Elemental fluorine to 8-fluoropurines in one step

Article abstract of DOI:10.1021/ja961456c

An efficient regiocontrolled approach to the synthesis of 8-fluoropurines by direct fluorination of purines with dilute elemental fluorine is described. The one-step procedure produced regiospecific substitution of the C(8)-hydrogen of purine derivatives in isolated yields close to 30% with protected purines in CHCl₃. Fluorination yields with unprotected purines in EtOH were reduced to less than 10%. The electrophilic fluorination procedure has a broad scope of applicability and permits a ready and easy access to 8-fluoropurine derivatives, for the first time, for evaluation of their biochemical and pharmacological properties.

Full text of DOI:10.1021/ja961456c

10408
J. Am. Chem. Soc. 1996, 118, 10408-10411
Elemental Fluorine to 8-Fluoropurines in One Step
Jorge R. Barrio,* Mohammad Namavari, Michael E. Phelps, and
Nagichettiar Satyamurthy
Contribution from the Department of Molecular and Medical Pharmacology,
The Crump Institute of Biological Imaging and the Laboratory of Structural Biology and
Molecular Medicine, UCLA School of Medicine, Los Angeles, California 90095
ReceiVed May 2, 1996X
Abstract: An efficient regiocontrolled approach to the synthesis of 8-fluoropurines by direct fluorination of purines
with dilute elemental fluorine is described. The one-step procedure produced regiospecific substitution of the C(8)-
hydrogen of purine derivatives in isolated yields close to 30% with protected purines in CHCl3. Fluorination yields
with unprotected purines in EtOH were reduced to less than 10%. The electrophilic fluorination procedure has a
broad scope of applicability and permits a ready and easy access to 8-fluoropurine derivatives, for the first time, for
evaluation of their biochemical and pharmacological properties.
Introduction
sugar2 side chain, but access to 8-fluoropurines has remained
limited.11 As a result, knowledge on the enzymatic and/or
pharmacological activities of 8-fluoropurines has been hampered
by the difficulties encountered in their synthesis. We now report
the first reaction of purines with elemental fluorine to generate
efficiently 8-fluoropurines.12
The interest in fluorinated purine and pyrimidine derivatives
in the last four decades stems from the unique properties
displayed by fluorine-substituted bioactive molecules.1 In terms
of size, replacement of a hydrogen for fluorine would produce
minimum steric perturbations upon the binding of the analogue
molecules to receptors or enzymes. However, the strong
electron-withdrawing properties of fluorine may substantially,
yet in a predictable manner, alter the chemical stability2 or
enzymatic activity3 of substrate molecules. Stimulated by this
combination, and the increased stability of the carbon-fluorine
bond relative to the carbon-hydrogen bond, new procedures for
the synthesis of fluoropurines,1,4,6-10 and fluoropyrimidines (and
their nucleosides)1,5 with anticancer and antiviral activities1 were
developed. The fluorine atom has been successfully introduced
at the 2- and 6-positions of the purine ring system1,4 and the
Results and Discussion
Chemistry. A variety of novel fluorinating reagents has been
introduced for site-specific fluorination in the past 15 years.13
However, regiospecific fluorination reactions involving elemen-
tal fluorine are still rare. Surprisingly, the reaction of 9-sub-
stituted purines 1 with F2 (1% in He) in polar solvents proceeds
smoothly at room temperature. In general, the C(8)-fluorinated
derivative was the major isolated product in moderate yields.
The simplicity of this one-step fluorination reaction, however,
makes it very attractive to produce otherwise inaccessible
8-fluoropurine derivatives. The fluorination reactions are easily
carried out with regular glassware using mixtures of F2 and He.
The flow rate of the F2/He mixture is controlled (∼5-10 µmol
of F2/min) to assure a fine dispersion of the gas mixture in the
solvents used.14 Progress of the reaction, namely the conversion
of 1a-f to 2a-f (Table 1), was monitored by TLC, 1H NMR,
and 19F NMR. Disappearance of the C(8)-hydrogen singlet at
7.7-8.0 ppm in 1H NMR and the concomitant appearance of a
singlet in the 19F NMR at -102.3 to -108.2 ppm (Table 2)
enabled the progress of the reaction and incidentally confirmed
a reaction at carbon-8 of the purine ring.
