Synthesis of ethyl (1S,2R,3S,4S,5S)-2,3-O-(isopropylidene)-4-hydroxy- bicyclo[3.1.0]hexane-carboxylate from l-ribose: A versatile chiral synthon for preparation of adenosine and P2 receptor ligands

Article abstract of DOI:10.1080/15257770701845253

Substitution of the ribose moiety of various nucleosides and nucleotides with the (N)-methanocarba ring system increases the potency and selectivity as ligands at certain subtypes of adenosine and P2 receptors. We have prepared a key intermediate in the synthesis of these derivatives, ethyl (1S,2R,3S,4S,5S)-2,3-O-(isopropylidene)-4-hydroxybicyclo[3.1.0] hexane-carboxylate (15), starting from L-ribose (8) as a readily available, enantiopure building block. L-ribose was converted to the corresponding 5′-iodo derivative (9), which was cleaved reductively with Zn. Improvements were made in subsequent steps corresponding to a published route to biologically important (N)-methanocarba 5′-uronamido nucleosides, and new steps were added to prepare related 5′-nucleotides. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15257770701845253

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Synthesis of Ethyl (1S,2R,3S,4S,5S)-2,3-
O-(Isopropylidene)-4-Hydroxy-
Bicyclo[3.1.0]Hexane-Carboxylate from
L-Ribose: A Versatile Chiral Synthon
for Preparation of Adenosine and P2
Receptor Ligands
a
a
b
Bhalchandra V. Joshi , Artem Melman , Richard L. Mackman
&
a
Kenneth A. Jacobson
a
Molecular Recognition Section, Laboratory of Bioorganic Chemistry,
National Institute of Diabetes and Digestive and Kidney Diseases,
National Institutes of Health, Bethesda, Maryland, USA
b
Gilead Sciences, Inc., Foster City, California, USA
Version of record first published: 14 Feb 2008.
To cite this article: Bhalchandra V. Joshi , Artem Melman , Richard L. Mackman & Kenneth
A. Jacobson (2008): Synthesis of Ethyl (1S,2R,3S,4S,5S)-2,3-O-(Isopropylidene)-4-Hydroxy-
Bicyclo[3.1.0]Hexane-Carboxylate from L-Ribose: A Versatile Chiral Synthon for Preparation of
Adenosine and P2 Receptor Ligands, Nucleosides, Nucleotides and Nucleic Acids, 27:3, 279-291
To link to this article: http://dx.doi.org/10.1080/15257770701845253
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Nucleosides, Nucleotides, and Nucleic Acids, 27:279–291, 2008
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1525-7770 print / 1532-2335 online
DOI: 10.1080/15257770701845253
SYNTHESIS OF ETHYL (1S,2R,3S,4S,5S)-2,3-O-(ISOPROPYLIDENE)-4-
HYDROXY-BICYCLO[3.1.0]HEXANE-CARBOXYLATE FROM L-RIBOSE:
A VERSATILE CHIRAL SYNTHON FOR PREPARATION OF ADENOSINE
AND P2 RECEPTOR LIGANDS
1
1
2
Bhalchandra V. Joshi, Artem Melman, Richard L. Mackman, and
Kenneth A. Jacobson1
1Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of
Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda,
Maryland, USA
2
Gilead Sciences, Inc., Foster City, California, USA
2
Substitution of the ribose moiety of various nucleosides and nucleotides with the (N)-
methanocarba ring system increases the potency and selectivity as ligands at certain subtypes
of adenosine and P2 receptors. We have prepared a key intermediate in the synthesis of
these derivatives, ethyl (1S,2R,3S,4S,5S)-2,3-O-(isopropylidene)-4-hydroxybicyclo[3.1.0]hexane-
carboxylate (15), starting from L-ribose (8) as a readily available, enantiopure building block.
ꢁ
L-ribose was converted to the corresponding 5 -iodo derivative (9), which was cleaved reductively
with Zn. Improvements were made in subsequent steps corresponding to a published route to
ꢁ
biologically important (N)-methanocarba 5 -uronamido nucleosides, and new steps were added to
ꢁ
prepare related 5 -nucleotides.
Keywords G protein–coupled receptor; Nucleosides; Nucleotides; Purines; Pyrim-
idines; Iodination
INTRODUCTION
In solution, the conformation of the ribose ring of nucleosides and
nucleotides exists in a rapid, dynamic two-state equilibrium between a (N)
Received 18 June 2007; accepted 13 November 2007.
Mass spectral measurements were carried out by Dr. John Lloyd and NMR measurements by Wesley
White (NIDDK). This research was supported in part by the Intramural Research Program of the NIH,
National Institute of Diabetes and Digestive and Kidney Diseases. BVJ thanks Gilead Sciences (Foster
City, CA) for research support.
Present address for B. V. J.: Nektar Therapeutics, Huntsvile, Alabama, USA.
Address correspondence to Kenneth A. Jacobson, Molecular Recognition Section, Bldg. 8A, Rm.
B1A-19, NIH, NIDDK, LBC, Bethesda, MD 20892-0810. E-mail: kajacobs@helix.nih.gov
2
79

