Design and synthesis of dually branched 5′-norcarbocyclic adenosine phosphonodiester analogue as a new anti-HIV prodrug

Article abstract of DOI:10.1080/15257770.2010.509645

A novel 3′,4′-dimethyl-5′-norcarbocyclic adenosine phosphonic acid was prepared using acyclic stereoselective route from 4-hydroxybutan-2-one (4). To improve the cellular permeability and enhance the anti-HIV activity of this phosphonic acid, a (bis)SATE

Full text of DOI:10.1080/15257770.2010.509645

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Design and Synthesis of Dually
Branched 5′-Norcarbocyclic Adenosine
Phosphonodiester Analogue as a New
Anti-HIV Prodrug
Chang Hyun Oh a , Lian Jin Liu b & Joon Hee Hong b
a Medicinal Chemistry Research Center , Korea Institute of Science
and Technology , Seoul, Republic of Korea
b BK21–Project Team, College of Pharmacy , Chosun University ,
Kwangju, Republic of Korea
Published online: 04 Oct 2010.
To cite this article: Chang Hyun Oh , Lian Jin Liu & Joon Hee Hong (2010) Design and Synthesis of
Dually Branched 5′-Norcarbocyclic Adenosine Phosphonodiester Analogue as a New Anti-HIV Prodrug,
Nucleosides, Nucleotides and Nucleic Acids, 29:10, 721-733, DOI: 10.1080/15257770.2010.509645
To link to this article: http://dx.doi.org/10.1080/15257770.2010.509645
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Nucleosides, Nucleotides and Nucleic Acids, 29:721–733, 2010
Copyright ꢀC Taylor and Francis Group, LLC
ISSN: 1525-7770 print / 1532-2335 online
DOI: 10.1080/15257770.2010.509645
DESIGN AND SYNTHESIS OF DUALLY BRANCHED
5ꢀ-NORCARBOCYCLIC ADENOSINE PHOSPHONODIESTER
ANALOGUE AS A NEW ANTI-HIV PRODRUG
Chang Hyun Oh,1 Lian Jin Liu,2 and Joon Hee Hong2
1Medicinal Chemistry Research Center, Korea Institute of Science and Technology, Seoul,
Republic of Korea
2BK21–Project Team, College of Pharmacy, Chosun University, Kwangju, Republic of Korea
A novel 3ꢁ,4ꢁ-dimethyl-5ꢁ-norcarbocyclic adenosine phosphonic acid was prepared using acyclic
2
stereoselective route from 4-hydroxybutan-2-one (4). To improve the cellular permeability and en-
hance the anti-HIV activity of this phosphonic acid, a (bis)SATE phosphonodiester nucleoside pro-
drug (20) was prepared and its chemical stability was evaluated. The newly synthesized bis(SATE)
analogue (20) and its parent nucleoside phosphonic acid (18) were assayed for anti-HIV activity
using an in vitro assay system in a CEM cell line.
Keywords Anti-HIV agent; bis(SATE) derivative; nucleoside adenine phosphonic acid
INTRODUCTION
Emerging drug-resistant viral strains and drug toxicity are major prob-
lems in antiviral chemotherapy,[1] which has lead to research for structurally
modified nucleosides. Although the pharmacophore of nucleoside antiviral
activity is not completely defined, 5ꢁ-nornucleoside phosphonic acid ana-
logues such as d4AP (1),[2] 2ꢁ-Fd4AP (2),[3] and 4ꢁ-ethynyl-cpAP (3)[4] as
potential anti-HIV agents have encouraged the search for novel nucleosides
in this class of compounds (Figure 1).
A nucleotide is essentially a nucleoside monophosphate analogue. How-
ever, the phosphonate has certain advantages over its phosphate counterpart
as it is metabolically stable due to the fact that its phosphorus-carbon bond is
not susceptible to hydrolytic cleavage.[5] Moreover, a nucleotide can skip the
requisite first phosphorylation, which is a crucial step for the activation of
nucleosides. Although triphosphates of the nucleoside analogues exhibit ex-
cellent antiviral potency, only a few nucleoside derivatives exhibit biological
Received 18 May 2010; accepted 15 July 2010.
Address correspondence to Joon Hee Hong, College of Pharmacy, Chosun University, Kwangju
501-759, Republic of Korea. E-mail: hongjh@chosun.ac.kr
721

