Synthesis of novel spiro[2.3]hexane carbocyclic nucleosides via enzymatic resolution

Article abstract of DOI:10.1021/ol0491989

(Matrix Presented) Novel R- and S-spiro[2.3]hexane nucleosides have been synthesized. The key step involved the Pseudomonas cepacia lipase catalyzed resolution of racemic compound 2, synthesized in seven steps starting from diethoxyketene and diethyl fuma

Full text of DOI:10.1021/ol0491989

ORGANIC
LETTERS
2004
Vol. 6, No. 15
2531-2534
Synthesis of Novel Spiro[2.3]hexane
Carbocyclic Nucleosides via Enzymatic
Resolution
Lavanya Bondada,† Giuseppe Gumina,† Ranjeet Nair,† Xing Hai Ning,†
,†
Raymond F. Schinazi,‡ and Chung K. Chu*
Department of Pharmaceutical and Biomedical Sciences, College of Pharmacy,
The UniVersity of Georgia, Athens, Georgia 30602, and Emory School of
Medicine/Veterans Affairs Medical Center, Decatur, Georgia 30033
dchu@rx.uga.edu
Received April 30, 2004
ABSTRACT
Novel R- and S-spiro[2.3]hexane nucleosides have been synthesized. The key step involved the Pseudomonas cepacia lipase catalyzed resolution
of racemic compound 2, synthesized in seven steps starting from diethoxyketene and diethyl fumarate, to give (+)-acetate 3 and (−)-alcohol
13. (+)-Acetate 3 and (−)-acetate 14 were converted to R- and S-9-(6-hydroxymethylspiro[2.3]hexane)-4-adenine, respectively.
During the last two decades, treatment of viral infections
has advanced remarkably, driven particularly by the search
for effective agents for the treatment of herpes, AIDS, and
viral hepatitis. In recent years, new and emerging viruses,
such as new strains of hepatitis and herpes viruses, Ebola,
West Nile, and SARS, have shown their lethal potential.
Furthermore, the threat that viruses and other microorganisms
could be used as biological weapons in warfare or bioter-
rorism has brought antiviral research to the forefront.
Although vaccination is a valuable preventative tool for
certain viral infections, new and effective antiviral agents
are needed to prevent acute and chronic viral infections.
Nucleosides are the most frequently used effective class
of antiviral agents, with over 20 drugs currently approved
for the treatment of viral diseases and a number of candidates
in the clinical trials.1
Since oxetanocin-A2 was isolated and its antiviral activity
described,2,3 continuous studies have been made to explore
the chemistry and biological activity of four-membered-ring-
containing nucleosides (Figure 1). Since Honjo et al.4 first
(1) (a) Antiviral nucleosides: Chiral Synthesis and Chemotherapy; Chu,
C. K., Eds.; Elsevier: New York, 2003. (b) Recent advances in nucleo-
sides: Chemistry and Chemotherapy; Chu, C. K., Ed.; Elsevier: New York,
2002.
(2) Shimada, N.; Hasegawa, S.; Harada, T.; Tomosawa, T.; Fujji, A.;
Takita, T. J. Antibiot. 1986, 39, 1623-1625.
(3) Hoshino, H.; Shimizu, N.; Shimada, M.; Tokito, T.; Takeuchi, T.; J.
Antibiot. 1986, 39, 1077-1078.
† The University of Georgia.
‡ Emory School of Medicine/Veterans Affairs Medical Center.
