Fluorescent charge-neutral analogue of xanthosine: Synthesis of a 2′-deoxyribonucleoside bearing a 5-aza-7-deazaxanthine base

Article abstract of DOI:10.1021/jo005743h

A concise route is described to prepare the 5-aza-7-deazapurine 2′-deoxyriboside (4), which presents the puADA hydrogen-bonding pattern, analogous to the hydrogen-bonding pattern presented by 2′-deoxyxanthosine (2). The route begins with the commercially

Full text of DOI:10.1021/jo005743h

5012
J . Org. Chem. 2001, 66, 5012-5015
F lu or escen t Ch a r ge-Neu tr a l An a logu e of Xa n th osin e: Syn th esis of
a 2′-Deoxyr ibon u cleosid e Bea r in g a 5-Aza -7-d ea za xa n th in e Ba se
Photon Rao† and Steven A. Benner*,†,‡
Department of Chemistry and Department of Anatomy and Molecular Biology, University of Florida,
Gainesville, Florida 32611
benner@chem.ufl.edu
Received November 23, 2000
A concise route is described to prepare the 5-aza-7-deazapurine 2′-deoxyriboside (4), which presents
the puADA hydrogen-bonding pattern, analogous to the hydrogen-bonding pattern presented by
2′-deoxyxanthosine (2). The route begins with the commercially available 1-R-chloro-2-deoxy-3-5-
bistoluoyloxyribofuranose (10), which proves to be a versatile point of entry to â-2′-deoxyribofura-
nosides. In the first step, 2-nitroimidazole (8) is coupled with 10 to yield intermediate 11. Reduction
of the nitro group to an amino group yields 12, which is treated with phenyl isocyanatoformate to
complete the nucleobase to yield 13. Removal of the toluoyloxy protecting groups of 13 yields the
target nucleoside 4 in 40% overall yield in four steps. In an alternative strategy, convergent coupling
of 14 with 10 under basic conditions was attempted but found to yield the heterocycle glycosylated
at the undesired position. Compound 13 displays potentially useful fluorescence properties. After
excitation at 250 nm, a solution of 13 in MeCN shows a fluorescence emission with a maximum at
410 nm. Furthermore, 13 is neutral at physiological pH, a property that it shares with natural
nucleobases but not xanthosine itself, which is an acid with a pKa of ca. 5.6. Furthermore, as part
of the design, 4 is made capable of presenting an unshared pair of electrons to the DNA minor
groove.
In tr od u ction
This includes functionalized derivatives and, most desir-
ably, fluorescent derivatives, well-known to be useful in
studies of natural DNA.8
To generate a fluorescent probe, we focused on the base
pair joined by the pyDAD-puADA hydrogen-bonding
scheme (Figure 1b).1b,9 As originally implemented, this
pair was formed between 2,4-diaminopyrimidine and
xanthine, represented by the C-nucleoside (1) and deoxy-
xanthosine (2) or its 7-deaza analogue (3).10 Neither of
these components is fluorescent. Other heterocycles can
The Watson-Crick nucleobase pair in DNA follows two
rules of complementarity: size complementarity (large
purines pair with small pyrimidines) and hydrogen-
bonding complementarity (hydrogen-bond donors from
one partner complement hydrogen-bond acceptors from
the other partner). Ten years ago, we noted that 12
nucleobases forming six base pairs joined by mutually
exclusive hydrogen-bonding patterns are possible within
the geometry of the Watson-Crick base pair (Figure 1)
and that these might be functionalized to enable a single
biopolymer capable of both genetics and catalysis.1
Expanded genetic alphabets have now been further
explored in a variety of laboratories, and the possibility
of a fully artificial genetic system has recently been
advanced.2-7
Artificial genetic systems require their own set of
manipulative tools, including polymerases to copy DNA
analogues containing artificial nucleoside derivatives,
kinases to phosphorylate them, and chemical methods
to synthesize them. In addition, an artificial genetic
system needs its own set of biophysical tools to permit
us to explore their structure, conformation, and behavior.
(3) (a) Piccirilli, J . A.; Krauch, T.; MacPherson, L. J .; Benner, S. A.
