9532 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Chatgilialoglu et al.
Table 3. Vertical Optical Transitions Computed at the UB3LYP/
mixture of R- and â-anomers, in benzene (10 mL) kept under argon,
were consecutively added tris(trimethylsilyl)silane (342 mg; 1.38 mmol)
and AIBN (0.09 mmol; 15 mg). The reaction mixture was heated at
reflux for 2 h. TLC (eluent: n-hexane/ethyl acetate 7/3) showed the
consumption of the starting material. The solvent was evaporated under
vacuum affording a syrup (R:â diastereoisomeric mixture of the reduced
product) which was separated by silica gel flash-chromatography (n-
hexane/ethyl acetate 8/2). A first fraction containing the R-anomer 16
(150 mg; 0.32 mmol; 35% yield) was eluted and evaporated under
vacuum. Addition of hexane resulted in precipitation of white crystals.
Mp 162-164 °C. 1H NMR (CDCl3): δ 0.01, 0.04, 0.13, 0.14 (s, each
3H, SiMe3), 0.79, 0.94 (s, each 9H, SiBut), 2.94 (m, 2H, 2′-H), 3.74 (t,
2H, J ) 2.8 Hz, 5′-H), 4.36 (m, 1H, 4′-H), 4.44 (m, 1H, 3′-H), 5.72
(d, 1H, J ) 8.5 Hz, 5-H), 7.57 (d, 1H, J ) 8.5 Hz, 6-H), 8.27 (bs, 1H,
NH). 13C NMR (CDCl3): δ -5.7, -5.4, -5.1, -5.0 (each CH3), 17.6,
18.3 (each C), 25.4, 25.9 (each 3 × CH3), 47.4, 62.7 (each CH2), 72.4
(CH), 88.2 (C), 93.4 (CH), 101.8 (CH), 115.4 (C), 137.9 (CH), 149.4,
163.0 (C). MS: m/z (rel intensity), 482 (5) (M + 1)+, 424 (18), 261
(19), 187 (39), 169 (25), 89 (51), 73 (100). HR-MS: calcd for (M -
t-Bu)+, 424.1724; found, 424.1719. A second fraction was eluted
containing the â-anomer (220 mg; 0,47 mmol; 51% yield) which had
spectral characteristics identical to those reported.26c
Preparation of Compound 14. To a magnetically stirred solution
of 16 (150 mg; 0.31 mmol) in dry THF (0.5 mL), kept under argon at
-78 °C, was slowly added tert-butyllithium (0.93 mmol; 0.6 mL of a
1.6 M solution in hexane) via syringe. The reaction was stirred for 30
min, then quenched by adding a saturated solution of 0.1 N HCl at
low temperature and worked up by partitioning with ethyl acetate and
water. The organic phase was dried with anhydrous sodium sulfate,
and solvent was evaporated to afford an oil which was separated by
silica gel flash chromatography (ethyl acetate/hexane 7/3). The first
fraction gave the silylated compound 14 (16 mg; 20% yield based on
the consumption of starting material). The silylated 14 was immediately
dissolved in dry MeOH (5 mL), in a Wheaton reactor equipped with a
stopper, with an excess of NH4F (16 mg) and stirred at 60 °C overnight.
The solvent was evaporated under vacuum, and the residue was
chromatographed on silica gel with ethyl acetate increasing the amount
of methanol up to 5%. The ketone 14 (7 mg; 77% yield) was further
purified by preparative TLC with ethyl acetate/MeOH (95/5) as eluent.
1H NMR (acetone-d6): δ 1.15 (s, 9H, But), 2.12 (dd, 1H, J ) 2.4,
14.4, 2′-H), 3.42 (dd, 1H, J ) 5.6, 14.4 Hz, 2′--H), 3.49 (m, 2H, 5′-
H), 4.44 (m, 2H, 3′-H, 4′-H), 5.66 (d, 1H, J ) 8.4 Hz, 5-H), 7.88 (d,
1H, J ) 8.4 Hz, 6-H), 10.1 (bs, 1H, NH). 13C NMR (CD3CN): δ 29.5
(3 × CH3), 42.2 (CH2), 43.8 (C), 62.1 (CH2), 71.8 (CH), 92.4 (CH),
100.7 (C), 101.4 (CH), 141.3 (CH), 151.4, 163.9, 204.1 (C). Anal. Calcd
for C14H20N2O6 (C, H, N): 53.84, 6.45, 8.97. Found: 53.98, 6.44, 8.94.
Product Studies from Continuous Photolysis. A Pyrex tube
containing the ketone 13 or 14 (0.01 M) and the appropriate thiol
dissolved in H2O (200 mL) was deoxygenated and photolyzed in a
Rayonet photochemical reactor (equipped with six lamps of 350 nm)
for 90 min at ca. 40 °C. The consumption of the ketone 13 or 14 and
the formation of uracil and its derivatives 19 and 20 were followed by
using HPLC analysis on a reverse-phase analytical column (Waters
Spherisorb S5ODS2, 150 × 4.6 mm, 3 mm) using authentic samples
as references. Absolute concentrations were interpolated from external
standard calibration curves. In some experiments, the reaction mixture
was lyophilized followed by 1H NMR analysis. In all cases the
agreement between the two techniques was found to be excellent. The
consumption of the starting ketone varied between 40 and 60%, whereas
the yields of 2′-deoxyuridine were higher than 80% based on the
disappearance of the starting material. In all experiments, a small
amount of free base was obtained. Table 1 shows the details of these
experiments.
