Synthesis of dihydroisoxazole nucleoside and nucleotide analogs

Article abstract of DOI:10.1021/jo961779r

The dihydroisoxazole nucleosides as well as their phosphonate derivatives were efficiently prepared via 1,3-dipolar cycloaddition reactions of nitrile oxides with corresponding vinyl nucleoside bases for antiviral studies.

Full text of DOI:10.1021/jo961779r

8
8
J . Org. Chem. 1997, 62, 88-92
Syn th esis of Dih yd r oisoxa zole Nu cleosid e a n d Nu cleotid e An a logs
†
Hung-J ang Gi, Yuejun Xiang, Raymond F. Schinazi, and Kang Zhao*
Department of Chemistry, New York University, New York, New York 10003, Georgia Research Center for
AIDS and HIV Infections, Veterans Affairs Medical Center, Decatur, Georgia 30033, and
Laboratory of Biochemical Pharmacology, Department of Pediatrics, Emory University School of
Medicine, Atlanta, Georgia 30322
Received September 17, 1996X
The dihydroisoxazole nucleosides as well as their phosphonate derivatives were efficiently prepared
via 1,3-dipolar cycloaddition reactions of nitrile oxides with corresponding vinyl nucleoside bases
for antiviral studies.
In tr od u ction
Naturally occurring nucleosides provide structural
1
insight for the development of potent antiviral agents.
The unique feature of the known antiviral drugs, derived
from nucleoside analogs, consists of a hydroxymethylated
five-membered ring that is cis connected to a nucleoside
base generally via a glycosidic bond. Chemically, the
parent class belongs to the five-carbon cyclopentane
F igu r e 1.
clude the glycosidic bond and the hydroxymethyl-group
cis relationship with the nucleoside base.
It has been synthetically challenging to introduce both
compounds (C
5
), which are occasionally found in nature,2
and some of the derivatives show interesting antiviral
or anticancer activities. From the cyclopentane system,
a simple one-carbon replacement with oxygen atom
3
oxygen and nitrogen into the five-membered ring (C ON)
while keeping a glycosidic bond.5 For example, the classic
problem of using system 2c (X ) NR) is the unstable
nature of the aminal moiety probably due to the easy
protonation of the nitrogen atom under physiological
4
generates furanose (C O) systems that are widely dis-
tributed in nature. Actually, the most studied antiviral
drugs derive from the furanose-constituted 2′,3′-dideoxy-
nucleosides 1 (Figure 1), and the modification of the
6
-8
conditions. We are interested in the N-O compounds
such as dihydroisoxazolidine 4 and the mutual induction
effect of two heteroatoms exerting several desired proper-
ties. Firstly, the nitrogen atom becomes less basic to
prevent its protonation; therefore, the compounds 4 are
expected to be stable at physiological pH. Secondly, the
oxygen atom may also become less nucleophilic, making
the glycosidic bond of 4 more resistant to acidic hydroly-
furanose substituents has resulted in several excellent
anti-HIV drugs such as AZT, ddC, d4T, and ddI.3
The introduction of a second heteroatom into the
furanose ring would result in several new classes of
nucleoside analogs. The presence of multihetero atoms
in the five-membered ring is desired to retain the
required conformation for the recognition between the
8
sis. In our previous paper, we had reported the first
preparation of anti-HIV-active dihydroisoxazolidinyl
nucleoside and the target enzyme (reverse tran-
scriptase).2
a,3c
Indeed, the use of oxygen or sulfur atoms
6
-chloropurine and adenine via 1,3-dipolar addition. This
has produced several diheteroatom-substituted analogs
(2a , X ) O; 2b, X ) S), and some of the dioxolanyl
) or oxathiolanyl (C OS) nucleosides are potent virus
present paper reports the recent studies on other base-
modified dihydroisoxazolidine nucleosides 4 as well as
their corresponding phosphonate derivatives.
2
(C
3
O
2
3
4
inhibitors. Of note is that the newly approved antiviral
agent 3 (B ) cytosine) is a nucleoside analog, but its ring
constitution is markedly different from the furanose of
nucleoside. The conserved structural features of 3 in-
Resu lts a n d Discu ssion
Nitr ile Oxid e Ad d ition to Vin yl 6-Ch lor op u r in e.
Dipolar cycloaddition reactions of nitrile oxides to olefins
provide efficient means for the preparation of dihy-
†
Georgia Research Center for AIDS and HIV Infections and Emory
droisoxazole.8 The olefins, which are substituted with
University School of Medicine.
9
X
alkyl groups, react smoothly with nitrile oxides. The
Abstract published in Advance ACS Abstracts, December 15, 1996.
(
1) Huryn, D. M.; Okabe, M. Chem. Rev. 1992, 92, 1475.
rate of reactions can be increased by substituting olefins
with either electron-withdrawing groups (e.g., CH d
2
(2) (a) De Clercq, E. AIDS Res. Human Retrovir. 1992, 8, 119. (b)
De Clercq, E. Nucleosides Nucleotides 1994, 13, 1271.
3) For selected recent reviews, see: (a) De Clercq, E. J . Med. Chem.
995, 38, 2491. (b) Nair, V.; J ahnke, T. S. Antimicrob. Agents
Chemother. 1995, 39, 1017. (c) de Clercq, E. Ann. New York Acad. Sci.
994, 438.