722
C. H. Oh et al.
NH2
N
NH2
N
N
N
N
N
O
N
O
P
O
N
O
HO
P
O
OH
O
HO
OH
F
GS-9148 (2)
d4AP (1)
NH2
N
N
N
O
N
P
O
HO
OH
4'-Ethynyl-cpAP ( 3)
FIGURE 1 Structures of 5ꢁ-nornucleoside analogues as potent anti-HIV agents.
activity in cell culture assays. This might be due to poor cellular penetration
coupled with insufficient metabolism of these nucleoside derivatives to 5ꢁ-
triphosphates. The poor oral bioavailability of these nucleoside analogues is
due to the negative charges of the phosphonate present in nucleoside phos-
phonic acid at physiological pH. Therefore, temporary masking of these
charges with neutral groups to form more lipophilic derivatives capable of
crossing the gastrointestinal wall and reverting back to the parent nucleoside
phosphonic acid was attempted.[6]
Stimulated by the finding that 5ꢁ-nornucleosides and their phosphonic
acid derivatives exhibit excellent antiviral activities, we sought to synthe-
size a novel class of nucleosides comprising 3ꢁ,4ꢁ-dimethyl carbocyclic 5ꢁ-
norcarbocyclic phosphonic acid and its bis(SATE) phosphonodiester deriva-
tive. Here we report on the synthesis, antiviral activity, and chemical stability
of the bis(SATE) prodrug.
The target compounds were prepared from commercially available start-
ing material (4) as shown in Scheme 1. The alcohol functional group of (4)
was temporarily protected with t-butyldimethylchlorosilane (TBDMSCl) to
give the ketone derivative (5), which was subject to carbonyl addition using
isopropenylmagnesium bromide [CH2 C(CH3)MgBr] to yield the tertiary
alcohol derivative (6). In order to differentiate between the two hydroxyl
groups, the protective silicon group on the primary hydroxyl was replaced
with a benzoyl group by sequential desilylation and selective benzoylation
to provide (8). The tertiary hydroxyl group of (8) was successfully silylated

3ꢁ,4ꢁ-Dimethyl-5ꢁ-norcarbocyclic Adenosine Phosphodiester
723
HO
O
O
ii
i
H3C
H3C
OTBDMS
H3C
OTBDMS
OH
H3C
5
4
6
iii
TBDMSO
HO
HO
v
vi
H3C
H3C
H3C
H3C
H3C
OBz
OBz
OH
H3C
9
8
7
vi
TBDMSO
TBDMSO
TBDMSO
OH
viii
vii
H3C
H3C
H3C
H3C
H3C
H3C
O
OH
11
12
10
SCHEME 1 Synthesis of divinyl intermediate 12. Reagents: i) TBDMSCl, imidazole, CH2CI2; ii) iso-
propenylMgBr, THF; iii) TBAF, THF; iv) BzCI, DMAP, pyridine; v) TBDMSOTf, DMAP, TEA, CH2Cl2;
vi) NH3/MeOH; vii) (COCI)2, DMSO, TEA, CH2CI2; viii) vinylMgBr, THF.
using trimethylsilyl trifluoromethanesulfonate (TMSOTf) to give the fully
protected compound (9). The benzoyl protection group of the primary hy-
droxyl was removed under methanolic ammonia conditions to provide the
alcohol derivative (10), which was oxidized to the aldehyde (11) using Swern
oxidation conditions (DMSO, oxalyl chloride, TEA). The aldehyde (11) was
subjected to nucleophilic Grignard conditions by vinylmagnesium bromide
to yield divinyl (12).
Without separation of the diastereomeric mixture, divinyl (12) was sub-
jected to ring-closing metathesis (RCM) conditions using second-generation
Grubbs catalyst[78] to provide cyclopentenol (13a) (36%) and (13b) (35%),
which were readily separated by silica gel column chromatography. The rela-
tive stereochemical assignments of the two isomers were made readily based
on NOE comparisons. Upon irradiation of C1-H, weak NOE patterns were
observed at the proximal hydrogens of compound (13b) [C4-CH3 (0.09%)]
versus those of compound (13a) [C4-CH3 (0.21%)] (Figure 2).
To synthesize the desired carbocyclic nucleoside analogues, the pro-
tected cyclopentenol (13b) was treated with 6-chloropurine in the presence
of diisopropyl azodicarboxylate (DIAD) and PPh3 to give (14) with the cor-
rect configuration through a chirality inversion.[9] The oxygen at C4 of (14)
was phosphonated after the silyl protection was removed. Treatment of the

