A facile and efficient synthesis of (purin-6-yl)alanines

Article abstract of DOI:10.1021/jo048812r

(Purin-6-yl)alanines, a new class of amino acid-nucleobase conjugates, were synthesized by palladium-catalyzed cross-coupling reactions of protected iodozincalanines with 6-iodopurines (9-Bn-6-iodopurine and 9-THP-6-iodopurine as well as acyl-protected 6-

Full text of DOI:10.1021/jo048812r

A Facile and Efficient Synthesis of (Purin-6-yl)alanines
Petr Cˇ apek, Radek Pohl, and Michal Hocek*
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,
CZ-16610 Prague 6, Czech Republic
hocek@uochb.cas.cz
Received July 13, 2004
(Purin-6-yl)alanines, a new class of amino acid-nucleobase conjugates, were synthesized by
palladium-catalyzed cross-coupling reactions of protected iodozincalanines with 6-iodopurines
(9-Bn-6-iodopurine and 9-THP-6-iodopurine as well as acyl-protected 6-iodopurine ribonucleoside
and 2-deoxyribonucleoside). Free purine base and nucleosides bearing alanine in position 6 were
obtained after complete deprotection of the products of cross-coupling reactions.
Purines bearing carbon substituents attached to carbon
atoms of the purine ring (at positions 2, 6, or 8) are of
great interest because of their potential applications in
medicinal chemistry and chemical biology. Several ex-
amples of this class of compounds were reported to
possess cytostatic,1 antimicrobial,2 and A2-receptor ago-
nist activity,3 and some of them were used as artificial
nucleobases in the extension of the genetic alphabet.4
Cross-coupling reactions are a powerful tool5 for the
synthesis of these compounds but, in most cases, have
only been used for an introduction of simple unfunction-
alized alkyl, alkenyl, alkynyl, aryl, and hetaryl substit-
uents. Therefore, an extension of this methodology to
functionalized carbon substituents is a very challenging
target. One of the prominent functionalized substituents
of high biological relevance is undoubtedly an amino acid
residue. Our goal is to develop approaches toward the
synthesis of a novel class of compounds: carbon-carbon-
linked conjugates of amino acids and purines. Such
compounds may display biological activity and may be
used as building blocks in the synthesis of chemically and
enzymatically stable nucleic acids-peptide/protein con-
jugates. Very recently, protected (purin-6-yl)glycines were
prepared in our laboratory by Pd-catalyzed R arylation
of ethyl N-(diphenylmethylidene)glycinate, but because
of very limited stability, they could not be deprotected.6
This paper reports on the synthesis of (purin-6-yl)-
alanines, another important example of this type of
compound.
Racemic (9H-purin-6-yl)alanine,7 (9-methylpurin-6-yl)-
alanines,8 and (purin-2-yl)alanines9 were claimed to have
been prepared 4 decades ago by multistep approaches
using successive construction of the amino acid moiety
in position 6 or 2 of a purine base, but no spectral
characterization of the products was given and neither
biological activity nor any other use of these compounds
was ever reported. Moreover, these laborious multistep
syntheses gave low total yields and are not suitable for
the synthesis of enantiomerically pure amino acids.
Results and Discussion
Our synthesis of (purin-6-yl)alanines is based on Pd(0)-
catalyzed cross-coupling reactions of protected iodozin-
calanines 3 and 4 with 6-iodopurines. This approach
should enable a simple, single-step introduction of the
entire alanine moiety to the target molecule and should
be applicable for both racemic and enantiopure amino
acids. Suitable protecting groups for the amino acid
should be easily introduced, survive under the metalation
and cross-coupling conditions, and should be readily
cleaved under mild conditions without racemization.
Therefore, the groups of choice were benzyl ester for
carboxyl functions and Boc or Cbz groups for amino
functions.
(1) (a) Hocek, M.; Holy´, A.; Votruba, I.; Dvorˇa´kova´, H. J. Med. Chem.