2
80
B. V. Joshi et al.
CHART 1 Structures of biologically-important (N)-methanocarba nucleoside derivatives 1a, b that act
as adenosine A3 receptor ligands and nucleotides 2, 3 that act as P2 receptor ligands.
ꢁ
ꢁ
[1,2]
(
Northern, 2 -exo) or (S) (Southern, 2 -endo) pucker conformation.
It
is expected that the target receptors for these small molecules may prefer
one of these conformations. It follows logically that when the nucleoside
furanose ring is locked in either a (N) or (S) conformation an enhancement
of potency and selectivity towards the target receptors may occur. Re-
cently, we have successfully executed this approach by replacing the ribose
moiety with the (N)-methanocarba ring system in nucleosides/nucleotides
(Chart 1), and demonstrated enhanced selectivity and potency for therapeu-
tically important A adenosine receptors (ARs, e.g., 1) and P2 nucleotide
3
[
3–6]
receptors (e.g., 2, 3).
Both purine and pyrimidine nucleotides have been synthesized bearing
[
7]
the (N)-methanocarba modification of ribose. Our previously reported
synthetic route for the above (N)-methanocarba adenine nucleosides is
depicted in Scheme 1.[ By following this synthetic approach and by
3]
employing the methanocarba sugar moiety as key synthon we have re-
ꢁ
cently reported a highly A AR selective series of (N)-methanocarba 5 -N -
3
methyluronamide derivatives, prominent among them were MRS3558 1a
SCHEME 1 Previously reported initial steps in the synthesis of (N)-methanocarba nucleoside and
nucleotide derivatives 1–3.

Chiral Synthon for Receptor Ligands from L-Ribose
281
and MRS73602 1b.[4] In particular, MRS3558 has a K value in binding to
i
the human A AR of 0.3 nM with at least 900-fold selectivity for the A AR in
3
3
comparison to other subtypes, that is, A A , and A ARs. Recently, Matot
1,
2A
2B
and coworkers have shown that MRS3558 is effective in attenuating reperfu-
sion lung injury in cats.[ This result suggests that A AR agonists have po-
8]
3
tential as an effective therapy for ischemia/reperfusion-induced lung injury.
In addition, Fishman and coworkers have studied extensively A AR agonists
3
[
9]
as potential inhibitors in a model of adjuvant-induced arthritis in rats.
P2X and P2Y receptors are effectively targeted using related nucleotide
[
10,11]
derivatives, for example, MRS2339 2 and MRS2365 3, respectively.
Clearly, the potential therapeutic use of (N)-methanocarba nucleoside
and nucleotides that act as receptor ligands demands their easy access in
multigram amounts for further biological and clinical studies.
We previously reported a new synthetic route for the above
(
N)-methanocarba nucleosides starting from 2,3-O-isopropylidene-D-
[
3]
erythronolactone 4 as depicted in Scheme 1.
However, this scheme
includes three low temperature steps, an inconsistent, low-yielding Wittig
reaction to provide 6 and a subsequent tedious Swern oxidation step to
provide 7. The low and inconsistent yields in the Wittig reaction on large
scale may be attributed to the quality of potassium t-butoxide and also
an excess of Wittig salt in the reaction. In addition, the starting material
2, 3-O-isopropylidene-D-erythronolactone 4 is expensive and not readily
available. In this communication we delineate a modified procedure for
preparing (N)-methanocarba nucleosides, in particular 1a, on a reasonably
large scale. Toward this endeavor we evaluated L-ribose as a less expensive
and readily available starting material, using reactions similar to those
previously reported using D-ribose.[
12,13]
Synthesis of 7 from L-ribose
avoided problems of the Wittig reaction and Swern oxidation encountered
in the previous route. Improvements also were made in steps subsequent
to 7 corresponding to our published route to biologically important
(N)-methanocarba nucleosides and nucleotides.
METHODS
Chemical Synthesis
1
H NMR spectra (300 MHz) were obtained with a Varian Gemini
31
3
00 spectrometer (Varian, Inc., Palo Alto, CA, USA). P NMR spectra
were recorded at room temperature with a Varian XL 300 spectrometer
121.42 MHz); orthophosphoric acid (85%) was used as an external stan-
(
dard. Purity of compounds was checked using a Hewlett-Packard 1100 HPLC
equipped with a Luna 5 µm RP-C18(2) analytical column (250 mm ×
4
.6 mm; Phenomenex, Torrance, CA, USA). System A parameters consisted
of the following: linear gradient solvent system of H O/CH CN from 95/5
2
3