The protected oxopurines 1b,d,e and adenosine 1f all reacted
cleanly with F2, and the corresponding 8-fluoro counterparts
(2b,d,e,f) were obtained in 25-30% isolated yields. Unreacted
starting material (g20%) was always present at the end of the
reaction. Attempts to consume most of the starting material(s)
led to a decrease in the isolated product (2) yields, presumably
due to multiple fluorination and/or formation of oxidation
products. For example, in the fluorination of 1f, a byproduct
with a molecular weight of 446 (FAB MS) and a 19F NMR
* Address correspondence to Jorge R. Barrio, UCLA School of Medicine,
Department of Molecular and Medical Pharmacology, Box 956948, Los
Angeles, CA 90095-6948.
X Abstract published in AdVance ACS Abstracts, October 15, 1996.
(1) (a) Walsh, C. In AdVances in Enzymology; Meister, A., Ed.; John
Wiley and Sons: New York, 1983; Vol. 55, p 197. (b) Welch, J. T.
Tetrahedron 1987, 43, 3123. (c) SelectiVe Fluorination in Organic and
Bioorganic Chemistry; Welch, J. T., Ed.; American Chemical Society:
Washington, DC, 1991. (d) Barnette, W. E. CRC Crit. ReV. Biochem. 1984,
15, 210.
(2) Marquez, V. E.; Tseng, C. K.-H.; Mitsuya, H.; Aoki, S.; Kelley, J.
A.; Ford, H., Jr.; Roth, J. S.; Broder, S.; Johns, D. G.; Driscoll, J. S. J.
Med. Chem. 1990, 33, 978.
(3) Silverman, R. B. in Mechanism-Based Enzyme InactiVation: Chem-
istry and Enzymology; CRC Press, Inc.: Boca Raton, FL, 1988; Vol. I, p
59.
(4) See, for example: (a) Robins M. J.; Uznanski, B. Can. J. Chem.
1981, 59, 2608. (b) Montgomery, J. A.; Shortnacy, A. T.; Secrist, J. A., III
J. Med. Chem. 1983, 26, 1483. (c) Secrist, J. A., III; Shortnacy, A. T.;
Montgomery, J. A. J. Med Chem. 1985, 28, 1740. (d) Secrist, J. A., III;
Bennett, L. L., Jr.; Allan, P. W.; Rose, L. M.; Chang, C. H.; Montgomery,
J. A. J. Med. Chem. 1986, 29, 2069. (e) Secrist, J. A., III; Shortnacy, A.
T.; Montgomery, J. A. J. Med. Chem. 1988, 31, 405.
(5) (a) Robins M. J.; Naik, S. R. J. Am. Chem. Soc. 1971, 93, 5277. (b)
Robins, M. J.; MacCoss, M.; Naik, S. R.; Ramani, G. J. Am. Chem. Soc.
1976, 98, 7381. (c) Gerstenberger, M. R. C.; Haas, A. Angew Chem., Int.
Ed. Engl. 1981, 20, 647.
(6) Beaman, A. G.; Robins, R. K. J. Org. Chem. 1963, 28, 2310.
(7) Naik, S. R.; Witkowski, J. T.; Robins, R. K. J. Org. Chem. 1973,
38, 4353.
(8) Ikehara, M.; Yamada, S. Chem. Pharm. Bull. 1971, 19, 104.
(9) Kobayashi, Y.; Kumadaki, I.; Ohsawa, A.; Murakami, S. J. Chem.
Soc., Chem. Commun. 1976, 430.
(11) Ratsep, P. C.; Robins, R. K.; Vaghefi, M. M. Nucleosides Nucle-
otides 1990, 9, 197 and references cited therein.
(12) For a preliminary communication, see: Barrio, J. R.; Namavari,
M.; Phelps, M. E.; Satyamurthy, N. J. Org. Chem. 1996, 61, 6084.
(13) The chemistry of halides, pseudo-halides and azides; Patai, S.,
Rappoport, Z., Eds.; John Wiley and Sons: Chichester, U.K., 1995;
Supplement D2 Part 1, p 629.
(10) Sono, M.; Toyoda, N.; Shizuri, Y.; Tori, M. Tetrahedron Lett. 1994,
35, 9237.
(14) Rozen, S.; Gal, C.; Faust, Y. J. Am. Chem. Soc. 1980, 102, 6860.
