Regioselective synthesis of various prodrags of ganciclovir

Article abstract of DOI:10.1016/S0040-4039(99)02280-7

High-yield regioselective syntheses of 9-[(1,3-dihydroxy-2- propoxy)methyl]guanine mono-, di- and tri-substitution derivatives as potential prodrugs were accomplished via one or multisteps. Two amino acid esters of ganciclovir were synthesized as water-so

Full text of DOI:10.1016/S0040-4039(99)02280-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1131–1136
Regioselective synthesis of various prodrugs of ganciclovir
Hongwu Gao and Ashim K. Mitra ∗
Division of Pharmaceutical Sciences, School of Pharmacy, University of Missouri-Kansas City, 5005 Rockhill Road,
Kansas City, MO 64100-2499, USA
Received 27 September 1999; accepted 3 December 1999
Abstract
High-yield regioselective syntheses of 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine mono-, di- and tri-
substitution derivatives as potential prodrugs were accomplished via one or multisteps. Two amino acid esters
of ganciclovir were synthesized as water-soluble prodrugs, which form protonated cations in pH 7.4 phosphate
buffer. © 2000 Elsevier Science Ltd. All rights reserved.
9-[(1,3-Dihydroxy-2-propoxy)methyl]guanine (ganciclovir 1, Scheme 1) is a potent and selectively
active agent against cytomegalovirus.1 Since the oral administration of ganciclovir may not provide
optimal therapeutic levels, various prodrugs were synthesized and evaluated with the aim of improving its
delivery and apparent half-life.2–6 Ganciclovir has two identical primary hydroxyl groups and one amino
group which may result in five possible combinations of ester and amide prodrug derivatives (mono-
O-, mono-N2-, di-O-, di-N2,O-, tri-N2,O,O-acyl derivatives). In this paper, we wish to report various
strategies for regioselective syntheses of these ganciclovir derivatives as potential prodrugs to investigate
their effectiveness in improving pharmacokinetic properties and enhancing their intraocular retention
after direct intravitreal administration. Biochemical conversion and in vivo pharmacokinetic studies
of acyclovir prodrugs in ocular tissues have revealed that the isobutyrate prodrug was most effective
because of its optimum hydrolysis profile producing sustained therapeutic levels of acyclovir after
intravitreal administration.7 Thus, ganciclovir isobutyrate derivatives were selected for further studies
and development. The product structures were confirmed by their NMR chemical shifts, MS and HPLC
retention profiles.
The simple di-O-isobutyrate ester of ganciclovir 2a was prepared in a single-step reaction as reported
previously from this laboratory (Scheme 1).8 A solution of ganciclovir in dimethylformamide (DMF)
was treated with isobutyric anhydride in the presence of a catalytic amount of 4-dimethylaminopyridine
(DMAP) at room temperature for 4 hours. Chromatographic purification generated compound 2a in 80%
yield.
The synthesis of mono-O-propionate ester of ganciclovir 2b could be achieved by reaction of ganciclo-
vir with an excess of trimethyl orthopropionate followed by acidic hydrolysis of the cyclic orthoester
intermediate (Scheme 1).8
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02280-7
tetl 16208