2
82
B. V. Joshi et al.
to 20/80 in 20 minutes; flow rate of 1 mL/minute. System B parameters
consisted of the following: linear gradient solvent system of 5 mm
TBAP/CH CN from 80/20 to 20/80 in 20 minutes followed by isocratic for
3
2
minutes; flow rate of 1 mL/minute. Peaks were detected by UV absorption
with a diode array detector.
Materials
All reagents were from standard commercial sources and of analytical
grade.L-ribose was purchased from V-Labs, Inc. (Covington, LA, USA).
Ethyl (4S,5S)-3-[2,2-dimethyl-5-vinyl(1,3-dioxolan-4-yl)]-3-oxopropanoate (10).
Concentrated HCl (5 mL) was added to a suspension of L-ribose 8 (150 g,
1
mol) in acetone (400 mL) and methanol (400 mL) at room temperature.
The resulting reaction mixture was refluxed for 3 hours and cooled to
room temperature. It was then neutralized with pyridine and concentrated
in vacuo. The residue was partitioned between water (300 mL) and ethyl
acetate (300 mL three times). The combined organic phase was washed with
a saturated copper sulfate solution followed by brine, dried over sodium
sulfate, and concentrated. The crude residue was used in the next step
without further treatment.
The above crude methyl ethers (164 g) were dissolved in toluene
(750 mL) and the solution was treated with imidazole (85 g, 1.25 mol),
triphenylphosphine (243 g, 0.93 mol), and acetonitrile (200 mL). The
◦
resulting reaction mixture was heated to 70 C, and iodine (253 g, 1 mol)
was added carefully in small portions till the reaction mixture maintained
a dark brown appearance. The reaction temperature was held constant at
◦
7
0 C for 1 hour. The reaction mixture was filtered, and the residue was
washed with toluene (500 mL). The combined organic layer was washed with
% sodium thiosulfate solution, water, and brine and then concentrated.
5
The crude residue was first treated with a mixture of hexanes and ethyl
acetate (3:1). Solid triphenylphosphine oxide precipitated, and the mixture
was filtered and the filtrate concentrated. This process was repeated to
remove most of the triphenylphosphine oxide. A further purification of
the concentrated filtrate by flash column chromatography (silica gel, ethyl
acetate:hexanes, 1:10) afforded the iodo derivative 9 as the mixture of
anomers (216 g, colorless oil, 69% in two steps). αD25 + 3.1 (c = 2.7,
EtOH).
◦
Powdered Zn (9.75 g, 150 mmol) was added to the solution of the
iodides 9 (31.4 g, 100 mmol) in isopropanol (100 mL). To this resulting
reaction slurry, acetic acid (10.5 g, 175 mmol) was slowly added in 30 min-
utes (cold water bath was used to control the reaction temperature), and
◦
the reaction mixture was maintained at 30 C and stirred for an additional
2
hours. The solution was decanted and concentrated under reduced

Chiral Synthon for Receptor Ligands from L-Ribose
283
pressure. The residue obtained was partitioned with ethyl acetate (200 mL),
and water (100 mL) and stirred for 30 minutes. The organic layer was
separated, and the aqueous layer was extracted with ethyl acetate (100 mL).
The combined organic layer was washed with brine, dried and concentrated
to afford crude aldehyde 7 as a colorless oil (12.8 g), which was carried
forward for the next step.
Crude aldehyde 7 in dichloromethane (25 mL) was treated with anhy-
drous SnCl (1.9 g, 10 mmol) and ethyldiazoacetate (9.3 g, 82 mmol) in
2
dichloromethane (80 mL) was added drop wise over a period of 1 hour at
◦
0
C. The mixture was then allowed to stir at room temperature for 1 hour.
The reaction mixture was filtered through a pad of Celite and concentrated.
The desired keto ester was purified by column chromatography (silica gel,
hexanes: ethyl acetate, 95:5) to afford 10, (16.3 g, 68% based on the iodide
1
derivative) as an oil. H NMR (CDCl ) δ 11.92 (s, 0.1H, D O exchangeable,
3
2
enolic OH), 5.93–6.08 (m, 1H), 5.39–5.56 (m, 2H), 4.46–4.72 (m, 1H),
4
1
.13–4.38 (m, 3H), 3.65 (AB q, 2H, J = 17.4 Hz), 1.64–1.57 (m, 6H, 2),
−
.34–1.41 (m, 3H); HRMS (M − 1) : calculated 241.1076, found 241.1084.
Ethyl (4S,5S)-3-[2,2-dimethyl-5-vinyl(1,3-dioxolan-4-yl)]-2-diazo-3-oxopropano-
ate (11). Neat triethylamine (28.3 g, 280 mmol) was added dropwise to
a solution of keto ester 10 (34.0 g, 140 mmol) and tosyl azide (27.6 g,
◦
1
40 mmol) in acetonitrile (150 mL) at 0 C. The reaction mixture was stirred
◦
for 30 minutes at 0 C and then a further 30 minutes at room temperature
followed by evaporation. The residue was dissolved in chloroform (100 ml)
and the resulting precipitate was removed by filtration. The filtrate was
diluted with hexane (100 mL), filtered again, and evaporated. The residue
was purified by column chromatography (silica gel, 5% to 10% ethyl acetate-
hexane) to afford the title compound as pale yellow oil (36.0 g, 133 mmol,
1
9
5%). H NMR (CDCl ) δ 6.01 (ddd, 1H, J = 16.3, 10.4, 7.4 Hz), 5.70 (d, 1H,
3
J = 7.6 Hz), 5.60 (d, 1H, J = 17.1 Hz), 5.41 (d, 1H, J = 10.2 Hz), 4.97 (t, 1H,
J = 7.5 Hz), 4.30 (q, 2H, J = 7.1 Hz), 1.65(s, 3H), 1.42 (s, 3H), 1.32 (t, 3H,
+
25
◦
J = 7.1 Hz); FAB MS m/z (relative intensity) 269 (MH , 86). α
+ 6.1
D
(
c = 1.1).
Ethyl (1S,3S,4S,5S)-3,4-O-isopropylidene-2-oxobicyclo[3.1.0]hexanecarboxylate
12). A solution of diazo compound 11 (26.9 g, 100 mmol) in dry toluene
100 mL) was added through a dropping funnel over a period of 4 hours
(
(
to a refluxing mixture of toluene (25 mL) and CuI (3.8 g, 20 mmol).
The reaction mixture was refluxed for 1 hour. The reaction mixture
was cooled to room temperature, concentrated and purified by column
chromatography (silica gel; hexanes:ethyl acetate, 70:30) to provide bicylic
compound 12 as a colorless oil (12.24 g, 51%) and compound 13 as a pale
yellow oil (3.36 g, 14%).
1
1
2: H NMR (CDCl ) δ 5.13 (ddd, 1H, J = 9.3, 5.6, 1.2 Hz), 4.37 (d,
3
1
H, J = 8.3 Hz), 4.21 (q, 2H, J = 7.3 Hz), 2.75 (dt, 1H, J = 8.7, 5.4 Hz),

