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The Journal of Organic Chemistry
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1.05H, B); 13C{1H} NMR (126 MHz, CDCl3) δ 167.3 (A+B), 163.5
(A+B), 154.4 (A+B), 147.0 (A), 146.7 (B), 133.1 (A+B), 132.8 (A
+B), 128.4 (A+B), 118.0 (A), 116.7 (B), 96.1 (A+B), 94.3 (A), 93.4
(A), 88.1 (A), 87.0 (B), 84.4 (A), 83.8 (B), 80.8 (A), 80.2 (B), 61.5
(A), 61.1 (B), 51.7 (A), 50.3 (B). The spectroscopic properties of
these diastereomers were consistent with the data available in the
literature.25
the TBDMS protecting group are twofold: it provides a
lipophilic handle that enables reverse-phase flash chromato-
graphic purification of highly charged compounds that would
otherwise require more intensive purification methods, and its
traceless removal under mild conditions provides the
deprotected targets in good purity. Taken together, these
properties have facilitated a robust and scalable synthesis of
ddhCTP (2), providing useful quantities of this antiviral
metabolite and its prodrugs for biological studies.
4-N-Benzoyl-3′-deoxy-3′,4′-didehydro-cytidine-5′-alde-
hyde (6).9a The title compound was prepared according to a
9a
́
procedure adapted from Petrova et al. Alcohol 5 (12.2 g, 31.3
mmol) and EDC·HCl (12.6 g, 62.4 mmol) were placed under argon
and then suspended in DMF (65 mL). To this suspension at room
temperature was added DMSO (4.6 mL, 64 mmol), followed by a
solution of pyridine (2.6 mL, 32 mmol) and TFA (1.2 mL, 16 mmol)
in DMF (13 mL). The reaction mixture was stirred for 1 h, after
which it became a pale yellow homogeneous solution. The reaction
mixture was cooled to 0 °C, and Et3N (17.5 mL, 125 mmol) was
added, resulting in immediate precipitation of Et3N·HCl. Then, the
mixture was stirred for 10 min. The reaction was quenched by
addition of oxalic acid dihydrate (3.95 g, 31.3 mmol) and stirred for a
further 5 min at room temperature. The reaction mixture was filtered
through a pad of Celite, washed twice with DMF (5 mL), and then
the filtrate was carefully concentrated in vacuo at room temperature.
The crude residue was subjected to flash chromatography (silica gel,
0−20% MeOH−CHCl3), which afforded partially purified product
contaminated with DMSO. Trituration of this mixture with EtOAc
provided the title compound (4.07 g, 40%) as a pale yellow solid. 1H
NMR (500 MHz, DMSO-d6) δ 11.33 (s, 1H), 9.57 (s, 1H), 8.05−
7.93 (m, 3H), 7.65−7.61 (m, 1H), 7.55−7.48 (m, 2H), 7.40−7.28
(m, 1H), 6.51 (d, J = 2.8 Hz, 1H), 6.23 (d, J = 3.3 Hz, 1H), 6.16 (d, J
= 6.6 Hz, 1H), 5.18 (ddd, J = 6.4, 3.3, 2.8 Hz, 1H); 13C{1H} NMR
(126 MHz, DMSO-d6) δ 183.3, 167.4, 163.6, 155.6, 154.0, 146.4,
133.0, 132.8, 128.5, 128.4, 121.7, 96.9, 96.1, 77.0. The spectroscopic
properties were consistent with the data available in the literature.9a
3′-Deoxy-3′,4′-didehydro-cytidine (7).9a To a solution of
aldehyde 6 (1.10 g, 3.36 mmol) in MeOH (70 mL) at 0 °C was
added NaBH4 (67 mg, 1.8 mmol). The reaction was allowed to warm
to room temperature and stirred for 10 min, during which time a
colorless precipitate formed. Vacuum filtration afforded N-protected
ddhC nucleoside (1.02 g, 92%) as a colorless solid. The N-protected
ddhC nucleoside (890 mg, 2.70 mmol) was treated with NH3 (7 M in
MeOH, 40 mL), stirred at 50 °C for 3 h, and then concentrated in
vacuo. The crude residue was suspended in MeOH-Et2O (1:1) and
then collected by vacuum filtration and washed with MeOH-Et2O
(1:1, 3 × 1 mL) to afford the title compound (543 mg, 89%) as an
off-white solid. 1H NMR (500 MHz, D2O) δ 7.45 (d, J = 7.5 Hz, 1H),
6.34 (d, J = 2.0 Hz, 1H), 6.06 (d, J = 7.5 Hz, 1H), 5.44−5.39 (m,
1H), 5.01−4.98 (m, 1H), 4.38−4.30 (m, 2H); 13C{1H} NMR (126
MHz, D2O) δ 166.3, 161.2, 157.2, 140.9, 100.3, 96.4, 93.8, 78.7, 56.3.
The spectroscopic properties were consistent with the data available
in the literature.9a
EXPERIMENTAL SECTION
■
Silyl ether 10,12 methyltriphenoxyphosphonium iodide,21 difluorenyl
N,N-diisopropylphosphoramidite,22 tributylammonium phosphate,23
and bis(tributylammonium) pyrophosphate24 were prepared accord-
ing to literature procedures. Reactions requiring anhydrous conditions
were carried out in flame-dried glassware under a positive pressure of
argon in anhydrous solvents using standard Schlenk techniques.
Reaction temperatures above room temperature (22−23 °C) were
carried out in heating mantles with an internal temperature probe.