724
C. H. Oh et al.
OH
H
(0.09%)
PO
PO
H
H3C
H3C
OH
H3C
H3C
(0.21%)
13a
13b
FIGURE 2 NOE difference between the hydrogens of 13a and 13b.
nucleoside (15) with diisopropylphosphonomethyl bromide[10] yielded the
desired compound (16) (Scheme 2). The chlorine group of (16) was then
transformed to amine with methanolic ammonia in a steel bomb at 80◦C to
give the adenine phosphonate derivative (17). The nucleoside phosphonate
mimics the overall shape and geometry of a nucleoside monophosphate.
Hydrolysis of (17) by treatment with bromotrimethylsilane afforded the ade-
nine phosphonic acid (18).[11] To synthesize the thioester-protected ana-
logue, compound (18) was reacted with thioester (19)[12] in the presence of
1-(2-mesitylenesulfonyl)-3-nitro-1H-1,2,4-triazole (MSNT)[13] to provide the
bis(SATE) derivative as a target compound (20) (Scheme 3).
The newly synthesized bis(SATE) analogue (20) and its parent nucleo-
side phosphonic acid (18) were assayed for anti-HIV activity using an in vitro
Cl
N
TBDMSO
X
N
ii
i
N
TBDMSO
H3C
12
N
Y
H3C
H3C
13a: X = OH, Y = H
13b: X = H, Y = OH
14
iii
H3C
Cl
N
N
N
Cl
N
O
N
N
iv
HO
i-PrO P
i-PrO
N
N
O
H3C
H3C
15
H3C
H3C
16
SCHEME 2 Synthesis of carbocyclic adenine phosphonate 16. Reagents; i) Grubbs (II), benzene; ii)
6-chloropurine, DIAD, THF; iii) TBAF, THF; iv) (i-PrO)2POCH2Br, LiO-t-Bu, THF.

3ꢀ,4ꢀ-Dimethyl-5ꢀ-norcarbocyclic Adenosine Phosphodiester
725
NH2
N
R
O
N
O
N
N
HO P
HO O
ii
i-PrO P
N
N
N
N
i-PrO O
H3C
H3C
H3C
H3C
18
iii
16: R = Cl
17: R = NH2
i
O
OH
S
19
NH2
N
O
O
N
N
S
O P
O
N
O O
S
H3C
H3C
20
SCHEME 3 Synthesis of target bis(SATE) prodrug of adenine analogue 20. Reagents: i) NH3/MeOH,
steel bomb, 50◦C; ii) TMSBr, CH3CN; iii) thioester, 19, 1-(2-mesitylenesulfonyl)-3-nitro-1H-1,2,4-triazole,
pyridine.
assay system in a CEM cell line that is suitable for monitoring the anti-HIV
activities of compounds (Table 1).[14]
As shown in Table 1, nucleoside prodrug (20) exhibited an increased
toxicity-dependent anti-HIV activity over its parent nucleoside phosphonic
acid (18).
In conclusion, the anti-HIV activity of the bis(t-Bu-SATE) prodrug was
4-fold higher than that of the parent nucleoside phosphonic acid. The syn-
thesis of other nucleoside analogues (U,T,C), together with chemical and
enzymatic stability data will be reported elsewhere.
TABLE 1 Anti-HIV activity of synthesized compounds
Compound No.
Anti-HIV EC50 (µM)
Cytotoxicity CC50 (µM)
18
20
ABC
82.3
21.6
0.17
>100
81.5
>100
ABC: abacavir
EC50: concentration (µM) required to inhibit the replication of HIV-1 by 50%.
CC50: concentration (µM) required to reduce the viability of unaffected cells by 50%.

726
C. H. Oh et al.
EXPERIMENTAL
Melting points were determined on a Mel-temp II laboratory device and
are uncorrected. NMR spectra were recorded on a JEOL 300 Fourier trans-
form spectrometer (JEOL, Tokyo, Japan); chemical shifts are reported in
parts per million (δ) and signals are reported as s (singlet), d (doublet), t
(triplet), q (quartet), m (multiplet), and dd (doublet of doublets). Ultra-
violet (UV) spectra were obtained on a Beckman DU-7 spectrophotometer
(Beckman, South Pasadena, CA, USA). Mass spectrometry (MS) spectra were
collected in electrospray ionization (ESI) mode. The elemental analyses were
performed using a Perkin-Elmer 2400 analyzer (Perkin-Elmer, Norwalk, CT,
USA). Thin layer chromatography (TLC) was performed on Uniplates (silica
gel) purchased from Analtech Co. (7558, Newark, DE, USA). All reactions
were carried out under an atmosphere of nitrogen unless specified. Dry
dichloromethane, benzene and pyridine were obtained by distillation from
CaH2. Dry THF was obtained by distillation from Na and benzophenone
immediately prior to use.
4-(t-Butyldimethylsilanyloxy) butan-2-one (5)
To a solution of 4-hydroxy-butan-2-one 4 (10.0 g, 113.5 mmol) and im-
idazole (11.59 g, 170.24 mmol) in CH2Cl2 (250 mL), TBDMSCl (18.82 g,
124.85 mmol) was added slowly at 0◦C and stirred overnight at room temer-
pature. The reaction solvent was evaporated under reduced pressure. The
residue was poured into water (200 mL) and extracted with ethyl acetate
(200 mL) two times. The combined organic layer was dried over anhydrous
MgSO4, filtered, and concentrated under reduced pressure. The residue was
purified by silica gel column chromatography (EtOAc/hexane, 1:25) to give
compound 5 (21.36 g, 93%) as a colorless oil: 1H NMR (CDCl3, 300 MHz) δ
3.91 (t, J = 7.0 Hz, 2H), 2.72 (t, J = 7.0 Hz, 2H), 2.17 (s, 3H), 0.81 (s, 9H),
0.01 (s, 6H);13C NMR (CDCl3) δ 208.6, 59.3, 46.2, 25.5, 18.7, −5.5.
( )-5-(t-Butyldimethylsilanyloxy)-2,3-dimethyl-pent-1-en-3-ol (6)
To a solution of 5 (1.11 g, 5.5 mmol) in dry THF (15 mL) was slowly added
isopropenylMgBr (11.10 mL, 0.5 M solution in THF) at −20◦C and stirred
for 4 hours at the same temperature. Saturated NH4Cl solution (10 mL) was
added to the mixture, which was slowly warmed to room temperature. The
mixture was further diluted with water (50 mL) and extracted with EtOAc
(50 mL) two times. The combined organic layer was washed with brine, dried
over MgSO4, filtered, and evaporated. The residue was purified by silica gel
column chromatography (EtOAc/hexane, 20) to give 6 (968 mg, 72%) as a
1
colorless oil: H NMR (CDCl3, 300 MHz) δ 5.38 (d, J = 2.0 Hz, 1H), 5.12
(t, J = 1.9 Hz, 1H), 3.78 (dd, J = 10.4, 1.2 Hz, 2H), 1.74 (s, 3H), 1.62–1.55