1132
Scheme 1. Reagents and conditions: (i) ((CH3)2CHCO)2O, DMAP, DMF, rt, 4 h, 2a: 80%; (ii) CH3CH2C(OCH3)3, TFA, DMF,
rt, 2 h, then, H2O, overnight, 2b: 87%; (iii) ((CH3)2CHCO)2O, DMAP, pyridine, reflux, 1 h, 2c: 99%; (iv) 8 M NaOMe, MeOH,
rt, 16 min, 2d: 79%
Triisobutyl derivatization of ganciclovir 2c was successfully carried out with isobutyric anhydride in
refluxing pyridine using DMAP as a catalyst (Scheme 1). During the synthesis of acyclovir prodrugs,
the use of strong base (8 M NaOCH3/CH3OH) resulted in a selective O-deacylation in the presence of
N2-acyl as reported previously.9 The same protocol when applied to compound 2c produced mono-N2-
isobutyrate amide of ganciclovir 2d in 79% yield.
To avoid potential difficulty in selective hydrolysis of O-acyl groups, preparation of mono-N2-pivalate
amide of ganciclovir 5 was accomplished in a three-step procedure (Scheme 2). This preparation required
the protection of both primary hydroxyl functions, which was reported by direct silylation of ganciclovir
in DMF with tert-butyldimethyl silyl chloride (TBDMSCl) in the presence of imidazole.2 However,
our investigation revealed that TBDMS groups were unstable at reflux in pivalic anhydride. The more
acidic stable silyl protecting group, tert-butyldiphenylsilyl, was satisfactory. Treatment of the bis-O-tert-
butyldiphenylsilyl derivative 3 with trimethyl acetic anhydride in pyridine, followed by removal of the
silyl protecting group with tetrabutylammonium fluoride (TBAF) in DMF, gave the desired mono-N2-
pivaloyl derivative 5 in 52% yield.
Scheme 2. Reagents and conditions: (i) TBDPSCl, imidazole, DMF, rt, 24 h, 3: 91%; (ii) trimethylacetic anhydride, DMAP,
pyridine, reflux, 1 h, 4; (iii) TBAF, THF, DMF, 1 h, 5: 52% for two steps
Synthesis of di-N2,O-diisobutyl ganciclovir derivative 9 required selective protection of one of two
primary hydroxyl functions in ganciclovir (Scheme 3). This was achieved by treating mono-O-propionate
ester 2b in DMF with TBDPSCl in the presence of imidazole, followed by removal of the propionate
ester with strong base (8 M NaOCH3/CH3OH). The resulting mono-O-tert-butyldimethylsilyl derivative
7 was then treated with isobutyric anhydride in refluxing pyridine in the presence of DMAP to give
fully protected product 8. Removal of the TBDPS silyl group with TBAF provided the desired di-N2,O-
diisobutyl ganciclovir derivative 9 in 62% yield.
Two di-O-amino acid esters (12a and 12b) were prepared in two steps according to the same method
as for preparation of acyclovir amino acid derivatives (Scheme 4).10,11 A solution of ganciclovir in DMF
was treated with the N-carbobenzyloxy-protected amino acid (10a and 10b) using the coupling agent
DCC and DMAP as a catalyst. The resulting N-protected ester derivatives (11a and 11b) were purified

1133
Scheme 3. Reagents and conditions: (i) TBDPSCl, imidazole, DMF, rt, 24 h, 6: 93%; (ii) 8 M NaOMe, MeOH, rt, 16 min, 7:
85%; (iii) ((CH3)2CHCO)2O, DMAP, pyridine, reflux, 1 h, 8; (iv) TBAF, THF, DMF, 1 h, 9: 62% for two steps
by column chromatography on silica gel and subsequently subjected to catalytic hydrogenation in the
presence of HCl in order to avoid hydrolysis of the labile ester function. The target esters, 12a and 12b,
as the hydrochloride salts were isolated from a water:ethanol mixture (80:20) in 97% and 96% yields,
respectively (Scheme 4).
Scheme 4. Reagents and conditions: (i) acid 10a–10b, DCC, DMAP, DMF, rt, 24 h, then, recharge acid, DCC, DMAP, rt, 24 h,
11a: 81%, 11b: 68%; (ii) H2, 10% Pd/C, ethanol/water, rt, 4 h, 12a: 97%, 12b: 96%
The 1H NMR spectral data and assignments for compounds 2a–2d, 3–5 and 6–9, 11a–12b are listed
in Tables 1 and 2, respectively. The 13C NMR spectral data and assignments for compounds 2a–2d,
3–5 and 6–9, 11a–12b are listed in Tables 3 and 4, respectively. The physiological data and MS data of
compounds 2a–2d, 3–9 and 11a–12b are listed in Ref. 12.
Table 1
1H NMR data for ganciclovir and its derivatives (2a–2d, 3–5)

