Synthesis of an o-nitrobenzyl attached A1 adenosine receptor antagonist, a prodrug approach

Article abstract of DOI:10.1081/SCC-120021980

The o-nitrobenzyl group, possessing distinct advantage of being photolabile under mild conditions, was successfully connected to 8-(5,6-epoxynorbornan-2-yl)-1,3-dipropylxanthine (5), a high specific A₁ adenosine receptor antagonist. The resulting compound 4 would have potential use as a prodrug.

Full text of DOI:10.1081/SCC-120021980

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Synthesis of an o-Nitrobenzyl Attached A1 Adenosine
Receptor Antagonist, a Prodrug Approach
HeXi Chang a , Carol Ensinger a , Robert D. McCargar a & Bruno M. Vittimberga b
a Chemical Process Research and Development , Biogen Inc. , Cambridge, Massachusetts,
USA
b Department of Chemistry , University of Rhode Island , Kingston, Rhode Island, USA
Published online: 17 Aug 2006.
To cite this article: HeXi Chang , Carol Ensinger , Robert D. McCargar & Bruno M. Vittimberga (2003) Synthesis of an o-
Nitrobenzyl Attached A1 Adenosine Receptor Antagonist, a Prodrug Approach, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 33:15, 2605-2611, DOI: 10.1081/SCC-120021980
To link to this article: http://dx.doi.org/10.1081/SCC-120021980
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 15, pp. 2605–2611, 2003
Synthesis of an o-Nitrobenzyl Attached A1
Adenosine Receptor Antagonist,
a Prodrug Approach
1
1
HeXi Chang,1, Carol Ensinger, Robert D. McCargar,
*
and Bruno M. Vittimberga2
1Chemical Process Research and Development,
Biogen Inc., Cambridge, Massachusetts, USA
2Department of Chemistry, University of Rhode Island,
Kingston, Rhode Island, USA
ABSTRACT
The o-nitrobenzyl group, possessing distinct advantage of being
photolabile under mild conditions, was successfully connected to
8-(5,6-epoxynorbornan-2-yl)-1,3-dipropylxanthine (5), ahigh specific
A1 adenosine receptor antagonist. The resulting compound 4 would
have potential use as a prodrug.
*Correspondence: HeXi Chang, Chemical Process Research and Development,
Biogen Inc., 14 Cambridge Center, Cambridge, Massachusetts 02142, USA;
E-mail: hexi_chang@biogen.com.
2605
DOI: 10.1081/SCC-120021980
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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2606
Chang et al.
Key Words: Photolysis; o-Nitrobenzyl group; A1 adenosine recep-
tor antagonist.
Since adenosine receptors were identified as drug targets,[1] avariety
of xanthine derivatives have been prepared and biologically screened in
order to find potent, receptor selective adenosine antagonists.[2] It is
interesting to note that, 8-(5,6-epoxynorbornan-2-yl)-1,3-dipropyl-
xanthine (5), a potential drug candidate for congestive heart failure,
possesses high affinity and selectivity for the A1 adenosine receptor.[3]
The o-Nitrobenzyl group, a special nitrogen protective group that is
stable under most of conditions, can be cleaved by irradiation at 320 nm
for 1 h in nearly quantitative yield.[4] This protective group has been used
[5]
widely in avariety of structure syntheses.
It is important to mention
that besides traditional photolytic conditions, removal of the o-nitroben-
zyl group could also be carried out efficiently using direct sunlight.[6]
The use of different kinds of prodrugs have been an effective way to
improve certain drug’s absorption, distribution, as well as to achieve
prolonged pharmaceutical effects by controlling drug-release.[7] Adding
the o-nitrobenzyl group to the N-7 position of compound 5 could
produce acompound ( 4) with some prodrug activity. By controlling
exposure of 4 to sunlight, it is possible to release the drug gradually
and therefore dramatically change its pharmaceutical properties.
A possible route to desired prodrug 4 could be the alkylation of
1,3-dipropyl-8-(5-norbornen-2-yl)xanthine[8] with o-nitrobenzyl bromide
in the presence of absae (either NEt
or NaNH2), and subsequent
3
epoxidation of the norbornenyl double dond. But the chance to complete
the alkylation reaction is little in considering of the high stereo-hindrance
of norbornenyl ring and relative large o-nitrobenzyl group. The
route that we eventually followed in preparing prodrug 4 is outlined in
Sch. 1.
1,3-Dipropyl-5,6-diaminouracil, prepared according to the literature
method,[9] was alkylated with o-nitrobenzyl chloride. The following acyl-
ation was then accomplished with (ꢀ)(1S,2S)-5-norborbene-2-carboxylic
acid chloride yielding the corresponding amide 2. Regarding this reac-
tion, we found that when small quantities of reactants are used for the
acylation, the order of reagents addition was not significant. Either
adding compound 1 to the acid chloride or vice versa, will basically
give the same results. However it is significant to note that when
the reaction was scaled-up, only the addition of the acid chloride
slowly to compound 1 produced the desired product in high yield as
described in the Experimental section. The reverse addition will result