2
84
B. V. Joshi et al.
2
3
2
.10 (dd, 1H, J = 8.6, 5.1 Hz), 1.81(t, 1H, J ∼ 5.6 Hz), 1.52 (s, 3H), 1.31(s,
+
H), 1.29 (t, 3H, J = 7.1 Hz); HRMS (M + 1) : calculated 241.1076, found
2
5
◦
1
41.1092; αD + 9.5 (c = 1.1, EtOH). 13: H NMR (CDCl ) δ 4.87 (d, 1H, J
3
=
5.1 Hz), 4.48 (dd, 1H, J = 4.6, 1.6 Hz), 4.31 (dq, 2H, J = 7.1, 1.5 Hz), 2.92
(
dd, 1H, J = 8.7, 5.8 Hz), 2.31 (ddd, 1H, J = 8.8, 5.6, 1.7 Hz), 1.56 (s, 3H),
1
.41 (s, 3H), 1.32 (t, 3H, J = 4.1 Hz), 182 (t, 1H, J = 5.1 Hz); HRMS (M +
+
25
◦
1) : calculated 241.1076, found 241.1082; α
+ 4.9 (c = 0.65, EtOH).
D
Ethyl (1S,2R,3S,4S,5S)-3,4-O-isopropylidene-2-hydroxybicyclo[3.1.0]hexane-
carboxylate (14). NaBH (0.38 g, 10 mmol) was added to a stirred solution
4
of 12 (2.4 g, 10 mmol) in ethanol (30 mL) at room temperature, and
stirring was continued for an additional 1 hour. The reaction mixture
was then treated with acetone (4 mL) and concentrated to dryness. The
residue was purified by column chromatography (silica gel; hexanes:ethyl
acetate, 70:30) to give compound 14 (1.68 g, 69%) as a white solid. 14:
◦
1
m.p. 111 C (cyclohexane); H NMR (CDCl ) δ 5.05 (t, 1H, J = 7.5 Hz),
3
4
1
1
2
.91 (t, 1H, J = 6 Hz), 4.66 (t, 1H, J = 7 Hz), 4.14–4.30, (m, 2H), 2.46 (d,
H, J = 12 Hz, OH), 2.16–2.29(m, 1H), 1.38–1.54(m, 5H), 1.31 (s, 3H),
+
.24 (t, 3H, J = 6.5 Hz). HRMS (M + 1) : calculated 243.1232, found
43.1233.
Ethyl (1S,2R,3S,4S,5S)-2,3-O-(isopropylidene)-4-hydroxybicyclo[3.1.0]hexane-
carboxylate (15). A solution of 14 (0.96 g, 4.0 mmol) and trifluoroacetic
◦
acid (0.68 g, 6 mmol) in acetone (20 mL) was stirred at 25 C for 6 hours.
The reaction mixture was then concentrated under reduced pressure. The
residue was dissolved in CHCl (50 mL) and washed with 10% sodium
3
bicarbonate solution. The organic layer was concentrated to furnish a
mixture of isomerized alcohols 14 and 15 in a 6:4 ratio based on NMR.
This crude mixture was further purified by crystallization from cyclohexane
(20 mL), in which alcohol 14 separates out as crystals. The mother liquor
was concentrated to afford the requisite 15 (0.340 g, 35%) as a colorless
solid with purity >80%. The analytical sample of alcohol 15 can be obtained
◦
by repeated recrystallization from cyclohexane. 15: m.p. 83 C (cyclohex-
1
ane). H NMR (CDCl ) δ 5.35 (d, 1H, J = 5.8 Hz), 4.38–4.56 (m, 2H),
3
4
1
2
.04–4.19 (m, 2H), 2.36–2.41 (m, 2H), 1.41–1.62 (m, 5H), 1.33 (s, 3H),
+
.13 (t, 2H, J = 3.2 Hz); HRMS (M + 1) : calculated 243.1232, found
41.1245.
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
Ethyl (1 S,2 R,3 S,4 R,5 S)-4 -(2,6-dichloropurin-9-yl]-2 ,3 -O-(isopropylidene)-
bicyclo[3.1.0]hexanecarboxylate (16). A mixture of triphenyl phosphine
(1.04 g, 4.0 mmol) and 2,6-dichloropurine (0.756 g, 4.0 mmol) in dry THF
(5 mL) was treated with diisopropyl azodicarboxylate (0.80 g, 4.0 mmol)
at room temperature. After stirring for 20 minutes, a solution of the
methanocarba sugar 15 (0.48 g, 2.0 mmol) in THF (5 mL) was added
and the mixture was stirred an additional 8 hours. Concentration and
purification of the residue by repeated column chromatography (silica gel;

