Synthesis of (-)-DAPD

Article abstract of DOI:10.1081/NCN-200040643

A new synthesis of (-)-DAPD, suitable for large scale development, is described.

Full text of DOI:10.1081/NCN-200040643

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Synthesis of (-)‐DAPD
Dr. Marcos L. Sznaidman a b c , Jinfa Du a d , Amir Pesyan a e , Darryl G. Cleary a f , Kevin P.
Hurley a f , Frank Waligora a g & Merrick R. Almond a h
a Division of Organic Chemistry , Triangle Pharmaceuticals Inc. , 4611 University Dr.,
Durham, North Carolina, 27717, USA
b Norak Biosciences , P.O. Box 14769, RTP, North Carolina, 27709‐4769, USA
c Norak Biosciences , 7030 Kit Creek Road, Morrisville, North Carolina, 27560, USA
d Pharmasset, Inc. , 1860 Montreal Rd., Tucker, Georgia, 30084, USA
e NPS Pharmaceuticals , 420 Chipeta Way, Salt Lake City, Utah, 84108, USA
f 6410 Falconbridge Rd., Chapel Hill, North Carolina, 27514, USA
g Idenix Pharmaceuticals , 763 E. Concord Ave., Cambridge, Massachusetts, 02138, USA
h Chimerix, Inc. , 5015 Southpark Drive, Suite 240, Durham, North Carolina, 27713, USA
Published online: 17 Aug 2006.
To cite this article: Dr. Marcos L. Sznaidman , Jinfa Du , Amir Pesyan , Darryl G. Cleary , Kevin P. Hurley , Frank Waligora &
Merrick R. Almond (2004) Synthesis of (-)‐DAPD, Nucleosides, Nucleotides and Nucleic Acids, 23:12, 1875-1887, DOI: 10.1081/
NCN-200040643
To link to this article: http://dx.doi.org/10.1081/NCN-200040643
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NUCLEOSIDES, NUCLEOTIDES & NUCLEIC ACIDS
Vol. 23, No. 12, pp. 1875–1887, 2004
Synthesis of (À)-DAPD
Marcos L. Sznaidman,1,2,3, Jinfa Du,1,4 Amir Pesyan,1,5
*
Darryl G. Cleary,1,6 Kevin P. Hurley,1,6 Frank Waligora,1,7
and Merrick R. Almond1,8
1Division of Organic Chemistry, Triangle Pharmaceuticals Inc.,
Durham, North Carolina, USA
2Norak Biosciences, RTP, North Carolina, USA
3Norak Biosciences, Morrisville, North Carolina, USA
4Pharmasset, Inc., Tucker, Georgia, USA
5NPS Pharmaceuticals, Salt Lake City, Utah, USA
6Chapel Hill, North Carolina, USA
7Idenix Pharmaceuticals, Cambridge, Massachusetts, USA
8Chimerix, Inc., Durham, North Carolina, USA
ABSTRACT
A new synthesis of (À)-DAPD, suitable for large scale development, is described.
Key Words: DAPD; HIV; HBV; Dioxolane nucleosides.
INTRODUCTION
(À)-DAPD is a potent and selective inhibitor of HIV in vitro and in vivo[1] and
HBV replication in vitro.[1] Structurally it belongs to a class of compounds known as
the dioxolane nucleosides.
*
Correspondence: Marcos L. Sznaidman, Norak Biosciences, P.O. Box 14769, RTP, NC 27709,
USA; E-mail: msznaidman@norakbio.com.
1875
DOI: 10.1081/NCN-200040643
1525-7770 (Print); 1532-2335 (Online)
www.dekker.com
Copyright D 2004 by Marcel Dekker, Inc.

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Sznaidman et al.
Scheme 1. Synthesis of (+/À) DAPD by Belleau et al.
In 1988 Belleau et al. filed patent application U.S. Patent No. 5,041,449[2] which
disclosed the generic group of racemic 1,3-dioxolane nucleosides for the treatment of
HIV and portions of this work were presented in 1989 at the Fifth International
Conference on AIDS.[3] Racemic DAPD was synthesized as depicted in Scheme 1.
Belleau et al. reacted glycerol and chloroacetaldehyde to generate a dioxolane
intermediate. After chlorine displacement with a benzoic acid salt, oxidation of the
primary alcohol to a carboxylic acid and Baeyer-Villiger rearrangement with
m-chloroperbenzoic acid, the corresponding racemic dioxolane benzoate was obtained.
This compound was then coupled with 2-amino-6-chloropurine and the resulting
nucleoside intermediate was reacted with ammonia under pressure to afford racemic
DAPD. In 1989, Norbeck et al.[4] concurrently published an article which described the
synthesis of racemic cis-1,3-dioxolane thymidine which also had anti-HIV activity in
vitro. Norbeck et al.[4] condensed ( )-methyl glycerate and benzyloxyacetaldehyde
dimethylacetal, which after saponification and oxidative decarboxylation afforded a
racemic dioxolane acetate. The same racemic dioxolane acetate intermediate was
synthesized by Liotta et al.[5,6] starting from cis-2-buten-1,4-diol (Scheme 2). After
protection and ozonolysis of the double bond an aldehyde was isolated which, due to its
unstable nature, was immediately used in the next step. Condensation with glycolic acid
afforded a racemic dioxolane lactone, which was reduced and acetylated to afford the
racemic dioxolane acetate. The drawback of these procedures, shown in Scheme 2,
is that they involve the synthesis of an unstable aldehyde and/or a tedious oxida-
tive step(s).
In 1990, Chu and Shinazi filed U.S. Patent No. 5,179,104 which disclosed a
method to obtain enantiomerically pure b-D-1,3-dioxolane nucleosides via a
stereospecific synthesis and Kim et al.[7,8] subsequently published a paper entitled
‘‘1,3-Dioxolanylpurine Nucleosides (2R,4R) and (2R,4S) with Selective Anti-HIV-1
Activity in Human Lymphocytes’’. The synthesis of (À)-DAPD was described as a
thirteen-step process from 1,6-anhydro-D-mannose. 1,6-Anhydro-D-mannose was
converted to a chiral dioxolane acetate in nine steps (Scheme 3). After coupling of
the acetate under Vorbruggen conditions, several purification and deprotection steps,
(À)-DAPD was obtained in modest yield. While this is an asymmetric synthesis, it is
too long and involves difficult oxidation steps that make the process unsuitable for
commercial development.