Helv. Chim. Acta 1991, 74, 397. (b) Voegel, J . J .; Altorfer, M. M.;
Benner, S. A. Helv. Chim. Acta 1993, 76, 2061. (c) Voegel, J . J .; von
Krosigk, U.; Benner, S. A. J . Org. Chem. 1993, 58, 7542. (d) Heeb, N.
V.; Benner, S. A. Tetrahedron Lett. 1994, 35, 3045. (e) Voegel, J . J .;
Benner, S. A. J . Am. Chem. Soc. 1994, 116, 6929. (f) von Krosigk, U.;
Benner, S. A. J . Am. Chem. Soc. 1995, 117, 5361. (g) Baeschlin, D. K.;
Daube, M.; Blaettler, M. O.; Benner, S. A.; Richert, C. Tetrahedron
Lett. 1996, 37, 1591. (h) Voegel, J . J .; Benner, S. A. Helv. Chim. Acta
1996, 79, 1863. (i) J urczyk, S. C.; Battersby, T. R.; Kodra, J . T.; Park,
J . H.; Benner, S. A. Helv. Chim. Acta 1999, 82, 1005.
(4) (a) Schweitzer, B. A.; Kool, E. T. J . Am. Chem. Soc. 1995, 117,
1863. (b) Ren, R. X. F.; Chaudhuri, N. C.; Paris, P. L.; Rumney, S.;
Kool, E. T. J . Am. Chem. Soc. 1996, 118, 7671. (c) Morales, J . C.; Kool,
E. T. Nat. Struct. Biol. 1998, 5, 950.
(5) (a) Berger, M.; Ogawa, A. K.; McMinn, D. L.; Wu, Y. Q.; Schultz,
P. G.; Romesberg, F. E. Angew. Chem., Intl. Ed. 2000, 3, 2940. (b) Wu,
Y. Q.; Ogawa, A. K.; Berger, M.; McMinn, D. L.; Schultz, P. G.;
Romesberg, F. E. J . Am. Chem. Soc. 2000, 122, 7621.
* To whom correspondence should be addressed. Tel: +352-392-
7773; fax: +352-846-2095.
(6) Service, R. F. Science 2000, 232, 5477.
(7) Seela, F.; Wei, C. F. Chem. Commun. 1997, 1869.
(8) Strassler, C.; Davis, N. E.; Kool, E. T. Helv. Chim. Acta 1999,
82, 2160.
† Department of Chemistry.
‡ Department of Anatomy and Molecular Biology.
(1) (a) Switzer, C. Y.; Moroney, S. E.; Benner, S. A. J . Am. Chem.
Soc. 1989, 111, 8322. (b) Piccirilli, J . A.; Krauch, T.; Moroney, S. E.;
Benner, S. A. Nature (London) 1990, 343, 33.
(2) (a) Incorporation into RNA: Piccirilli, J . A.; Moroney, S. E.;
Benner, S. A. Biochemistry 1991, 30, 10350. (b) Switzer, C. Y.; Moroney,
S. E.; Benner, S. A. Biochemistry 1993, 32, 10489. (c) Voegel, J . J .;
Benner, S. A. Helv. Chim. Acta 1996, 79, 1881. (d) Lutz, M.; Horlacher,
J .; Benner, S. A. Bioorg. Med. Chem. Lett. 1998, 8, 499.
(9) pyDAD: Pyrimidine analogue carrying a H-bond donor-accep-
tor-donor motif. Similarly, puADA: purine analogue carrying an
acceptor-donor-acceptor motif.
(10) (a) Horlacher, J .; Hottiger, M.; Podust, V. N.; Huebscher, U.;
Benner, S. A. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 6329. (b) Lutz,
M. J .; Held, H. A.; Hottiger, M.; Hubscher, U.; Benner, S. A. Nucleic
Acids Res. 1996, 24, 1308. (c) J urczyk, S. C.; Horlacher, J .; Devine, K.
G.; Benner, S. A.; Battersby, T. R. Helv. Chim. Acta 2000, 83, 1517.
