Synthesis of 2′,3′-Dideoxy-2′-difluoromethyl Azanucleosides

Article abstract of DOI:10.1055/s-2004-815932

Methyl (2S,4S)-N-tert-butoxycarbonyl-4-difluoromethylpyroglutamate (9a) was synthesized from trans-4-hydroxy-L-proline (5). Compound 9a was converted to (5S,3S)-N-benzyloxycarbonyl-5-(tert-butyldimethylsilyloxymethyl-3- difluoromethyl-2-pyrrolidone (15) o

Full text of DOI:10.1055/s-2004-815932

334
PAPER
Synthesis of 2 ,3 -Dideoxy-2 -difluoromethyl Azanucleosides
S
ynthesis of 2,3-D
i
ideoxy
a
o
n
-
ucleoside
L
s
ong Qiu,a Feng-Ling Qing*a,b
a
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai 200032, P. R. China
b
College of Chemistry and Chemical Engineering, Donghua University, 1882 West Yanan Lu, Shanghai 200051, P. R. China
Fax +86(21)64166128; E-mail: flq@mail.sioc.ac.cn
Received 30 October 2003; revised 4 December 2003
Abstract: Methyl (2S,4S)-N-tert-butoxycarbonyl-4-difluorometh-
ylpyroglutamate (9a) was synthesized from trans-4-hydroxy-L-pro-
line (5). Compound 9a was converted to (5S,3S)-N-
benzyloxycarbonyl-5-(tert-butyldimethylsilyloxymethyl-3-difluo-
romethyl-2-pyrrolidone (15) over 4 steps in 66% yield, which was
used as a key intermediate for the synthesis of 2 ,3 -dideoxy-2 -dif-
luoromethyl azanucleosides
Key words: fluorine-containing nucleoside, azanucleoside, pyro-
glutamate, difluoromethylenation, glycosylation reaction
The small and highly electronegative fluorine atom has
found many applications in medicinal and physical organ-
ic chemistry.1 The strong carbon–fluorine bond in fluo-
rine-containing molecules is particularly resistant to
metabolic transformation,2 which plays a great role in
medical designation. Thus introduction of fluorinated
groups into potential bioactive molecules such as amino
Figure 1 Several highly bioactive fluorinated nucleosides
acids, peptides and nucleosides attracts the attention of groups on the biological activities of azanucleosides), we
many chemists.
embarked on the synthesis of 2 ,3 -dideoxy-2 -difluoro-
methyl azanucleosides.
In recent years, nucleosides and nucleotides, known to be
DNA and RNA subunits, have become an interestingly Our retrosynthetic analysis (Scheme 1) was based on the
important subject of research in the field of pharmaceuti- thought that the target molecules 2 ,3 -dideoxy-2 -difluo-
cal science,3 and many studies have dealt with the synthe- romethyl azanucleosides 1 could be prepared from pro-
sis and biological activity of novel types of nucleosides tected pyrrolidine 2 by the glycosylation reactions, which
and nucleotides including fluorine-containing nucleo- could be reached from 3 by protection of two hydroxy
sides. Fluorinated nucleosides are the attractive nucleo- groups of 3 step by step. Compound 3 could be prepared
side analogues because of their antiviral and anticancer from the corresponding fluorinated pyroglutamate 4 by
activities. Up to date, many fluorinated nucleoside ana- reduction of the ester and lactam with DIBAL-H in one
logues have been synthesized and characterized. Some step according to the reported methods.8 Compound 4
highly bioactive fluorinated nucleosides have been pre- could be obtained from trans-4-hydroxy-L-proline (5) us-
pared including FMAU,4 FLT,5 FIAC4b and Gemcitabine6 ing the similar methods reported recently by us.9
(Figure 1).
According to our retrosynthetic analysis, the methyl
Azanucleosides, in which the oxygen atom of pyranose or (2S,4S)-N-tert-butoxycarbonyl-4-difluoromethylpyro-
furanose ring is replaced by nitrogen atom, are an impor- glutamate (9a) was synthesized first from the trans-4-hy-
tant class of modified nucleosides. The synthesis and bio- droxy-L-proline (5) under the similar synthetic route
logical activities of azanucleosides were summarized by (Scheme 2).9 Thus the commercially available amino acid
Yokoyama et al in 1999.3 However, among the many aza- 5 was converted to the protected 4-oxo-L-proline 6 in
nucleosides reported to date, only a few examples of flu- three steps.10 Difluoromethylenation11 of 6 under CF2Br2/
orinated azanucleosides have been synthesized and HMPT/Zn condition afforded the unstable compound 7 in
investigated.7 In connection with our studies on fluorinat- 50% yield. However, hydrogenation of 7 with Pd/C in
ed amino acids, fluorinated peptides and fluorinated nu- EtOH gave the compound 8 in 94% yield with moderate
cleosides (investigation on the effects of fluorinated diastereoselectivity (cis/trans = 7:1). When the ester me-
thyl group in 7 was replaced by a benzyl group, the hydro-
genation proceeded with high diastereoselectivity.12 In
our opinion, this was caused by the different block effect
of two groups. Finally, oxidation of the fluorinated proline
SYNTHESIS 2004, No. 3, pp 0334–0340
Advanced online publication: 26.01.2004
DOI: 10.1055/s-2004-815932; Art ID: F10303SS.pdf
© Georg Thieme Verlag Stuttgart · New York
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Synthesis of 2,3-Dideoxy-2-difluoromethyl Azanucleosides
335
Scheme 1 Retrosynthetic analysis of 2 , 3 -dideoxy-2 -difluoromethyl-N-azanucleosides.
8 with RuO2·xH2O/NaIO4/EtOAc system yielded the cor-
responding pyroglutamate 9. Slightly surprisingly, the
two diastereoisomers of 9 could be separated by flash
chromatography. Cis-9a and trans-9b were obtained in 68
and 11% yields, respectively.