10.1021/ol0491989 CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/30/2004

Table 1. Different Enzymes Tested for Maximum Optical
Purity
lipases
3 [R]D
13 [R]D
time (h)
Candida antarctica
Lipozyme (Mucor mechei)
Porcine pancreatic
1.40
2.40
11.90
42.00
-3.6
-4.2
-19.0
-48.10
13-14
13-14
13-14
2
Pseudomonas cepacia
Figure 1. Analogues of oxetanocin.
of nonracemic cyclobutyl adenine. However, the large
number of steps involved as well as the moderate enanti-
oselectivity of enzyme-catalyzed reactions resulted in rela-
tively low overall yield. Consequently, the need exists for
more efficient and simple methods for obtaining chiral forms
with high enantiomeric excess.
reported the synthesis of cyclobutyladenine, the carbocyclic
analogue of oxetanocin with its interesting biological activity5
has prompted numerous subsequent syntheses of both parent
and related compounds.6
Zemlicka and co-workers7 described a new class of
nucleoside analogues, the spiro[3.3]heptane and spiro[2.2]-
pentane nucleosides. They found that adenosine analogue
displayed some activity against human cytomegalovirus in
vitro.8 However, only limited examples of spiro nucleosides
have been reported.7,8 Herein, we wish to describe a facile
method for the synthesis of novel R- and S-spiro[2.3]hexane
nucleosides.
The observation of different pharmacological as well as
toxicological properties of opposite enantiomers highlights
the need for asymmetric synthesis.9 In early studies, chiral
resolutions were performed to obtain nonracemic intermedi-
ates.10 Ichikawa et al.11 employed a chiral titanium complex
as the catalyst in an asymmetric [2 + 2] cyclization to
provide a functionalized cyclobutyl intermediate. Jung and
Sledeski12 utilized an enzymatic desymmetrization of a meso
cyclobutane as the enantioselective step in their formation
In this paper, we describe the use of Pseudomonas cepacia
lipase promoted enzymatic resolution of intermediate 2 for
the synthesis of spiro[2.3]hexane nucleosides. The synthesis
of cyclobutyl precursor 2 was achieved as described in the
literature.13 Compound 2 was subjected to enzymatic resolu-
tion by using different lipases (Table 1). Among the various
lipases studied, P. cepacia (PS) gave the highest optical and
chemical yield on a multigram scale. The reaction progress
1
was monitored by H NMR. After 2 h, a 1:1 ratio for the
H-3 proton was observed and (+)-(1S,2S,3S)-3-O-acetyl-1,2-
O-cyclohexylidene-2,3-bis(hydroxymethyl)-1-cyclobutanol 3
and (-)-(1R,2R,3R)-1,2-O-cyclohexylidene-3-hydroxy-2,3-
bis(hydroxymethyl)-1-cyclobutanol 13 were obtained. Vinyl
benzoate and vinyl acetate were studied as an acylating
agents; however, the later one was found to give better
enantioselectivity.
To synthesize R-spiro[2.3]hexane carbocyclic nucleoside
12, compound 3 was hydrolyzed with Amberlite IR 120 (H+)
in methanol, followed by the tritylation of primary alcohol
to give compound 4 (Scheme 1). Subsequent silylation of
the secondary alcohol gave compound 5. Deprotection of
the trityl group was effected using BF3‚Et2O to furnish 6.
Iodination of compound 6 with CH3P(OPh)3I in THF gave
iodide 7. DBU-mediated elimination of 7 in THF under
reflux conditions gave compound 8. Attempts for cyclopro-
panation on compound 8 were unsuccessful; hence, the
TBDPS protecting group was first deprotected using TBAF
in THF to give compound 9. Cyclopropanation of compound
9 using (C2H5)2Zn and CH2I2 under reflux conditions in
ether gave (R)-6-acetoxymethylspiro[2.3]hexane-4-ol 1014
in 53% yield. The alcohol 10 was condensed with 6-chlo-
ropurine under Mitsunobu conditions to give compound
11 in 63% yield. The 6-chloro derivative was converted
to compound 12 by treatment with a saturated solution
of ammonia in methanol in a steel bomb at 100 °C for
24 h. Deprotection of the primary alcohol as well as
ammonolysis took place under the same conditions to give
(R)-9-(6-hydroxymethylspiro[2.3]hexane)-4-adenine 12.15 Fol-
lowing a similar procedure, the (-)-acetate 14 was converted
(4) Honjo, M.; Maruyama, T.; Sato, Y.; Morii, T. Chem. Pharm. Bull.
