Synthesis of conformationally locked carbocyclic nucleosides built on an oxabicyclo[3.1.0]hexane system

Article abstract of DOI:10.1016/S0040-4020(02)01528-4

The rigid 6-oxobicyclo[3.1.0]hexane scaffold, characteristic of the natural antibiotic neplanocin C (3), was used to build prototypes of conformationally locked deoxynucleosides in the North hemisphere of the pseudorotational cycle. The purine analogues 6 and 7 are conformationally equivalent to carbocyclic nucleosides built with the bicyclo[3.1.0]hexane template. The pyrimidine nucleosides were unstable and underwent a facile intramolecular epoxide ring-opening reaction leading to heterocycle 22. Only the deoxyguanosine analogue 7 showed antiviral activity against EBV.

Full text of DOI:10.1016/S0040-4020(02)01528-4

Tetrahedron 59 (2003) 295–301
Synthesis of conformationally locked carbocyclic nucleosides built
on an oxabicyclo[3.1.0]hexane system
a
Marıa J. Comin, Juan B. Rodriguez, Pam Russ and Victor E. Marquez
a
b
b
,
,
*
*
´
a
´ ´ ´
Departamento de Quımica Organica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellon 2,
Ciudad Universitaria, 1428 Buenos Aires, Argentina
bLaboratory of Medicinal Chemistry, Center for Cancer Research, National Cancer Institute at Frederick, 376 Boyles St, Bldg 376, Rm 104,
Frederick, MD 21702-1201, USA
Received 27 September 2002; revised 25 November 2002; accepted 26 November 2002
Abstract—The rigid 6-oxobicyclo[3.1.0]hexane scaffold, characteristic of the natural antibiotic neplanocin C (3), was used to build
prototypes of conformationally locked deoxynucleosides in the North hemisphere of the pseudorotational cycle. The purine analogues 6 and
7 are conformationally equivalent to carbocyclic nucleosides built with the bicyclo[3.1.0]hexane template. The pyrimidine nucleosides were
unstable and underwent a facile intramolecular epoxide ring-opening reaction leading to heterocycle 22. Only the deoxyguanosine analogue
7 showed antiviral activity against EBV. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction and background
As defined by the concept of pseudorotation, nucleosides in
solution exist in a dynamic equilibrium characterized by the
rapid change of the furanose ring between a North-type
geometry and the corresponding antipodal South-type
geometry.1 Normally, a conformationally unrestricted
furanose ring can adopt a number of envelope (E) or twist
(T) forms, which can be conveniently described by the value
of P in the pseudorotational cycle. By convention, a phase
angle P¼08 corresponds to an absolute North conformation
possessing a symmetrical twist form 3T2, whereas its South
antipode, T2, is represented by P¼1808 (Fig. 1). For the
3
typical North conformation, the value of P can fluctuate
between 3428 and 188 (2E! T2! E) and for the typical
3
3
antipodal South conformation P values range between 1628
2
and 1988 (2E! T3!3E). Despite this dynamism in solution,
only one preferred conformer is found in the solid state, and
only one conformer is responsible for optimal molecular
recognition in drug–enzyme complexes. In order to identify
the preferred conformation in drug–enzyme interactions,
we and others have proposed the use of cyclopentane
nucleosides that are locked into one of the two extreme
antipodes of the pseudorotational cycle by virtue of the
presence of a fused three-member ring.2 With this approach,
we have shown that the bicyclo[3.1.0]hexane template can
adopt a value of P that corresponds closely to either a North
(2E) or a South (3E) conformation, as demonstrated in the
case of North-methanocarba adenosine (1) and South-
Figure 1. Fixed location of the bicyclo[3.1.0]hexane (X¼CH2) and 6-
oxobicyclo[3.1.0]hexane (X¼O) templates in the pseudorotational cycle
(nucleoside numbering).
methanocarba adenosine (2).3 A naturally occurring and
conformationally equivalent template found in the antibiotic
neplanocin C (3)4 is 6-oxobicyclo[3.1.0]hexane. Neplano-
cin C is a minor component of the neplanocin family of
antibiotics isolated from Ampullariela regularis which
according to its X-ray structure corresponds to a confor-
mationally locked nucleoside with an E North envelope
2
conformation (P¼338.038 and nmax¼21.898).4e An enantio-
selective synthesis of neplanocin C has been recently
achieved employing D-ribonolactone as a chiral starting
material.5 A crucial point in this synthesis was the
Keywords: carbocyclic nucleosides; conformationally locked nucleosides;
6-oxobicyclo[3.1.0]hexane; 20-deoxyneplanocin C; antiviral activity, EBV.
*
Corresponding authors; e-mail: marquezv@dc37a.nci.nih.gov
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01528-4

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M. J. Comin et al. / Tetrahedron 59 (2003) 295–301
remarkable stability of the epoxide ring during the
conversion of the 6-chloropurine ring into adenosine
under harsh reaction conditions, such as methanolic
ammonia at high temperature.5 This remarkable stability
of the epoxide ring was also observed in simpler, closely
related carbanucleosides.6
was prepared according to our published method,3b was
employed for the preparation of all purine and pyrimidine
20-deoxyneplanocin C analogues. This compound, in turn,
was synthesized from the readily available chiral template
(3R,4S)-4-phenylmethoxy-3-[(phenylmethoxy)methyl]-
cyclopent-1-ene (11).12
2.1. Synthesis of purine analogues
The corresponding 20-deoxy adenosine derivative 6 was
synthesized following a convergent approach (Scheme 1).