6-311G**//HF/6-31G* Level with the TD-DFT Methoda
species
λ (nm)
f
transitionb
expt (nm)
320
17
378
296
400
308
405
318
236
0.006
0.010
0.013
0.094
0.011
0.106
0.167
a
b
a
b
a
b
c
26
6
330
27
261c
a Wavelengths (λ) and oscillator strengths (f) for transitions with f
> 0.005. b Transition {spin}: a ) SOMO f π*(base){R}; b ) π(base)
f SOMO{â}; c ) π(base) f π*(base). c Reference 31.
similar absorbances. The kinetics obtained with the 2′-deoxy-
uridin-1′-yl radical are likely to provide good estimates for rate
constants of reactions of other C1′ nucleoside radicals, especially
with thiols that effect “repair” reactions. The good agreement
between the structural features of the 2′-deoxyuridin-1′-yl radical
found computationally and those indicated by the ESR spectrum
of this radical17 is noteworthy, particularly the pyramidalization
at the radical center. This agreement suggests that structural
features of other C1′ nucleoside radicals computed at the level
of theory applied here will be reliable.
Experimental Section
(5-Methyl-2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-yl)ethanoic Acid
2-Thioxo-2H-pyridin-1-yl Ester (7). A solution of (5-methyl-2,4-
dioxo-3,4-dihydro-2H-pyrimidin-1-yl)ethanoic acid47 (0.18 g, 0.98
mmol) and 2,2′-dithiobis(pyridine-N-oxide) (0.26 g, 1.07 mmol) in THF
(10 mL) was cooled to 0 °C. The flask was shielded from light.
Tributylphosphine (0.27 mL, 1.1 mmol) was added dropwise via
syringe. The mixture was stirred at 0 °C for 1 h. Addition of benzene
(1 mL) resulted in precipitation of 7. The precipitate was filtered off
and washed with hexane and benzene to give 7 as a yellow solid in
1
0.175 g (61%) yield. The sample was judged to be >95% pure by H
1
NMR. Mp 152-153 °C dec with gas evolution. H NMR (acetone-
d6-DMSO-d6): δ 1.84 (d, J ) 1.2 Hz, 3 H), 5.00 (s, 2 H), 6.88 (td, J
) 7.0, 1.8 Hz, 1 H), 7.43 (ddd, J ) 8.5, 7.0, 1.5 Hz, 1 H), 7.56 (q, J
) 1.2 Hz, 1H), 7.59 (dd, J ) 8.5, 1.8 Hz, 1 H), 8.17 (dd, J ) 7.3, 1.2
Hz, 1 H). 13C NMR (CDCl3-DMSO-d6): δ 10.3 (CH3), 45.0 (CH2),
108.0 (C), 111.5 (CH), 133.1 (CH), 134.2 (CH), 137.1 (CH), 138.8
(CH), 149.1 (C), 162.5 (C), 163.0 (C), 172.6 (C).
se-Phenyl (5-Methyl-2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-yl)-
selenoethanoate (8). Tributylphosphine (0.7 mL, 0.0028 mol) was
added dropwise to a solution of methyl (5-methyl-2,4-dioxo-3,4-
dihydro-2H-pyrimidin-1-yl)ethanoate47a (0.400 g, 0.0022 mol) and
N-(phenylseleno)phthalimide (0.85 g, 0.00282 mol) in anhydrous DMF
(15 mL) at 10 °C. The solution was stirred overnight at room
temperature. DMF was removed under vacuum. Chloroform (4 mL)
was added to the residue, and the resulting mixture was filtered to
remove precipitated phthalimide. The chloroform was removed under
vacuum, and the resulting crude product was chromatographed on silica
gel (hexanes-acetone, 7:3) to give 8 contaminated with tributylphos-
phine oxide. Recrystallization from ethyl acetate-hexanes gave pure
8 as a white solid (0.28 g, 40%). Mp 181-182.5 °C. 1H NMR (DMSO-
d6): δ 1.76 (s, 3 H), 4.75 (s, 2 H), 7.38-7.48 (m, 5 H), 7.58 (s, 1 H),
11.52 (bs, 1 H). 13C NMR (DMSO-d6): δ 12.0 (CH3), 59.0 (CH2),
109.3 (C), 124.5 (C), 129.4 (CH), 129.7 (CH), 135.8 (CH), 141.2 (CH),
150.9 (C), 164.3 (C), 197.4 (C). MS (ESI): 324.87 (M + H)+, 346.84
(M + Na)+, 670.87 (2M + Na)+. MS (EI): m/z (rel intensity), 167
(49), 157 (23), 139 (48), 96 (100), 78 (68), 77 (51), 51 (33). HRMS
(CI): calcd for C13H13N2O3Se (M + H)+, 325.0091; found, 325.0088.
Radical Reduction of a Mixture of Diastereoisomers of 15. To a
magnetically stirred solution of 15 (500 mg; 0.92 mmol), as a 1:1.5
Laser Flash Photolysis. Laser flash photolysis experiments were
performed on an Applied Photophysics LK-50 kinetic spectrometer
employing a Nd:YAG laser with a 7 ns pulse with power of ca. 40
mJ/pulse at 266 nm and ca. 75 mJ/pulse at 355 nm. Dilute, He-sparged
solutions of 10 or 12 in THF or of 13 in water (absorbances of ca. 0.4
AU at the irradiating wavelength) at (22 ( 2) °C were allowed to flow
through the spectrometer cell. The solutions contained various con-
centrations of thiols or salts. Spectra were obtained in 2 or 5 nm steps.
(47) (a) Dueholm, K. L.; Egholm, M.; Behrens, C.; Christensen, L.;
Hansen, H. F.; Vulpius, T.; Petersen, K. H.; Berg, R. H.; Nielsen, P. E.;
Buchardt, O. J. Org. Chem. 1994, 59, 5767-5773. (b) Kosynkina, L.; Wang,
W.; Liang, T. C. Tetrahedron Lett. 1994, 35, 5173-5176.