8-Benzyltetrahydropyrazino[2,1-f]purinediones: Water-soluble tricyclic xanthine derivatives as multitarget drugs for neurodegenerative diseases

Article abstract of DOI:10.1002/cmdc.201402082

8-Benzyl-substituted tetrahydropyrazino[2,1-f]purinediones were designed as tricyclic xanthine derivatives containing a basic nitrogen atom in the tetrahydropyrazine ring to improve water solubility. A library of 69 derivatives was prepared and evaluated

Full text of DOI:10.1002/cmdc.201402082

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DOI: 10.1002/cmdc.201402082
8-Benzyltetrahydropyrazino[2,1-f]purinediones: Water-
Soluble Tricyclic Xanthine Derivatives as Multitarget Drugs
for Neurodegenerative Diseases
Andreas Brunschweiger,[a] Pierre Koch,[a] Miriam Schlenk,[a] Felipe Pineda,[b] Petra Kꢀppers,[a]
Sonja Hinz,[a] Meryem Kçse,[a] Stefan Ullrich,[a] Jçrg Hockemeyer,[a] Michael Wiese,[b]
Jag Heer,[c] and Christa E. Mꢀller*[a]
8-Benzyl-substituted
tetrahydropyrazino[2,1-f]purinediones
hydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (72) (human AR:
Ki A1 217 nm, A2A 233 nm; IC50 MAO-B: 508 nm). Dichlorinated
compound 36 [8-(3,4-dichlorobenzyl)-1,3-dimethyl-6,7,8,9-
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione] was found to
be the best triple-target drug in rat (Ki A1 351 nm, A2A 322 nm;
IC50 MAO-B: 260 nm), and may serve as a useful tool for pre-
clinical proof-of-principle studies. Compounds that act at multi-
ple targets relevant for symptomatic as well as disease-modify-
ing treatment of neurodegenerative diseases are expected to
show advantages over single-target therapeutics.
were designed as tricyclic xanthine derivatives containing
a basic nitrogen atom in the tetrahydropyrazine ring to im-
prove water solubility. A library of 69 derivatives was prepared
and evaluated in radioligand binding studies at adenosine re-
ceptor (AR) subtypes and for their ability to inhibit monoamine
oxidases (MAO). Potent dual-target-directed A1/A2A adenosine
receptor antagonists were identified. Several compounds
showed triple-target inhibition; one of the best compounds
was 8-(2,4-dichloro-5-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetra-
Introduction
Adenosine is a modulator of many physiological and patho-
physiological processes exhibiting central nervous system
(CNS) depressant, cardiodepressant, antidiuretic, and immuno-
modulatory effects.[1] The nucleoside exerts its effects through
activation of specific G protein-coupled cell membrane recep-
tors termed A1, A2A, A2B, and A3 adenosine receptors (ARs).[2]
The different AR subtypes show distinct tissue and cell distri-
bution. The dominant subtypes in the central nervous system
(CNS) are A1 and A2A, whereas A2B and A3ARs typically display
low expression levels in the brain. A1ARs are highly expressed
in many regions of the brain including cortex, hippocampus,
and caudate–putamen, but also in several peripheral organs
and tissues, such as the heart, lung, kidney, and fat cells.[2] Po-
tential therapeutic applications of selective A1AR antagonists
include renal and cardiac failure, and the treatment of cogni-
tive dysfunction, as observed in Alzheimer’s disease (AD), due
to their CNS stimulatory effects.[3,4] A2AARs show a restricted ex-
pression in the brain with high levels in the basal ganglia,
where they are co-expressed and form heteromers with dopa-
mine D2 receptors.[5–7] Blockade of A2AARs has shown beneficial