Treatment of 12 with m-chloroperbenzoic acid at 08C gave
exclusively the a-epoxy derivative 13, which according to
Hembest’s rule resulted from the OH-directed attack of the
incoming electrophile.13 Despite the potential involvement
of the epoxide ring, coupling 6-chloropurine to epoxy
alcohol 13 under Mitsunobu conditions14 successfully gave
the desired product 14 in low but reproducible yields (ca.
36%). The desired N-9 alkylated product was the only
compound isolated with no traces of the alternative N-7
alkylated product observed. Ammonolysis of 14 produced
the adenosine analogue 15, which under catalytic-transfer
hydrogenation conditions with palladium black and formic
acid afforded the expected target 6 in good yield.
The Mitsunobu reaction between 13 and 2-amino-6-
benzyloxypurine was more efficient than that with 6-chloro-
purine, and the guanosine precursor 16 was obtained in 65%
yield (Scheme 1). Cleavage of the benzyl groups by
The use of bicyclo[3.1.0]hexane templates has already
confirmed the existence of a conformational preference for a
number of enzymes such as adenosine deaminase (ADA),3b,7
HIV reverse transcriptase,8 DNA (cytosine-C5) methyl
transferase,9 and several subtypes of adenosine receptors.10
Furthermore, the conformationally locked antipodes of
thymidine, North-methanocarba-T (4) and South-methano-
carba-T (5), represent yet another example of a clear
conformational discrimination, with the final outcome being
that only the North antipode is endowed with activity
against herpes infections caused by HSV-1 and HSV-2
viruses.11
Since for the most part North analogues have proven to be
the more effective as antiviral agents, while the South
derivatives have shown weakly inhibitory activities,2e we
were encouraged to investigate the synthesis and antiviral
activity of 20-deoxycarbocyclic nucleosides 6–10, which
contain a similar pseudosugar template as that found in
neplanocin C. Unfortunately, due to the extremely facile
intramolecular ring opening of the epoxide in compounds
with pyrimidine bases (thymine and uracil), only the target
purines 6 and 7 were obtained.
Scheme 1. (a) m-CPBA, CH2Cl2, 08C, 61%; (b) 6-chloropurine, PPh3,
DEAD, THF, 08C!rt 2 h, 36%; (c) NH3/MeOH, 708C, 4 h, 47%; (d) Pd
black, MeOH/96% HCO2H (24:1),rt, 16 h, 64%; (e) 2-amino-6-benzyloxy-
purine, PPh3, DEAD, THF, 08C!rt 16 h, 65%; (f) H2, 42 psi, 5% Pd/C,
MeOH, rt, 4 h, 80%.
2. Results and discussion
The carbocyclic intermediate (1R,4S)-4-phenylmethoxy-3-
[(phenylmethoxy)methyl]cyclopent-2-en-1-ol (12), which

M. J. Comin et al. / Tetrahedron 59 (2003) 295–301
297
catalytic hydrogenation afforded the corresponding guano-
sine derivative 7 in 80% yield.
Both the deoxyadenosine (6) and deoxyguanosine (7)
analogues appear to be conformationally equivalent to the
bicyclo[3.1.0]hexane nucleosides as judged by the charac-
teristic doublet (J¼7.6 (6) and 7.7 Hz (7)) at d 4.77–5.07
observed in the 1H NMR spectra of these compounds.0This
doublet is typical of the pseudoanomeric proton (H-4 (for
compound 6) and H-20 (for compound 7)) in this class of
conformationally locked nucleosides. Additionally, these
compounds display the distinctive epoxide singlet (d 3.67
(for compound 6) and at d 3.57 (compound 7)), which is
unique to the 6-oxobicyclo[3.1.0]hexane system.
2.2. Synthesis of pyrimidine analogues
A similar attempt to synthesize pyrimidine derivatives 8–10
was complicated by this ring’s tendency to participate in an
intramolecular epoxide ring-opening reaction. Initially
N 3-benzoylthymine15 was coupled to 13 by a Mitsunobu
reaction employing a large excess of triphenylphosphine
and diethyl azodicarboxilate (Scheme 2). Under these
conditions, only the desired N-1 alkylated product 17 was
obtained. In contrast to other structurally related carbocyclic
rings where competition between O-alkylation and N-alkyl-
ation is markedly noticeable,2a,b the O-alkylated product
was not observed. Unfortunately, following the removal of
the N 3-benzoyl group with methanolic ammonia, even at
08C, an ensuing intramolecular nucleophilic opening of the
epoxide ring resulted in the exclusive formation of the
corresponding anhydride 18 (80%). Removal of the benzyl
protecting groups by catalytic hydrogenation afforded the
free anhydride 19 in 82% yield.
Under similar Mitsunobu conditions with N 3-benzoyl-
uracil,15 again only the N-alkylated product 20 was obtained
(Scheme 2). However, the epoxy group in this compound
appears to be slightly more stable than in the case of 17
since under the same deprotection conditions (methanolic
ammonia at 08C) some epoxide 21 could be obtained along
with compound 22. Confirmation that this intramolecular
reaction is temperature dependent was revealed after the
exclusive isolation of compound 21 when removal of the
Scheme 2. (a) N 3-benzoylthymine, PPh3, DEAD, THF, 2458C, 30 min!rt
16 h, 48%; (b) NH3/MeOH, 08C, 1 h, 80%; (c) H2, 42 psi, 5% Pd/C, MeOH,
rt, 4 h, 82%; (d) N 3-benzoyluracil, PPh3, DEAD, THF, 2458C, 30 min!rt
16 h, 52%; (e) NH3/MeOH, 2458C, 24 h, 84%.
signals of the nucleobase, the spectra of 17, 20, and 21
appeared nearly identical to those of closely related
compounds with an intact epoxide ring.2a,c,5,6 On the other
hand, the 1H NMR spectra of anhydrides 18 and 22 differed
substantially. For example, the pseudoanomeric proton of
anhydride 18 appeared as a triplet (d¼4.79, J¼7.3 Hz)
while the corresponding pseudoanomeric proton of epoxide
17 was a doublet of doublets (d¼4.96, J¼6.1, 2.7 Hz)
reflecting that important conformational changes had taken
place.