S0002-7863(96)01456-4 CCC: $12.00 © 1996 American Chemical Society

Elemental Fluorine to 8-Fluoropurines
J. Am. Chem. Soc., Vol. 118, No. 43, 1996 10409
Table 1
In the reaction with F2, a key element in the success of the
fluorination reaction resides in the modulation of the electro-
philic character of F2 reactivity.20 The use of polar solvents
for the fluorination reactions lowers the activation energy of
the transition state intermediate below that of homolytic cleavage
of the F-F bond (39 kcal/mol).20d It also provides a hydrogen
acceptor to the counterion of the electrophile (e.g., fluoride ion
with F2 and acetate ion for AcOF).20b,c
The effective use of polar solvents for fluorine substitution
reactions has been observed earlier. The synthesis of 5-fluo-
rouracil21 and its nucleosides21,22 using elemental fluorine in
polar solvents has been proposed as initiated by syn addition
of fluorine across the C(5)-C(6) double bond with solvent-
assisted eliminations of fluoride ion. The use of CHCl3 also
allowed the controlled fluorination of benzoate esters,23 but
similar aromatic substitutions with diluted elemental fluorine
(<1% F2 in N2) in solvents favoring homolytic F2 cleavage (e.g.,
CFCl3) produced very low yields (<0.1%) of fluorinated
products.24 Nevertheless, regiospecific monofluorinations of
aromatic substrates with elemental fluorine (or AcOF) are
unfavored and frequently require the directing effects of Group
IVA metals (Si, Ge, Sn).25,26 Even more rare are reports of
selective F2-mediated monofluorination of nitrogen-containing
heterocyclic compounds.27 It is noteworthy in this regard that
fluorination of both imidazole and stannylated derivatives
thereof produced essentially trace fluorinated products as judged
by 19F NMR.13,28 In any event, it is quite conspicuous that the
direct fluorination of purines has not yet been reported while
other halogens have been successfully used in their elemental
form to produce regiospecific substitution of the C(8)-hydrogen
of purines29 and in many instances their use constitutes the most
convenient procedure for regiospecific C(8)-halogen substitution.
Effects of C(8) Fluorine Substitution. The lack of easy
access to 8-fluoropurines has limited the understanding of their
biochemical and pharmacological properties. It could be
anticipated, for example, that the electronegative effects of the
fluorine atom at C(8) in these analogues may be significant on
the substrate activity of 8-fluoropurine ribosides with several
enzymes (e.g., N-ribosylhydrolases and transferases that cleave
the C-N ribosidic bond by an SN1-like mechanism).2,30,31
Theoretical and experimental approaches32 have indicated the
existence of a marked flexibility about the glycosidic bond of
isolated
yields
(%)
compda
R
X
Y
a
b
cb
d
e
â-D-ribofuranosyl
NH2 OH
7
30
10
28
27
25
2′,3′,5′-tri-O-acetyl-â-D-ribofuranosyl NHAc OH
(2-hydroxyethoxy)methyl
(2-acetoxyethoxy)methyl
2′,3′,5′-tri-O-acetyl-â-D-ribofuranosyl H
2′,3′,5′-tri-O-acetyl-â-D-ribofuranosyl H
NH2 OH
NHAc OH
OH
NH2
f
a The cyclic amide tautomers of the indicated hydroxy(lactim)
heterocycles predominate in aqueous solution. b Reaction also performed
with acetyl hypofluorite in HOAc (see the Experimental Section).
Table 2. 19F and H NMR Chemical Shifts for Purine and
Anomeric (or N-CH2-O) Protons
1
19F NMRa
1H NMR
C(2)-H
compd
C(8)-F
C(8)-H
7.93
C(1′)-H
solvent
1a
2a
1b
2b
1c
2c
1d
2d
1e
2e
1f
5.68
5.61
6.10
6.08
DMSO-d6
DMSO-d6
CDCl3
-102.3
-107.5
-108.2
-107.8
-103.3
-102.9
7.77
7.80b
7.78
8.00
7.95
CDCl3
5.33c,d
5.32c,d
5.47c
5.39c
6.15
DMSO-d6
DMSO-d6
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
8.26
8.20
8.36
8.34
6.05
6.17
6.05
2f
a Typical chemical shifts for fluoropurines are as follows: C(2)-F,
-50 ppm; C(6)-F, -60 to -79 ppm.49,11 b C(8)-H chemical shift in
CD3OD: 7.76. c Chemical shifts for the N-CH2-O protons (singlet).
d C(1′)-H chemical shifts in CD3OD: 1c, 5.40; 2c, 5.32.
chemical shift of -51.24 was also observed. Thus, the isolated
yields reported herein reflect these conditions.