3ꢁ,4ꢁ-Dimethyl-5ꢁ-norcarbocyclic Adenosine Phosphodiester
727
(m, 2H), 1.35 (s, 3H), 0.82 (s, 9H), 0.01 (s, 6H);13C NMR (CDCl3) δ 152.2,
109.4, 76.6, 57.4, 48.6, 25.4, 24.3, 18.5, 13.8, −5.2.
( )-3,4-Dimethyl-pent-4-ene-1,3-diol (7)
To a solution of 6 (1.2 g, 4.9 mmol) in THF (10 mL) at 0◦C, TBAF
(7.3 mL, 1.0 M solution in THF) was added. The mixture was stirred
overnight at room temperature and concentrated. The residue was purified
by silica gel column chromatography (EtOAc/hexane, 3:1) to give 7 (523 mg,
82%): 1H NMR (CDCl3, 300 MHz) δ 5.35 (d, J = 2.1 Hz, 1H), 5.15 (d, J =
2.0 Hz, 1H), 3.67 (dd, J = 10.2, 1.4 Hz, 2H), 1.73 (s, 3H), 1.61–1.54 (m, 2H),
1.36 (s, 3H);13C NMR (CDCl3) δ 152.7, 105.2, 77.5, 56.7, 45.2, 24.9, 14.1.
( )-Benzoic Acid 3-hydroxy-3,4-dimethyl-pent-4-enyl Ester (8)
To a solution of 7 (1.4 g, 10.76 mmol) in anhydrous pyridine (15 mL) was
added benzoyl chloride (1.66 g, 11.83 mmol) and dimethylamino pyridine
(DMAP) (262 mg, 2.15 mmol) at 0∞C. The reaction mixture was stirred for
2 hours at 0∞C and further stirred overnight at room tmperature. The reac-
tion mixture was quenched with saturated NaHCO3 solution (6 mL), stirred
for 20 minutes and concentrated under reduced pressure. The residue was
diluted with water (60 mL) and extracted with EtOAc (60 mL) twice. The
combined organic layer was washed with brine, dried over MgSO4, filtered,
and concentrated. The residue was purified by silica gel column chromatog-
raphy (EtOAc/hexane, 1:10) to give 8 (1.76 g, 70%) as a colorless syrup: 1H
NMR (CDCl3, 300 MHz) δ 8.04 (m, 2H), 7.58 (m, 1H), 7.47 (m, 2H), 5.34
(t, J = 2.0 Hz, 1H), 5.18 (d, J = 2.0 Hz, 1H), 4.21 (dd, J = 9.4, 1.8 Hz, 2H),
1.89–1.79 (m, 2H), 1.73 (s, 3H), 1.37 (s, 3H);13C NMR (CDCl3) δ 167.3,
151.4, 132.7, 130.4, 128.4, 107.2, 78.3, 59.2, 43.7, 25.2, 14.2; Anal. Calc. for
C14H18O3: C, 71.77; H, 7.74; Found: C, 71.75; H, 7.71.
( )-Benzoic Acid 3-(tert-butyl-dimethyl-silanyloxy)-
3,4-dimethyl-pent-4-enyl Ester (9)
To a solution of 8 (984 mg, 4.2 mmol) in anhydrous CH2Cl2 (15 mL)
was added TEA (1.01 g, 9.99 mmol) and DMAP (131 mg, 1.07 mmol) at 0◦C.
t-Butyldimethylsilyl trifluomethane-sulfonate (TBDMSOTf) (1.45 g, 5.49
mmol) was added to this mixture; the reaction mixture was stirred overnight
at room temperature and quenched with cold H2O (5 mL). The mixture was
concentrated under reduced pressure, diluted with water (60 mL), and ex-
tracted with EtOAc (60 mL) two times. Combined organic layer was washed
with brine, dried over MgSO4, filtered, and concentrated. The residue was
purified by silica gel column chromatography (EtOAc/hexane, 1:20) to give
9 (1.21 g, 83%) as a colorless syrup: 1H NMR (CDCl3, 300 MHz) δ 8.01 (m,
2H), 7.46 (m, 1H), 7.39 (m, 2H), 5.38 (d, J = 2.1 Hz, 1H), 5.12 (t, J = 2.0

