A1 adenosine receptor antagonists as ligands for positron emission tomography (PET) and single-photon emission tomography (SPET).

Article abstract of DOI:10.1021/jm9705465

The high affinity of 8-cyclopentyl-1,3-dipropylxanthine (CPX) for the A1 adenosine receptor (A1AR) provides a good lead for developing radioligands suitable for positron emission tomography (PET) and single-photon emission tomography (SPET). This study tested the hypothesis that the kinds of chemical modifications made in the synthesis of CPX analogues containing carbon-11, fluorine-18, or radioiodine will not alter affinity for the A1AR. This report describes the synthesis and radioligand binding assays of unlabeled CPX analogues having methyl, 2-methoxyethyl, 2-fluoropropyl, or 3-fluoropropyl substituents, respectively, at either N-1 (13a-d) or N-3 (8a-d) or an (E)-3-iodoprop-2-en-1-yl substituent at N-3 (8f). Compounds 8d,f and 13b,d antagonized the binding of [3H]CPX to the A1AR of rat brain with affinities similar to those of CPX; compound 8c was twice as potent as CPX. Analogues 8a,b and 13a were less potent than CPX, but for each the Ki of antagonism was > or = 0.5 nM. Attempts to iodinate the 8-(4-hydroxyphenyl) analogue of CPX failed, probably because the xanthine substituent strongly deactivated the phenol toward electrophilic iodination. In summary, several of the modifications of the propyl groups of CPX needed to produce ligands for imaging by PET and SPET preserve or enhance affinity for the A1AR.

Full text of DOI:10.1021/jm9705465

555
J . Med. Chem. 1998, 41, 555-563
A1 Ad en osin e Recep tor An ta gon ists a s Liga n d s for P ositr on Em ission
Tom ogr a p h y (P ET) a n d Sin gle-P h oton Em ission Tom ogr a p h y (SP ET)†
Marcus H. Holschbach,‡ Thomas Fein,§ Christof Krummeich,‡ Robert G. Lewis, Walter Wutz,‡ Ulrich Schwabe,§
Dieter Unterlugauer,‡ and Ray A. Olsson*,‡,
Institut fu¨r Nuklearchemie, Forschungszentrum J u¨lich GmbH, 52425 J u¨lich, Germany, Pharmakologisches
Institut der Ruprecht-Karls Universita¨t Heidelberg, 69120 Heidelberg, Germany, and Department of Internal Medicine,
University of South Florida, Tampa, Florida 33612
Received August 14, 1997
The high affinity of 8-cyclopentyl-1,3-dipropylxanthine (CPX) for the A1 adenosine receptor
(A1AR) provides a good lead for developing radioligands suitable for positron emission
tomography (PET) and single-photon emission tomography (SPET). This study tested the
hypothesis that the kinds of chemical modifications made in the synthesis of CPX analogues
containing carbon-11, fluorine-18, or radioiodine will not alter affinity for the A1AR. This report
describes the synthesis and radioligand binding assays of unlabeled CPX analogues having
methyl, 2-methoxyethyl, 2-fluoropropyl, or 3-fluoropropyl substituents, respectively, at either
N-1 (13a -d ) or N-3 (8a -d ) or an (E)-3-iodoprop-2-en-1-yl substituent at N-3 (8f). Compounds
8d ,f and 13b,d antagonized the binding of [3H]CPX to the A1AR of rat brain with affinities
similar to those of CPX; compound 8c was twice as potent as CPX. Analogues 8a ,b and 13a
were less potent than CPX, but for each the Ki of antagonism was g0.5 nM. Attempts to iodinate
the 8-(4-hydroxyphenyl) analogue of CPX failed, probably because the xanthine substituent
strongly deactivated the phenol toward electrophilic iodination. In summary, several of the
modifications of the propyl groups of CPX needed to produce ligands for imaging by PET and
SPET preserve or enhance affinity for the A1AR.
Single-photon emission tomography (SPET) and
positron emission tomography (PET) are widely used
techniques for medical imaging. PET also serves as a
means for assessing tissue metabolism in vivo1 and for
studying receptor density and function.2 Imaging a
pharmacological receptor requires a ligand that has (a)
selectivity and high affinity for the receptor, ideally a
KD < 1 nM, so that imaging can distinguish specific
binding to the receptor from unspecific binding to
“background” structures; (b) pharmacodynamic proper-
ties that enable it to traverse the boundaries between
compartments, for example, the blood-brain barrier,
and thereby to achieve effective concentrations at the
receptors of interest; and (c) metabolism that is either
negligible or generates metabolites that clear rapidly
and do not interfere with measurements of specifically
bound ligand. The design of ligands must also take into
account (d) the requirements of no-carrier-added (nca)
radiosynthesis that ensures the high specific radioactiv-
ity on which the imaging of bound ligand depends and,
(e) since the medically useful â+-emitting radionuclides
have short half-lives, the provisions for incorporating
longer-lived γ-emitting isotopes such as those of iodine
for studies of tissue distribution and metabolism as well
as for SPET.
The clinical importance of the A1 adenosine receptor
(A1AR) makes it an attractive target for radionuclide
imaging. The A1AR is neuromodulatory, inhibiting
synaptic transmission.3 In the brain those tonic inhibi-
tory influences may prevent seizures.4 Brief periods of
hypoxia can reduce receptor density greatly.5 Recent
work indicates that the A1AR plays an essential role in
cerebral protection by ischemic preconditioning.6 In the
heart A1ARs coupled to muscarinic potassium channels
mediate the bradycardia and heart block caused by
ischemia7,8 and perhaps the bradyarrhythmias that
occur in a substantial number of patients after heart
transplantation.9 A1ARs in the renal juxtaglomerular
apparatus inhibit renin release,10 and receptors coupled
to bicarbonate channels in the epithelium of the proxi-
mal tubule promote sodium reabsorption.11
This report describes the synthesis and structure-
activity relationships of some A1AR antagonists that,
if radiolabeled, would be of interest as potential ligands
for SPET and PET. The ligands are analogues of
8-cyclopentyl-1,3-dipropylxanthine (CPX), which, owing
to its high selectivity and affinity for the A1AR,14,15 is
the prototypical A1AR antagonist.16 The ligands for
PET include some methyl analogues of CPX obtainable
by carbon-11 methylation of a suitable precursor, as well
as surrogates for fluorine-18-labeled radioligands. The
single ligand suitable only for SPET has an N-3 sub-
stituent that contains iodine. Although the present
project aims ultimately at the development of radioli-
gands, the intermediate steps described here evaluate
only the pharmacological impact of the chemical modi-
fications caused by radiolabeling and do so with unla-
beled compounds. The aim was to obtain target ligands
† Dedicated to Professor G. Sto¨cklin on the occasion of his 65th
birthday.
* Address for correspondence: Department of Internal Medicine,
Box 19, 12901 Bruce B. Downs Blvd., Tampa, FL 33612. Tel: (813)
974-4067. Fax: (813) 974-2189. E-mail: rolsson@com1.med.usf.edu.
‡ Institut fu¨r Nuklearchemie.
§ Pharmakologisches Institut der Ruprecht-Karls Universita¨t Heidel-
berg.
University of South Florida.
S0022-2623(97)00546-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/17/1998

556 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 4
Holschbach et al.
Sch em e 1
as directly as possible, and so some of the syntheses are
inappropriate for nca syntheses of radioligands. How-
ever, in those instances we prepared some of the
precursors for radiolabeling.
Pharmacodynamic studies in rats support the candi-
dacy of CPX as a brain-imaging agent. [3H]CPX ad-
ministered intravenously penetrated into the brain to
the extent of 0.8% of the injected dose/g at 5 min. That
level remained stable for 15 min. Unlabeled CPX
displaced 45-70% of bound [3H]CPX.12 Ishikawa et al.13
synthesized [11C]KF15372, an 8-dicyclopropylmethyl
analogue of CPX, and demonstrated specific uptake in
mouse brain.
Ch em istr y
The syntheses of the 1- and 3-substituted 8-cyclopen-
tylxanthines followed a well-known route, namely, the
condensation of a urea with cyanoacetic acid, cyclization
in alkali to form a uracil, nitrosation and reduction of
the nitroso group to form a diaminouracil that is
acylated, and, finally, cyclization of the amidouracil to
form a xanthine.17-19 The syntheses had two features
in common. First, they proceeded from unsymmetrical
ureas, either N-benzylurea or N-benzyl-N′-propylurea.
Second, both syntheses called for alkylations at N-3 that
required prior protection of N-7 to ensure that the
alkylations at N-3 would be regioselective.
Proceeding from benzylureas served two purposes.
First, the benzyl group exerted steric control over the
condensation of the urea with cyanoacetic acid. The
condensation of cyanoacetic acid with an unsymmetrical
urea occurs at either of the nitrogens but favors the
nitrogen bearing the smallest (least hindering) sub-
stituent.17 Because a benzyl group is so much larger
than a hydrogen or propyl substituent, the nitrogen
forming the cyanoacetamide is more likely to be N-3 of
the intermediate uracil and, ultimately, N-1 of the
xanthine. Proceeding from N-benzyl-N′-propyluracil
appeared by TLC to give only one xanthine, but reverse-
phase HPLC showed that the product was a 4:1 mixture
of two xanthines. Reference to an authentic sample
established that 3-benzyl-8-cyclopentyl-1-propylxan-
thine, 1, was the more polar major product. Presum-
ably, the other isomer was 1-benzyl-8-cyclopentyl-3-
propylxanthine. Fractional crystallization of the isomers
was not successful, but the 7-POM derivatives (see
below) separated easily on crystallization from metha-
nol. By contrast, benzylurea gave one product, 3-benzyl-
8-cyclopentylxanthine, 2. Second, the benzyl group also
served as a protecting group that is easily removable
by catalytic transfer hydrogenation (CTH).20
Alkylation at N-7 of 1 with di-tert-butyl pyrocarbon-
ate, (boc)2O, chloromethyl methyl ether, MOM-Cl, or
chloromethyl pivalate, POM-Cl, generated the tert-
butoxycarbonyl (BOC), methoxymethyl (MOM), and
pivaloyloxymethyl (POM) derivatives 3a -c, respec-
tively, for evaluation of the suitability of those protecting
groups. In a subsequent step hydrogenolysis cleaved
the t-BOC and MOM groups, and thus, they were not
useful. The POM group resisted hydrogenolysis and,
just as importantly, underwent removal by alkali.
The initial plan to synthesize the 3-substituted 8-cy-
clopentyl-1-propylxanthines called for debenzylation of
1 to form 8-cyclopentyl-1-propylxanthine, 4, which was
then to undergo protection of N-7 followed by alkylation
at N-3 (Scheme 1). The reaction of POM-Cl with 4
could, in principle, produce three products: the desired
7-POM derivative 5a as well as the 3-POM derivative
5b and the 3,7-bis-POM derivative 5c. The original
description21 of the protection of xanthines by the POM
group indicated that the reaction of 1-methylxanthine
with POM-Cl gives a mixture of 5a ,c. On the basis of
that information, 4, the monosubstituted product of the
reaction with POM-Cl, underwent alkylation with io-
domethane to form 6a , which was then deprotected.
Unexpectedly, that product, 6b, was 300-fold less potent
than CPX. 1H NMR analysis of the products of the
reaction of POM-Cl with 4 showed that the monosub-
stituted product was the 3-POM derivative 5b rather
than the expected 7-POM derivative 5a . That struc-
tural assignment was based on the following evidence:
(a) The resonance for the proton on the unsubstituted
nitrogen was at 13.5 ppm, which is characteristic of an
N-7 rather than an N-3 proton;22 (b) the unambiguous
synthesis of 8-cyclopentyl-1-propyl-7-POM-xanthine, 5a ,
by the debenzylation of 3c gave a product that had an
NH resonance at 11.7 ppm, characteristic of a 3-H
xanthine;22 (c) the alkylation of the mono-POM deriva-
tive with methyl iodide gave a product, 6a , having a
methyl proton resonance at 3.84 ppm, that expected of
a 7-methyl rather than a 3-methyl substituent, which
would have a resonance at about 3.4 ppm;22,23 (d) the
methylation of authentic 5a gave a product, 8-cyclopen-
tyl-1-propyl-3-methyl-7-POM-xanthine, 7a , that had the
expected methyl resonance at 3.38 ppm; and (e) in
parallel with the shifts of the methyl resonances, the
resonance of the methylene moiety of the POM group
of 5a at 6.25 ppm was further downfield than that

Adenosine Receptor Antagonists as PET/ SPET Ligands
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 4 557
Sch em e 2a
Sch em e 3a
a
(i) NH4CO2H, PD/C, MeOH, reflux; (ii) RX, K2CO3, DMF; (iii)
a
(i) (E)-Bu3Sn-CHdCh-CH2-OTos, DMF, K2CO3, rt; (ii) I2, DCM,
NAOH, reflux.
rt; (iii) NaOH, reflux.
Sch em e 4a
resonance of 5b, which was at 6.15 ppm. Thus, pro-
ceeding from 5b had yielded 8-cyclopentyl-7-methyl-3-
POM-1-propylxanthine, 6a , and, after deprotection,
8-cyclopentyl-7-methyl-1-propylxanthine, 6b. Further,
that result showed that to use the POM group for the
protection of N-7, it was essential to first protect N-3
with a benzyl group.
The synthesis of the 3-substituted CPX analogues
(Scheme 2) began with the fractional crystallization of
3c to remove the unwanted 1-benzyl-3-propyl isomer.
1
The H NMR spectrum of 3c was a convenient way to
follow that separation. Two crystallizations usually
sufficed to remove the unwanted isomer, as shown by
the disappearance of its benzylic resonance at 5.15 ppm.
Removal of the 3-benzyl group by CTH21 gave 8-cyclo-
pentyl-7-(pivaloyloxymethyl)-1-propylxanthine, 5a. CTH
required elevated temperatures (bath temperatures of
140 °C) and anhydrous conditions, specifically, anhy-
drous methanol as the solvent and excess ammonium
formate dried over P4O10 as the hydrogen donor. Re-
gioselective alkylation with either methyl iodide, 2-chlo-
roethyl methyl ether, 1-bromo-2-fluoropropane, or 1-bro-
mo-3-fluoropropane provided 7a -d , respectively.
Because the scope of the project included iodinated
ligands and because vinylic iodides tend to resist
metabolism,24 we alkylated 5a with (E)-3-(tri-n-butyl-
stannyl)prop-2-en-1-yl p-toluenesulfonate,25, 26 obtaining
the intermediate (E)-3-[3-(tributylstannyl)-1-prop-2-en-
1-yl]xanthine, 7e, which was then reacted with iodine
to give the (E)-3-(3-iodoprop-2-en-1-yl)xanthine, 7f
(Scheme 3). Alkaline cleavage of the POM groups of
7a -f completed the syntheses of 8a -f.
The reactions in Scheme 2 include the preparations
of precursors for the nca synthesis of carbon-11-labeled
8a ,b and fluorine-18-labeled 8c,d . The precursor for
carbon-11-labeled 8a is 5a . The alkylation of 5a with
chloroethanol followed by deprotection of the intermedi-
ate 7g gave 8g, the precursor of carbon-11-labeled 8b.
The synthesis of 8c,d labeled with fluorine-18 pro-
ceeds from the 2- and 3-hydroxypropyl analogues, 8h ,i.
Since 1-halo-2-propanols cyclize to the oxiranes under
the conditions of alkylation, 5a was alkylated with
chloroacetone and then reduced by CTH in methanol
to yield 7h as a mixture of isomers. The alkylation of
a
(i) POM-Cl, K2CO3, DMF; (ii) RX, K2CO3, DMF; (iii) NH4CO2H,
Pd/C, MeOH, reflux; (iv) CH3(CH2)2I, K2CO3, DMF; (v) 2 N NaOH,
reflux.
5a with 3-bromopropanol yielded 7i. Deprotection of
7h ,i in alkali gave 8h ,i. Alkylation of 5a by chloroeth-
anol gave intermediate 7g and, following deprotection,
8g. Compound 7g serves as a precursor for carbon-11-
labeled 8b. An alternative approach suited to the nca
synthesis of 8f began with the alkylation of 5a with
propargyl chloride to give the 3-(prop-2-yn-1-yl) deriva-
tive 7j. However, 7j did not react with tributyltin
hydride, and so we abandoned that approach.
The synthesis of the 1-substituted CPX analogues
(Scheme 4) from 3-benzyl-8-cyclopentylxanthine, 2,
began with the protection of N-7 by a POM group to
give 9. Alkylation of 9 at N-1 with either iodomethane,

558 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 4
Holschbach et al.
2-chloroethyl methyl ether, 1-bromo-2-fluoropropane, or
1-bromo-3-fluoropropane formed 10a -d . Successive
steps included debenzylation to the 3-H xanthines, 11a -
d , alkylation with 1-iodopropane to form the 3-propyl-
xanthines, 12a -d , and removal of the POM groups to
form the 1-methyl, 1-ethoxymethyl, 1-(2-fluoropropyl),
and 1-(3-fluoropropyl) analogues of CPX, 13a -d . The
syntheses of the 1-(2-hydroxyethyl) and 1-(3-hydroxy-
propyl) analogues of CPX, 13e,g, precursors of carbon-
11-labeled 13b and fluorine-18-labeled 13d , followed
those of 8g,i. The synthesis of the 1-(2-hydroxypropyl)
analogue of CPX, 13f, began with the alkylation of 9
with chloroacetone, but differed from the synthesis of
8h in that CTH simultaneously reduced the 2-keto
group during debenzylation rather than in a separate
step.
Exp er im en ta l Section
Melting points were measured on an Electrothermal ap-
paratus and are uncorrected. Elemental analyses were per-
formed by MHW Laboratories, Tucson, AZ, and by the Zen-
tralabteilung fu¨r chemische Analysen at the Forschungszentrum
J u¨lich, and are within (0.4% of calculated composition. Thin-
layer chromatography employed precoated silica gel sheets
(Polygram, Macherey-Nagel, Du¨ren, Germany) developed with
ethyl acetate-hexane, 1:1 (solvent A) or 3:1 (solvent B), or
chloroform-methanol, 95:5 (solvent C). A Bruker WP-80
spectrometer provided 80-MHz NMR spectra. Resonances are
reported as chemical shifts (δ) downfield from a TMS internal
standard. Refluxing over Mg turnings and distillation dried
methanol for CTH. Drying of ammonium formate was over
P4O10 at room temperature and atmospheric pressure. CTH
employed equal weights of substrate and 10% Pd-C and a 10-
fold molar excess of ammonium formate for times and tem-
peratures indicated in individual experiments. Purification
of dimethylformamide (DMF) consisted of distillation and
storage in a light-proof container over 4A molecular sieves.
Storage over 4A molecular sieves dried CH2Cl2 and toluene.
Other solvents and reagents were used as supplied by the
vendors. Analytical HPLC used a 250- × 4-mm column of C-18
silica gel, eluted with methanol-water (64:36, v/v). Table 1
lists the solvents used for purification by crystallization or
liquid chromatography.
The project plan included iodophenyl derivatives of
CPX because 8-phenyltheophylline and its 1,3-dialkyl
congeners are useful investigational tools. Since phe-
nols are strongly activated for iodination, the known27
1,3-dipropyl-8-(4-hydroxyphenyl)xanthine, 14, seemed
a suitable precursor. Surprisingly, 14 resisted the usual
methods for electrophilic iodination.28 Those methods
included (a) elemental iodine applied to a solution of
the xanthine in 35% aqueous ammonia or (b) NaI with,
as an oxidant, either NaOCl,29 chloramine T, iodogen,30
or HgO-HBF4 supported on silica gel.31 The best
explanation for the unreactivity of the phenolic moiety
of 14 is that the xanthine must be a profoundly
inactivating substituent.
3-Ben zyl-8-cyclop en tyl-1-p r op ylxa n th in e (1). A solu-
tion of N-benzyl-N′-propylurea (96.1 g, 0.5 mol) and cyanoacetic
acid (40.2 g, 0.55 mol) in acetic anhydride (200 mL) was stirred
for 3 h at 70 °C and evaporated. The syrupy residue was
mixed with water (500 mL) and then basified with 5 N NaOH.
Refrigeration overnight deposited a solid that was filtered off
and crystallized from methanol-water. Yield 123 g, 95%, of
the 6-aminouracil as silky white needles. A solution of the
aminouracil (30 g, 116 mmol) in 80% acetic acid (200 mL) and
ethanol (400 mL) was stirred at 40 °C during the dropwise
addition of a solution of NaNO2 (12 g, 174 mmol) in water (100
mL) over a 2-h period. Stirring continued for an additional
30 min, at which time TLC (solvent C) showed the reaction
was complete. Cooling on ice and filtration collected the
nitrosouracil, which was washed with water and dried. Yield
28.5 g, 85%. The nitrosouracil was suspended in water (100
mL), heated to 85 °C and, stirred during the addition of solid
sodium dithionite in portions until the red color disappeared.
The cooled solution was extracted with ethyl acetate (5 × 50
mL), and the organic phases were pooled, dried over MgSO4,
and evaporated to a thick syrup that was then dried at high
vacuum to remove traces of moisture. Yield of the diaminou-
racil was 25.9 g, 95%. A solution of cyclopentanecarbonyl
chloride, freshly prepared from cyclopentanecarboxylic acid
(11.3 g, 99 mmol) and SOCl2 (8.0 mL, 110 mmol) and dissolved
in dry CH2Cl2 (50 mL), was added dropwise to a stirred
solution of the diaminouracil in dry CH2Cl2 (50 mL). After 15
min HPLC showed that a more polar product had completely
replaced the starting material. The gummy residue after
evaporation was dissolved in ethanol (100 mL) and 1 N NaOH
(100 mL) and refluxed overnight. Cooling and acidification
with HCl precipitated essentially pure product that was
filtered off and crystallized. 1H NMR (CDCl3) δ: 0.85 (t, 3H,
CH2CH2CH3), 1.2-2.3 (m, 10H, CH2CH2CH3 and cyclopentyl
CH2), 2.85-3.45 (m, 1H, cyclopentyl H-1), 3.85 (m, 2H, CH2-
CH2CH3), 5.25 (s, 2H, CH2C6H5), 7.2-7.5 (m, 5H, C6H5), 13.08
(br s, 1H, N7H).
Resu lts a n d Discu ssion
Table 1 lists the chemical characteristics of the CPX
analogues.
Table 2 summarizes the results of the radioligand
binding assays. Modifications of the 3-propyl group
(analogues 8a -d ,f) were well-tolerated. All of the
analogues had values of Ki < 0.5 nM. (()-8-Cyclopentyl-
3-(2-fluoropropyl)-1-propylxanthine, 8c, was twice as
potent as CPX, and the least active, 8-cyclopentyl-3-(2-
methoxyethyl)-1-propylxanthine, 8b, was only 3-fold less
potent than CPX. Because 8c is a racemate, the
possibility remains that one isomer might be even more
potent. The iodinated analogue, 8f, was only 1.4-fold
less potent than CPX. Generally, the modifications of
the 1-propyl group (analogues 13a -d ) reduced affinity,
though only modestly. The least potent analogue, 13c,
had a Ki slightly above 1 nM and was only 8-fold less
potent than CPX. The other 1-substituted analogues
had values of Ki in the subnanomolar range.
In summary, several chemical modifications of the 1-
and 3-propyl groups of CPX have little impact on affinity
for the A1 adenosine receptor. Since the CPX analogues
studied here are surrogates for ligands labeled with
carbon-11, fluorine-18, or radioiodine, such radioligands
could be useful agents for imaging by PET and SPET.
CPX analogues radiolabeled in the C-8 substituent are
not promising because the present experiments show
that 1,3-dipropyl-8-(4-hydroxyphenyl)xanthine is refrac-
tory to iodination. The low potency of 1,3-dipropyl-8-
(3-fluorocyclopentyl)xanthine32 suggests that fluorina-
tion of the cyclopentane moiety affects potency.
3-Ben zyl-8-cyclop en t ylxa n t h in e (2). A solution of 6-
amino-1-benzyluracil15 (34.2 g, 157 mmol) in glacial acetic acid
(100 mL), ethanol (100 mL), and water (200 mL) was cooled
in an ice bath and stirred during the dropwise addition of a
solution of NaNO2 (14 g, 200 mmol) in water (50 mL). Stirring
continued for an additional 30 min. The nitrosouracil was
filtered off, washed with water, suspended in water (250 mL),
and heated to 85 °C. The addition of solid sodium dithionite
discharged the purple color and precipitated a bright-green
solid that was filtered off and freeze-dried. Yield 27.0 g, 74%.

Adenosine Receptor Antagonists as PET/ SPET Ligands
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 4 559
Ta ble 1. Characteristics of Xanthines
no.
R1-R3-R7
yield, %
purificationa
mp, °C
formula
anal.b
1
2
3a
3b
3c
4
5a
5b
5c
6a
6b
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
8a
8b
8c
8d
8e
8f
8g
8h
8i
9
10a
10b
10c
10d
10e
10f
10g
11a
11b
11c
11d
11e
11f
11g
12a
12b
12c
12d
12e
12f
12g
13a
13b
13c
13d
13e
13f
13g
Pr-Bz-H
64c
54c
100
56
92
83
92
12
35
91
77
93
78
39
46
95
92
72
88
67
73
92
88
93
82
92
91
78
85
89
94
91
78
84
76
79
87
72
95
89
93
86
91
91
86
86
82
76
69
75
82
76
94
87
67
91
84
82
89
M
Et-W
DCM
PE
M
M
M
221
>300
131
131
134
>300
127
157
141
69-71
151
93
oil
oil
105
oil
oil
C20H24N4O2
C17H18N4O2
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
C,H,N
H-Bz-H
Pr-Bz-BOC
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
25H32N4O4
22H28N4O3
26H34N4O4‚DMF
13H18N4O2
19H28N4O4
19H28N4O4
25H38N4O6
20H30N4O4
14H20N4O2
20H30N4O4
22H28N4O2
22H33FN4O4
22H33FN4O4
34H58SnN4O4
22H31IN4O4
25H32N4O5
22H34N4O5
22H34N4O5
22H30N4O4
14H20N4O2
16H24N4O3
16H23FN4O2
Pr-Bz-MOM
Pr-Bz-POM
Pr-H-H
Pr-H-POM
Pr-POM-H
Pr-POM-POM
Pr-POM-Me
Pr-H-Me
Pr-Me-POM
Pr-MeOEt-POM
Pr-2FPr-POM
Pr-3FPr-POM
Pr-TBSP-POM
Pr-IP-POM
LC H-E, 1:1
LC H-E, 1:1
PE
EC
H
H
H
H
DCM
LC H-E, 4:1
H
M
H
M
M
M-H
H
Pr-HOEt-POM
Pr-2OHPr-POM
Pr-3OHPr-POM
Pr-propyn-POM
Pr-Me-H
oil
114
89
158-60
182-4
173-5
206-8
190-3
69-71
211-2
104-5
106-7
88-9
142
146
121
116
124
oil
Pr-MeOEt-H
Pr-2FPr-H
Pr-3FPr-H
H
C16H23FN4O2
Pr-TBSP-H
LC H-E, 3:2
C
C
C
C
C
C
28H48SnN4O2
16H21IN4O2
21H32N4O5
22H34N4O5
22H34N4O5
23H28N4O4
Pr-IP-H
M-H
Pr-HOEt-H
M-H
Pr-2OHPr-H
Pr-3OHPr-H
H-Bz-POM
H
H
M
Et2O
M-W
M
E
E
M
E
Me-Bz-POM
MeOEt-Bz-POM
2FP-Bz-POM
3FP-Bz-POM
HOEt-Bz-POM
2OdPr-Bz-POM
3OHPr-Bz-POM
Me-H-POM
MeOEt-H-POM
2FPr-H-POM
3FPr-H-POM
HOEt-H-POM
2OHPr-H-POM
3OHPr-H-POM
Me-Pr-POM
MeOEt-Pr-POM
2FPr-Pr-POM
3FPr-Pr-POM
HOEt-Pr-POM
2OHPr-Pr-POM
3OHPr-Pr-POM
Me-Pr-H
C24H30N4O4
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
24H34N4O5
26H33FN4O4
26H33FN4O4
25H32N4O5
26H32N4O5
26H34N4O5
17H24N4O4
19H28N4O5
19H27FN4O4
19H27FN4O4
18H26N4O5
19N28N4O5
19H28N4O5
20H30N4O4
22H24N4O5
22H33FN4O4
22H33FN4O4
21H32N4O5
22H34N4O5
22H34N4O5
14H20N4O2
16H24N4O2
16H23NFN4O2
16H23FN4O2
15H24N4O3
16H26N4O3
16H26N4O3
126
oil
M
196
121
144
159
69
97
oil
97
63
oil
oil
oil
oil
M-W
M-W
M
M-H
M-W
M
M-W
M-W
E
E
E
E
E
M
M
oil
197
188
oil
212
198
oil
MeOEt-Pr-H
2FP-Pr-H
DMSO-W
EE
M
DMSO-W
DMSO-W
3FP-Pr-H
HOEt-Pr-H
2OHPr-Pr-H
3OHPr-Pr-H
218
a
Solvents for crystallization or liquid chromatography. Abbreviations are: DCM, dichloromethane; DMSO, methyl sulfoxide; Et, ethanol;
b
E, ethyl acetate; EC, tetrachloroethane; H, hexane, LC, liquid chromatography; PE, petroleum ether, and W, water. Analyses for C, H
and N agreed with calculated composition by ( 0.4%. c Based on the 6-aminouracil starting material.
A suspension of the green diaminouracil in dry CH2Cl2 was
treated with cyclopentanecarbonyl chloride, freshly prepared
from cyclopentanecarboxylic acid (15.4 g, 124 mmol) and SOCl2
(16.2 g, 136 mmol). The addition of the acid chloride turned
the suspension a deep red. The mixture was stirred overnight;
a white solid was filtered off and boiled for 12 h in 2 N NaOH
(100 mL). The clear solution was brought to pH 5 with
concentrated HCl, which precipitated a solid that was filtered
off, washed with water, and crystallized. 1H NMR (DMSO-
d6) δ: 1.35-2.20 (m, 8H, cyclopentyl CH2), 2.85-3.5 (br m, 1H,
cyclopentyl H-1), 5.08 (s, 2H, C6H5CH2), 7.30 (m, 5H, C6H5),
11.00 (s, 1H, N1H), 13.08 (s, 1H, N7H).

560 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 4
Holschbach et al.
Ta ble 2. Antagonism of Binding of [3H]CPX to Bovine Brain
Cortex Membranes
8-Cyclop en tyl-1-p r op ylxa n th in e (4). For CTH a 500-mL
flask was flushed with argon and charged with 2 (5.2 g, 16.8
mmol), 10% Pd-C (5.2 g), dry ammonium formate (10.6 g, 168
mmol), and absolute methanol (250 mL). The contents were
stirred in an oil bath heated to 140 °C. After 4 h TLC (solvent
A) showed the reaction was complete. Filtration through sea
sand removed the catalyst, and methanol was evaporated. The
filter cake was washed three times with 1 N NaOH (3 × 80
mL), and the washings and residue from the evaporation were
combined. Adding crushed ice to the alkaline solution followed
by careful acidification to pH 3-4 with dilute HCl precipitated
crude product. Dissolution in alkali and reprecipitation gave
3.7 g (83%) of essentially pure product. Crystallization from
methanol gave an analytical sample. 1H NMR (DMSO-d6) δ:
0.88 (t, 3H, CH2CH2CH3), 1.2-2.3 (m, 10H, CH2CH2CH3 and
cyclopentyl CH2), 2.90-3.35 (m, 1H, cyclopentyl H-1), 3.85 (t,
2H, CH2CH2CH3), 12.51 (s, 1H, N3H), 13.54 (s, 1H, N7H).
8-Cyclop e n t yl-7-(p iva loyloxym e t h yl)-1-p r op ylxa n -
th in e (5a ). The debenzylation of 3c (10 g, 2.15 mmol) followed
the general method for CTH. Workup consisted of removal of
catalyst by filtration, washing the filter cake with CHCl3 (3 ×
100 mL), and evaporation of the filtrate and washings to a
yellowish oil that solidified on standing. Two crystallizations
from methanol gave 7.4 g (92%) of white crystals, mp 127 °C.
1H NMR (CDCl3) δ: 0.95 (t, 3H, CH2CH2CH3), 1.20 (s, 9H,
C(CH3)3), 1.4-2.2 (m, 10H, CH2CH2CH3 and cyclopentyl CH2),
3.05-3.45 (m, 1H, cyclopentyl H-1), 3.98 (t, 2H, CH2CH2CH3),
6.25 (s, 2H, N7CH2OPOM), 11.7 (br s, 1H, N3H).
Rea ction of 4 w ith P iva loyloxym eth yl Ch lor id e. 8-Cy-
clop en tyl-3-(p iva loyloxym eth yl)-1-p r op ylxa n th in e (5b)
a n d 3,7-Bis(p iva loyloxym eth yl)-8-cyclop en tyl-1-p r op yl-
xa n th in e (5c). A suspension of 4 (1.56 g, 6.0 mmol) and
anhydrous Na2CO3 (630 mg, 5.94 mmol) in anhydrous DMF
(40 mL) was stirred during the dropwise addition of POM-Cl
(1.0 g, 6.6 mmol) in dry DMF (10 mL). Stirring continued for
16 h at room temperature. The suspension was filtered, the
filtrate was evaporated, and the residue was taken up in
acetone. The insoluble educt was filtered off, and the filtrate
was taken to dryness. MPLC on a 1.2- × 23-cm column of
silica gel eluted with solvent A gave two products. The fraction
eluting between 30 and 50 mL contained 5c (1 g, 35%). 1H
NMR (CDCl3) δ: 0.99 (m, 3H, CH2CH2CH3), 1.2 (br s, 18H, 2
× C(CH3)3), 1.15-2.10 (m, 10H, CH2CH2CH3 and cyclopentyl
CH2), 3.05-3.42 (m, 1H, cyclopentyl H-1), 3.97 (t, 2H, CH2-
CH2CH3), 6.10 (s, 2H, N3CH2OPOM), 6.23 (s, 2H, N7CH2-
OPOM). The second fraction eluting between 60 and 95 mL
contained 5b (0.27 g, 12%). 1H NMR (CDCl3) δ: 1.0 (t, 3H,
CH2CH2CH3), 1.20 (s, 9H, C(CH3)3), 1.48-2.25 (m, 10H,
CH2CH2CH3 and cyclopentyl CH2), 3.05-3.46 (m, 1H, cyclo-
pentyl H-1), 4.05 (t, 2H, CH2CH2CH3), 6.15 (s, 2H, N3CH2-
OPOM), 13.5 (br s, 1H, N7H).
8-Cyclop en t yl-7-m et h yl-3-(p iva loyloxym et h yl)-1-p r o-
p ylxa n th in e (6a ). Gen er a l Meth od A. A mixture of 5b
(376 mg, 1 mmol), Na2CO3 (106 mg, 1 mmol), and iodomethane
(125 µL, 2 mmol) in dry DMF (4 mL) was stirred for 1 h at
room temperature. Workup began with dilution with water
(10 mL), acidification with 2 N HCl, and extraction with CH2-
Cl2 (4 × 25 mL). The combined extracts were washed with
brine, dried over Na2SO4, and evaporated to give a semisolid
residue that was crystallized from an appropriate solvent
(Table 1). Yield 355 mg, 91%. 1H NMR (CDCl3) δ: 0.98 (t,
3H, CH2CH2CH3), 1.20 (s, 9H, C(CH3)3), 1.48-2.31 (m, 10H,
CH2CH2CH3 and cyclopentyl CH2), 3.05-3.49 (m, 1H, cyclo-
pentyl H-1), 3.84 (s, 3H, N7CH3), 4.07(t, 2H, CH2CH2CH3), 6.13
(s, 2H, N3CH2OPOM).
a
no.
R1-R3-R7
Ki, nM
95% confidence limits
6
8a
8b
8c
8d
8f
Pr-H-Me
Pr-Me-H
149
132-166
0.310
0.493
0.094
0.183
0.237
0.170
0.526
0.286
1.23
0.269-0.356
0.377-0.644
0.085-0.103
0.163-0.207
0.217-0.258
0.161-0.180
0.420-0.659
0.259-0.315
1.08-1.40
Pr-MeOEt-H
Pr-2FPr-H
Pr-3FPr-H
Pr-Ipren-H
Pr-Pr-H
CPXb
13a
13b
13c
13d
CPXc
Me-Pr-H
MeOEt-Pr-H
2FPr-Pr-H
3FPr-H
0.216
0.155
0.181-0.257
0.127-0.180
Pr-Pr-H
a
Abbreviations: Me, methyl; Et, ethyl; Pr, propyl; 2FPr,
2-fluoropropyl; 3FPr, 3-fluoropropyl; Ipren, (E)-3-iodoprop-2-en-
1-yl. b Standard for comparison of 8a -d ,f. c Standard for compari-
son of 13a -d .
3-Ben zyl-7-(ter t-bu toxyca r bon yl)-8-cyclop en tyl-1-p r o-
p ylxa n th in e (3a ). A solution of 1 (14.0 g, 39.3 mmol) and
triethylamine (4.0 g, 39.5 mmol) in dry CH2Cl2 (50 mL) was
stirred during the portionwise addition of di-tert-butyl pyro-
carbonate (8.6 g, 39.4 mmol). When gas evolution ceased the
clear solution was filtered through a 1- × 3-cm column of silica
gel to remove a polar impurity, and the filtrate was evaporated.
A solution of the residue in CH2Cl2 was washed with water
(100 mL), 1 N NaOH (3 × 100 mL), and water (100 mL).
Drying over MgSO4 and evaporation gave 18 g (100%) of
product as an off-white solid. 1H NMR (CDCl3) δ: 0.91 (t, 3H,
CH2CH2CH3), 1.40-2.25 (m, 19H, CH2CH2CH3, cyclopentyl
CH2 and C(CH3)3), 3.55 (t, 1H, cyclopentyl H-1), 3.95 (m, 2H,
CH2CH2CH3), 5.25 (s, 2H, PhCH2), 7.30 (m, 5H,C6H5).
3-Ben zyl-8-cyclop en t yl-7-(m et h oxym et h yl)-1-p r op yl-
xa n th in e (3b). A suspension of 1 (103 mg, 0.4 mmol) in
hexamethyldisilazane (10 mL) containing a catalytic amount
of ammonium sulfate was refluxed for 90 min. During that
time the mixture became homogeneous. Evaporation gave a
liquid residue that was treated with methyl bromomethyl ether
(MOM-Br; 350 µL, 4 mmol) in dry THF (4 mL). After stirring
for 20 h at room temperature the reaction mixture was treated
with methanol (20 mL) and stirred for 30 min. The solution
was concentrated nearly to dryness and was treated with 33%
ammonia (2 mL) and acetone (25 mL). Crystals of NH4Br were
filtered off, and the filtrate was evaporated to dryness. The
residue was taken up in ethyl acetate, washed with water,
dried over Na2SO4, and evaporated to yield a yellow oil.
Crystallization from petroleum ether gave fluffy white needles.
1H NMR (CDCl3) δ: 0.95 (t, 3H, CH2CH2CH3), 1.45-2.27 (m,
10H, CH2CH2CH3 and cyclopentyl CH2), 3.0-3.3 (m, 1H,
cyclopentyl H-1), 3.40 (s, 3H, OCH3), 3.95 (t, 2H, CH2CH2CH3),
5.75 (s, 2H, N7CH2O).
3-Ben zyl-8-cyclop en t yl-7-(p iva loyloxym et h yl)-1-p r o-
p ylxa n th in e (3c). A mixture of 1 (20.6 g, 58.5 mmol),
chloromethyl pivalate (POM-Cl; 17.7 mL, 122.6 mmol), and
anhydrous Na2CO3 (13.0 g, 122.6 mmol) in anhydrous DMF
(150 mL) was stirred overnight at 50 °C. TLC (solvent A)
showed complete conversion of the starting material (Rf ) 0.68)
to a less polar product (Rf ) 0.91). Evaporation in vacuo gave
a gummy residue that was taken up in CH2Cl2, washed with
water, dried over Na2SO4, and evaporated. Crystallization
gave 25.1 g (92%) of product. 1H NMR (CDCl3) δ: 0.95 (t, 3H,
CH2CH2CH3), 1.20 (s, 9H, C(CH3)3), 1.40-2.20 (m, 10H,
CH2CH2CH3 and cyclopentyl CH2), 3.05-3.45 (m, 1H, cyclo-
pentyl H-1), 3.97 (t, 2H, CH2CH2CH3), 5.25 (s, 2H, C6H5CH2),
6.25 (s, 2H, N7CH2OPOM), 7.2-7.7 (m, 5H, C6H5).
8-Cyclop en tyl-7-m eth yl-1-p r op ylxa n th in e (6b). A sus-
pension of 6a in 4 N aqueous NaOH (4 mL, 16 mmol) was
stirred at reflux for 6 h. Cooling and acidification to pH 3
precipitated a yellowish solid that was dried over P4O10
.
Crystallization gave white crystals. Yield 107 mg, 77%. 1H
NMR (CDCl3) δ: 0.93 (t, 3H, CH2CH2CH3), 1.50-2.36 (m, 10H,
CH2CH2CH3 and cyclopentyl CH2), 3.06-3.50 (m, 1H, cyclo-
pentyl H-1), 3.81 (s, 3H, N7CH3), 4.02 (t, 2H, CH2CH2CH3),
9.79 (s, 1H, N3H).

Adenosine Receptor Antagonists as PET/ SPET Ligands
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 4 561
8-Cyclop en t yl-3-m et h yl-7-(p iva loyloxym et h yl)-1-p r o-
p ylxa n th in e (7a ). Alkylation by general method A employed
iodomethane as the alkylating reagent and a reaction time of
24 h at 80 °C. Yield 363 mg, 93%, white crystals.
8-Cyclopen tyl-3-(m eth oxyeth yl)-7-(pivaloyloxym eth yl)-
1-p r op ylxa n th in e (7b). General method A employed 2-chlo-
roethyl methyl ether (182 µL, 2 mmol) as the alkylation
reagent and a reaction time of 24 h at 80 °C. Yield 338 mg,
78%, colorless oil.
8-Cyclopen tyl-3-(2-flu or opr opyl)-7-(pivaloyloxym eth yl)-
1-pr opylxan th in e (7c). General method A employed 1-bromo-
2-fluoropropane (282 mg, 2 mmol) as the alkylation reagent
and a reaction time of 24 h at 80 °C. Yield 170 mg, 39%,
colorless oil.
for 5 min at room temperature. Propargyl chloride (145 mg,
2 mmol) was added via syringe, and the reaction mixture was
stirred for 6 h at 85 °C. After cooling and addition of water
(30 mL), the mixture was extracted with chloroform (3 × 50
mL). The combined organic phases were washed with water
(2 × 25 mL), dried over Na2SO4, filtered, and evaporated, and
the yellowish solid was recrystallized from methanol to yield
583 mg (73%) of 7j as white crystals. 1H NMR (CDCl3) δ: 0.95
(t, 3H, CH2CH2CH3), 1.20 (s, 9H, C(CH3)3), 1.45-2.18 (m, 10H,
CH2CH2CH3 and cyclopentyl CH2), 2.47 (t, 1H, CH2CtCH),
3.0-3.50 (m, 1H, cyclopentyl H-1), 3.97 (t, 2H, CH2CH2CH3),
4.85 (d, 2H, CH2CtCH), 6.25 (s, 2H, N7CH2OPOM).
8-Cyclop en tyl-3-m eth yl-1-p r op ylxa n th in e (8a ). Gen -
er a l Meth od B. To 1 mmole of 7a -d dissolved in DMSO (5-
10 mL/mmol), was added 4 N aqueous NaOH (4 mL, 16 mmol).
The solution was stirred at room temperature for 30 min. Slow
dilution with water precipitated the 7-H xanthine. Recrys-
tallization from an appropriate solvent yielded the analytically
pure compound. Yield 254 mg, 92%. 1H NMR (CDCl3) δ: 1.00
(t, 3H, CH2CH2CH3), 1.50-2.38 (m, 10H, CH2CH2CH3 and
cyclopentyl CH2), 3.09-3.53 (m, 1H, cyclopentyl H-1), 3.61 (s,
3H, N3CH3), 4.05 (t, 2H, CH2CH2CH3), 12.76 (s, 1H, N7H).
8-Cyclopen tyl-3-(3-flu or opr opyl)-7-(pivaloyloxym eth yl)-
1-pr opylxan th in e (7d). General method A employed 1-bromo-
3-fluoropropane (282 mg, 2 mmol) as the alkylation reagent
and a reaction time of 24 h at 60 °C. Yield 201 mg, 46%, white
crystals.
8-Cyclop en tyl-3-[(E)-3-(tr i-n -bu tylsta n n yl)p r op -2-en -1-
yl]-7-(p iva loyloxym eth yl)-1-p r op ylxa n th in e (7e). A sus-
pension of 5a (545 mg, 1.5 mmol) and K2CO3 (249 mg, 1.8
mmol) in dry DMF (15 mL) was stirred for 5 min at room
temperature before the addition of (E)-3-(tri-n-butylstannyl)-
prop-2-en-1-yl tosylate23,24 (867 mg, 1.8 mmol). Stirring con-
tinued at room temperature for 3 h. Acidification of the
reaction mixture with 2 N HCl, extraction with CH2Cl2 (3 ×
25 mL), drying of the combined organic phases over Na2SO4,
and evaporation of the solvent yielded 1.12 g (95%) of 7e as a
colorless oil. 1H NMR (CDCl3) δ: 0.5-1.0 (m, 12H, CH3 of
POM and CH2CH2CH3), 1.1-1.75 (m, 28H, CH2-Bu and
cyclopentyl CH2 and CH2CH2CH3), 1.20 (s, 9H, C(CH3)3), 3.0-
3.50 (m, 1H, cyclopentyl H-1), 3.88-4.15 (q, 2 H, CH3CH2CH2),
4.62-4.75 (q, 2H, CH2-propenyl), 5.97-6.17 (m, 2H, CHdCH),
6.25 (s, 2H, N7CH2OPOM).
8-Cyclop en t yl-3-[(E)-3-iod op r op -2-en yl]-7-(p iva loyl-
oxym eth yl)-1-p r op ylxa n th in e (7f). A solution of I2 (127 mg,
0.5 mmol) in CH2Cl2 (7.5 mL) was added dropwise to a solution
of 7e (350 mg, 0.5 mmol) in CH2Cl2 (5 mL). After the addition
of a stochiometric amount of iodine the color of iodine persisted.
Evaporation of the solvent left an oily residue for purification
by MPLC (silica gel 60, 25- × 1.5-cm, eluted with hexanes-
ethyl acetate, 80:20; flow 15 mL/min; UV detection at 254 nm).
Evaporation of effluent collected between 6:40 and 9:25 min
yielded a colorless oil (246 mg, 92%). 1H NMR (CDCl3) δ: 0.95
(t, 3H, CH2CH2CH3), 1.20 (s, 9H, C(CH3)3), 1.28-2.15 (m, 12H,
cyclopentyl CH2 and CH3CH2CH2), 3.05-3.47 (m, 1H, cyclo-
pentyl H-1), 3.85-4.15 (q, 2H, CH3CH2CH2-), 4.5-4.65 (t, 2H,
CH2-propenyl), 6.25 (s, 2H, N7CH2OPOM), 6.35-6.83 (m, 2H,
CHdCH).
8-Cyclop en tyl-3-(2-h yd r oxyeth yl)-7-(p iva loyloxym eth -
yl)-1-p r op ylxa n th in e (7g). General method A employed
2-chloroethanol (134 µL, 2 mmol) as the alkylation reagent and
a reaction time of 24 h at 80 °C. Yield 310 mg, 72%, colorless
oil.
8-Cyclop en tyl-3-(2-h yd r oxyp r op yl)-7-(p iva loyloxym e-
th yl)-1-p r op ylxa n th in e (7h ). General method A employed
chloroacetone (160 µL, 2 mmol) as the alkylation reagent and
a reaction time of 24 h at 60 °C. Workup yielded a yellowish
solid (415 mg, 96%, >95% pure as shown by TLC), which
underwent CTH in dry methanol (5 mL) at a bath temperature
of 140 °C for 10 min. Filtration of the catalyst, washing the
filter cake three times with hot methanol (3 × 10 mL), and
evaporation of the solvent gave a solid residue which was
recrystallized from methanol. Yield 382 mg, 88%, white
crystals.
8-Cyclopen tyl-3-(m eth oxyeth yl)-1-pr opylxan th in e (8b).
Yield 282 mg, 88%; mp 173-175 °C (MeOH-hexane), white
crystals.
8-Cyclopen tyl-3-(2-flu or opr opyl)-1-pr opylxan th in e (8c).
Yield 300 mg, 93%; mp 206-208 °C (hexane), white crystals.
8-Cyclopen tyl-3-(3-flu or opr opyl)-1-pr opylxan th in e (8d).
Yield 264 mg, 82%; mp 190-193 °C (hexane), white crystals.
8-Cyclop en tyl-3-[(E)-3-(tr i-n -bu tylsta n n yl)p r op -2-en -1-
yl]-1-p r op ylxa n th in e (8e). A solution of the POM tin
compound 7e (175 mg, 0.25 mmol) in DMSO (10 mL) contain-
ing 4 N NaOH (3 mL) was stirred for 5 min at room
temperature. After dilution with water (100 mL) the aqueous
phase was extracted with ethyl acetate (3 × 50 mL) and the
combined organic phases were dried over Na2SO4. The result-
ing oily residue was purified by MPLC, on a 1.5- × 25-cm
column of silica gel 60, eluted with hexanes-ethyl acetate, 60:
40, at 20 mL/min, UV detection at 254 nm. Evaporation of
the fraction eluted from 2:5 to 4:1 min yielded 544 mg (92%)
of a colorless oil which solidified upon standing, mp 69-71
°C. 1H NMR (CDCl3) δ: 0.68-0.98 (m, 12H, C(CH3)3 and CH3-
CH2CH2), 1.0-1.53 (m, 18H, CH2-C(CH3)3), 1.54-2.36 (m, 10H,
cyclopentyl CH2 and CH3CH2-CH2), 3.05-3.48 (m, 1H, cyclo-
pentyl H-1), 3.85-4.26 (t, 2H, CH2CH2CH3), 4.75 (d, 2H, CH2-
propenyl), 5.97-6.17 (m, 2H, CHdCH), 12.63 (br s, 1H, N7H).
8-Cyclop en t yl-3-[(E)-3-iod op r op -2-en yl]-1-p r op ylxa n -
th in e (8f). Compound 7f (108 mg, 0.2 mmol) was stirred in 2
N NaOH (5 mL) for 12 h at 100 °C. Acidification with 6 N
HCl, extraction with CH2Cl2 (3 × 20 mL), drying of the
combined organic phases over Na2SO4, and rotary evaporation
of the solvent yielded a solid residue which was recrystallized
from methanol-hexane. Yield 80 mg, 91%. 1H NMR (CDCl3)
δ: 0.95 (t, 3H, CH3CH2CH2), 1.25-2.3 (m, 10H, cyclopentyl
CH2 and CH3CH2CH2), 3.05-3.45 (m, 1H, cyclopentyl H-1),
3.75-4.15 (t, 2H, CH2CH2CH3), 4.55-4.75 (d, 2H, CH2-
propenyl), 6.45-6.95 (m, 2H, CHdCH), 12.6 (br s, 1H, N7H).
8-Cyclopen tyl-3-(2-h ydr oxyeth yl)-1-pr opylxan th in e (8g).
Yield 239 mg, 78%, white crystals.
8-Cyclop en tyl-3-(2-h yd r oxyp r op yl)-1-p r op ylxa n th in e
(8h ). Yield 272 mg, 85%, white crystals.
8-Cyclop en tyl-3-(3-h yd r oxyp r op yl)-1-p r op ylxa n th in e
(8i). Yield 285 mg, 89%, white crystals.
3-Be n zyl-8-cyclop e n t yl-7-(p iva loyloxym e t h yl)xa n -
th in e (9). A mixture of 2 (15.5 g, 50 mmol), anhydrous Na2-
CO3 (10.6 g, 100 mmol), and pivaloyl chloromethyl ester (15
g, 100 mmol) in dry DMF (100 mL) was stirred overnight at
room temperature, diluted with water (200 mL), and extracted
with CH2Cl2 (3 × 50 mL). The combined extracts were washed
with water and brine, dried over Na2SO4, and evaporated.
Product crystallized from a solution of the residue in methanol.
Yield 20 g, 94%. 1H NMR (DMSO-d6) δ: 1.10 (s, 9H, C(CH3)3),
1.40-2.20 (m, 8H, cyclopentyl CH2), 3.40 (br s, 1H, cyclopentyl
8-Cyclop en tyl-3-(3-h yd r oxyp r op yl)-7-(p iva loyloxym e-
th yl)-1-p r op ylxa n th in e (7i). General method A employed
3-bromo-1-propanol (278 mg, 2 mmol) as the alkylation reagent
and a reaction time of 24 h at 80 °C. Yield 291 mg, 67%, white
crystals.
8-Cyclop en t yl-7-(p iva loyloxym et h yl)-3-(1-p r op yn -3-
yl)xa n th in e (7j). A solution of 5a (720 mg, 2 mmol) in dry
DMF (10 mL) containing Na2CO3 (212 mg, 2 mmol) was stirred

562 J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 4
Holschbach et al.
H-1), 5.1 (s, 2H, CH2C6H5), 6.20 (s, 2H, N7CH2OPOM), 7.30
(m, 5H, C6H5), 11.2 (s, 1H, N1H).
Yield 336 mg, 86%, white crystals. 1H NMR (CDCl3) δ: 0.95
(t, 3H, CH3CH2CH2), 1.21 (s, 9H, C(CH3)3), 1.45-2.20 (m, 10H,
CH3CH2CH2 and cyclopentyl CH2), 3.00-3.50 (m, 1H, cyclo-
pentyl H-1), 3.40 (s, 3H, CH3), 4.07 (t, 2H, CH3CH2CH2), 6.25
(s, 2H, N7CH2OPOM).
8-Cyclop en t yl-1-(2-m et h oxyet h -1-yl)-7-(p iva loyloxy-
m eth yl)-3-p r op ylxa n th in e (12b). Yield 356 mg, 82%, white
crystals.
8-Cyclopen tyl-1-(2-flu or opr opyl)-7-(pivaloyloxym eth yl)-
3-p r op ylxa n th in e (12c). Yield 334 mg, 76%, colorless oil.
8-Cyclopen tyl-1-(3-flu or opr opyl)-7-(pivaloyloxym eth yl)-
3-p r op ylxa n th in e (12d ). Yield 301 mg, 69%, colorless oil.
8-Cyclop en tyl-1-(2-h yd r oxyeth yl)-7-(p iva loyloxym eth -
yl)-3-p r op ylxa n th in e (12e). Yield 315 mg, 75%, colorless oil.
8-Cyclop en tyl-1-(2-h yd r oxyp r op yl)-7-(p iva loyloxym e-
th yl)-3-p r op ylxa n th in e (12f). Yield 359 mg, 82%, colorless
oil.
3-Ben zyl-8-cyclopen tyl-1-m eth yl-7-(pivaloyloxym eth yl)-
xa n th in e(10a ). Gen er a l Meth od C. A solution of 9 (412
mg, 1 mmol) in dry DMF (5 mL) was treated with K2CO3 (138
mg, 1 mmol) and iodomethane (125 µL, 2 mmol). After stirring
for 4 h at room temperature, the mixture was concentrated in
vacuo (bath temperature < 70 °C), the residue taken up in
ethyl acetate (50 mL), the organic phase washed with water
(6 × 30 mL), dried over Na2SO4, and filtered, and the solvent
evaporated. The residue was recrystallized from an appropri-
ate solvent. Yield 384 mg, 91%, white crystals. 1H NMR
(CDCl3) δ: 1.16 (s, 9H, C(CH3)3), 1.50-2.27 (m, 8H, cyclopentyl
CH2), 3.05-3.47 (m, 1H, cyclopentyl H-1), 3.35 (s, 3H, CH3),
5.25 (s, 2H, CH2C6H5), 6.25 (s, 2H, N7CH2OPOM), 7.17-7.65
(m, 5H, C6H5).
3-Ben zyl-8-cyclop en t yl-1-(2-m et h oxyet h yl)-7-(p iva l-
oyloxym et h yl)xa n t h in e (10b ). Alkylation with 1-chloro-
ethyl methyl ether proceeded for 24 h at 80 °C. Yield 364 mg,
78%; mp 121 °C (MeOH-H2O).
8-Cyclop en tyl-1-(3-h yd r oxyp r op yl)-7-(p iva loyloxym e-
th yl)-3-p r op ylxa n th in e (12g). Yield 330 mg, 76%, colorless
oil.
3-Be n zyl-8-cyclop e n t yl-1-(2-flu or op r op yl)-7-(p iva l-
oyloxym eth yl)xa n th in e (10c). Alkylation with 1-bromo-2-
fluoropropane proceeded for 48 h at 80 °C. Yield 394 mg, 84%,
white crystals; mp 116 °C (MeOH).
3-Be n zyl-8-cyclop e n t yl-1-(3-flu or op r op yl)-7-(p iva l-
oyloxym eth yl)xa n th in e (10d ). Alkylation with 1-bromo-3-
fluoropropane proceeded for 24 h at 80 °C. Yield 357 mg, 76%,
white crystals; mp 123.9 °C (EtOH).
8-Cyclop en tyl-1-m eth yl-3-p r op ylxa n th in e (13a ). Gen-
eral method B served for the deprotection of 13a -g. The yield
of 13a was 260 mg, 94%, white crystals. 1H NMR (CDCl3) δ:
0.98 (t, 3H, CH3CH2CH2), 1.45-2.30 (m, 10H, CH3CH2CH2 and
cyclopentyl CH2), 3.05-3.60 (m, 1H, cyclopentyl H-1), 3.45 (s,
3H, CH3), 4.10 (t, 2H, CH3CH2CH2), 12.63 (br s, 1H, N7H).
8-Cyclop en t yl-1-(2-m et h oxyet h yl)-3-p r op ylxa n t h in e
(13b). Yield 219 mg, 87%, white crystals.
3-Ben zyl-8-cyclop en t yl-1-(2-h yd r oxyet h yl)-7-(p iva l-
oyloxym et h yl)xa n t h in e (10e). Alkylation with 2-chloro-
ethanol proceeded for 24 h at 80 °C. Yield 351 mg, 79%,
yellowish oil.
8-Cyclop e n t yl-1-(2-flu or op r op yl)-3-p r op ylxa n t h in e
(13c). Yield 214 mg, 67%, colorless oil.
8-Cyclop e n t yl-1-(3-flu or op r op yl)-3-p r op ylxa n t h in e
(13d ). Yield 293 mg, 91%, white crystals.
3-Be n zyl-8-cyclop e n t yl-1-(2-oxop r op yl)-7-(p iva loyl-
oxym eth yl)xa n th in e (10f). Alkylation with chloroacetone
proceeded for 24 h at 60 °C. Yield 404 mg, 87%, white crystals;
mp 126.1 °C (MeOH).
3-Ben zyl-8-cyclop en t yl-1-(3-h yd r oxyp r op yl)-7-(p iva l-
oyloxym eth yl)xa n th in e (10g). Alkylation with 1-bromo-3-
hydroxypropane proceeded for 24 h at 80 °C. Yield 387 mg,
72%, colorless oil.
8-Cyclop en t yl-1-(2-h yd r oxyet h yl)-3-p r op ylxa n t h in e
(13e). Yield 251 mg, 84%, white crystals.
8-Cyclop en tyl-1-(2-h yd r oxyp r op yl)-3-p r op ylxa n th in e
(13f). Yield 262 mg, 82%, colorless oil.
8-Cyclop en tyl-1-(3-h yd r oxyp r op yl)-3-p r op ylxa n th in e
(13g). Yield 285 mg, 89%, white crystals.
Ra d ioliga n d Bin d in g Stu d ies. Competitive radioligand
binding assays were performed at room temperature in a total
volume of 250 µL in 50 mM Tris‚HCl, 0.02% CHAPS, pH 7.4,
containing 0.2 nM [3H]CPX (4 Gbq/µmol; New England Nuclear,
Bad Homburg, Germany), 25 µg of bovine cerebral cortex
membranes, 0.2 U/mL adenosine deaminase, and a CPX
analogue. Binding was initiated by the addition of the cerebral
cortex membranes. Incubations were terminated after 2 h by
filtration of a 200-µL aliquot through Whatman GF/B glass
fiber filters, and filter-bound radioactivity was determined by
liquid scintillation counting. Binding in the presence of 10
µM (R)-PIA (N6-(1R)-methyl-2-phenethyladenosine) defined
unspecific binding. Equilibrium binding data were analyzed
by nonlinear curve fitting using the program SCTFIT described
by De Lean et al.33 Reported values of Ki represent the
geometric means and 95% confidence limits of 3-5 indepen-
dent experiments performed on duplicate samples.
8-Cyclop e n t yl-1-m e t h yl-7-(p iva loyloxym e t h yl)xa n -
th in e (11a ). Gen er a l Meth od D. A mixture of 1 mmol of
10a -g, 10% Pd-C (0.5 g/g of benzyl compound), and dry
ammonium formate (0.94 g, 20 mmol) in dry methanol (10 mL)
was stirred in an oil bath for 4 h at 140 °C. After cooling the
mixture was filtered through sea sand, the filter cake was
washed three times with warm methanol (3 × 25 mL), and
the combined filtrates were evaporated. The resulting residue
was recrystallized from an appropriate solvent. Yield 331 mg,
95%, white crystals. 1H NMR (CDCl3) δ: 1.20 (s, 9H, C(CH3)3),
1.65-2.00 (m, 8H, cyclopentyl CH2), 3.00-3.60 (br s, 1H,
cyclopentyl H-1), 3.63 (s, 3H, CH3), 6.25 (s, 2H, N7CH2OPOM),
9.20 (s, 1H, N3H).
8-Cyclop en t yl-1-(2-m et h oxyet h -1-yl)-7-(p iva loyloxy-
m eth yl)xa n th in e (11b). Yield 349 mg, 89%, white crystals.
8-Cyclop en tyl-1-(2-flu or op r op yl)-7-(p iva loyloxym eth -
yl)xa n th in e (11c). Yield 361 mg, 93%, white crystals.
8-Cyclop en tyl-1-(3-flu or op r op yl)-7-(p iva loyloxym eth -
yl)xa n th in e (11d ). Yield 339 mg, 86%, white crystals.
8-Cyclop en tyl-1-(2-h yd r oxyeth yl)-7-(p iva loyloxym eth -
yl)xa n th in e (11e). Yield 334 mg, 91%, white crystals.
8-Cyclop en tyl-1-(2-h yd r oxyp r op yl)-7-(p iva loyloxym e-
th yl)xa n th in e (11f). Yield 351 mg, 91%, white crystals.
8-Cyclop en tyl-1-(3-h yd r oxyp r op yl)-7-(p iva loyloxym e-
th yl)xa n th in e (11g). Yield 338 mg, 86%, yellowish oil.
8-Cyclop en t yl-1-m et h yl-7-(p iva loyloxym et h yl)-3-p r o-
p ylxa n th in e (12a ) Gen er a l Meth od E. A solution of 11a
(348 mg, 1 mmol) in dry DMF (5 mL) was treated with K2CO3
(138 mg, 1 mmol) and 1-iodopropane (195 µL, 2 mmol). The
mixture was stirred for 12 h at room temperature and then
concentrated in vacuo (bath temperature < 70 °C). A solution
of the residue in ethyl acetate (50 mL) was washed with water
(6 × 30 mL) to remove DMF, dried over Na2SO4, and filtered
and the solvent evaporated. The residue was recrystallized.
Su p p or tin g In for m a tion Ava ila ble: Additional NMR
data for compounds mentioned in the text (8 pages). Ordering
information is given on any current masthead page.
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