Chiral Synthon for Receptor Ligands from L-Ribose
285
ethyl acetate:hexanes, 3:1) provided provided pure 16 (0.346 g, 42%) as a
colorless amorphous solid.
1
1
6: H NMR (CDCl ) δ 8.06 (s, 1H), 5.81 (d, 1H, 6.7 Hz), 4.92 (s, 1H),
3
4
1
4
.68 (d, 1H, J = 5.5 Hz), 4.05–4.36 (m, 2H), 2.14–2.2 (m, 1H), 1.75–1.82 (m,
+
H), 1.52–1.62 (m, 4H), 1.11–1.35 (m, 6H); HRMS (M + 1) : calculated
2
5
◦
13.0783, found 413.0780; α
+ 2.6 (c = 1.4, EtOH).
D
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(
1 S,2 R,3 S,4 S,5 S)-4 -[6-(3-Chlorobenzylamino)-2-chloropurin-9-yl]-2 ,3 -O-
ꢁ
isopropylidenebicyclo[3.1.0]hexane-1 -carboxylic Acid Ethyl Ester (17). 3-Chloro-
benzylamine (0.705 g, 5.0 mmol) was added to a solution of 16 (0.413 g,
1
.0 mmol) and triethylamine (2 mL) in methanol (5 mL). The mixture was
stirred at room temperature for 3 hours, and then concentrated in vacuo
to dryness. The residue was purified by column chromatography (silica
gel; chloroform:methanol, 10:1) to give 17 as a pale yellow oil (0.316 g,
1
6
1%). H NMR (CDCl ) δ 1.18–1.37 (m, 3H), 1.41–1.85 (m, 8H), 2.18–2.29
3
(
5
m, 1H), 4.05–4.41 (m, 2H), 4.78–4.87 (m, 3H), 5.39 (d, J = 4.8 Hz, 1H),
.82 (d, J = 7.6 Hz, 1H), 6.24 (br s, 1H), 7.21–7.42 (m, 4H), 7.68 (s, 1H).
+
25
◦
HRMS (M + 1) : calculated 518.1362, found 518.1357; α
+ 3.0 (c =
D
0
.4, EtOH).
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(
1 S,2 R,3 S,4 S,5 S)-4 -[6-(3-Chlorobenzylamino)-2-chloropurin-9-yl]-2 ,3 -di-
ꢁ
hydroxybicyclo[3.1.0]hexane-1 -carboxylic Acid Methylamide (1a). Compound
17 (0.518 g, 1 mmol) was dissolved in methanol (5 mL) and treated with
an aqueous solution of methylamine (2 mL, 40%). This mixture was stirred
at room temperature for 12 hours, then evaporated to dryness. This crude
intermediate was treated directly with a solution of trifluoroacetic acid in
MeOH (15 mL, 10%) and H O (0.5 mL), and the mixture was heated at
2
◦
7
0 C for 3 hours. The solution was cooled, and the solvent was removed to
dryness by coevaporation with acetonitrile in vacuo. The white residue was
purified by flash column chromatography (silica gel, chloroform:methanol,
8
1
1
4
7
:2) to give the final product 1a as a white powder (0.250 g, 54%). M.p.
◦
1
22 C (decomp). H NMR (CDCl ) δ 1.18–1.39 (m, 1H), 1.46–1.63 (m,
3
H), 2.04–2.09 (m, 1H), 2.83 (d, J = 4.8 Hz, 1H), 3.87–4.08 (m, 2H),
.78–5.05 (m, 5H), 6.73 (br s, 1H), 7.04 (br s, 1H), 7.13–7.31 (m, 4H),
+
.75(s, 1H). HRMS (M + 1) : calculated 463.1052, found 463.1053. HPLC
system A) 14.8 minutes (99%), (system B) 12.5 minutes (99%). αD2 + 1.2
5
◦
(
(
c = 1.0, EtOH).
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(
1 S,2 R,3 S,4 R,5 S)-4-(2,6-dichloro-purin-9-yl)-1-[hydroxymethyl]bicyclo-
[
(
3.1.0]-hexane-2,3-(O-isopropylidine) (18). A stirred solution of ester 16
◦
0.207 g, 0.5 mmol) in CH Cl (10 mL) cooled to −78 C was treated
2
2
dropwise with DIBAL-H (1.5 mL, 1.5 M solution in toluene) and stirred at
that temperature for 1 hour. Methanol (1.5 mL) and 1 M cold sulfuric acid
(10 mL) were added carefully to adjust the reaction mixture to pH 4–5. The
reaction mixture was filtered through a pad of Celite, and the filtrate was
separated. The aqueous phase was extracted with dichloromethane, and the

