Alternative route towards the convergent synthesis of a human purine nucleoside phosphorylase inhibitor-forodesine HCl

Article abstract of DOI:10.1016/j.tetlet.2009.06.137

Forodesine HCl is being investigated as a potential target for the control of T-cell proliferation. Herein we present an alternative route for the synthesis of the target molecule with addition of lactam to the lithiated deazahypoxanthine (generated in situ). The lactam was synthesized in five steps starting from l-pyroglutamic acid.

Full text of DOI:10.1016/j.tetlet.2009.06.137

Tetrahedron Letters 50 (2009) 5198–5200
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Tetrahedron Letters
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Alternative route towards the convergent synthesis of a human purine
nucleoside phosphorylase inhibitor—forodesine HCl
*
Vivekanand P. Kamath , Jie Xue, Jesus J. Juarez-Brambila, Philip E. Morris Jr.
BioCryst Pharmaceuticals Inc., 2190 Parkway Lake Drive, Birmingham, AL 35244, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Forodesine HCl is being investigated as a potential target for the control of T-cell proliferation. Herein we
present an alternative route for the synthesis of the target molecule with addition of lactam to the lith-
Received 26 March 2009
Revised 18 June 2009
Accepted 29 June 2009
Available online 8 July 2009
iated deazahypoxanthine (generated in situ). The lactam was synthesized in five steps starting from
pyroglutamic acid.
L-
Ó 2009 Elsevier Ltd. All rights reserved.
Purine nucleoside phosphorylase (PNP) enzyme has been investi-
gated widely due to its importance in the purine salvage pathway.
PNP catalyzes the reversible phosphorolysis of ribonucleosides and
L
-Pyroglutamic acid was converted to the ester followed by reduc-
tion of the ester with sodium borohydride to give the alcohol, 1 in
75% yield.13,14 Compound 1 was treated with benzaldehyde in the
presence of an acid catalyst (p-TsOH) to furnish 2 as a solid in 80%
yield.15 Transformation of compound 2 to the unsaturated lactam,
3 was accomplished using the Myers reagent in 65% yield.16 Cis-
dihydroxylation of compound 3 was accomplished using a catalytic
20-deoxyribonucleosides to the respective base and deoxyribose-
1-phosphate.1
a-
The importance of PNP enzyme to the integrity of the immune
system has become apparent with the discovery of a rare form of
immune deficiency found in children who are genetically deficient
in PNP enzyme.2,3 Children lacking the enzyme have severe T-cell
immunodeficiency and in most cases they maintain a normal or
high B-cell function.
O
H
N
NH
In the past decade, Schramm and co-workers have identified
several potent inhibitors of PNP enzyme.4,5 This has led to the dis-
covery of several novel classes of inhibitors that are extremely po-
tent against PNP. These compounds are the aza-C-nucleosides, and
one of them, forodesine is currently being investigated by BioCryst
Pharmaceuticals Inc. for control of T-cell proliferation (see Fig. 1).
A number of earlier publications by Tyler and co-workers have
described the synthesis of forodesine.6–10 The key step in the syn-
thesis involved addition of the imine (generated in situ from the
aza-sugar) to the lithiated 9-deazahypoxanthine, followed by the
removal of the protecting groups to furnish forodesine. The synthe-
sis of aza-sugar was first reported by Fleet and co-workers starting
N
H
N
HO
HCl
OH
HO
Figure 1. Structure of forodesine HCl.
Ph
Ph
O
O
H
N
PhSO2Me
PhCHO
N
N
3
O
from D-gulonolactone. Since D-gulonolactone is no longer available
O
p-TsOH
90%
O
65%
HO
commercially a new synthetic route to make the aza-sugar had to
be explored.11 The coupling of the imine with the lithiated 9-
deazahypoxanthine gave reproducible results, but the process in-
volved cumbersome manipulation that was not suitable for
large-scale operations. Hence there was a need to explore other
synthetic routes to make the aza-sugar and forodesine HCl.
Herein we describe an alternate route to make the aza-sugar
1
2
Ph
Ph
O
O
DMP/H+
OsO4
N
O
N
5
Acetone
70%
O
90%
using L L-Pyroglutamic acid
-pyroglutamic acid (see Scheme 1).12
is commercially available and cheap source of starting material.
HO
OH
4
O
O
H
Ph
* Corresponding author. Tel.: +1 205 444 4600; fax: +1 205 444 4640.
E-mail address: vkamath@biocryst.com (V.P. Kamath).
Scheme 1. Synthesis of the aza-sugar moiety (5).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.06.137

