Biocatalytic separation of N -7/ N -9 guanine nucleosides

Article abstract of DOI:10.1021/jo101565e

Vorbrueggen coupling of trimethylsilylated 2-N-isobutanoylguanine with peracetylated pentofuranose derivatives generally gives inseparable N-7/N-9 glycosyl mixtures. We have shown that the two isomers can be separated biocatalytically by Novozyme-435-mediated selective deacetylation of the 5′-O-acetyl group of peracetylated N-9 guanine nucleosides.

Full text of DOI:10.1021/jo101565e

pubs.acs.org/joc
3
,4
that are difficult to separate. The changes in experimental
variables and the use of a selectively modified guanine
moiety, such as 2-N-acetyl-6-O-diphenylcarbamoylguanine
in nucleoside coupling reactions, affect the isomeric ratio but
do not eliminate the formation of the N-7 isomer together
Biocatalytic Separation of N-7/N-9 Guanine
Nucleosides
†
Sunil K. Singh, Vivek K. Sharma, Carl E. Olsen,
†
‡
§ †,§
Jesper Wengel, Virinder S. Parmar, and
Ashok K. Prasad*
5
,
†
with the desired N-9 isomer. In this paper, we report the
synthesis of guanine nucleosides (mixture of 9- and 7-glycosyl
derivatives) derived from D-ribose, D-arabinose, and L-arabinose
sugars and for the first time their highly efficient separation
mediated by Novozyme-435 lipase-catalyzed removal of one of
the acetoxy functions of the peracetylated N-9 guanine nucleo-
sides.
†
Bioorganic Laboratory, Department of Chemistry,
‡
University of Delhi, Delhi-110 007, India, University of
Copenhagen, Faculty of Life Sciences, Department of Natural
Sciences, DK- 1871 Frederiksberg C, Denmark, and Nucleic
§
Acid Centre, Department of Physics and Chemistry,
University of Southern Denmark, DK-5230 Odense M,
Denmark
6
The coupling of 2-N-isobutanoylguanine (2) with 1,2,3,5-
7,8
tetra-O-acetyl-D-ribofuranose (3), 1,2,3,5-tetra-O-acetyl-
9
D-arabinofuranose (4), or 1,2,3,5-tetra-O-acetyl-L-arabino-
ashokenzyme@yahoo.com
Received August 9, 2010
8-10
furanose (5)
acid catalyst following a standard Vorbr u€ ggen
in the presence of TMSOTf as Lewis
5
,11
coupling
protocol afforded mixtures of 2,3,5-tri-O-acetylated 9- and
7
-β-D-ribofuranosylguanines 6 and 7, 9- and 7-R-D-arabino-
furanosylguanines 8 and 9, and 9- and 7-R-L-arabinofura-
nosylguanines 10 and 11 in ratios of 87:13, 63:37, and 76:24,
respectively, in 60-65% yields (Scheme 1 and Table 1). The
ratio of regioisomers N-9 and N-7 in the above guanine
nucleoside mixtures 6 and 7, 8 and 9, and 10 and 11 were
calculated on the basis of the integration of the correspond-
1
ing anomeric protons in the H NMR spectra (400 MHz) of
the mixtures (Table 1). Our various attempts of separation of
N-9 and N-7 guanine nucleosides from the mixtures 6 and 7, 8
and 9, and 10 and 11 by repeated column chromatography
on silica gel were unsuccessful.
Some lipases have been found to selectively acylate/deacylate
primary hydroxyl over secondary hydroxyl group(s) of sugars
12
Vorbr u€ ggen coupling of trimethylsilylated 2-N-isobuta-
noylguanine with peracetylated pentofuranose derivatives
generally gives inseparable N-7/N-9 glycosyl mixtures. We
have shown that the two isomers can be separated biocat-
alytically by Novozyme-435-mediated selective deacetyla-
(
(
3) Zhong, M.; Robins, M. J. Tetrahedron Lett. 2003, 44, 9327–9930.