effects in animal models and in clinical studies of Parkinson’s
disease (PD).[8–10] In addition, A2AAR antagonists were found to
exhibit neuroprotective effects in preclinical studies and may
therefore exert disease-modifying properties in neurodegener-
ative disorders such as PD and AD.[11–14]
[a] Dr. A. Brunschweiger,+ Dr. P. Koch,++ Dr. M. Schlenk, Dr. P. Kꢀppers,
Dr. S. Hinz, Dr. M. Kçse, Dr. S. Ullrich, Dr. J. Hockemeyer,
Prof. Dr. C. E. Mꢀller
Pharmaceutical Chemistry I, Pharmaceutical Institute
PharmaCenter Bonn, University of Bonn
Monoamine oxidases A and B (MAO-A and MAO-B) are fla-
vine adenine dinucleotide (FAD)-dependent mitochondrial en-
zymes. They catalyze the oxidative deamination of various
amine neurotransmitters, such as (nor)epinephrine, dopamine,
and serotonin, and of xenobiotic arylalkylamines, including
2-phenylethylamine and tryptamine.[15] Nonselective inhibitors
of MAO, for example, tranylcypromine, and selective MAO-A in-
hibitors, for example, moclobemide, are used for the treatment
of depression. Selective MAO-B inhibitors are applied as ad-
junctive therapeutics for PD in combination with the prodrug
levodopa to increase its bioavailability and that of its active
metabolite dopamine. Besides the irreversible MAO-B inhibitors
selegiline (1a) and rasagiline, reversible inhibitors such as laza-
bemide (1b) and safinamide (1c, Figure 1) have been devel-
An der Immenburg 4, 53121 Bonn (Germany)
E-mail: christa.mueller@uni-bonn.de
[b] Dr. F. Pineda, Prof. Dr. M. Wiese
Pharmaceutical Chemistry II, Pharmaceutical Institute
PharmaCenter Bonn, University of Bonn
An der Immenburg 4, 53121 Bonn (Germany)
[c] Dr. J. Heer
CNS Research, Medicinal Chemistry & Lead Generation, UCB S.A.
Chemin du Foriest, 1420 Braine l’Alleud (Belgium)
[+] Current address: Faculty for Chemistry and Chemical Biology
TU Dortmund, Otto-Hahn-Straße 6, 44227 Dortmund (Germany)
++] Current address: Institute of Pharmaceutical Sciences, Faculty of Science
[
Eberhard Karls Universitꢁt Tꢀbingen
Auf der Morgenstelle 8, 72076 Tꢀbingen (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cmdc.201402082.
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4a) failed to show cognitive-enhancing effects when tested
under the same conditions. These results support the hypothe-
sis that a dual A2A/A2AR antagonist may provide additional
benefit to PD patients relative to selective A2AARs antagonists,
because of their positive effects on cognitive impairment asso-
ciated with the disease. Such drugs may also be useful for AD
therapy.
Another dual-target approach was reported by Chen et al.
who evaluated the A2AAR antagonist (E)-8-(3-chlorostyryl)caf-
feine (CSC, 4b) for inhibition of MAO-B.[38] The authors sug-
gested that the neuroprotective properties of compound 4b
in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)
mouse model of PD may be partly due to MAO-B inhibition in
synergy with its A2AAR antagonistic activity.[38,39] Recently,
Rivara et al.[40] reported a new series of 9-deazaxanthines based
on previous studies by our group.[41,42] They showed that the
9-deaza analogue of CSC also acted as a relatively potent dual
target A2AAR/MAO-B inhibitor (Ki human A2AAR: 260 nm; IC50
human MAO-B: 200 nm).
Thus, the approach to design compounds that inhibit MAO-
B in addition to blocking A2AARs (dual-target approach), and
ancillary A1AR antagonistic activity (triple-target approach) may
result in synergistic or additive effects in the treatment of PD.