10.1021/jo005743h CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/30/2001

Charge-Neutral Analogue of Xanthosine
J . Org. Chem., Vol. 66, No. 15, 2001 5013
Sch em e 1a
a
Key: (a) K2CO3, TDA-1, MeCN, rt, 80%; (b) 10% Pd-C, H2 (1
atm), THF, rt, quantitative; (c) PhOC(O)NCO, dioxane, rt, 55%;
(d) MeOH-NH3, rt, 60%.
Sch em e 2a
F igu r e 1. (a) Complementary Watson-Crick pairing between
pyDAD (1) and puADA derivatives 2 and 3. (b) Imidazo[1,2-
a]triazine (purine in parentheses) and sugar numberings
showed in 4. (c) The rare tRNA constituent wyosine. (d) The
puDAA analogue 7.
support a puADA hydrogen-bonding pattern, however.
In particular, 4, a 5-aza-7-deazapurine[imidazo[1,2-a]-
pyrimidine] analogue, was attractive. We speculated that
since the base moiety of the rare tRNA constituent
wyosine (5), carrying a 5-aza-7-deazapurine substructure,
is fluorescent, then 4 might also be fluorescent. The ribo
analogue 6 of this molecule was prepared over 20 years
ago by Moffat and his colleagues, but its photophysical
properties were not mentioned.11
Synthetic approaches to the 5-aza-7-deazapurine 2′-
deoxyriboside (7) were reported in these laboratories,3b
but the synthetic routes used by both this laboratory and
Moffat’s for analogous compounds suggested that more
efficient approaches were conceivable. This paper reports
a simple and direct synthesis to prepare the 2′-deoxyri-
bonucleoside (4) carrying imidazo[1,2-a]-1,3,5-triazine-
2(8H),4(3H)-dione (5-aza-7-deazaxanthine).
a
Key: (a) PhOC(O)NCO, NaH, DMF, rt, 60%; (b) K2CO3, TDA-
1, MeCN, 10, rt, 24%.
Reduction of the nitro group to the amino group was
carried out quantitatively by hydrogenation using 10%
Pd-C as the catalyst. The amino derivative (12) was
cyclized using phenyl isocyanatoformate in dioxane to
yield 13, which possesses the required 5-aza-7-deaza
skeleton, in 50% yield. Deprotection of the toluoyloxy
functionalities was performed in methanolic ammonia
over 3 days to yield the nucleoside (4) in 80% yield.
In addition to the linear synthesis, a convergent
protocol was also evaluated for the synthesis of 4 (Scheme
2). Here, the nucleobase imidazo[1,2-a]-1,3,5-triazine-2,4-
(3H,8H)-dione (5-aza-7-deazaxanthine, 14) was synthe-
sized from 9 and obtained in situ from 2-aminoimidazo-
lium sulfate (15) and phenyl isocyanatoformate. Conceiva-
bly, 14 may exist as the tautomer 16. Interestingly,
tautomer 16 was calculated to be favored by ca. 5 kcal/
mol over 14 in the gas phase, following semiempirical
AM1 calculations.14 The base (14/16) was reacted with
chlorosugar (8) under phase-transfer conditions. Resolu-
tion of the reaction mixture by silica gel column chro-
matography using 5-10% MeOH/CH2Cl2 yielded a single
Resu lts a n d Discu ssion
Previously, Vorbrueggen ribosylation of 2-nitroimida-
zole (8) or 2-aminoimidazole (9) had been used as the
stereoselective step toward building the ADA (5-aza-7-
deaza) backbone in the riboside 6.10 However, the deoxy-
genation at the 2′ position necessary to generate the 2′-
deoxyribo derivative 4 makes this process lengthy and
low-yielding. We decided to employ glycosylation of 8 as
the key step, making use of the commercially available
1-R-chloro-2′-deoxyribose derivative (10) as the glycosyl
donor. Reaction of 8 with 10 in the presence of K2CO3
and tris[2-(2-methoxyethoxy)ethyl]amine (TDA-1) (phase-
transfer conditions) yielded the nitrosugar (11) in 80%
yield (Scheme 1).12,13 K2CO3 was chosen because other
bases (e.g., NaH and KOH) have been reported to yield
mixtures of the 1′-R and the 1′-â epimers.
1
component (17). H NMR showed that both the nucleo-
base and the toluoyloxy-protected 2′-deoxyribofuranose
moiety were present. Nucleophilic substitutions by the
5-aza-7-deaza ring system may be preferred at the 1
(purine-3) position however.3b Comparison with the un-
ambiguously synthesized nucleoside (4) suggested that
the derivative (17) had its sugar joined at the 1′ position
to the “1 position” of the nucleobase. The riboderivative
(18) of 17 is known in the literature, and the convergent
synthesis outlines another possible route to this class of
nucleosides.8a The “1 vs 8” selectivity in alkylation of
5-aza-7-deazapurine analogues is influenced by the sub-
stituents at the 2 and 3 positions because it is reported
that the imidazopyrimidine derivative (19) reacts with
10 in the presence of K2CO3 and TDA-1 to yield derivative
(11) (a) Prisbe, E. J .; Verheyden, J . P. H.; Moffatt, J . G. J . Org.
Chem. 1978, 43, 4774. (b) Prisbe, E. J .; Verheyden, J . P. H.; Moffatt,
J . G. J . Org. Chem. 1978, 43, 4784.
(12) Seela, F.; Gumbiowski, R. Liebigs Ann. Chem. 1992, 7, 679.
(13) The â-stereochemistry at the 1′ position was confirmed by a
NOESY experiment following the observation of NOE between 1′-H
and 4′-H and the absence of the same between 1′-H and 3′-H.
(14) Details of the MO calculations will be published elsewhere.

5014 J . Org. Chem., Vol. 66, No. 15, 2001
Rao and Benner
Sch em e 3a
a
Key: (a) K2CO3, TDA-1, MeCN, 10, rt (ref 11). (b) Na2CO3/
DMF/BnBr, rt (ref 3b).
20 (with an 8-1′ linkage, Scheme 3) while the thio
analogue (21) of 14 reacts under similar conditions to
yield 22 (with an 1-1′ linkage).11,3b
To learn about the fluorescence properties of the
imidazo[1,2-a]-1,3,5-triazine-2(8H),4(3H)-dione moiety,
the UV-visible absorption spectrum and the fluorescence
emission spectrum of 13 (Figure 2panels a and b,
respectively) were obtained. Compound 13 fluoresces
with a blue color after excitation at 250 nm, with two
emission maxima (λmax ) 410 and 580 nm).
Glycosylation of bases or base precursors is complicated
due to the ambiguity of the stereochemical outcome (R/
â) of the bond formed. The synthesis of 4 is further
complicated by the regiochemistry of the newly formed
bond (1-1′ or 8-1′). Employing the deoxyribose deriva-
tive 10 determines both the desired â-stereochemistry
and the desired regiochemistry (1-8′) in the very first
step.
F igu r e 2. (a) UV-visible absorption spectrum of 13 in MeCN.
(b) Emission spectrum of 13 in MeCN (λmax ) 410 nm).
Compound 4 turns out to have other advantages over
2/3 as a component of an expanded genetic alphabet.
Xanthosine is an acid with a pKa of ca. 5.6.15,16 Therefore,
the nucleobase is almost fully deprotonated at pH 7.4 and
introduces a negative charge into the stacked nucleobases
in the duplex. In contrast, 4 lacks a charge at physiologi-
cal pH; an 80 mM solution of 4 in water shows a pH of 7,
while a 15 mM aqueous solution of xanthosine, in
contrast, shows (as expected) a pH of 4.
Furthermore, as part of the design, 4 is capable to
present an unshared pair of electrons to the minor groove
in DNA. We and others have suggested that this might
be a recognition feature demanded by many polymer-
ases.17-20 Xanthosine and 7-deazaxanthosine derivatives
(such as 2 and 3) lack the sp2 lone pair of electrons at
the 3 position of the purine.
Exp er im en ta l Section
Gen er a l. Chlorosugar 10 was purchased from Berry &
Associates. Measurement of pH was performed using color-
pHast indicator strips purchased from EM Science. Anhyd
MeCN, THF, DMF, and dioxane were purchased from Aldrich
Chemical Co. Silica gel (200-450 mesh) was purchased from
Fischer Scientific. NMR spectra: Varian XL-300 spectrometer
at 300 MHz referenced to SiMe4 (1H), at 75.4 MHz referenced
to CDCl3, DMSO-d6, or MeOD-d4 (13C). MS analyses: Finnigan
MAT-95Q apparatus. UV-visible absorption spectra: Varian
Cary 1-Bio UV-visible spectrophotometer. Emission spectra:
Perkin-Elmer LS50B luminescence spectrophotometer. All
reactions were performed under Ar. Volatiles were removed
using a rotary evaporator and, finally, in vacuo. Chemical
yields reported refer to the yields of pure isolated materials.