Synthesis of (À)-DAPD
1877
Scheme 2. Synthesis of (+/À) DAPD by Norbeck et al.
In 1992 Belleau et al.[9] published a synthesis of enantiomerically pure 2’,3’-
dideoxy-3-oxacytidine stereoisomers. The derivatives were synthesized in eight steps
from L-ascorbic acid. While this route was stereospecific, the use of lead tetraacetate
made the route unsuitable for scale-up.
Relevant chemistry was found in a patent filed by Liotta and Shinazi, WO
92/14729[10] and in a publication by Jin et al.[11] Liotta and Shinazi found that racemic
Scheme 3. Synthesis of (À) DAPD by Kim et al.

1878
Sznaidman et al.
2-O-protected-5-O-acylated-1,3-dioxolanes could be coupled with purine or pyrimidine
bases in the presence of a titanium containing Lewis acid to predominately generate the
racemic b-isomers. In addition, Jin et al. published that titanium tetrachloride and tin
tetrachloride promoted the racemization of enantiomerically pure 2’-deoxy-3’-oxaribo-
sides during coupling reactions but that the use of the Lewis acids trimethylsilyltriflate,
trimethylsilyl iodide and TiCl2(Oi-Pr)2 provided enantiomerically pure cytosine
dioxolane nucleosides in low diastereoselectivity.
While there are other papers in the literature which involve the synthesis of
dioxolane nucleosides,[12–14] the chemistry was not applicable to the synthesis of (À)-
DAPD on large scale. For these reasons we carried out the following work.
RESULTS AND DISCUSSION
In pursuit of a facile and economical synthesis of (À)-DAPD we envisioned the
following strategy: preliminary results showed that properly substituted racemic
dioxolane lactones, like the ones shown in Scheme 2, could be resolved by chiral
chromatography (unpublished results). As previously mentioned, one of our goals was
to avoid oxidative steps and/or to avoid the production of unstable aldehydes as
depicted in Scheme 2. For these reasons, we proposed a simple two-step synthesis of a
racemic dioxolane lactone from commercially available 2,2-dialkoxy-ethanol
(Scheme 4). After resolution and reductive acetylation chiral dioxolane acetates would
be obtained. Any suitable R group should comply with all of the following conditions:
1) the corresponding lactone should be resolvable, 2) the corresponding ester should be
stable under the reductive acetylation conditions, 3) after coupling, the corresponding
anomers should be easily separated, preferentially by crystallization, 4) should be easy
to remove at the end of the synthesis, 5) have a low molecular weight to avoid carrying
large mass during the process and, 6) be commercially available and inexpensive. Two
groups that satisfied all of these conditions were: iso-butyryl and p-methoxy benzoyl.
The results are summarized in Scheme 5.
Scheme 4. Proposed synthesis of dioxolane lactone.

Synthesis of (À)-DAPD
1879
Scheme 5. Synthesis of (À) DAPD.
Initially a 2,2-dialkoxy-ethanol (1) was esterified with the corresponding acid
chloride in quantitative yields. In the case of the iso-butyryl (2a) ester, cyclization to
the corresponding dioxolane lactone (3a), was affected with glycolic acid in the
presence of a Lewis acid (BF3Á Et2O) in 80% yield. However, under the same
conditions, the p-methoxy benzoate ester (2b) afforded the corresponding lactone in
low yield and purity. Much better results were achieved in a two step procedure in
which the acetal was first hydrolyzed to the corresponding aldehyde (2c), which then
was subjected to similar cyclization conditions, affording the corresponding lactone
(3b) in 80% yield as a solid. The lactones were resolved by chiral chromatography in