The same concept may be extended to the treatment of AD,
for which several multitarget drug approaches including MAO-
B inhibition have recently been developed.[43,44] The resulting
drugs will exhibit a broad spectrum of activities showing ef-
fects on several symptoms of these complex neurodegenera-
tive diseases.[39]
Figure 1. Structures of standard MAO-B inhibitors.
oped for the treatment of PD.[16–20] MAO inhibitors decrease
the production of hydrogen peroxide, a product of the MAO
reaction, and may therefore protect the brains of PD patients
that are treated with levodopa from oxidative stress.[15,20] MAO-
B inhibitors have also been proposed as novel, neuroprotective
therapeutics for AD.[21]
Therapies that act at multiple targets and provide both
symptomatic and neuroprotective effects may be more effec-
tive in treating complex neurodegenerative diseases such as
AD or PD than drugs interacting with a single target.[22–27] Re-
cently, two dual-target approaches for the treatment of PD
have been suggested and both include A2AAR blockade. Dual
antagonism of A1 and A2AARs has been found to be highly ef-
fective in different animal models of PD: A1/A2AAR antagonists
improve motor disabilities and may be neuroprotective by
A2AAR blockade and may improve cognitive function by A1AR
antagonism.[28–32]
´
Examples of AR antagonists are shown in Figure 2. The xan-
thine derivatives caffeine (2a) and theophylline (2b) are about
equally active at all human AR subtypes.[33,34] In epidemiologi-
cal studies caffeine intake has been associated with a lower in-
Some of us—in collaboration with the group of K. Kiec-Ko-
nonowocz (Jagiellonian University, Krakꢂw, Poland)—have pre-
viously reported the development of tetrahydropyrimido[2,1-
f]purinediones 5 (Figure 3), a class of compounds that can be
Figure 3. Tetrahydropyrimido[2,1-f]purinediones 5 and related
tetrahydropyrazino[2,1-f]purinedione 6.
envisaged as tricyclic caffeine derivatives.[45–50] They represent
structural analogues of 8-styrylxanthine derivatives, which are
sterically constrained by annulation of a tetrahydropyrimidine
ring to the 7,8-position of caffeine mimicking the 8-E-styryl
substructure of A2AAR antagonists 4a and 4b. So far, this ap-
proach has led to relatively potent A2AAR antagonists, some of
which showed good selectivity over A1ARs.[46] However, a draw-
back of this class of compounds is their low water solubility. In
continuing efforts to develop improved, more water-soluble
A2AAR antagonists, structure 6 has been designed. In 6, the ni-
trogen atom at position 9 of the tricyclic compounds 5 was
formally shifted to position 8. The nitrogen atom in position 8
of compounds 6 is expected to exhibit increased basicity as it
Figure 2. Structures of adenosine receptor antagonists including dual- and
multitarget drugs.
cidence of PD and AD.[35,36] The aminopyrazine ASP-5854 (3),
a dual-target A1/A2AAR antagonist has been extensively charac-
terized in several models of PD and cognition.[28] In monkeys,
ASP-5854 reversed haloperidol-induced catalepsy with an ED50
value of 0.1 mgkgꢀ1 p.o.[37] ASP-5854 produced positive results
in the rat passive avoidance test, a model of cognition,[31]
whereas the A2A-selective antagonist istradefylline (KW-6002,
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is not connected to an aromatic ring, provided the substituent
R is not aromatic. Consequently, compounds 6 should exhibit
better water solubility than the related tricyclic xanthine deriv-
atives 5 at physiological pH values.
pounds. The purity of the tested compounds was confirmed
by HPLC coupled to electrospray ionization mass spectrometry
(ESI-MS) using two different methods (see the Experimental
Section below for details) and shown to be generally >96%.