Solubility of the unprotected purines 1a,c precluded the use
of the most favorable solvent conditions (e.g., CHCl3), but we
found that these fluorination reactions could be carried out in
EtOH with acceptable results [the addition of base improved
their solubility in EtOH (see the Experimental Section)].15 We
have also evaluated the effectiveness of acetyl hypofluorite
(AcOF) for selective C(8)-hydrogen substitution with 1c. The
milder AcOF (generated in-situ)16 also reacted but with lower
efficiency. However, in our hands, initial attempts to fluorinate
these substituted purines with XeF2 and other recently introduced
fluorinating agents failed. Thus, reactions of 1b,d,e,f with
XeF2,17 N-fluoropyridinium triflate, N-fluoro-3,5-dichloropyri-
dinium triflate, N-fluoro-2,4,6-trimethylpyridinium triflate,18 and
N-fluoro-N-(chloromethyl)triethylenediamine bis (tetrafluorobo-
rate) (Select fluor reagent)19 were all unsuccessful.
(18) Umemoto, T.; Fukami, S.; Tomizawa, G.; Harasawa, K; Kawada,
K.; Tomita, K. J. J. Am. Chem. Soc. 1990, 112, 8563.
(19) Lal, G. S. J. Org. Chem. 1993, 58, 2791.
(20) See, for example: (a) Gal, C.; Rozen, S. Tetrahedron Lett. 1984,
25, 449. (b) Rozen, S.; Gal, C. J. Org. Chem. 1987, 52, 2769. (c) Rozen,
S. Acc. Chem. Res. 1988, 21, 307. (d) Pross, A. AdV. Phys. Org. Chem.
1985, 21, 99.
(21) Cech, D.; Holy, A. Collect. Czech. Chem. Commun. 1976, 41, 3335.
(22) Purrington, S. T.; Kagen, B. S.; Patrick, T. B. Chem. ReV. 1986,
86, 997.
(23) Grakauskas, V. J. Org. Chem. 1970, 35, 723.
(24) Cacace, F.; Giacomello, P.; Wolf, A. P. J. Am. Chem. Soc. 1980,
102, 3511.
(25) (a) Namavari, M.; Bishop, A.; Satyamurthy, N.; Bida, G. T.; Barrio,
J. R. Appl. Radiat. Isot. 1992, 43, 989. (b) Namavari, M.; Satyamurthy, N.;
Phelps, M. E.; Barrio, J. R. Appl. Radiat. Isot. 1993, 44, 527. (c)
Satyamurthy, N.; Namavari, M.; Barrio, J. R. CHEMTECH 1994, 24, 25.
(d) Lacan, G.; Satyamurthy, N.; Barrio, J. R. J. Org. Chem. 1995, 60, 227.
(26) Coenen, H. H.; Moerlein, S. M. J. Fluorine Chem. 1987, 36, 63.
(27) See, for example: (a) Van Der Puy, M.; Nalewajek, D.; Wicks, G.
E. Tetrahedron Lett. 1988, 29, 4389. (b) Kosower, E. M.; Hebel, D.; Rozen,
S.; Radkowski, A. E. J. Org. Chem. 1985, 50, 4152. (c) Luxen, A.; Barrio,
J. R.; Satyamurthy, N.; Bida, G. T.; Phelps, M. E. J. Fluorine Chem. 1987,
36, 83.
(28) Bryce, M. R.; Chambers, R. D.; Mullins, S. T.; Parkin. A. Bull.
Chim. Soc. Fr. 1986, 930.
(29) Fused Pyrimidines. Brown, D. J., Ed.; Part II, Purines; Meister, J.
H., Ed.; Wiley-Interscience: New York, 1971; Chapter V, p 135.
(30) Mazzella, L. J.; Parkin, D. W.; Tyler, P. C.; Furneaux, R. H.;
Schramm, V. L. J. Am. Chem. Soc. 1996, 118, 2111.
(15) With protected purines (1b,d,e,f), about 80% of the starting material
was consumed under the conditions described in the Experimental Section.
For reaction with purines 1a,c in EtOH, larger amounts of starting material
could be recovered from the reaction mixture. Longer reaction times,
however, revealed the formation of side products in detriment of the yield
of the C(8)-fluorinated products. Also, solvents such as water and HOAc
provided much lower yields than that observed in EtOH medium.
(16) Shiue, C.-Y.; Salvadori, P. A.; Wolf, A. P.; Fowler, J. S.; MacGregor,
R. R. J. Nucl. Med. 1982, 23, 899.
(17) Reference 13, p 821.

10410 J. Am. Chem. Soc., Vol. 118, No. 43, 1996
Barrio et al.
