ethyl acetate/methanol 4:1 (v/v), to give the product (3)11 as white
powder (0.456 g, 100% yield): 1H NMR (D2O) δ 7.79 (d, 1H, J )
8.1 Hz), 5.87 (s, 1H), 5.76 (d, 1H, J ) 8.1 Hz), 3.90 (m, 2H), 3.74
(m, 2H), 1.07 (s, 3H); 13C NMR (D2O) δ 165.9, 151.4, 141.3, 102.0,
91.4, 81.5, 78.9, 72.1, 59.2, 18.7.
SCHEME 4
3′,5′-O-(Tet r a isop r op yld isiloxa n e-1,3-d iyl)-2′-C-â-m et h -
ylu r id in e (4). To a stirred mixture of 2′-C-â-methyluridine
(0.437 g, 1.69 mmol) and imidazole (0.688 g, 10.11 mmol) in DMF
(15 mL) under argon was added 1,3-dichloro-1,1,3,3-tetraiso-
propyldisiloxane (0.637 g, 2.02 mmol). After the mixture was
stirred at room temperature for 2 h, water (1.0 mL) was added,
and the mixture was concentrated under reduced pressure. The
residue was dissolved in chloroform (100 mL), and the solution
was washed with water and dried over MgSO4. After solvent
was removed, the residue was purified by silica gel chromatog-
raphy, eluting with 35% ethyl acetate in hexane to give the
product as white foam (0.546 g, 65% yield): 1H NMR (CDCl3/
TMS) δ 10.18 (br s, 1H), 7.80 (d, 1H, J ) 8.0 Hz), 6.03 (s, 1H),
5.73 (d, 1H, J ) 8.0 Hz), 4.24 (d, 1H, J ) 12.0 Hz), 4.12 (m, 1H),
4.02 (m, 1H), 3.98 (m, 1H), 3.37 (s, 1H), 1.25 (s, 3H), 1.15-0.85
(m, 28H); 13C NMR (CDCl3) δ 163.6, 150.6, 139.5, 102.2, 90.6,
81.5, 78.8, 72.6, 59.7, 20.3, 17.3, 17.2, 17.1, 17.0, 16.9, 16.8, 16.8,
16.7, 13.6, 12.7, 12.3; HRMS calcd for C22H41N2O7Si2 [MH+]
501.2452, found 501.2439.
NOESY NMR spectra of 11 that the major isomer has
the 2′-â-configuration. We observed a strong NOE be-
tween 1′-H (δ 6.20) and 2′-H (δ 2.52), weaker NOE’s from
1′-H (δ 6.20) to 2′-C-Me (δ 0.95) and 4′-H (δ 3.72), and
no NOE from 1′-H (δ 6.20) to either 3′-H (δ 3.88) or 5′-H
(δ 3.78, 3.93). These results suggest that 1′-H and 2′-H
reside on the same face of the ribose ring. We also
observed a strong NOE between 6-H (δ 8.08) and 3′-H (δ
3.88) and a weaker NOE between 6-H (δ 8.08) and 2′-C-
Me (δ 0.95), consistent with the proposed preference of
2′-deoxy-2′-C-â-methylnucleosides for the 3′-endo confor-
mation.4
3′,5′-O-(Tetr a isop r op yld isiloxa n e-1,3-d iyl)-2′-d eoxy-2′-C-
â-m eth ylu r id in e (5). Methyl chlorooxoacetate (0.132 mL, 1.44
mmol) was added to a solution of 3′,5′-O-(tetraisopropyldisilox-
ane-1,3-diyl)-2′-C-â-methyluridine (0.534 g, 0.96 mmol) and
DMAP (0.235 g, 1.92 mmol) in dry acetonitrile (10 mL). The
reaction mixture was stirred at room temperature for 1 h under
an argon atmosphere, at which time TLC showed that the
reaction was complete. The reaction mixture was diluted with
ethyl acetate (50 mL) and washed with saturated aqueous
NaHCO3 (10 mL), water (10 mL), and brine (10 mL). The organic
phase was dried over magnesium sulfate. After the solvent was
removed, the residue (white foam) was dried under vacuum
overnight and used directly in the next reaction without further
purification. The white foam was dissolved in dry toluene (20
mL). To the resulting solution were added 2,2′-azobisisobuty-
ronitrile (20 mg) and tributyltin hydride (382 µL, 1.44 mmol).
The reaction mixture was heated to reflux for 2 h. After the
solvent was removed, the residue was purified by silica gel
chromatography, eluting with 30% ethyl acetate in hexane to
give the product as a white foam (0.502 g, 97% yield) with â/R
selectivity of 93.4:6.6: 1H NMR (CDCl3/TMS) δ 10.43 (br s, 1H),
7.83 (d, 1H, J ) 8.1 Hz), 6.27 (d, 1H, J ) 7.2 Hz), 5.73 (d, 1H,
J ) 8.1 Hz), 4.18 (m, 1H), 4.10-3.90 (m, 2H), 3.77 (dd, 1H, J )
1.6, 7.8 Hz), 2.70 (m, 1H), 3.37 (s, 1H), 1.25 (s, 3H), 1.15-0.85
(m, 31H); 13C NMR (CDCl3) δ 163.8, 150.8, 140.2, 101.7, 85.7,
84.0, 71.9, 59.4, 44.0, 17.3, 17.14, 17.10, 17.0, 16.83, 16.8, 16.7,
13.5, 12.8, 12.7, 12.2, 11.0; HRMS calcd for C22H41N2O6Si2 [MH+]
485.2503, found 485.2491.