4) (a) Schaffer, H. J.; Beauchamp, L.; Miranda, P. de.; Elion, G. B.;
Bauer, D. J.; Collins, P. Nature 1978, 272, 583–585. (b) Robins, M. J.;
Hatfield, P. W. Can. J. Chem. 1982, 60, 547–553. (c) Boryski, J.; Golankiewicz,
B. Nucleosides Nucleotides 1987, 6, 385–386. (d) Matsumoto, H.; Kaneko, C.;
Yamada, K.; Takeuchi, T.; Mori, T.; Mizuno, Y. Chem. Pharm. Bull. 1988, 36,
1153–1157. (e) Boryski, J. J. Chem. Soc., Perkin Trans. 2 1997, 649–652. (f ) Clair,
A. S.; Xiang, G.; McLaughlin, L. W. Nucleosides Nucleotides 1998, 17, 925–937.
0
tion of the 5 -O-acetyl group of peracetylated N-9 guanine
nucleosides.
(
g) Boryski, J.; Golankiewicz, B. Nucleosides Nucleotides 1989, 8, 529–536.
(h) Geen, G.; Grinter, T. J.; Kincey, P. M.; Jarvest, R. L. Tetrahedron 1990,
46, 6903–6914. (i) Izawa, K.; Shiragami, H. Pure Appl. Chem. 1998, 70, 313–318.
(
j) Li, N. -S.; Piccirilli, J. A. Synthesis 2005, 17, 2685–2870. (k) Ferenc, G.;
Kele, Z.; Kov ꢀa cs, L. Rapid Commun. Mass Spectrom. 2005, 19, 236–240.
5) (a) Vorbr u€ ggen, H.; Krolikiewicz, K.; Bennua, B. Chem. Ber. 1981, 114,
1234–1255. (b) Robins, M. J.; Zou, R.; Guo, Z.; Wnuk., S. F. J. Org. Chem. 1996,
1, 9207–9212. (c) Garner, P.; Ramakanth, S. J. Org. Chem. 1988, 53, 1294–1298.
d) Zou, R.; Robins, M. J. Can. J. Chem. 1987, 65, 1436–1437. (e) Cheung
A. W.-H.; Sidduri, A.; Garofalo, L. M.; Goodnow, R. A., Jr. Tetrahedron Lett.
The discovery of 9-[(2-hydroxyethoxy)methyl]guanine
acyclovir) against herpes simplex type 1 and type 2 viruses
(
(
6
(
and its low mammalian toxicity triggered the synthesis of a
series of guanine nucleosides, e.g., penciclovir, famciclovir,
valaciclovir, valganciclovir, abacavir, etc., for the treatment
2000, 41, 3303–3307.
1
,2
(6) Milecki, J.; Foldesi, A.; Fischer, A.; Adamiak, R. W.; Chattopadhyaya, J.
J. Labelled Compd. Radiopharm. 2001, 44, 763–783.
(7) Wright, G. E.; Dudycz, L. W. J. Med. Chem. 1984, 27, 175–181.
(8) Shi, C.-J.; Zhang, J.; Fu, J.; Tang, J. Lett. Org. Chem. 2006, 3, 932–935.
of various viral diseases. The most problematic chemistry
and difficulties in manipulation of all five common bases
found in DNA and RNA occur with the polyfunctional
guanine (pKa1 -1.7, pKa2 -9.2) nucleosides and nucleotides.
The coupling of guanine with peracetylated sugar derivatives
generally produces N-7/N-9 isomeric mixtures of nucleosides
(9) Gupta, P.; Maity, J.; Shakya, G.; Prasad, A. K.; Parmar, V. S.;
Wengel, J. Org. Biomol. Chem. 2009, 7, 2389–2401.
(10) Kam, B. L.; Barascut, J. L.; Imbach, J. L. Carbohydr. Res. 1979, 69,
1
35–142.
(
(
11) Vorbr u€ ggen, H.; H o€ fle, G. Chem. Ber. 1981, 114, 1256–1268.
12) (a) Prasad, A. K.; Kalra, N.; Yadav, Y.; Kumar, R.; Sharma, S. K.;
(
1) Elion, G. B.; Furman, P. A.; Fyfe, J. A.; Miranda, P. De.; Beauchamp, L.;
Schaeffer, H. J. Proc. Natl. Acad. Sci. U.S.A. 1977, 74, 5716–5720.
2) (a) Herdewijn, P. Modified Nucleosides. In Biochemistry, Biotechnology
Patkar, S.; Lange, L.; Wengel, J.; Parmar, V. S. Chem. Commun. 2007, 2616–
2617. (b) Maity, J.; Shakya, G.; Singh, S. K.; Ravikumar, V. T.; Parmar,
V. S.; Prasad, A. K. J. Org. Chem. 2008, 73, 5629–5632. (c) Sharma, R. K.;
Aggarwal, N.; Arya, A.; Olsen, C. E.; Parmar, V. S.; Prasad, A. K.