2000, 43, 1817-1825. (b) Hocek, M.; Holy´, A.; Votruba, I.; Dvorˇa´kova´,
H. Collect. Czech. Chem. Commun. 2001, 66, 483-499.
(2) (a) Bakkestuen, A. K.; Gundersen, L.-L.; Langli, G.; Liu, F.;
Nolsøe, J. M. J. Bioorg. Med. Chem. Lett. 2000, 10, 1207-1210. (b)
Andresen, G.; Gundersen, L.-L.; Nissen-Meyer, J.; Rise, F.; Spilsberg,
B. Bioorg. Med. Chem. Lett. 2002, 12, 567-569. (c) Gundersen, L.-L.;
Nissen-Meyer, J.; Spilsberg, D. J. Med. Chem. 2002, 45, 1383-1386.
(3) (a) Review: Mu¨ller, C. E. Curr. Med. Chem. 2000, 7, 1269-1288.
(b) Recent example: Volpini, R.; Constanzi, S.; Lambertucci, C.; Taffi,
S.; Vittori, S.; Klotz, K.-N.; Cristalli, G. J. Med. Chem. 2002, 45, 3271-
3279.
A preparation of iodozincalanines was recently devel-
oped by Jackson involving metalation of protected â-iodo-
alanines (e.g., 1),10 available in three steps from serine,
with a zinc/copper10 couple or activated zinc dust.11 The
zincation proceeds smoothly in several minutes, and use
(7) Woenckhaus, C.; Stock, A. Z. Naturforsch. B: Anorg. Chem., Org.
Chem., Biochem., Biophys., Biol. 1965, 20, 400.
(4) Hirao, I.; Ohtsuki, T.; Fujiwara, T.; Mitsui, T.; Yokogawa, T.;
Okuni, T.; Nakayama, H.; Takio, K.; Yabuki, T.; Kigawa, T.; Kodama,
K.; Yokogawa, T.; Nishikawa, K.; Yokoyama, S. Nat. Biotechnol. 2002,
20, 177-182.
(8) (a) Chaman, E. S.; Golovchinskaya, E. S. Zh. Obshch. Khim.
1963, 33, 3342-3349. (b) Chaman, E. S.; Golovchinskaya, E. S. Zh.
Obshch. Khim. 1966, 36, 1608-1613.
(5) For reviews, see: (a) Hocek, M. Eur. J. Org. Chem. 2003, 245-
254. (b) Agrofoglio, L. A.; Gillaizeau, I.; Saito, Y. Chem. Rev. 2003,
103, 1875-1916.
(9) Ovcharova, I. M.; Golovchinskaya, E. S. Zh. Obshch. Khim. 1964,
34, 3254-3259.
(10) Jackson, R. F. W.; Wishart, N.; Wood, A.; James, K.; Wythes,
M. J. J. Org. Chem. 1992, 57, 3397-3404.
(6) Hocek, M. Heterocycles 2004, 63, 1673-1677.
10.1021/jo048812r CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/20/2004
J. Org. Chem. 2004, 69, 7985-7988
7985

Cˇ apek et al.
TABLE 1. Synthesis of (Purine-6-yl)alanines
SCHEME 1. Synthesis of Alanylzinc Iodides
SCHEME 2. Synthesis of (Purin-6-yl)alanines
entry
purine
organozinc
config
product
yield (%)
1
2
3
4
5
6
7
8
5a
5b
5c
5d
5c
5d
5c
5d
3
R,S
R,S
R,S
R,S
R,S
R,S
S
6a
95
88
75
94
80
96
72
95
3
6b
3
6c
3
6d
4
7c
4
7d
(S)-4
(S)-4
(S)-7c
(S)-7d
S
temperature with a Pd(0) catalyst prepared from tris-
(dibenzylideneacetone)dipalladium and tri-o-tolylphos-
phine (4 equiv to Pd2dba3) to give the (purin-6-yl)alanines
6a-6d, 7c, 7d, (S)-7c, and (S)-7d in excellent yields
(Table 1). The slightly lower preparative yields (Table 1:
entries 3, 5, and 7) of triacetylated derivatives 6c, 7c,
and (S)-7c were caused by limited stability of the acetate
protective groups during the final workup of the reaction
mixtures.