2
86
B. V. Joshi et al.
combined organic layer was evaporated and concentrated under reduced
pressure. The residue was purified by flash column chromatography (silica
1
gel, CHCl :MeOH, 10:1) to give alcohol 18 (0.103 g, 56%) as a solid. H
3
NMR (CDCl ) δ 1.05–1.08 (m, 1H), 1.21–138 (m, 7H), 1.70–1.74 (m, 1H),
3
3
.51(d, J = 12 Hz, 1H), 4.21–4.27 (m, 1H), 4.63 (d, J = 7.5 Hz, 1H), 4.96
+
(
3
s, 1H), 5.51 (d, J = 12 Hz, 1H), 8.32 (s, 1H). HRMS (M + 1) : calculated
71.0678, found 371.0669.
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(
1 S,2 R,3 S,4 R,5 S)-4-(6-amino-2-chloro-purin-9-yl)-1-[hydroxymethyl]bicyclo-
3.1.0]hexane-2,3-(O-isopropylidine) (19). Compound 18 (0.186 g, 0.5 mmol)
was treated with 2 M ammonia in isopropyl alcohol (2 mL) and stirred for
hours. The resulting reaction mixture was concentrated and the residue
[
8
was purified by silica gel column chromatography (CHCl : MeOH, 80:20)
3
1
to afford 19 (0.08 g, 46%) as a colorless solid. H NMR (CDCl ) δ 7.80 (s,
3
1
H), 5.89 (bs, 2H), 5.56 (d, 1H, J = 6.4 Hz), 4.76 (s, 1H), 4.66 (d, 1H, J =
7
.4 Hz), 4.26 (d, 1H, J = 11.5 Hz), 3.37 (d, 1H, J = 11.5 Hz), 1.78–1.70
(m, 1H), 1.55 (s, 3H), 1.26 (s, 3H), 1.14–1.20 (m, 1H), 0.96–1.20 (m, 1H).
+
HRMS (M + 1) : calculated 352.1176, found 353.1163.
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(
1 S,2 R,3 S,4 R,5 S)-4-(6-Amino-2-chloro-9H-purin-9-yl)-1-[(di-tert-butyl
phosphate)methyl]bicyclo[3.1.0]hexane-2,3-(O-isopropylidene) (20). Neat di-t-
butyl N ,N -diethylphosphoramidite (0.075 g, 0.3 mmol) was added to a
stirred solution of 19 (0.035 g, 0.1 mmol) in anhydrous THF (2 mL) at room
temperature followed by addition of tetrazole (0.063 g, 0.9 mmol). After
◦
1
hour the reaction mixture was cooled to −78 C, and m-chloroperbenzoic
◦
acid (0.050 g, 77%) was added. The reaction mixture was warmed to 0 C
and stirred for 15 minutes and then treated with triethylamine (0.5 mL).
Purification was accomplished by preparative thin-layer chromatography
using 4:1 chloroform:methanol to give 20 as a solid (0.025 g, 45%). H
NMR (CDCl ): δ H NMR (CDCl ) δ 7.93 (s, 1H), 5.98 (bs, 2H), 5.35
1
1
3
3
(
d, 1H, J = 6.1 Hz), 4.94 (d, 1H, J = 5.7 Hz), 4.65 (d, 1H, J = 6.1 Hz),
4
1
5
.30–4.45 (m, 2H), 1.76–1.84 (m, 1H), 1.22–1.61 (m, 25H), 1.37–1.42 (m,
+
H), 1.13–1.19 (m, 1H). HRMS (M + 1 + Na) : calculated 566.1911, found
25
◦
66.1916. α
+ 0.6 (c = 0.15, EtOH).
D
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
(
1 S,2 R,3 S,4 R,5 S)-4-(6-Amino-2-chloro-9H-purin-9-yl)-1-[phosphoryloxy-
methyl]bicyclo[3.1.0]hexane-2,3-diol (2). To
a
solution containing 20
(
0.028 g, 0.05 mmol) in MeOH (3 mL) and water (3 mL) was added
◦
Dowex-50 resin (0.050 g). The mixture was stirred for 3 hours at 70 C
and the resin removed by filtration. The filtration was then treated with
triethylammonium bicarbonate buffer (1 mL, 1 M) and concentrated
under reduced pressure. The residue was purified by ion-exchange column
chromatography using Sephadex-DEAE A-25 resin with linear gradient
(
(
0.01–0.5 M) of 0.5 M ammonium bicarbonate as mobile phase to give 2
1
0.005 g, 25%) as a white solid. H NMR (D O) δ 8.44 (s, 1H), 4.72 (s, 1H),
2
4
(
.42–4.52 (m, 1H), 4.00 (d, 1H, J = 6.6 Hz), 3.62–3.76 (m, 1H), 1.84–1.96
m, 1H), 1.53–1.62 (m, 1H), 0.96–1.05 (m, 1H). 31P NMR (D O) 0.677.
2

