then quenched by pouring into saturated
NaH2PO4 (50 mL) and extracted to EtOAc (3 ꢀ 50 mL). Crude
product was chromatographed on silica gel in gradient hexanes
to 2% EtOAc in hexanes to give 12a (250 mg, 92%) as colorless
oil. HRMS (ESI) C29H58NO4Si3: [M þ H] calcd 568.3668, found
1β-(6-Bromopyridin-2-yl)-1-O-acetyl-2,3,5-tri-O-(tert-butyl-
dimethylsilyl)-D-ribofuranose (4). n-BuLi (10.1 mL, 16.3 mmol,
1.6 M solution in hexane, 4 equiv) in dry hexanes (50 mL) was
added within 7 min to a vigorously stirred solution of 2,
6-dibromopyridine (3.86 g, 16.3 mmol, 4 equiv) in dry THF
(40 mL) at -78 °C, and the resulting brown-yellow solution was
stirred for 15 min at -78 °C. A precooled (-78 °C) solution of 1
(2.0 g, 4.07 mmol) in THF (20 mL) was then added via cannula
to the reaction mixture in one portion followed by additional
stirring for 15 min at -78 °C and dropwise addition of Ac2O
(2.6 mL, 27.7 mmol, 6.8 equiv). The resulting red mixture was
stirred for 10 min at -78 °C, slowly warmed to rt, quenched by
pouring into satd NaHCO3 (150 mL), extracted with Et2O (3 ꢀ
250 mL), and dried with Na2SO4. After evaporation of solvents,
crude product was dissolved in Ac2O (100 mL) and extracted
with hexanes (2 ꢀ 300 mL). Collected hexane layers were washed
with satd NaHCO3 (500 mL) and dried over Na2SO4. Evapora-
tion under reduced pressure and chromatography in gradient
hexanes to 6% Et2O in hexanes furnished compound 4 (2.01 g,
70%) as a colorless oil, which crystallized upon standing. Mp
78-82 °C. HRMS (ESI) C30H56BrNO6Si3Na: [M þ Na] calcd
712.2491, found 712.2492. 1H NMR (500 MHz, CDCl3): -0.39,
-0.12, 0.06, 0.08, and 0.09 (6 ꢀ s, 6 ꢀ 3H, CH3Si); 0.88, 0.89,
1
568.3668. H NMR (500 MHz, CDCl3): -0.24, -0.08, 0.072,
0.073, 0.10, and 0.11 (6 ꢀ s, 6 ꢀ 3H, CH3Si); 0.82, 0.92, and 0.93
(3 ꢀ s, 3 ꢀ 9H, (CH3)3C); 2.51 (s, 3H, CH3); 3.77 (dd, 1H, Jgem
=
0
0
0
11.1 Hz, J5 a,4 =3.40Hz, H-5 a); 3.86 (dd, 1H, Jgem=11.1 Hz,
0
J5 b,4 =4.2 Hz, H-5 b); 4.06 (dt, 1H, J4 ,5 b =4.2 Hz, J4 ,5 a
0
0
0
0
0
=
0
0
0
0
0
0
0
0
J
4 ,3 =3.4 Hz, H-4 ); 4.13 (dd, 1H, J3 ,2 =4.4 Hz, J3 ,4 =3.5 Hz,
0
H-30); 4.17 (dd, 1H, J2 ,1 =6.0 Hz, J2 ,3 =4.4 Hz, H-2 ); 4.89 (d,
0
0
0
0
0
0
1H, J1 ,2 =6.0 Hz, H-1 ); 7.01 (bd, 1H, J5,4=7.6 Hz, H-5); 7.35
(dp, 1H, J3,4=7.8 Hz, J3,5=J3,CH3=0.5 Hz, H-3); 7.50 (t, 1H,
J
4,5=J4,3=7.7 Hz, H-4). 13C NMR (125.7 MHz, CDCl3): -5.5,
-5.4, -5.2, -4.6, and -4.4 (CH3Si); 18.0, 18.1, and 18.4
((CH3)3C); 24.4 (CH3); 25.8, 25.9, and 26.0 ((CH3)3C); 63.2
(CH2-50); 73.1 (CH-30); 78.6 (CH-20); 84.7 (CH-10); 85.1 (CH-40);
118.5 (CH-3); 122.0 (CH-5); 136.3 (CH-4); 157.4 (C-6); 159.7 (C-
2). IR spectrum (CCl4): 3063, 2956, 2896, 1594, 1579, 1472,
1462, 1406, 1389, 1375, 1362, 1254, 1154, 1127, 1104, 1078, 1046,
and 0.94 (3 ꢀ s, 3 ꢀ 9H, (CH3)3C); 2.11 (s, 3H, CH3CO); 3.77
0
0
(dd, 1H, Jgem=11.3 Hz, J5 a,4 =2.1 Hz, H-5 a); 3.95 (dd, 1H,
0
0
0
0
0
0
J
gem=11.3 Hz, J5 b,4 =2.9 Hz, H-5 b); 4.08 (d, 1H, J2 ,3 =5.2 Hz,
0
1004, 996, 989, 967, 940, 879, 838, 681, 671 cm-1
.
1β-(6-Phenylpyridin-2-yl)-1-deoxy-2,3,5-tri-O-(tert-butyldime-
thylsilyl)-D-ribofuranose (12d). Compound 6 (406 mg, 0.641
H-20); 4.18 (dd, 1H, J3 ,2 =5.2 Hz, J3 ,4 =2.1 Hz, 0H-3 ); 4.27
0
0
0
0
0
0
0
0
0
(ddd, 1H, J4 ,5 b=2.9 Hz, J4 ,5 a=J4 ,3 =2.1 Hz, H-4 ); 7.34 (dd,
0
J. Org. Chem. Vol. 75, No. 2, 2010 447

ꢀ
Stefko et al.
JOCArticle
mmol), K2CO3 (178 mg, 1.28 mmol, 2 equiv), Pd(PPh3)4 (37 mg,
0.032 mmol, 5 mol %), and PhB(OH)2 (157 mg, 1.28 mmol, 2
equiv) were dissolved in toluene (19 mL) under argon, and the
mixture was stirred at 100 °C for 12 h. The reaction mixture was
concentrated under reduced pressure, and crude product was
chromatographed on silica gel in toluene to give 12d (370 mg,
92%) as a colorless oil. HRMS (ESI) C34H60NO4Si3: [M þ H]
calcd 630.3825, found 630.3825. 1H NMR (500 MHz, CDCl3):
-0.07, 0.01, 0.07, 0.08, 0.115, and 0.123 (6 ꢀ s, 6 ꢀ 3H, CH3Si);
mixture was stirred at 60 °C for 3.5 h. After being cooled to room
temperature, the reaction mixture was diluted with Et2O
(10 mL), and 2 M HCl (0.5 mL) was added. The resulting hetero-
geneous mixture was stirred for an additional 5 min and then
transferred into a saturated NaHCO3 (30 mL) and extracted to
Et2O (3 ꢀ 100 mL). Crude product was chromatographed on
silica gel in gradient hexanes to 9% Et2O in hexanes followed by
gradient of 6% EtOAc in hexanes to 11% EtOAc in hexanes to
give 12h (324 mg, 86%) as yellow oil. HRMS (ESI) C28H57N2O4-
1
0.86, 0.92, and 0.94 (3 ꢀ s, 3 ꢀ 9H, (CH3)3C); 3.80 (dd, 1H,
Si3: [M þ H] calcd 569.3621, found 569.3621. H NMR (500
0
0
0
Jgem=11.1 Hz, J5 a,4 =3.4 Hz, H-5 a); 3.97 (dd, 1H, Jgem=11.1
MHz, CDCl3): -0.15, -0.04, 0.06, 0.07, 0.106, and 0.112 (6 ꢀ s,
0
0
0
0
0
0
0
=
Hz, J5 b,4 = 3.9 Hz, H-5 b); 4.13 (dt, J4 ,3 = 5.4 Hz, J4 ,5 a
0
6 ꢀ 3H, CH3Si); 0.84, 0.91, and 0.93 (3 ꢀ s, 3 ꢀ 9H, (CH3)3C);
J
4 ,5 b=3.6 Hz, H-40); 4.19 (dd, 1H, J3 ,4 =5.4 Hz, J3 ,2 =4.2 Hz,
0
0
0
0
0
0
0
0
3.76 (dd, 1H, Jgem=11.1 Hz, J5 a,4 =3.5 Hz, H-5 a); 3.84 (dd, 1H,
H-30); 4.35 (t, 1H, J2 ,3 =J2 ,1 =4.2 Hz, H-2 ); 5.03 (d, 1H, J1 ,2
=
J
Hz, J4 ,5 a=J4 ,3 =3.7 Hz, H-4 ); 4.11 (bt, 1H, J3 ,2 =J3 ,4 =4.1
0
0
0
0
0
0
0
0
0
0
gem=11.1 Hz, J5 b,4 =4.5 Hz, H-5 b); 4.04 (dt, 1H, J4 ,5 b=4.6
0
0
4.2 Hz, H-10); 7.40 (m, 1H, H-p-Ph); 7.46 (m, 2H, H-m-Ph); 7.55
0
0
0
0
0
0
0
0
0
Hz, H-30); 4.14 (dd, 1H, J2 ,1 =5.6 Hz, J2 ,3 =4.30Hz, H-2 ); 4.37
0
0
(ddd, 1H, J3,4=7.5 Hz, J3,5=1.2 Hz, J3,1 =0.5 Hz, H-3); 7.64
0
0
0
0
0
(bs, 2H, NH2); 4.72 (d, 1H, J1 ,2 =5.6 Hz, H-1 ); 6.38 (d, 1H,
0
(dd, 1H, J5,4=7.9 Hz, J5,3=1.2 Hz, H-5); 7.69 (t, 1H, J4,5=J4,3
=
7.7 Hz, H-4); 8.07 (m, 2H, H-o-Ph). 13C NMR (125.7 MHz,
CDCl3): -5.39, -5.38, -4.8, -4.7, -4.6, and -4.3 (CH3Si);
18.0, 18.1, and 18.5 ((CH3)3C); 25.85, 25.89, and 26.1 ((CH3)3C);
62.7 (CH2-50); 72.0 (CH-30); 78.4 (CH-20); 83.9 (CH-40); 85.9
(CH-10); 118.8 (CH-5); 120.1 (CH-3); 126.8 (CH-o-Ph); 128.5
(CH-m-Ph); 128.8 (CH-p-Ph); 137.0 (CH-4); 139.2 (C-i-Ph);
156.1 (C-6); 160.3 (C-2). IR spectrum (CCl4): 3067, 3090,
3067, 3036, 2956, 2896, 1592, 1579, 1572, 1496, 1472, 1462,
1462, 1449, 1406, 1389, 1362, 1327, 1254, 1217, 1155, 1104, 1078,
J5,4=8.1 Hz, H-5); 6.90 (d, 1H, J3,4=7.4 Hz, H-3); 7.37 (dd, 1H,
J4,5=8.1 Hz, J4,3=7.5 Hz, H-4). 13C NMR (125.7 MHz, CDCl3):
-5.44, -5.38, -5.0, -4.61, -4.58, and -4.4 (CH3Si); 18.0, 18.1,
and 18.4 ((CH3)3C); 25.8, 25.9, and 26.0 ((CH3)3C); 63.1 (CH2-
50); 72.8 (CH-30); 78.1 (CH-20); 84.6 (CH-10); 84.7 (CH-40); 107.5
(CH-5); 112.0 (CH-3); 138.0 (CH-4); 157.6 (C-6); 158.2 (C-2). IR
spectrum (CCl4): 3510, 3409, 3175, 3063, 2956, 2896, 1610, 1591,
1579, 1471, 1462, 1406, 1391, 1389, 1336, 1253, 1155, 1129, 1101,
1079, 998, 979, 838, 682, 672 cm-1
.
1041, 1029, 1006, 996, 968, 940, 878, 838, 672 cm-1
.
1β-[6-(Dimethylamino)pyridin-2-yl]-1-deoxy-2,3,5-tri-O-(tert-
butyldimethylsilyl)-D-ribofuranose (12i). Toluene (2.1 mL) was
added to a flame-dried and argon-purged flask containing 6 (468
mg, 0.740 mmol), Pd2(dba)3 (17 mg, 0.019 mmol, 5 mol %), (2-
biphenyl)di-tert-butylphosphine (22 mg, 0.074 mmol), sodium
1β-[6-(2-Thienyl)pyridin-2-yl]-1-deoxy-2,3,5-tri-O-(tert-butyl-
dimethylsilyl)-D-ribofuranose (12e). DMF (3.1 mL) was added to
a flame-dried and argon-purged flask containing 6 (419 mg,
0.662 mmol) and PdCl2(PPh3)2 (23 mg, 0.033 mmol, 5 mol %).
After 5 min of stirring at room temperature, tributyl(thiophen-
2-yl)stannane (0.27 mL, 0.860 mmol, 1.3 equiv) was added, and
the reaction vessel was immersed in an oil bath (100 °C). After
1.5 h, the reaction mixture became dark-brown, and the reaction
was complete (monitored by TLC hexanes/EtOAc 10:1). The
crude reaction mixture was diluted with Et2O (300 mL), washed
with 2 M HCl (80 mL) and saturated NaHCO3 (100 mL), and
dried over MgSO4. After evaporation of solvents under reduced
pressure, the crude product was chromatographed on silica gel
in gradient hexanes to 4% EtOAc in hexanes to obtain 12e (380
mg, 90%) as a colorless oil. HRMS (ESI) C32H58NO4SSi3: [M þ
H] calcd 636.3389, found 636.3389. 1H NMR (500 MHz,
CDCl3): 0.04, 0.05, 0.06, 0.11, and 0.13 (6 ꢀ s, 6 ꢀ 3H, CH3Si);
tert-butoxide (426 mg, 4.44 mmol, 6 equiv), and Me2NH HCl
3
(302 mg, 3.70 mmol, 5 equiv). The resulting mixture was stirred
at 40 °C for 4.5 h and then diluted with Et2O (5 mL), poured into
water (100 mL), extracted with Et2O (3 ꢀ 100 mL), and dried
over MgSO4. After removal of the solvent under reduced pre-
ssure, crude product was chromatographed on silica in gradient
hexanes to 4% EtOAc in hexanes to give 12i (387 mg, 86%) as a
colorless oil. HRMS (ESI) C30H61N2O4Si3: [M þ H] calcd
597.3934, found 597.3934. 1H NMR (500 MHz, CDCl3): 0.00,
0.02, 0.04, 0.05, 0.089, and 0.093 (6 ꢀ s, 6 ꢀ 3H, CH3Si); 0.88,
0.90, and 0.92 (3 ꢀ s, 3 ꢀ 9H, (CH3)3C); 3.07 (s, 6H, (CH3)2N);
0
0
3.75 (dd, 1H, Jgem=11.1 Hz, J5 a,4 =3.9 Hz, H-5 a); 3.92 (dd, 1H,
0
0
0
0
0
0
J
gem=11.1 Hz, J5 b,4 =4.2 Hz, H-5 b); 4.07 (dt, 1H, J4 ,3 =6.3
0
Hz, J4 ,5 a=J4 5 b=4.0 Hz, H-40); 4.13 (dd, 1H, J3 ,4 =6.3 Hz,
0
0
0
0
0
0.89, 0.91, and 0.94 (3 ꢀ s, 3 ꢀ 9H, (CH3)3C); 3.78 (dd, 1H,
0
0
0
gem=11.2 Hz, J5 a,4 =3.2 Hz, H-5 a); 3.98 (dd, 1H, Jgem=11.2
0
0
0
0
0
0
0
J
Hz, J5 b,4 =3.4 Hz, H-5 b); 4.10 - 4.15 (m, 2H, H-3 ,4 ); 4.36 (t,
J
3 ,2 =4.2 Hz, H-3 ); 4.31 (dd, 1H, J2 ,3 =4.2 Hz, J2 ,1 =3.3 Hz,
0
0
0
0
H-20); 4.79 (d, 1H, J1 ,2 =3.3 Hz, H-1 ); 6.39 (d, 1H, J5,4=8.4 Hz,
H-5); 6.82 (d, 1H, J3,4=7.3 Hz, H-3); 7.38 (dd, 1H, J4,5=8.5 Hz,
0
0
0
0
0
0
0
0
0
0
0
0
1H, J2 ,1 =J2 ,3 =3.6 Hz, H-2 ); 4.96 (d, 1H, J1 ,2 =3.5 Hz, H-1 );
7.10 (dd, 1H, J3,5=5.1 Hz, J4,3=3.7 Hz, H-4-thienyl); 7.37 (dd,
1H, J5,4=5.1 Hz, J5,3=1.2 Hz, H-5-thienyl); 7.47 (dm, 1H, J3,4=
J
4,3=7.3 Hz, H-4). 13C NMR (125.7 MHz, CDCl3): -5.4, -4.8,
-4.7, -4.6, and -4.3 (CH3Si); 18.0, 18.1, and 18.5 ((CH3)3C);
25.9 and 26.1 ((CH3)3C); 38.1 ((CH3)2N); 62.8 (CH2-50); 71.8
(CH-30); 77.7 (CH-20); 83.0 (CH-40); 86.5 (CH-10); 104.5 (CH-5);
109.6 (CH-3); 137.3 (CH-4); 158.1 (C-2); 158.8 (C-6). IR spec-
trum (CCl4): 2956, 2896, 1597, 1572, 1499, 1472, 1463, 1431,
1405, 1389, 1374, 1362, 1253, 1154, 1126, 1101, 1078, 941, 838,
7.7 Hz, H-3); 7.52 (dd, 1H, J5,4=7.9 Hz, J5,3=1.1 Hz, H-5); 7.58
(dd, 1H, J3,4=3.7 Hz, J3,5=1.2 Hz, H-3-thienyl); 7.62 (t, 1H,
J
4,3=J4,5=7.8 Hz, H-4). 13C NMR (125.7 MHz, CDCl3): -5.4,
-4.8, -4.6, -4.5, and -4.3 (CH3Si); 18.0, 18.1, and 18.5
((CH3)3C); 25.88, 25.9, and 26.1 ((CH3)3C); 62.6 (CH2-50);
71.6 (CH-30); 78.2 (CH-20); 83.5 (CH-40); 86.0 (CH-10); 117.1
(CH-5); 119.8 (CH-3); 124.2 (CH-3-thienyl); 127.4 (CH-
5-thienyl); 127.8 (CH-4-thienyl); 136.9 (CH-4); 145.4 (C-2-
thienyl); 151.7 (C-6); 160.3 (C-2). IR spectrum (CCl4): 3074,
2956, 2896, 1588, 1572, 1535, 1525, 1472, 1458, 1458, 1434, 1406,
1389, 1362, 1287, 1254, 1103, 1076, 1045, 1006, 995, 968, 940,
683 cm-1
.
1β-[6-(Carbamoyl)pyridin-2-yl]-1-deoxy-2,3,5-tri-O-(tert-butyl-
dimethylsilyl)-D-ribofuranose (12j). A flame-dried septum-sealed
flask (10 mL) containing 6 (220 mg, 0.348 mmol), Pd(OAc)2
(3.9 mg, 0.018 mmol, 5 mol %), Xantphos (20 mg, 0.035 mmol,
10 mol %), NH4Cl (74 mg, 1.39 mmol, 4 equiv), and K3PO4
(369 mg, 1.74 mmol, 5 equiv) was evacuated and backfilled with
CO(g). Then, toluene (0.6 mL) and DMSO (0.6 mL) were added
via syringe. The reaction mixture was stirred at room tempera-
ture for 5 min and then immersed into a preheated oil bath
(80 °C) and vigorously stirred for 3 h. After the mixture was
cooled to room temperature, Et2O (8 mL) was added, and the
reaction mixture was filtered through a plug of Celite (eluting
879, 854, 838,702, 682, 671 cm-1
.