The reaction was applied to both racemic and enan-
tiopure amino acids. The enantiopure amino acids are
the ones that will be used as building blocks for the
conjugates, and therefore their racemization-free syn-
thesis was crucial. However, the racemic amino acids
could also be of some advantage, particularly for biologi-
cal activity prescreening (parallel testing of both enan-
tiomers). The use of racemic zincated amino acids in the
synthesis of nucleosides leads to diastereomeric mixtures.
This phenomenon can be used as proof that no racemi-
zation occurs during cross-coupling and deprotection
using the enantiopure (S)-4 by simply comparing the
NMR spectra of the diastereomeric mixtures to those of
the pure diastereoisomers. In the case of the diastereo-
meric mixtures of nucleosides 6c, 6d, 7c, and 7d, two
sets of signals were clearly seen in the NMR spectra (see
the Supporting Information), while the pure diastereo-
isomers (S)-6 and (S)-7 gave a single set of signals.
However, these diastereomeric mixtures were not sepa-
rable on achiral C-18 HPLC. On the other hand, the THP-
protected derivatives 6b and 7b, though also diastereo-
meric mixtures because of an additional chiral center at
the THP, gave only one set of signals in the NMR spectra
and were chromatographically homogeneous. This is
probably caused by a very high conformational flexibility
of the THP group together with a relatively long distance
between the two chiral centers.
of obtained iodozincalanines in Pd-catalyzed cross-
coupling reactions with aryl10-13 or hetaryl10,14 halides led
to numerous aryl- or hetarylalanine derivatives. It was
also established that DMF11 is an excellent solvent for
zincation as well as for subsequent cross-coupling and
that the use of ultrasound10 could increase the rate of
the zincation.
We prepared racemic and enantiopure (S)-iodozincala-
nines 3 and 4 and (S)-4 (Scheme 1) by sonication of the
solution of iodoalanines 1 and 2 and (S)-2 in DMF at room
temperature with zinc dust activated only13 by trimeth-
ylsilyl chloride (instead of the classical sequence11 of 1,2-
dibromoethane and trimethylsilyl chloride). These orga-
nozinc reagents were used in Pd-catalyzed cross-coupling
reactions with 6-iodopurines 5a-5d (9-Bn-6-iodopurine
5a was used as an example of 9-alkylated purines,
9-THP-6-iodopurine 5b is a 9-protected purine base, and
derivatives 5c and 5d are examples of acyl-protected
ribonucleosides or 2-deoxyribonucleosides; Scheme 2).
The reactions were carried out in DMF at ambient
Unlike with the (purin-6-yl)glycines,6 we have man-
aged to deprotect all (purin-6-yl)alanines to give free
amino acids (Table 2). (9-Benzyl-purin-6-yl)alanine 9a
and (9H-purin-6-yl)alanine 9b were obtained from Boc-
protected intermediates 6a and 6b, respectively, by Pd/
C-catalyzed hydrogenolysis of benzyl ester (surprisingly
with simultaneous cleavage of THP in 6b) followed by
treatment of the products with HCl in ethyl acetate or
trifluoracetic acid in DCM (Scheme 3 and Table 2, entries
1-6). All products of these deprotecting sequences were
isolated simply by crystallization from the reaction
solvents.
Similarly, we have tried to deprotect Boc derivatives
6c and 6d and succeeded with cleavage of the Boc group
from ribonucleoside derivative 6c under acidic conditions
(TFA in DCM). Nevertheless, when the same conditions
were applied to 2-deoxyribonucleoside derivative 6d,
(11) Jackson, R. F. W.; Moore, R. J.; Dexter, C. S.; Elliott, J.;
Mowbray, C. E. J. Org. Chem. 1998, 63, 7875-7884.
(12) (a) Jackson, R. F. W.; James, K.; Wythes, M. J.; Wood, A.
Tetrahedron Lett. 1989, 30, 5941-5944. (b) Smyth, M. S.; Burke, T.
R., Jr. Tetrahedron Lett. 1994, 35, 551-554. (c) Fraser, J. L.; Jackson,
R. F. W.; Porter, B. Synlett 1994, 379-380. (d) Dow, R. L.; Bechle, B.
M. Synlett 1994, 293-294. (e) Jung, M. E.; Starkey, L. S. Tetrahedron
1997, 53, 8815-8824. (f) Malan, C.; Morin, C. Synlett 1996, 167-168.