1989, 37, 1413-1416.
(5) (a) Norbeck, D. W.; Kern, E.; Hayashi, S.; Rosenbrook, W.; Sham,
H.; Henin, T.; Plattner, J. J.; Erickson, J.; Clement, J.; Shannon, W.;
Shipkowitz, N.; Hardy, D.; Marsh, K.; Arnett, G.; Shannon, W.; Broder,
S.; Mitsuya, H. J. Med. Chem. 1990, 33, 1281-1285. (b) Maruyama, T.;
Hanai, Y.; Sato, Y.; Snoeck, R.; Andrei, G.; Hosoya, M.; Balzarini, J.;
Clercq, E. D. Chem. Pharm. Bull. 1993, 41, 516-521.
(6) (a) Somekawa, K.; Hara, R.; Kinnami, K.; Muraoka, S, T.; Shimo,
T. Chem. Lett. 1995, 407-408. (b) Bisacchi, G. S.; Singh, J.; Godfrey, J.
D., Jr.; Kissick, T. P.; Mitt, T.; Malley, M. F.; Di Marco, J. D.; Gougoutas,
J. Z.; Mueller, R. H.; Zahler, R. J. Org. Chem. 1995, 60, 2902-2905.
(c) Maruyama, T.; Hanai, Y.; Sato. Y. Nucleosides Nucleotides. 1992,
11, 855-864. (d) Maruyama, T.; Hanai, Y.; Sato. Y.; Snoeck, R.; Andrei,
G.; Hosoya, M.; Balzarini, J.; Clercq, D. Chem. Pharm. Bull. 1993, 41,
516-521. (e) Maruyama, T.; Sato, Y.; Horii, T.; Shiota, H.; Nitta, K.;
Shirasaka, T.; Mitsuya, H.; Honjo, M. Chem. Pharm. Bull. 1990, 38, 2719-
2725.
(7) Guan, H. P.; Ksebati, M. B.; Cheng, Y. C.; Drach, J. C.; Kern, E.;
Zemlicka, J. J. Org. Chem. 2000, 65, 1280-1290.
(8) Jones, B. C. N. M.; Drach, J. C.; Corbett, T. H.; Kessel, D.; Zemlicka,
J. J. Org. Chem. 1995, 60, 6277-6287.
(9) (a) Wang, P.; Gullen, B.; Newton, M. G., Cheng, Y. C.; Schinazi,
R. F.; Chu, C. K. J. Med. Chem. 1999, 42, 3390-3399. (b) Wang, P.;
Agrofoglio, L. A.; Newton, H. G.; Chu, C. K. J. Org. Chem. 1999, 64,
4173-4178.
(10) Bisacchi, G. S.; Braitman, A.; Cianci, C. W.; Clark, J. M.; Field,
A. K.; Hagen, M. E.; Hockstein, D. R.; Malley, M. F.; Mitt, T.; Slusarchyk,
W. A.; Sundeen, J. E.; Terry, B. J.; Tuomari, A. V.; Weaver, E. R.; Young,
M. G.; Zahler, R. J. Med. Chem. 1991, 34, 1415-1421.
(11) Ichikawa, Y.; Narita, A.; Shiozawa, A.; Hayashi, Y.; Narasaka, K.
J. Chem. Soc., Chem. Commun. 1989, 43, 1919-1921.
(12) Jung, M. E.; Sledeski, A. W. J. Chem. Soc., Chem. Commun. 1993,
589-591.
(13) (a) Slusarchyk, W. A.; Young, M. G.; Bisacchi, G. S.; Hockstein.