With 9a in hand, the sequential reduction of the ester and
lactam in one step with DIBAL-H was carried out accord-
Scheme 3
ing to the reported methods.8 However, when compound
9a was treated with DIBAL-H in THF at 0 °C (Scheme 3),
protected with a benzyloxycarbonyl (Cbz) group, by
Kikugawa’s method,13 the reaction proceeded smoothly.
The desired compound 15 was obtained in 83% yield
along with the defluorinated compound 16 in 9% yield.
the reaction was complicated and the desired compound
10 was not observed. In our opinion, the existence of the
difluoromethyl group may be responsible for the failure of
the reaction.
Thus, reduction of compound 15 with LiBEt3H in THF at
–78 °C yielded two diastereoisomers 17a and 17b (17a/
17b = 9.2:1) in 88% yield. The two isomers could be sep-
arated on silica gel chromatography (Scheme 5). The
acetylation of 17a with acetic anhydride in CH2Cl2 afford-
ed the compound 18 in 92% yield. Coupling of 18 with si-
lylated uracil and thymine under Vorbrüggen’s
conditions14 (glycosylation reaction) gave the desired si-
lyl-protected azanucleosides. Following deprotection of
the silyl protection groups with TBAF in THF afforded
our target molecules 19a, 19b, 20a and 20b. The ratio of
19a, 19b and that of 20a, 20b were different and opposite.
In view of above failure, we changed our strategies, that
is, step by step reduction of the ester and the lactam. Thus,
deprotection of N-Boc group of 9a with trifluoroacetic
acid in CH2Cl2, followed by reduction of the ester with
NaBH4, and further protection of the resulting hydroxyl
group with TBDMSCl afforded compound 11 in 80%
yield over three steps (Scheme 4). However, protection of
the amino group of 11 with tert-butoxycarbonyl group un-
der usual conditions only afforded the desired compound
12 in 9% yield, and the diastereoisomeric compound 13
(9%) and defluorinated compound 14 (25%) were also
isolated. Fortunately, when the amino group of 11 was
Scheme 2 Reagents and conditions: (a) i. SOCl2, MeOH; ii. Boc2O, Et3N, DMAP, CH2Cl2; iii. Swern oxidation; (b) CF2Br2/HMPT/Zn; (c)
Pd/C, H2, EtOH, r.t., 1 atm.; (d) RuO2·xH2O, NaIO4, EtOAc, H2O.
Synthesis 2004, No. 3, 334–340 © Thieme Stuttgart · New York

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X.-L. Qiu, F.-L. Qing
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Scheme 4 Reagents and conditions: (a) i. TFA, CH2Cl2; ii. NaBH4, MeOH, –78 → 0 °C; iii. TBDMSCl, imidazole, DMAP, CH2Cl2; (b)
Boc2O, Et3N, CH2Cl2, DMAP, r.t.; (c) LHMDS, CbzCl, THF, –78 °C, 10 min.
This was probably due to the different block effects of be-
tween methyl group and benzyloxycarbonyl group and
between hydrogen and benzyloxycarbonyl group. The ab-
solute configuration of compound 20b was confirmed by
X-ray (Figure 2).
In summary, we have synthesized the novel fluorinated
azanucleosides: 2 ,3 -dideoxy-2 -difluoromethyl azanu-
cleosides 19a, 19b, 20a, 20b from trans- 4-hydroxy-L–
proline (5). The investigations of their biological activities
are in progress.
THF was distilled from sodium metal. CH2Cl2 and pyridine were
distilled from CaH2. All the melting points and optical rotations are
1
uncorrected. H NMR and 13C NMR spectra were recorded on a
Bruker AM 300 spectrometer. 19F NMR spectra were recorded on a
Bruker AM 300 spectrometer (CFCl3 as external standard and low
field is positive). Chemical shifts ( ) are reported in ppm, and cou-
pling constants (J) are in Hz.
Figure 2 X-ray crystal structure of 20b.
Scheme 5 Reagents and conditions: (a) LiBEt3H, THF, –78 °C; (b) Ac2O, DMAP, pyridine; (c) i. silylated uracil, or silylated thymine,
TMSOTf, MeCN; ii. TBAF, THF.
Synthesis 2004, No. 3, 334–340 © Thieme Stuttgart · New York

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Synthesis of 2,3-Dideoxy-2-difluoromethyl Azanucleosides
337
Methyl (2S)-N- tert-Butoxycarbonyl-4-oxoprolinate (6)
ane–EtOAc, 10:1) to afford 8 as a colorless oil; yield: 2.626 g
SOCl2 (6.5 mL, 89.58 mmol) was added dropwise to an ice-cold so-
lution of trans-4-hydroxy-L-proline (5; 10.00 g, 76.26 mmol) in
MeOH (100 mL) under anhydrous conditions. After the addition,
the mixture was heated to reflux for 2 h. Then the reaction mixture
was stirred overnight at r.t. The mixture was then concentrated in
vacuo to afford a crude white solid (13.78 g), which was used with-
out further purification. To a mixture of the white solid and DMAP
(2.0 g, 16.39 mmol) was added CH2Cl2 (110 mL) followed by Et3N
(25 mL). The mixture was cooled in an ice-bath and a solution of
Boc2O (20.0 g, 91.6 mmol) in CH2Cl2 (50 mL) was added dropwise
and it was stirred overnight at r.t. The reaction was then quenched
with H2O (100 mL) and the pH of the mixture was adjusted to 2–3
by the addition of dil. HCl. The organic phase was separated and the
aqueous phase was extracted with CH2Cl2 (2 × 100 mL). The com-
bined organic phases were washed with brine and dried (Na2SO4).
Concentration and purification by column chromatography (hex-
ane–EtOAc, 5:1, 2:1) afforded a light yellow oil (15.715 g). Oxalyl
chloride (9.0 mL, 102.7 mmol) was added to a mixture of anhyd
CH2Cl2 (100 mL) and DMSO (9.5 mL, 133.97 mmol) at –78 °C. Af-
ter 10 min, a solution of above yellowish oil (15.715 g) in CH2Cl2
(120 mL) was added dropwise at a rate by keeping the reaction tem-
perature below –60 °C. After the addition, the mixture was stirred
at –78 °C for 2 h, then Et3N (28 mL) was added dropwise. After the
addition, the mixture was gradually warmed to r.t. and quenched
with H2O (60 mL). The organic layer was separated and the aqueous
layer was extracted with CH2Cl2 (2 × 50 mL). The combined organ-
ic phases were washed with brine and dried (Na2SO4). Concentra-
tion and purification by silica gel chromatography (hexane–EtOAc,
2:1) gave 6 as a yellowish oil; yield: 11 g (59% from 5).