3′,5′-O-(Tetr a isop r op yld isiloxa n e-1,3-d iyl)-2′-d eoxy-2′-C-
â-m eth ylcytid in e (6). Triethylamine (0.14 mL, 1.0 mmol) was
added to a mixture of 3′,5′-O-(tetraisopropyldisiloxane-1,3-diyl)-
2′-deoxy-2′-C-â-methyluridine (0.371 g, 0.687 mmol), 2,4,6-
triisopropylbenzenesulfonyl chloride (0.606 g, 2.0 mmol), and
DMAP (0.244 g, 2.0 mmol) in acetonitrile (20 mL). After the
mixture was stirred at room temperature for 53 h, concentrated
ammonium hydroxide (28%, 30 mL) was added, and the mixture
was stirred at room temperature for 3 h. The solvent was
removed, and the aqueous phase was extracted with chloroform
(3 × 20 mL). The chloroform solution was washed with brine
and dried over magnesium sulfate. The solvent was removed,
and the residue was purified by silica gel chromatography,
eluting with 7.5% ethanol in chloroform to give the product as
a white foam (0.337 g, 91% yield): 1H NMR (CDCl3/TMS) δ 8.14
(br s, 1H), 7.66 (d, 1H, J ) 7.4 Hz), 6.47 (br s, 2H), 6.24 (d, 1H,
J ) 7.4 Hz), 5.73 (d, 1H, J ) 7.47 Hz), 4.06 (m, 1H), 3.93 (m,
1H), 3.86 (m, 1H), 3.65 (m, 1H), 2.55 (m, 1H), 1.10-0.80 (m,
31H); 13C NMR (CDCl3) δ 165.7, 156.3, 140.9, 94.6, 85.8, 83.6,
72.5, 59.8, 44.0, 17.3, 17.22, 17.17, 17.1, 16.93, 16.85, 16.8, 13.5,
12.8, 12.7, 12.3, 11.4; HRMS calcd for C22H42N3O5Si2 [MH+]
484.2663, found 484.2667.
In conclusion, 2′-C-â-methyl-2′-deoxynuclesides may
provide a means to eliminate the conformational ambigu-
ity inherent to 2′-deoxynucleoside substitution experi-
ments in functional RNA molecules. As part of our efforts
to test this hypothesis, we have established more facile
access to the phosphoramidite derivative of 2′-deoxy-2′-
C-â-methylcytidine from a sugar substrate: 1,2,3,5-tetra-
O-benzoyl-2-C-â-methylribofuranose. This approach has
greater stereoselectivity and a higher overall yield than
previously published methods. NOESY experiments sup-
port previous arguments based on NMR coupling con-
stants4 that 2′-deoxy-2′-C-â-methylnucleosides populate
the 3′-endo conformation.
Exp er im en ta l Section
2′,3′,5′-Tr i-O-ben zoyl-2′-C-â-m eth ylu r id in e (2). Under an
argon atmosphere, a stirred suspension of uracil (1.12 g, 10.0
mmol) and (NH4)2SO4 (25 mg) in 1,1,1,3,3,3-hexamethyldisila-
zane (25 mL) was heated at reflux until a clear solution formed.
The clear solution was concentrated under vacuum, and the
residue was dried under high vacuum (<0.1 mmHg) for 2 h. The
crude bis(trimethylsilyl)uracil obtained was dissolved in dry
acetonitrile (75 mL), and 1,2,3,5-tetra-O-benzoyl-2-C-methyl-R-
(and â)-D-ribofuranose (1) (2.838 g, 4.89 mmol) was added. Under
argon, SnCl4 (1.18 mL, 10.0 mmol) was added in one portion
with vigorous stirring. After being stirred at room temperature
for 20 h, the reaction mixture was heated to reflux for 1 h. The
mixture was cooled to room temperature, and the reaction was
carefully quenched with saturated aqueous NaHCO3 (50 mL).
The mixture was extracted with ethyl acetate (3 × 50 mL), and
the organic layers were combined, washed with brine, and dried
over magnesium sulfate. The solvent was removed, and the
residue was purified by silica gel chromatography, eluting with
50% ethyl acetate in hexane to give product (2)11 as white foam
(2.663 g, 96% yield): 1H NMR (CDCl3/TMS) δ 9.22 (br s, 1H),
8.08 (m, 4H), 7.89 (d, 2H, J ) 7.7 Hz), 7.65-7.35 (m, 8H), 7.32-
7.18 (m, 2H), 6.54 (s, 1H), 5.79 (d, 1H, J ) 4.7 Hz), 5.75 (d, 1H,
J ) 8.2 Hz), 4.95-4.70 (m, 2H), 4.64 (m, 1H), 1.74 (s, 3H); 13C
NMR (CDCl3) δ 166.2, 165.3, 165.2, 163.0, 149.9, 140.8, 133.63,
133.58, 133.5, 129.9, 129.8, 129.6, 129.4, 129.3, 128.5, 128.4,
102.3, 89.5, 84.4, 80.4, 75.4, 63.3, 17.9.
2′-C-â-Meth ylu r id in e (3). A solution of 2′,3′,5′-tri-O-benzoyl-
2′-C-â-methyluridine (1.007 g, 1.77 mmol) in methanol (100 mL)
was saturated with ammonia gas at 0 °C, and the mixture was
stored at 4 °C for 2 days. The solvent was removed, and the
residue was purified by silica gel chromatography, eluting with
J . Org. Chem, Vol. 68, No. 17, 2003 6801