4) For selected recent references, see: (a) Wilson, L. J .; Choi, W.-
(
1
(5) Tronchet, J . M. J .; Zs e´ ly, M.; Sultan, N. Nucleosides Nucleotides
1994, 13, 2071 and references therein.
(6) Sun, L.; Li, P.; Landry D. W.; Zhao, K. Tetrahedron Lett. 1996,
37, 1547.
1
(
B.; Spurling, T.; Liotta, D. C.; Schinazi, R. F.; Cannon, D.; Painter, G.
R.; St. Clair, M.; Furman, P. A. Bioorg. Med. Chem. Lett. 1993, 3, 169.
(7) (a) Xiang, Y.; Chen, J .; Schinazi, R. F.; Zhao, K. Tetrahedron
Lett. 1995, 36, 7193. (b) Xiang, Y.; Gong, Y.; Zhao, K. Tetrahedron Lett.
1996, 37, 4877. (c) Xiang, Y.; Schinazi, R. F.; Zhao, K. Bioorg. Med.
Chem. Lett., in press.
(8) Xiang, Y.; Chen, J .; Schinazi, R. F.; Zhao, K. Bioorg. Med. Chem.
Lett. 1996, 6, 1051.
(9) For selected reviews, see: (a) Kozikowski, A. P. Acc. Chem Res.
1984, 17, 410. (b) Caramella, P.; Gr u¨ nanger, P. In 1,3-Dipolar
Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: Chichester, 1984; Vol.
1, p 291. (c) Curran, D. P. Adv. Cycloaddition 1988, 1, 129.
(
b) Kim, H, O.; Schinazi, R. F.; Shanmuganathan, K.; J eong, L. S.;
Beach, J . W.; Nampalli, S.; Cannon, D. L.; Chu, C. K. J . Med. Chem.
993, 36, 519. (c) Schinazi, R. F.; Gosselin, G.; Faraj, A.; Korba, B. E.;
1
Liotta, D. C.; Chu, C. K.; Math e´ , C.; Imbach, J .-L.; Sommadossi, J .-P.
Antimicrob. Agents Chemother. 1994, 38, 2172. (d) J in, H.; Siddigui,
M. A.; Evans, C. A.; Tse, H. L. A.; Mansour, T. S. J . Org. Chem. 1995,
6
0, 2621. (e) Liang, C.; Lee, D. W.; Newton, M. G.; Chu, C. K. J . Org.
Chem. 1995, 60, 1546 and references therein.
S0022-3263(96)01779-3 CCC: $14.00 © 1997 American Chemical Society

Synthesis of Dihydroisoxazole Nucleoside Analogs
J . Org. Chem., Vol. 62, No. 1, 1997 89
Sch em e 1
Sch em e 3
Sch em e 2
nucleoside analog 7b in 60% overall yield. The uridine
analog 7c was prepared by using a similar procedure.
The preparation of cytidine analog 7d , based on the
similar procedure, requires the 4-amine protection of
cytosine (Scheme 3). Bis(trimethylsilyl)cytosine (11) was
prepared from cytosine by treatment with excess hex-
2 2
CHCO Et) or electron-donating groups (e.g., CH d
CHOAc). However, the preparation of nucleoside analogs
from vinyl nucleoside bases has not been well investi-
gated (Scheme 1). We have recently disclosed that vinyl-
6
-chloropurine (5a ), which was prepared by using a
1
0
modified procedure of Pitha and Ts’O, behaves similarly
to other vinyl compounds to undergo the 1,3-dipolar
cycloaddition. The phenyl isocyanate-promoted1 reaction
amethyldisilazane in the presence of a catalytic amount
13
of ammonium sulfate.
Kaye and co-worker reported
1
14
that the N-vinylation of 11 with vinyl acetate, mercuric
acetate, and sulfuric acid for 2 days gave 12 in 14% yield,
wherein vinyl acetate behaved as both vinylating and
acetylating agent. Besides, the acetylation proceeded
more slowly than the vinylation because of the steric
hindrance. It was of interest to find that the amino
derivative 1-vinylcytosine 13 can be directly obtained in
2 2 2
of THPOCH CH NO with vinyl bases 5a at room tem-
perature produced a pair of enantiomers 6a . The THP-
protected product 6a , which existed as two diastereomers
due to the presence of the THP group, was converted to
+
the hydroxymethyl derivative by using Dowex 50 (H )
in methanol. The adenosine analog 7a was obtained
through steps of azide displacement, triphenylphosphine
reduction, and acetic acid hydrolysis. The survival of the
glyosidic bond of 7a under these acidic conditions (acidic
Dowex in methanol and aqueous acetic acid at 100 °C),
which was atypical, confirmed our original expectation
that compound 7a could be acid-stable due to the mutual
induction effect through the oxygen-nitrogen bond.
Nitr ile Oxid e Ad d ition to Vin ylp yr im id in es. Sev-
eral procedures have been developed for the preparation
of vinylpyrimidines, which are the common intermediates
for the constructions of nucleoside polymers. The direct
exchange of the acetyl group of vinyl acetate with uracil
or thymine was not very successful in our hands, pre-
sumably due to the presence of four nucleophilic centers.
A multistep procedure was adapted to prepare vinylthy-
mine (5b) and vinyluracil (5c) (Scheme 2). The N-
monoalkylated thymine 9a (R ) Me) was obtained by the
reaction of thymine 8a with ethylene carbonate in the
presence of a catalytic amount of sodium hydroxide, and
6
5% yield by increasing the amounts of two catalysts and
decreasing the reaction time, and in addition, this condi-
tion produced the acetate 12 in 9% yield, which was
useful for the subsequent reactions. Although the direct
1
,3-dipolar reaction of 12 with corresponding nitrile oxide
gave the N-acetylated cytidine 15b (E ) Ac), the final
15
step of deacetylation failed under basic conditions.