(94%).
IR (film): 1751, 1706, 1480, 1396, 1165 cm–1.
1H NMR (300 MHz, CDCl3): = 6.02–6.21 (1 H, m), 4.38–4.26 (1
H, m), 3.77–3.66 (4 H, m), 3.52–3.44 (1 H, m), 2.73–2.67 (1 H, m),
2.53–2.43 (1 H, m), 2.07–1.97 (1 H, m), 1.47, 1.41 (9 H, 2 s).
19F NMR (282 MHz, CDCl3): = –120 (m).
MS (EI): m/z = 280 (M+ + 1), 178 (M+ – Boc, 27), 57 (100).
Anal. Calcd for C12H19F2NO4: C, 51.62; H, 6.81; N, 5.02. Found: C,
51.76; H, 7.03; N, 4.82.
Methyl (2S,4S)-N-tert-Butoxycarbonyl-4-difluoromethylpyro-
glutamate (9a) and Methyl (2S,4R)-N-tert-Butoxycarbonyl-4-di-
fluoromethylpyroglutamate (9b)
To a solution of NaIO4 (4.780 g, 22.34 mmol) in H2O (50 mL) was
added RuO2·xH2O (240 mg, 1.73 mmol) under N2. The resulting
greenish-yellow solution was stirred for 5 min, followed by addition
of 8 (2.435 g, 8.727 mmol) in EtOAc (35 mL) in one portion. The
mixture was stirred vigorously at r.t. Additional aliquots of 10% aq
NaIO4 were added to maintain a yellow-colored solution during the
reaction. After 30 h, the resulting mixture was then diluted with
EtOAc and filtered. The filtrate was washed with sat. aq NaHSO3
solution. The organic layer was washed with brine and dried
(Na2SO4). After removal of the solvent in vacuo, the resulting resi-
due was purified by silica gel chromatography (hexane–EtOAc,
10:1, then 7:1) to give 9b (less polar) and 9a (more polar).
9b
White solid; yield: 280 mg (11%); mp 102–103 °C; [ ]D20 –17.5
1H NMR (300 MHz, CDCl3): = 4.83–4.70 (1 H, dd, J = 9.9, 9.9
Hz), 3.91–3.88 (2 H, d, J = 7.8 Hz), 3.76 (3 H, s), 2.98–2.88 (1 H,
m), 2.62–2.55 (1 H, m), 1.48, 1.46 (9 H, 2s).
(c = 0.78, CHCl3).
IR (film): 1781, 1745, 1706, 1687, 1324 cm–1.
1H NMR (300 MHz, CDCl3): = 6.41–6.04 (1 H, td, J = 54.6, 2.1
Hz), 4.68–4.64 (1 H, dd, J = 2.4, 2.4 Hz), 3.80 (3 H, s), 3.27–3.19
(1 H, m), 2.62–2.50 (1 H, m), 2.21–2.14 (1 H, m), 1.50 (9 H, s).
19F NMR (282 MHz, CDCl3): = –123.97 to –125.20 (1 F, ddd,
J = 287.0, 55.0, 6.0 Hz), –127.19 to –128.49 (1 F, ddd, J = 286.0,
55.0, 26.0 Hz).
Methyl (2S)-N-tert-Butoxycarbonyl-4-difluoromethyleneproli-
nate (7)
CF2Br2 (3.9 mL, 42.69 mmol) and HMPT (8.0 mL, 42.32 mmol)
were added at 0 °C to a solution of 6 (4.85 g, 19.46 mmol) in THF
(100 mL). The mixture was warmed to r.t. and stirred for 30 min.
Zinc dust (2.85 g, 43.85 mmol) and HMPT (100 L) were added.
The mixture was heated to reflux for 20 min, then H2O and Et2O
were added. The mixture was separated and the aqueous layer was
extracted with Et2O (3 × 100 mL). The combined organic phases
were washed with sat. aq CuSO4 solution, H2O and brine, and dried
(Na2SO4). Concentration and purification on silica gel chromato-
graphy (hexane–EtOAc, 15:1) gave 7; oil; yield: 2.777 g (50%);
[a]D20 –27.2 (c = 1.45, CHCl3).
MS (ESI): m/z = 316 (M+ + Na).
Anal. Calcd for C12H17F2NO5: C, 49.15; H, 5.80; N, 4.78. Found: C,
49.12; H, 5.57; N, 4.52.
9a
White solid; yield: 1.745 g (68%); mp 112–114 °C; [ ]D20 –30.7
(c = 0.46, CHCl3).
IR (film): 1776, 1750, 1699, 1478 cm–1.
IR (film): 1788, 1753, 1708, 1480, 1396, 1170 cm–1.
1H NMR (300 MHz, CDCl3): = 4.57–4.43 (1 H, dd, J = 9.6, 9.3
Hz), 4.18–4.04 (2 H, m), 3.75 (3 H, s), 3.00–2.87 (1 H, m), 2.70–
2.64 (1 H, m), 1.48, 1.43 (9 H, 2s).
19F NMR (282 MHz, CDCl3): = –88.01 to –88.44 (1 F, m), –90.97
to –91.23 (1 F, m).
1H NMR (300 MHz, CDCl3): = 6.37–5.99 (1 H, td, J = 55.0, 2.4
Hz), 4.65–4.60 (1 H, dd, J = 6.6, 6.0 Hz), 3.80 (3 H, s), 3.19–3.09
(1 H, m), 2.61–2.49 (1 H, m), 2.32–2.22 (1 H, m), 1.51 (9 H, s).