1880
Sznaidman et al.
almost quantitative yield.a Selective reduction of the chiral lactones (4a and 4b) with
LiAlH(OtBu)3 and subsequent treatment with acetic anhydride afforded the
corresponding acetates (5a and 5b) in reasonable yields, after purification by flash
column chromatography. Coupling with 2,6-dichloropurine under Vobruggen conditions
afforded crude mixtures of a:b anomers of the correponding nucleosides (6a and 6b).
The iso-butyrate ester was first purified by column chromatography and then crystallized
from MeOH to afford the pure b anomer (7a) in a 24% yield. The p-methoxy benzoate
ester nucleoside did not require chromatography, and was purified by crystallization
from MeOH to afford pure b anomer (7b) in 46% yield. The chlorines were replaced
with amino groups in a two-step procedure by treatment with sodium azide and then
hydrogenation. The corresponding intermediates (9a and 9b) were isolated and fully
described. Removal of the protecting groups was achieved with n-butylamine in
refluxing MeOH to afford (À)-DAPD (10) in 93% and 90% yield respectively.
The total overall yields from the corresponding chiral acetates (5a and 5b) were:
18.7% for the iso-butyrate and 36.4 % for the p-methoxy benzoate. Much better than
the overall 10.6% yield previously reported from the t-butyl-di-phenyl-silyl chiral
acetate.[7,8] The total overall yield from 1 was 4.5% using route a, and 8.7% using route
b, in 8 steps, using one chiral resolution and one regular column chromatography in the
case of the p-methoxy benzoate protecting group. The overall yield of (À)-DAPD
previously described in the literature[7,8] was 1.1% in 14 steps from D-Mannose, using
six chromatographic separations. Preliminary results in our laboratory showed that this
synthesis could be successfully repeated in a kilogram scale, without using any
chromatographic separation.b
EXPERIMENTAL SECTION
General methods. Melting points were determined in open glass capillaries by
1
use of a Melt-Temp II apparatus with a digital thermometer. H-NMR spectra were
recorded at 400 MHz with a Varian XL-400 spectrometer. Evaporations were
aThese are typical conditions for the chiral resolution of 5 grams of either 3a or 3b: Column:
Chiralpak Semi-prep. AD-RH, 250 Â 10 mm. Mobile phase: Methanol. Gradient: Isocratic.
Detection: UV at 280 nm. The recovery of both enantiomers of 3a and 3b was almost quantitative
(close to 50% for each enantiomer for both compounds).
bIn an exploratory large scale run racemic iso-butyrate lactone 3a (3.6 kg, 19 mol) was obtained in
80% yield from 1. The lactone was resolved enzymaticaly using Chirazyme-L2 in isopropanol:
water as a solvent at 0°C. Chiral lactone 4a (1.4 kg, 7.4 mol) was obtained in 40% yield. This
lactone was converted to the crude acetate 5a, which without any further purification was coupled
with 2,6-dichloroputine to afford 7a (432 g, 1.2 mol) as almost pure b anomer in 16% yield from
4a. This material was carried through the next steps as described before to afford the final product:
(À)-DAPD (10, 217 g, 0.86 mol) in 72% yield from 7a. The total overall yield for this procedure
from 1 was 3.7%, a little bit lower than the 4.5% we reported in this manuscript for the smaller
scale, but no chromatographic resolution of any type was required and, since this was our first run,
further improvements can be made to improve the yield. We also believe that further improvements
will also be made by using p-methoxybenzoate as the protecting group that gave much better yields
and purity than the iso-butyrate.

Synthesis of (À)-DAPD
1881
performed under diminished pressure using a Buchi rotatory evaporator at 40°C unless
otherwise indicated. Solutions were dried over anhydrous Na2SO4. TLC was performed
on precoated glass plates (0.25 mm) with Silica Gel 60F254 (E. Merck, Darmstad).
Flash column chromatography was performed with Silica Gel 60 (230–400 mesh, E.
Merck, Darmstad). Elemental analyses were performed by Atlantic Microlab (Atlanta,
GA). High Resolution Mass Spectra (HRMS) were performed by Analytical Instrument
Group Inc. (Raleigh, NC).
Gas chromatography analysis. Samples were prepared by dissolving 1 mg
sample per 1 mL THF, or other suitable solvent. A 1 ml injection volume was used.
Samples were separated using a 30 m HP-5 (crosslinked 5% PH ME siloxane) capillary
column, Hewlett Packard part #19091J-413. The inlet temperature was set at 200°C,
and the oven temperature as follows: 35°C for 1 minute, ramp to 250°C at 12.5°C/
minute, hold at 250°C for 1.8 minutes. The flame ionization detector temperature was
set at 250°C. The carrier gas was nitrogen set at a nominal flow of 8.0 mL/minute.
HPLC analysis.
Method A. Column: Chiralpak AD-RH, 150 Â 4.6 mm. Mobile phase: Methanol.
Gradient: Isocratic. Flow rate: 0.5 mL/min. Run time: 30 min. Detection: UV at 280 nm.
Method B. Column: Aquasil C18, 150 Â 4.6 mm. Mobile phase: solvent A:
acetonitrile, solvent B: 50 mmol NH4OAc, 0.1% AcOH in water. Gradient: time: 0 min.,
A: 1%, B: 99%; time: 17 min., A: 50%, B: 50%, then isocratic. Flow rate: 1.0 mL/min.
Run time: 30 min. Detection: UV at 290 nm.
Method C. Column: Chiralpak AD, 250 Â 4.6 mm. Mobile phase: Methanol.
Gradient: Isocratic. Flow rate: 0.8 mL/min. Run time: 20 min. Detection: UV at 254 nm.
Isobutyric acid 2,2-dimethoxy-ethyl ester (2a).
4-Methoxy-benzoic acid 2,2-diethoxy-ethyl ester (2b). To a well-stirred
solution of 2,2-diethoxy or 2,2-dimethoxy-ethanol (1, 100 mmol), DMAP (61 mg,
0.5 mmol) and Et3N (16 mL, 11.64 g, 115 mmol) in EtOAc or tert-butylmethyl ether
(50 mL) at 0°C was slowly added the corresponding acid chloride (105 mmol). After
stirring for 16 h at room temperature the reaction mixture was diluted with EtOAc
(50 mL), and successively washed with: (c) NaHCO3 (2 Â 100 mL), brine (2 Â 100 mL),
dried, filtered and evaporated to afford:
Isobutyric acid 2,2-dimethoxy-ethyl ester (2a, 99%) as a yellow liquid that was
1
used in the next step without any further purification. GC (Rt = 5.24 min., 98%); H-
NMR (CDCl3) d: 4.57 (1H, t, J = 5.2, (MeO)2CHCH2–), 4.11 (2H, d, J = 5.2,
(MeO)2CHCH2–), 3.40 (6H, s, CH3O–), 2.60 (1H, m, OCOCH(CH3)2), 1.54 (6H, d,
J = 6.8, OCOCH(CH3)2).
4-Methoxybenzoic acid 2,2-diethoxy-ethyl ester (2b, 100%) as a syrup that was
used in the next step without any further purification. GC (Rt = 13.7 min, 99%);
1H-NMR (CDCl3) d: 7.98 (2H, d, J = 9.0, ArH), 6.89 (2H, d, J = 9.0, ArH), 4.79 (1H,
t, J = 5.6, (EtO)2CHCH2–), 4.28 (2H, d, J = 5.6, (EtO)2CHCH2–), 3.82 (3H, s,

