samples were monitored over 20 h by reversed-phase HPLC.
The time required for 50% completion was determined by
integration at 254 nm giving the data shown below in Table
1.
conditions analogous to those employed here. These NMR
results provide new insight into guanine reactivity, and the
experimental procedures for N-acylation provide a new, high-
yield route for preparation of these important compounds.12,13
NMR of specifically labeled compounds is a straightfor-
ward method for monitoring formation of unstable interme-
diates such as 2. The 15N NMR experiments reported here
are the first uses of this approach to obtain direct evidence
for transient derivatization of the guanine O6 and amino
groups. The O6 derivatization is consistent with known
reactivity of the O6 position, including reaction with other
silyl compounds.8e We are not aware of previous reports of
silylation of the guanine amino group, at least under mild
Acknowledgment. This work was supported by a grant
from the National Institutes of Health (EB002809).
Supporting Information Available: Spectra (1H NMR,
13C NMR, 15N NMR, MS, and UV). This material is available
OL049096I
(13) Preparation of 6a and 6b. 3′,5′-O-Di-tert-butylsilylene-2′-O-TBS-
guanosine (5, 2.69 g, 5 mmol) was dried three times by azeotropic
evaporation of pyridine, the last time leaving 25 mL. Dry dichloromethane
(125 mL) was added under N2, and the mixture was cooled in an ice bath
with stirring. TMSCl (3.8 mL, 30 mmol, 6 equiv) was added over 2 min,
and the mixture was stirred at 0 °C for 15 min, by which time it had become
homogeneous. Acetyl chloride (0.39 mL, 5.5 mmol, 1.1 equiv) or phe-
noxyacetyl chloride (0.76 mL, 5.5 mmol, 1.1 equiv) was added over 10
min. The mixture was stirred at 0 °C for 1.5 h with acetyl chloride or for
3 h with phenoxyacetyl chloride. The solution was then poured into 150
mL of 5% aq NaHCO3, and the organic layer was isolated. It was washed
with an additional 150 mL of 5% aq NaHCO3 and then concentrated to a
thick oil. N-Acetyl-3′,5′-O-di-tert-butylsilylene-2′-O-TBS-guanosine (6a)
was crystallized by addition of 100 mL of ethyl ether and stirring in an ice
bath for 3 h and filtered to give pure product in 82% yield: mp 174-176
(12) Preparation of 4a and 4b. Guanosine hydrate (3, 1.5 g, 5 mmol)
was dried three times by azeotropic evaporation of pyridine, the last time
leaving 25 mL. Dry dichloromethane (100 mL) was added under N2, and
the mixture was cooled in an ice bath with stirring. TMSCl (5.7 mL, 45
mmol, 9 equiv) was added over 2 min, and the flask was removed from the
ice bath and allowed to stir for 2 h, by which time it had become
homogeneous. The flask was then cooled again in an ice bath, and acetyl
chloride (0.39 mL, 5.5 mmol, 1.1 equiv) or phenoxyacetyl chloride (0.76
mL, 5.5 mmol, 1.1 equiv) was added over 10 min. The mixture was stirred
at 0 °C for 1.5 h with acetyl chloride or for 3 h with phenoxyacetyl chloride.
Methanol (20 mL) was then added, and the solution was stirred at room
temperature for 12 h to complete the desilylation. The mixture was
evaporated to an oil, and the product was crystallized. For N-acetylguanosine
(4a), 100 mL of methanol was added and the mixture was stirred at 0 °C
for 1 h and filtered to give pure product in 98% yield: mp 157-159 °C.
1
°C, softens 157 °C. UV λmax 259 nm with shoulder at 277 nm. H NMR
(CDCl3, 400 MHz): δ 11.93 (s, 1H), 8.41 (s, 1H), 7.66 (s, 1H), 5.76 (s,
1H), 4.45 (dd, J ) 9.2 Hz, J ) 4.8 Hz, 1H), 4.36 (d, J ) 4.4 Hz, 1H), 4.25
(dd, J ) 9.2 Hz, J ) 4.4 Hz, 1H), 4.15 (td, J ) 9.8 Hz, J ) 4.8 Hz, 1H),
3.96 (t, J ) 9.8 Hz, 1H), 2.28 (s, 3H), 1.05 (s, 9H), 1.01 (s, 9H), 0.88 (s,
9H), 0.10 (s, 3H), 0.09 (s, 3H). 13C NMR (CDCl3, 75.4 MHz): δ 171.9,
155.6, 147.7, 147.5, 136.8, 121.8, 91.6, 76.4, 76.1, 74.7, 68.0, 27.7, 27.3,
26.1, 24.8, 23.1, 20.6, 18.6, -4.0, -4.7. Anal. Calcd for C26H45N5O6Si2:
C, 53.86; H, 7.82; N, 12.08. Found: C, 53.79; H, 7.78; N, 12.02.