19F NMR (282 MHz, CDCl3): = –121.70 to –122.0 (1 F, ddd,
J = 287.0, 55.0, 7.6 Hz), –125.20 to –126.53 (1 F, ddd, J = 285.0,
55.0, 25.0 Hz).
MS (ESI) : m/z = 300.2 (M+ + Na).
MS (EI): m/z 278 (M+ – 15, <1), 192 (M+ – Boc, 24), 57 (100).
HRMS (ESI): m/z calcd for C12H17F2NNaO4: 300.1018; found:
300.1009.
Anal. Calcd for C12H17F2NO5: C, 49.15; H, 5.80; N, 4.78. Found: C,
49.14; H, 5.91; N, 4.65.
Anal. Calcd for C12H17F2NO4: C, 51.99; H, 6.14; N, 5.05. Found: C,
51.58; H, 6.28; N, 5.51.
(5S,3S)-5-tert-Butyldimethylsiloxymethyl-3-difluoromethylpyr-
rolidin-2-one (11)
Methyl (2S)-N-tert-Butoxycarbonyl-4-difluoromethylprolinate
(8)
5% Pd/C (1.00 g) was added to a solution of 7 (2.777 g, 10.03
mmol) in EtOH (100 mL). Then the mixture was hydrogenated
overnight at r.t. and 1 atm. After filtration and removal of the EtOH
in vacuo, the residue was purified by flash chromatography (hex-
TFA (0.88 mL) was added dropwise to a solution of 9a (771 mg,
2.63 mmol) in CH2Cl2 (30 mL) at –78 °C. The mixture was then
warmed to r.t. and stirred overnight. The reaction mixture was
cooled to 0 °C and quenched with sat. aq NaHCO3 (40 mL). The or-
ganic phase was separated and the aqueous phase was extracted
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338
X.-L. Qiu, F.-L. Qing
PAPER
with CH2Cl2 (2 × 30 mL). The combined organic phases were
washed with brine and dried (Na2SO4). After removal of the solvent
in vacuo, the residue was dissolved in MeOH (20 mL) and the solu-
tion was cooled to –78 °C and NaBH4 (120 mg, 3.16 mmol) was
added. The mixture was then warmed to 0 °C and stirred for 2 h. The
reaction was quenched with conc. HCl and the mixture was then fil-
tered and concentrated to give a yellowish solid. To a cooled solu-
tion of DMAP (30 mg, 0.25 mmol) and imidazole (718 mg, 10.56
mmol) in CH2Cl2 (10 mL) was added a solution of the above yel-
lowish solid in DMF (3 mL). Then a solution of TBDMSCl (1.230
g, 8.16 mmol) in CH2Cl2 (10 mL) was added dropwise. After the
mixture was stirred overnight at r.t., it was quenched with sat. aq
NH4Cl in an ice-bath. The organic phase was separated and the
aqueous layer was extracted with CH2Cl2 (2 × 40 mL). The com-
bined organic phases were washed with brine and dried (Na2SO4).
After removal of the solvent in vacuo, the resulting residue was pu-
rified by flash chromatography (hexane–EtOAc, 5:1, then 1:1) to
give 11; colorless oil; yield: 587 mg (80%); [ ]D20 +60.9 (c = 0.89,
CHCl3).
13
Colorless oil; yield: 78 mg (9%); [ ]D20 –55.0 (c = 0.26, CHCl3).
IR (film): 1791, 1755, 1720, 1473, 1369 cm–1.
1H NMR (300 MHz, CDCl3): = 6.39–6.01 (1 H, td, J = 56.8, 2.1
Hz), 4.24–4.21 (1 H, d, J = 9.3 Hz), 4.04–3.99 (1 H, dd, J = 3.0, 2.4
Hz), 3.71–3.67 (1 H, dd, J = 2.7, 1.8 Hz), 3.40–3.31 (1 H, m), 2.43–
2.32 (1 H, m), 2.14–2.07 (1 H, m), 1.52 (9 H, s), 0.88 (9 H, s), 0.05,
0.04 (6 H, 2s).
19F NMR (282 MHz, CDCl3): = –118.67 to –119.98 (1 F, ddd,
J = 283.6, 27.6, 26.5 Hz), –112.99 to –123.22 (1 F, ddd, J = 283.97,
6.1, 6.3 Hz).
MS (ESI): m/z = 402.1 (M+ + Na).
HRMS (ESI): m/z calcd for C17H31F2NO4NaSi: 402.1883; found:
402.1892.
Anal. Calcd for C17H31F2NO4Si: C, 53.83; H, 8.18; N, 3.69. Found:
C, 53.51; H, 8.26; N, 3.52.
IR (film): 3223, 3111, 1701, 1256 cm–1.
14
20
White solid; yield: 216 mg (25%); mp 32–34 °C; [ ]D –31.5
(c = 1.36, CHCl3).
IR (film): 1718, 1687, 1473, 1366 cm–1.
1H NMR (300 MHz, CDCl3): = 7.57–7.29 (1 H, dt, J = 79.5, 2.6
Hz), 4.25–4.19 (1 H, m), 3.85–3.81 (1 H, dd, J = 3.9, 3.6 Hz), 3.67–
3.64 (1 H, dd, J = 1.8, 2.1 Hz), 2.84–2.70 (2 H, m), 1.53 (9 H, s),
0.84 (9 H, s), 0.09–0.04 (6 H, m).
1H NMR (300 MHz, CDCl3): = 6.51 (1 H, s), 6.35–5.97 (1 H, td,
J = 55.0, 2.4 Hz), 3.79–3.72 (1 H, m), 3.69–3.64 (1 H, dd, J = 4.2,
4.2 Hz), 3.47–3.41 (1 H, dd, J = 5.1, 5.4 Hz), 3.06–2.93 (1 H, m),
2.31–2.20 (1 H, m), 1.95–1.85 (1 H, m), 0.88 (9 H, s), 0.05 (6 H, s).