Indian J. Chem. 2009, 48B, 1727–1731.
(
and Medicine; Wiley-VCH: New York, 2008. (b) Chu, C. K. Antiviral Nucleo-
sides: Chiral Synthesis and Chemotherapy; Elsevier: New York, 2003.
7
932 J. Org. Chem. 2010, 75, 7932–7935
Published on Web 10/25/2010
DOI: 10.1021/jo101565e
r 2010 American Chemical Society

Singh et al.
JOCNote
SCHEME 1. Procedure for the Synthesis of Guanine Nucleosides
FIGURE 1. Selective deacetylation studies on nucleosides 6 and 7,
8
435 and other lipases.
and 9, and 10 and 11 in THF at 40-42 °C catalyzed by Novozyme-
TABLE 1. Glycosylation of 2-N-Isobutanoylguanine with Peracety-
lated Sugar Derivatives 3-5
a
ratio of N-9/ N-7
nucleosides
reaction
time (h)
yields
(%)
reactants
products
SCHEME 2. Novozyme-435-Catalyzed Deacetylation Reaction:
Separation of N-7 and N-9 Guanine Nucleosides
2
2
2
þ 3
þ 4
þ 5
4
6
6
6 and 7
8 and 9
10 and 11
87:13
63:37
76:24
60
62
65
a
Ratios of N-9 and N-7 nucleoside regioisomers were calculated from
1
the H NMR spectral (400 MHz) data of the mixtures.
1
3
and nucleosides. In the recent past, lipases have been used for
1
4
the separation of the nucleosides from their anomeric mixtures
0
15
and for the resolution of β-D/L-2 - deoxynucleosides. Stimu-
latedbytheseresults, wedecidedto explore an enzymatic route
for the separation of N-9 and N-7 guanine nucleosides. Five
lipases, viz. Candida antarctica lipase-B immobilized on poly-
acrylate, Lewatit (Novozyme-435 or CAL-B), Theremomyces
lanuginosus lipase immobilized on silica (Lipozyme TL IM),
Amano PS lipase, Candida rugosa lipase (CRL), and porcine
pancreatic lipase (PPL) were screened for the selective dea-
cetylation of one regioisomer over the other in N-9/N-7
guanine nucleoside mixtures 6 and 7, 8 and 9, and 10 and
products. Lipozyme TL IM, Amano PS, and CRL did not
show any reaction on the peracetylated N-9-, N-7-glycosyl-
guanine nucleoside substrate mixtures.
In a typical reaction, a solution of N-9/N-7 nucleoside
mixture 6and 7(200 mg) in tetrahydrofuran (20 mL) containing
a small amount of n-butanol was incubated with Novozyme-
11 in five sets of organic solvents, i.e., tetrahydrofuran (THF),
acetonitrile (CH CN), toluene, diisopropyl ether (DIPE),
and acetone using n-butanol as acetyl acceptor at 40-42 °C
and at 150 rpm in an incubator shaker (Figure 1).
It was observed that lipase Novozyme-435 in THF at
3
435 (substrate-enzyme ratio 1:0.5 w/w) in an incubator shaker
at 40-42 °C. The reaction mixture was shaken for 4-6 h until a
prominent spot of the product appeared on analytical TLC at
much lower R value than the starting compounds. The
f
4
0-42 °C selectively and most efficiently deacetylates the
0
acetoxy group at the C-4 -hydroxymethyl moiety of N-9-
glycosylguanine over the other acetoxy groups of N-9 and
N-7 isomers of nucleosides (Scheme 2). The Novozyme-435-
catalyzed deacetylation reaction carried out in other sol-
vents, i.e., acetonitrile, toluene, DIPE, and acetone, did not
yield any product. Although PPL in THF initially showed
16
reaction was quenched by filtering off the enzyme, and the
solvent was removed under reduced pressure. The crude
product thus obtained was purified by silica gel column
chromatography using methanol in chloroform as gradient
solvent system to afford the pure N-9 glycosylguanine 12
0
selectivity for the deacetylation of the C-4 -acetoxymethyl
0
having a C-5 hydroxyl group in 72% yield (Scheme 2).