Alternatively, we used the N-BOC-protected derivative
1
4, derived from 13 by refluxing with di-tert-butyl dicar-
bonate in (50%) THF/CH Cl for 48 h, in the 1,3-dipolar
2
2
cycloaddition, and the final treatment of 15a (E ) t-BOC)
with 44% formic acid in methanol removed both protect-
ing groups to achieve the synthesis of 7d in 29% overall
yield.
P r ep a r a tion of Isoxa zole Nu cleotid e P h osp h o-
n a tes. Nucleoside phosphonates have been considered
as isosteric alternatives for nucleoside phosphates as
2,3a
antiviral agents.
The potential value of nucleoside
phosphonates makes it attractive to investigate the limit
of the cycloaddition reactions of vinyl bases with a
phosphonate starting material. Since the isocyanate-
dehydrate method requires the 3-nitropropyl phospho-
nate derivatives that are difficult to prepare, we used the
a small amount of the catalyst is the key to avoid a large
amount of N,N-dialkylated product.1
0,12
Chlorination of
9
a with excess thionyl chloride gave 1-(2-chloroethyl)-
uracil 10a in 89% yield, followed dehydrochlorination by
potassium tert-butoxide in dimethyl sulfoxide at room
temperature, provided 5b (R ) Me) in 83% yield. The
16
well-known NCS-oxidation method of oximes, and the
required starting material is the readily available oxime
1
,3-dipolar addition of 5b with THPOCH
2 2 2
CH NO gave
6
b (Scheme 1), which was deprotected to give the desired
(
(
13) Nishimura, T.; Iwai, I. Chem. Pharm. Bull. Tokyo 1964, 12, 352.
14) Kaye, H.; Chang, S.-H. Tetrahedron 1970, 26, 1369.
(
10) Pitha, J .; Ts’O, P. O. P. J . Org. Chem. 1968, 33, 1341.
(15) Thymine was the only product isolated upon treatment of
thymidine analog 7b with methanolic ammonia at room temperature
for 4 h. Cytosine was similarly obtained by the use of 7d .
(16) (a) Larsen, K. E.; Torssell, K. B. G. Tetrahedron 1984, 40, 2985.
(b) Stevens, R. V.; Albizati, K. F. Tetrahedron Lett. 1984, 25, 4587. (c)
De Sarlo, F.; Brandi, A. J . Heterocycl. Chem. 1983, 20, 1505.
(11) Carrea, G.; De Amici, M.; DeMichei, C.; Cateni; F.; Liverani,
P.; Carnielle, M.; Riva, S. Tetrahedron: Asymmetry 1993, 4, 1063.
12) (a) Lapienis, G.; Penczek, S. J . Polym. Sci. Polym. Chem. 1990,
8, 1519. (b) Ueda, N.; Kondo, K.; Kono, M.; Takemoto, K.; Imoto, M.
Makromol. Chem. 1968, 120, 13.
(
2

9
0
J . Org. Chem., Vol. 62, No. 1, 1997
Gi et al.
Sch em e 4
acetate/petroleum ether to give 5a (2.98 g, 82%) as a white
solid:10 1H NMR (DMSO-d
, 200 MHz) δ 8.78 (s, 1H), 8.32 (s,
H), 7.26 (dd, J ) 9.1, 16.0 Hz, 1H), 6.02 (dd, J ) 1.8, 16.8
Hz, 1H), 5.30 (dd, J ) 1.8, 9.2 Hz, 1H).
-Ch lor o-9-[3-[(t et r a h yd r op yr a n yl-2-oxy)m et h yl]-4,5-
d ih yd r o-1,2-isoxa zol-5-yl]p u r in e (6a ). A mixture of 5 (0.72
g, 4 mmol), THPO(CH NO (0.95 g, 6 mmol), triethylamine
0.2 mL), and phenyl isocyanate (1.12 mL, 10 mmol) in benzene
20 mL) was stirred overnight at room temperature. To the
6
1
6
2
)
2
2
(
(
mixture was added water (20 mL), and the resulting solution
was stirred for 1 h. After filtration, the filtrate was washed
with brine, dried, and concentrated. The residue was chro-
matographed on silica gel with 50% ethyl acetate/petroleum
ether to give 6a (0.92 g, 68%) as a clear oil: 1H NMR (CDCl
,
3
2
(
00 MHz) δ 8.74 (1 H, s), 8.29 (s, 0.5 H), 8.21 (s, 0.5 H), 6.86
m, 1H), 4.64 (m, 3H), 3.52-3.87 (m, 4H), 1.53-1.79 (m, 6H).
Anal. Calcd for C14 : C, 49.78; H, 4.77; N, 20.73.
1
6 (Scheme 4). The nitrile oxide 17 can be prepared by
the NCS oxidation of 16 in the presence of pyridine and
H
5 3
16ClN O
reacts smoothly with vinyl nucleoside bases to produce
Found: C, 49.29; H, 4.84; N, 20.47.