1β-(6-Aminopyridin-2-yl)-1-deoxy-2,3,5-tri-O-(tert-butyldime-
thylsilyl)-D-ribofuranose (12h). LiN(SiMe3)2 (1.2 mL, 1 M solu-
tion in THF 1.18 mmol, 1.8 equiv) was added to a flame-dried
and argon-purged flask containing 6 (416 mg, 0.6573 mmol),
Pd2(dba)3, (15 mg, 0.016 mmol, 5 mol %), and (2-biphenyl-
)dicyclohexylphosphine (23 mg, 0.036 mmol, 10 mol %), and the
448 J. Org. Chem. Vol. 75, No. 2, 2010

ꢀ
Stefko et al.
JOCArticle
0
0 0 0 0
J5 a,4 =4.4 Hz, H-5 a); 3.63 (dd, 1H, Jgem=11.8 Hz, J5 b,4 =3.1
with Et2O) and concentrated under reduced pressure. The crude
product was purified by flash chromatography on silica gel in
gradient hexanes to 17% EtOAc in hexanes to give 12j (160 mg,
78%) as a colorless oil. HRMS (ESI) C29H57N2O5Si3: [M þ H]
calcd 597.3537, found 597.3557. 1H NMR (500 MHz, CDCl3):
-0.24, -0.04, 0.066, 0.070, 0.12, and 0.13 (6 ꢀ s, 6 ꢀ 3H,
Hz, H-50b); 3.79 (ddd, 1H, J4 ,3 =6.1 Hz, J4 ,5 a=4.3 Hz, J4 ,5 b
0
0
0
0
0
0
=
3.1 Hz, H-40); 3.82 (dd, 1H, J3 ,4 =6.1 Hz, J3 ,2 =4.7 Hz, H-3 );
0
0
0
0
0
0
0
0
0
0
0
0
3.94 (t, 1H, J2 ,1 =J2 ,3 =4.4 Hz, H-2 ); 4.47 (d, 1H, J1 ,2 =4.2 Hz,
H-10); 4.80-5.20 (m, 3H, OH-20,30,50); 5.89 (s, 2H, NH2); 6.31
(dd, 1H, J5,4=8.2 Hz, J5,3=1.0 Hz, H-5); 6.61 (dm, 1H, J3,4=7.2
Hz, H-3); 7.32 (dd, 1H, J4,5 =8.2 Hz, J4,3 =7.2 Hz, H-4). 13C
NMR (125.7 MHz, DMSO-d6): 62.2 (CH2-50); 71.3 (CH-30); 76.6
(CH-20); 84.0 (CH-40); 85.8 (CH-10); 107.3 (CH-5); 109.3 (CH-3);
137.8 (CH-4); 159.0 (C-2); 159.4 (C-6). IR spectrum (KBr): 3432,
3366, 3230, 2000, 1628, 1607, 1576, 1471, 1339, 1106, 1082, 1046,
993, 795, 739 cm-1. [R]20D=-43.5 (c 3.33, MeOH). Anal. Calcd
CH3Si); 0.84, 0.92, and 0.94 (3 ꢀ s, 3 ꢀ 9H, (CH3)3C); 3.78 (dd,
0
0
0
1H, Jgem=11.2 Hz, J5 a,4 =2.6 Hz, H-5 a); 3.92 (dd, 1H,0Jgem
=
0
0
0
0
0
11.2 Hz, J5 b,4 =3.2 Hz, H-5 b); 4.08 - 4.13 (m, 3H, H-2 ,3 ,4 );
0
0
0
4.95 (d, 1H, J1 ,2 =4.8 Hz, H-1 ); 5.77 (d, 1H, Jgem =4.0 Hz,
NH2a); 7.81 (dd, 1H, J4,3=7.9 Hz, J4,5=7.0 Hz, H-4); 7.84 (dd,
1H, J3,4=7.9 Hz, J3,5=1.9 Hz, H-3); 7.85 (m, 1H, NH2b); 8.10
(dd, 1H, J5,4=7.0 Hz, J5,3=1.9 Hz, H-5). 13C NMR (125.7 MHz,
CDCl3): -5.43, -5.41, -5.1, -4.7, -4.5, and -4.4 (CH3Si);
17.9, 18.1, and 18.4 ((CH3)3C); 25.7, 25.9, and 26.0 ((CH3)3C);
62.7 (CH2-50); 72.4 (CH-30); 78.9 (CH-20); 84.8 (CH-40); 84.9
(CH-10); 121.3 (CH-5); 124.3 (CH-3); 137.8 (CH-4); 148.4 (C-6);
