7432 J. Am. Chem. Soc., Vol. 119, No. 32, 1997
Allerson et al.
allowed to stir for an additional 3 h, and was then poured into 250 mL
of EtOAc. This solution was washed with 5% (w/v) aqueous NaHCO3
(2 × 150 mL), dried over anhydrous Na2SO4, filtered, and concentrated
in vacuo to yield a pale yellow oil. Silica gel chromatography (silica
was pretreated with 90:8:2 hexanes/EtOAc/Et3N) with 7:3 hexanes/
EtOAc (Rf(2c) ) 0.15, 0.12 (two diastereomers) in 8:2 hexanes/EtOAc)
yielded 894 mg (0.899 mmol, 83.3%) as a white foam. 1H NMR
(CDCl3, 500 MHz): δ (ppm) 8.40 (s, H2, diastereomer 1), 8.38 (s,
H2, diastereomer 2), 8.27 (s, H8, diastereomer 1), 8.24 (s, H8,
diastereomer 2), 7.47-7.32 (m, 5H, DMT H2′′/H3′′/H4′′), 7.40 (m,
2H, phenyl H3), 7.33 (m, 4H, DMT H2/H2′), 7.21 (m, 2H, phenyl H2),
6.80 (m, 4H, DMT H3/H3′), 6.11 (d, H1′, diastereomer 2), 6.05 (d,
H1′, diastereomer 1), 5.07 (m, 1H, H2′), 4.44-4.36 (m, 2H, H3′/H4′),
3.92-3.58 (m, 2H, diisopropyl methines), 3.77, 3.76 (s, 6H, DMT
methoxy), 3.61 (t, 2H, cyanoethyl H1), 3.52, 3.35 (m, 2H, H5′/H5′′),
2.30 (t, 2H, cyanoethyl H2), 1.25-1.05 (m, 12H, diisopropyl methyls),
0.76, 0.73 (2s, 9H, TBDMS t-Bu methyls), -0.02, -0.03, -0.19, -0.21
(4s, 6H, TBDMS methyls). 13C NMR (CDCl3, 125 MHz): δ (ppm)
159.8 (phenyl C1), 158.6, 153.1 (C6), 152.0 (C2), 150.8 (C4), 144.5,
144.3, 142.4, 142.3 (C8), 135.7, 135.6, 135.5, 135.4, 131.1 (phenyl
C4), 130.1, 130.0, 129.7 (phenyl C3), 128.2, 127.9, 127.0, 123.2 (phenyl
C2), 122.1 (C5), 117.5, 117.2 (CN), 113.2, 113.1, 88.5, 88.3 (C1′),
86.9, 86.6, 84.3, 83.9 (C4′),75.8, 74.7 (C2′), 73.5, 73.4, 72.8, 72.7 (C3′),
63.3, 63.2 (C5′), 58.9, 58.7, 57.7, 57.5 (cyanoethyl C1), 55.2 (DMT
methoxy), 43.5, 43.4, 43.0, 42.9, 25.6, 25.5 (TBDMS t-Bu methyls),
24.7, 24.6, 24.5 (diisopropyl methyls), 20.4, 20.1 (cyanoethyl C2), 17.9,
-4.67, -5.16 (TBDMS methyls). 31P NMR (CDCl3, 202 MHz): δ
(ppm) 151.9, 149.8. FAB-HRMS: calcd for C52H64ClN6O8PSi (M +
H+) 995.4055, obsd 995.4059.
2′,3′,5′-Tris(O-triethylsilyl)-N2-(triethylsilyl)guanosine (10b). To
a suspension of guanosine (10a) (2.86g, 10 mmol) and imidazole (8.5
g, 120 mmol, 12 equiv) in 12 mL of dry DMF was added chlorotri-
ethylsilane (10.1 mL, 60 mmol, 6 equiv) in three portions over 2 h at
0 °C. The reaction mixture was allowed to warm to room temperature
over 8 h and then was poured into 75 mL of Et2O and washed with
10% (w/v) aqueous LiBr (3 × 100 mL). The combined organic layers
were dried with Na2SO4, filtered, and concentrated to a pale oil. Silica
gel chromatography of this residue (14:5:1 CH2Cl2/EtOAc/MeOH)
(Rf(10b) ) 0.45, 9:1 CH2Cl2/MeOH) afforded 4.7 g of 10b (64%) as
a white solid. 1H NMR (CDCl3, 500 MHz): δ (ppm) 7.89 (s, 1H,
H8), 5.97 (d, 1H, H1′), 4.78 (brs, 1H, N2-H), 4.43 (m, 1H, H2′), 4.24
(m, 1H, H3′), 4.06 (m, 1H, H4′), 3.82-3.72 (m, 2H, H5′ and H5′′),
1.02-0.96 (m, 27H), 0.89-0.86 (m, 6H), 0.81-0.78 (m, 9H), 0.71-
0.63 (m, 12H), 0.43-0.35 (m, 6H). 13C NMR (CDCl3, 125 MHz): δ
(ppm) 160.0, 154.4, 152.8, 135.4, 117.1, 87.1, 85.4, 77.1, 63.0, 7.0,
6.8, 6.7, 6.5, 5.0, 4.6, 4.2, 4.0. FAB-HRMS: calcd for C34H69N5O5Si4
(M + Na+) 762.4269, obsd 762.4274.