1
benzoyl group from 20 was performed at 2458C. The H
NMR singlet at d 3.43 indicated that the epoxide ring in 21
was intact. Unfortunately, on standing the compound
gradually converted to 22. The instability of these
compounds thwarted our hopes of obtaining the correspond-
ing pyrimidine analogues 8–10.
Ab initio energy calculations of optimized conformers for
compounds 8 (target thymidine analogue) and the corre-
sponding anhydride 19 with Gaussian 98W employing a
HF/6-31Gdp basis set16 indicated that 19 is 2.88 kcal/mol
more stable than 8. These results are in line with our
inability to isolate stable pyrimidine analogues containing a
6-oxobicyclo[3.1.0]hexane pseudosugar. The corresponding
pesudorotational parameters for 8 (P¼3508 (2E–3T2),
nmax¼17.78) and 19 (P¼24.68 (3E–3T4), nmax¼39.88)
revealed substantial conformational differences between
epoxides and anhydrides. These results are in agreement
2.3. Antiviral results
In contrast to North-methanocarba-T (4), anhydride 19 was
devoid of antiviral activity against HSV-1 and HSV-2. The
purine analogues 6 and 7 were also inactive against these
viruses. Surprisingly, 19 showed good antiviral activity
against EBV although with only a 40-fold selectivity index
(EC50¼1.2 mg/mL, SI.40). The guanosine derivative 7 was
much more potent than 19 against EBV and also less toxic
(EC50¼0.34 mg/mL, SI.150). This value was ca. 6-fold
1
with the experimental H NMR data where save for the

298
M. J. Comin et al. / Tetrahedron 59 (2003) 295–301
better than that obtained with the reference standard,
acyclovir (EC50¼2 mg/mL). Anti-EBV activity in Daudi
cells was determined by the viral capsid antigen (VCA)
Elisa assay.17
1H), 4.53 (s, 2H), 4.59 (d, J¼12.1 Hz, 1H), 7.30 (m, 10H);
13C NMR (CDCl3) d 33.6, 62.0, 65.5, 66.9, 69.6, 71.8, 73.6,
75.0, 127.7, 127.8, 127.8, 128.4, 128.4, 137.7, 138.0. Anal.
calcd for C20H22O4: C, 73.60; H, 6.79. Found: C, 73.30; H,
6.72.
4.1.2. (1S,2S,4S,5R)-[4-(6-Chloropurin-9-yl)-6-oxa-2-
(phenylmethoxy)bicyclo-[3.1.0]hexyl](phenylmethoxy)-
methane (14). Diethyl azodicarboxylate (DEAD, 1.04 mL,
5.94 mmol) was added dropwise to a 08C stirred solution of
triphenylphosphine (1.71 g, 6.53 mmol) in anhydrous THF
(12 mL). While still at 08C, the resulting solution was added
to a stirred suspension of 13 (0.925 g, 2.83 mmol) and
6-chloropurine (0.922 g, 5.94 mmol) in anhydrous THF
(10 mL). The resulting mixture was stirred at 08C for 30 min
and at room temperature for 2 h. The solvent was
evaporated and the product was purified by column
chromatography (silica gel) eluting with hexanes/EtOAc
(4:1) to give 1.10 g (36%) of 14 which was contaminated
with traces of reduced DEAD: 1H NMR (CDCl3) d 2.25 (m,
2H), 3.69 (s, 1H), 3.77 (d, J¼10.7 Hz, 1H), 4.20 (d,
J¼10.7 Hz, 1H), 4.63 (m, 4H), 4.76 (t, J¼7.9 Hz, 1H), 5.33
(d, J¼7.6 Hz, 1H), 7.36 (m, 10H), 8.26 (s, 1H), 8.74 (s, 1H);
13C NMR (CDCl3) d 34.7, 53.7, 59.3, 66.1, 67.0, 72.5, 73.6,
76.4, 127.6, 127.8, 128.2, 128.3, 131.8, 137.2, 137.5, 143.5,
151.1, 151.8. This compound was used directly in the next
reaction.
3. Conclusion
We have achieved the convergent syntheses of deoxy-
adenosine (6) and deoxyguanosine (7) analogues of natural
product neplanocin C via Mitsunobu coupling from a
common precursor 13. Conformationally, these compounds
are equivalent to known carbocyclic nucleosides built with
the bicyclo[3.1.0]hexane template. The epoxide in the
6-oxobicyclo[3.1.0]hexane ring was stable during the
synthesis of purine analogues 6 and 7. However,
the corresponding pyrimidine nucleosides are unstable and
subject to a facile intramolecular epoxide ring-opening
reaction leading to the formation of anhydrides. Only the
deoxyguanosine analogue 7 demonstrated important anti-
viral activity against EBV.