group of N-9-glycosylguanine, the reaction was too slow to
be of any practical use, and deacetylation of other acetoxy
groups of the N-7 and N-9 regioisomers were observed
leading to the formation of a mixture of compounds. When
the Novozyme-435 catalyzed reaction was conducted at a
higher temperature than 40-42 °C, the enhancement in the
rate was observed, but it resulted in the formation of multiple
Similarly, lipase-catalyzed deacetylation reactions on nucle-
oside mixtures 8 and 9 and 10 and 11 led to the formation of
the selectively deacetylated compounds 13 and 14 in 50 and
61% yields, respectively, along with the unreacted peracetylated
pure N-7 glycosylated guanines 9 and 11 in 30 and 20% yields,
1
7
respectively. These reactions did not yield any product
when carried out in the absence of lipase. All the enzyma-
tically deacetylated N-9 glycosylguanine nucleosides 12-14
(
13) (a) Prasad, A. K.; Trikha, S.; Parmar, V. S. Bioorg. Chem. 1999, 27,
35–154. (b) Ciuffreda, P.; Casati, P.; Santaniello, E. Bioorg. Med. Chem.
Lett. 1999, 9, 1577–1582. (c) Ferrero, M.; Gotor, V. Chem. Rev. 2000, 100,
319–4347.
14) (a) Damkjaer, D. L.; Petersen, M.; Wengel, J. Nucleosides Nucleo-
1
(16) Completion of the reaction was monitored by TLC in methanol--
chloroform as solvent system. Partial separation in N-9- and N-7-glycosy-
lated nucleosides was observed after five to six runs of the TLC plate in the
solvent.
(17) Yields of N-9 glycosylated guanines 12-14 and N-7 glycosylated
guanines 9 and 11 were calculated on the basis of the consideration of N-9 and
N-7 regioisomers as 100% in the mixtures.
4
(
tides 1994, 13, 1801–1807. (b) Garcia, J.; Diaz-Rodriguez, A.; Fernandez, S.;
Sanghvi, Y.; Ferrero, M.; Gotor, V. J. Org. Chem. 2006, 71, 9765–9771.
(15) Garcia, J.; Fernandez, S.; Ferrero, M.; Sanghvi, Y.; Gotor, V. Org.
Lett. 2004, 6, 3759–3762.
J. Org. Chem. Vol. 75, No. 22, 2010 7933

Singh et al.
JOCNote
SCHEME 3. Deacetylation of Guanine Nucleosides 12-14
and 11. The solution of N-9/N-7 guanine nucleosides 6 and 7, 8
and 9, or 10 and 11 (200 mg, 0.42 mmol) in tetrahydrofuran
(20 mL) containing a small amount of n-butanol as acetyl acceptor
was incubated with C. antarctica lipase B (Novozyme-435, 100 mg)
in an incubator shaker at 40-42 °C. After 4-6 h of incubation, a
f
prominent spot of the product appeared at much lower R value
1
6
than the starting compounds. The reaction was stopped by
filtering off the enzyme, and the solvent was removed under
reduced pressure. The crude product thus obtained was purified
through a silica gel column using a gradient solvent system of
0
methanol in chloroform (1:99 to 1.5:98.5) to afford pure 5 -
hydroxylated N-9 glycosylguanine nucleosides 12, 13, and 14 in
7
2%, 50%, and 61% yields, respectively. The recovered un-
reacted N-7 guanine nucleosides 9 and 11 were isolated in pure
form in 30 and 20% yields, respectively, whereas the recovered N-7
nucleoside 7 was contaminated with N-9 guanine nucleoside.
0
0
2
9
12). Compound 12 was obtained as a white sticky solid (0.13 g,
-(2 ,3 -Di-O-acetyl-β-D-ribofuranosyl)-N -isobutanoylguanine
(
2
5
7
2% yield): R
þ2.2 (c 0.07, methanol); IR (KBr) ν
f
= 0.5 (10% methanol in chloroform); [R]
3220, 2936, 1751, 1684,
609, 1560, 1405, 1375, 1250, 1157, 1105, 949, 785 cm ; H
D
=
max
-
1 1
1
3
NMR (CD OD, 400 MHz) δ 1.23 (6H, d, J = 6.8 Hz), 2.03, 2.12
(
(
(
6H, 2s), 2.70-2.73 (1H, m), 3.78-3.88 (2H, m), 4.26-4.28
and unreacted, recovered peracetylated N-7 glycosylguanine
nucleosides 9and 11 were unambiguously identified on the basis
1H, m), 5.56-5.58 (1H, m), 5.82 (1H, t, J = 5.6 Hz), 6.18
1
3
3
1H, d, J = 6.0 Hz), 8.29 (1H, s); C NMR (CD OD,
1
of their spectral (IR, H and C NMR) and HRMS data.