728
C. H. Oh et al.
Hz, 1H), 4.32–4.24 (m, 2H), 1.93–1.86 (m, 2H), 1.70 (s, 3H), 1.39 (s, 3H),
0.82 (s, 9H), 0.01 (s, 6H); 13C NMR (CDCl3) δ 166.5, 152.0, 133.5, 130.8,
128.9, 128.1, 105.8, 78.7, 58.5, 42.5, 25.8, 24.6, 18.4, 14.0, −5.3; Anal. Calc.
for C20H32O3Si: C, 68.92; H, 9.25; Found: C, 68.89; H, 9.21.
( )-3-(tert-Butyl-dimethyl-silanyloxy)-3,4-dimethyl-pent-4-en-1-ol
(10)
A solution of 9 (2.2 g, 6.31 mmol) was dissolved in saturated methanolic
ammonia (20 mL) at 0◦C and stirred overnight at room temperature. The
mixture was concentrated under reduced pressure and the residue was pu-
rified by silica gel column chromatography (EtOAc/hexane, 1:4) to give 10
1
(1.4 g, 91%) as a colorless syrup: H NMR (CDCl3, 300 MHz) δ 5.37 (t, J
= 2.0 Hz, 1H), 5.14 (d, J = 2.0 Hz, 1H), 3.61–3.53 (m, 2H), 1.72 (s, 3H),
1.63–1.55 (m, 2H), 1.49 (s, 3H), 0.81 (s, 9H), 0.01 (s, 6H); 13C NMR (CDCl3)
δ 151.7, 109.3, 71.9, 55.0, 46.6, 26.2, 25.4, 18.5, 13.8, −5.4; Anal. Calc. for
C13H28O2Si: C, 63.87; H, 11.55; Found: C, 63.90; H, 11.52.
( )-3-(tert-Butyl-dimethyl-silanyloxy)-3,4-dimethyl-pent-4-enal
(11)
To a stirred solution of oxalyl chloride (0.0864 mL, 0.979 mmol) in
CH2Cl2 (8 mL) was added a solution of DMSO (0.0864 mL, 1.296 mmol) in
CH2Cl2 (1.2 mL) dropwise at −78◦C. The resulting solution was stirred at
−78◦C for 10 minutes, and a solution of alcohol 10 (133 mg, 0.545 mmol) in
CH2Cl2 (5 mL) was added dropwise. The mixture was stirred at −78◦C for 20
minutes and TEA (0.384 mL, 2.71 mmol) was added. The resulting mixture
was warmed to 0◦C and stirred for 30 minutes. H2O (7 mL) was added, and
the solution was stirred at room temperature for 20 minutes. The mixture
was diluted with water (60 mL) and then extracted with EtOAc (60 mL) two
times. The combined organic layer was washed with brine, dried over MgSO4
and filtered. The filtrate was concentrated under reduced pressure and the
residue was purified by silica gel column chromatography (EtOAc/hexane,
1
1:25) to give aldehyde compound 11 (123 mg, 93%) as a colorless oil: H
NMR (CDCl3, 300 MHz) δ 9.91 (s, 1H), 5.36 (d, J = 2.1 Hz, 1H), 5.15 (d,
J = 2.0 Hz, 1H), 2.51 (dd, J = 10.2, 6.2 Hz, 2H), 1.74 (s, 3H), 1.51 (s, 3H),
0.81 (s, 9H), 0.02 (s, 6H); 13C NMR (CDCl3) δ 200.9, 151.8, 110.3, 70.3, 56.2,
26.0, 25.7, 18.6, 13.9, −5.6.
(rel)-(3R and 3S,5S)-5-(tert-Butyl-dimethyl-silanyloxy)-5,6-
dimethyl-hepta-1,6-dien-3-ol (12)
To a solution of 11 (4.2 g, 17.32 mmol) in dry THF (20 mL) was slowly
added vinylMgBr (20.78 mL, 1.0 M solution in THF) at −20◦C. After 5
hours, saturated NH4Cl solution (20 mL) was added to the mixture, and