Chiral Synthon for Receptor Ligands from L-Ribose
287
SCHEME 2 Improved synthesis of a key (N)-methanocarba bicyclic pseudosugar intermediate 15
starting from L-ribose 8.
+
HRMS (M − 1) : calculated 390.0357, found 390.0359. HPLC (system B)
8
.2 minutes (98%). For optical rotation measurements a crystalline salt with
◦
25
◦
triethylamine was crystallized. M.p. 215 C (decomp); α
+ 0 (c = 0.16,
D
80% EtOH).
RESULTS AND DISCUSSION
The key step in our published route to (N)-methanocarba nucleosides
involves intramolecular cyclopropanation of the diazo derivative 11 to
afford the requisite bicyclo[3.1.0]hexan-2-one derivative 12 as the major
isomer (Scheme 2). Isomerization of the isopropylidene group in 14
provided the key (N)-methanocarba bicyclic pseudosugar intermediate,
ethyl
(1S,2R,3S,4S,5S)-2,3-O-(isopropylidene)-4-hydroxybicyclo[3.1.0]-
hexane-carboxylate 15 which was converted into requisite (N)-
methanocarba nucleosides by following a standard set of reactions in
good yields (Scheme 3).
We explored the use of commercially available L-ribose 8 as an enantiop-
ure building block for the construction of 15. Accordingly, 8 was converted
into a 5:1 mixture of iodides 9, which were isolated as a colorless oil
in 69% overall yield. An important attribute of this iodide preparation
was the trituration of the crude product with hexanes/ethyl acetate (3:1)

2
88
B. V. Joshi et al.
SCHEME 3 Synthesis of the (N)-methanocarba nucleoside derivative MRS3558 1a and nucleotide
derivative MRS2339 2.
to precipitate the triphenylphosphine oxide byproduct thereby allowing
ꢁ
facile removal by filtration. The mixture of 5 -iodoribose derivatives was
initially cleaved reductively with Zn in refluxing methanol according to a
literature procedure.[
12,13]
The purification of the crude aldehyde 7 was
[
13]
attempted by using distillation under reduced pressure,
which resulted
in polymerization of the product. To overcome this problem the aldehyde
was generated in situ and was reacted further without purification. We
also employed isopropyl alcohol as a co-solvent along with methanol in
this reaction. It is presumed that the formation of the undesired methyl
hemiacetal was minimized to almost negligible levels due to the presence
of a large excess of isopropyl alcohol. At this stage the reaction mixture
was simply filtered through Celite to remove an excess of Zn, and the
alcoholic layer containing 7 was concentrated to dryness. The residue was
redissolved in dichloromethane and reacted with ethyl diazoacetate to
afford the keto ester 10 in 42% yield based on the iodoribose mixture
9. Although we obtained some improvement in yields using this reaction
protocol we continued to probe this reaction sequence further. We observed
that the resulting aldehyde was very unstable under the present reaction
conditions, that is, refluxing in i-PrOH/MeOH. So, our aim was to carry out
this reaction sequence at lower temperature to avoid the decomposition of
the aldehyde. Accordingly, when the same reaction was carried out in the
presence of acetic acid we observed that the Zn-mediated reduction pro-
ceeded smoothly at room temperature. We believe that this acid-catalysed
reduction occurs upon protonation of the methoxy group followed by the
rate-determining reductive elimination of the “MeOH” moiety to afford the