13
100.6 MHz): δ 19.36, 20.11, 20.50, 37.03, 62.35, 72.78, 75.38,
87.11, 85.34, 121.44, 139.74, 150.07, 150.63, 157.49, 171.11,
0
The 5 -OH group of selectively deacetylated N-9-glycosy-
lated guanine nucleosides 12-14 was acetylated using acetic
anhydride and DMAP as a catalyst in THF resulting in the
þ
171.64, 181.81; HR-ESI-TOF-MS m/z 460.1432 ([M þ Na] ),
þ
calcd for [C18
H
23
5
N O
8
þ Na] 460.1439.
0
0
2
0
0
0
9-(2 ,3 -Di-O-acetyl-r-D-arabinofuranosyl)-N -isobutanoyl-
guanine (13). Compound 13 was obtained as a white solid
(
mp 180-182 °C; [R]
ν
1
1
formation of 2 ,3 ,5 -tri-O-acetylated guanine nucleosides 6,
, and 10 in 82, 84, and 85% yields, respectively. The NMR
spectral data of pure tri-O-acetylated N-9 glycosylguanines
8
f
90 mg, 50% yield): R = 0.5 (10% methanol in chloroform);
25
= þ3.9 (c 0.1, methanol); IR (KBr)
18
,
D
6
8, and 10 and pure N-7 glycosylguanines 9 and 11 were
max 3196, 2975, 2935, 1737, 1718, 1690, 1615, 1567, 1370,
247, 1137, 1043, 908, 800 cm ; H NMR (CD OD, 400 MHz): δ
compared with the NMR spectral data of N-9/N-7 glycosy-
lated guanine mixtures 6 and 7, 8 and 9, and 10 and 11 to
further confirm the identity of the individual compounds.
To confirm the structures of Novozyme-435 deacetylated
nucleosides, hydrolysis of compounds 12-14 was performed
-1 1
3
.22 (6H, d, J = 6.8 Hz), 2.05, 2.09 (6H, 2s), 2.71-2.74
(1H, m), 3.73-3.83 (2H, m), 4.56-4.59 (1H, m), 5.43-5.45
(1H, m), 5.85 (1H, t, J = 4.0 Hz), 6.22 (1H, d, J = 3.6 Hz) and
1
3
8.13 (1H, s); C NMR (CD
0.05, 36.99, 62.41, 76.93, 81.08, 86.16, 89.04, 121.65, 139.85,
OD, 100.6 MHz) δ 19.36, 20.02,
3
5
2
150.06, 150.68, 157.49, 170.54, 171.49, 181.87; HR-ESI-
in methanolic ammonia to yield guanosine (15), R-D-arabi-
9
nofuranosylguanine 16, and R-L-arabinofuranosylguanine
þ
TOF-MS m/z 460.1426 ([M þ Na] ), calcd for [C H N O
1
8
23
5
8
1
7 in 90, 92, and 95% yields, respectively (Scheme 3). The
þ
0
þ Na] 460.1439.
hydrolyzed products 15-17 were unambiguously identified
0
2
9
guanine (14). Compound 14 was obtained as a white solid
-(2 ,3 -Di-O-acetyl-r-L-arabinofuranosyl)-N -isobutanoyl-
1
13
on the basis of their spectral (IR, H and C NMR spectra)
and HRMS data analysis. The physical and spectral data of
known compounds 15 and 16 were found to be identical to
the reported data; the spectral data of compound 17 has not
been previously reported.