1
8 in good yield. In the case of cytosine analog, the
9-[3-(Hyd r oxym eth yl)-4,5-d ih yd r o-1,2-isoxa zol-5-yl]a d -
en in e (7a ). The mixture of 6a (0.50 g, 1.48 mmol), Dowex 50
deprotection was achieved by using formic acid in metha-
nol to afford the desired phosphonate 18f in 34% overall
yield. The nucleosides 7 and their phosphonate deriva-
tive 18 are being evaluated for anticancer and antiviral
activities.
+
(H ) (0.50 g), and methanol (20 mL) was stirred at room
temperature for 1 h and then filtered. The filtrate was
concentrated and chromatographed on silica gel with 10%
MeOH/CHCl
3
to give the corresponding alcohol (0.37 g, 99%)
1
as a white solid: mp 188-189 °C; H NMR (DMSO-d
6
, 200
MHz) δ 8.83 (s, 1H), 8.76 (s, 1H), 7.01 (dd, J ) 4.1, 8.5 Hz,
Con clu sion
1H), 5.50 (t, J ) 5.9 Hz, 1H), 4.44-4.47 (m, 2H), 3.79-3.83
(
2
m, 2H). Anal. Calcd for C
7.61. Found: C, 42.66; H, 3.23; N, 27.62.
The mixture containing 6-chloro-9-[3-(hydroxymethyl)-4,5-
9 8 5 2
H ClN O : C, 42.62; H, 3.17; N,
Reaction of (â-alkoxyethyl)nitrile oxides with vinylpy-
rimidines or -purines, via 1,3-dipolar cycloaddition reac-
tions, allows the efficient synthesis of the hydroxymeth-
ylated isoxazolidine nucleosides 7 in which the furanose
ring is replaced by an N-O-containing heterocycle. The
usefulness of this reaction for the related analogs has also
been demonstrated by the easy access to the correspond-
ing phosphonate derivatives 18. The connection of
oxygen and nitrogen atoms provides a possibility to
introduce the nucleophilic nitrogen into the furanose ring
with a minimum of steric manipulation, providing op-
portunities to modify the furanose ring of nucleoside
analogs with novel structural features. Consequently,
it becomes possible to design other related N-O-contain-
ing derivatives for biological studies. Future direction
should also include addressing the important problem of
enantiostereoselective preparation of these new and
related analogs.
dihydro-1,2-isoxazol-5-yl]purine (0.25 g, 1 mmol), sodium azide
(0.32 g, 5 mmol), ethanol (10 mL), and water (0.5 mL) was
refluxed for 1 h. After the reaction mixture was cooled to room
temperature, triphenylphosphine (0.40 g, 1.5 mmol) was added
and the resulting solution was stirred at room temperature
for 30 min. Concentration of the mixture gave a residue that
was dissolved in acetic acid (5 mL) and water (2 mL) and
refluxed for 30 min. The mixture was concentrated and
coevaporated with toluene (2 × 5 mL) to dryness. The crude
product was chromatographed on silica gel with 10% MeOH/
CHCl
3
to yield 7a (0.12 g, 51%) as a white solid: mp 126-127
1
°
C; H NMR (DMSO-d , 200 MHz) δ 8.19 (s, 1H), 8.18 (s, 1H),
6
7
1
.34 (s, 2H), 6.85 (dd, J ) 8.8, 4.3 Hz, 1H), 5.48 (t, J ) 6.1 Hz,
H), 4.43 (d, J ) 3.2 Hz, 1H), 4.40 (d, J ) 3.2 Hz, 1Hz), 3.72
(d, J ) 9.1 Hz, 1H), 3.69 (d, J ) 4.3 Hz, 1H). Anal. Calcd for
.
9 10 6 2 2
C H N O 0.25H O: C, 45.28; H, 4.43; N, 35.20. Found: C,
45.43; H, 4.37; N, 35.24.
1-(2-Hyd r oxyeth yl)u r a cil (9b). Uracil (9.9 g, 88 mmol),
dried previously in vacuo at 120 °C for 4 h, was heated in dry
DMF (200 mL) with a catalytic amount of NaOH until all
uracil was dissolved. To this reaction mixture was added
ethylene carbonate (8.8 g, 100 mmol), and the resulting
mixture was allowed to reflux for 1.5 h. After cooling, the
solution was concentrated and the residue was diluted with
300 mL of water. Unreacted uracil formed a precipitate and
Exp er im en ta l Section
Gen er a l P r oced u r es. All reagents and solvents were
purchased from Aldrich, Fisher, or Acros Chimica and, except
when otherwise stated, were used without further purification.
All reactions involving air-sensitive agents were conducted
under an argon atmosphere. The drying of an organic solvent
+
was filtered. After neutralization with Dowex 50 (H ), the
over Na
2
SO
4
or MgSO
4
followed by filtration is referred to
water was evaporated in vacuo. The residue was chromato-
simply as “dried”. Removal of solvents under reduced pressure
on a rotary evaporator is referred to simply as “concentrated”.
Thin layer chromatography (TLC) was done on E. Merck silica
gel (0.25 mm thickness) 60 F254 glass plates. Plates were
viewed under UV light (254 nm) or developed in an iodine
chamber. Column chromatography was performed on Mallinck-
rodt silica gel 60 (230-400 mesh). Melting points were
recorded in open capillary tubes and are uncorrected. The 200
MHz proton NMR data were collected on a Varian instrument.
graphed on silica gel with 20% methanol/CHCl
3
to give 9b (8.54
, 200 MHz) δ
g, 54%) as a white solid:10 1H NMR (DMSO-d
6
11.23 (s, 1H), 7.55 (d, J ) 8.0 Hz, 1H), 5.54 (d, J ) 8.1 Hz,
1H), 4.91 (t, J ) 5.3 Hz, 1H), 3.85-3.44 (m, 4H).