Recently, we reported the synthesis and biological evalua-
tion of a first representative of this novel class of AR an-
tagonists, namely 1,3-dimethyl-8-isopropyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (6a).[51] This class of
compounds has been poorly investigated so far, and only very
few derivatives have been described.[52,53]
Biological evaluation and determination of water solubility
The synthesized tetrahydropyrazino[2,1-f]purinediones were in-
itially evaluated in radioligand binding assays for their affinity
to A1ARs in rat brain cortical membrane and to A2AARs in rat
brain striatal membrane preparations. Selected compounds
were further investigated for their affinity to human A1 and
A2AARs recombinantly expressed in Chinese hamster ovary
(CHO) cells. All compounds were additionally investigated for
their affinity to human A2B and A3ARs recombinantly expressed
in CHO cells, to determine their AR subtype selectivity. [3H]2-
Chloro-N6-cyclopentyladenosine ([3H]CCPA),[55] [3H]3-(3-hydroxy-
propyl)-8-(m-methoxystyryl)-7-methyl-1-propargylxanthine
Herein, we report on the synthesis of a large series of novel
tetrahydropyrazino[2,1-f]purinedione derivatives 6 and their
evaluation as antagonists at ARs and inhibitors of MAO iso-
enzymes, MAO-B and MAO-A. Benzyl residues bearing a variety
of substituents were introduced at position 8 of the tetra-
hydropyrazino[2,1-f]purinedione scaffold 6 to modulate the
compounds’ biological activities while keeping the basicity of
the nitrogen atom N8 to allow protonation.
([3H]MSX-2),[56]
[3H]8-(4-(4-(4-chlorophenyl)piperazine-1-sul-
fonyl)phenyl)-1-propylxanthine ([3H]PSB-603),[57] and [3H]2-
phenyl-8-ethyl-4-methyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2,1-
i]purine-5-one ([3H]PSB-11)[58] were used as radioligands in A1,
A2A, A2B, and A3AR binding studies, respectively. Functional
studies were not performed as it is well known that all xan-
thine derivatives lacking a ribose moiety, including tricyclic
compounds, can only act as antagonists, but never as agonists,
at ARs.
Results and Discussion
Chemistry
Starting from 1,3-dimethyl-8-hydroxymethylxanthine (7),[54] the
synthesis of a library of tetrahydropyrazino[2,1-f]purinediones
10–78 was carried out in a three-step procedure (Scheme 1).
All compounds were tested for inhibition of human MAO-B
at a concentration of 10 mm. For compounds that showed in-
hibition >70%, full concentration–inhibition curves were re-
corded and IC50 values were determined. Two select com-
pounds, 32 and 72, were also tested for inhibition of rat MAO-
B to investigate their suitability for animal studies in rodents.
Potent inhibitors were additionally investigated for inhibition
of human MAO-A to assess their selectivity. Results are present-
ed in Tables 1 and 2. Data of standard ligands are included for
comparison. Water solubility of selected compounds at differ-
ent pH values was determined by thermodynamic solubility
measurements (Supporting Information Table S1).
Scheme 1. Synthesis of 8-substituted 1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-diones 10–78. Reagents and conditions:
a) 1,2-dibromoethane, DMF, NEt3, 808C, 6 h; b) PBr3, CH2Cl2, 08C!RT, 1 h;
c) dimethoxyethane, N,N-diisopropylethylamine, RT, 16 h.
SAR at adenosine receptors
Initial studies showed that the unsubstituted parent com-
pound of this new class of AR antagonists, 8-benzyl-1,3-
dimethyltetrahydropyrazino[2,1-f]purinedione (10), was more
potent than its isomer, 9-benzyl-1,3-dimethyltetrahydro-
pyrimido[2,1-f]purinedione (5a)[44] especially at the A1AR (see
Table 1). Compound 10 was found to be a potent, selective A1
antagonist in rat (Ki rat A1: 79.3 nm), but somewhat less potent
at human A1 receptors (Ki human A1: 265 nm). It was also able
to block A2AARs (Ki human A2A: 1060 nm, rat A2A: 598 nm), but
was inactive at human A2B and A3ARs. Structure–activity rela-
tionships (SAR) were subsequently explored by systematic sub-
stitution of the benzyl residue: 1) through introduction of one,
two, or three substituents into the phenyl ring, 2) by bioiso-
steric replacement of the phenyl ring, and 3) by substitution
Xanthine 7 was alkylated at position 7 by treatment with 1,2-
dibromoethane in the presence of triethylamine. The hydroxy
function of 8 was subsequently converted into the correspond-
ing bromide by treatment with phosphorus tribromide. The re-
sulting 7-(2-bromoethyl)-8-(bromomethyl)-1,3-dimethylpurine-
2,4-dione (9) was not isolated, but directly reacted in a parallel
manner with a large number of mono-, di-, or trisubstituted
benzylamines under basic conditions to afford a library of
tetrahydropyrazino[2,1-f]purinediones 10–78 in moderate to
very good yields (24–81%, calculated over two steps from 8).