V. P. Kamath et al. / Tetrahedron Letters 50 (2009) 5198–5200
5199
see entry 7). The ratio was determined by HPLC using authentic
forodesine HCl as a reference.6
BOM OMe
N
N
Ph
Ph
O
BOM OMe
Based on the results from Table 1, 8 was reacted with BH3–Me2S
O
Et2O/
Anisole
reagent to furnish 9 as a mixture with b/a
ratio of 4:1.22 Hydrolysis
N
N
N
N
NH
BBr3
DCM
O
+
-20 0
55%
C
of 9 was carried out in MeOH and concd HCl under reflux condi-
tions for 1 h to give product 10 in 90% yield. The two compounds
were finally isolated by column chromatography.23 The b-anomer
was characterized and was found to be identical in all respects
with the compound published in the literature.10
In conclusion, an alternative route to synthesize forodesine HCl
has been demonstrated. Work on improving the synthetic route is
still in progress.
O
N
O
O
O
O
Li
Ph
Ph
H
7
H
6
5
OMe
OMe
N
O
H
H
H
N
N
N
NH
N
N
MeOH/H+
BH3Me2S
N
N
H
H
N
H
DMF
HO
90%
HO
N
N
8
HO
Acknowledgement
H
OH
HO
OH
HO
HO
10
OH
We thank Dr. Ken Belmore at The University of Alabama, Tusca-
loosa for the high-field NMR spectral analysis.
β/α
9
= 4/1
Scheme 2. Progress towards the synthesis of forodesine HCl (1).
References and notes
1. Pegg, A. E.; Williams-Ashman, H. G. Biochem. J. 1969, 115, 241.
2. Ragione, F. D.; Oliva, A.; Gragnaniello, V.; Russo, G. L.; Palumbo, R.; Zappia, V. J.
Biol. Chem. 1990, 265, 6241.
Table 1
Reaction conditions for reduction of the imine using various reducing agents
3. Toorchen, D.; Miller, R. L. Biochem. Pharmacol. 1991, 41, 2023.
4. (a) Miles, R. W.; Tyler, P. C.; Evans, G. B.; Furneaux, R. H.; Parkin, D. W.;
Schramm, V. L. Biochemistry 1999, 38, 13147; (b) Miles, R. W.; Tyler, P. C.;
Bagdassarian, C. K.; Furneaux, R. H.; Schramm, V. L. Biochemistry 1998, 37, 8615.
5. (a) Horenstein, B. A.; Parkin, D. W.; Estupinan, B.; Schramm, V. L. Biochemistry
1991, 30, 10788; (b) Horenstein, B. A.; Schramm, V. L. Biochemistry 1993, 32,
7089.
6. Evans, G. B.; Furneaux, R. H.; Gainsford, G. J.; Schramm, V. L. Tetrahedron 2000,
56, 3053.
7. Tyler, P. C.; Furneaux, R. H. J. Org. Chem. 1999, 64, 8411.
8. Tyler, P. C.; Limberg, G.; Schramm, R. H.; Furneaux, V. L. Tetrahedron 1997, 53,
2915.
OMe
BOM
N
OMe
BOM
N
N
N
N
N
H
N
N
HO
HO
H
O
O
O
O
9. Tyler, P. C.; Furneaux, R. H.; Schramm, V. L. Bioorg. Med. Chem. 1999, 7, 2599.
10. Evans, G. B.; Tyler, P. C.; Furneaux, R. H.; Hutchison, T. L.; Kezar, H. S.; Morris, P.
E.; Schramm, V. L. J. Org. Chem. 2001, 66, 5723.
11. Fleet, G. W. J.; Son, J. C. Tetrahedron 1988, 44, 2637.
12. (a) Yokoyama, M.; Momotake, A. Synthesis 1999, 1541; (b) Najera, C.; Yus, M.
Tetrahedron: Asymmetry 1999, 10, 2245.
13. Coopola, G. M.; Schuster, H. F. Asymmetric Synthesis. Construction of Chiral
Molecules Using Amino Acids; John Wiley: New York, 1987.
14. Silverman, R. B.; Levy, M. A. J. Org. Chem. 1980, 45, 815.
15. Thottahill, J. F.; Moniot, J. L.; Mueller, R. H.; Wong, M. K. Y.; Kissick, T. P. J. Org.
Chem. 1986, 51, 3140.
11
12
No.
Solvent
Reducing reagent
NaCNBH3
b/
a
ratio
1
2
3
4
5
6
7
THF
THF
THF
THF
THF
THF
THF
1: 1
1:4
1:4
1:4
1:4
1:1
2:1
L-Selectride + NaCNBH3
R-MeCBS + BH3Me2S
S-MeCBS + BH3Me2S
STAB
NaBH4
BH3Me2S
16. Resek, J. E.; Myers, A. I. Tetrahedron Lett. 1995, 36, 7051.
17. (a) Hamada, Y.; Tanada, Y.; Yokokawa, F.; Shioiri, T. Tetrahedron Lett. 1982, 23,
5435; (b) Couturier, M.; Andresen, B. M.; Jorgensen, J. B.; Tucker, J. L.; Busch, F.
R.; Brenek, S. J.; Dube, P.; AmEnde, D. J.; Negri, J. T. Org. Process Res. Dev. 2002, 6,
42.
18. Hamada, Y.; Hara, O.; Kawai, A.; Kohno, Y.; Shioiri, T. Tetrahedron 1991, 47,
8635.
19. Shimojima, Y.; Hayashi, H.; Ooka, T.; Shibukawa, M.; Iitaka, Y. Tetrahedron Lett.
1982, 23, 5435.
amount of OsO4 and N-methylmorpholine N-oxide (NMO) in aque-