4. Experimental
4.1. General procedures
All moisture sensitive reactions were performed under a dry
atmosphere of argon, and all the glassware used in air and/or
moisture sensitive reactions was flame-dried. Unless other-
wise noted, all chemicals were commercially available and
used without further purification. Solvents were distilled
before use. THF was distilled from sodium/benzophenone
ketyl and CH2Cl2 was distilled from P2O5 and stored over
4.1.3. (1R,2S,4S,5S)-9-{6-Oxa-4-(phenylmethoxy)-5-
[(phenylmethoxy)methyl]-bicyclo[3.1.0]hex-2-yl}purine-
6-ylamine (15). Compound 14 (1.00 g, 2.38 mmol) was
treated with a saturated solution of methanolic ammonia
(15 mL) in a sealed tube at 708C for 4 h. The mixture was
cooled to 2708C, the tube was opened, and nitrogen was
bubbled to eliminate the ammonia. Analysis by TLC
revealed that in addition to product some polar compounds
were formed. The solvent was evaporated and the residue
was purified by column chromatography (silica gel) with
CHCl3/MeOH (97:3) as eluant to give 0.50 g (47%) of pure
15 as a white solid: mp 1728C; [a]2D3¼28.88 (c 0.4, CHCl3);
1H NMR (CD3OD) d 2.09 (m, 1H), 2.30 (dd, J¼13.4,
7.8 Hz, 1H), 3.67 (s, 1H), 3.76 (d, J¼11.0 Hz, 1H), 4.04 (d,
J¼11.0 Hz, 1H), 4.55 (m, 4H), 4.81 (t, J¼7.8 Hz, 1H), 5.17
(d, J¼7.6 Hz, 1H); 7.29 (m, 10H), 8.07 (s, 1H), 8.13 (s, 1H);
13C NMR (CD3OD) d 35.6, 54.8, 61.1, 67.9, 68.2, 73.5,
74.5, 78.5, 120.2, 128.7, 128.8, 128.8, 129.0, 129.3, 129.4,
139.2, 139.5, 140.8, 150.4, 153.7. Anal. calcd for
C25H25N5O3: C, 67.70; H, 5.68; N, 15.79. Found: C,
67.54; H, 5.72; N, 15.84.
˚
freshly activated 4 A molecular sieves. Anhydrous DMF
was used as supplied. Column chromatography was
performed on silica gel 60 (230–400 mesh) and analytical
TLC was performed on commercial 0.2 mm aluminum
coated silica gel plates (Kieselgel 60 F254) and visualized by
UV light (254 nm) or by immersion in ethanolic 5% H2SO4.
Melting points are uncorrected. Routine optical rotations, IR
and 1H and 13C NMR spectra were recorded using standard
methods. Low-resolution mass spectra were obtained at
70 eV (direct inlet). Elemental analyses were conducted by
Atlantic Microlab Inc., Norcross, Georgia.
4.1.1. (1R,2R,4S,5S)-6-Oxa-4-(phenylmethoxy)-5-[(phe-
nylmethoxy)methyl]-bicyclo[3.1.0]hexan-2-ol (13).
A
stirred solution of 12 (2.00 g, 6.5 mmol)3b in methylene
chloride (100 mL) was treated with a solution of 50%
m-chloroperbenzoic acid (2.20 g) in methylene chloride
(25 mL) at 08C. The mixture was allowed to reach room
temperature and stirred for a total of 3 h. The organic phase
was washed with a saturated solution of NaHCO3
(3£70 mL), water (3£70 mL) and dried (MgSO4). The
solvent was evaporated and the residue was purified by
column chromatography (silica gel) eluting with hexanes/
EtOAc (4:1) to give 1.30 g (61%) of pure 13 as a white
4.1.4. (1S,2S,4S,5R)-4-(6-Aminopurin-9-yl)-1-(hydroxy-
methyl)-6-oxabicyclo-[3.1.0]hexane-2-ol (6). A solution
of compound 15 (0.128 g, 0.29 mmol) in MeOH (24 mL)
and 96% formic acid (1 mL) was stirred at room
temperature overnight in the presence of palladium black
(0.25 g) under argon. The solvent was evaporated and the
residue was purified by column chromatography (silica gel)
eluting with CHCl3/MeOH (9:1) to afford 0.036 g (64%) of
pure 6 as a white solid: mp 215–2188C (d); [a]2D3¼233.08
1
solid: mp 101–1038C; [a]2D3¼26.058 (c 1.1, CHCl3); H
1
(c 1.0, MeOH); H NMR (DMSO-d6) d 1.91 (m, 1H), 2.17
NMR (CDCl3) d 1.41 (ddd, J¼12.5, 8.3, 4.1 Hz, 1H), 2.30
(ddd, J¼12.3, 7.5, 5.0 Hz, 1H), 3.48 (s, 1H), 3.56 (d,
J¼11.6 Hz, 1H), 3.99 (d, J¼11.6 Hz, 1H), 4.00 (distorted t,
J¼8.0 Hz, 1H), 4.03 (t, J¼8.0 Hz, 1H), 4.49 (d, J¼12.1 Hz,
(dd, J¼14.0, 8.0 Hz, 1H), 3.63 (d, J¼12.2 Hz, 1H), 3.67 (s,
1H), 4.04 (d, J¼12.2 Hz, 1H), 4.73 (t, J¼8.1 Hz, 1H), 5.07
(d, J¼7.6 Hz, 1H), 7.26 (s, 2H), 8.12 (s, 1H), 8.19 (s, 1H);

M. J. Comin et al. / Tetrahedron 59 (2003) 295–301
299
13C NMR (DMSO-d6) d 36.2, 52.2, 57.3, 59.3, 68.5, 69.0,
118.8, 138.7, 149.0, 152.3, 155.9. Anal. calcd for
C11H13N5O3: C, 50.12; H, 4.98; N, 26.60. Found: C,
50.24; H, 4.91; N 26.38.