The structures of all products were confirmed by NMR and MS
analyses. Melting points were determined for all new com-
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Table 1. Adenosine receptor affinities of 1,3-dimethyltetrahydropyrazino[2,1-f]purinediones and standard compounds.
Compd
R
Ki ꢁSEM [nm] (h, human; r, rat)
A2A vs. [3H]MSX-2[a] A2B vs. [3H]PSB-603[b]
A1 vs. [3H]CCPA[a]
A3 vs. [3H]PSB-11[b]
Standard compounds[c]
caffeine (1a)
44900 (h)
41000 (r)
841 (h)
23400 (h)
32500 (r)
12 (h)
4.46 (r)
38.0ꢁ11.0 (h)
54 (r)
33800 (h)
30000 (r)
>10000 (h)
13300 (h)
>10000 (r)
4470 (h)
istradefylline (4a)
CSC (4b)
230 (r)
>10000 (14%)[d] (h)
28000 (r)
8200 (h)
>10000 (h)
9-Benzyl-1,3-dimethyltetrahydropyrimido[2,1-f]purinedione[e]
5a 3580 (r)
1090 (r)
nd[f]
nd[f]
8-Benzyl-1,3-dimethyltetrahydropyrazino[2,1-f]purinedione
265ꢁ68 (h)
1060ꢁ300 (h)
598ꢁ102 (r)
10
>1000 (h) (15%)[d]
>10000 (h) (21%)[d]
79.3ꢁ12.0 (r)
Monosubstituted 8-benzyl-1,3-dimethyltetrahydropyrazino[2,1-f]purinediones
152ꢁ21 (h)
11
1260ꢁ370 (r)
1330ꢁ110 (r)
1250ꢁ60 (r)
>1000 (h) (5%)[d]
>1000 (h) (5%)[d]
>300 (h) (3%)[d]
>1000 (h) (31%)[d]
>10000 (h) (11%)[d]
>10000 (h) (39%)[d]
40.5ꢁ5.1 (r)
80.6ꢁ20.0 (h)
12
42.0ꢁ6.5 (r)
427ꢁ25 (h)
13
54.2ꢁ2.9 (r)
218ꢁ74 (h)
15.5ꢁ2.8 (r)
14
15
16
>1000 (r) (29%)[d]
>1000 (h) (1%)[d]
>300 (h) (6%)[d]
>1000 (h) (1%)[d]
>1000 (h) (18%)[d]
>10000 (h) (30%)[d]
>10000 (h) (13%)[d]
215ꢁ24 (h)
962ꢁ156 (h)
61.9ꢁ19.7 (r)
>1000 (r) (34%)[d]
>1500 (r) (22%)[d]
>1000 (r) (36%)[d]
85.7ꢁ13.6 (h)
40.1ꢁ18.8 (r)
291ꢁ59 (h)
257ꢁ36 (r)
17
18
19
20
21
22
>1000 (h) (11%)[d]
>1000 (h) (7%)
>1000 (h) (4%)[d]
>10000 (h) (29%)[d]
>1000 (h) (21%)[d]
>1000 (h) (24%)[d]
>1000 (h) (14%)[d]
>10000 (h) (25%)[d]