1882
Sznaidman et al.
CH3O–), 3.73 (2H, m, CH3CH2O–), 3.59 (2H, m, CH3CH2O–), 1.22 (6H, t, J = 6.9,
CH3CH2O–).
Isobutyric acid 4-oxo-[1,3]-dioxolan-2-yl methyl ester (3a). To a well stirred
solution of the corresponding acetal (2a, 30 mmol) and a-hydroxy acetic acid (3.42 g,
45 mmol) in acetonitrile (30 mL) at 0°C was slowly added BF3Á Et2O (6.38 g, 5.70 mL,
45 mmol). The solution was left at room temperature overnight with stirring. The
solution was partitioned between EtOAc (150 mL) and (c) NaHCO3 (150 mL). The
organic solution was successively washed with (c) NaHCO3 (150 mL), brine (2 Â
150 mL), dried, filtered and evaporated to afford:
Isobutyric acid 4-oxo-[1,3]-dioxolan-2-yl methyl ester (3a, 80%) as a colorless
1
syrup. GC (Rt = 7.89 min., 95%); H-NMR (CDCl3) d: 5.83 (1H, s, H-2), 4.35–4.20
(4H, m, H-5, H-5’ and –CH2OCO–), 2.62 (1H, m, (CH3)2CHCOO–), 1.19 (6H, d,
J = 7.0, (CH3)2CHCOO–).
Calculated mass for C8H13O5 (M + 1)+: 189.0763. Found: (H.R.F.A.B.M.S.):
189.0763.
4-Methoxybenzoic acid 4-oxo-[1,3]-dioxolan-2-yl methyl ester (3b). A solution
of 4-methoxybenzoic acid 2,2-diethoxy-ethyl ester (2b, 7.5 g, 28 mmol) in Cl2CH2
(75 mL) was treated with TFA (16.7 g, 11.3 mL, 140 mmol) and water (7.5 g, 7.5 mL,
28 mmol). The homogeneous solution was stirred for 3.5 hours at room temperature
until GC showed complete reaction. The solution was concentrated in vacuo at 40°C
and then diluted with hexane and concentrated in vacuo several times to remove traces
of TFA. The product, 4-methoxybenzoic acid 2-oxo-ethyl ester (2c, 5.9 g, 28 mmol,
100%) was isolated as an amorphous white solid and was used in the next step without
1
any further purification. GC (Rt = 11.0 min, 95%); H-NMR (CDCl3) d: 9.72 (1H, s,
HCO–), 8.06 (2H, d, J = 8.8, ArH), 6.95 (2H, d, J = 8.8, ArH), 4.87 (2H, s,
HCOCH2O–), 3.87 (3H, s, OCH3).
To a well stirred solution of the crude aldehyde (2c, 5.9 g, 28 mmol) and a-
hydroxy acetic acid (5.2 g, 68 mmol) in DME (100 mL) at 0°C was slowly added
BF3 Á Et2O (12.3 g, 11.0 mL, 85 mmol). The solution was left at room temperature
overnight with stirring. The solution was partitioned between EtOAc (150 mL) and (c)
NaHCO3 (150 mL). The organic solution was successively washed with (c) NaHCO3
(150 mL), brine (2 Â 150 mL), dried, filtered and evaporated to afford a syrup. The
syrup was treated with DME (15 mL) and a solid precipitated. After stirring for
30 minutes, the solid was filtered to afford 4-methoxybenzoic acid 4-oxo-[1,3]-dioxolan-
2-yl methyl ester (3b, 5.7 g, 23 mmol, 80%) as a white granular solid; GC (Rt = 14.7 min,
99%); mp: 61–63°C; 1H-NMR (CDCl3) d: 7.97 (2H, d, J = 9.2, ArH), 6.91 (2H, d, J = 9.2,
ArH), 5.95 (1H, t, J = 3.0, H-2), 4.57 (1H, dd, J = 3.0 and J = 12.6, –CH2OCO–), 4.50
(1H, dd, J = 3.0 and J = 12.6, –CH2OCO–), 4.41 (1H, d, J = 15.0, H-5), 4.31 (1H, d,
J = 15.0, H-5), 3.87 (3H, s, OCH3).
Calculated for C12H12O6, C, 57.14; H, 4.80. Found: C, 57.40; H, 4.93.
Isobutyric acid 4-oxo-[1,3]-dioxolan-2(R)-yl methyl ester (4a). Racemic
isobutyric acid 4-oxo-[1,3]-dioxolan-2-yl methyl ester (3a) was resolved by chiral
chromatographya to afford two fractions corresponding to each one of the enantiomers.
Both fractions were colorless syrups. The first fraction corresponded to the R