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A1 Adenosine Receptor Antagonist
2607
Scheme 1.
in the formation of a di-acylated side product due to the presence of
excess amount of acid chloride.
The amide 2 underwent ring closure when it was heated at reflux in
basic alcohol solution to form the xanthine derivative 3. This compound
was then subjected to epoxidation with peracetic acid to give the expected
prodrug 4. Other epoxidation reagents that were tried include magnesium
monoperoxyphthalate hexahydrate (MMPP) and urea hydrogen perox-
ide, but unlike peracetic acid, both of these cannot drive the reaction to
completion.
Photolytic deprotection of 4 was carried out by irradiating a solution
of 4 in either THF or sec-BuOH with a450 W UV lamp for 3–6 h. Under
these reaction conditions no racemization was observed, which was
further confirmed by measuring the HPLC chiral purity of product 5
obtained.
In conclusion, an o-nitrobenzyl attached A1 adenosine receptor
antagonist 4 has been prepared. The target compound 4 has potential
as a prodrug and further pharmaceutical related activities are currently
under investigation.

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2608
Chang et al.
EXPERIMENTAL
Melting points were determined on an Electrothermal Digital
Melting Point Apparatus (IA9300). 1H-NMR spectrawere mesaured
on a Bruker spectrometer (400 MHz). Elemental analysis was performed
by QTI, Whitehouse, NJ. HPLC analyses were performed on an Alliance
Waters 2690 Separations Module system with Waters 996 Photodiode
Array Detector. The chemical purity was measured by reversed-phase
HPLC using aKeystone BDS Hypersil C8 column (150 ꢁ 2.0 mm,
5 mm). The chiral purity was measured by normal phase HPLC using a
Chiralcel OD column (250 ꢁ 4.6 mm, 10 mm).
6-Amino-1,3-dipropyl-5-(o-nitrobenzyl)aminouracil HCl salt (1). To
the mixture of 1,3-dipropyl-5,6-diaminouracil (24 g, 0.1 mmol), o-nitro-
benzyl chloride (20 g, 0.12 mol) in ethanol (400 mL) was added triethyl
amine (16 g, 0.16 mol). The mixture was heated at reflux in an oil bath for
12 h. After removing the solvents in vacuo, water (300 mL) was added to
the residue. The mixture was extracted with ethyl acetate (2 ꢁ 150 mL)
and the organic layer was dried with MgSO4. After removing the
solvent under vacuum, THF (30 mL) was added to the residue and
the mixture was cooled in an ice bath. Then HCl gas was passed through
the solution to the saturation point and the resultant suspension was
stirred with cooling for 1 h. The solid was filtered and washed with
THF to give product 1 as yellow solid (27.8 g, 67% yield); m.p.
193.8–194.9ꢂC. 1H-NMR (DMSO): (ppm) 7.80–7.30 (m, 4H, Ph-H),
4.30 (s, 2H, N-CH2-Ph), 3.53 (t, 2H, N-CH2-C), 3.41 (t, 2H, N-CH2-
C), 1.25 (m, 2H, C-CH2-C), 1.15 (m, 2H, C-CH2-C), 0.65 (t, 3H, CH3),
0.55 (t, 3H, CH3).
Analysis: C17H24ClN5O4: Calcd. C, 51.32; H, 6.04; N, 17.61.
Found: C, 51.73; H, 6.09; N, 17.24.
6-Amino-1,3-dipropyl-5-{[(1S,2S)-5-norbornen-2-yl-acyl]-(o-nitroben-
zyl)}aminouracil (2). To the solution of (ꢀ)(1S,2S)-5-norborbene-2-
carboxylic acid[10] (5 g, 36 mmol) in CH2Cl2 (125 mL), was added dropwise
diluted oxalyl chloride (5 mL in 125 mL CH2Cl2). After stirring at r.t.
for 2 h, the mixture was concentrated under vacuum and then 200 mL
of CH2Cl2 was added to the residue. The resultant solution was concen-
trated again as before and the residue was dissolved in 125 mL of THF.
In a separate flask, a slurry of diamine salt (1) (12.75 g, 32 mmol), triethyl
amine (11 g, 110 mmol) in THF (250 mL) was stirred at r.t. for 2 h. The
mixture was then cooled with an ice bath and the acid chloride solution in
THF prepared above was added dropwise. The resultant yellow suspen-
sion was stirred overnight. The mixture was then diluted with water
(1500 mL) and stirred for an additional 3 h. The product was isolated