D. R.; Zahler, R. Tetrahedron Lett. 1989, 30, 6453-6456. (b) Brannock,
K. C.; Burpitt, R. D.; Thweatt, J. G. J. Org. Chem. 1964, 29, 940.
2532
Org. Lett., Vol. 6, No. 15, 2004

Scheme 1. Synthesis of (R)-D- and (S)-L-Spiro[2.3]hexane Carbocyclic Nucleosidea
a Reagents and conditions: (a) P. cepacia lipase, AcOCHdCH2, 28 °C, 2 h; (b) Ac2O, py, CH2Cl2; (c) (i) Amberlite IR-120 (H+),
MeOH, rt, 1 h, (ii) Ph3CCl, Py, rt, overnight; (d) TBDPS-Cl, imidazole, CH2Cl2, rt, 1 h; (e) BF3‚OEt2, MeOH, CH2Cl2, rt, 1 h; (f) Me3P(OPh3)I,
DMF, 1 h, rt; (g) DBU, THF reflux, 24 h; (h) TBAF, THF, RT, 1 h; (i) (C2H5)2Zn, CH2I2, ether, 0-45 °C; (j) 6-chloropurine, PPh3, DIAD,
THF, 0 °C to rt, overnight; (k) NH3, MeOH, 100 °C, 24 h.
to (S)-9-(6-hydroxymethylspiro[2.3]hexane)-4-adenine 2316
in 10 steps. The optical purity was determined by chiral
HPLC on a CHIRALPAK AD-H column using 2-propanol-
hexane (1:1) as an eluent and found to be 98.2% ee for
compound 12 and >99.9% ee for compound 23 (Figure 2).
The antiviral activity of the synthesized spiro nucleosides
12 and 23 was evaluated against HIV-1 in human PBM
cells17 The (R)-adenine analogue 12 exhibited moderately
potent anti HIV activity (EC50 22.4 µM) without cytotoxicity
up to 42.4 µM in PBM, 100 µM in CEM, and Vero cells.
The S-enantiomer 23 also showed some anti-HIV activity
(EC50 48.6 µM) (Table 2).
(14) Compound 10: colorless oil; [R]D +119.05 (c 0.15, CHCl3); 1H
NMR (CDCl3); δ 0.34-0.44 (m, 2H), 0.73 (m, 1H), 0.86 (m, 1H), 2.08 (s,
3H), 2.13 (m, 1H), 2.33 (m, 1H), 2.46 (m, 1H), 4.08 (d, J ) 6.83 Hz, 2H),
4.31 (t, J ) 6.83 Hz, 1H); 13C NMR (CDCl3) δ 6.7, 7.1, 21.0, 33.9, 34.2,
66.8, 70.3, 171.9. Anal. Calcd for C16H17NO6 (p-nitrobenzoyl derivative):
C, 60.18; H, 5.37; N. 4.39. Found: C. 60.40; H. 5.59; N. 4.24. Compound
21: colorless oil; [R]D -127.82 (c 0.4, CHCl3); 1H NMR (CDCl3) δ 0.35-
0.46 (m, 2H), 0.72 (m, 1H), 0.84 (m, 1H), 2.07 (s, 3H), 2.14 (m, 1H), 2.32
(m, 1H), 2.44 (m, 1H), 4.06 (d, J ) 6.73 Hz, 2H), 4.29 (t, J ) 6.73 Hz,
1H); 13C NMR (CDCl3) δ 6.8, 7.1, 21.0, 29.9, 33.6, 34.1, 66.9, 70.3, 172.0.
Anal. Calcd for C16H17NO6 (p-nitrobenzoyl derivative): C, 60.28; H, 5.37;
N, 4.39. Found: C, 60.36; H, 5.68; N, 4.19.