Synthesis of (À)-DAPD
1883
enantiomer (4a, isobutyric acid 4-oxo-[1,3]-dioxolan-2(R)-yl methyl ester); HPLC
20
(Method A, Rt = 7.80 min., 100%); [a]D = 20.20° (c 0.25, MeOH). The second
fraction corresponded to the S enantiomer (isobutyric acid 4-oxo-[1,3]-dioxolan-2(S)-yl
20
methyl ester); HPLC (Method A, Rt = 9.30 min., 100%); [a]D = À 14.40° (c 0.25,
MeOH).
4-Methoxybenzoic acid 4-oxo-[1,3]-dioxolan-2(R)-yl methyl ester (4b). Race-
mic 4-methoxybenzoic acid 4-oxo-[1,3]-dioxolan-2-yl methyl ester (3b) was resolved
by chiral chromatographya to afford two fractions corresponding to each one of the
enantiomers. Both fractions were white solids. The first fraction corresponded to the R
enantiomer (4b, 4-methoxybenzoic acid 4-oxo-[1,3]-dioxolan-2(R)-yl methyl ester);
20
mp: 76–78°C; HPLC (Method A, Rt = 13.59 min., 99%); [a]D = 12.20° (c 0.25,
MeOH). The second fraction corresponded to the S enantiomer (4-methoxybenzoic acid
4-oxo-[1,3]-dioxolan-2(S)-yl methyl ester); mp: 76–78°C; HPLC (Method A,
20
Rt = 20.76 min., 99%); [a]D = À13.50° (c 0.25, MeOH).
Isobutyric acid 4-acetoxy-[1,3]-dioxolan-2(R)-yl methyl ester (5a).
4-Methoxybenzoic acid 4-acetoxy-[1,3]-dioxolan-2(R)-yl methyl ester (5b). To
a well stirred solution of the corresponding lactone (4a or 4b, 15 mmol) in dry THF
(45 mL) at À10°C, was slowly added a 1.0 M solution of LiAlH(OtBu)3 in THF
(19.5 mL, 19.5 mmol) over a period of 20 minutes, maintaining the temperature
between À15°C and À10°C. The reaction was followed by GC and was stirred for
30 minutes at room temperature (complete disappearance of starting material). The
solution was again cooled to À10°C and DMAP (0.92 g, 7.50 mmol) was added in one
portion followed by the dropwise addition of Ac2O (15.3 g, 14.20 mL, 150 mmol). The
reaction was further stirred for 1 h at À15°C, and then overnight at room temperature.
The solution was cooled to À15°C and quenched with MeOH (40 mL). After stirring
for 20 minutes at room temperature, the reaction was concentrated in vacuo to a red
syrup that was purified by flash column chromatography (250 g silica, hexane:EtOAc
3:1) to afford:
Isobutyric acid 4-acetoxy-[1,3]-dioxolan-2(R)-yl methyl ester (5a, 61%) as a
1
yellow syrup. H-NMR (CDCl3) shows almost a 1:1 mixture of anomers, d: 6.40 (d,
J = 3.8) and 6.37 (d, J = 3.8) corresponding to H-4 a and H-4 b; 5.41 (t, J = 3.7) and
5.32 (t, J = 3.7) corresponding to H-2 a and H-2 b.
4-Methoxybenzoic acid 4-acetoxy-[1,3]-dioxolan-2(R)-yl methyl ester (5b, 61%) as
1
a yellow syrup. H-NMR (CDCl3) shows almost a 1:1 mixture of anomers, d: 6.44 (dd,
J = 2.1 and J = 4.2) and 6.37 (d, J = 4.0) corresponding to H-4 a and H-4 b; 5.54 (t,
J = 3.6) and 5.45 (t, J = 4.0) corresponding to H-2 a and H-2 b.
Isobutyric acid 4(R)-(2,6-dichloro-purin-9-yl)-[1,3]-dioxolan-2(R)-yl methyl
ester (7a).
4-Methoxybenzoic acid 4(R)-(2,6-dichloro-purin-9-yl)-[1,3]-dioxolan-2(R)-yl
methyl ester (7b). A suspension of 2,6-dichloropurine (1.27 g, 6.73 mmol), am-
monium sulfate (38 mg, 0.29 mmol) and 1,1,1,3,3,3-hexamethyldisilazane (8.45 g,
11.04 mL, 52.3 mmol) was heated at reflux for 2.5 h. The resulting solution was cooled