19F NMR (282 MHz, CDCl3): = –122.78 to –124.02 (1 F, ddd,
J = 284.0, 55.0, 3.1 Hz), –126.26 to –127.56 (1 F, ddd, J = 283.0,
57.0, 27.0 Hz).
19F NMR (282 MHz, CDCl3): = –123.57 to –123.86 (1 F, d,
MS (ESI): m/z = 302 (M+ + Na).
J = 80.5 Hz).
MS (EI): m/z = 360 (M+ + 1, 1), 57 (100).
Anal. Calcd for C12H23F2NO2Si: C, 51.61; H, 8.24; N, 5.02. Found:
C, 51.56; H, 8.14; N, 4.80.
Anal. Calcd for C17H30FNO4Si: C, 56.82; H, 8.36; N, 3.90. Found:
C, 56.79; H, 8.23; N, 3.65.
(5S,3S)-N-tert-Butoxycarbonyl-5-tert-butyldimethylsilyloxyme-
thyl-3-difluoromethyl-2-pyrrolidone (12), (5S,3R)-N-tert-But-
oxycarbonyl-5-tert-butyldimethylsilyloxymethyl-3-difluoro-
methyl-2-pyrrolidone (13), and (5S)-N-tert-Butoxycarbonyl)-5-
tert-butyldimethylsilyloxymethyl-3-[(Z)-fluoromethylidene]-2-
pyrrolidone (14)
(5S,3S)-N-Benzyloxycarbonyl-5-tert-butyldimethylsilyloxyme-
thyl-3-difluoromethyl-2-pyrrolidone (15) and (5S)-N-Benzyl-
oxycarbonyl-5-tert-butydimethylsilyloxymethyl-3-[(Z)-
fluoromethylidene]-2-pyrrolidone (16)
To a cooled solution of 11 (664 mg, 2.38 mmol), DMAP (126 mg,
1.03 mmol) and Et3N (1.5 mL) in CH2Cl2 (15 mL) was added Boc2O
(1.283 g, 5.87 mmol) in CH2Cl2 (5 mL) dropwise. Then the mixture
was warmed to r.t. and stirred overnight. The reaction was quenched
with sat. aq NH4Cl and the aqueous layer was extracted with CH2Cl2
(2 × 30 mL). The combined organic phases were washed with dil.
HCl, brine and dried (Na2SO4). After removal of the solvent in vac-
uo, the residue was purified by flash chromatography (hexane–
EtOAc, 20:1, then 15:1) to give 12 (most polar), 13 (less polar) and
14 (more polar).
To a solution of 11 (220 mg, 0.79 mmol) in THF (10 mL) at –78 °C
was added LHMDS (0.79 mL, 1 M in THF, 0.79 mmol) dropwise.
After the mixture was stirred for 10 min, CbzCl (0.17 mL, 1.19
mmol) was added dropwise. The mixture was stirred for further 10
min at –78 °C and quenched with sat. aq NH4Cl (6 mL). The mix-
ture was warmed to r.t. and extracted with EtOAc (3 × 30 mL). The
combined organic phases were washed with brine and dried
(Na2SO4). After removal of the solvent, the resulting residue was
purified by flash chromatography (hexane–EtOAc, 10:1, then 7:1)
to give 15 and 16.
12
15
20
White solid; yield: 82 mg (9%); mp 83–85 °C; [ ]D –52.3
20
White solid; yield: 270 mg (83%); mp 111–112 °C; [ ]D –53.4
(c = 0.41, CHCl3).
IR (film): 2960, 1715, 1307, 1275 cm–1.
1H NMR (300 MHz, CDCl3): = 7.44–7.33 (5 H, m), 6.36–5.97 (1
H, td, J = 55.8, 3.0 Hz), 5.35–5.23 (2 H, m), 4.21–4.19 (1 H, m),
3.91–3.86 (1 H, dd, J = 5.1, 4.5 Hz), 3.75–3.70 (1 H, dd, J = 2.4, 2.1
Hz), 3.12 (1 H, m), 2.32–2.25 (2 H, m), 0.84 (9 H, s), 0.07 (6 H, s).
19F NMR (282 MHz, CDCl3): = –121.53 to –122.76 (1 F, ddd,
J = 285.8, 9.7, 7.8 Hz), –123.38 to –124.67 (1 F, ddd, J = 285.7,
24.8, 20.6 Hz).
(c = 0.81, CHCl3).
IR (film): 1766, 1689, 1474 cm–1.
1H NMR (300 MHz, CDCl3): = 6.34–5.95 (1 H, td, J = 56.0, 3.0
Hz), 4.15–4.10 (1 H, m), 3.91–3.86 (1 H, dd, J = 5.4, 5.1 Hz), 3.76–
3.72 (1 H, dd, J = 3.0, 2.7 Hz), 3.11–3.00 (1 H, m), 2.28–2.21 (2 H,
m), 1.52 (9 H, s), 0.86 (9 H, s,), 0.05, 0.04 (6 H, 2 s).
19F NMR (282 MHz, CDCl3): = –121.18 to –122.41 (1 F, ddd,
J = 284.1, 8.5, 9.0 Hz), –123.71 to –124.36 (1 F, ddd, J = 284.3,
22.9, 14.7 Hz).
MS (ESI): m/z = 402.2 (M+ + Na).
MS(ESI): m/z = 436 (M+ + Na).
HRMS (ESI): m/z calcd for C17H31F2NO4NaSi: 402.1883; found:
402.1874.
Anal. Calcd for C20H29F2NO4Si: C, 58.11; H, 7.02; N, 3.39. Found:
C, 58.12; H, 6.96; N, 3.13.
Synthesis 2004, No. 3, 334–340 © Thieme Stuttgart · New York

PAPER
16
Synthesis of 2,3-Dideoxy-2-difluoromethyl Azanucleosides
339
was extracted with CH2Cl2 (3 × 30 mL). The combined organic
phases were washed with dil. HCl, brine and dried (Na2SO4). After
removal of the solvent, the resulting residue was purified by silica
gel chromatography (hexane–EtOAc, 15:1, then 7:1) to give 18
(540 mg, 92%); white solid; yield: 540 mg (92%); mp 87–88 °C;
[ ]D20 –56.4 (c = 0.67, CHCl3).