(15) Compound 12: white solid; mp 197-198 °C; [R]D -39.09 (c 0.13,
MeOH); UV (MeOH) λmax 261.0 (ꢀ 11 097, pH 2), 261.0 (ꢀ 12 030, pH 7),
1
261.0 (ꢀ 8277.5, pH 11); H NMR [(CD3)2SO] δ 0.02 (m, 1H), 0.85 (m,
2H), 1.06 (m, 1H), 2.88 (m, 2H), 3.80 (t, J ) 4.88 Hz, 2H), 4.84 (t, J )
4.88 Hz, 1H), 5.42 (t, J ) 8.30 Hz, 1H), 8.28 (s, 1H), 8.64 (s, 1H); 13C
NMR [(CD3)2SO] δ 4.9, 10.4, 28.6, 30.3, 36.3, 50.9, 63.2, 119.2, 140.3,
150.1, 152.8, 156.4. Anal. Calcd for C12H15N5O: C, 58.76; H, 6.16; N,
28.55. Found: C, 58.62; H, 6.20; N, 28.30.
(16) Compound 23: white solid; mp 197-199 °C; [R]D +38.44 (c 0.12,
MeOH); UV (MeOH) λmax 261.0 (ꢀ 11 640, pH 2), 261.0 (ꢀ 13 767, pH 7),
261.0 (ꢀ 16 727.5, pH 11); 1H NMR [(CD3)2SO] δ 0.01 (m, 1H), 0.90 (m,
2H), 1.05 (m, 1H), 2.85 (m, 2H), 3.8 (t, J ) 5.37 Hz, 2H), 4.85 (t, J )
5.37 Hz, 1H), 5.24 (t, J ) 8.30 Hz, 1H), (s, 2H), 8.40 (s, 1H), 8.65 (s, 1H);
13C NMR [(CD3)2SO] δ 5.1, 10.4, 28.6, 30.3, 36.3, 51.1, 63.0, 119.2, 140.4,
150.1, 153.0, 156.5. Anal. Calcd for C12H15N5O: C, 58.76; H, 6.16; N,
28.55. Found: C, 58.34; H, 6.14; N, 28.01.
(17) Schinazi, R. F.; Sommadossi, J. P.; Saalmann, V.; Cannon, D. L.;
Xie, M. W.; Hart, G. C.; Smith, G. A.; Hahn, E. F. Activity of 3′-azido-
3′-deoxythymidine nucleotide dimers in primary lymphocytes infected with
human immunodeficiency virus type 1. Antimicrob. Agents Chemother.
1990, 34, 1061-1067.
Figure 2. Chiral HPLC of enantiomers 1, 2, and 1 + 2.
CHIRALPAK AD-H column (250 × 4.6 mm, 2-propanol/hexane
(1:1) as eluent, flow rate 0.2 mL/min, detection at 260 nm,
concentrated ) 1 mg/mL, volume injected ) 5 µL. RT ) retention
time.
Org. Lett., Vol. 6, No. 15, 2004
2533

intermediates 10 and 21 are being utilized for the synthesis
of other nucleosides with natural as well as unnatural
heterocyclic moieties.
Table 2. Anti-HIV-1 Activity and Cytotoxicity of Compounds
12 and 23 in Different Cells
IC50 (mM)
Acknowledgment. This research was supported by the
U.S. Public Health Service Research Grants AI 32351 and
AI 56540 from the National Institute of Allergy and
Infectious Diseases, NIH, and the Department of Veterans
Affairs.
compd
EC50 (mM)
PBM
42.4
>100
>100
CEM
Vero
12
23
AZT
22.4
48.6
0.004
>100
>100
14.3
>100
>100
29.0
Supporting Information Available: Complete Experi-
mental Section with full characterization of all new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
In summary, we have described the preparative scale
enzymatic resolution of compound 2 and the synthesis of
two new versatile intermediates 10 and 21. Condensation of
these intermediates with 6-chloropurine, followed by am-
monolysis readily provided novel spiro[2.3]hexane nucleo-
sides 12 and 23 in high optical purity. Furthermore,
OL0491989
2534
Org. Lett., Vol. 6, No. 15, 2004