eluant to give the desired N-alkylated product 17 (0.180 g,
1
48%) as a colorless oil: [a]2D3¼þ32.58 (c 1.3, CHCl3); H
NMR (500 MHz, CDCl3) d 1.69 (d, J¼0.9 Hz, 3H), 2.09 (m,
2H), 3.47 (s, 1H), 3.59 (d, J¼10.5 Hz, 1H), 4.14 (d,
J¼10.2 Hz, 1H), 4.49–4.63 (m, 5H), 4.96 (dd, J¼6.1,
2.7 Hz, 1H), 7.12 (d, J¼0.9 Hz, 1H), 7.24–7.37 (m, 10H),
7.48 (t, J¼7.9 Hz, 2H), 7.63 (t, J¼7.5 Hz, 1H), 7.89 (dd,
J¼8.3, 1.0 Hz, 2H); 13C NMR (CDCl3) d 12.2, 35.1, 55.6,
60.0, 67.0, 67.3, 72.5, 73.9, 76.2, 111.6, 127.7, 127.9, 127.9,
128.1, 128.5, 128.5, 129.1, 130.4, 131.6, 135.0, 137.1,
137.4, 149.7, 162.5, 168.8. Anal. calcd for C32H30O6N2: C,
71.36; H, 5.61; N, 5.20. Found C, 70.89; H, 5.74; N, 5.16.
4.1.5. (1R,2S,4S,5S)-6-Benzyloxy-9-(4-benzyloxy-5-ben-
zyloxymethyl-6-oxabicyclo-[3.1.0]-hex-2-yl)-9H-purine
(16). Diethyl azodicarboxylate (DEAD, 270 mL,
1.69 mmol) was added dropwise to a 08C stirred solution
of triphenylphosphine (0.444 g, 1.69 mmol) in anhydrous
THF (5 mL). While still at 08C, the resulting solution was
added to a stirred suspension of 13 (0.240 g, 0.73 mmol) and
2-amino-6-benzyloxypurine (0.248 g, 1.03 mmol) in anhy-
drous THF (5 mL). The resulting mixture was vigorously
stirred at 08C for 30 min and at room temperature overnight.
The solvent was evaporated and the residue was purified by
column chromatography (silica gel) using hexanes/EtOAc
(3:2) as eluant to give 0.26 g (65%) of the desired product
16 slightly contaminated with DEAD. An analytical sample
was obtained after further purification by column chroma-
tography using CH2Cl2/MeOH (99:1) as eluant to afford
pure 16 as a colorless oil: [a]2D3¼þ45.18 (c 0.91, CHCl3); 1H
NMR (CDCl3) d 2.12 (m, 2H), 3.54 (s, 1H), 3.65 (d,
J¼11.0 Hz, 1H), 4.08 (d, J¼10.6 Hz, 1H), 4.52 (d,
J¼11.9 Hz, 1H), 4.55 (m, 2H), 4.60 (d, J¼11.9 Hz, 1H),
4.67 (t, J¼8.0 Hz, 1H), 5.01 (t, J¼4.4 Hz, 1H), 5.55 (m,
2H), 7.23–7.36 (m, 13H), 7.49 (d, J¼7.2 Hz, 2H), 7.61 (s,
1H); 13C NMR (CDCl3) d 34.6, 52.7, 60.1, 66.6, 66.9, 68.0,
72.9, 73.7, 76.6, 127.8, 127.8, 127.9, 128.2, 128.3, 128.4,
128.5, 136.4, 137.5, 137.9, 153.7, 159.1, 161.1. Anal. calcd
for C32H31O4N5·0.25H2O: C, 69.36; H, 5.73; N, 12.64.
Found: C, 69.40; H 5.96; N, 12.21.
4.1.8. (7S,8S,5aS,8aS)-8-Hydroxy-3-methyl-7-(phenyl-
methoxy)-8-[(phenylmethoxy)methyl]-5,6,7,8,5a,8a-
hexahydro-5aH,8aH-cyclopenta[1,2-d]pyrimidino[2,1-
b]1,3-oxazolidin-2-one (18). Compound 17 (0.15 g,
0.28 mmol) was treated with methanolic ammonia (5 mL,
saturated at 2788C) and stirred in a pressure vessel at 08C
for 1 h. The solvent was evaporated and the residue was
purified by column chromatography (silica gel) using
EtOAc/MeOH (95:5) as eluant to give 0.097 g (80%) of
pure 18 as a white solid: mp 166–1678C; [a]2D3¼þ37.08 (c
1
1.8, CHCl3); H NMR (CDCl3) d 1.95 (d, J¼1.1 Hz, 3H),
2.22 (dd, J¼13.3, 6.9 Hz, 1H), 2.31 (ddd, J¼14.1, 10.0,
7.1 Hz, 1H), 3.06 (br s, 1H), 3.72 (m, 2H), 3.93 (dd, J¼10.0,
6.8 Hz, 1H), 4.55 (m, 2H), 4.58 (m, 2H), 4.79 (t, J¼7.3 Hz,
1H), 5.02 (d, J¼7.7 Hz, 1H), 7.00 (d, J¼1.1 Hz, 1H),
7.2127.36 (m, 10H); 13C NMR (CDCl3) d 14.1, 34.9, 59.1,
70.5, 73.2, 74.0, 78.7, 80.5, 85.2, 119.6, 127.8, 127.9, 127.9,
128.3, 128.5, 128.6, 130.3, 137.0, 137.4, 159.8, 172.1. Anal.
calcd for C25H26O5N2: C, 69.11; H, 6.03; N, 6.45. Found: C,
68.98; H, 6.01; N, 6.54.