(
0.11 g, 61% yield): R = 0.5 (10% methanol in chloroform);
f
2
5
mp 176-178 °C; [R]
ν
D
= -4.2 (c 0.1, methanol); IR (KBr)
3193, 2976, 2950, 1737, 1719, 1691, 1612, 1566, 1407,
max
-1 1
1370, 1247, 1137, 1071, 909, 801 cm ; H NMR (CD
400 MHz) δ 1.22 (6H, d, J = 6.8 Hz), 2.09, 2.05 (6H, 2s),
.71-2.74 (1H, m), 3.76-3.80 (2H, m), 4.57-4.59 (1H, m),
3
OD,
In summary, it has been established that the lipase Novo-
zyme-435 can selectively hydrolyze the acetoxy function
2
0
5.43-4.45 (1H, m), 5.85 (1H, t, J = 4.0 Hz), 6.22 (1H, d, J =
derived from the primary hydroxyl group at the C-5 -posi-
1
3
4
2
1
.0 Hz), 8.13 (1H, s); C NMR (CD OD, 100.6 MHz) δ 19.36,
3
tion of tri-O-acetylated N-9 glycosylated guanines over the
acetoxy moieties derived from the two secondary hydroxyl
groups presentinthe same molecule andalsooverthree acetoxy
functions of N-7 glycosylguanines. This enzymatic methodol-
ogy enables efficient separation of mixtures of N-9 and N-7
glycosylated guanines, which are otherwise almost impossible to
separate by column chromatography or other techniques.
0.02, 20.05, 36.99, 62.41, 76.93, 81.08, 86.16, 89.04, 121.65,
39.85, 150.06, 150.68, 157.49, 170.54, 171.49, 181.87;
þ
HR-ESI-TOF-MS m/z 460.1426 ([M þ Na] ), calcd for
þ
[C H N O þ Na] 460.1439.
1
8
23
5
8
0
0
0
2
7
-(2 ,3 ,5 -Tri-O-acetyl-r-D-arabinofuranosyl)-N -isobutanoyl-
guanine (9). Compound 9 was obtained as a white sticky solid
(59 mg, 30% yield): R = 0.5 (7% methanol in chloroform);
= þ1.7 (c 0.1, methanol); IR (KBr) ν 3188, 2976, 1751,
f
2
5
[
R]
D
max
-
1 1
1
4
2
5
4
1
3
686, 1610, 1375, 1226, 1054, 781 cm ; H NMR (CD OD,
Experimental Section
00 MHz) δ 1.21 (6H, d, J = 6.8 Hz), 1.99-2.10 (9H, 3s),
.70-2.74 (1H, m), 4.27-4.40 (2H, m), 4.85-4.86 (1H, m),
.37-5.39 (1H, m), 5.88 (1H, t, J = 4.0 Hz), 6.41 (1H, d, J =
General Procedure of Lipase-Catalyzed Selective Deacetyla-
tion of Mixtures of N-9/N-7 Nucleosides 6 and 7, 8 and 9, and 10
1
3
.0 Hz), 8.35 (1H, s); C NMR (CD
3
OD, 100.6 MHz) δ
9.36, 20.13, 20.70, 20.92, 36.96, 64.39, 76.98, 81.59, 83.35,
(18) Rigoli, J. W.; Østergaard, M. E.; Canady, K. M.; Guenther, D. C.;
Hrdlicka, P. J. Tetrahedron Lett. 2009, 50, 1751–1753.
91.38, 112.03, 144.30, 149.61, 154.81, 160.03, 171.51, 172.39,
7
934 J. Org. Chem. Vol. 75, No. 22, 2010

Singh et al.
JOCNote
þ
1
81.93; HR-ESI-TOF-MS m/z 502.1535 ([M þ Na] ), calcd for
2.10 (9H, 3s), 2.73 (1H, m), 4.32-4.36 (2H, m), 4.79-4.86
þ
[
C
20 25 5
H N O
9
þ Na] 502.1544.