1-(2-Ch lor oeth yl)u r a cil (10b). Pyridine (0.2 mL) was
added to a solution of 9b (1.87 g, 12 mmol) in dry, hot dioxane
(30 mL). Thionyl chloride (5 mL) in dioxane (5 mL) was then
added dropwise, and the solution was refluxed for 1 h. The
solvent was evaporated in vacuo, and the residue was redis-
6
-Ch lor o-9-vin ylp u r in e (5a ). A solution of concentrated
solved in 100 mL of CHCl
water and extracted with CHCl
were dried over Na SO , filtered, concentrated, and chromato-
3
. The solution was partitioned over
sulfuric acid (0.3 mL) in ethyl acetate (2 mL) was added to a
suspension of mercuric acetate (0.64 g, 2 mmol) in vinyl acetate
3
to collect the organics, which
2
4
(
15 mL) to form a clear solution. This procedure avoids the
coloring of acetate by the direct addition of acid. The powdered
-chloropurine (3.1 g, 20 mmol) was then added to the reaction
graphed on silica gel with 50% ethyl acetate/petroleum ether
to give 10b (1.80 g, 86%) as a white solid:1 H NMR (DMSO-
0 1
6
6
d , 200 MHz) δ 11.35 (s, 1H), 7.67 (d, J ) 7.8 Hz, 1H), 5.59 (d,
J ) 7.9 Hz, 1H), 4.02 (t, J ) 5.9 Hz, 2H), 3.86 (t, J ) 5.5 Hz,
2H).
1-Vin ylu r a cil (5c). The chloride 10b (1.5 g, 8.6 mmol)
dissolved in dry dimethyl sulfoxide (20 mL) was added drop-
mixture, followed by an additional portion of vinyl acetate (10
mL) under argon. The reaction mixture was refluxed for 2 h
before it was filtered. The filtrate was concentrated, and the
residue was chromatographed on silica gel with 60% ethyl

Synthesis of Dihydroisoxazole Nucleoside Analogs
J . Org. Chem., Vol. 62, No. 1, 1997 91
wise to the stirred solution of potassium tert-butoxide (3.26 g,
2.5 mmol) in a mixture of THF (15 mL) and benzene (6 mL)
under argon atmosphere were added phenyl isocyanate (0.80
mL, 7 mmol) and triethylamine (0.1 mL). The reaction
mixture was stirred for 1 h at room temperature, and 1-vi-
nylthymine (5b) (0.39 g, 2.5 mmol) was added. The mixture
was stirred overnight at room temperature, filtered, and dried.
After evaporation of solvent, the crude product was chromato-
graphed on a silica gel column with 50% ethyl acetate/
petroleum ether to give crude 6b (0.45 g). A mixture of 6b
2
9 mmol) in dimethyl sulfoxide (20 mL). After the reaction
mixture was stirred for 1 h at room temperature, 60 mL of
cold water was added to it, and the solution was made slightly
+
acidic by adding Dowex 50 (H ). The solution was filtered and
evaporated in vacuo (0.2 mm) at 60 °C to give a residue. The
residue was chromatographed on silica gel with 50% ethyl
acetate/petroleum ether to give 5c (1.06 g, 74%) as a white
1
0 1
solid: H NMR (DMSO-d , 200 MHz) δ 11.51 (s, 1H), 8.01 (d,
6
+
J ) 8.1 Hz, 1H), 7.09 (dd, J ) 9.3, 16.0 Hz, 1H), 5.73 (d, J )
(0.45 g) and Dowex 50 (H ) (1.16 g) in methanol (20 mL) was
stirred for 2 h at room temperature. The filtered solution was
evaporated in vacuo, and the residue was chromatographed
8
9
.0 Hz, 1H), 5.36 (dd, J ) 2.3, 16.1 Hz, 1H), 4.91 (dd, J ) 2.3,
.2 Hz, 1H).
1
-[3-(H yd r oxym et h yl)-4,5-d ih yd r o-1,2-isoxa zol-5-yl)]-
on silica gel with 10% methanol/CH
as a white solid: mp 170-171 °C; H NMR (DMSO-d , 200
2
Cl
2
to give 7b (0.37 g, 66%)
1
u r id in e (7c). To a solution of THPO(CH NO (0.40 g, 2.3
2
)
2
2
6
mmol) in a mixture of THF (15 mL) and benzene (6 mL) under
an argon atmosphere were added phenyl isocyanate (0.80 mL,
MHz) δ 11.42 (s, 1H), 7.12 (1s, 1H), 6.62 (dd, J ) 3.6, 10.0 Hz,
1H), 5.44 (t, J ) 6.1 Hz, 1H), 4.28 (dd, J ) 3.1, 5.8 Hz, 1H),
3.60 (dd, J ) 10.0, 18.8 Hz, 1H), 3.21 (dd, J ) 3.8, 18.9 Hz,
7
mmol) and triethylamine (0.1 mL). The reaction mixture
was stirred for 1 h at room temperature, and 1-vinyluracil (5c)
0.34 g, 2.5 mmol) was added to this solution. The resulting
1H), 1.79 (s, 3H); 13C NMR (DMSO-d
) δ 12.5, 40.7, 56.2, 84.2,
110.7, 135.8, 150.4, 160.5, 163.9; HRMS (EI) m/ z calcd for
(C ) 225.2057, found 225.0754.