2′,3′,5′-Tris(O-triethylsilyl)-N2-(triethylsilyl)-6-O-(4-nitropheneth-
yl)guanosine (11). A mixture of 2′,3′,5′-Tris(O-triethylsilyl)-N2-
(triethylsilyl)guanosine (10b) (102 mg, 0.16 mmol), 4-nitrophenethyl
alcohol (44.7 mg, 0.24 mmol, 1.5 equiv), and triphenylphosphine (88.5
mg, 0.33 mmol, 2 equiv) was suspended in 1.6 mL of dry dioxane in
the dark. To this suspension was added diethyl azodicarboxylate (52
mL, 0.32 mmol, 2 equiv). After stirring at room temperature overnight,
the mixture was concentrated to a white solid suspended in a pink-
orange oil. The residue was dissolved in 20 mL of CHCl3, washed
with saturated NaCl (2 × 20 mL) and 5% (w/v) aqueous NaHCO3 (2
× 20 mL), dried over Na2SO4, filtered, and concentrated in vacuo to a
pink-orange oil. Silica gel flash chromatography with 3:2 hexanes/
EtOAc (Rf(11) ) 0.89, 1:1 hexanes/EtOAc) afforded 108.2 mg (87%)
of 11 as a yellow oil. 1H NMR (CDCl3, 500 MHz): δ (ppm) 8.16 (d,
2H), 7.94 (s, 1H, H8), 7.47 (d, 2H), 5.94 (d, 1H, H1′), 4.72 (t, 2H),
4.60 (t, 1H, H2′), 4.47 (s, 1H, N2-H), 4.29 (m, 1H, H3′), 4.07 (m, 1H,
H4′), 3.90-3.73 (m, 2H, H5′ and H5′′), 3.28 (t, 2H), 1.00-0.94 (m,
27H), 0.82-0.77 (m, 15H), 0.69-0.62 (m, 12H), 0.48-0.36 (m, 6H).
13C NMR (CDCl3, 125 MHz): δ (ppm) 160.5, 160.3, 154.3, 146.1,
137.9, 129.8, 123.7, 87.1, 85.4, 75.9, 72.6, 65.7, 62.3, 35.2, 7.0, 6.8,
6.7, 6.6, 4.9, 4.6, 4.5, 4.4, 4.3, 4.2. FAB-HRMS: calcd for C42H76N6O7-
Si4 (M + H+) 889.4927, obsd 889.4931.
a separate tube, 2′,3′,5′-tris(O-triethylsilyl)-N2-(triethylsilyl)-6-O-(4-
nitrophenethyl)guanosine (11) (1.13 g, 1.3 mmol) was dissolved in 5.5
mL of the 60% HF/pyridine solution at -42 °C. To this solution was
added t-butyl nitrite (0.215 mL, 1.8 mmol, 1.4 equiv) in a dropwise
fashion. After stirring for 3 h, the reaction mixture was diluted with
20 mL of CHCl3, poured slowly onto 20 g of K2CO3, and diluted with
water. The aqueous layer was separated and extracted with CHCl3 (2
× 20 mL). The combined organic layers were washed with NaHCO3
(40 mL) and water (40 mL), dried over Na2SO4, filtered, and
concentrated in vacuo to an orange oil. Silica gel flash chromatography
of this residue with 12:1 CH2Cl2/MeOH (Rf(3) ) 0.45, 9:1 CH2Cl2/
MeOH) afforded 424 mg (0.97 mmol, 75%) of 3 as a yellow solid. 1H
NMR (CD3OD, 400 MHz): δ (ppm) 8.53 (s, 1H, H8), 8.15 (d, 2H),
7.59 (d, 2H), 5.98 (d, 1H, H1′), 4.85 (t, 2H), 4.62 (t, 1H, H2′), 4.32
(m, 1H, H3′), 4.12 (m, 1H, H4′), 3.89-3.73 (m, 2H, H5′ and H5′′),
3.32 (t, 2H). 13C NMR (CD3OD, 100 MHz): δ (ppm) 163.5, 163.4,
160.3, 158.2, 154.4, 154.2, 148.3, 147.3, 144.1, 131.3, 124.5, 90.1,
87.4, 75.8, 71.9, 62.8, 35.7. FAB-HRMS: calcd for C18H18N5O7F (M
+ H+) 436.1268, obsd 436.1269.