(g) Liu, S.; Dockendorff, C.; Taylor, S. D. Org. Lett. 2001, 3, 1571-
1574. (h) Jackson, R. F. W.; Rilatt, I.; Murray, P. J. Org. Biomol. Chem.
2004, 2, 110-113.
(13) Deboves, H. J. C.; Montalbetti, C. A. G. N.; Jackson, R. F. W.
J. Chem. Soc., Perkin Trans. 1 2001, 1876-1884.
(14) (a) Ye, B.; Burke, T. R., Jr. J. Org. Chem. 1995, 60, 2640-2641.
(b) Swahn, B.-M.; Claesson, A.; Pelcman, B.; Besidski, Y.; Molin, H.;
Sandberg, M. P.; Berge, O.-G. Bioorg. Med. Chem. Lett. 1996, 6, 1635-
1640. (c) Ye, B.; Otaka, A.; Burke, T. R., Jr. Synlett 1996, 459-460.
(d) Walker, M. A.; Kaplita, K. P.; Chen, T.; King, H. D. Synlett 1997,
169-170. (e) Kawata, S.; Ashizava, S.; Hirama, M. J. Am. Chem. Soc.
1997, 119, 12012-12013. (f) Schmidt, B.; Ehlert, D. K. Tetrahedron
Lett. 1998, 39, 3999-4002. (g) Tabanella, S.; Valancogne, I.; Jackson,
R. F. W. Org. Biomol. Chem. 2003, 1, 4254-4261.
7986 J. Org. Chem., Vol. 69, No. 23, 2004

Synthesis of (Purin-6-yl)alanines
TABLE 2. Deprotection of (Purine-6-yl)alanines
11c, 11d, (S)-11c, and (S)-11d from reaction mixtures
nor their separation on ion-exchange resins was success-
ful. Finally, we used preparative reversed-phase HPLC
on C-18 with water/methanol as an eluent for the final
isolation and purification of these products. All com-
pounds were fully characterized including complete as-
signment of all NMR signals using a combination of H,H-
COSY, H,C-HSQC, and H,C-HMBC methods. Again, the
diastereomeric mixtures 10c, 10d, 11c, and 11d gave two
sets of signals in the NMR spectra, and in the case of
the fully deprotected nucleosides 11c and 11d, a reason-
able analytical separation was achieved in HPLC on an
L-proline-based chiral column. The diastereomerically
pure compounds (S)-10c, (S)-10d, (S)-11c, and (S)-11d
gave one set of signals, giving proof that no racemization
occurred during the cross-coupling and deprotection
sequence.
entry
reagent
methoda
product
yield (%)
1
2
6a
A
A
B
B
C
C
D
D
D
D
E
E
E
E
8a
89
84
72
86
84
96
97
85
96
95
80
73
81
70
6b
8b
3
8a
9a‚TFA
9b‚TFA
9a‚HCl
9b‚2HCl
10c
4
8b
5
8a
6
8b
7
7c
8
7d
10d
9
(S)-7c
(S)-7d
10c
10d
(S)-10c
(S)-10d
(S)-10c
(S)-10d
11c
10
11
12
13
14
11d
(S)-11c
(S)-11d
a Methods: (A) H2, Pd/C, EtOH; (B) TFA in DCM; (C) HCl in
EtOAc; (D) aqueous NaOH (2-3 equiv) in THF/MeOH; (E) H2, Pd/
C, Dioxane/H2O.
Conclusions
SCHEME 3. Deprotection of (Purin-6-yl)alaninesa
We have developed a simple and efficient approach for
the synthesis of (purin-6-yl)alanines, a new class of
artificial amino acids or modified purines applicable for
the synthesis of both racemic and enantiopure com-
pounds. Though analogous reactions were used before10-14
for the synthesis of some simple aryl- or hetarylalanines,
this was the first successful use in the functionalization
of protected nucleobases and nucleosides. Furthermore,
we have found suitable combinations of protecting groups
and mild racemization-free conditions for the deprotec-
tion of nucleosides-amino acids conjugates containing
acidolabile nucleosidic bonds. Title (purinyl)alanines 9
and 11 were subjected to biological activity screening. No
considerable antiviral (HCV) or antiproliferative (L1210,
HL-60, HeLa S3, and CCRF-CEM cell lines) activity was
observed. Further efforts will focus on the application of
this methodology in other positions of the purine moiety
and on the synthesis of purine-peptide (or nucleic acid-
peptide) conjugates.
a (i) H2, Pd/C, EtOH. (ii) TFA in DCM. (iii) HCl in EtOAc.