3ꢁ,4ꢁ-Dimethyl-5ꢁ-norcarbocyclic Adenosine Phosphodiester
729
the reaction mixture was slowly warmed to room temperature. The mixture
was further diluted with water (100 mL) and extracted with EtOAc (100
mL) two times. The combined organic layer was washed with brine, dried
over MgSO4, filtered, and evaporated. The residue was purified by silica gel
column chromatography (EtOAc/hexane, 1:15) to give 12 (3.84 g, 82%) as
1
a colorless oil: H NMR (CDCl3, 300 MHz) δ 5.75 (dd, J = 16.2, 10.2 Hz,
1H), 5.21 (m, 2H), 5.04 (d, J = 10.3 Hz, 1H), 3.85–3.78 (m, 1H), 1.65–1.56
(m, 2H), 1.73 (s,s, 3H), 1.50 (s, 3H), 0.83 (s, 9H), 0.01 (m, 6H); 13C NMR
(CDCl3) δ 151.3, 142.2, 115.2, 105.2, 71.7, 66.5, 50.6, 26.6, 25.4, 18.4, 13.9,
−5.3.
(rel)-(1S and 4S)-4-(tert-Butyl-dimethyl-silanyloxy)-3,4-dimethyl-
cyclopent-2-enol (13a) and (rel)-(1R and 4S)-4-(tert-Butyl-
dimethyl-silanyloxy)-3,4-dimethyl-cyclopent-2-enol (13b)
To a solution of 12 (1.39 g, 5.16 mmol) in dry benzene (8 mL) was added
second-generation Grubbs catalyst (76.2 mg 0.09 mmol). The reaction mix-
ture was refluxed overnight and cooled to room temperature. The mixture
was concentrated in vacuo, and the residue was purified by silica gel column
chromatography (EtOAc/hexane, 1:10) to give cyclopentenol 13a (450 mg,
36%) and 13b (437 mg, 35%) as colorless oils, respectively. Cyclopentenol
1
13a: H NMR (CDCl3, 300 MHz) δ 5.41 (s, 1H), 4.09 (dd, J = 6.4, 2.0 Hz,
1H), 2.16 (dd, J = 12.2, 8.6 Hz, 1H), 1.98 (dd, J = 12.2, 6.8 Hz, 1H), 1.70
(s, 3H), 1.53 (s, 3H), 0.81 (s, 9H), 0.01 (s, 6H); 13C NMR (CDCl3) δ 143.2,
122.9, 77.5, 68.4, 44.8, 26.8, 25.5, 18.5, 14.3, -5.6; Anal. Calc. for C13H26O2Si:
C, 64.41; H, 10.81; Found: C, 64.43; H, 10.84; Cyclopentenol 13b: 1H NMR
(CDCl3, 300 MHz) δ 5.39 (s, 1H), 4.12 (t, J = 6.8 Hz, 1H), 2.15 (dd, J =
10.8, 8.8 Hz, 1H), 2.01 (dd, J = 10.7, 7.2 Hz, 1H), 1.75 (s, 3H), 1.52 (s, 3H),
0.82 (s, 9H), 0.01 (s, 6H); 13C NMR (CDCl3) δ 142.9, 123.1, 78.2, 69.0, 45.6,
25.9, 25.2, 18.2, 14.8, −5.7; Anal. Calc. for C13H26O2Si: C, 64.41; H, 10.81;
Found: C, 64.38; H, 10.80.
(rel)-(1ꢀR,4ꢀS)-9-[4-(t-Butyldimethylsilyloxy)-3,4-dimethyl-
cyclopent-2-en-1-yl] 6-chloropurine (14)
To a solution containing compound 13b (131 mg, 0.542 mmol), triph-
enylphosphine (570 mg, 2.17 mmol) and 6-chloropurine (167 mg, 1.085
mmol) in anhydrous THF (6 mL), diisopropyl azodicarboxylate (DIAD)
(219 mg, 1.085 mmol) was added dropwise at −20◦C for 30 minutes under
nitrogen. The reaction mixture was stirred for 3 hours at 0◦C and further
stirred overnight at room temperature under nitrogen. The solvent was con-
centrated under reduced pressure and the residue was purified by silica gel
column chromatography (EtOAc/hexane, 2.5:1) to give compound 14 (86
mg, 42%) as a white solid: m.p. 167–169◦C;1H NMR (CDCl3, 300 MHz) δ