N-Phenoxyacetyl-3′,5′-O-di-tert-butylsilylene-2′-O-TBS-guanosine (6b) was
crystallized by addition of 30 mL of ethyl ether, followed by 20 mL of
hexane, and stirring in an ice bath for 1 h and filtered to give pure product
in 90% yield: mp 163-164 °C; UV λmax 260 nm with a shoulder at 276
1
UV λmax 259 nm with shoulder at 279 nm. H NMR (DMSO, 300 MHz):
δ 12.04 (s, 1H), 11.71 (s, 1H), 8.26 (s, 1H), 5.80 (d, J ) 5.7 Hz, 1H),
5.70-4.60 (br, 3H), 4.43 (t, J ) 5.4 Hz, 1H), 4.13 (t, J ) 4.2 Hz, 1H),
3.90 (q, J ) 3.7 Hz, 1H), 3.64 (dd, J ) 12 Hz, J ) 4.0 Hz, 1H), 3.54 (dd,
J ) 12 Hz, J ) 3.9 Hz, 1H), 2.18 (s, 3H). 13C NMR (DMSO, 75.4 MHz):
δ 173.4, 154.8, 148.7, 148.1, 137.4, 120.0, 86.6, 85.2, 73.8, 70.2, 61.1,
23.9. Anal. Calcd for C12H15N5O6‚1.25CH3OH: C, 43.56; H, 5.52; N, 19.17.
Found: C, 43.67; H, 5.36; N, 19.13. For N-phenoxyacetylguanosine (4b),
residual pyridine was removed by partial evaporation three times with water,
and the mixture was then stirred in about 50 mL of water at 0 °C for 1 h
and filtered to give pure product in 98% yield: mp 159-160 °C. UV λmax
1
1
260 nm with shoulder at 276 nm. H NMR (DMSO, 300 MHz): δ 11.83
nm. H NMR (CDCl3, 400 MHz): δ 11.74 (s, 1H), 8.91 (s, 1H), 7.70 (s,
(s, 1H), 11.78 (s, 1H), 8.29 (s, 1H), 7.35-7.28 (m, 2H), 7.02-6.95 (m,
3H), 5.82 (d, J ) 5.7 Hz, 1H), 5.70-4.90 (br, 3H), 4.87 (s, 2H), 4.45 (t, J
) 5.3 Hz, 1H), 4.14 (t, J ) 4.2 Hz, 1H), 3.92 (q, J ) 3.9 Hz, 1H), 3.65
(dd, J ) 12 Hz, J ) 4.2 Hz, 1H), 3.55 (dd, J ) 12 Hz, J ) 4.1 Hz, 1H).
13C NMR (DMSO, 75.4 MHz): δ 170.8, 157.4, 154.8, 148.5, 147.2, 137.7,
129.4, 121.2, 120.3, 114.4, 86.7, 85.3, 74.0, 70.2, 66.2, 61.1. Anal. Calcd
for C18H19N5O7: C, 51.80; H, 4.59; N, 16.78. Found: C, 51.62; H, 4.42;
N, 16.42.
1H), 7.36 (m, 2H), 7.10 (m, 1H), 6.96 (m, 2H), 5.81 (s, 1H), 4.69 (s, 2H),
4.50 (dd, J ) 9.2 Hz, J ) 5.2 Hz, 1H), 4.39 (d, J ) 4.4 Hz, 1H), 4.21 (td,
J ) 10 Hz, J ) 5.2 Hz, 1H), 4.12 (dd, J ) 9.6 Hz, J ) 4.4 Hz, 1H), 4.00
(t, J ) 9.8 Hz, 1H), 1.02 (s, 9H), 1.01 (s, 9H), 0.93 (s, 9H), 0.14 (s, 3H),
0.13 (s, 3H). 13C NMR (CDCl3, 75.4 MHz): δ 169.4, 156.3, 155.2, 146.9,
146.2, 136.7, 130.2, 123.4, 122.8, 114.9, 92.2, 76.3, 76.1, 74.8, 68.0, 67.2,
27.7, 27.3, 26.2, 23.1, 20.7, 18.7, -3.8, -4.6. Anal. Calcd for C32H49N5O7-
Si2: C, 57.20; H, 7.35; N, 10.42. Found: C, 56.80; H, 7.30; N, 10.23.
Org. Lett., Vol. 6, No. 15, 2004
2557