SCHEME 4. Deprotection of (Purin-6-yl)alaninesa
Experimental Section
Preparation of Organozinc Reagents from Iodo-
alanines and Their Cross-Coupling Reactions with
6-Iodopurines, - General Procedure. Trimethylsilyl
chloride (60 µL, 0.5 mmol) was added through a septum
to an argon-purged flask containing a suspension of zinc
dust (1.95 g, 30 mmol) in DMF (3 mL). The mixture was
sonicated at ambient temperature for 15 min. Then a
solution of protected 2-amino-3-iodopropanoate (6.0 mmol)
in DMF (20 mL), prepared under argon, was added
through a septum to the suspension of activated zinc,
sonication continued for another 40 min at ambient
temperature, and then the zinc was allowed to settle. The
supernatant was transferred through a septum to a
mixture of appropriate 6-iodopurine (4.5 mmol), Pd2dba3
(95 mg, 0.1 mmol), and tri-o-tolylphosphine (122 mg, 0.4
mmol) in DMF (15 mL) prepared under argon. The
reaction mixture was stirred at ambient temperature for
8 h and allowed to stand overnight, and then the solvent
was evaporated in vacuo. The residue was diluted by
ethyl acetate (90 mL) and washed with water (2 × 80
mL) and brine (80 mL). The organic phase was evapo-
rated and the residue chromatographed on a silica gel
column (ethyl acetate/hexane) to give (purine-6-yl)ala-
nines.
a (i) Aqueous NaOH (2-3 equiv) in THF/MeOH. (ii) H2, Pd/C,
Dioxane/H2O.
decomposition of the starting compound occurred because
of the lability of its nucleosidic bond under acidic condi-
tions. Because our goal was to find a universal combina-
tion of protective groups for both ribonucleosides and
2-deoxyribonucleosides, we focused on the intermediates
7c and 7d, where the amino group is protected by
hydrogenolytically cleavable benzyl carbamate. Basic
hydrolysis of compounds 7c and 7d (epimeric mixtures)
by aqueous sodium hydroxide (2-3 equiv), which cleaved
all of the ester groups, was followed by Pd/C-catalyzed
hydrogenolysis of benzyl carbamate to afford free nucleo-
sides 11c and 11d (Scheme 4 and Table 2, entries 7 and
8 and entries 11 and 12). This sequence was also applied
for enantiomerically pure compounds (S)-7c and (S)-7d
to give (S)-11c and (S)-11d (Table 2, entries 9 and 10
and entries 13 and 14) with no racemization of the amino
acid moiety. A suitable isolation technique had to be
found for the highly hydrophilic amino acids. Neither
crystallization of compounds 10c, 10d, (S)-10c, (S)-10d,
J. Org. Chem, Vol. 69, No. 23, 2004 7987

Cˇ apek et al.
Benzyl (S)-3-[9-(2,3,5-Tri-O-acetyl-â-D-ribofurano-
syl)purin-6-yl]-2-[(benzyloxycarbonyl)amino]pro-
panoate, (S)-7c. (S)-7c was prepared from (S)-2 (1.15
g, 2.6 mmol) and 5c (1.00 g, 2.0 mmol). Yield: 990 mg
(72%) of (S)-7c as a colorless, amorphous solid. MS (FAB,
m/z): 690 (45, M + 1); 432 (24, M - AcRf + 2); 259 (33);
65.55 (CH2-Ph); 70.60 (CH-3′); 73.83 (CH-2′); 85.97 (CH-
4′); 87.79 (CH-1′); 127.79 (m-CH-Ph); 127.97 (p-CH-Ph);
128.53 (o-CH-Ph); 133.13 (C-5); 137.17 (i-C-Ph); 144.54
(CH-8); 150.56 (C-4); 151.84 (CH-2); 156.08 (CO-Cbz);
158.22 (C-6); 173.11 (CO). [R]20D: -33.5 (c 8.56, MeOH).