(1H, m), 5.36-5.38 (1H, m), 5.83-5.85 (1H, m), 6.23 (1H, d,
0
0
0
2
13
7
-(2 ,3 ,5 -Tri-O-acetyl-r-L-arabinofuranosyl)-N -isobutanoyl-
3
J = 3.2 Hz), 7.90 (1H, s); C NMR (CD OD, 100.6 MHz) δ
guanine (11). Compound 11 was obtained as a white sticky solid
40 mg, 20% yield): R = 0.5 (7% methanol in chloroform);
= -1.5 (c 0.1, methanol); IR (KBr) ν-max 3434, 2927, 1751,
19.36, 20.54, 20.68, 37.01, 64.07, 77.17, 80.79, 83.56, 89.23,
121.67, 139.60, 150.07, 150.37, 157.48, 171.29, 171.38, 172.39,
(
f
2
5
þ
[
R]
D
181.86; HR-ESI-TOF-MS m/z 502.1538 ([M þ Na] ), calcd for
1
1
þ
1
3
2
685, 1611, 1551, 1372, 1224, 1055, 781 cm ; H NMR (CDCl
00 MHz) δ 1.24 (6H, d, J = 6.8 Hz), 2.03-2.15 (9H, 3s),
.83-2.85(1H, m), 4.29-4.38(2H, m), 4.79(1H, brs), 5.31(1H, t,
3
,
[C20
H
25
N
5
O
9
þ Na] 502.1544.
General Procedure for the Hydrolysis of Nucleosides 12-14.
A solution of selectively deacetylated nucleosides 12-14 (0.1 g,
0.22 mmol) in 20% methanolic ammonia solution (15 mL) was
stirred at 25-28 °C for 10 h. On completion of the reaction
(examination on analytical TLC), the excess solvent was removed
under reduced pressure and the residue thus obtained was
washed with hot ethyl acetate several times to afford guanosine
(15), 9-R-D-arabinofuranosyl guanine (16), and 9-R-L-arabino-
furanosyl guanine (17) as white solids in 90, 92, and 95% yields,
respectively.
J = 3.6 Hz), 5.83 (1H, t, J = 3.2 Hz), 6.43 (1H, d, J = 3.0 Hz),
1
3
8
7
9
1
.01 (1H, s), 9.86 (1H, brs), 12.36 (1H, brs); C NMR (CDCl3,
5.5 MHz) δ 19.05, 20.68, 20.79, 36.15, 62.95, 75.96, 80.36, 82.55,
0.26, 111.31, 141.58, 148.14, 152.99, 157.84, 169.36, 169.74,
70.61, 179.65; HR-ESI-TOF-MS m/z 502.1537 ([M þ Na] ),
þ
þ
5
9
calcd for [C H N O þ Na] 502.1544.
20
25
5
9
General Procedure of Acetylation of Enzymatically Selectively
Deacetylated Guanine Nucleosides 12-14. To a stirred solution of
nucleosides 12-14 (0.1 g, 0.21 mmol) in THF (8 mL) was added
acetic anhydride (0.25 mmol), followed by the addition of catalytic
amount of DMAP, and the reaction mixture was stirred for 4-5 h
at 28 °C. On completion (examination on analytical TLC), crushed
ice was added, and the reaction mixture was extracted with ethyl
acetate (2 ꢀ 30 mL). The combined organic layer was washed with
5
Guanosine (15) . Compound 15 was obtained as a white solid
5
(57.6 mg, 90% yield): mp 238-240 °C dec (lit. mp 240 °C dec);
2
5
[R]
D
= -40.7 (c 0.1, CH
3
SOCH
H
13
3
); HR-ESI-TOF-MS m/z
þ H] 284.0989.
þ
þ
284.0988 ([M þ H] ), calcd for [C10
N
5
O
5
9
9-r-D-Arabinofuranosylguanine (16) . Compound 16 was ob-
tained as a white solid (58.9 mg, 92% yield): R = 0.3 (25%
= þ21.8
f
3
2
D
saturated aqueous NaHCO
3
solution (2 ꢀ 30 mL) and dried over
methanol in chloroform); mp 210 °C dec; [R]
þ
anhydrous Na SO
2
4
, and the solvent was removed under reduced
pressure. The residue thus obtained was purified by column
chromatography on silica gel using methanol/chloroform (1:99)
(c 0.1, MeOH); HR-ESI-TOF-MS m/z 306.0804 ([M þ Na] ),
þ
calcd for [C H N O þ Na] 306.0809.