730
C. H. Oh et al.
8.72 (s, 1H), 8.50 (s, 1H), 5.41 (s, 1H), 4.56 (dd, J = 7.8, 2.2 Hz, 1H), 2.52
(dd, J = 10.6, 8.6 Hz, 1H), 2.13 (dd, J = 10.6, 6.8 Hz, 1H), 1.76 (s, 3H), 1.54
(s, 3H), 0.81 (s, 9H), 0.01 (s, 6H); 13C NMR (CDCl3) δ 153.2, 151.7, 149.6,
144.3, 137.7, 133.2, 123.2, 78.3, 53.6, 41.7, 26.2, 25.1, 18.5, 17.5, 13.5, −5.4;
Anal. Calc. for C18H27ClN4OSi: C, 57.05; H, 7.18; N, 14.78; Found: C, 57.09;
H, 7.22; N, 14.81.
(rel)-(1ꢀR,4ꢀS)-9-[4-Hydroxy-3,4-dimethylcyclopent-2-en-1-yl]
6-chloropurine (15)
To a solution of 14 (180 mg, 0.475 mmol) in THF (6.0 mL), TBAF
(0.569 mL, 1.0 M solution in THF) was added at 0◦C. The mixture was stirred
overnight at room temperature and concentrated. The residue was purified
by silica gel column chromatography (MeOH/Hexane/EtOAc, 0.1:4:1) to
give 15 (85 mg, 87%) as a white solid: m.p. 164–166◦C;1H NMR (DMSO-
d6, 300 MHz) δ 8.76 (s, 1H), 8.55 (s, 1H), 5.40 (s, 1H), 5.21 (s, 1H, D2O
exchangeable), 4.52 (t, J = 7.2 Hz, 1H), 2.60 (dd, J = 10.8, 8.7 Hz, 1H), 2.12
(dd, J = 10.9, 6.8 Hz, 1H), 1.75 (s, 3H), 1.51 (s, 3H); 13C NMR (DMSO-d6)
δ 153.4, 151.7, 159.2, 147.7, 137.5, 134.6, 124.4, 78.3, 56.9, 43.5, 26.4, 17.5,
13.5; Anal. Calc. for C12H13ClN4O · 0.5 MeOH: C, 53.48; H, 5.38; N, 19.96;
Found: C, 53.51; H, 5.36; N, 19.93.
(rel)-(1ꢀR,4ꢀS)- Diisopropyl [9-(4-Hydroxy-3,4-dimethylcyclopent-
2-en-1-yl)] 6-chloropurine] Methylphosphonate (16)
Both LiOt-Bu (7.6 mL of 1.0 M solution in THF, 3.8 mmol) and a solution
of diisopropyl bromomethylphosphonate (0.832 g, 3.21 mmol) in 6 mL of
THF were slowly added to a solution of the carbocyclic purine analogue
15 (627 mg, 2.37 mmol) in 6 mL of THF at 0◦C and stirred for 5 hours at
room temperature under anhydrous conditions. The mixture was quenched
by adding water (6 mL) and further diluted with additional H2O (70 mL).
The aqueous layer was extracted with EtOAc (3 × 70 mL). The combined
organic layer was washed with brine, dried over MgSO4, and concentrated
in vacuo. The residue was purified by silica gel column chromatography
(MeOH/Hexane/EtOAc, 0.05:3:1) to give 16 (619 mg, 59%) as a foamy
syrup: m.p. 121–123◦C;1H NMR (CDCl3, 300 MHz) δ 8.73 (s, 1H), 8.52 (s,
1H), 5.45 (s, 1H), 4.72 (m, 2H), 4.52 (dd, J = 7.4, 1.8 Hz, 1H), 3.79 (dd, J =
8.8, 2.8 Hz, 2H), 2.51 (dd, J = 10.8, 8.7 Hz, 1H), 2.08 (dd, J = 10.8, 6.8 Hz,
1H), 1.76 (s, 3H), 1.52 (s, 3H), 1.33 (m, 12H); 13C NMR (CDCl3) δ 152.9,
152.4, 150.3, 147.8, 137.7, 134.7, 124.6, 84.5, 72.2, 63.6, 53.8, 38.4, 24.2, 22.5,
14.4; Anal. Calc. for C19H28ClN4O4P · 1.0 MeOH: C, 50.64; H, 6.80; N, 11.81;
Found: C, 50.67; H, 6.82; N, 11.79.