8-substituted purine analogues with substituents having a van
der Waals radius of less than 2 Å.33 The conformation of the
base about the glycosidic bond (e.g., syn or anti) is frequently
related to the chemical shift of C(1′)H, but this chemical shift
is also markedly dependent on the anisotropic effect of the C(8)
substituent.31d In this regard, the solvents effects cannot be
excluded either.31d Therefore, little information about their
potential as enzyme substrates can be extracted at the present
time from the observed chemical shifts of the â-D-ribofuranosyl
anomeric proton (C(1′)H) (or the CH2-N(9) proton) in 8-fluo-
ropurines, when compared with the C(8)-H substituted coun-
terparts (Table 2).34 It is also unlikely that the preferential
conformer population based on torsion around the C(1′)-N(9)
bond may play a significant role in the ability of 8-fluoropurine
analogues to bind to enzymes, transporters, or receptor sites.
More likely, modification of biological activity could be
attributed to the electron-withdrawing properties of the fluorine
atom, rather than to any conformational changes around the
glycosidic bond.31d,32
To gain initial insight into the significance of 8-fluoro sub-
stitution, we evaluated the 8-fluoroacycloguanine (2c) in its
ability to act as a substrate for Herpes Virus Simplex I thymidine
kinase (HSV tk).35,36 The specificity of 2c for HSV tk and its
unique in-vivo stability and rapid plasma clearance have also
permitted the use of fluorine-18-labeled 2c37 to image with
positron emission tomography the expression of HSV tk
transplanted genes in living animals as reported in detail
elsewhere.36
of 8-fluoropurine derivatives. This reliable and direct synthetic
approach now makes accessible a variety of 8-fluoro-substituted
purines for determination of their biochemical and pharmaco-
logical properties. As an indication of the potential of these
derivatives, the ability of the 8-fluoroacycloguanine derivative
2c to act as a substrate for HSV tk has already provided a new
approach to monitor gene expression in-vivo.36
Experimental Section
Melting points were determined on an electrothermal melting point
apparatus and are uncorrected. The 1H, 13C, and 19F NMR spectra were
recorded with a 360 MHz instrument. The 1H and 13C chemical shifts
are expressed in parts per million downfield from tetramethylsilane
(TMS). The 19F chemical shifts are referenced to an external fluorot-
richloromethane standard. The concentrations of all the samples for
NMR analysis were maintained at 50 mM. Electron impact and direct
chemical ionization (DCI) high-resolution mass spectral (HRMS) data
were recorded on a VG Analytical Autospec mass spectrometer. Fast
atom bombardment (FAB) high-resolution mass spectral data were
obtained on a ZAB SE mass spectrometer. The preparative HPLC
purification were carried out on a Beckman 110 system equipped with
a UV detector. Ultraviolet spectra were recorded with a Beckman DU-
640 spectrophotometer. TLC were run on silica gel plates (Whatman
PE SIL G/UV) in CHCl3:CH3OH:H2O. Solvent proportions were as
follows: 1a, 2a, 1c, 2c (60:36:4); 1b, 2b, 1d, 2d (80:18:2); 1f, 2f (90:
9:1); and 1e, 2e (100% EtOAc).
Caution: Fluorine and acetyl hypofluorite are highly toxic and
reactiVe gases. HoweVer, they can be handled safely by following the
procedures deVeloped specifically for such gases.38
Direct Fluorination of Unprotected Purine Nucleosides with F2.
Fluorine (1% in He, 0.6 mmol) was bubbled into a solution of the
unprotected purine derivative (0.3 mmol) in absolute ethanol (6.0 mL)
and tetraethylammonium hydroxide (0.34 mL of 20% aqueous solu-
tion) at room temperature over a period of 1 h. The reaction mixture
was neutralized with 1 N HOAc (0.46 mL), concentrated, and
chromatographed on silica gel (CHCl3:CH3OH:H2O ) 90:9:1). Earlier
fractions contained the required fluoro analogue, and from the later
fractions, the unreacted starting material was recovered. Fractions
containing product were pooled, and the solvents were evaporated to
give the 8-fluoropurine nucleoside analogue, which was further purified
by preparative HPLC (column: Alltech Econosil, C-18, 5µ, 50 × 1
cm; mobile phase: 5% CH3OH in water, flow rate: 5 mL/min; UV:
254 nm).
8-Fluoro-9-[(2-hydroxyethoxy)methyl)]guanine (8-Fluoroacyclo-
vir) (2c). 10% isolated yield (52% yield based on the starting material
recovered); mp 212 °C (dec). 1H NMR (CD3OD): δ 3.53-3.62 (m,
4H), 5.31 (s, 2H) ppm. UV (H2O) λmax (H2O): 242 nm (ꢀ 9530), 275
(7100). Electron impact HRMS calcd for C8H10N5O3F: 243.0768.
Found: 243.0772.
Conclusion. The use of elemental fluorine provides for the
first time a procedure of broad applicability for the synthesis
(31) (a) In the only example available to date on the ability of an
8-fluoropurine to act as a substrate for an enzymatic reaction (Ikehara, M.;
Fukui, T. Biochim. Biophys. Acta 1974, 338, 512), it was reported that
adenosine deaminase (EC 3.5.4.4), an enzyme known to bind adenosine in
the anti conformation,31b,c hydrolyzed 8-fluoroadenosine only at 1% of the
rate of adenosine. Caution should exist in the interpretation of these data,
however. Kobayashi et al.9 reported that the product claimed by Ikehara
and Yamada8 was not 8-fluoroadenosine. Further, Stolarski et al.31d reported
their unsuccessful attempts to synthesize 8-fluoroadenosine by following
the procedure of Ikehara and Yamada.8 Our spectroscopic data for
8-fluoroadenosine (2f) agree with those reported by Kobayashi et al.9 (see
the Experimental Section). (b) Hampton, A.; Harper, P. J.; Sasaki, T.
Biochemistry 1972, 11, 4736. (c) Zemlicka, J. J. Am. Chem. Soc. 1975, 97,
5896. (d) Stolarski, R.; Pohorille, A.; Dudycz, L.; Shugar, D. Biochim.
Biophys. Acta 1980, 610, 1.
(32) (a) Orozco, M.; Lluis, C.; Mallol, J.; Canela, E. I.; Franco, R. J.
Pharm. Sci. 1990, 79, 133. (b) Ikehara, M.; Uesugi, S.; Yoshida, K.
Biochemisty 1972, 11, 830.
(33) Fluorine has a small van der Waals radius (1.35 Å) which closely
resembles that of hydrogen (1.2 Å). See: Pauling, L. The Nature of the
Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960; p
260.
8-Fluoroguanosine (2a). 7% yield (46% yield based on the starting
material recovered); mp 238 °C (dec). 1H NMR (DMSO-d6):
δ
(34) More definitive results should await studies on the 1H, 13C, and 19
F
3.42-3.56 (m, 2H), 3.81-3.84, 4.02-4.06 (2m, 2H), 4.54-4.59
(m, 1H), 4.88 (t, J ) 5.8 Hz, 1H), 5.15 (d, J ) 5.0 Hz, 1H), 5.49 (d,
J ) 5.7 Hz, 1H), 5.61 (d, J ) 6.6 Hz, 1H), 6.58 (br s, 2H), 10.83 (br
s, 1H) ppm. UV (methanol) λmax(H2O): 243 nm (ꢀ 9500), 275 (7100).
FAB HRMS (M+H)+ calcd for C10H13N5O5F: 302.0901. Found:
302.0905.
Fluorination of Acyclovir (1c) with AcOF. To a solution of
AcOF16 [prepared by bubbling of 0.12 mmol of 1% F2 in He into a
solution of aqueous ammonium hydroxide (0.03 mL) in glacial acetic
acid (5 mL)] was added 25 mg of acyclovir (2c)39 (0.11 mmol) in 1
mL of acetic acid. The reaction mixture was stirred at room temperature
NMR of nucleotides in these series. Specific influences of the phosphate-
(s) on the sugar base torsion angle (XCN) have been recognized with purine
nucleotides: (a) Schweizer, M. P.; Broom, A. D.; Ts’o, P. O. P.; Hollis, D.
P. J. Am. Chem. Soc. 1968, 90, 1042. (b) Jardetzky, C. D.; Jardetzky, O. J.
Am. Chem. Soc. 1960, 82, 222 and other nucleotide analogues. (c) Barrio,
J. R.; Liu, F.-T.; Keyser, G. E.; VanDer Lijn, P.; Leonard, N. J. J. Am.
Chem. Soc. 1979, 101, 1564.
(35) (a) Robins, M. J.; Hatfield, P. W.; Balzarini, J.; De Clercq, E. J.
Med. Chem. 1984, 27, 1486. (b) Gosselin, G.; Bergogne, M.-C.; De Rudder,
J.; De Clercq, E.; Imbach, J.-L. J. Med. Chem. 1987, 30, 982.