1
0
13
5
5
9-r-L-Arabinofuranosylguanine (17). Compound 17 was ob-
1
7
as eluent to afford the pure triacetylated guanine nucleosides 6,
, and 10 in 82, 84, and 85% yields, respectively.
tained as a white solid (60.8 mg, 95% yield); R
methanol in chloroform); mp 238 °C dec; [R]
f
= 0.3 (25%
= -25.6 (c 0.1,
3368, 3200, 2933, 1729, 1693, 1609,
3
2
8
D
0 0 0
-(2 ,3 ,5 -Tri-O-acetyl-β-D-ribofuranosyl)-N -isobutanoylgua-
18
2
9
MeOH); IR (KBr) ν
max
-1 1
nine (6) . Compound 6 was obtained as a white sticky solid (90 mg,
1534, 1398, 1052, 781 cm ; H NMR (CD
3
OD, 400 MHz)
2
5
8
2% yield): R
f
= 0.5 (7% methanol in chloroform); [R]
D
=
δ 3.44-3.59 (2H, m), 3.93-3.96 (1H, m), 4.06-4.10 (1H, m),
1
3
þ5.2 (c 0.1, methanol); HR-ESI-TOF-MS m/z 502.1533 ([M þ
4.46-4.48 (1H, m), 5.67 (1H, d, J = 4.8 Hz), 7.91 (1H, s);
C
þ
þ
Na] ), calcd for [C H N O þ Na] 502.1544.
NMR (CD OD, 100.6 MHz): δ 60.90, 75.03, 79.38, 84.84, 87.31,
2
0
25
5
9
3
0
0
0
2
9
-(2 ,3 ,5 -Tri-O-acetyl-r-D-arabinofuranosyl)-N -isobutanoyl-
guanine (8). Compound 8 was obtained as a white sticky solid
92 mg, 84% yield): R = 0.5 (7% methanol in chloroform);
116.53, 135.96, 151.09, 153.32, 156.26; ESI-TOF-MS m/z
þ Na] 306.0809.
þ
þ
306.0805 ([M þ Na] ), calcd for [C10
H
13
N
5
O
5
(
f
2
5
[
R]
= þ4.2 (c 0.1, methanol); IR (KBr) ν
3167, 2976, 2936,
749, 1683, 1609, 1560, 1403, 1373, 1224, 1101, 1053, 907,
D
max
Acknowledgment. We acknowledge the financial support
from the University of Delhi (DU-DST Purse Grant to
A.K.P. and V.S.P.) and the DBT, New Delhi (under the
Indo-Danish collaboration in Biotechnology to A.K.P., V.S.P.,
and J.W.). S.K.S. and V.K.S. thank the Council of Scientific
and Industrial Research (CSIR, New Delhi) for the award of
research fellowships. J.W. thanks the Danish National Research
Foundation for financial support.
1
7
6
-
1 1
85 cm ; H NMR (CD
3
OD, 400 MHz) δ 1.22 (6H, d, J =
.8 Hz), 2.05, 2.06, 2.10 (9H, 3s), 2.73 (1H, m), 4.32-4.36
2H, m), 4.79-4.86 (1H, m), 5.36-5.38 (1H, m), 5.84 (1H, t,
(
13
J = 3.6 Hz), 6.23 (1H, d, J = 3.2 Hz), 8.11 (1H, s); C NMR
CD OD, 100.6 MHz) δ 19.36, 20.54, 20.68, 37.01, 64.07, 77.17,
0.79, 83.56, 89.23, 121.67, 139.60, 150.07, 150.37, 157.48,
(
3
8
1
71.29, 171.38, 172.39, 181.86; HR-ESI-TOF-MS m/z 502.1526
þ Na] 502.1544.
þ
þ
(
[M þ Na] ), calcd for [C20
25
H N
5
O
9
0
0
0
2
9
-(2 ,3 ,5 -Tri-O-acetyl-r-L-arabinofuranosyl)-N -isobutanoyl-
guanine (10). Compound 10 was obtained as a white sticky solid
(93 mg, 85% yield): R
Supporting Information Available: General experimental
methods, procedure for the preparation of compounds 6 and 7, 8
and 9, and 10 and 11, and H- and C NMR spectra of mixtures
of 6 and 7, 8 and 9, and 10 and 11, 6, and 8-17. This material is
available free of charge via the Internet at http://pubs.acs.org.
2
5
f
= 0.5 (7% methanol in chloroform); [R]
1
13
D
= -4.6 (c 0.1, methanol); IR (thin film) νmax 3153, 2928, 1747,
683, 1609, 1563, 1402, 1373, 1225, 1158, 1053, 908, 757 cm ; H
-
1 1
1
NMR (CD OD, 400 MHz) δ 1.22 (6H, d, J = 6.8 Hz), 2.05, 2.06,
3
J. Org. Chem. Vol. 75, No. 22, 2010 7935