Reaction progress was monitored by thin layer chromatography
(TLC) on Merck Aluminum-backed silica gel coated TLC plates (60
Å, F254 indicator). TLC plates were visualized by exposure to
ultraviolet light (254 nm) and/or KMnO4. Flash column chromatog-
raphy was performed either manually in glass columns using Sigma-
Aldrich silica gel (35−75 μm particle size) or with a Buchi Pure C-
̈
815 Flash automated flash chromatography system using prepacked
FlashPure cartridges containing either silica gel (50 μm irregular) or
C18 silica gel (50 μm spherical), using ACS grade solvents. All yields
refer to chromatographically and spectroscopically (1H and 13C{1H}
NMR) pure material. NMR spectra were recorded using a Bruker 500
MHz spectrometer. All chemical shifts (δ) are reported in parts per
million (ppm) and referenced to residual protium or the carbon
resonance of the NMR solvent, respectively. 31P{1H} NMR spectra
were referenced to H3PO4 as an external standard. Data are
represented as follows: chemical shift, multiplicity (br = broad, s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling
constants (J) in Hertz (Hz), integration. High resolution electrospray
ionization (ESI) mass spectra were undertaken on a Waters Q-TOF
Premier Tandem Mass spectrometer fitted with a Waters 2795 HPLC.
Optical rotations were measured on a Rudolph Research Analytical
Autopol IV automatic polarimeter. Crystallographic data for
compound 11 were collected on an Agilent SuperNova diffractometer
fitted with an EOS S2 detector.
4-N-Benzoyl-2′,3′-di-O-methoxymethylidene-cytidine (5).25
To a suspension of 4-N-benzoylcytidine (4) (15.0 g, 43.2 mmol) in
anhydrous DMF (100 mL) at room temperature were added
trimethyl orthoformate (14 mL, 130 mmol) and PTSA monohydrate
(750 mg, 4.32 mmol). The reaction mixture was stirred at 50 °C for 2
h, after which a homogeneous solution was obtained. The reaction
mixture was neutralized by addition of MeOH-washed Amberlyst A21
(6 g) and stirred for 5 min, then filtered and concentrated in vacuo.
The crude solid obtained was suspended in EtOAc (75 mL) and
heated at 70 °C for 30 min and cooled to room temperature, and the
solid product was collected by filtration. The mother liquor was left to
stand for 24 h, upon which further product precipitated and was
collected by filtration. The two crops of product were combined and
dried under high vacuum to afford the title compound (12.5 g, 74%)
as a colorless powder. The product was a 1.9:1 mixture of
diastereomers at the orthoformate stereocenter, designated as A
4-N-Benzoyl-2′-O-(tert-butyldimethylsilyl)-3′-deoxy-3′,4′-
didehydrocytidine-5′-aldehyde (8). Method 1: Aldehyde 6 (400
mg, 1.22 mmol) and imidazole (170 mg, 2.44 mmol) were dissolved
in dry DMF (10 mL), to which was added TBDMSCl (250 mg, 1.61
mmol) at room temperature. The mixture was stirred for 3 h before
more TBDMSCl (80 mg, 0.51 mmol) was added. The mixture was
stirred for a further 2 h before the reaction was quenched by addition
of a few drops of water, and the mixture was stirred for 20 min. Water
and EtOAc were added, and the aqueous layer was extracted with
EtOAc; then, the combined organic phases were washed once with
water, then brine, dried over MgSO4, and concentrated to dryness. To
the residue was added toluene (20 mL), and the toluene solution was
concentrated to dryness to remove residual DMF; this process was
repeated three times before the crude was purified by column
chromatography (silica gel, 0−5% MeOH−CHCl3) to afford the title
compound (528 mg, 98%) as a colorless solid. [α]2D0 − 67.5 (0.20,
1
(major) and B (minor). H NMR (500 MHz, DMSO-d6) δ 11.26 (s,
1H, A+B), 8.31−8.26 (m, 1H, A+B), 8.01 (d, J = 7.3 Hz, 2H, A+B),
7.63 (t, J = 7.4 Hz, 1H, A+B), 7.52 (t, J = 7.8 Hz, 2H, A+B), 7.38−
7.33 (m, 1H, A+B), 6.11 (s, 0.35H, B), 6.01 (s, 0.65H, A), 5.97 (d, J =
2.4 Hz, 0.65H, A), 5.85 (d, J = 2.0 Hz, 0.35H, B), 5.12 (t, J = 5.2 Hz,
0.35H, B), 5.07 (t, J = 5.3 Hz, 0.65H, A), 5.04−5.00 (m, 1H, A+B),
4.89 (dd, J = 6.3, 3.4 Hz, 0.35H, B), 4.83 (dd, J = 7.1, 3.4 Hz, 0.65H,
A), 4.31 (q, J = 4.6 Hz, 0.65H, A), 4.22 (q, J = 4.3 Hz, 0.35H, B),
3.71−3.58 (m, 2H, A+B), 3.33 (s, A, obscured by H2O), 3.22 (s,
1
CHCl3); H NMR (500 MHz, CDCl3) δ 9.56 (s, 1H), 9.08 (s, 1H),
7.86 (d, J = 7.5 Hz, 2H), 7.57−7.50 (m, 2H), 7.50−7.38 (m, 3H),
6.16−6.12 (m, 2H), 5.25 (t, J = 2.6 Hz, 1H), 0.86 (s, 9H), 0.12 (s,
8846
J. Org. Chem. 2021, 86, 8843−8850