3ꢁ,4ꢁ-Dimethyl-5ꢁ-norcarbocyclic Adenosine Phosphodiester
731
(rel)-(1ꢀR,4ꢀS)-Diisopropyl [9-(4-Hydroxy-3,4-dimethylcyclopent-
2-en-1-yl)] adenine] Methylphosphonate (17)
A solution of 16 (187 mg, 0.422 mmol) in saturated methanolic ammonia
(8 mL) was stirred overnight on a steel bomb at 50◦C, and the volatile
components were evaporated. The residue was purified by silica gel column
chromatography (MeOH/CH2Cl2, 1:7) to give 17 (123 mg, 69%) as a solid:
m.p. 145–147◦C;1H NMR (DMSO-d6, 300 MHz) δ 8.28 (s, 1H), 8.14 (s, 1H),
5.45 (s, 1H), 4.70 (m, 2H), 4.49 (t, J = 7.8 Hz, 1H), 3.73 (dd, J = 8.6, 6.4 Hz,
2H), 2.57 (dd, J = 10.6, 8.6 Hz, 1H), 2.15 (dd, J = 10.6, 7.0 Hz, 1H), 1.78 (s,
3H), 1.53 (s, 3H), 1.33 (m, 12H); 13C NMR (DMSO-d6) δ 154.8, 152.7, 151.5,
147.4, 137.5, 130.6, 118.8, 83.5, 72.6, 66.7, 54.2, 38.6, 26.4, 22.6, 14.3; Anal.
Calc. for C19H30N5O4P · 1.0 MeOH: C, 52.74; H, 7.52; N, 15.37; Found: C,
52.73; H, 7.49; N, 15.39.
(rel)-(1ꢀR,4ꢀS)-[9-(4-Hydroxy-3,4-dimethylcyclopentyl)] adenine]
Methylphosphonic Acid (18)
To a solution of the phosphonate 17 (103 mg, 0.244 mmol) in CH3CN
(10 mL) was added trimethylsilyl bromide (407 mg, 2.66 mmol). The mixture
was heated under reflux for 24 hours and then concentrated under reduced
pressure. The residue was partitioned between CH2Cl2 (50 mL) and distilled
H2O (50 mL). The aqueous layer was washed out with CH2Cl2 and then
freeze-dried to give target compound 18 (69 mg, 84%) as a yellowish foamy
1
solid. m.p. 109–111◦C; H NMR (DMSO-d6, 300 MHz) δ 8.29 (s, 1H), 8.13
(s, 1H), 7.15 (br s, 2H), 5.33 (s, 1H), 4.50 (d, J = 7.8 Hz, 1H), 3.79 (dd,
J = 8.8, 6.8 Hz, 2H), 2.54 (dd, J = 10.8, 8.8 Hz, 1H), 2.12 (dd, J = 10.8,
6.8 Hz, 1H), 1.73 (s, 3H), 1.46 (s, 3H); 13C NMR (DMSO-d6) δ 154.4, 152.3,
151.1, 146.7, 137.4, 124.7, 118.7, 84.7, 66.8, 54.2, 38.5, 23.2, 14.3; Anal. Calc.
for C13H18N5O4P · 1.0 H2O: C, 43.69; H, 5.64; N, 19.60; Found: C, 43.73; H,
5.61; N, 19.55.
(rel)-(1ꢀR,4ꢀS)-Bis(SATE) phosphoester of [9-(4-
methyloxyphosphonate-3,4-dimethylcyclopentyl)] Adenine (20)
A solution of adenine phosphonic acid derivative 18 (72 mg, 0.212 mmol)
and tri-n-butylamine (117 mg, 0.636 mmol) in methanol (4.5 mL) was mixed
for 30 minutes and concentrated under reduced pressure. The residue was
thoroughly dried with anhydrous ethanol and toluene. The resulting foamy
solid was dissolved in anhydrous pyridine (15 mL) to which thioester 19
(649 mg, 4.0 mmol) and 1-(2-mesitylenesulfonyl)-3-nitro-1H-1,2,4-triazole
(251 mg, 0.848 mmol) were added. The mixture was stirred overnight at
room temperature and quenched with tetrabutylammonium bicarbonate
buffer (12.0 mL, 1 M solution, pH 8.0). The mixture was concentrated
under reduced pressure and the residue was diluted with water (70 mL) and

732
C. H. Oh et al.
extracted with CHCl3 (80 mL) two times. The combined organic layer was
washed with brine, dried over Na2SO4, filtered, and evaporated. The residue
was purified by silica gel column chromatography (MeOH/Hexane/EtOAc,
0.05:4:1) to give 20 (46 mg, 35%) as a white solid: m.p. 121–123◦C; UV
(MeOH) λmax 262.5 nm;1H NMR (CDCl3, 300 MHz) δ 8.31 (s, 1H), 8.16 (s,
1H), 5.39 (s, 1H), 4.49 (d, J = 6.8 Hz, 1H), 4.02 (m, 4H), 3.56 (d, J = 9.6 Hz,
2H), 3.16 (t, J = 6.4 Hz, 4H), 2.21–2.13 (m, 2H), 1.71 (s, 3H), 1.50 (s, 3H),
1.22–1.16 (s, 18H); 13C NMR (CDCl3) δ 204.8, 154.4, 152.7, 148.6, 145.5,
125.4, 118.6, 83.6, 62.6, 60.3, 53.4, 46.6, 38.4, 30.1, 26.8, 23.4, 13.9; Anal.
Calc. for C27H42N5O6PS2 · 1.0 MeOH: C, 49.15; H, 7.03; N, 10.61. Found: C,
49.20; H, 6.97; N, 10.58.
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