8-(â-D-2′-Deoxyr ib ofu r a n osyl)im id a zo[1,2-a ]-1,3,5-t r i-
a zin e-2(8H),4(3H)-d ion e (4). Solid 13 (0.25 g, 0.5 mmol) was
dissolved in methanolic ammonia (50 mL) and stirred at rt
for 3 days. The volatiles were removed, and the crude material
was purified by column chromatography on silica gel using
10-20% EtOH/CH2Cl2 as eluent to yield 0.11 g (80%) of 4 as
a white solid. 1H NMR (300 MHz, D2O): δ 2.50-2.60 (m, 2H),
3.75 (dd, 1H, J ) 5 Hz, J ) 13 Hz), 3.85 (dd, 1H, J ) 5 Hz, J
) 13 Hz), 4.05-4.15 (m, 1H), 4.50-4.60 (m, 1H), 6.27 (t, 1H,
(15) Miles, H. Y.; Roy, K. B. Nucleosides Nucleotides 1983, 2, 231.
(16) The use of 2 appears to have some limitations due to its rather
high acidity causing ionization, which seems to disrupt the base pairing
between
1 and 2 in duplex DNA at neutral pH. Also, charged
nucleobases may be undesirable polymerase substrates. Battersby, T.;
Geyer, R.; Benner, S. A. Unpublished results.
(17) Diederichsen, U. Angew. Chem., Intl. Ed. 1998, 37, 1655 and
references therein.
(18) Cosstick, R.; Li, X.; Tuli, D. K.; Williams, D. M.; Connolly, B.
A.; Newman, P. C. Nucleic Acids Res. 1990, 18, 447.
(19) Morales, J . C.; Kool, E. T. J . Am. Chem. Soc. 2000, 122, 1001.
(20) Evaluation of 3-deaza analogues of 2′-deoxyadenosine and 2′-
deoxyguanosine derivatives are being pursued in our laboratory toward
gaining more insight into this problem.

Charge-Neutral Analogue of Xanthosine
J . Org. Chem., Vol. 66, No. 15, 2001 5015
J ) 9 Hz), 7.40 (1H, d, J ) 2.5 Hz), 7.44 (1H, d, J ) 2.5 Hz).
13C NMR (75 MHz, D2O): 39.1, 62.0, 71.5, 84.8, 88.0, 109.6,
117.3, 147.3, 150.8, 158.6. FAB (positive) MS (% relative
abundance): 269 (M+, 5), 221 (50), 207 (50), 149 (60), 147 (100).
HRMS-FAB (m/z): [M+] calcd for C10H13N4O5, 269.0886;
found, 269.0878.
NMR (300 MHz, MeOD-d4): δ 2.40 (s, 3H), 2.43 (s, 3H), 2.36-
2.56 (m, 1H), 2.78-2.82 (m, 1H), 4.58-4.70 (m, 3H), 5.70-
5.76 (m, 1H), 6.35 (t, 1H, J ) 6.5 Hz), 7.22-7.34 (m, 6H), 7.90
(d, 2H, J ) 8.0 Hz), 7.98 (d, 1H, J ) 3.3 Hz). 13C NMR (75
MHz, CDCl3): δ 20.0, 38.1, 64.4, 74.8, 86.0, 87.1, 109.1, 114.8,
125.6, 126.7, 129.0, 129.3, 129.6, 130.0, 143.3, 144.0, 145.0,
150.0, 155.3, 166.2, 166.4. FAB (positive) MS (% relative
abundance): 505 (MH+, 5), 353 (M - free base C5H3O2N4, 20),
154 (10), 153, (10), 137 (10), 136 (10), 119 (70), 91 (20), 81 (100).