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A1 Adenosine Receptor Antagonist
2609
by filtration as a yellow solid, washed with water, and air dried (14.5 g,
94% yield); m.p. 186.8–189.2ꢂC.
Analysis: C25H31N5O5: Calcd. C, 62.37; H, 6.44; N, 14.55.
Found: C, 62.00; H, 6.45; N, 14.68.
1,3-Dipropyl-7-o-nitrobenzyl-8-[(1S,2S)-5-norbornen-2-yl]xanthine (3).
To the suspension of compound 2 (33.5 g, 70 mmol) in MeOH (300 mL)
was added aqueous NaOH (1N, 300 mL). The mixture was refluxed for
15 h, followed by adding 600 mL of water. The resulting mixture was
cooled in an ice bath and stirred for an additional 2 h. Product 3 was
isolated by filtration as a light yellow solid (14 g, 43% yield); m.p 134.9–
138.1ꢂC; 97.58% HPLC chemical purity. H-NMR (CDCl3): (ppm) 8.23
1
(d, 1H, Ph-H), 7.55 (t, 1H, Ph-H), 7.47 (t, 1H, Ph-H), 6.59 (d, 1H, Ph-H),
6.20 (dd, 1H, C6H), 6.00 (s, 2H, N-CH2-Ph), 5.80 (dd, 1H, C5H), 4.05 (t,
2H, N-CH2-C), 3.83 (t, 2H, N-CH2-C), 3.17 (dt, 1H, C2H), 3.00 (s, 1H,
C1H), 2.92 (s, 1H, C4H), 2.02 (m, 1H, C3aH), 1.79 (m, 2H, C-CH2-C),
1.70–1.48 (m, 3H, C3bH þ C-CH2-C), 1.45 (d, 1H, C7aH), 1.30 (d, 1H,
C7bH), 0.97 (t, 3H, CH3), 0.87 (t, 3H, CH3).
Analysis: C25H29N5O4: Calcd. C, 64.79; H, 6.26; N, 15.12.
Found: C, 64.56; H, 6.29; N, 15.03.
1,3-Dipropyl-7-o-nitrobenzyl-8-[5,6-exo-epoxy-(1S,2S)-norbornan-2-
yl]xanthine (4). To asolution of compound 3 (4 g, 8.65 mmol) in CH2Cl2
(100 mL) was added peracetic acid (32% Wt., 8.4 g, 35.37 mmol). The
clear, light yellow solution was stirred at r.t. for 16 h, and then saturated
aqueous NaHCO3 (20 mL) was added. The organic layer was separated
and washed with saturated NaHCO3 (100 mL), Na2S2O3 (1M, 100 mL),
water (100 mL), and then dried over Na2SO4. After removing solvent,
ether (15 mL) was added to the residue and the mixture was stirred at r.t.
for half an hour. The product was obtained as a white solid by filtration
(1.8 g, 44% yield); m.p. 181.4–183.6ꢂC; 99.42% HPLC chemical purity.
1H-NMR (CDCl3): (ppm) 8.22 (d, 1H, Ph-H), 7.57 (t, 1H, Ph-H), 7.48 (t,
1H, Ph-H), 6.59 (d, 1H, Ph-H), 5.97 (s, 2H, N-CH2-Ph), 4.08 (t, 2H, N-
CH2-C), 3.87 (t, 2H, N-CH2-C), 3.26 (s, 2H, C5H þ C6H), 3.08 (dt, 1H,
C2H), 2.60 (s, 1H, C1H), 2.52 (s, 1H, C4H), 1.90 (m, 2H, C3H), 1.80 (m,
2H, C-CH2-C), 1.70–1.51 (m, 3H, C7aH þ C-CH2-C), 1.48 (d, 1H, C7bH),
0.97 (t, 3H, CH3), 0.89 (t, 3H, CH3).
Analysis: C25H29N5O5: Calcd. C, 62.63; H, 6.05; N, 14.61.
Found: C, 62.68; H, 6.01; N, 14.54.
1,3-Dipropyl-8-[5,6-exo-epoxy-(1S,2S)-norbornan-2-yl]xanthine (5). A
solution of 4 (200 mg, 0.42 mmol) in 10 mL of THF or sec-BuOH was
deaerated with nitrogen for 10 min in a Pyrex testing tube. The solution
was irradiated for 6 h at 45–50ꢂC with a450-W Hanoviaphotochemical
lamp. The resulting solution was concentrated in vacuo to a brown oil,

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2610
Chang et al.
which was then dissolved in ethyl acetate (10 mL) and filtered through a
silica gel pad. The filtrate was washed with water (10 mL) and dried with
Na2SO4. After removing solvent under vacuum, the residue was stirred
with hexane (5 mL) at r.t. for half an hour. The de-protected product 5
was collected by filtration as an off-white solid (76 mg, 53% yield); m.p.
135.0–138.5ꢂC; 95.02% HPLC chemical purity; 100% HPLC chiral
1
purity. H-NMR (CDCl3): (ppm) 4.09 (t, 2H, N-CH2-C), 3.98 (t, 2H,
N-CH2-C), 3.42 (dt, 1H, C2H), 3.23 (d, 1H, C6H), 3.10 (d, 1H, C5H),
3.05 (s, 1H, C1H), 2.65 (s, 1H, C4H), 2.04 (m, 2H, C3H), 1.90–1.60
(m, 5H, 2C-CH2-C þ C7aH), 1.54 (d, 1H, C7bH), 0.99 (t, 3H, CH3),
0.94 (t, 3H, CH3).
Analysis: C25H29N5O5: Calcd. C, 62.72; H, 6.97; N, 16.26.
Found: C, 62.91; H, 6.91; N, 16.13.
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Received in Japan September 21, 2002

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