(S)-3-[9-(â-D-Ribofuranosyl)purin-6-yl]-2-amino-
propanoic Acid Monohydrate, (S)-11c. Hydrogen was
bubbled through a solution of (S)-10c (379 mg, 0.80
mmol) in water (20 mL), dioxane (20 mL), and acetic acid
(0.2 mL) in the presence of Pd/C catalyst (10 wt %, 40
mg) for 4 h. The catalyst was filtered off on a Celite pad
and the filtrate evaporated in vacuo. The crude product
was purified by preparative HPLC on a C18 column with
water/methanol as the mobile phase. The product was
lyophilized from water to give 230 mg (81%) of (S)-11c
(monohydrate) as a white solid. MS (FAB, m/z): 340 (81,
M + 1); 307 (72); 215 (100). 1H NMR (500 MHz, D2O, δ):
3.73 (dd, 1H, Jgem ) 15.5 Hz, JCH2bCH ) 7.3 Hz, CH2-b);
3.78 (dd, 1H, Jgem ) 15.5 Hz, JCH2aCH ) 5.3 Hz, CH2-a);
3.83 (dd, 1H, Jgem ) 12.8 Hz, J5′b4′ ) 4.0 Hz, H-5′b); 3.90
(dd, 1H, Jgem ) 12.8 Hz, J5′a4′ ) 2.9 Hz, H-5′a); 4.27 (q,
1H, J4′3′ ) 4.0 Hz, J4′5′b ) 4.0 Hz, J4′5′a ) 2.9 Hz, H-4′);
4.35 (dd, 1H, JCHCH2b ) 7.3 Hz, JCHCH2a ) 5.3 Hz, CH);
4.44 (t, 1H, J3′2′ ) 5.3 Hz, J3′4′ ) 4.0 Hz, H-3′); 4.81
(t, 1H, J2′1′ ) 5.6 Hz, J2′3′ ) 5.3 Hz, H-2′); 6.17 (d, 1H,
J1′2′ ) 5.6 Hz, H-1′); 8.66 (s, 1H, H-8); 8.85 (s, 1H, H-2).
13C NMR (125.8 MHz, D2O, ref(dioxane) ) 67.19 ppm):
33.02 (CH2); 53.42 (CH); 61.89 (CH2-5′); 70.99 (CH-3′);
74.39 (CH-2′); 86.20 (CH-4′); 89.06 (CH-1′); 133.24 (C-5);
145.60 (CH-8); 150.74 (C-4); 152.42 (CH-2); 157.71 (C-
6); 173.64 (CO). [R]20D: -22.2 (c 2.70, H2O). Anal. Calcd
for C13H19N5O7: C, 43.70; H, 5.36; N, 19.60. Found: C,
43.35; H, 5.28; N, 19.48.
1
139 (100). H NMR (500 MHz, CDCl3, δ): 2.08 (s, 3H,
CH3); 2.11 (s, 3H, CH3); 2.16 (s, 3H, CH3); 3.66 (dd, 1H,
J ) 4.2 and 16.0 Hz, PuCHAHB); 3.92 (dd, 1H, J ) 5.4
and 16.0 Hz, PuCHAHB); 4.38-4.47 (m, 3H, H-4′ + H-5′);
5.04 (m, 1H, CH from alanine); 5.10 (s, 2H, CH2Ph); 5.12
(s, 2H, CH2Ph); 5.67 (t, 1H, J ) 4.9 Hz, H-3′); 5.96 (t,
1H, J ) 5.4 Hz, H-2′); 6.20 (d, 1H, J ) 5.1 Hz, H-1′); 6.45
(d, 1H, J ) 8.7 Hz, NH); 7.21-7.34 (m, 10H, arom.); 8.15
(s, 1H, H-8); 8.74 (s, 1H, H-2). 13C NMR (125.8 MHz,
CDCl3, δ): 20.35, 20.49, and 20.70 (3 × CH3CO); 34.39
(PuCH2); 52.08 (CH from alanine); 62.98 (CH2-5′); 66.93
and 67.15 (2 × CH2Ph); 70.52 (CH-3′); 72.95 (CH-2′);
80.38 (CH-4′); 86.42 (CH-1′); 128.04, 128.14, 128.18,
128.35, and 128.43 (CH-arom.); 133.42 (C-5); 135.29 and
136.25 (2 × C-arom.); 142.59 (CH-8); 150.27 (C-4); 152.23
(CH-2); 156.08 (NCO); 158.04 (C-6); 169.28, 169.50, and
170.24 (3 × COCH3); 171.08 (COOBn). [R]20D: -26.3 (c
6.27, CHCl3). Anal. Calcd for C34H35N5O11: C, 59.21; H,
5.12; N, 10.15. Found: C, 58.95; H, 5.25; N, 9.80.