HRMS-FAB (m/z): [MH+] calcd for C26H25O7N4, 505.1723;
found, 505.1738.
Im id a zo[1,2-a ]-1,3,5-tr ia zin e-2,4(3H,8H)-d ion e (14). To
a slurry of 15 (0.5 g, 1.9 mmol) in anhyd DMF (5 mL) was
added a 60% dispersion of NaH in oil (0.46 g, 11.4 mmol), and
it was stirred for 2 h at rt. Phenyl isocyanatoformate (1.0 mL,
7.6 mmol) was added, and stirring was continued for another
8 h. Water (1 mL) was added, the volatiles were removed, and
the residue was crystallized from cold water to yield 14 as an
off-white powder (0.5 g, 90% yield). 1H NMR (300 MHz, DMSO-
d6): δ 7.01 (d, 1H, J ) 2 Hz), 7.32 (d, 1H, J ) 2 Hz), 11. 4 (s,
1H). 13C NMR (75 MHz, DMSO-d6): δ 110.0, 125.0, 146.0,
146.8, 152.5.
1-(â-D-2′-Deoxy-3′,5′-b is(4-t olu oyloxy)r ib ofu r a n osyl)-
im id a zo[1,2-a ]-1,3,5-t r ia zin e-2(1H ),4(3H )-d ion e (17). A
slurry of 14 (0.1 g, 0.7 mmol) in anhyd MeCN (50 mL) was
stirred along with K2CO3 (0.5 g, 3.62 mmol) at rt and TDA-1
(10 µL, 0.02 mmol) for 1 h after which period of time the sugar
10 (0.54 g, 1.4 mmol) was added, and stirring was continued
for 2 h. The mixture was filtered off, and the volatiles were
removed. The crude material was purified by column chroma-
tography on silica gel using 5-10% MeOH/CH2Cl2 as eluent
to yield 0.08 g (24%) of 17 as a white solid. 1H NMR (300 MHz,
MeOD-d4): δ 2.30 (s, 3H), 2.35 (s, 3H), 2.65 (d, 1H, J ) 13.6
Hz), 2.80-3.00 (m, 1H), 4.40-4.50 (m, 2H), 4.90-4.95 (m, 1H),
5.58 (d, 1H, J ) 6.5 Hz), 6.25 (d, 1H, J ) 6.5 Hz), 7.15 (d, 2H,
J ) 8.0 Hz), 8.22 (d, 2H, J ) 8.0 Hz), 7.30 (d, 1H, J ) 3.3 Hz),
7.38 (d, 1H, J ) 3.3 Hz), 7.70 (d, 2H, J ) 8.0 Hz), 7.85 (d, H,
J ) 8.0 Hz). 13C NMR (300 MHz, MeOD-d4): δ 22.0, 38.0, 66.2,
76.6, 86.2, 88.5, 109.0, 118.5, 127.1, 127.6, 129.5, 130.0, 145.1,
145.6, 146.6, 158.5, 168.1, 168.5. FAB (positive) MS (% relative
abundance) 505 (MH+, 6), 353 (25).
1-(â-D-2′-Deoxy-3,5-bis(4-tolu oyloxy)r ibofu r a n osyl)-2-
n itr oim id a zole (11). A slurry of 8 (1.0 g, 8.9 mmol) was
stirred in anhyd MeCN (500 mL) at rt along with K2CO3 (4.0
g, 29 mmol) and TDA-1 (100 µL, 0.21 mmol) for 1 h, after which
period of time sugar 10 (4.5 g, 11.6 mmol) was added, and
stirring continued for 2 h. The mixture was filtered off, and
the volatiles were removed. The crude material was purified
by column chromatography on silica gel using 1-2% Me2CO/
CH2Cl2 as eluent to yield 3.3 g (80%) of 11 as a white foam.