1884
Sznaidman et al.
to ambient temperature whereby a thick solid precipitated. The solids were redissolved
through the addition of dry dichloromethane (14.9 mL). The solution was then cooled
to À10°C, and a solution of the corresponding acetate (5a or 5b, 8.6 mmol) in dry
dichloromethane (10 mL) was slowly added over a period of 20 minutes, maintaining
the temperature between À10°C and À5°C. Then, TMSOTf (2.5 g, 2.08 mL, 10.4 mmol)
was slowly added over a period of 20 minutes. The reaction was left overnight with
stirring at room temperature. The solution was diluted with dichloromethane (120 mL)
and quenched with water (150 mL). The organic layer was separated and successively
washed with water (150 mL), (c) NaHCO3 (2 Â 150 mL), water (2 Â 150 mL), dried,
filtered and evaporated to a syrup (crude 6a) or a yellow solid (crude 6b).
Crude 6a was further purified by column chromatography (Hexane:AcOEt 4:1) to
1
afford 6a as a 1.2:1 b:a mixture of anomers, according to H-NMR. The mixture was
slowly crystallized from MeOH (10 mL) to afford isobutyric acid 4(R)-(2,6-dichloro-
purin-9-yl)-[1,3]-dioxolan-2(R)-yl methyl ester (7a, 24%) as a white solid and
20
characterized as the b anomer; mp: 145–147°C; [a]D = À47.67° (c 0.25, MeOH);
1H-NMR (CDCl3) d: 8.52 (1H, s, H-8), 6.55 (1H, d, J = 4.9, H-4), 5.34 (1H, t, J = 5.7,
H-2), 4.58–4.30 (4H, m, –CH2COO–, H-5 and H-5’), 2.61 (1H, m, (CH3)2CHCOO–),
1.19 (3H, d, J = 6.8, (CH3)2CHCOO–), 1.14 (3H, d, J = 6.8, (CH3)2CHCOO–).
Calculated for C13H14Cl2N4O4: C, 43.23; H, 3.91; N, 15.51. Found: C, 43.39; H,
3.91; N, 15.58.
Crude 6b was first washed with hot hexane to afford 4-methoxy-benzoic acid 4-
(2,6-dichloro-purin-9-yl)-[1,3]-dioxolan-2(R)-yl methyl ester (6b) as a yellow solid.
1H-NMR indicated a 2:1 mixture of b:a anomers. This mixture was slowly crystallized
from MeOH (100 mL) to afford 4-methoxybenzoic acid 4(R)-(2,6-dichloro-purin-9-yl)-
[1,3]-dioxolan-2(R)-yl methyl ester (7b, 46%) as a yellow solid and characterized as
20
the b anomer; mp: 154–156°C; HPLC (Method B, Rt = 20.73 min.); [a]D = À53.80°
1
(c 0.25, MeOH); H-NMR (Cl3CD) d: 8.41 (1H, s, H-8), 7.91 (2H, d, J = 8.5, ArH),
6.92 (2H, d, J = 8.5, ArH), 6.53 (1H, d, J = 4.8, H-4), 5.44 (1H, bs, H-2), 4.70–4.30
(4H, m, H-5, H-5’ and –CH2OCO–), 3.85 (3H, s, OCH3).
Calculated mass for C17H15Cl2N4O5 (M + 1)+: 425.0419. Found (H.R.F.A.B.M.S.):
425.0420.
Isobutyric acid 4(R)-(2,6-diazido-purin-9-yl)-[1,3]-dioxolan-2(R)-yl methyl es-
ter (8a).
4-Methoxybenzoic acid 4(R)-(2,6-diazido-purin-9-yl)-[1,3]-dioxolan-2(R)-yl
methyl ester (8b). To a well-stirred solution of the corresponding dichloropurine
nucleoside (7a or 7b, 2.9 mmol) in dry DMF (13.5 mL) was added NaN3 (390 mg,
6.0 mmol). The reaction mixture was left at room temperature with stirring for 4 h. The
mixture was filtered through celite to afford a solution of:
Isobutyric acid 4(R)-(2,6-diazido-purin-9-yl)-[1,3]-dioxolan-2(R)-yl methyl ester
(8a) or 4-methoxybenzoic acid 4(R)-(2,6-diazido-purin-9-yl)-[1,3]-dioxolan-2(R)-yl
methyl ester (8b) in DMF (70 mL) which was used in the next step without any
further purification.
Isobutyric acid 4(R)-(2,6-diamino-purin-9-yl)-[1,3]-dioxolan-2(R)-yl methyl
ester (9a).