4.1.6. (1R,2S,4S,5S)-2-Amino-9-[4-hydroxy-5-(hydroxy-
methyl)-6-oxabicyclo-[3.1.0]hex-2-yl]hydropurin-6-one
(7). A solution of 16 (0.069 g, 0.13 mmol) in methanol
(5 mL) was in a hydrogenated at 42 psi in the presence of
5% Pd/C (5 mg). After 4 h, the mixture was filtered off and
the solvent evaporated. The residue was purified by reverse
phase column chromatography using water as eluant to give
0.030 g of pure 7 (80%) as a white solid: mp .2708C;
[a]2D3¼236.48 (c 0.5, MeOH); 1H NMR (DMSO-d6) d 1.78
(m, 1H), 2.04 (dd, J¼14.1, 8.0 Hz, 1H), 3.55 (d, J¼12.3 Hz,
1H), 3.57 (s, 1H), 3.99 (d, J¼12.1 Hz, 1H), 4.59 (t,
J¼8.0 Hz, 1H), 4.77 (d, J¼7.7 Hz, 1H), 6.49 (s, 2H), 7.68
(s, 1H); 13C NMR (DMSO-d6) d 35.3, 53.4, 57.0, 58.9, 68.1,
69.3, 110.0, 135.4, 149.8, 154.1, 155.0. Anal. calcd for
C11H13O4N5·1.25H2O: C, 43.78; H, 5.18; N, 23.21. Found:
C, 44.01; H, 5.16; N 23.02.
4.1.9. (7S,8S,5aS,8aS)-7,8-Dihydroxy-8-(hydroxy-
methyl)-3-methyl-5,6,7,8,5a,8a-hexahydro-5aH,8aH-
cyclopenta[1,2-d]pyrimidino[2,1-b]1,3-oxazolidin-one
(19). A solution of 18 (0.083 g; 0.19 mmol) in methanol
(5 mL) was hydrogenated at 42 psi in the presence of 5%
Pd/C (5 mg) for 4 h. The mixture was filtered and the
solvent was evaporated. The residue was purified by column
chromatography (silica gel) with CH2Cl2/MeOH (85:15) as
eluant to give pure 19 (0.040 g, 82%) as a white solid: mp
2308C (d); [a]2D3¼þ82.98 (c 0.69, MeOH); 1H NMR (D2O) d
1.87 (d, J¼0.9 Hz, 3H), 2.23 (ddd, J¼14.1, 11.2, 6.8 Hz,
1H), 2.33 (dd, J¼14.1, 7.1 Hz, 1H), 3.70 (d, J¼12.3 Hz,
1H), 3.81 (d, J¼12.5 Hz, 1H), 4.03 (dd, J¼11.4, 7.1 Hz,
1H), 5.04 (distorted t, J¼7.4 Hz, 1H), 5.12 (d, J¼8.0 Hz,
1H), 7.64 (d, J¼0.9 Hz, 1H); 13C NMR (CD3OD) d 13.9,
37.7, 60.8, 63.7, 72.3, 81.9, 87.6, 119.3, 135.1, 161.8, 175.4.
Anal. calcd for C11H14O5N2·H2O: C, 48.53; H, 5.92; N,
10.29. Found: C, 48.17; H, 5.80; N, 9.99.
4.1.7. (1R,2S,4S,5S)-5-Methyl-1-{6-oxa-4-(phenyl-
methoxy)-5-[(phenylmethoxy)-methyl]bicyclo[3.1.0]hex-
2-yl}-3-(phenylcarbonyl)-1,3-dihydropyrimidine-2,4-
dione (17). A stirred solution of triphenylphosphine
(0.569 g, 2.17 mmol) in anhydrous THF (8 mL) was treated
with diethyl azodicarboxylate (0.34 mL, 2.17 mmol) at 08C
for 20 min. After cooling to 2458C, a solution of
N 3-benzoylthymine (0.40 g, 1.74 mmol) and 13 (0.283 g,
0.87 mmol) in THF (8 mL) was added via canula over a
period of 10 min. The reaction mixture was stirred at 2458C
for 30 min and then at room temperature overnight. After
volatiles were removed the residue was purified by column
chromatography (silica gel) using hexanes/EtOAc (7:3) as
4.1.10. (1R,2S,4S,5S)-1-{6-Oxa-4-(phenylmethoxy)-5-
[(phenylmethoxy)methyl]-bicyclo-[3.1.0]hex-2-yl}-3-
(phenylcarbonyl)-1,3-dihydropyrimidine-2,4-dione (20).
According to the procedure used for 17, triphenylphosphine
(0.444 g, 1.69 mmol), DEAD (0.27 mL, 1.69 mmol),
N 3-benzoyluracil (0.294 g, 1.36 mmol) and 13 (0.221 g,
0.68 mmol), were reacted to give a crude product which was
purified by column chromatography (silica gel) using
hexanes/EtOAc (6:4) as eluant to give compound 20 still
contaminated with reduced DEAD and triphenylphosphine

300
M. J. Comin et al. / Tetrahedron 59 (2003) 295–301
Nucleic Acid Structure. Springer: New York, 1984; pp
51–104.
oxide. The product was repurified by column chromato-
graphy (silica gel) using a mixture of CH2Cl2/MeOH (99:1)
to afford pure 20 (0.186 g, 52%) as a colorless oil:
2. (a) Rodriguez, J. B.; Marquez, V. E.; Nicklaus, M. C.; Barchi,
J. J. Tetrahedron Lett. 1993, 34, 6233–6236. (b) Altman,
K.-H.; Kesselring, R.; Francotte, E.; Rihs, G. Tetrahedron
Lett. 1994, 35, 2331–6234. (c) Rodriguez, J. B.; Marquez,
V. E.; Nicklaus, M. C.; Mitsuya, H.; Barchi, J. J. J. Med.
Chem. 1994, 37, 3389–3399. (d) Altman, K.-H.;
Imwinkelried, R.; Kesselring, R.; Rihs, G. Tetrahedron Lett.
1995, 35, 7625–7628. (e) Ezzitouni, A.; Marquez, V. E.