1-Vin ylcytosin e (13). Cytosine (2.3 g, 21 mmol) was
6
(
mixture was stirred overnight at room temperature, filtered,
and dried. After evaporation of solvent, the crude product was
chromatographed on silica gel with 50% ethyl acetate/
9 11 3 4
H N O
heated at 140-150 °C with hexamethyldisiliazane (16 mL) and
a trace of ammonium sulfate for 24 h. When a clear solution
was formed, the solution was cooled to room temperature and
concentrated in vacuo. The resulting residue was refluxed
with vinyl acetate (30 mL), mercuric acetate (0.25 g, 0.8 mmol),
and concd sulfuric acid (0.1 mL) under argon atmosphere for
24 h. The solvent was removed under reduced pressure, and
methanol (50 mL) was added, followed by addition of enough
ammonium hydroxide to make the solution slightly basic. The
insoluble residue was extracted twice with boiling methanol
(15 mL), and the methanol solutions were combined and
concentrated. The residue was chromatographed on silica gel
petroleum ether to give crude 6c (0.40 g). A mixture of 6c
+
(
0.40 g) and Dowex 50 (H ) (1.16 g) in methanol (20 mL) was
stirred for 2 h. The filtered solution was evaporated under
reduced pressure, and the residue was chromatographed on
silica gel with 10% methanol/CH
as a white solid: mp ) 191-192 °C; H NMR (DMSO-d
MHz) δ 11.41 (s, 1H), 7.30 (d, J ) 8.1 Hz, 1H), 6.59 (dd, J )
.4, 9.7 Hz, 1H), 5.66 (d, J ) 8.1 Hz, 1H), 5.42 (bs, 1H), 4.28
d, J ) 3.7 Hz, 2H), 3.61 (dd, J ) 10.2, 19.1 Hz, 1H), 3.28 (dd,
2 2
Cl to give 7c (0.33 g, 68%)
1
6
, 200
3
(
1
3
J ) 10.4, 13.7 Hz, 1H); C NMR (DMSO-d
1
6
) δ 41.0, 56.1, 84.5,
02.8, 140.6, 150.3, 160.7, 163.2; HRMS (EI) m/ z calcd for
(C
8
H
9
N
3
O
4
) 211.1786, found 211.0589.
with 10% methanol/CHCl
3
to give 13 (1.86 g, 65%) and 12 (0.34
, 200 MHz) 13 δ
g, 9%) as white solids:14 1H NMR (DMSO-d
1
-(2-Hyd r oxyeth yl)th ym in e (9a ). Thymine (12.6 g, 100
6
mmol), dried previously in vacuum at 120 °C for 4 h, was
heated in dry DMF (200 mL) with a small amount of NaOH
until all uracil was dissolved. To this solution was added
ethylene carbonate (8.8 g, 100 mmol), and the mixture was
refluxed for 1.5 h. Upon cooling, the mixture was concentrated
and the residue was dissolved in 300 mL of water. Unreacted
uracil formed a precipitate and was filtered off. After neu-
7.92 (d, J ) 7.4 Hz, 1H), 7.40 (s, 2H), 7.24 (dd, J ) 9.2, 16.2
Hz, 1H), 5.82 (d, J ) 7.3 Hz, 1H), 5.22 (dd, J ) 1.2, 16.2 Hz,
1H), 4.78 (dd, J ) 1.3, 9.2 Hz, 1H); 12 δ 11.00 (s, 1H), 8.36 (d,
J ) 7.3 Hz, 1H), 7.77-7.30 (m, 3H), 5.55 (d, J ) 16.1 Hz, 1H),
5.10 (d, J ) 9.1 Hz, 1H).
4
N -(ter t-Bu tyloxyca r bon yl)-1-vin ylcytosin e (14). Di-
tert-butyl dicarbonate (1.70 g, 8 mmol) was added to the
solution of 1-vinylcytosine (13) (1.0 g, 7 mmol) in THF (20 mL)
+
tralization with Dowex 50 (H ), the water was evaporated in
vacuo. The residue was chromatographed on silica gel with
2 2
and CH Cl (20 mL), and the reaction mixture was refluxed
2
0% methanol/chloroform to give 9a (13.1g, 70%) as a white
for 48 h. The solvent was removed under reduced pressure,
1
0 1
solid: H NMR (DMSO-d
J ) 1.1 Hz, 1H), 4.89 (t, J ) 5.3 Hz, 1H), 3.69 (t, J ) 5.1 Hz,
H), 3.58 (t, J ) 4.8 Hz, 2H), 1.75 (s, 3H).