(S)-3-[9-(â-D-Ribofuranosyl)purin-6-yl]-2-[(benzyl-
oxycarbonyl)amino]propanoic Acid, (S)-10c. An aque-
ous solution of NaOH (0.59 M, 6.5 mL) was slowly added
at 0 °C to the solution of (S)-7c (799 mg, 1.16 mmol) in
THF (13 mL)/methanol (10 mL), and the reaction mixture
was stirred at ambient temperature for 25 min. The pH
was adjusted to 7 by aqueous HCl (3%), and the solvents
were evaporated in vacuo. The crude product was purified
by preparative HPLC on a C-18 column with water/
methanol as the mobile phase to give 525 mg (96%) of
(S)-10c as a white solid. MS (FAB, m/z): 474 (100, M +
Acknowledgment. This work is a part of the
research project Z4 055 905. It was supported by the
Grant Agency of the Czech Republic (Grant 203/03/0035)
and by Sumika Fine Chemicals, Co. Ltd. (Osaka,
Japan). The authors also thank Kamila Havl´ıcˇkova´ for
technical assistance and Dr. Ivan Votruba for cytostatic
activity screening.
1
1); 342 (77, M - Rf + 2). H NMR (400 MHz, DMSO-d6,
δ): 3.47 (dd, 1H, Jgem ) 15.0 Hz, JCH2bCH ) 8.2 Hz, CH2-
b); 3.58 (dd, 1H, Jgem ) 12.0 Hz, J5′b4′ ) 4.1 Hz, H-5′b);
3.62 (dd, 1H, Jgem ) 15.0 Hz, JCH2aCH ) 6.1 Hz, CH2-a);
3.70 (dd, 1H, Jgem ) 12.0 Hz, J5′a4′ ) 4.2 Hz, H-5′a); 3.99
(q, 1H, J4′5′a ) 4.2 Hz, J4′5′b ) 4.1 Hz, J4′3′ ) 3.5 Hz, H-4′);
4.20 (dd, 1H, J3′2′ ) 5.0 Hz, J3′4′ ) 3.5 Hz, H-3′); 4.66
(t, 1H, J2′1′ ) 5.8 Hz, J2′3′ ) 5.0 Hz, H-2′); 4.87 (dt, 1H,
JCHNH ) 8.5 Hz, JCHCH2b ) 8.2 Hz, JCHCH2a ) 6.1 Hz, CH);
4.99 (s, 2H, CH2-Ph); 6.04 (d, 1H, J1′2′ ) 5.8 Hz, H-1′);
7.26-7.38 (m, 5H, Ph); 7.70 (bd, 1H, JNHCH ) 8.5 Hz, NH);
8.80 (s, 1H, H-8); 8.85 (s, 1H, H-2). 13C NMR (100.6 MHz,
DMSO-d6, δ): 34.57 (CH2); 52.42 (CH); 61.55 (CH2-5′);
Supporting Information Available: General methods;
synthesis of starting compounds; synthetic procedures; char-
acterization data for compounds 6a-6d, 7c, 7d, (S)-7d, 8a,
8b, 9a, 9b, 10c, 10d, (S)-10d, 11c, 11d, and (S)-11d; and
deprotection of 6c under acidic conditions are available free
of charge via the Internet at http://pubs.acs.org.
JO048812R
7988 J. Org. Chem., Vol. 69, No. 23, 2004