5′-O-(4,4′-Dimethoxytrityl)-2-fluoro-6-O-(4-nitrophenethyl)-
inosine (3a). 2-fluoro-6-O-(4-nitrophenethyl)inosine (3) (1.5 g, 3.4
mmol) and 4-(dimethylamino)pyridine (43.3 mg, 0.34 mmol, 0.10
equiv) were dissolved in 30 mL of dry pyridine. To this solution was
added 4,4′-dimethoxytrityl chloride (1.5 g, 4.4 mmol, 1.3 equiv) and
triethylamine (2.8 mL, 20.4 mmol, 6 equiv). After stirring at room
temperature for 16 h, the solution was quenched with 15 mL of
methanol and concentrated in vacuo to an orange oil. This residue
was dissolved in 50 mL of CH2Cl2 and washed with 5% (w/v) aqueous
NaHCO3 (3 × 50 mL). The combined aqueous layers were back-
extracted with CH2Cl2 (2 × 50 mL), and the combined organic phases
were dried over Na2SO4, filtered, and concentrated in vacuo to an orange
oil. Silica gel flash chromatography (silica was pretreated with 3%
triethylamine in CH2Cl2) with 19:1 CH2Cl2/MeOH (Rf(3a) ) 0.75, 9:1
CH2Cl2/MeOH) afforded 1.88 g (2.55 mmol, 74%) of 3a as an off-
white foam. 1H NMR (CDCl3 and TEA, 400 MHz): δ (ppm) 8.16 (d,
2H), 8.11 (s, 1H, H8), 7.47 (d, 2H), 7.36 (d, 2H), 7.28-7.14 (m, 7H),
6.75 (m, 4H), 5.97 (d, 1H, H1′), 4.82 (t, 2H), 4.70 (t, 1H, H2′), 4.44
(m, 1H, H3′), 4.24 (m, 1H, H4′), 3.75 (s, 6H), 3.45-3.28 (m, 4H, H5′
and H5′′ and p-NPE CH2). 13C NMR (CDCl3, 125 MHz): δ (ppm)
162.1, 162.0, 158.8, 158.6, 156.7, 153.2, 153.0, 147.0, 145.3, 144.6,
141.7141.7, 135.7, 130.1, 130.0, 128.1, 127.9, 126.9, 123.8, 120.1,
120.0, 113.2, 89.6, 86.5, 84.8, 77.3, 74.6, 70.8, 67.6, 63.4, 55.2, 35.0.
FAB-HRMS: calcd for C39H36N5O9F (M + Na+) 760.2392, obsd
760.2395.
5′-O-(4,4′-Dimethoxytrityl)-2′-O-(tert-butyldimethylsilyl)-2-fluoro-
6-O-(4-nitrophenethyl)inosine (3b). A mixture of 5′-O-(4,4′-dimethoxy-
trityl)-2-fluoro-6-O-(4-nitrophenethyl)inosine (3a) (1.58 g, 2.14 mmol)
and imidazole (1.02 g, 15 mmol, 7 equiv) was dissolved in 3 mL of
dry DMF at 0 °C. To this solution was added tert-butyldimethylsilyl
chloride (386 mg, 2.57 mmol, 1.2 equiv). After stirring for 9 h at
room temperature, the reaction mixture was diluted with 30 mL of ether
and washed with 10% (w/v) aqueous LiBr (3 × 30 mL). The combined
aqueous layers were back-extracted with ether (2 × 40 mL). The
combined organic layers were then dried over Na2SO4, filtered, and
concentrated in vacuo to a yellow oil. Silica gel flash chromatography
of this residue (silica gel was pretreated with 3% triethylamine in
hexanes) with a gradient of 2:1 to 1:1 hexanes/EtOAc (Rf(3b) ) 0.85,
19:1 CH2Cl2/MeOH) afforded 346 mg (0.41 mmol, 19%) of 3b as a
white foam. 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.17 (d, 2H), 8.13
(s, 1H), 7.49 (d, 2H), 7.42 (d, 2H), 7.33-7.18 (m, 7H), 6.81 (d, 4H),
6.00 (d, 1H, H1′), 4.83 (m, 3H), 4.32 (m, 1H), 4.25 (m, 1H), 3.77 (s,
6H), 3.46 (dd, 2H, H5′/H5′′), 3.32 (t, 2H), 0.84 (s, 9H), 0.01 (s, 3H),
-0.13 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 162.2, 162.0,
159.0, 158.6, 156.9, 153.4, 153.3, 147.0, 145.3, 144.5, 141.3, 135.5,
130.1, 129.9, 128.1, 128.0, 127.0, 123.8, 120.0, 119.9, 113.3, 88.3,
86.8, 84.4, 76.0, 71.6, 67.6, 63.4, 55.2, 35.0, 25.5, 17.9, -5.0, -5.1.
FAB-HRMS: calcd for C45H50N5O9FSi (M + Na+) 874.3256, obsd
874.3260.
5′-O-(4,4′-Dimethoxytrityl)-3′-O-(2-cyanoethoxy)-N,N-diisopro-
pylamino)phosphino] 2′-O-(tert-butyldimethylsilyl)-2-fluoro-6-O-(4-
nitrophenethyl)inosine (3c). 5′-O-(4,4′-Dimethoxytrityl)-2′-O-(tert-
butyldimethylsilyl)-2-fluoro-6-O-(4-nitrophenethyl)inosine (3b) (360
2-Fluoro-6-O-(4-nitrophenethyl)inosine (3). A solution of 60%
HF in pyridine was generated by diluting 19 mL of 70% HF/pyridine
with 3.3 mL of distilled pyridine in a 50 mL polypropylene tube. In