20
White solid; yield: 29 mg (9%); mp 47–48 °C; [ ]D –37.0
(c = 0.40, CHCl3).
IR (film): 2957, 1738, 1712, 1685, 1294 cm–1.
1H NMR (300 MHz, CDCl3): = 7.61–7.44 (1 H, dt, J = 46.4, 1.7
Hz), 7.43–7.32 (5 H, m), 5.36–5.26 (2 H, m), 4.31–4.27 (1 H, m),
3.85–3.80 (1 H, dd, J = 3.9, 3.9 Hz), 3.66–3.62 (1 H, dd, J = 1.8, 2.1
Hz), 2.81–2.75 (2 H, m), 0.82 (9 H, s), 0.07 (6 H, s).
19F NMR (282 MHz, CDCl3): = –122.39 to –122.69 (1 F, dt,
J = 80.2, 3.6 Hz).
IR (film): 1718, 1399, 1086 cm–1.
1H NMR (300 MHz, acetone-d6): = 7.39–7.35 (5 H, m), 6.86–5.84
(1 H, d, J = 5.1 Hz), 6.18–5.78 (1 H, td, J = 56.6, 6.0 Hz), 5.15–5.12
(2 H, br), 4.01–3.91 (3 H, m), 2.88–2.84 (1 H, m), 2.25–2.18 (2 H,
m), 2.00 (3 H, s), 0.90 (9 H, s), 0.03 (6 H, s).
19F NMR (282 MHz, acetone-d6): = –115.47 to –116.71 (1 F, dd,
J = 52.45, 60.9 Hz), –120.02 to –125.67 (1 F, m).
MS (ESI): m/z = 416 (M+ + Na), 394.1 (M+ + 1).
Anal. Calcd for C20H28FNO4Si: C, 61.07; H, 7.12; N, 3.56. Found:
C, 61.16; H, 7.35; N, 3.31.
MS (ESI): m/z 480.1 (M+ + Na).
Benzyl (2R,3S,5S)-5-tert-Butyldimethylsilyloxymethyl-3-difluo-
romethyl-2hydroxypyrrolidine-1-cabboxylate (17a) and Benzyl
(2S,3S,5S) -5- tert-Butyldimethylsilyloxymethyl-3-difluorome-
thyl-2-hydroxypyrrolidine-1-carboxylate (17b)
HRMS (ESI): m/z calcd for C22H33F2NNaO5Si: 480.1988; found:
480.1991.
Anal. Calcd for C22H33F2NO5Si: C, 57.77; H, 7.22; N, 3.06. Found:
C, 58.01; H, 7.30; N, 2.80.
To a solution of 15 (670 mg, 1.622 mmol) in THF (20 mL) at –78 °C
was added LiBEt3H (6.4 mL, 1 M in THF, 6.4 mmol) dropwise. The
mixture was stirred at –78 °C for 3.5 h. The reaction was then
quenched with H2O (5 mL) and warmed up to r.t. The mixture was
extracted with CH2Cl2 (3 × 60 mL) and the combined organic phas-
es were washed with brine and dried (Na2SO4). After removal of the
solvent, the resulting residue was purified by flash chromatography
(hexane–EtOAc, 15:1, then 7:1) to give 17a (less polar) and 17b
(more polar).
Benzyl (2R,3S,5S)-5-Hydroxymethyl-2-[2,4-dioxo-3,4-dihydro-
pyrimidin-1(2H)-yl]-3-(difluoromethyl)pyrrolidine-1-carboxy-
late (19a) and Benzyl (2S,3S,5S)-5-Hydroxymethyl-2-[2,4-di-
oxo-3,4-dihydropyrimidin-1(2H)-yl]-3-(difluoromethyl)pyrrol-
idine-1-carboxylate (19b); Typical Procedure
To a stirred solution of 18 (306 mg, 0.669 mmol) and uracil (220
mg, 1.96 mmol) in anhyd MeCN (30 mL) was added N,O-bis (trim-
ethylsilyl)acetamide (1.0 mL, 3.03 mmol). The reaction mixture
was stirred under reflux for 30 min. After cooling to 0 °C, TMSOTf
(0.33 mL, 1.58 mmol) was added dropwise and the solution was
stirred at r.t. for further 30 min. The reaction was quenched with
cold sat. aq NaHCO3 and the resulting mixture was extracted with
CH2Cl2 (3 × 50 mL). The combined organic phases were washed
with brine and dried (Na2SO4). After removal of the solvent, the re-
sulting residue was purified by flash chromatography (hexane–
EtOAc, 4:1, 3:1 then 2:1) to give two compounds, the less polar
compound (90 mg, white foam) and the more polar compound (150
mg, white foam). A stirred solution of the above less polar com-
pound (90 mg) in THF (10 mL) was treated with 1 M solution of
TBAF (0.24 mL, 0.24 mmol) at 0 °C. After stirring at r.t. for 8.5 h,
the reaction was quenched with H2O and the mixture was extracted
with CH2Cl2 (3 × 30 mL). The combined organic phases were
washed with brine and dried (Na2SO4). After removal of the solvent,
the resulting residue was purified by flash chromatography (hex-
ane–EtOAc, 1:1) to give 19a. The more polar compound was also
treated with TBAF under the same conditions to give 19b.
17a
20
Light yellow oil; yield: 534 mg (79%); [ ]D –39.7 (c = 0.99,
CHCl3).
IR (film): 3422, 2957, 1709, 1406, 1097 cm–1.
1H NMR (300 MHz, acetone-d6): = 7.38–7.30 (5 H, m), 6.00–5.64
(1 H, td, J = 56.1, 7.2 Hz), 5.34 (1 H, br), 5.24–5.08 (2 H, m), 4.21–
4.08 (1 H, m), 3.79–3.74 (1 H m), 3.57–3.46 (1 H, m), 2.48–2.41 (1
H, m), 2.21–2.01 (2 H, m), 0.89, 0.86 (9 H, 2 s), 0.09, 0.07 (6 H, 2 s).