ous acetone to furnish the diol, 4 with the desired stereochemistry
in 70% yield.17 The diol was further protected as a benzylidene ace-
tal mixture, 5 under standard conditions in 90% yield.18
20. (a) Momotake, A.; Togo, H.; Yokoyama, M. J. Chem. Soc., Perkin Trans. 1 1999,
1193; (b) Evans, G. B.; Furneaux, R. H.; Lewandowicz, A.; Schramm, V. L.; Tyler,
P. C. J. Med. Chem. 2003, 46, 3412.
The key step was the addition of the lactam to the lithiated 9-
deazahypoxanthine (see Scheme 2). Compound
6 (generated
in situ by reacting the 9-bromo deazahypoxanthine10 with n-BuLi)
was coupled to the N,O-protected lactam, 5 at ꢀ20 °C and stirred
for 20 min. The reaction mixture was slowly warmed to 0 °C and
then stirred further for 1 h.19 Workup of the reaction furnished
the crude product. The crude was purified by column chromatogra-
phy to furnish 7 as a foam in 55% yield.20 Compound 7 was treated
with BBr3 in DCM and stirred at rt for 2 h. Workup of the reaction
mixture gave the cyclized adduct, 8 which was taken directly to the
next step without any further purification.21
At this stage a number of reducing agents and reaction condi-
tions were attempted on a model compound 1110 to selectively re-
duce the imine to furnish the target compound (see Table 1).
Reduction of the imine, 11 with bulky reducing agents predomi-
21. 9-Bromo deazahypoxanthine (1 g, 2.9 mmol) dissolved in Et2O/anisole
mixture at ꢀ20 °C was treated with n-butyl lithium solution (1.6 M,
1.9 mL, 3.0 mmol) and stirred for 10 min. Compound
5 (0.9 g, 2.9 mmol)
pre-dissolved in Et2O/anisole mixture was added slowly via a cannula to
the reaction mixture containing 6 (formed in situ) at ꢀ20 °C for 20 min and
then warmed to 0 °C and stirred further for 1 h. Upon completion of the
reaction, the mixture was quenched with satd NH4Cl solution, extracted in
Et2O, washed with H2O, dried and evaporated to dryness. The product was
purified by column chromatography using hexanes/EtOAc 1:9 as eluent. The
appropriate fractions were pooled together to furnish compound
7 as a
foam (0.95 g, 55%). 1H NMR (300 MHz, CDCl3) d 8.70 (s, 1H), 8.15 (s, 1H),
7.11–7.62 (m, 15H), 5.60–5.80 (m, 6H), 4.42–4.71 (m, 5H), 4.15 (s, 3H,
OMe), 3.90 (dd, 1H).
22. Compound 7 (0.25 g, 0.43 mmol) was dissolved in DCM (10 mL) followed by
slow addition of BBr3 (0.16 mL, 1.75 mmol) and stirred at rt for 2 h. Upon
completion of the reaction (as indicated by TLC) the reaction mixture was
neutralized with solid NaHCO3. The solids were filtered and the filtrate was
concentrated. The crude was taken directly to the next step without any
further purification.
nantly gave the undesired a-anomer as the major product. This re-
sult was further supported by molecular modelling studies of
compound 11. The best anomeric ratio was obtained using BH3–
23. Compound 8 (87 mg, 0.3 mmol) was dissolved in DMF (5 mL) followed by
addition of BH3–Me2S (39
lL, 0.6 mmol) and stirred for 1 h. Workup of the
reaction furnished 9 which was then hydrolyzed using MeOH/concd HCl (1:1,
Me2S reagent to give 12 with b/
a anomeric ratio of 2:1 (Table 1,

5200
V. P. Kamath et al. / Tetrahedron Letters 50 (2009) 5198–5200
(m, H-40, 1H); 13C NMR (125 MHz, D2O) d 155.3, 143.3, 142.7, 129.8, 117.3,
2 mL) under reflux conditions to furnish 10 as a mixture. Compound 10 (
a-
anomer) was isolated by column chromatography and fully characterized.
1H NMR (500 MHz, D2O) d 7.96 (s, H-2, 1H), 7.75 (s, H-8, 1H), 4.97 (d, H-
10,1H), 4.41 (m, H-30, 1H), 4.37 (m, H-20, 1H), 3.94, 3.96 (dd, H-50, 2H), 3.69
106.7, 72.2, 71.9, 62.0, 58.6, 56.3; IR (thin film)
m
3087.9, 2941.4,
1661.6 cmꢀ1
267.1080.
.
HRMS: Calcd for C11H15N4O4 (M+H+): 267.1082; Found: