Reactions of trimethylsilyl fluorosulfonyldifluoroacetate with purine and pyrimidine nucleosides

Article abstract of DOI:10.1016/j.jfluchem.2008.12.004

Difluorocarbene, generated from trimethylsilyl fluorosulfonyldifluoroacetate (TFDA), reacts with the uridine and adenosine substrates preferentially at the enolizable amide moiety of the uracil ring and the 6-amino group of the purine ring. 2′,3′-Di-O-ben

Full text of DOI:10.1016/j.jfluchem.2008.12.004

Journal of Fluorine Chemistry 130 (2009) 321–328
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Reactions of trimethylsilyl fluorosulfonyldifluoroacetate with purine and
pyrimidine nucleosides
Magdalena Rapp a,1, Xiaohong Cai b, Wei Xu b, William R. Dolbier Jr.b, Stanislaw F. Wnuk a,
*
a Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, United States
b Department of Chemistry, University of Florida, Gainesville, FL 32611, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
Difluorocarbene, generated from trimethylsilyl fluorosulfonyldifluoroacetate (TFDA), reacts with the
uridine and adenosine substrates preferentially at the enolizable amide moiety of the uracil ring and the
6-amino group of the purine ring. 20,30-Di-O-benzoyl-30-deoxy-30-methyleneuridine reacts with TFDA to
produce 4-O-difluoromethyl product derived from an insertion of difluorocarbene into the 4-hydroxyl
group of the enolizable uracil ring. Reaction of the difluorocarbene with the adenosine substrates having
the unprotected 6-amino group in the purine ring produced the 6-N-difluoromethyl derivative, while
reaction with 6-N-benzoyl protected adenosine analogues gave the difluoromethyl ether product
derived from the insertion of difluorocarbene into the enol form of the 6-benzamido group. Treatment of
the 6-N-phthaloyl protected adenosine analogues with TFDA resulted in the unexpected one-pot
conversion of the imidazole ring of the purine into the corresponding N-difluoromethylthiourea
derivatives. Treatment of the suitably protected pyrimidine and purine nucleosides bearing an
exomethylene group at carbons 20, 30 or 40 of the sugar rings with TFDA afforded the corresponding
spirodifluorocyclopropyl analogues but in low yields.
Received 18 October 2008
Received in revised form 10 December 2008
Accepted 12 December 2008
Available online 25 December 2008
Keywords:
Carbohydrates
Difluorocarbene
Difluorocyclopropanes
Nucleosides
Purines
Pyrimidines
Spiro compounds
ß 2008 Elsevier B.V. All rights reserved.
1. Introduction
used to differentiate the contrasting conformational preferences
between kinases and polymerases [12].
Attachment of fluorine atoms on the sugar and heterocyclic
moieties of nucleoside analogues can impart potent anticancer and
antiviral activity [1,2]. The small size and powerful electronega-
tivity of fluorine can cause significant effects on the metabolic
pathway as compared to the natural nucleoside analogues. The
best known examples are 5-fluorouracil (inhibitor of thymidylate
synthase) [3], 20-deoxy-20-(fluoromethylene)cytidine [4] and 20-
deoxy-20-difluorocytidine [5] [gemcitabine, inhibitors of ribonu-
cleotide diphosphate reductase (RDPR)], and 40,50-unsaturated 50-
fluoroadenosine (inhibitor of the adenosylhomocysteine hydro-
lase) [6] among others [1,2]. Robins and co-workers [7a] and
Czernecki et al. [7b] prepared 20-deoxy-20-spirocyclopropane
nucleosides (e.g., A) as putative inhibitors of the RDPR (Fig. 1).
Other fused- and spiro-cycloalkyl derivatives of naturally occur-
ring carbohydrate and nucleoside analogues with restrictions in
their conformational flexibility were also designed and synthe-
sized [7–11]. Marquez and coworkers elegantly demonstrated that
bicyclo[3.1.0]hexane scaffold of carbocyclic nucleosides can be
It is well known that geminal fluorine substituents on
cyclopropane give rise to a substantial increase in ring strain
which leads to an increasing ease of cleavage [13]. The kinetic
effect of the 9–10 kcal/mol of incremental strain is derived largely
from a homolytic cleavage of the carbon–carbon bond which is
opposite to the CF2 group. We designed gem-difluoro substituted
20-deoxyuridine 20-spirocyclopropane B as a potential inhibitor of
RDPR since the abstraction of H30 by the radical initiator (Cys439)
from C30 of the 50-diphosphate C and distal (to the fluorine
substituents) ring opening of the spiro 2,2-difluorocyclopropyl-
carbinyl radical D would result in generation of the corresponding
2,2-difluoro-3-butenyl radical species
E which might cause
inactivation of RDPR. The calculated activation barrier of
1.9 kcal/mol and an estimated ring opening rate constant of
1.5 ꢀ 1011 sꢁ1 at 25 8C qualifies the difluorocyclopropylcarbinyl
system [13b] as a ‘‘hypersensitive’’ probe of reactions involving
radicals as intermediates.
Recently, Nowak and Robins reported meticulous studies on the
addition of difluorocarbene [generated from either Hg(CF3)2 or
PhHgCF3 and NaI] to double bonds placed between carbons 30-40
and 40-50 in the ribose rings of nucleosides (all electron rich vinyl
ethers) to give derivatives with the 30,40-fused and 40-spiro
difluorocyclopropane rings, respectively [11]. We now report
studies on the reactions of difluorocarbene, generated from the
* Corresponding author. Tel.: +1 305 348 6195; fax: +1 305 348 3772.
E-mail address: wnuk@fiu.edu (S.F. Wnuk).
1
Present address: Faculty of Chemistry, Adam Mickiewicz University, Poznan,
Poland.
0022-1139/$ – see front matter ß 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2008.12.004

322
M. Rapp et al. / Journal of Fluorine Chemistry 130 (2009) 321–328
Fig. 1. Possible generation of the active intermediates from the
spirodifluorocyclopropyl nucleoside analogues by the ribonucleoside diphosphate
reductase.
trimethylsilyl fluorosulfonyl-difluoroacetate (TFDA) [14], with the
Scheme 2. (a) BzCl/pyridine, (b) BzCl/Et(i-Pr)2N/pyridine, (c) TFDA/NaF/benzene/
3 h/
D.
purine and pyrimidine nucleosides bearing
a less reactive
exomethylene double bond on the ribose ring. The TFDA reagent
was selected because of (a) its lower toxicity as compared to the
Hg-based reagents, (b) its capability of generating difluorocarbene
at a lower temperature than is usually required for most reagents,
and (c) the fact that all the products of the fluoride-catalyzed
thermal decomposition of TFDA, other than :CF2 and the recycled
fluoride, are gases which are lost from the reaction mixture.
product’s 19F NMR spectrum and the triplet at
d 7.41 (J = 71.0 Hz) in
its 1H NMR spectrum are characteristic of the diastereotopic
fluorines of the OCF2H group. In addition to 5, a sugar byproduct
whose structure was assigned as ribosyl fluoride [17] 6 (8%) was
also isolated. A similar insertion of difluorocarbene, generated
from Seyferth’s phenyl(trifluoromethyl) mercury reagent, into the
4-hydroxyl group of a uracil ring has been observed [18]. When the
reaction between 4a and TFDA was carried out in toluene (110 8C)
the ratio of products changed [5 (30%) and 6 (23%)].
2. Results and discussion
In an attempt to prevent difluorocarbene insertion into the 4-
hydroxyl of the uracil ring the substrates 4b and 7b with
unenolizable uracil rings protected with benzoyl and methyl
groups at N3 were prepared. Unfortunately, treatment of 4b with
TFDA resulted mainly in removal of the N-benzoyl group from the
uracil aglycone giving 4a (70%) plus 5 (13%).
We chose diacetone 3-deoxy-3-methyleneglucose [15] 1 as a
convenient substrate to study the feasibility of the addition of
difluorocarbene to exomethylene double bonds of pentofuranose
sugar rings that are not electron rich vinyl ethers. Treatment of 1
with difluorocarbene generated in situ from ‘‘acid free’’ trimethyl-
silyl fluorosulfonyldifluoroacetate (TFDA) [14] produced spirodi-
fluorocyclopropane derivatives 2a and 2b (20%, 2:1; Scheme 1).
The 19F NMR signals for the geminal fluorine atoms in 2a appeared
Difluorocyclopropanation of 3-N-20,50-O-trimethylated deri-
vative 7b did lead to formation in a low yield of the desired 30-
spirodifluorocyclopropyl uridine analogues 8a and 8b [10%,
ꢂ1:1, 1H and 19F NMR (crude reaction mixture)], but most of 7b
was recovered unchanged (Scheme 3). The stereochemical
assignments for separated 8a(30R) and 8b(30S) were confirmed
by NOESY experiments. Thus, the correlation between one of
at
d
ꢁ128.83 (ddd, J = 4.3, 13.5, 165.6 Hz) and ꢁ134.56 (dd, J = 11.2,
165.0 Hz) whereas for 2b they were located at
d
ꢁ127.72 (ddt,
J = 7.0, 13.8, 159.0 Hz) and ꢁ135.64 (dd, J = 11.3, 159.0 Hz). The
values of the larger coupling constants observed for these two
compounds are typical for AB couplings of unsymmetrical gem-
difluorocyclopropanes. Attempted reaction of 1 with TFDA at a
higher temperature (toluene/110 8C) produced only traces of 2a
and 2b.
the cyclopropyl protons (H30,
d (d 4.26) was
1.34) and H40
observed for isomer 8a ( -face addition), whereas isomer 8b (a-
b
face addition) exhibited a correlation between H30/300 and H50
and H20. Attempts of difluorocyclopropanation in refluxing
benzene or cyclohexanone resulted only in the recovery of
substrate 7b.
NOESY experiments supported the stereochemical assignment
at C3 in 2a as being 3R since correlations were observed between
one of the cyclopropyl protons (H30,
well as between the other cyclopropyl proton (H300,
d
1.38) and H2 (
d
d
4.31–4.34) as
1.84) and H5
Based on the known greater reactivity of the 20-deoxy-20-
methylene nucleosides than the corresponding 30-methylene
counterparts in 1,3-dipolar cycloaddition reactions with diazo-
methane [7c], we turned our attention to difluorocyclopropantion
of the 20-deoxy-20-methyleneuridine derivatives. Unfortunately,
reaction of the trimethylated 9c with TFDA in benzene failed to
give the corresponding 20-spirodifluorocyclopropyl products. Use
of higher temperatures (toluene) and extended reaction times
(
d
3.80), whereas a weak correlation was noted between H30 and
H4 for 2b(3S). The difluorocarbene appears to have added
preferentially to the more hindered -face to give 2a as the major
isomer. This is in contrast to the stereoselective -face reduction
a
b
that is observed for the NaBH4 reduction of the 3-keto and 3-
deoxy-3-methylene derivatives of diacetoneglucose [16].
The attempted difluorocyclopropanation reaction of closely
related nucleoside analogue 30-deoxy-30-methyleneuridine 4a
produced instead 4-O-difluoromethyl product 5 (68%), derived
from an insertion of difluorocarbene into the 4-hydroxyl group of
the enolizable uracil ring (Scheme 2). The pair of doublet of
doublets at
d
ꢁ90.66 and ꢁ91.20 (J = 71.0, 176.0 Hz) in the
Scheme 3. (a) CH2N2/Et2O/EtOH, (b) (MeO)2SO2/NaH/THF, (c) TFDA/NaF/toluene/
6 h/D.
Scheme 1. (a) TFDA/NaF/benzene/3 h/D.

M. Rapp et al. / Journal of Fluorine Chemistry 130 (2009) 321–328
323
Scheme 4. (a) CH2N2/Et2O/EtOH, (b) (MeO)2SO2/NaH/THF, (c) TFDA/NaF/toluene/D.
Scheme 6. (a) BzCl/pyridine, (b) TFDA/NaF/toluene/D.
(14 h) led to hydrolysis of the glycosylic bond and insertion
of difluorocarbene at 2 position of the pyrimidine base to give
2-O-difluoromethyl-3-N-methyluracil 10 (17%) in addition to 11
(11%; Scheme 4). Traces of acid present in the TFDA reagent and
the high temperature of these reactions presumably cause
glycosylic bond cleavage, particularly for the less stable 20-deoxy
substrates.
based on the UV, 13C and 2D NMR (HMBC) spectra in agreement
with the literature value for the N7/N9 methylated adenosine and
6-N,N-dimethyladenine analogues. Thus, the chemical shifts for
C4 and C5 in 16 are at
d
150.5 and 119.3, respectively, similar to the
149.3 (C4) and 119.6 (C5)] rather
-ribofuranosyl)adenine [
reported values for adenosine [
d
than for the N7 isomer, e.g., 7-(
160.7 (C4) and 110.3 (C5)] [21b].
b
-
D
d
The 5,6-double bond of the uracil ring of 7b or 9c was also
unreactive to difluorocyclopropanation under these conditions,
with no products containing a difluoro[4.1.0]bicyclic ring being
observed. Additions of (di)halocarbenes generated from the
corresponding di(or tri)haloacetic acid or acetates to uracil ring
are known [19], although no example of such a difluorocarbene
addition has been reported. It is also worth mentioning that an
attempted cyclopropanation of the 3-N-benzoyl-30,50-di-O-ben-
zoyl-20-deoxy-20-difluoromethyleneuridine [20] with diazo-
methane, followed by the photochemical extrusion of nitrogen
as demonstrated by Robins and co-workers [7a] and Czernecki
et al. [7b] for the synthesis of unfluorinated spirocyclopropyl
derivatives, also failed to give the desired spirodifluorocyclopropyl
analogues.
We also performed reactions of purine nucleosides with TFDA.
To test the interaction of difluorocarbene with the unprotected
exo amino group in the purine ring, we carried out reactions
with 50-O-benzoyl-20,30-O-isopropylideneadenosine 12. Thus,
treatment of 12 with TFDA in toluene led to the formation of
6-N-difluoromethyl derivative 13 (18%) and 6-N-formyladenosine
analogue 14 (33%, Scheme 5). Although we did not investigate this
reaction in details, based on the NMR and TLC analysis of the
aliquots taken during the reaction, it appears that 13 is formed
initially and then is slowly converted to 14.
Moreover, the UV spectrum of 16 (
those of 15a (lmax 277 nm) and 6-N,N-dimethylamino-9-methy-
ladenine ( max 276 nm) while a bathochromic shift was observed
for the 6-N,N-dimethylamino-7-methyladenine regioisomer (lmax
291 nm) [21c].
lmax 276 nm) is identical to
l
Next, reactions of highly electrophilic difluorocarbene with
nucleosides containing an electron rich vinyl ether unit were
examined. However, treatment of 20,30-di-O-acetyl-6-N-benzoyl
protected 18b with TFDA produced the difluoromethyl ether 19
(68%), as a result of the insertion of difluorocarbene into the enol
form of the 6-benzamido group, together with two minor
byproducts containing fluorine (ꢂ10%, 1H and 19F NMR) which
also had intact exomethylene units present (Scheme 7).
In order to eliminate the interaction of the difluorocarbene
with the 6-N-benzoyl protecting group, we tested 6-N-phthaloyl
protection [22] for the exo amino group in the adenine ring,
since a phthaloyl strategy for the protection of an amino group
had previously been used for the reaction with TFDA [14]. Thus,
treatment of 20b [prepared (76%) by reaction of 9-(2,3-O-
isopropylidene-5-deoxy-b-D-erythro-pent-4-enofuranosyl)
adenine [23] 20a with phthaloyl dichloride in pyridine] with
TFDA followed by column chromatography afforded 21a and 21b
in 11% and 6% yield, respectively (Scheme 8). The 19F NMR
spectrum for each product showed two sets of signals, one from
the spirodifluorocyclopropane ring and the second from a CF2H
unit. Slightly different 1H NMR spectra for both products 21a
Treatment of the adenosine analogue 15b with the exo amino
group of the adenine ring protected with diethyl groups [21a]
yielded 6-N,N-diethyl-9-difluoromethyladenine 16 (21%) in
addition to the ribofuranosyl fluoride 17 (9%; Scheme 6). The
position of difluoromethyl group at N9 in 16 was established
Scheme 7. (a) Ac2O/pyridine, (b) TFDA/NaF/benzene/D.
Scheme 5. (a) TFDA/NaF/toluene/D.

324
M. Rapp et al. / Journal of Fluorine Chemistry 130 (2009) 321–328
3. Conclusions
In summary, we have shown that difluorocarbene, generated
from trimethylsilyl fluorosulfonyldifluoroacetate (TFDA), reacts
with the uridine substrates to produce 4-O-difluoromethyl
products derived from the insertion of difluorocarbene into the
4-hydroxyl group of the enolizable uracil ring. Similar reactions of
difluorocarbene with adenosine substrates having unprotected 6-
amino group of the purine ring produced a 6-N-difluoromethyl
derivative, while reaction with 6-N-benzoyl protected substrates
gave difluoromethyl ether products derived from the insertion of
difluorocarbene into the enol form of the 6-benzamido group.
Treatment of the 6-N-phthaloyl protected adenosine analogues
with TFDA resulted in one-pot conversion of the imidazole ring of
the purine into the corresponding N-difluoromethylthiourea
derivatives. Treatment of suitably protected carbohydrates as
well as pyrimidine and purine nucleosides bearing an exomethy-
lene group at carbons 20, 30 or 40 of the sugar rings with TFDA gave
the corresponding spirodifluorocyclopropyl analogues, but in low
yields.
Scheme 8. (a) Phthaloyl dichloride/pyridine, (b) TFDA/NaF/toluene/D.
and 21b showed only singlets for purine H2 proton and a
characteristic triplet (J = 58 Hz) for the CF2H groups. On the basis
of the overall spectroscopic evidence and studies of the reaction
of TFDA with model imidazole compounds, including an X-ray
structure determination, a thiourea entity bearing a difluoro-
methyl group attached to N7 was proposed as the structure of 21a
[24]. A structure for the second product was tentatively assigned
as 21b based on: (i) the UV spectrum of 21b (lmax 278 nm) did
not exhibit bathochromic shifts as observed for thione 21a (lmax
319 nm); and (ii) NOESY experiment showed correlations
4. Experimental section
1H (Me4Si) NMR spectra at 400 MHz, 13C (Me4Si) at 100.6 MHz
and 19F (CCl3F) at 376.4 MHz were determined with solutions in
CDCl3 unless otherwise noted. The NOESY, HMQC and HMBC
experiments were recorded at 600 MHz instruments. Mass spectra
(MS) were obtained by atmospheric pressure chemical ionization
(APCI) techniques and HRMS using FAB mode unless otherwise
noted. Reagent grade chemicals were used and solvents were dried
by reflux over and distillation from CaH2, except toluene and
benzene (sodium) and THF (potassium), under an argon atmo-
sphere. TLC was performed on Merck Kieselgel 60-F254 with MeOH/
CHCl3 (1:9, 1:19), EtOAc/hexane (1:2) or EtOAc/i-PrOH/H2O (4:1:2,
upper layer; S1) as developing systems, and products were
visualized under 254 nm light or with I2. Merck Kieselgel 60
(230–400 mesh) was used for column chromatography. An ‘‘acid
free’’ trimethylsilyl 2-fluorosulfonyl-2,2-difluoroacetate (TFDA)
was prepared by treatment of the equimolar quantity of TFDA
with triethylamine as reported [14]. The purity of TFDA was
between H50 and H30 confirming the
a-face addition of
difluorocarbene to exomethylene double bond while such
correlation was not observed for 21a.
The formation of the N-difluoromethylthiourea-type product
23 (18%) was also observed in the reaction of the protected
adenosine derivative 22b with TFDA, although the 1-fluoro sugar
byproduct 17 (32%) was a major product isolated from the reaction
mixture (Scheme 9).
We also attempted difluorocyclopropanation of
a ribose
derivative having an exomethylene group at C2 next to the
anomeric carbon. Thus, treatment of methyl-3,5-di-O-(4-chloro-
benzyl)-2-deoxy-2-methylene-a-D-ribofuranoside 24 [25] with
TFDA afforded methyl furanoside 25a(2R) and furanose 25b(2S)
with a free hydroxyl group at C1 (35%, 7:1; Scheme 10). A NOESY
experiment with 25a showed a correlation between one of the
protons from the cyclopropyl ring (H20) and H1 and H3, while no
such correlation was observed with 25b confirming
a
-face and
b
-
conveniently evaluated by 19F NMR in CDCl3 (dried over molecular
˚
face addition of difluorocarbene.
sieves 4 A) where TFDA shows peak at
from 2-fluorosulfonyl-2,2-difluoroacetic acid (FDA) appears at
d
ꢁ103.20 while the signal
d
ꢁ103.91. TFDA and FDA were purchased from Aldrich Co. Sodium
fluoride (NaF) was dried in the oven. Reactions with TFDA were
carried out in 10 or 25 mL three-necked flasks equipped with a
magnetic stirrer, a funnel with nitrogen (N2) inlet and a water-
cooled condenser with gas outlet. Carbohydrates and nucleosides
substrates have to be well dried prior to use.
3-Deoxy-(R/S)-3,3-C-(1,1-difluoroethane-1,2-diyl)-1,2;5,6-di-O-
isopropylidene-a-D-ribo-hexofuranose (2a/2b). A solution of 1 [15]
(150 mg, 0.58 mmol) in anhydrous benzene (1.5 mL) containing
dried NaF (1.4 mg, 0.03 mmol) was heated at 80 8C (oil bath) for
30 min. An ‘‘acid free’’ trimethylsilyl fluorosulfonyldifluoroacetate
[14] (TFDA; 0.30 mL, 380 mg, 1.5 mmol) was then added dropwise
via a Teflon needle at the rate of 0.15 mL/h (controlled by syringe
pump). The solution was refluxed for 3 h and then cooled to
ambient temperature and was evaporated to give crude mixture of
2a/2b (ꢂ20%, 2:1; 1H and 19F NMR). The reaction mixture was
partitioned (NaHCO3/H2O//CHCl3) and the separated organic layer
was washed (brine), dried (Na2SO4), evaporated and was carefully
column chromatographed (0 ! 20% EtOAc/hexane) to give 2a(3R)
(10 mg, 6%) and 2b(3S) (5 mg, 3%) as slightly yellow oils and
recovered 1 (102 mg, 68%). Compound 2a [slightly more polar than
Scheme 9. (a) BzCl/pyridine, (b) TFDA/NaF/toluene/D.
1 on TLC (EtOAc/hexane, 3:7)] had: 1H NMR
3H), 1.30 (s, 3H), 1.38 (ddd, J = 8.1, 11.7, 13.5 Hz, 1H), 1.52 (s, 3H),
d 1.23 (s, 3H), 1.27 (s,
Scheme 10. (a) TFDA/NaF/benzene/5 h/D.

M. Rapp et al. / Journal of Fluorine Chemistry 130 (2009) 321–328
325
1.84 (ddd, J = 4.6, 8.1, 12.0 Hz, 1H), 3.80 (ddd, J = 4.8, 6.3, 7.7 Hz,
1H), 3.96 (dd, J = 4.9, 8.8 Hz, 1H), 4.02 (dd, J = 6.4, 8.8 Hz, 1H), 4.31–
4.34 (m, 1H), 4.35 (d, J = 8.0 Hz, 1H), 5.89 (d, J = 3.6 Hz, 1H); 19F
129.3, 129.7, 129.9, 130.3, 133.9, 134.0, 141.5, 145.6, 154.4, 166.0,
166.5, 168.1 (t, J = 3.9 Hz); HRMS calcd for C25H21F2N2O7 [M+H]+:
499.1317; found: 499.1316.
NMR
d
ꢁ128.83 (ddd, J = 4.3, 13.5, 165.6 Hz, 1F), ꢁ134.56 (dd,
17.8 (t, J = 10.5 Hz), 25.3, 26.4,
Also isolated was sugar byproduct whose structure was
tentatively assigned as 2,5-di-O-benzoyl-3-deoxy-3-methylene-
J = 11.2, 165.0 Hz, 1F); 13C NMR
d
26.9, 27.3, 37.7 (t, J = 11.0 Hz), 67.2, 76.0, 77.2, 77.9, 104.6, 110.0,
110.8 (t, J = 286.2 Hz), 113.3; HRMS calcd for C14H21F2O5 [M+H]+:
307.1357; found: 307.1350. Compound 2b [less polar than 1 on
b
-
D
-erythro-pentofuranosyl fluoride [6; 4 mg, 8%; less polar on TLC
(EtOAc/hexane, 1:1) than 5 and 4a]: 1H NMR
d
4.40 (dd, J = 6.8,
12.0 Hz, 1H), 4.56 (dd, J = 3.7, 12.0 Hz, 1H), 5.15–5.20 (m, 1H), 5.53
(br s, 1H), 5.78 (d, J = 5.1 Hz, 1H), 5.82 (d, J = 2.5 Hz, 1H), 5.88 (d,
J = 60.8 Hz, 1H), 7.39 (t, J = 7.7 Hz, 4H), 7.48–7.55 (m, 2H), 7.96 (d,
J = 8.3 Hz, 2H), 8.03 (d, J = 8.3 Hz, 2H); 19F NMR
J = 5.0 Hz, 60.9 Hz, 1F); 13C NMR
J = 2.5 Hz), 112.5 (d, J = 225.3 Hz), 118.0 (d, J = 3.8 Hz), 128.8, 128.9,
129.5, 130.0, 130.1, 130.2, 133.6, 134.0, 141.7, 165.7, 166.7; MS m/z
357 (M+H+); HRMS calcd for C20H17FLiO5 [M+Li]+: 363.1220;
found: 363.1219.
Treatment of 4a (65 mg, 0.14 mmol) with TFDA (0.25 mL,
320 mg, 1.28 mmol) in toluene [1.5 mL; 110 8C (oil bath)/3 h]
instead of benzene gave 5 (22 mg, 30%) and 6 (12 mg, 23%).
Analogous treatment of 4b (150 mg, 0.27 mmol) with TFDA
(toluene, 3 h) gave 5 (18 mg, 13%), 4a (85 mg, 70%) and unchanged
4b (10 mg, 7%).
30-Deoxy-3-N-methyl-20,50-di-O-methyl-30-methyleneuridine
(7b). Step a. An ethereal solution of the excess diazomethane
[generated from (N-methyl-N-nitroso)-p-toluenesulfonamide
(DIAZALD; 0.55 g, 2.6 mmol)] was added to a stirred solution of
30-deoxy-30-methyleneuridine [26] (3; 184 mg, 0.77 mmol) in
EtOH (2 mL). After 1 h, the volatiles were evaporated to give 30-
deoxy-3-N-methyl-30-methyleneuridine (7a; 195 mg, 100%) which
was directly used in next step: 1H NMR
d 3.29 (s, 3H), 3.65–3.75 (m,
2H), 3.85–3.95 (m, 1H), 4.66–4.74 (m, 1H), 5.19 (s, 1H), 5.39 (s, 1H),
5.69 (d, J = 5.6 Hz, 1H), 5.74 (d, J = 8.0 Hz, 1H), 7.70 (d, J = 8.0 Hz,
1H); MS m/z 255 (MH+). Step b. NaH (123 mg, 3.1 mmol, 60%
TLC (EtOAc/hexane, 3:7)] had: 1H NMR
d 1.24 (s, 3H), 1.26 (s, 3H),
1.34 (s, 3H), 1.48 (s, 3H), 1.64 (ddd, J = 3.2, 8.3, 13.6 Hz, 1H), 1.74
(ddd, J = 6.5, 8.3, 11.4 Hz, 1H), 3.85 (ddd, J = 1.0, 5.1, 8.7 Hz, 1H),
4.00 (dd, J = 6.1, 8.6 Hz, 1H), 4.10–4.18 (m, 1H), 4.31 (‘‘t’’, J = 7.7 Hz,
d
ꢁ116.66 (dt,
d
67.2, 78.1 (d, J = 40.4 Hz), 81.0 (d,
1H), 4.51 (d, J = 4.2 Hz, 1H), 5.78 (d, J = 4.1 Hz, 1H); 19F NMR
d
ꢁ127.72 (ddt, J = 7.0, 13.8, 159.0 Hz, 1F), ꢁ135.64 (dd, J = 11.3,
159.0 Hz, 1F); 13C NMR
d
15.6 (t, J = 10.9 Hz), 25.5, 27.0, 27.1, 27.1,
37.6 (t, J = 8.3 Hz), 68.1 (d, J = 1.7 Hz), 74.2 (d, J = 3.7 Hz), 79.4, 81.1
(d, J = 4.8 Hz), 104.6 (d, J = 1.6 Hz), 110.0, 112.3, 113.7 (t,
J = 286.0 Hz); HRMS calcd for C14H21F2O5 [M+H]+: 307.1357;
found: 307.1347.
20,50-Di-O-benzoyl-30-deoxy-30-methyleneuridine
(4a).
BzCl
(217 L, 263 mg, 1.87 mmol) was added to a stirred solution of
m
30-deoxy-30-methyleneuridine [26] (3; 150 mg, 0.62 mmol) in
pyridine (3 mL) at 0 8C (ice-bath) and the resulting mixture was
stirred overnight at ambient temperature. Volatiles were evapo-
rated in high-vacuum (<5 8C) and the residue was partitioned (1N
HCl/H2O//CHCl3). The separated organic layer was washed with
NaHCO3/H2O, brine, dried (Na2SO4), evaporated and column
chromatographed (EtOAc/hexane, 2:1) to give 4a (230 mg, 82%)
as a white powder: 1H NMR
d 4.62 (dd, J = 4.8, 12.2 Hz, 1H), 4.73
(dd, J = 3.0, 12.3 Hz, 1H), 5.10–5.14 (m, 1H), 5.47–5.49 (m, 1H),
5.61–5.64 (m, 2H), 5.93–5.97 (m, 1H), 6.23 (d, J = 4.8 Hz, 1H), 7.42–
7.49 (m, 5H), 7.60 (‘‘q’’, J = 7.4 Hz, 2H), 8.02–8.07 (m, 4H), 9.57 (br s,
1, NH); MS m/z 449 (MH+). Anal. calcd for C24H20N2O7 (448.43): C,
64.28; H, 4.50; N, 6.25. Found: C, 64.53; H, 4.82; N, 6.37.
3-N-Benzoyl-20,50-di-O-benzoyl-30-deoxy-30-methyleneuridine
dispersion in oil) and dimethyl sulfate (146
mL, 193 mg,
1.53 mmol) were added to a stirred solution of 7a (195 mg,
0.77 mmol) in THF (8 mL) at ambient temperature. After 14 h,
glacial AcOH was added to neutralize reaction mixture to pH ꢂ6
and then volatiles were evaporated. The residue was partitioned
(NaHCO3/H2O//CHCl3) and the organic layer was washed (brine),
dried (Na2SO4), evaporated and chromatographed (20 ! 50%
EtOAc/hexane) to give 7b (174 mg, 80%) as a transparent oil: 1H
(4b). BzCl (166
solution of 4a (428 mg, 0.95 mmol) in pyridine (8 mL) containing
ethyldiisopropylamine (830 L, 615 mg, 4.76 mmol) at 0 8C (ice-
mL, 201 mg, 1.43 mmol) was added to a stirred
m
bath). After 16 h, volatiles were evaporated and the residue was
partitioned (1N HCl/H2O//CHCl3). The separated organic layer was
washed with NaHCO3/H2O, brine, dried (Na2SO4), evaporated and
column chromatographed (EtOAc/hexane, 1:1) to give 4b (442 mg,
NMR
d 3.23 (s, 3H), 3.32 (s, 3H), 3.33 (s, 3H), 3.56 (dd, J = 3.6,
84%) as a slightly yellow foam: 1H NMR
d
4.62 (dd, J = 4.6, 12.3 Hz,
10.9 Hz, 1H), 3.71 (dd, J = 2.9, 10.9 Hz, 1H), 4.06–4.09 (m, 1H),
4.65–4.69 (m, 1H), 5.21 (t, J = 1.6 Hz, 1H), 5.33 (t, J = 1.6 Hz, 1H),
5.65 (d, J = 8.1 Hz, 1H), 5.96 (d, J = 2.9 Hz, 1H), 7.63 (d, J = 8.1 Hz,
1H), 4.78 (dd, J = 2.7, 12.8 Hz, 1H), 5.13–5.17 (m, 1H), 5.48–5.51 (m,
1H), 5.64–5.67 (m, 1H), 5.77 (d, J = 8.2 Hz, 1H), 5.94–5.98 (m, 1H),
6.18 (d, J = 5.2 Hz, 1H), 7.42–7.53 (m, 6H), 7.57–7.67 (m, 4H), 7.97
(d, J = 7.5 Hz, 2H), 8.02 (d, J = 7.4 Hz, 2H), 8.08 (d, J = 7.4 Hz, 2H); MS
m/z 553 (MH+). Anal. calcd for C31H24N2O8 (552.53): C, 67.39; H,
4.38; N, 5.07. Found: C, 67.04; H, 4.67; N, 5.17.
1H); 13C NMR
d 27.6, 56.8, 59.4, 73.0, 79.9, 85.7, 88.7, 101.7, 112.3,
138.1, 142.9, 151.1, 162.8; HRMS calcd for C13H19N2O5 [M+H]+:
283.1294; found: 283.1297.
3-N-Methyl-1-[3-deoxy-(R/S)-3,3-C-(1,1-difluoroethane-1,2-
20,50-Di-O-benzoyl-30-deoxy-4-O-difluorometyl-30-methyleneuri-
dine (5). A solution of 4a (65 mg, 0.14 mmol) in anhydrous benzene
(1.5 mL) containing NaF (1.4 mg, 0.03 mmol) was heated at 80 8C
(oil bath) for 30 min. TFDA (0.25 mL, 320 mg, 1.28 mmol) was
added dropwise via a Teflon needle at the rate of 0.15 mL/h
(controlled by syringe pump). The solution was refluxed for 3 h and
then was cooled to ambient temperature. The reaction mixture
was partitioned (NaHCO3/H2O//CHCl3) and the separated organic
layer was washed (brine), dried (Na2SO4), evaporated and column
chromatographed (20 ! 40% EtOAc/hexane) to give 5 (49 mg, 68%)
diyl)-2,5-di-O-methyl-b-D-erythro-pentofuranosyl]uracil (8a/8b). A
solution of 7b (70 mg, 0.25 mmol) in anhydrous toluene (1.0 mL)
containing NaF (1.4 mg, 0.03 mmol) was refluxed for 15 min. TFDA
(0.10 mL, 127 mg, 0.51 mmol) was then added dropwise via a
Teflon needle at the rate of 0.15 mL/h (controlled by syringe
pump). The solution was refluxed for 6 h and then was cooled to
ambient temperature and was evaporated to give mainly
unchanged substrate 7b in addition to two minor less polar
products 8a and 8b (TLC, 19F NMR). The reaction mixture was
partitioned (NaHCO3/H2O//CHCl3) and the separated organic layer
was washed (brine), dried (Na2SO4), evaporated and column
chromatographed (20 ! 50% EtOAc/hexane) to give in order of
elution 8a(30R) (2 mg, 2.5%), 8b(30S) (2 mg, 2.5%) and 7b (29 mg,
as an oil: 1H NMR
d 4.62 (dd, J = 4.9, 12.2 Hz, 1H), 4.67 (dd, J = 3.0,
12.4 Hz, 1H), 5.11–5.14 (m, 1H), 5.40 (s, 1H), 5.60 (s, 1H), 5.82 (d,
J = 7.3 Hz, 1H), 5.87–5.91 (m, 1H), 6.20 (d, J = 3.9 Hz, 1H), 7.36–7.43
(m, 4H), 7.41 (t, J = 71.0 Hz, 1H), 7.49–7.57 (m, 2H), 7.92–8.00 (m,
41%) as slightly yellow oils. Compound 8a had: 1H NMR
d 1.34 (ddd,
J = 4.6, 8.4, 13.3 Hz, 1H), 1.91 (ddd, J = 5.2, 8.4, 13.2 Hz, 1H), 3.28 (s,
3H), 3.32 (s, 3H), 3.38 (s, 3H), 3.47 (ddd, J = 2.2, 3.7, 10.6 Hz, 1H),
3.52 (ddd, J = 1.5, 2.6, 10.8 Hz, 1H), 3.83 (d, J = 4.6 Hz, 1H), 4.26
4H), 7.99 (d, J = 7.3 Hz, 1H); 19F NMR
d
ꢁ90.66 (dd, J = 71.0,
176.0 Hz, 1F), ꢁ91.20 (dd, J = 71.0, 176.0 Hz, 1F); 13C NMR
d 65.2,
78.0, 79.8, 90.1, 95.2, 113.1 (t, J = 259.9 Hz), 114.9, 128.9, 129.0,

326
M. Rapp et al. / Journal of Fluorine Chemistry 130 (2009) 321–328
(‘‘dt’’, J = 3.1, 5.3 Hz, 1H), 5.78 (d, J = 8.1 Hz, 1H), 6.07 (d, J = 4.7 Hz,
13 had: 1H NMR
d 1.39 (s, 3H), 1.61 (s, 3H), 4.47 (dd, J = 4.8, 11.8 Hz,
1H), 7.63 (d, J = 8.1 Hz, 1H); 19F NMR
d
ꢁ132.06 (ddt, J = 4.8, 12.8,
1H), 4.61 (q, J = 3.7 Hz, 1H), 7.66 (dd, J = 3.9, 11.8 Hz, 1H), 5.06 (dd,
J = 3.4, 6.3 Hz, 1H), 5.33 (dd, J = 6.3, 2.4 Hz, 1H), 5.99 (d, J = 2.4 Hz,
1H), 7.41 (t, J = 7.4 Hz, 2H), 7.56 (t, J = 7.5 Hz, 1H), 7.71 (s, 1H), 7.83
(s, 1H), 7.93 (d, J = 7.1 Hz, 2H), 7.94 (t, J = 59.9 Hz, 1H); 19F NMR
d
164.1 Hz, 1F), ꢁ134.83 (ddd, J = 4.5, 13.4, 164.3 Hz, 1F); 13C NMR
(150.9 MHz)
d 16.3 (t, J = 10.9 Hz), 27.8, 34.0 (t, J = 10.7 Hz), 58.9,
59.2, 72.1, 81.1, 82.6, 88.4, 102.5, 111.7 (t, J = 288.1 Hz), 137.5,
151.2, 162.6; HRMS (FAB) calcd for
14H19F2N2O5 [M+H]+:
333.1257; found: 333.1260. Compound 8b had: 1H NMR
1.30
C
ꢁ101.53 (dd, J = 59.7, 228.3 Hz, 1F), ꢁ102.35 (dd, J = 59.7, 228.3 Hz,
d
1F); 13C NMR
d
25.7, 27.5, 64.3, 81.6, 85.0, 85.1, 91.7, 107.9 (t,
(ddd, J = 4.2, 8.5, 12.8 Hz, 1H), 2.01 (ddd, J = 4.4, 8.5, 13.0 Hz, 1H),
3.26 (s, 3H), 3.32 (s, 3H), 3.46 (‘‘dt’’, J = 1.7, 11.7 Hz, 1H), 3.50 (br s,
1H), 3.53 (s, 3H), 3.77 (dd, J = 2.9, 11.7 Hz, 1H), 4.59–4.62 (m, 1H),
5.67 (d, J = 8.1 Hz, 1H), 5.92 (s, 1H), 8.13 (d, J = 8.1 Hz, 1H); 19F NMR
J = 252.8 Hz), 115.3, 124.1, 128.7, 129.6, 129.9, 133.8, 138.2, 140.1,
141.4 (t, J = 5.1 Hz), 152.9, 166.3; MS m/z 462 (MH+); HRMS calcd
for C21H21F2N5LiO5 [M+Li]+: 468.1671; found: 468.1670. Com-
pound 14 had: 1H NMR
d 1.37 (s, 3H), 1.60 (s, 3H), 4.47 (dd, J = 4.7,
d
ꢁ131.59 (ddd, J = 3.8, 13.0, 171.0 Hz, 1F), ꢁ137.64 (ddd, J = 3.5,
11.5 Hz, 1H), 4.67 (dd, J = 3.9, 11.5 Hz, 1H), 4.70 (‘‘dd’’, J = 3.9,
7.1 Hz, 1H), 5.17 (dd, J = 2.9, 6.2 Hz, 1H), 5.63 (dd, J = 6.3, 2.1 Hz,
1H), 6.19 (d, J = 2.1 Hz, 1H), 7.36 (t, J = 7.5 Hz, 2H), 7.45 (t, J = 7.4 Hz,
1H), 7.82 (d, J = 7.1 Hz, 2H), 8.36 (s, 1H), 8.62 (s, 1H), 9.89 (d,
12.6, 170.9 Hz, 1F); 13C NMR
d
20.4 (t, J = 10.6 Hz), 27.4, 34.5 (t,
J = 10.7 Hz), 58.1, 59.3, 69.9 (d, J = 3.6 Hz), 78.6, 89.1, 90.1, 100.8,
111.2 (t, J = 288.6 Hz,), 137.6, 151.0, 162.9; HRMS (FAB) calcd for
C
14H19F2N2O5 [M+H]+: 333.1257; found: 333.1261.
20-Deoxy-3-N-methyl-30,50-di-O-methyl-20-methyleneuridine
J = 10.3 Hz, 1H), 9.95 (br d, J = 10.3 Hz, 1H); 13C NMR
d 25.8, 27.5,
64.6, 82.0, 84.6, 85.6, 92.2, 115.2, 121.7, 128.6, 129.6, 129.8, 133.6,
143.4, 149.5, 151.4, 152.9, 163.2, 166.3; HRMS calcd for
(9c). Step a. An ethereal solution of the excess of diazomethane
[generated as described for 7b (step a)] was added to a stirred
solution of 20-deoxy-20-methyleneuridine [26] (9a; 411 mg,
1.71 mmol) in EtOH (5 mL). After 1 h, the volatiles were evaporated
to give 20-deoxy-3-N-methyl-20-methyleneuridine (9b; 435 mg,
C
21H22N5O6 [M+H]+: 440.1570; found: 440.1563.
Notes: 19F NMR of the crude reaction mixture showed formation
of other byproducts having CF2H group as judged by their
characteristic splitting pattern. Reaction of 12 with TFDA (3 h,
120 8C, oil bath) gave recovered 12 (20 mg, 50%) and 13 (10 mg,
22%).
50-O-Benzoyl-6-N,N-diethyl-20,30-O-isopropylideneadenosine
(15b). Treatment of 6-N,N-diethyl-20,30-O-isopropylideneadeno-
100%) which was directly used in next step: 1H NMR
d 3.23 (s, 3H),
3.75 (dd, J = 3.2, 12.9 Hz, 1H), 3.83–3.91 (m, 2H), 4.73 (d, J = 7.0 Hz,
1H), 5.35 (s, 1H), 5.43 (s, 1H), 5.66 (d, J = 8.1 Hz, 1H), 6.52 (s, 1H),
7.70 (d, J = 8.1 Hz, 1H); MS m/z 255 (MH+). Step b. NaH (356 mg,
8.9 mmol, 60% dispersion in oil) and dimethyl sulfate (0.42 mL,
564 mg, 4.47 mmol) was added to a stirred solution of 9b (435 mg,
1.71 mmol) in THF (5 mL) at ambient temperature. After 14 h,
glacial AcOH was added to neutralize reaction mixture to pH ꢂ6
and then the volatiles were evaporated. The residue was
partitioned (NaHCO3/H2O//CHCl3) and the organic layer was
washed (brine), dried (Na2SO4), evaporated and chromatographed
(20 ! 40% EtOAc/hexane) to give 9c (383 mg, 79%) as a white solid:
sine [21] [15a; 235 mg, 0.65 mmol; UV (MeOH)
BzCl (113 L, 136 mg, 0.97 mmol), as described for 4a (column
chromatography: 40 ! 60% EtOAc/hexane), gave 15b (250 mg,
83%) as a white foam: 1H NMR
1.23 (t, J = 7.0 Hz, 6H), 1.37 (s, 3H),
lmax 277 nm] with
m
d
1.60 (s, 3H), 3.90 (br s, 4H), 4.47 (dd, J = 6.7, 12.9 Hz, 1H), 4.57–4.62
(m, 2H), 5.16 (dd, J = 3.1, 6.3 Hz, 1H), 5.57 (dd, J = 2.0, 6.3 Hz, 1H),
6.08 (d, J = 2.0 Hz, 1H), 7.32 (t, J = 8.0 Hz, 2H), 7.47 (t, J = 7.4 Hz, 1H),
7.80 (s, 1H), 7.87 (d, J = 8.2 Hz, 2H), 8.28 (s, 1, H); 13C NMR
d 13.7 (br
s), 25.6, 27.4, 43.3 (br s), 64.8, 82.0, 84.5, 85.2, 91.6, 114.6, 120.5,
128.5, 129.6, 129.8, 133.3, 137.5, 149.9, 152.9, 153.9, 166.2; MS m/z
468 (MH+). Anal. calcd for C24H29N5O5 (467.52): C, 61.66; H, 6.25;
N, 14.98. Found: C, 61.46; H, 6.62; N, 14.61.
1H NMR
d
3.26 (s, 3H), 3.33 (s, 3H), 3.36 (s, 3H), 3.54 (dd, J = 3.3,
10.7 Hz, 1H), 3.58 (dd, J = 3.0, 10.7 Hz, 1H), 4.06 (‘‘q’’, J = 3.3 Hz, 1H),
4.21–4.25 (m, 1H), 5.18 (‘‘t’’, J = 1.6 Hz, 1H), 5.46 (‘‘t’’, J = 1.6 Hz,
1H), 5.72 (d, J = 8.1 Hz, 1H), 6.65–6.67 (m, 1H), 7.43 (d, J = 8.1 Hz,
1H); 13C NMR
d
27.9, 56.7, 59.4, 73.1, 81.1, 82.6, 85.5, 102.3, 114.9,
6-N,N-Diethyl-9-N-difluoromethyladenine
(16).
Treatment
139.1, 145.1, 151.8, 162.9; HRMS calcd for C13H19N2O5 [M+H]+:
283.1294; found: 283.1299.
[toluene (1.5 mL), NaF (1.4 mg, 0.03 mmol), 120 8C (oil bath),
5.5 h] of 15b (55 mg, 0.12 mmol) with TFDA (0.18 mL, 229 mg,
0.91 mmol; adding at the rate of 0.10 mL/h) as described for 8a/8b
(column chromatography: 20 ! 80% AcOEt/hexane) gave recov-
ered 15b (30 mg, 55%) and 16 (6 mg, 21%) and 17 (3 mg, 9%).
2-O-Difluoromethyl-3-N-methyluracil (10). A solution of 9c
(70 mg, 0.25 mmol) in dry toluene (1.5 mL) containing NaF
(1.4 mg, 0.03 mmol) was heated at 110 8C for 30 min. TFDA
(0.10 mL, 127 mg, 0.51 mmol) was added dropwise via a Teflon
needle at the rate of 0.05 mL/h (controlled by syringe pump). The
resulting solution was refluxed overnight and then was cooled to
ambient temperature. The reaction mixture was partitioned
(NaHCO3/H2O//CHCl3) and the separated organic layer was washed
(brine), dried (Na2SO4), evaporated and column chromatographed
Compound 16 had: UV (MeOH) lmax 276 nm; 1H NMR
d 1.23 (t,
J = 7.0 Hz, 6H), 3.87 (br s, 4H), 7.44 (t, J = 60.0 Hz, 1H), 7.88 (s, 1H),
8.24 (s, 1H); 19F NMR
(br s), 43.6 (br s), 106.9 (t, J = 249.5 Hz), 119.3 (C5), 133.4 (C8),
150.0 (C4), 153.4 (C2), 153.5 (C6); MS m/z 242 (MH+); HRMS (FAB)
calcd for C10H13F2LiN5 [M+Li]+: 248.1299; found: 248.1297.
d
ꢁ95.45 (d, J = 59.8 Hz, 2F); 13C NMR
d 13.1
(30 ! 50% EtOAc/hexane) to give 10 [18] (7.5 mg, 17%): 1H NMR
d
The 5-O-benzoyl-2,3-O-isopropylidene-
b-D-ribofuranosyl fluor-
3.35 (s, 3H), 5.98 (d, J = 8.2 Hz, 1H), 7.43 (d, J = 8.2 Hz, 1H), 7.46 (t,
ide 17 had: 1H NMR
d
1.34 (s, 3H), 1.48 (s, 3H), 4.36 (dd, J = 6.9,
J = 59.7 Hz, 1H); 19F NMR
d
ꢁ102.38 (d, J = 56.5 Hz, 2F); HRMS calcd
11.7 Hz, 1H), 4.42 (dd, J = 5.8, 11.8 Hz, 1H), 4.68 (ddt, J = 3.1, 5.9,
6.2 Hz, 1H), 4.83–4.87 (m, 2H), 5.83 (d, J = 61.7 Hz, 1H), 7.45 (t,
J = 7.9 Hz, 2H), 7.57 (t, J = 7.6 Hz, 1H), 8.07 (d, J = 8.2 Hz, 2H); 19F
for C6H7F2N2O2 [M+H]+: 177.0476; found: 177.0471.
Further elution yielded 3-N-methyluracil (11; 4 mg, 11%) with
data identical to commercial sample [1H NMR (CD3OD)
3H), 5.96 (d, J = 8.3 Hz, 1H), 7.48 (d, J = 8.3 Hz, 1H); MS m/z 127
(MH+)].
d
3.35 (s,
NMR
d
ꢁ115.85 (‘‘d quintet’’, J = 4.0, 61.6 Hz,); 13C NMR
d 25.1, 26.5,
64.8, 81.2, 85.2 (d, J = 40.6 Hz), 86.6 (d, J = 2.8 Hz), 113.4, 115.5 (d,
J = 223.1 Hz), 128.7, 129.7, 130.0, 133.5, 166.3; MS m/z 297 (MH+);
HRMS (FAB) calcd for C15H17FLiO5 [M+Li]+: 303.1220; found:
303.1220.
50-O-Benzoyl-6-N-difluoromethyl-20,30-O-isopropylideneadeno-
sine (13) and 50-O-benzoyl-6-N-formyl-20,30-O-isopropylideneadeno-
sine (14). Treatment [toluene (1.5 mL), NaF (1.4 mg, 0.03 mmol),
120 8C (oil bath), 5.5 h] of 50-O-benzoyl-20,30-O-isopropylidenea-
denosine [27] (12; 40 mg, 0.10 mmol) with TFDA (0.25 mL, 320 mg,
1.28 mmol, adding with the rate of 0.10 mL/h), as described for
8a/8b (column chromatography: 20 ! 50% EtOAc/hexane), gave
13 (8 mg, 18%) and slightly more polar 14 (14 mg, 33%). Compound
9-(2,3-Di-O-acetyl-5-deoxy-
N-benzoyladenine (18b). Ac2O (28
to solution of 6-N-benzoyl-9-(5-deoxy-
b
-
D
-erythro-pent-4-enofuranosyl)-6-
L, 31 mg, 0.3 mmol) was added
-erythro-pent-4-eno-
m
b
-D
furanosyl)adenine [28] (18a, 35 mg, 0.1 mmol) in pyridine (2 mL)
at 0 8C (ice-bath) and the resulting mixture was stirred overnight at
ambient temperature. Volatiles were evaporated in high-vacuum

M. Rapp et al. / Journal of Fluorine Chemistry 130 (2009) 321–328
327
(<5 8C) and the residue was partitioned (1N HCl/H2O//CHCl3). The
separated organic layer was washed with NaHCO3/H2O, brine,
dried (Na2SO4), evaporated and column chromatographed
278 nm, 302 nm (sh), 321 nm (sh),
l
min 259 nm; 1H NMR
d
1.38 (s,
3H), 1.52 (s, 3H), 1.70 (ddd, J = 6.0, 10.2, 14.7 Hz, 1H), 1.79 (ddd,
J = 6.8, 10.2, 14.8 Hz, 1H), 5.20 (d, J = 6.0 Hz, 1H), 5.37 (d, J = 6.0 Hz,
1H), 6.28 (s, 1H), 7.18 (t, J = 57.1 Hz, 1H), 7.77–7.94 (m, 4H), 8.79 (s,
(40 ! 60% EtOAc/hexane) to give 18b (37 mg, 85%): 1H NMR
d
2.06 (s, 3H), 2.16 (s, 3H), 4.51 (d, J = 2.9 Hz, 1H), 4.70 (d, J = 2.9 Hz,
1H), 6.04 (t, J = 5.6 Hz, 1H), 6.14 (d, J = 5.6 Hz, 1H), 6.44 (d,
J = 5.6 Hz, 1H), 7.48 (t, J = 7.7 Hz, 2H), 7.58 (t, J = 7.7 Hz, 1H), 8.00 (d,
J = 7.3 Hz, 2H), 8.16 (s, 1H), 8.76 (s, 1H), 9.32 (s, 1, NH); HRMS calcd
for C21H22N5O6 [M+H]+: 438.1414; found: 438.1422.
1H); 19F NMR
d
ꢁ98.82 (‘‘dm’’ J = 58.4 Hz, 2F), ꢁ135.48 (ddd, J = 6.1,
14.7 Hz, 163.7 Hz, 1F), ꢁ144.45 (ddd, J = 6.8, 14.4, 163.6 Hz, 1F);
13C NMR
17.2 (t, J = 11.0 Hz), 25.6, 26.4, 72.8 (t, J = 10.7 Hz), 79.4,
d
84.6, 87.6, 107.2 (t, J = 247.2 Hz), 109.8 (t, J = 285.6 Hz), 114.1,
117.2, 123.6, 124.4, 131.8, 134.4, 134.9, 149.6, 151.3, 153.7, 165.3,
165.6; HRMS (ESI) calcd for C23H17F4LiN5O5 [M+Li]+: 526.1320;
found: 526.1328.
9-(2,3-Di-O-acetyl-5-deoxy-
N-[( -difluorometoxy)benzylidene]adenine (19). Treatment of 18b
(65 mg, 0.15 mmol) with TFDA (60 L, 76 mg, 0.30 mmol) as
described for 2a/2b [column chromatography (20 ! 60% EtOAc/
hexane)] gave 19 (49 mg, 68%) as slightly yellow oil: 1H NMR
2.09
b-D-erythro-pent-4-enofuranosyl)-6-
a
m
50-O-Benzoyl-20,30-O-isopropylidene-6-N-phthaloyladenosine
(22b). BzCl (36
mL, 44 mg, 0.31 mmol) was added to a stirred
d
solution of 20,30-O-isopropylidene-6-N-phthaloyladenosine [29]
(22a, 120 mg, 0.27 mmol; prepared by standard protection of 6-N-
phthaloyladenosine [22] with acetone) in pyridine (3 mL) at 0 8C.
After 16 h, the volatiles were evaporated and the residue was
partitioned (1N HCl/H2O//CHCl3). The organic layer was separated
and was washed (NaHCO3/H2O//brine), dried (Na2SO4), evaporated
and chromatographed (30 ! 70% AcOEt/hexane) to give 22b
(110 mg, 74%) as a white foam: UV (MeOH) lmax 275 nm, lmin
(s, 3H), 2.17 (s, 3H), 4.53 (d, J = 2.8 Hz, 1H), 4.71 (d, J = 2.7 Hz, 1H),
5.81 (t, J = 5.6 Hz, 1H), 6.03 (d, J = 5.5 Hz, 1H), 6.23 (d, J = 5.7 Hz,
1H), 7.43–7.48 (t, J = 7.8 Hz, 2H), 7.53–7.57 (m, 1H), 7.77 (s, 1H),
8.00 (t, J = 59.4 Hz, 1H), 8.10 (d, J = 7.8 Hz, 2H), 8.24 (s, 1H); 19F
NMR
NMR
d
d
ꢁ101.75 (d, J = 60.0 Hz, 1F), ꢁ101.80 (d, J = 60.0 Hz, 1F); 13
C
20.6, 21.0, 69.6, 72.8, 86.8, 90.5, 108.6 (t, J = 249.6 Hz, 1H),
122.5, 128.7, 130.0, 133.1, 134.7, 139.1, 141.8 (t, J = 3.6 Hz), 144.1,
144.3, 156.1, 169.5, 169.8, 177.1; HRMS calcd for C22H20F2N5O6
[M+H]+: 488.1380; found: 488.1380.
Notes: Analogous treatment of 18b (0.15 mmol) with TFDA in
toluene also gave 19 (65%).
255 nm; 1H NMR
d 1.37 (s, 3H), 1.63 (s, 3H), 4.52 (dd, J = 6.6,
13.1 Hz, 1H), 4.63–4.67 (m, 2H), 5.17 (dd, J = 3.2, 6.3 Hz, 1H), 5.53
(dd, J = 2.3, 6.3 Hz, 1H), 6.21 (d, J = 2.3 Hz, 1H), 7.37 (t, J = 7.9 Hz,
2H), 7.52 (t, J = 7.5 Hz, 1H), 7.79–7.83 (m, 2H), 7.89 (d, J = 7.5 Hz,
2H), 7.97–8.01 (m, 2H), 8.28 (s, 1H), 8.97 (s, 1H); 13C NMR
d 25.4,
27.3, 64.2, 81.5, 84.3, 85.1, 91.6, 115.1, 124.5, 125.4, 128.6, 129.4,
129.7, 130.3, 132.0, 133.5, 135.0, 144.7, 144.9, 152.8, 153.1, 165.6,
166.1; MS m/z 542 (MH+). Anal. calcd for C28H23N5O7 (541.51): C,
62.10; H, 4.28; N, 12.93. Found: C, 61.86; H, 4.56; N, 12.61.
9-(5-Deoxy-2,3-O-isopropylidene-
syl)-6-N-phthaloyladenine (20b). Phthaloyl dichloride (75
105 mg, 0.52 mmol) was added to a stirred solution of 9-(5-
deoxy-2,3-O-isopropylidene- -erythro-pent-4-enofuranosyl)
b-D-erythro-pent-4-enofurano-
m
L,
b
-D
adenine [23] (20a; 75 mg, 0.26 mmol) in pyridine (1.0 mL). After
2 h, the volatiles were evaporated under oil-pump vacuum and the
residue was partitioned (1% CH3CO2H/H2O//CHCl3). The organic
layer was washed (NaHCO3/H2O//brine), dried (Na2SO4), evapo-
rated and chromatographed (0 ! 5% MeOH/CHCl3) to give 20b [24]
9-(5-O-Benzoyl-2,3-O-isopropylidene-b-D-ribofuranosyl)-7-
difluoromethyl-6-(phthalimido)purine-8(7H,9H)-thione (23). Treat-
ment [toluene (1.5 mL), NaF (1.4 mg, 0.03 mmol), 120 8C (oil bath),
8 h] of 22b (40 mg, 0.07 mmol) with TFDA (0.24 mL, 305 mg,
1.22 mmol, at the rate of 0.05 mL/h) as described for 2a/2b
[column chromatography, 20 ! 60% EtOAc/hexane] gave recov-
ered 22b (16 mg, 40%), 23 (8 mg, 18%) and 17 (7 mg, 32%):
(82 mg, 76%): UV (MeOH) lmax 276 nm, lmin 245 nm; 1H NMR
d
1.46 (s, 3H), 1.62 (s, 3H), 4.65 (d, J = 2.6 Hz, 1H), 4.69 (d, J = 1.9 Hz,
1H), 5.38 (d, J = 6.0 Hz, 1H), 5.61 (d, J = 6.0 Hz, 1H), 6.37 (s, 1H),
7.81–7.83 (m, 2H), 7.99–8.02 (m, 2H), 8.20 (s, 1H), 9.00 (s, 1H); 13
C
compound 23 had: UV (MeOH)
d 1.37 (s, 3H), 1.63 (s, 3H), 4.51–4.57 (m, 2H), 4.67–4.74 (m, 1H),
lmax 320 nm, l
min 276 nm; 1H NMR
NMR
d
25.9, 27.0, 79.8, 82.9, 89.6, 91.1, 114.7, 124.7, 130.2, 132.1,
135.2, 144.7, 144.9, 153.1, 153.2, 161.6, 165.7; MS m/z 420 (MH+);
HRMS (ESI) calcd for C21H17LiN5O5 [M+Li]+: 426.1386; found:
426.1384.
5.25 (dd, J = 4.6, 6.4 Hz, 1H), 5.55 (dd, J = 2.1, 6.5 Hz, 1, H), 6.88 (d,
J = 2.2 Hz, 1H), 7.44 (t, J = 7.4 Hz, 2H), 7.56 (t, J = 7.5 Hz, 1H), 7.74–
7.87 (m, 2H), 7.92 (t, J = 58.1 Hz, 1H), 7.98–8.02 (m, 2H), 8.05 (d,
9-[(S)-4,4-C-(1,1-Difluoroethane-1,2-diyl)-2,3-O-isopropylidene-
J = 8.3 Hz, 2H), 8.75 (s, 1H); 19F NMR
1F), ꢁ100.6 (dd, J = 58.1, 224.1 Hz, 1F); 13C NMR
d
ꢁ99.7 (dd, J = 58.2, 224.1 Hz,
b
-
D
-erythrofuranosyl]-7-difluoromethyl-6-(phthalimido)purine-
8(7H,9H)-thione (21a) and 9-[(R)-4,4-C-(1,1-difluoroethane-1,2-
diyl)-2,3-O-isopropylidene- -erythrofuranosyl]-8-difluoromethyl-
d 25.7, 27.6, 64.1,
81.5, 83.1, 85.2, 89.8, 109.8 (t, J = 252.0 Hz), 115.5, 123.7, 124.6,
124.7, 128.6, 129.1, 130.0, 132.03, 132.05, 133.4, 134.9 (d,
J = 1.4 Hz), 135.0, 135.2, 153.4, 154.2, 165.7, 166.40, 166.45,
172.0; MS m/z 624 (MH+); HRMS calcd for C29H25F2N5O7S
[M+H]+: 624.1359; found: 624.1358.
b
-D
6-N-phthaloyladenine (21b). Treatment [toluene (2.0 mL), over-
night] of 20b (75 mg, 0.18 mmol) with TFDA (0.10 mL, 127 mg,
0.5 mmol, added at the rate of 0.10 mL/h) as described for 2a/2b
(column chromatography: 30 ! 70% EtOAc/hexane) gave crude
21a (19 mg, 19%) and 21b (8 mg, 8%) as yellow oils. Repurification
of the crude 21a (column chromatography: 20 ! 50% EtOAc/
Methyl-3,5-di-O-(4-chlorobenzyl)-2-deoxy-(R)-2,2-C-(1,1-difluor-
oethane-1,2-diyl)-
chlorobenzyl)-2-deoxy-(S)-2,2-C-(1,1-difluoroethane-1,2-diyl)-
erythro-pentofuranose (25b). Treatment [benzene (2.0 mL), 80 8C,
5 h] of methyl-3,5-di-O-(4-chlorobenzyl)-2-deoxy-2-methylene-
-ribofuranoside [25] 24 (150 mg, 0.37 mmol) with TFDA [(0.3 mL,
a-D-erythro-pentofuranoside(25a)and3,5-di-O-(4-
a-D-
hexane) gave 21a [24] (11 mg, 11%): UV (MeOH)
l
max 319 nm, lmin
272 nm; 1H NMR
d
1.40 (s, 3H), 1.57 (s, 3H), 1.70 (ddd, J = 5.9, 10.1,
b-
14.8 Hz, 1H), 1.80 (ddd, J = 7.1, 10.3, 15.0 Hz, 1H), 5.32 (d, J = 5.9 Hz,
1H), 5.43 (d, J = 6.1 Hz, 1H), 6.87 (s, 1H), 7.77–7.82 (m, 2H), 7.83 (t,
D
380 mg, 1.5 mmol; added at the rate of 0.15 mL/h], as described for
2a/2b, gave25a(2R)(51 mg,30%)and25b(2S)(8 mg,5%):compound
25a: 1H NMR
d 1.58–1.65 (m, 2H), 3.40 (s, 3H), 3.50 (dd, J = 3.3,
10.9 Hz, 1H), 3.53 (dd, J = 3.3, 10.9 Hz, 1H), 3.92 (d, J = 5.4 Hz, 1H),
4.32 (‘‘q’’, J = 4.2 Hz, 1H), 4.42 (s, 2H), 4.47 (s, 2H), 4.76 (s, 1H), 7.14–
J = 58.2 Hz, 1H), 7.90–7.96 (m, 2H), 8.84 (s, 1H); 13C NMR
d
17.0 (t,
J = 10.6 Hz), 25.6, 26.5, 73.6 (t, J = 9.3 Hz), 79.9, 85.0, 90.9, 109.65 (t,
J = 281.7 Hz), 109.69 (t, J = 255.9 Hz), 114.2, 124.4, 124.5, 131.7,
131.8, 133.7, 134.7, 135.0, 135.1, 153.1, 154.1, 165.3,165.7, 172.1;
19F NMR
d
ꢁ99.52 (dd, J = 57.8, 224.1 Hz, 1F), ꢁ100.84 (dd, J = 59.0,
7.27 (m, 8H); 19F NMR
d
ꢁ135.17 (ddd, J = 7.1, 10.3, 154.0 Hz, 1F),
223.9 Hz, 1F), ꢁ135.01 (ddd, J = 6.2, 15.0, 163.5 Hz, 1F), ꢁ143.92
(ddd, J = 6.4, 14.1, 164.1 Hz, 1F); MS m/z 552 (100, MH+); HRMS
calcd for C23H18F4N5O5S [M+H]+: 552.0959; found: 552.0957.
Repurification of the crude 21b (column chromatography:
20 ! 50% EtOAc/hexane) gave 21b (6 mg, 6%): UV (MeOH) lmax
ꢁ135.90 (ddd, J = 6.6, 11.7, 154.3 Hz, 1F); 13C NMR
d 14.4 (t,
J = 10.9 Hz), 39.7 (t, J = 9.9 Hz), 55.3, 69.6, 71.8, 74.6, 77.1, 82.2, 102.6
(d, J = 4.8 Hz), 111.7 (t, J = 285.0 Hz), 128.5, 128.6, 129.1, 129.2,
133.6, 133.6, 136.1, 136.2; HRMS calcd for C22H2335Cl2F2O4 [M+H]+:
459.0942; found: 459.0948. Compound 25b: 1H NMR
d 1.54 (‘‘ddd’’,

328
M. Rapp et al. / Journal of Fluorine Chemistry 130 (2009) 321–328
[7] (a) V. Samano, M.J. Robins, J. Am. Chem. Soc. 114 (1992) 4007;
(b) S. Czernecki, L. Mulard, J.M. Valery, A. Commercon, Can. J. Chem. 71 (1993) 413;
(c) V. Samano, M.J. Robins, Tetrahedron Lett. 30 (1994) 3445.
[8] (a) L.A. Paquette, R.T. Bibart, C.K. Seekamp, A.L. Kahane, Org. Lett. 3 (2001) 4039;
(b) L.A. Paquette, D.R. Owen, R.T. Bibart, C.K. Seekamp, Org. Lett. 3 (2001) 4043.
[9] M.K. Gurjar, S.V. Ravindranadh, K. Sankar, S. Karmakar, J. Cherian, M.S. Chorghade,
Org. Biomol. Chem. 1 (2003) 1366.
[10] J. Gagneron, G. Gosselin, C. Mathe, J. Org. Chem. 70 (2005) 6891.
[11] (a) I. Nowak, M.J. Robins, J. Org. Chem. 71 (2006) 8876;
(b) I. Nowak, J.F. Cannon, M.J. Robins, J. Org. Chem. 72 (2007) 532;
(c) I. Nowak, M.J. Robins, J. Org. Chem. 72 (2007) 3319.
J = 6.5, 10.7, 11.4 Hz, 1H), 1.61 (‘‘ddd’’, J = 5.7, 10.7, 11.0 Hz, 1H), 3.43
(dd, J = 4.5, 10.6 Hz, 1H), 3.49 (dd, J = 4.1, 10.6 Hz, 1H), 3.89 (d,
J = 5.7 Hz, 1H), 4.34 (‘‘q’’, J = 4.6 Hz, 1H), 4.32–4.43 (m, 3H), 4.67 (d,
J = 12.0 Hz, 1H), 4.96 (s, 1H), 7.12–7.23 (m, 8H); 19F NMR
d
ꢁ135.09
(ddd, J = 6.3, 10.5, 154.3 Hz,1F), ꢁ135.76(ddd,J = 6.8, 12.0, 154.3 Hz,
1F);13CNMR
14.5(t, J = 10.9 Hz), 39.6(t, J = 9.9 Hz), 68.2,69.3, 71.7,
d
72.3, 82.1, 100.7(d, J = 4.1 Hz), 111.7(t, J = 285.0 Hz), 128.62, 128.64,
129.0, 129.2, 129.3, 133.5, 133.6, 136.1, 136.3; HRMS calcd for
C
21H2135Cl2F2O4 [M+H]+: 445.0785; found: 445.0788.
[12] (a) V.E. Marquez, T. Ben-Kasus, J.J. Barchi, K.M. Green, M.C. Nicklaus, R. Agbaria, J.
Am. Chem. Soc. 126 (2004) 543;
(b) M.J. Comin, R. Agbaria, T. Ben-Kasus, M. Huleihel, C. Liao, G. Sun, M.C.
Nicklaus, J.R. Deschamps, D.A. Parrish, V.E. Marquez, J. Am. Chem. Soc. 129
(2007) 6216.
Acknowledgments
[13] (a) W.R. Dolbier Jr., Acc. Chem. Res. 14 (1981) 195;
We thank NIGMS (S06GM08205) and NCI (SC1CA138176)
programs for supporting this research. The support of US Army
Research Office (W911NF-04-1-0022) in the purchase of 600 MHz
NMR spectrometer is gratefully acknowledged. We also thank Dr.
Vered Marks (FIU) for assistance with NMR spectra.
(b) F. Tian, M.D. Bartberger, W.R. Dolbier Jr., J. Org. Chem. 64 (1999) 540;
(c) F. Tian, M.A. Battiste, W.R. Dolbier Jr., Org. Lett. 1 (1999) 193.
[14] W.R. Dolbier Jr., F. Tian, J.X. Duan, A.R. Li, S. Ait-Mohand, O. Bautista, S. Buathong,
J.M. Baker, J. Crawford, P. Anselme, X.H. Cai, A. Modzelewska, H. Koroniak, M.A.
Battiste, Q.-Y. Chen, J. Fluorine Chem. 125 (2004) 459.
[15] E.M. Acton, R.N. Goerner, H.S. Uh, K.R.E. Ryan, D.W. Henry, C.E. Cass, G.A. LePage, J.
Med. Chem. 22 (1979) 518.
[16] M. Xie, D.A. Berges, M.J. Robins, J. Org. Chem. 61 (1996) 5178.
[17] S. Kobayashi, A. Yoneda, T. Fukuhara, S. Hara, Tetrahedron 60 (2004) 6923.
[18] C.-D. Pein, D. Cech, Z. Chem. 25 (1985) 328.
References
[19] (a) H.P.M. Thiellier, G.J. Koomen, U.K. Pandit, Tetrahedron 33 (1977) 1493;
(b) H.P.M. Thiellier, G.J. Koomen, U.K. Pandit, Tetrahedron 33 (1977) 2603.
[20] P.J. Serafinowski, C.L. Barnes, Tetrahedron 32 (1996) 7929.
[21] (a) D.N. Reinhoudt, D.M. Rudkevich, F. de Jong, J. Am. Chem. Soc. 118 (1996) 6880;
(b) M.-T. Chenon, R.J. Pugmire, D.M. Grant, R.P. Panzica, L.B. Townsend, J. Am.
Chem. Soc. 97 (1975) 4636;
[1] K.W. Pankiewicz, Carbohydr. Res. 327 (2000) 87.
[2] W.D. Meng, F.L. Qing, Curr. Top. Med. Chem. 6 (2006) 1499.
[3] (a) P.V. Danenberg, H. Malli, S. Swenson, Semin. Oncol. 26 (1999) 621;
(b) P.M. Navolanic, J.A. McCubrey, Int. J. Oncol. 27 (2005) 1341;
(c) L.M. Abraham, D. Selva, R. Cassonand, I. Leibovitch, Drugs 67 (2007) 237.
[4] J.R. McCarthy, D.P. Matthews, D.M. Stemerick, E.W. Huber, P. Bey, B.J. Lippert, R.D.
Snyder, P.S. Sunkara, J. Am. Chem. Soc. 113 (1991) 7439.
(c) L.B. Townsend, R.K. Robins, R.N. Loeppky, N.J. Leonard, J. Am. Chem. Soc. 86
(1964) 5320.
[5] (a) L.W. Hertel, J.S. Kroin, J.W. Misner, J.M. Tustin, J. Org. Chem. 53 (1988)
2406;
[22] M. Beier, W. Pfleiderer, Helv. Chim. Acta 82 (1999) 633.
[23] J.R. McCarthy Jr., R.K. Robins, M.J. Robins, J. Am. Chem. Soc. 90 (1968) 4993.
[24] W. Xu, K.A. Abboud, I. Ghiviriga, W.R. Dolbier Jr., M. Rapp, S.F. Wnuk, Org. Lett. 8
(2006) 5549.
[25] N.-S. Li, J.A. Piccirilli, J. Org. Chem. 69 (2004) 4751.
[26] V. Samano, M.J. Robins, Synthesis (1991) 283.
[27] K. Anzai, J. Uzawa, J. Org. Chem. 49 (1984) 5076.
[28] S.D. Dimitrijevich, J.P.H. Verheyden, J.G. Moffatt, J. Org. Chem. 44 (1979) 400.
[29] H. Bazin, J. Heikkila, J. Chattopadhyaya, Acta Chem. Scand. B 39 (1985) 391.
(b) C.H. Baker, J. Banzon, J.M. Bollinger, J. Stubbe, V. Samano, M.J. Robins, B.
Lippert, E. Jarvi, R. Resvick, J. Med. Chem. 34 (1991) 1879;
(c) C. Manegold, Exp. Rev. Anticancer Ther. 4 (2004) 345.
[6] (a) J.R. McCarthy, E.T. Jarvi, D.P. Matthews, M.L. Edwards, N.J. Prakash, T.L. Bowlin,
S. Mehdi, P.S. Sunkara, P.J. Bey, Am. Chem. Soc. 111 (1989) 1127;
(b) M.J. Robins, S.F. Wnuk, K.B. Mullah, N.K. Dalley, C.S. Yuan, Y. Lee, R.T.
Borchardt, J. Org. Chem. 59 (1994) 544.