160.0 (C-2); 166.7 (CO). IR spectrum (CCl4): 3528, 3459, 3401,
3275, 3150, 3063, 2897, 2895, 1699, 1594, 1574, 1556, 1472, 1462,
1435, 1404, 1388, 1362, 1256, 1155, 1127, 1075, 972, 940, 887,
for C10H14N2O4 1/2 H2O: C, 51.06; H, 6.43; N, 11.91. Found: C,
3
51.11; H, 6.31; N, 11.55.
For all other synthetic procedures and for complete charac-
terization of all compounds, see the Supporting Information.
Single Crystal X-ray Structure Analysis. The diffraction data
of single crystals of 13i (colorless, 0.09 ꢀ 0.23 ꢀ 0.40 mm) and 5
(colorless, 0.21 ꢀ 0.32 ꢀ 0.51 mm) were collected on Xcalibur
˚
X-ray diffractometr with CuKR (λ=1.54180 A) at 150 and 298 K,
838, 680 cm-1
.
General Procedure for the Deprotection of TBS Group.
respectively. Both structures were solved by direct methods with
SIR9227 and refined by full-matrix least-squares on F with
CRYSTALS.28 All hydrogen atoms were located in a difference
map, but those attached to carbon atoms were repositioned
geometrically and then refined with riding constraints, while all
other atoms were refined anisotropically in both cases.
Method A. Et3N 3HF (163 μL, 1.00 mmol, 10 equiv) was added
3
to the solution of silylated compound 12a-l (0.10 mmol) in
THF (1.00 mL), and the resulting mixture was stirred at 40 °C
for 2 days. After the reaction was complete (monitored by TLC
eluted in CHCl3/MeOH 8:2), solvent was removed under re-
duced pressure, the crude product was dissolved in water, and
solid NaHCO3 was added until pH 8. Solvents were removed
under reduced pressure, and crude product was purified by
reversed-phase chromatography (H2O/MeOH as a eluent) to
obtain free C-ribonucleosides 13a-l.
Crystal data for 5: C30H55Br2NO6Si3, orthorhombic, space
˚
group P212121, a = 10.8289(9) A, b = 17.5540(19) A, c =
˚
3
˚
˚
21.7052(19) A, V = 4126.0(7) A , Z = 4, M = 769.82, 22646
reflections measured, 8654 independent reflections. Final R=
0.0481, wR=0.0757, GoF=1.1709 for 5600 reflections with I
> 2σ(I) and 381 parameters. CCDC 752141.
1β-(6-Phenylpyridin-2-yl)-1-deoxy-D-ribofuranose (13d). Com-
pound 13d was prepared from 12d (371 mg, 0.589 mmol) accord-
ing to general procedure, in 89% yield, as a colorless oil, which
after lyophylization furnished white hygroscopic powder. HRMS
(ESI) C16H18NO4: [M þ H] calcd 288.1230, found 288.1232.