19F NMR (282 MHz, acetone-d6): = –118.05 to –119.41 (1 F, m),
–127.23 to –128.35 (1 F, m).
MS (ESI): m/z = 438 (M+ + Na).
HRMS (ESI): m/z calcd for C20H31F2NNaO4Si: 438.1883; found:
438.1899.
17b
Light yellow oil; yield: 58 mg (9%); [ ]D20 –53.3 (c = 0.45, CHCl3).
IR (film): 3238, 1698, 1498, 1355, 1081 cm–1.
19a
White foam; yield: 64 mg (34% from 18); [ ]D20 +15.3 (c = 0.59,
1H NMR (300 MHz, acetone-d6): = 7.43–7.31 (5 H, m), 6.26–5.86
(1 H, td, J = 56.6, 6.3 Hz), 5.40 (1 H, br), 5.27–5.04 (3 H, m), 4.04–
3.98 (1 H, m), 3.79–3.77 (1 H, br), 2.54–3.44 (2 H, m), 2.04–1.99
(1 H, m), 0.87 (9 H, s), 0.04, (6 H, s).
19F NMR (282 MHz, acetone-d6): = –119.89 to –121.19 (1 F, m),
–123.41 to –125.20 (1 F, m).
CHCl3).
IR (film): 3444, 3203, 3061, 1687, 1465 cm–1.
1H NMR (300 MHz, methanol-d4): = 8.46 (1 H, br), 7.25 (5 H, br),
6.45–6.42 (1 H, d, J = 7.2 Hz), 6.15–5.78 (1 H, t, J = 55.2 Hz),
5.56–5.53 (1 H, d, J = 7.8 Hz), 5.18–4.92 (2 H, m), 4.37 (1 H, br),
3.89–3.84 (1 H, t, J = 8.6 Hz), 3.65–3.61 (1 H, d, J = 12.0 Hz),
3.13–2.93 (1 H, m), 2.39–2.27 (1 H, m), 2.11–2.02 (1 H, m).
19F NMR (282 MHz, methanol-d4): = –117.61 to –118.84 (1 F,
m), –125.93 to –127.15 (1 F, m).
MS (ESI): m/z = 438 (M+ + Na).
HRMS (ESI): m/z calcd for C20H31F2NNaO4Si: 438.1883; found:
438.1885.
13C NMR (75.5 MHz, methanol-d4): = 166.1, 156.2, 152.6, 143.7,
137.3, 129.6, 129.3, 128.9, 116.0 (t, J = 239.8 Hz), 102.2, 70.6,
68.8, 61.5, 60.8, 46.3 (t, J = 22.7 Hz), 24.8.
MS (EI): m/z 395 (M+, <1), 284 (M+ – uracil, 12), 91 (84), 43 (100).
HRMS (EI): m/z calcd for C14H16F2NO3 (M+ – uracil): 284.1098;
Benzyl (2R,3S,5S)-2-Acetyloxy-5-tert-butyldimethylsilyloxyme-
thyl-3-difluoromethylpyrrolidine-1-carboxylate (18)
To a mixture of 17a (534 mg, 1.286 mmol), DMAP (20 mg, 0.164
mmol), pyridine (1.60 mL, 19.78 mmol) in CH2Cl2 (20 mL) was
added Ac2O (1.21 mL, 11.95 mmol) dropwise. After the mixture
was stirred overnight at r.t., the reaction was quenched with sat. aq
NaHCO3. The organic phase was separated and the aqueous phase
found: 284.1105.
Synthesis 2004, No. 3, 334–340 © Thieme Stuttgart · New York

340
19b
X.-L. Qiu, F.-L. Qing
PAPER
J = 5.1, 3.6 Hz), 3.07–2.98 (1 H, br), 2.52–2.43 (1 H, m), 1.93–1.70
20
White foam; yield: 38 mg (14% from 18); [ ]D –75.4 (c = 0.67,
(2 H, m), 1.63 (3 H, s).
CHCl3).
19F NMR (282 MHz, CDCl3): = –120.30 to –121.67 (1 F, m),
–123.49 to –125.01 (1 F, m).
IR (film): 3424, 3201, 3063, 1690, 1463, 1409 cm–1.
1H NMR (300 MHz, CDCl3): = 9.52 (1 H, br), 7.32–7.26 (5 H, m),
6.85–6.83 (1 H, d, J = 6.6 Hz), 6.17–5.79 (1 H, t, J = 56.3 Hz), 5.65
(1 H, br), 5.38–5.37 (1 H, d, J = 5.1 Hz), 5.25–5.21 (1 H, d, J = 11.4
Hz), 4.96–4.92 (1 H, br), 4.37 (1 H, br), 3.97–3.93 (1 H, d, J = 12.0
Hz), 3.73–3.69 (1 H, br), 3.02 (1 H, br), 2.50 (1 H, br), 1.80 (1 H,
br).
MS (EI) : m/z = 409 (M+, <1), 284 (M+ – thymine, 18), 91 (100).
MS (ESI) : m/z = 410 (M+ + 1), 432.2 (M+ + Na).
HRMS (EI): m/z calcd for C14H16F2NO3 (M+ – thymine,): 284.1098;
found: 284.1115.
HRMS (ESI): m/z calcd for C19H21F2N3NaO5 (M+ + Na,): 432.1341;
19F NMR (282 MHz, CDCl3): = –120.69 to –121.57 (1 F, m),
–123.58 to –124.95 (1 F, m).
found: 432.1336.
MS (EI): m/z = 395 (M+, <1), 364 (M+ – CH2OH, <1), 284 (M+ –
uracil, 24), 91 (100).
Acknowledgment
We thank the National Natural Science Foundation of China for fi-
nancial support.
HRMS (EI): m/z calcd for C14H16F2NO3 (M+ – uracil): 284.1098;
found: 284.1147.