(36) With K1735M2 murine melanoma cells stably transfected with
the HSV tk gene, 2c was 6 times more effective than 1c in competing for
HSV tk dependent incorporation of [3H]1c into cells. Moreover, 2c
effectively killed cells expressing HSV tk (50% survival at 0.2 µM), with
no substantial killing of cells transfected with the empty vector. In-ViVo in
mice, 2c showed unusual metabolic stability. 19F NMR analysis of
urine accumulated for 60 min after intravenous administration showed the
presence of a single molecular entity [19F NMR (CD3OD/CFCl3): δ -108.2
ppm] consistent with unaltered 2c. Srinivasan, A.; Gambhir, S.; Barrio, J.
R.; Wu, L.; Namavari, M.; Satyamurthy, N.; Sharfstein, S.; Cherry, S.;
Green, A.; Berk, A.; Phelps, M. E.; Herschman, H.R. Submitted for
publication in Science.
(37) Radiofluorinated 2b,c,d were synthesized by bubbling cyclotron-
produced [18F]F2 (Bishop, A. J.; Satyamurthy, N.; Bida, G. T.; Hendry, G.;
Phelps, M. E.; Barrio, J. R. Nucl. Med. Biol. 1996, 23, 189) (1% in Ar)
with 1b,c,d, respectively (specific activity: 2.5 Ci/mmol; t1/2 of 18F is 109.7
min).
(38) Matheson Gas Data Book; Braker, W., Mossman, A. L., Eds.;
Matheson: East Rutherford, NJ, 1971; p 261.
(39) Barrio, J. R.; Bryant, J. D.; Keyser, G. E. J. Med. Chem. 1980, 23,
572.

Elemental Fluorine to 8-Fluoropurines
J. Am. Chem. Soc., Vol. 118, No. 43, 1996 10411
for 15 min and evaporated to dryness. 8-Fluoroacyclovir (1c) (5%
isolated yield; 43% yield based on the starting material recovered) was
purified by column chromatography followed by preparative HPLC
(see above).
Direct Fluorination of Protected Purine Nucleosides with F2.
Fluorine (1% in He, 0.6 mmol) was bubbled into a solution of the
protected purine derivative (1b,d,e,f) (0.4 mmol) in CHCl3 (6.0 mL)
at room temperature over a period of 1 h. The reaction mixture was
concentrated and chromatographed (silica gel). The initial fraction
afforded the fluoronucleoside, and from the later fractions, the unreacted
starting material was recovered.
N2-Acetyl-8-fluoro-9-[(2-acetoxyethoxy)methyl]guanine (2d). Ace-
tonitrile was used as the solvent for the fluorination. Chromatography
eluent: CHCl3:CH3OH:H2O (95:4.5:0.5); 28% yield; mp 196-198 °C.
1H NMR (CD3OD): δ 1.97 (s, 3H), 2.23 (s, 3H), 3.82 (t, J ) 4.5 Hz,
2H), 4.15 (t, J ) 4.5 Hz, 2H), 5.47 (s, 2H) ppm. 13C NMR (DMSO-
d6): δ 20.38, 23.70, 62.53, 67.08, 70.97, 113.50 (d, J ) 12.2 Hz),
147.29, 148.68, 149.31 (d, J ) 244.1 Hz), 153.92, 170.09, 173.53 ppm.
DCI HRMS (M+H)+ calcd for C12H15N5O5F: 328.1057. Found:
328.1056.