Chiral Synthon for Receptor Ligands from L-Ribose
289
aldehyde. We also observed that instead of simply filtering to remove the
Zn, an aqueous workup at this stage was necessary to remove the unwanted
ZnI from the reaction mixture. The resulting aldehyde was subjected to
further reaction with a tin-mediated ethyl diazoacetate addition reaction to
afford 10 in 68% yield based on the iodoribose mixture 8. It is pertinent
to mention here that longer reaction times with ethyl diazoacetate led to
formation of unwanted side products that made the column chromato-
graphic separation of 10 difficult as well as lowering the overall yields. It
is to be noted here that the combined yield of 10 by this modified acid
catalyzed protocol is 47% based on L-ribose, while our earlier approach
had only a 19% overall yield based on expensive 2, 3-O-isopropylidene-D-
erythronolactone.
The keto ester 10 was reacted with tosyl azide to afford the diazo deriva-
tive 11 in 95% yield. The byproduct p-toluenesulfonamide generated in this
reaction separates out easily when the residue was dissolved in chloroform.
The diazo derivative 11 then underwent a thermally-induced intramolecular
cyclopropanation to give bicyclo[3.1.0]hexan-2-one derivatives 12 and 13 in
a combined 48% yield as per our earlier procedure. However we observed
that the newly formed sterically strained α-keto cyclopropyl derivatives
12 and 13 were unstable under these reaction conditions. Based on this
observation the slow addition (over 4 hours) of the diazo derivative 11
was employed during this reaction. This modification has indeed resulted
in considerable improvement in the yields, and a favorable diastereoiso-
meric ratio (4:1) for the desired isomer 12 (51%) was obtained. The
small amount of the starting diazo compound 11 was also recovered from
this reaction. This bicyclo derivative 12 was isolated chromatographically
and reduced stereoselectively with NaBH to give alcohol 14 as a single
4
product in 69% yield. Alcohol 14 was then subjected to trifluoroacetic
acid-catalyzed equilibration at room temperature to produce the requisite
isomeric acetonide 15. The isolation of 15 was achieved by crystallization
from cyclohexane, in which the mother liquor was rich in the requisite
isomer 15 with purity >80%. The crystalline isomer 14 could be recycled
by re-subjecting it to the above equilibration procedure. Pure 15 could
be obtained by repeated recrystallizations from cyclohexane. However, for
most applications it was practical to use 15 at ∼80% purity in subsequent
reactions.
ꢁ
Our initial studies towards nucleophilic substitution of either the 1 -
ꢁ
mesylate or 1 -triflate derivative of 15, with the sodium salt of purines
under a variety of conditions failed presumably due to steric reasons. The
best result was obtained when alcohol 15 was subjected to a Mitsunobu
coupling reaction using 2,6-dichloropurine as the base to afford the con-
densed product 16 (Scheme 3). However, we observed that this condensed
product was contaminated with byproducts arising from the Mitsunobu

2
90
B. V. Joshi et al.
reaction. Repeated purification (at least three times) with silica gel column
chromatography afforded the pure compound 16 in 42% yield. We observed
that a slightly impure product could be carried forward in reaction with
3-chlorobenzylamine to provide 17 in 61% yield after silica gel column
chromatography. Finally, 17 was treated with excess methylamine followed
by acetonide deprotection with 10% TFA to produce 1a (MRS 3558) in 54%
yield over two steps whose spectral properties matched those of the identical
compound reported in the literature.[
4]
This modified procedure starting from L-ribose was also used to pre-
ꢁ
pare MRS2339 2, a (N)-methanocarba derivative of 2-chloroadenosine 5 -
monophosphate. Recently, Liang and coworkers have shown that com-
pound 2 acts as a P2X receptor agonist and when administered in a mouse
model of heart failure it reduces cardiac myocyte hypertrophy and enhances
[
10]
survival.
1
For its preparation we first reduced the ester functionality in
◦
6 using diisobutylaluminum hydride (DIBAL-H) at −78 C, to afford the
alcohol 18 in 56% yield. Initially, the chloro group at the 6-position was
replaced with an amino functionality by exposure to methanolic ammo-
nia, however the reaction was also accompanied by the formation of the
6
-methoxy substituted derivative. This problem was overcome by treating 18
with 2 M ammonia in isopropyl alcohol to afford the 6-amino derivative 19
in moderate yields. (N)-Methanocarba nucleoside 19 was phosphorylated
by first reacting with di-t-butyl N,N -diethylphosphoramidite and tetrazole
followed by treatment with m-chloroperbenzoic acid to afford the di-t-butyl
phosphate derivative 20 in 45% yield. Both t-butyl groups and the acetonide
group in 20 were deprotected simultaneously by using Dowex-50 resin in
the acid form, to afford the crude (N)-methanocarba nucleotide 2. The
final purification of the resulting monophosphate was accomplished by
using Sephadex ion exchange column chromatography to afford 2 as the
ammonium salt in 25% yield. The spectral properties of 2 were identical to
those reported previously.[
14]
In conclusion, we have demonstrated the feasibility of employing readily
available L-ribose as a precursor in the preparation of (N)-methanocarba
nucleosides and nucleotides of biological interest. In this regard the mod-
ified synthesis bypasses the pitfalls experienced in our earlier approach
thereby allowing a highly efficient and convergent synthesis of purine
receptor ligands MRS 3558 and MRS 2339. In addition to certain subtypes of
adenosine and P2 receptors, the (N)-methanocarba nucleosides have been
[
15]
explored and found to interact effectively with adenosine deaminase,
and the ENT1nucleoside transporter.[ Furthermore, nucleosides bearing
16]
the (N)-methanocarba modification have been shown to be substrates for
[
7 17]
cellular and viral DNA polymerases. , At present, use of this modified
synthetic procedure to synthesize (N)-methanocarba nucleosides with po-
tential antiviral activity is underway in our laboratories.

Chiral Synthon for Receptor Ligands from L-Ribose
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1
1
1