J. Chem. Soc., Perkin Trans. 1 1997, 1073–1078. (f) Shin,
K. J.; Moon, H. R.; George, C.; Marquez, V. E. J. Org. Chem.
2000, 65, 2172–2178. (g) Moon, H. R.; Ford, Jr. H.; Marquez,
V. E. Org. Lett. 2000, 2, 3793–3796.
1
[a]2D3¼þ25.38 (c 0.97, CHCl3); H NMR (CDCl3) d 2.10
(m, 2H), 3.48 (s, 1H), 3.59 (d, J¼10.7 Hz, 1H), 4.11 (d,
J¼10.5 Hz, 1H), 4.45–4.60 (m, 5H), 4.95 (s, 1H), 5.66 (d,
J¼8.0 Hz, 1H), 7.24 (d, J¼8.0 Hz, 1H), 7.29–7.35 (m,
10H), 7.48 (t, J¼7.7 Hz, 2H), 7.65 (t, J¼7.5 Hz, 1H), 7.91
(d, J¼8.2 Hz, 2H); 13C NMR (CDCl3) d 35.1, 56.1, 59.8,
66.8, 67.5, 72.5, 73.8, 76.3, 102.9, 127.7, 127.8, 128.0,
128.1, 128.5, 128.5, 129.2, 130.5, 131.4, 135.2, 137.5,
137.7, 141.4, 149.6, 161.7, 168.5. Anal. calcd for
(C31H28O6N2): C, 70.98; H, 5.38; N, 5.34. Found: C,
70.57; H, 5.45; N, 5.10.
3. (a) Siddiqui, M. A.; Ford, Jr. H.; George, C.; Marquez, V. E.
Nucleosides Nucleotides 1996, 15, 235–250. (b) Marquez,
V. E.; Russ, P.; Alonso, R.; Siddiqui, M. A.; Hernandez, S.;
George, C.; Nicklaus, M. C.; Dai, F.; Ford, Jr. H. Helv. Chim.
Acta 1999, 82, 2119–2129.
4.1.11. (1R,2S,4S,5S)-1-{6-Oxa-4-(phenylmethoxy)-5-
[(phenylmethoxy)methyl]-bicyclo-[3.1.0]hex-2yl}-1,3-
dihydropyrimidine-2,4-dione (21) and (7S,8S,5aS,8aS)-
8-hydroxy-7-(phenylmethoxy)-8-[(phenylmethoxy)-
methyl]-5,6,7,8,5a,8a-hexahydro-5aH,8aH-cyclo-
penta[1,2-d]pyrimidino[2,1-b]1,3-oxazolidin-2-one (22).
Compound 20 (0.018 g, 0.03 mmol) was treated with
methanolic ammonia (1.0 mL, saturated at 2788C) and
the reaction mixture was stirred in a sealed tube at 2458C
for 24 h. The solvent was evaporated and the residue was
purified by preparative TLC (silica gel 60, Merck) using
EtOAc/hexanes (4:1) as eluant to give 0.012 g of 21 (84%)
and 0.002 g (13%) of 22 as colorless oils.
4. (a) Yaginuma, S.; Muto, N.; Tsujino, M.; Sudate, Y.; Hayashi,
M.; Otani, M. J. Antibiot. 1981, 34, 359–366. (b) Isono, K.
J. Antibiot. 1988, 41, 1711–1739. (c) De Clercq, E.
Antimicrob. Agents Chemother. 1985, 28, 84–89. (d) De
Clercq, E.; Bernaerts, R.; Bregstrom, D. E.; Robins, M. J.;
Montgomery, J. A.; Holy, A. Antimicrob. Agents Chemother.
1986, 29, 482–487. (e) Kinoshita, K.; Yaginuma, S.; Hayashi,
M.; Nakatsu, K. Nucleosides Nucleotides 1985, 4, 661–668.
5. Comin, M. J.; Rodriguez, J. B. Tetrahedron 2000, 56,
4639–4649.
Compound 21. 1H NMR (CDCl3) d 2.04–2.08 (m, 2H), 3.43
(s, 1H), 3.59 (d, J¼10.5 Hz, 1H), 4.11 (d, J¼10.5 Hz, 1H),
4.45 (t, J¼7.8 Hz, 1H), 4.50 (d, J¼11.8 Hz, 1H), 4.52 (d,
J¼11.5 Hz, 1H), 4.56 (d, J¼11.5 Hz, 1H), 4.59 (d,
J¼11.8 Hz, 1H), 4.92 (d, J¼6.8 Hz, 1H), 5.56 (dd, J¼8.0,
2.1 Hz, 1H), 7.19 (d, J¼8.2 Hz, 1H), 7.26–7.35 (m, 10H),
8.26 (br s, 1H); 13C NMR (CDCl3) d 35.0, 55.8, 59.9, 66.8,
67.5, 72.5, 73.8, 76.3, 102.8, 127.7, 128.0, 128.0, 128.1,
128.5, 128.5, 137.5, 137.8, 141.7, 150.3, 162.4. On standing
this compound gradually converts to 22.
6. Comin, M. J.; Pujol, C. A.; Damonte, E. B.; Rodriguez, J. B.
Nucleosides Nucleotides 1999, 18, 2219–2231.
7. Marquez, V. E.; Russ, P.; Alonso, R.; Siddiqui, M. A.; Shin,
K. J.; George, C.; Nicklaus, M. C.; Dai, F.; Ford, Jr. H.
Nucleosides Nucleotides 1999, 18, 521–530.
8. (a) Marquez, V. E.; Ezzitouni, A.; Russ, P.; Siddiqui, M. A.;
Ford, Jr. H.; Feldman, R. J.; Mitsuya, H.; George, C.; Barchi,
Jr. J. J. Nucleosides Nucleotides 1998, 17, 1881–1884.