Benzyl (2R,3S,5S)-5-Hydroxymethyl-2-[5-methyl-2,4-dioxo-
3,4-dihydropyrimidin-1(2H)-yl]-3-difluoromethylpyrrolidine-
1-carboxylate (20a) and Benzyl (2S,3S,5S)-5-Hydroxymethyl-2-
[5-methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl]-3-difluo-
romethylpyrrolidine-1-carboxylate (20b)
Compounds 20a and 20b were prepared as white foams from com-
pound 18 (272 mg, 0.59 mmol) and thymine (230 mg, 1.82 mmol)
using the same conditions as described for compounds 19a and 19b.
Reference
(1) (a) Wilkinson, J. A. Chem. Rev. 1992, 92, 505.
(b) Organofluorine Compounds in Medicinal Chemistry and
Biomedical Application; Filler, R.; Kobayashi, Y.;
Yagupolskii, L. M., Eds.; Elsevier: Amsterdam, 1993.
(2) Harper, D. B.; O’Hagan, D. Nat. Prod. Rep. 1994, 11, 123.
(3) Yokoyama, M.; Momotake, A. Synthesis 1999, 1541.
(4) (a) Chu, C. K.; Ma, T.; Shanmuganathan, K.; Wang, C.;
Xiang, Y.; Pai, S. B.; Yao, G. Q.; Sommadossi, J. P.; Cheng,
Y. C. Antimicrob. Agents Chemother. 1995, 39, 979.
(b) Watanabe, K. A.; Reichman, U.; Hirota, K.; Lopez, C.;
Fox, J. J. J. Med. Chem. 1979, 22, 21.
(5) (a) Etzold, G.; Hintsche, R.; Kowollik, G.; Langen, P.
Tetrahedron 1971, 27, 2463. (b) Balzarini, J.; Baba, M.;
Pauwells, R.; Hetdewijn, P.; De Clercq, E. Biochem.
Pharmacol. 1988, 37, 2847.
(6) (a) Kotra, L. P.; Xiang, Y.; Newton, M. G.; Schinazi, R. F.;
Cheng, Y.-C.; Chu, C. K. J. Med. Chem. 1997, 40, 3635.
(b) Hertel, L. W.; Kroin, J. S.; Misner, J. W.; Tustin, J. M. J.
Org. Chem. 1988, 53, 2406.
20a
White foam; yield: 29 mg (12%); [ ]D20 +28.0 (c = 0.19, CHCl3).
IR (film): 3441, 3193, 3065, 1687 and 1471 cm–1.
1H NMR (300 MHz, MeOH-d4): = 8.40 (1 H, br), 7.24 (5 H, br),
6.43 (1 H, br), 6.12–5.75 (1 H, t, J = 55.1 Hz), 5.18–4.91 (2 H, m),
4.44 (1 H, br), 3.89–3.84 (1 H, t, J = 8.7 Hz), 3.64–3.60 (1 H, d,
J = 11.7 Hz), 3.11–2.91 (1 H, m), 2.42–2.30 (1 H, m), 2.10–2.01 (1
H, m), 1.74 (3 H, s).
19F NMR (282 MHz, MeOH-d4): = –115.58 to –116.75 (1 F, m),
–125.04 to –126.17 (1 F, m).
13C NMR (75.5 MHz, MeOH-d4): = 166.4, 156.2, 152.8, 139.5,
137.4, 129.6, 129.3, 128.9, 116.1 (t, J = 239.4 Hz), 111.0, 70.3,
68.7, 61.5, 60.7, 46.4 (t, J = 22.3 Hz), 24.8, 12.5.
MS (EI): m/z = 409 (M+, <1), 284 (M+ – thymine, 27), 91 (100).
MS (ESI): m/z = 410.1 (M+ + 1).
(7) Qing, F.-L.; Yu, J.; Fu, X.-K. Collect. Czech. Chem.
Commun. 2002, 67, 1267.
(8) Katoh, T.; Nagata, Y.; Kobayashi, Y.; Arai, K.; Minami, J.;
Terashima, S. Tetrahedron Lett. 1993, 34, 5743.
(9) Qiu, X.-L.; Qing, F.-L. J. Org. Chem. 2003, 68, 3614.
(10) (a) Lewis, A.; Wilkie, J.; Rutherford, T. J.; Gani, D. J. Chem.
Soc., Perkin Trans. 1 1998, 3777. (b) O’Connel, C. E.;
Rowell, C. A.; Ackermann, K.; Garcia, A. M.; Lewis, M. D.;
Kowalczyk, J. J. Chem. Pharm. Bull. 2000, 48, 740.
(11) Motherwell, W. B.; Tozer, M. J.; Ross, B. C. J. Chem. Soc.,
Chem. Commun. 1989, 1437.
HRMS (ESI): m/z calcd for C19H21F2N3NaO5 (M+ + Na,): 432.1341;
found: 432.1360.
20b
White foam; yield: 101 mg (41%); [ ]D20 –58.8 (c = 0.40, CHCl3).
IR (film): 3418, 3196, 1714, 1674, 1458, 1026 cm–1.
(12) Qiu, X.-L.; Qing, F.-L. J. Org. Chem. 2002, 67, 7162.
(13) Li, H.; Sakamoto, T.; Kato, M.; Kikugawa, Y. Synth.
Commun. 1995, 24, 4045.
(14) (a) Vorbrüggen, H.; Krolikiewicz, K.; Bennua, B. Chem.
Ber. 1981, 114, 1234. (b) Vorbrüggen, H.; Höfle, G. Chem.
Ber. 1981, 114, 1256.
1H NMR (300 MHz, CDCl3): = 9.07 (1 H, br), 7.35–7.24 (5 H, m),
6.62 (1 H, br), 6.14–5.78 (1 H, t, J = 54.8 Hz), 5.64 (1 H, br), 5.27–
5.23 (1 H, d, J = 11.4 Hz), 4.93–4.89 (1 H, d, J = 11.1 Hz), 4.41–
4.39 (1 H, br), 3.96–3.92 (1 H, d, J = 12.3 Hz), 3.75–3.70 (1 H, dd,
Synthesis 2004, No. 3, 334–340 © Thieme Stuttgart · New York