Synthesis of (À)-DAPD
1885
4-Methoxybenzoic acid 4(R)-(2,6-diamino-purin-9-yl)-[1,3]-dioxolan-2(R)-yl
methyl ester (9b). The solution from the previous reaction (8a or 8b) was
hydrogenated (Parr apparatus, 50 psi) at room temperature, in the presence of 10%
Pd/C (200 mg) overnight. The mixture was filtered through Celite and the filter cake
washed with additional DMF.
In the case of 8a, the solution was concentrated to dryness and purified by column
chromatography (Cl3CH:MeOH 9:1) to afford a white solid that was crystallized from
iso-propanol to afford isobutyric acid 4(R)-(2,6-diamino-purin-9-yl)-[1,3]-dioxolan-
2(R)-yl methyl ester (9a, 84% after correction for the presence of one molecule of
20
2-propanol that co-crystallizes) as a white solid; mp: 143–145°C; [a]D = À56.45°
(c 0.25, MeOH); 1H-NMR (CDCl3) d: 7.84 (1H, s, H-8), 6.33 (1H, dd, J = 1 and J = 5.1,
H-4), 5.29 (3H, t, J = 3.2, H-2 and NH2), 4.70 (2H, bs, NH2), 4.50 (1H, dd, J = 1 and
J = 9.0, H-5), 4.33 (2H, d, J = 3.2, –CH2COO–), 4.23 (1H, dd, J = 5.1 and J = 9.0, H-5’),
2.60 (1H, m, (CH3)2CHCOO–), 1.18 (3H, d, J = 6.2, (CH3)2CHCOO–), 1.14 (3H, d,
J = 7.2, (CH3)2CHCOO–).
Calculated for C13H18N6O4 Á 2-propanol: C, 50.25; H, 6.85; N, 21.98. Found: C,
50.16; H, 6.84; N, 21.92.
In the case of 8b, the solution was concentrated to a final volume of 15 mL in
vacuo at 60°C. The solution was diluted with water (150 mL) and after a few minutes a
solid precipitated. The product was filtered, washed with water, dried overnight at 50°C
under vacuum to afford 4-methoxybenzoic acid 4(R)-(2,6-diamino-purin-9-yl)-[1,3]-
dioxolan-2(R)-yl methyl ester (9b, 88%) as an amorphous solid; HPLC (Method B,
20
1
Rt = 15.41 min.); [a]D = À95.75° (c 0.25, MeOH); H-NMR (DMSO-d6) d: 7.82 (2H,
d, J = 8.8, ArH), 7.80 (1H, s, H-8), 7.02 (2H, d, J = 8.8, ArH), 6.77 (2H, bs, NH2),
6.23 (1H, dd, J = 5.5 and J = 1.6, H-4), 5.85 (2H, bs, NH2), 5.36 (1H, t, J = 3.3, H-2),
4.65 (1H, dd, J = 9.4 and J = 1.6, H-5), 4.46 (2H, d, J = 3.4, –CH2OCO–), 4.26 (1H,
dd, J = 9.4 and J = 5.5, H-5’), 3.84 (3H, s, OCH3).
Calculated mass for C17H19N6O5: 387.1417. Found (H.R.F.A.B.M.S.): 387.1417.
4(R)-[-(2,6-Diamino-purin-9-yl)-[1,3]-dioxolan-2(R)-yl]-methanol, [10, (À)-
DAPD] from 9a. A solution of isobutyric acid 4(R)-(2,6-diamino-purin-9-yl)-[1,3]-
dioxolan-2(R)-yl methyl ester (9a, 100 mg, 0.31 mmol) and n-butylamine (0.45 g,
0.62 mL, 6.2 mmol) in MeOH (10 mL) was heated at reflux for 4 h. The reaction was
cooled to ambient temperature and concentrated in vacuo to afford a solid that was
triturated with tert-butyl methyl ether and filtered to afford a solid that was crystallized
from EtOH:water (1:4.5 mL) to afford (À)-DAPD (10, 75 mg, 0.29 mmol, 93%) as a
white solid; mp: 237–239°C (lit.[1] mp: 236–237°C); HPLC (Method C, Rt = 8.7 min.,
an authentic sample of (À)-DAPD showed Rt = 8.7 min. and a sample of (+) DAPD
1
showed Rt = 6.0 min.); DAPD; H-NMR (DMSO-d6) d: 7.80 (1H, s, H-8), 6.74 (2H,
bs, NH2), 6.20 (1H, d, J = 5.5, H-4), 5.84 (2H, bs, NH2), 5.16 (1H, t, J = 6.3, OH), 5.03
(1H, t, J = 2.9, H-2), 4.42 (1H, d, J = 9.7, H-5), 4.18 (1H, dd, J = 9.7 and J = 5.5,
H-5’), 3.58 (2H, dd, J = 6.3 and J = 2.9, –CH2OH).
Calculated for C9H12N6O3: C, 42.86; H, 4.80; N, 33.32. Found: C, 42.88; H, 4.79;
N, 33.32.
4(R)-[-(2,6-Diamino-purin-9-yl)-[1,3]-dioxolan-2(R)-yl]-methanol, [10, (À)-
DAPD] from 9b. A suspension of 4-methoxybenzoic acid 4(R)-(2,6-diamino-purin-

1886
Sznaidman et al.
9-yl)-[1,3]-dioxolan-2(R)-yl methyl ester (9b, 1.0 g, 2.6 mmol) and n-butylamine (3.7 g,
5.0 mL, 50 mmol) in MeOH (25 mL) was heated at reflux for 4 hours. The reaction
was concentrated in vacuo to afford a yellow solid that was suspended in Cl3CH
at room temperature and filtered, to afford (À) DAPD (10, 590 mg, 2.3 mmol, 90%)
as a white solid. Physical properties were identical to the sample of (À)-DAPD ob-
tained above.
CONCLUSIONS
A new eight step scalable synthesis of (À)-DAPD was achieved in good overall
yield, using inexpensive reagents and starting materials.
REFERENCES
1. Furman, P.A.; Cleary, D.; Trost, L.C.; Bigley, J.W.; Painter, G.R. DAPD. Drugs
Future 2000, 25 (5), 454–461.
2. Belleau, B.; Dixit, D.; Nguyen, B.N. Preparation of Substituted 1,3-Oxathiolane
Nucleoside Analogs with Antiviral Properties. U.S. Patent No. 5,041,449, 1995.
3. Belleau, B.; Dixit, D.; Nguyen-Ba, N.; Kraus, J.L. Abstract of Papers, Fifth
International Conference on AIDS, Montreal; International Development Research
Centre: Ottawa, Ontario, 1989, T.C.O.1.
4. Norbeck, D.W.; Spanton, S.; Broder, S.; Mitsuya, H. ( )-Dioxolane-T (( )-1-[(2,4)-
2-(hydroxymethyl)-4-dioxolanyl]thymine): a new 2’, 3’-dideoxynucleoside proto-
type with in vitro activity against HIV. Tetrahedron Lett. 1989, 30 (46), 6263–
6266.
5. Choi, W.B.; Wilson, L.J.; Yeola, S.; Liotta, D.C.; Schinazi, R.F. In situ
complexation directs the stereochemistry of N-glycosylation in the synthesis of
thialanyl and dioxolanyl nucleoside analogs. J. Am. Chem. Soc. 1991, 113 (24),
9377–9379.
6. Wilson, L.J.; Choi, W.B.; Spurling, T.; Liotta, D.C.; Schinazi, R.F.; Cannon, D.;
Painter, G.R.; St. Clair, M.; Furman, P.A. The synthesis and anti-HIV activity of
pyrimidine dioxolanyl nucleosides. Bioorg. Med. Chem. Lett. 1993, 3 (2), 169–
174.
7. Kim, H.O.; Schinazi, R.F.; Nampalli, S.; Shanmuganathan, K.; Cannon, D.L.;
Alves, A.J.; Jeong, L.S.; Beach, J.W.; Chu, C.K. 1,3-Dioxolanylpurine nucleosides
(2R, 4R) and (2R, 4S) with selective anti-HIV-1 activity in human lymphocytes.
J. Med. Chem. 1993, 36 (1), 30–37.
8. Kim, H.O.; Ahn, S.K.; Alves, A.J.; Beach, J.W.; Jeong, L.S.; Choi, B.G.; Van Roey,
P.; Schinazi, R.F.; Chu, C.K. Asymmetric synthesis of 1,3-dioxolane-pyrimidine
nucleosides and their anti-HIV activity. J. Med. Chem. 1992, 35, 1987–1995.
9. Belleau, B.R.; Evans, C.A.; Tse, H.L.A.; Jin, H.; Dixit, D.M.; Mansour, T.S.
Oxidative degradation of L-ascorbic acid acetals to 2’, 3’-dideoxy-3’-oxaribofurano-
sides. Synthesis of enantiomerically pure 2’, 3’-dideoxy-3’-oxacytidine stereo-
isomers as potential antiviral agents. Tetrahedron Lett. 1992, 33 (46), 6949–6952.

Synthesis of (À)-DAPD
1887
10. Liotta, D.C.; Schinazi, R.F. Preparation of Antiviral 1,3-Dioxolane Nucleoside
Analogs. WO 92/14729, 1998.
11. Jin, H.; Tse, H.L.A.; Evans, C.A.; Mansour, T.S.; Beels, C.M.; Ravenscroft, P.;
Humber, D.C.; Jones, M.F.; Payne, J.J.; Ramsey, M.V.J. Unexpected effects of
Lewis acids in the synthesis of optically pure 2’-deoxy-3’-oxacytidine nucleoside
analogues. Tetrahedron: Asymmetry 1993, 4 (2), 211–214.
12. Chu, C.C.; Ahn, S.K.; Kim, H.O.; Chu, C.K.; Ahn, S.K.; Kim, H.O.; Beach, J.W.;
Alves, A.J.; Jeong, L.S.; Islam, Q.; Roey, P.V.; Schinazi, R.F. Asymmetric
synthesis of enantiomerically pure (À)-(1’R, 4’R)-dioxolane-thymine and its anti-
HIV activity. Tetrahedron Lett. 1991, 32 (31), 3791–3794.
13. Siddiqui, M.A.; Brown, W.L.; Nguyen-Ba, N.; Dixit, D.M.; Mansour, T.S. Antiviral
optically pure dioxolane purine nucleosides analogues. Bioorg. Med. Chem. Lett.
1993, 3 (8), 1543–1546.
14. Kim, H.O.; Smanmuganathan, K.; Alves, A.J.; Jeong, L.S.; Beach, J.W.; Schinazi,
R.F.; Chang, C.N.; Cheng, Y.C.; Chu, C.K. Potent anti-HIV and anti-HBV activities
of (À)-L-dioxolane-C and (+)-L-dioxolane-T and their asymmetric syntheses.
Tetrahedron Lett. 1992, 33 (46), 6899–6902.
Received April 19, 2004
Accepted August 25, 2004