-(2-Ch lor oeth yl)th ym in e (10a ). Pyridine (1.0 mL) was
added to a solution of 9a (6.91 g, 41 mmol) in dry, hot dioxane
150 mL). Thionyl chloride (30 mL) dissolved in dioxane (20
6
, 200 MHz) δ 11.19 (s, 1H), 7.43 (d,
and the residue was chromatographed on silica gel with 10%
1
methanol/CHCl
3
to give 14 (1.43 g, 83%): H NMR (DMSO-
2
d
6
, 200 MHz) δ 10.52 (s, 1H), 8.31 (d, J ) 7.6 Hz, 1H), 7.23
1
(dd, J ) 9.0, 16.2 Hz, 1H), 7.07 (d, J ) 7.3 Hz, 1H), 5.52 (d, J
1
3
) 16.0 Hz, 1H), 5.07 (d, J ) 9.1 Hz, 1H), 1.48 (s, 9H); C NMR
(CDCl ) δ 28.6, 83.3, 96.3, 104.5, 132.6, 143.1, 151.7, 154.3,
(
3
mL) was then added dropwise to this solution, and the
resulting solution was refluxed for 1 h. The solvent was
15 3 3
163.5; HRMS (EI) m/ z calcd for (C11H N O ) 237.2605, found
237.1110.
evaporated in vacuo and was redissolved in CHCl
The mixture was extracted over water (50 mL), and all the
organics were collected and dried over Na SO , filtered,
3
(500 mL).
1-[3-(H yd r oxym et h yl)-4,5-d ih yd r o-1,2-isoxa zol-5-yl)]-
cytid in e (7d ). To a solution of THPO(CH NO (0.25 g, 1.4
2
)
2
2
2
4
mmol) in a mixture of THF (15 mL) and benzene (6 mL) under
argon atmosphere were added phenyl isocyanate (0.50 mL, 4.6
mmol) and triethylamine (0.05 mL). The reaction mixture was
concentrated, and chromatographed on silica gel with 50%
ethyl acetate/petroleum ether to give 10a (7.52 g, 89%) as a
1
4
white solid: H NMR (DMSO-d
6
, 200 MHz) δ 11.33 (s, 1H),
stirred for 1 h at room temperature, and N -(tert-butyloxycar-
7
5
.58 (s, 1H), 3.97 (dd, J ) 1.2, 5.1 Hz, 2H), 3.87 (dd, J ) 1.7,
.2 Hz, 2H), 1.76 (s 3H).
-Vin ylth ym in e (5b). The chloride 10a (6.00 g, 32 mmol)
bonyl)-1-vinylcytosine (14) (0.30 g, 1.3 mmol) was added. The
mixture was stirred overnight at room temperature, filtered,
and dried. After evaporation of solvent, the crude product was
chromatoghaphed on silica gel column with 60% ethyl acetate/
petroleum ether to give crude 15a (0.34 g). 15a (0.34 g) was
dissolved in 44% formic acid/methanol (20 mL), and the
reaction mixture was stirred for 30 min at 40 °C. The solution
was evaporated under reduced pressure, and the residue was
1
dissolved in dry dimethyl sulfoxide (40 mL) was added drop-
wise to the stirred solution of potassium tert-butoxide (13.0 g,
1
was stirred at room temperature for 1 h, 80 mL of cold water
was added, and the mixture was made slightly acidic by adding
Dowex 50 (H ). After filtration, the solution was evaporated
in vacuo (0.2 mm) at 60 °C and the residue was chromato-
graphed on silica gel with 50% ethyl acetate/petroleum ether
to give 5c (4.0 g, 83%) as a white solid: H NMR (DMSO-d
2
Hz, 1H), 5.07 (dd, J ) 2.1, 16.1 Hz, 1H), 4.91 (dd, J ) 2.2, 9.1
Hz, 1H′), 1.98 (s, 3H).
16 mmol) in dimethyl sulfoxide (40 mL). After the mixture
+
3
chromatographed on silica gel with 20% methanol/CHCl to
1
give 7d (80 mg, 29%) as a white solid: mp 176-177 °C; H
NMR (DMSO-d , 200 MHz) δ 7.28 (m, 3H), 6.60 (dd, J ) 3.3,
6
1
0 1
6
,
9.5 Hz, 1H), 5.75 (d, J ) 7.3 Hz, 1H), 5.41 (bs, 1H), 4.26 (bs,
2H), 3.58 (dd, J ) 9.7, 18.8 Hz, 1H), 3.08 (dd, J ) 3.4, 18.5
00 MHz) δ 9.21 (s, 1H), 7.34 (s, 1H), 7.21 (dd, J ) 9.1, 16.0
8 10 4 3
Hz, 1H); HRMS (FAB) m/ z calcd for (C H N O P) 211.1778,
found 211.0822.
1
-[3-(H yd r oxym et h yl)-4,5-d ih yd r o-1,2-isoxa zol-5-yl)]-
Gen er a l P r oced u r e for th e Dih yd r oisoxa zole Nu cle-
th ym id in e (7b). To a solution of THPO(CH NO (0.45 g,
2
)
2
2
otid es 18a -d . To a stirring suspension of N-chlorosuccin-

9
2
J . Org. Chem., Vol. 62, No. 1, 1997
Gi et al.
imide in dry CH
diethyl 3-hydroiminopropyl phosphate 16 in dry CH
mL) in one portion. As the suspended N-chlorosuccinimide
completely disappeared, the vinyl base in dry CHCl (10 mL)
was added dropwise and the temperature was raised to 40-
0 °C. After 10 min of stirring at this temperature, triethyl-
amine was added dropwise over 15 min, and the mixture was
stirred for another 30 min. The solvent was evaporated under
reduced pressure, and the residue was chromatographed on
silica gel with methanol/methylene chloride to give 18. 18a
was converted to 18b using the same procedures as for 7a .
The final step of deprotection of 18e was achieved by 44%
formic acid in methanol to give 18f.