Crystal data for 13i: C12H18N2O4, orthorhombic, space group
˚
˚
˚
P212121, a=6.8208(3) A, b=9.0452(4) A, c=19.7411(9) A, V=
3
1217.93(10) A , Z=4, M=254.29, 11567 reflections measured,
˚
2580 independent reflections. Final R=0.0364, wR=0.0453,
GoF = 1.0933 for 2201 reflections with I > 2σ(I) and 165
parameters. CCDC 752142.
1H NMR (500 MHz, DMSO-d6): 3.55 (dd, 1H, Jgem=11.7 Hz,
0
0
0
0
0
J
5 a,4 =4.6 Hz, H-5 a); 3.67 (dd, 1H, Jgem=11.7 Hz, J5 b,4 =3.7
0
Hz, H-50b); 3.89 (m, 1H, H-40); 3.95 (bt, 1H,0J3 ,2 =J3 ,4 =5.2 Hz,
0
0
0
Acknowledgment. This work was supported by the Aca-
demy of Sciences of the Czech Republic (Z4 055 905), by the
Ministry of Education (LC 512), by the Grant Agency of the
ASCR (IAA400550902), and by Gilead Sciences, Inc. Cyto-
static activity was studied by Dr. I. Votruba (IOCB) and
Drs. Gabriel Birkus and Tomas Cihlar (Gilead Sciences,
Inc.) and anti-HCV activity by Dr. Richard Mackman
(Gilead Sciences, Inc.).
H-30); 4.11 (bt, 1H, J2 ,1 =J2 ,3 =4.9 Hz, H-2 ); 4.80 (d, 1H, J1 ,2
=
0
0
0
0
0
0
4.7 Hz, H-10); 4.80 - 5.30 (m, 3H, OH-20,30,50); 7.44 (m, 1H, H-p-
Ph); 7.50 (m, 2H, H-m-Ph); 7.53 (dd, 1H, J3,4=7.0 Hz, J3,5=1.6
Hz, H-3); 7.84 (dd, 1H, J5,4=7.9 Hz, J5,3=1.7 Hz, H-5); 7.87 (dd,
1H, J4,5=7.9 Hz, J4,3=7.0 Hz, H-4); 8.07 (m, 2H, H-o-Ph). 13
C
NMR (125.7 MHz, DMSO-d6): 62.1 (CH2-50); 71.4 (CH-30); 76.8
(CH-20); 84.5 (CH-40); 85.7 (CH-10); 119.4 (CH-5); 120.4 (CH-3);
126.9 (CH-o-Ph); 129.0(CH-m-Ph); 129.3(CH-p-Ph);138.0 (CH-
4); 138.9 (C-i-Ph); 155.5 (C-6); 160.7 (C-2). IR spectrum (KBr):
3415, 3066, 3066, 1627, 1605, 1593, 1579, 1570, 1497, 1458, 1449,
1334, 1161, 1104, 1074, 1049, 1029, 998, 816, 762, 697, 624
cm-1.[R]20D=-14.2 (c 2.61, MeOH). Anal. Calcd for C16H17-
Supporting Information Available: Detailed description of
unsuccessful approaches and lengthy optimizations, complete
experimental procedures, CIF files for crystal structures, con-
formational analysis, and copies of the 1H and 13C NMR spectra
of all new compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
NO4 1H2O: C, 65.05; H, 6.06; N, 4.79. Found: C, 65.31; H, 5.96;
3
N, 4.67.
1β-(6-Aminopyridin-2-yl)-1-deoxy-D-ribofuranose (13h). Com-
pound 13h was prepared from 12h (307 mg, 0.540 mmol) accord-
ing to the general procedure in 90% yield as a colorless oil, which
after lyophylization furnished white hygroscopic powder. HRMS
(ESI) C10H15N2O4: [M þ H] calcd 227.1026, found 227.1027.
1H NMR (500 MHz, DMSO-d6): 3.49 (dd, 1H, Jgem=11.8 Hz,
(27) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435–435.
(28) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.;
Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487–1487.
J. Org. Chem. Vol. 75, No. 2, 2010 449