(b) Marquez, V. E.; Ezzitouni, A.; Russ, P.; Siddiqui, M. A.;
Ford, Jr. H.; Feldman, R. J.; Mitsuya, H.; George, C.; Barchi,
Jr. J. J. J. Am. Chem. Soc. 1998, 120, 2780–2789.
1
Compound 22. [a]2D3¼þ28.68 (c 0.84, CHCl3); H NMR
9. (a) Wang, P.; Nicklaus, M. C.; Marquez, V. E.; Brank, A. S.;
Christman, J. K.; Banavali, N. K.; MacKerell, Jr. A. D. J. Am.
Chem. Soc. 2000, 122, 12422–12434. (b) Marquez, V. E.;
Wang, P.; Nicklaus, M. C.; Maier, M.; Manoharan, M.;
Christman, J. K.; Banavali, N. K.; MacKerell, Jr. A. D.
Nucleosides, Nucleotides Nucl. Acids 2001, 20, 451–459.
10. (a) Nandanan, E.; Jang, S.-Y.; Moro, S.; Kim, H.; Siddiqui,
M. A.; Russ, P.; Marquez, V. E.; Busson, R.; Herdewijn, P.;
Harden, T. K.; Boyer, J. L.; Jacobson, K. A. J. Med. Chem.
2000, 43, 829–842. (b) Jacobson, K. A.; Ji, X.; Li, A.;
Melman, N.; Siddiqui, M. A.; Shin, K. J.; Marquez, V. E.;
Ravi, R. G. J. Med. Chem. 2000, 43, 2196–2203. (c) Lee, K.;
Ravi, R.; Ji, X. D.; Marquez, V. E.; Jacobson, K. A. Bioorg.
Med. Chem. Lett. 2001, 11, 1333–1337. (d) Ravi, G.; Lee, K.;
Ji, X. D.; Kim, H. S.; Soltysiak, K. A.; Marquez, V. E.;
Jacobson, K. A. Bioorg. Med. Chem. Lett. 2001, 11,
2295–2300. (e) Kim, H. S.; Ravi, R. G.; Marquez, V. E.;
Maddileti, S.; Wihlborg, A.-K.; Erlinge, D.; Malsmjo, M.;
Boyer, J. L.; Harden, T. K.; Jacobson, K. A. J. Med. Chem.
2002, 45, 208–218.
(CDCl3) d 2.32 (m, 2H), 3.64 (d, J¼9.9 Hz, 1H), 3.73 (d,
J¼10.2 Hz, 1H), 3.92 (t, J¼8.2 Hz, 1H), 4.55 (m, 4H), 4.98
(m, 1H), 5.10 (d, J¼8.0 Hz, 1H), 5.99 (d, J¼7.3 Hz, 1H),
7.27 (m, 10H), 7.68 (d, J¼7.3 Hz, 1H); 13C NMR (CDCl3) d
34.9, 61.0, 72.6, 73.7, 74.8, 80.1, 81.8, 88.2, 109.6, 128.7,
128.9, 129.0, 129.1, 129.4, 139.3, 158.2, 162.4. Anal. calcd
for (C25H26O6N2O5·1.5H2O): C, 65.06; H, 6.33; N, 6.07.
Found: C, 64.93; H, 6.63; N, 6.07.
Acknowledgements
This work was supported in part by grants from the National
Research Council of Argentina CONICET (PIP 635/98),
and the Universidad de Buenos Aires (X-080) to J. B. R.
References
11. (a) Marquez, V. E.; Siddiqui, M. A.; Ezzitouni, A.; Russ, P.;
Wang, J.; Wagner, R. W.; Matteucci, M. J. Med. Chem. 1996,
39, 3739–3747. (b) Zalah, L.; Huleihel, M.; Manor, E.;
1. For a review of these concepts, see Saenger, W. Principles of

M. J. Comin et al. / Tetrahedron 59 (2003) 295–301
301
Konson, A.; Ford, Jr. H.; Marquez, V. E.; Johns, D. G.;
Agbaria, R. Antiviral Res. 2002, 55, 63–75.
Montgomery, J. A. Jr.; Stratmann, R. E.; Burant, J. C.;
Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.;
Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.;
Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford,
S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.;
Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7;
Gaussian, Inc.: Pittsburgh PA, 1998.
12. (a) Biggadike, K.; Borthwick, A. D.; Exall, A. M.; Kirk, B. E.;
Roberts, S. M.; Youds, P.; Slawin, A. M. Z.; Williams,
J. Chem. Soc., Chem. Commun. 1987, 255–256. (b) Biggadike,
K.; Borthwick, A. D.; Exall, A. M.; Kirk, B. E.; Roberts, S. M.;
Youds, P.; Slawin, A. M. Z.; Williams, D. J. J. Chem. Soc.,
Perkin Trans. 1 1988, 549–554. (c) Ezzitouni, A.; Russ, P.;
Marquez, V. E. J. Org. Chem. 1997, 62, 4870–4873.
13. Hembest, H. B.; Wilson, R. A. J. Chem. Soc. 1957,
1958–1965.
14. (a) Mitsunobu, O. Synthesis 1981, 1–28. (b) Jenny, T. F.;
Previsani, N.; Brenner, S. A. Tetrahedron Lett. 1991, 32,
7029–7032. (c) Jenny, T. F.; Horlacher, J.; Previsani, N.;
Brenner, S. A. Helv. Chim. Acta 1992, 75, 1944–1954.
15. Csuk, R.; Eversmann, L. Tetrahedron 1998, 54, 6445–6456.
16. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.;
17. Antiviral activity was obtained through Dr Christopher K.-H.
Tseng, NIAID, NIH. These assays were performed according
to the standard protocol developed in the laboratory of Dr Earl
R. Kern, University of Alabama.