1134
Table 2
1H NMR data for ganciclovir derivatives (6–9, 11a–12b)
Table 3
13C NMR data for ganciclovir and its derivatives (2a–2d, 3–5)

1135
Table 4
13C NMR data for ganciclovir derivatives (6–9, 11a–12b)
Acknowledgements
Supported by NIH grants 2R01EY09171-04 and 2R01EY10659-05. We are grateful to Dr. P. Brown at
Hoechst-Marion-Roussel for his help in obtaining the ESI mass spectra and Professor J. R. Dias from the
Chemistry Department at UMKC for his suggestions. The authors gratefully acknowledge the support
of the University of Missouri Research Board, UMKC Office of Research Administration and UMKC
College of Arts and Sciences for the NMR instrumentation.
References
1. (a) Freitas, V. R.; Smee, D. F.; Matthews, T. R. Antimicrob. Agents Chemother. 1985, 28, 240. (b) Cheng, Y. C.; Huang,
E.-S.; Lin, J.-C.; Mar, E.-C.; Pagano, J. S.; Dutschman, G. E.; Grill, S. P. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 2767.
2. Martin, J. C.; Tippie, M. A.; McGee, D. P. C.; Verheyden, J. P. H. J. Pharm. Sci. 1987, 76, 180.
3. Winkler, I.; Winkelmann, E.; Scholl, T.; Rosner, M.; Jahne, G.; Helsberg, M. Antiviral Res. 1990, 14, 61.
4. Dillon, M. P.; Cai, H.; Maag, H. Bioorg. Med. Chem. Lett. 1996, 6, 1653.
5. Kim, D.-K.; Kim, H.-K.; Chae, Y.-B. Bioorg. Med. Chem. Lett. 1994, 4, 1309.
6. Kim, D.-K.; Chang, K.; Im, G.-J.; Kim, H.-T.; Lee, N.; Kim, K. H. J. Med. Chem. 1999, 42, 324.

1136
7. Duggirala, S. M.; Mitra, A. K., unpublished work.
8. Gao, H.; Mitra, A. K. Magn. Reson. Chem. 2000, 38, 687.
9. Stimac, A.; Kobe, J. Synthesis 1990, 461.
10. Colla, L.; De Clercq, E.; Busson, R.; Vanderhaeghe, H. J. Med. Chem. 1983, 26, 602.
11. Beauchamp, L. M.; Orr, G. F.; de Miranda, P.; Burnette, T.; Krenitsky, T. A. Antiviral Chem. Chemother. 1992, 3, 157.
12. 2a: 80%. M.p. 208–210°C. MS (ESI): 396 [M+1]+ (100). 2b: 87%. M.p. 138–140°C. MS (ESI): 312 [M+1]+ (78), 126 (100).
2c: 99%. M.p. 153–155°C. MS (ESI): 466 [M+1]+ (100). 2d: 79%. Foamy solid. MS (ESI): 326 [M+1]+ (100). 3: 91%. M.p.
188–190°C. MS (ESI): 732 [M+1]+ (100). 4: Foamy solid. MS (ESI): 816 [M+1]+ (100). 5: 52%. M.p. 203–205°C. MS
(ESI): 340 [M+1]+ (100). 6: 93%. M.p. 158–160°C. MS (ESI): 550 [M+1]+ (100). 7: 85%. M.p. 223–225°C. MS (ESI): 494
[M+1]+ (100). 8: Foamy solid. MS (ESI): 634 [M+1]+ (100). 9: 62%. Foamy solid. MS (ESI): 396 [M+1]+ (100). 11a: 81%.
Foamy solid. MS (ESI): 638 [M+1]+ (50), 306 (100). 11b: 68%. Foamy solid. MS (ESI): 1048 [M+1]+ (40), 306 (100). 12a:
97%. Foamy solid. MS (ESI): 370 [M+1]+ (100). 12b: 96%. Foamy solid. MS (ESI): 512 [M+1]+ (100).