N2,2′,3′,5′-Tetraacetyl-8-fluoroguanosine (2b). Chromatography
eluent: CHCl3:CH3OH:H2O ) 98:1.8:0.2; 30% yield (yield based on
the recovered starting material: 37%); mp 89-92 °C. 1H NMR
(CDCl3): δ 2.06 (s, 3H), 2.11 (s, 3H), 2.16 (s, 3H), 2.32 (s, 3H), 4.41
(dd, J ) 11.0 and 7.1 Hz, 1H), 4.48 (q, J ) 5.4 Hz, 1H), 4.69 (dd, J
) 11.0 and 4.8 Hz, 1H), 5.85 (t, J ) 4.6 Hz, 1H), 6.06 (t, J ) 5.2 Hz,
1H), 6.08 (d, J ) 5.4 Hz, 1H) ppm. 13C NMR (CDCl3): δ 20.40,
20.60, 20.89, 24.23, 63.09, 70.93, 72.33, 80.02, 85.52, 115.81 (d, J )
14.2 Hz), 145.67, 147.68, 149.06 (d, J ) 250.0 Hz), 154.46, 169.48,
169.87, 171.74, 171.89 ppm. These data are in agreement with literature
values.10
J ) 3.4 Hz), 154.34, 169.47, 169.58, 170.55 ppm. FAB HRMS (M +
H)+ calcd for C16H19N5O7F: 412.1268. Found: 412.1264. These data
are in agreement with literature values.9
2′,3′,5′-Tri-O-acetyl-8-fluoroinosine (2e). Chromatography elu-
ent: EtOAc:hexane (9:1); 27% yield; mp 80-83 °C. 1H NMR
(CDCl3): δ 2.10 (s, 3H), 2.11 (s, 3H), 2.16 (s, 3H), 4.29 (dd, J ) 11.9
and 4.9 Hz, 1H), 4.39 (q, J ) 4.5 Hz, 1H), 4.47 (dd, J ) 11.9 and 3.5
Hz, 1H), 5.65 (t, J ) 5.5 Hz, 1H), 6.02-6.05 (m, 2H), 8.20 (s, 1H),
13.02 (br s, 1H) ppm. 13C NMR (CDCl3): δ 20.31, 20.46, 20.61, 62.82,
70.30, 71.89, 80.34, 85.20, 119.55 (d, J ) 12.3 Hz), 145.28, 147.24
(d, J ) 3.7 Hz), 150.07 (d, J ) 252.9 Hz), 157.74, 169.32, 169.47,
170.39 ppm. FAB HRMS (M + H)+ calcd for C16H18N4O8F: 413.1109.
Found: 413.1104.
General Procedure35b,40,41 for Deprotection of 2b,d. To the
protected 8-fluoropurine derivative (2b,d) (0.1 mmol) was added a
solution of methanolic ammonia (5 mL of 2 M solution), and the
reaction mixture was stirred at room temperature. The solution was
evaporated to dryness, and the residue was purified by column
chromatography as described above to provide the deblocked 8-fluo-
ronucleosides (2a: reaction time 5 h, 50% yield; 2c: reaction time
2-4 h, 30% yield).42
Acknowledgment. Dedicated to Professor Nelson J. Le-
onard, scientist, teacher, and friend on the occasion of his 80th
birthday. This work was supported in part by Department of
Energy Grant DE-FC0387-ER60615 and donations from the
Jennifer Jones Simon and Ahmansons Foundations.
JA961456C
(40) Gosselin, G.; Bergogne, M.-C.; Imbach, J.-L. J. Heterocycl. Chem.
1993, 30, 1229.
(41) Matsumoto, H.; Kaneko, C.; Yamada, K.; Takeuchi, T.; Mori, T.;
Mizuno, Y. Chem. Pharm. Bull. 1988, 36, 1153.
(42) Literature precedence indicates for a nucleophilic displacement of
C(8)-F in methanolic ammonia.11 Under the experimental conditions for
deprotection, 2b,d defluorinated with a t1/2 of decomposition of about 5 h.
Defluorination was easily demonstrated with radiofluorinated 2b,d.37
Appearance of [18F]fluoride ion, disappearance of starting materials (2b,d),
and product formation (2a,c) was verified by radio thin layer chromatog-
raphy (silica gel, CHCl3:CH3OH:H2O, 60:36:4)). Optimum reaction times
for product (2a,c) isolation varied from 2 to 5 h.
2′,3′,5′-Tri-O-acetyl-8-fluoroadenosine (2f). Chromatography
eluent: EtOAc:hexane (3:1); 25% yield; mp 98-101 °C. 1H NMR
(CDCl3): δ 2.08 (s, 6H), 2.15 (s, 3H), 4.29 (dd, J ) 12.1 and 5.0 Hz,
1H), 4.39 (q, J ) 4.5 Hz, 1H), 4.48 (dd, J ) 12.1 and 3.5 Hz, 1H),
5.73 (t, J ) 5.4 Hz, 1H), 5.79 (br s, 2H), 6.05 (d, J ) 5.6 Hz, 1H),
6.07 (t, J ) 5.3 Hz, 1H), 8.34 (s, 1H) ppm. 13C NMR (CDCl3): δ
20.42, 20.56, 20.67, 62.95, 70.52, 71.96, 80.39, 85.04, 114.24 (d, J )
12.2 Hz), 148.68 (d, J ) 4.0 Hz), 150.83 (d, J ) 254.5 Hz), 152.86 (d,