1H NMR (300 MHz, CDCl3): δ 2.40 (s, 3H), 2.44 (s, 3H), 2.40-
2.45 (m, 1H), 3.05-3.20 (m, 1H), 4.60-4.80 (m, 3H), 5.60-
5.67 (m, 1H), 6.80 (t, 1H, J ) 6.5 Hz), 7.12 (d, 1H, J ) 1.4
Hz), 7.23 (d, 2H, J ) 8.0 Hz), 7.29 (d, 2H, J ) 8.0 Hz), 7.59 (d,
1H, J ) 1.4 Hz), 7.84 (d, 2H, J ) 8.0 Hz), 7.96 (d, 2H, J ) 8.0
Hz). 13C NMR (75 MHz, CDCl3): δ 22.0, 41.5, 65.0, 74.1, 85.5,
90.0, 121.5, 126.0, 126.5, 129.0, 129.4, 129.6, 130.1, 145.0,
145.5, 167.2, 168.0. FAB (positive) MS (% relative abun-
dance): 353 (100), 112 (80).
1-(â-D-2′-Deoxy-3,5-bis(4-tolu oyloxy)r ibofu r a n osyl)-2-
a m in oim id a zole (12). A solution of 11 (0.50 g,1.1 mmol) in
THF (20 mL) was reduced in the presence of 10% Pd-C (0.42
g, 0.4 mmol) and H2 (1 atm) for 8 h at rt. The mixture was
filtered to remove the catalyst and washed with ethyl acetate
(2 × 10 mL). The volatiles were removed to obtain 0.50 g of
the amino sugar 12, which was used for the cyclization without
further purification. 1H NMR (300 MHz, CDCl3): δ 2.38 (s,
3H), 2.40 (s, 3H), 2.43-2.60 (m, 1H), 2.80-3.00 (m, 1H), 4.20-
4.40 (bs, 2H), 4.65 (s, 1H), 4.66-4.70 (m, 2H), 5.60-5.62 (m,
1H), 5.90-6.00 (m, 1H), 6.60 (s, 2H), 6.63 (d, 1H, J ) Hz),
7.22(d, 2H, J ) 8.0 Hz), 7.27(d, 2H, J ) 8.0 Hz), 7.86 (d, 2H,
J ) 8.0 Hz), 7.88 (d, 2H, J ) 8.0 Hz). 13C NMR (75 MHz,
CDCl3): δ 22.5, 35.9, 64.3, 75.0, 83.1, 85.2, 113.0, 125.4, 127.0,
127.1, 129.5, 129.6, 129.7, 130.3, 130.4, 144.8, 145.0, 149.0,
166.9, 167.0. FAB (positive) MS (% relative abundance): 436
(MH+, 20), 353 (5), 281 (5), 223 (5), 207 (5), 147 (15), 119 (65),
91 (20), 84 (free base + 2H+, 35), 81 (100).
8-(â-D-2′-Deoxy-3′,5′-b is(4-t olu oyloxy)r ib ofu r a n osyl)-
im id a zo[1,2-a ]-1,3,5-tr ia zin e-2(8H),4(3H)-d ion e (13). To a
solution of 12 (0.48 g, 1.1 mmol) in anhyd dioxane (2 mL) was
added phenyl isocyanatoformate (225 µL, 1.7 mmol), and it
was stirred for 8 h at rt. After addition of MeOH (5 mL), the
volatiles were removed, and the crude material was purified
by column chromatography on silica gel using 5-10% MeOH/
CH2Cl2 as eluent to yield 0.30 g (55%) of 13 as a white solid.
1H NMR (300 MHz, CDCl3): δ 2.35 (s, 3H), 2.45 (s, 3H), 2.45-
2.55 (m, 1H), 2.89-2.90 (m, 1H), 4.50-4.80 (m, 3H), 5.65 (dd,
1H, J ) 6 Hz, J ) 8.0 Hz), 6.96 (d, 1H, J ) 3.2 Hz), 7.14 (d,
1H, J ) 3.2 Hz), 7.23 (d, 2H, J ) 8.0 Hz), 7.27 (d, 2H, J ) 8.0
Hz), 7.88 (d, 2H, J ) 8.0 Hz), 7.94 (d, 2H, J ) 8.0 Hz). 1H
Ack n ow led gm en t. This work was supported in part
by grants from the NIH (GM 54068), the Office of Naval
Research (N00025-96-1-0362), and the NASA Astrobi-
ology Institute.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectral data (in CDCl3) for compounds 4, 11, 12, and 13 and
1H NMR spectral data (in MeOD-d4) for 13 and 17. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O005743H