2
Cl
2
(10 mL) was added a solution of O,O-
1-[3-[(Dieth oxylph osph in yl)eth yl]-4,5-dih yd r o-1,2-isox-
1
2
2
Cl (10
a zol-5-yl]u r a cil (18d ): yield 57%; H NMR (DMSO-d
6
, 200
MHz) δ 11.39 (s, 1H), 7.42 (d, J ) 8.1 Hz, 1H), 6.56 (dd, J )
3.7, 9.9 Hz, 1H), 5.63 (d, J ) 8.1, 1H), 4.03 (m, 4H), 3.57 (dd,
J ) 9.5, 19.1 Hz, 1H), 3.18 (m, 1H), 2.60 (m, 2H), 2.07 (m,
3
2H), 1.26 (t, J ) 7.0 Hz, 6H); 13C NMR (DMSO-d ) δ 16.6, 16.7,
5
6
20.4, 20.8, 20.9, 23.2, 42.3, 61.5, 61.6, 84.9, 102.6, 141.0, 150.4,
1
59.4, 159.8, 163.3; HRMS (EI) m/ z calcd for (C13
H) 346.3028, found 346.1172.
-[3-[(Dieth oxylph osph in yl)eth yl]-4,5-dih yd r o-1,2-isox-
20 3 6
H N O P +
1
1
a zol-5-yl)]ytosin e (18f): yield 34%; H NMR (DMSO-d
MHz) δ 7.34 (d, J ) 7.4 Hz, 1H), 7.25 (bs, 2H), 6.57 (dd, J )
.0, 9.6 Hz, 1H), 5.73 (d, J ) 8.1 Hz, 1H), 4.01 (m, 4H), 3.53
dd, J ) 7.5, 19.9 Hz, 1H), 3.19 (m, 1H), 2.62 (m, 2H), 2.06 (m,
6
, 200
4
(
2
2
1
6
6
-Ch lor o-9-[3-[(dieth oxylph osph in yl)eth yl]-4,5-dih ydr o-
1
1
d
,2-isoxa zol-5-yl]p u r in e (18a ): yield 84%; H NMR (DMSO-
, 200 MHz) δ 8.83 (s, 2H), 7.00 (m, 1H), 4.06 (m, 4H), 3.76
H), 1.25 (t, J ) 7.0 Hz, 6H); 13C NMR (CD
1.6, 22.1, 22.2, 24.5, 45.3, 63.8, 63.9, 88.3, 96.8, 142.5, 158.0,
61.1, 161.4, 168.0. Anal. Calcd (C13 P): C, 45.35; H,
OD) δ 17.1, 17.2,
3
6
(
7
+
m, 1H), 3.19 (m, 1H), 2.75 (m, 2H), 2.16 (m, 2H), 1.27 (t, J )
.0 Hz, 6H); HRMS (EI) m/ z calcd for (C14 P and M
H) 387.7656 and 388.7736, found 387.0854 and 388.0969.
-[3-[(Dieth oxylph osph in yl)eth yl]-4,5-dih ydr o-1,2-isox-
20 5 5
H N O
5 4
H19ClN O
.15; N, 16.27. Found: C, 45.52; H, 6.13; N, 16.36.
9
1
a zol-5-yl]a d en in e (18b): yield 50%; H NMR (DMSO-d
6
, 200
Ackn owledgm en t. K.Z. acknowledges financial sup-
port from the American Cancer Society (J FRA-517), the
NSF Faculty Early Career Development Program (CHE-
MHz) δ 8.27 (s, 1H), 8.15 (s, 1H), 7.33 (s, 2H), 4.06 (m, 4H),
3
6
6
.68 (m, 2H), 2.72 (m, 2H), 2.17 (m, 2H), 1.28 (t, J ) 7.1 Hz,
13
H); C NMR (CD
3.8, 64.0, 85.5, 141.2, 150.4, 154.3, 157.5, 160.8, 161.1. Anal.
P): C, 45.66; H, 5.75; N, 22.82. Found: C,
6.43; H, 5.73; N, 22.90.
-[3-(Dieth oxylp h osp h in yl)-4,5-d ih yd r o-1,2-isoxa zol-5-
3
OD) δ 17.1, 17.2, 21.7, 22.2, 22.3, 24.6, 43.8,
9
502068), and the donors of the Petroleum Research
Fund (28860-G1), administered by the American Chemi-
cal Society. R.S. acknowledges the Department of
Veterans Affairs for support.
Calcd (C14
4
20 6 4
H N O
1
1
6
yl]th ym in e (18c): yield 53%; H NMR (DMSO-d , 200 MHz)
δ 11.38 (s, 1H),7.30 (s, 1H), 6.59 (dd, J ) 3.7, 10.3 Hz, 1H),
Su p p or tin g In for m a tion Ava ila ble: Copies of NMR
spectra (7 pages). This material is contained in libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
4
4
.04 (m, 4H), 3.56 (dd, J ) 10.3, 18.7 Hz, 1H), 3.24 (dd, J )
.0, 19.1 Hz, 1H), 2.62 (m, 2H), 2.10 (m, 2H), 1.80 (s, 3H), 1.26
1
3
(
t, J ) 7.0 Hz, 6H); C NMR (CD
3
OD) δ 11.4, 15.7, 15.8, 20.2,
2
1
3
0.7, 20.8, 23.0, 42.6, 62.4, 62.5, 86.0, 111.1, 136.7, 150.8, 159.4,
59.7; HRMS (EI) m/ z calcd for (C14
22 3 6
H N O P) 359.3129, found
59.1230.
J O961779R