64.4ꢁ15.1 (h)
52.8ꢁ4.4 (r)
769ꢁ27 (h)
621ꢁ123 (r)
3860ꢁ220 (h)
1060ꢁ10 (r)
>1500 (r) (29%)[d]
>1000 (h) (20%)[d]
>1000 (h) (9%)[d]
>1000 (h) (ꢀ7%)[d]
>300 (h) (9%)[d]
87.7ꢁ28.6 (h)
25.5ꢁ4.4 (r)
834ꢁ155 (h)
384ꢁ80 (r)
>1500 (r) (38%)[d]
>1000 (r) (35%)[d]
127ꢁ20 (h)
26.0ꢁ1.8 (r)
533ꢁ199 (h)
476ꢁ51 (r)
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Table 1. (Continued)
Compd
R
Ki ꢁSEM [nm] (h, human; r, rat)
A1 vs. [3H]CCPA[a]
A2A vs. [3H]MSX-2[a]
A2B vs. [3H]PSB-603[b]
A3 vs. [3H]PSB-11[b]
216ꢁ18 (h)
74.4ꢁ4.4 (r)
653ꢁ177 (h)
387ꢁ127 (r)
23
24
25
>1000 (h) (20%)[d]
>1000 (h) (13%)[d]
392ꢁ123 (h)
51.1ꢁ7.3 (r)
1300ꢁ80 (r)
>1000 (h) (ꢀ2%)[d]
>1000 (h) (6%)[d]
>10000 (h) (28%)[d]
>10000 (h) (12%)[d]
>1500 (r) (21%)[d]
>1000 (r) (14%)[d]
76.2ꢁ9.2 (h)
98.8ꢁ5.6 (r)
1020ꢁ140 (h)
558ꢁ134 (r)
26
27
>1000 (h) (17%)[d]
>1000 (h) (9%)[d]
>10000 (h) (25%)[d]
6490ꢁ1380 (h)[a]
21.8ꢁ3.1 (h)
10.1ꢁ1.7 (r)
426ꢁ83 (h)
375ꢁ89 (r)
59.1ꢁ18.9 (h)
21.4ꢁ3.7 (r)
437ꢁ30 (h)
184ꢁ50 (r)
28
29
>1000 (h) (20%)[d]
>1000 (h) (4%)[d]
>10000 (h) (23%)[d]
>10000 (h) (16%)[d]
61.8ꢁ17.4 (h)
32.9ꢁ7.4 (r)
569ꢁ87 (h)
335ꢁ41 (r)
30
>1500 (r) (4%)[d]
1210ꢁ40 (r)
>1000 (h) (9%)[d]
>10000 (h) (10%)[d]
Disubstituted 8-benzyl-1,3-dimethyltetrahydropyrazino[2,1-f]purinediones
116 ꢁ15 (h)
94.3ꢁ10.4 (h)
993ꢁ81 (r)
31
>1000 (h) (ꢀ4%)[d]
>1000 (h) (27%)[d]
86.8ꢁ19.8 (r)
227ꢁ34 (h)
25.5ꢁ3.0 (r)
782ꢁ129 (h)
402ꢁ53 (r)
32
33
34
>300 (h) (1%)[d]
>300 (h) (12%)[d]
>300 (h) (20%)[d]
>1000 (h) (25%)[d]
7840ꢁ1850 (h)[a]
>1000 (h) (37%)[d]
296ꢁ34 (h)
33.5ꢁ4.0 (r)
662ꢁ154 (h)
174ꢁ13 (r)
257ꢁ71 (h)
44.4ꢁ7.5 (r)
2440ꢁ290 (h)
927ꢁ280 (r)
89.8ꢁ18.3 (h)
14.1ꢁ0.8 (r)
320ꢁ82 (h)
484ꢁ73 (r)
35
36
>300 (h) (8%)[d]
>1000 (h) (10%)[d]
>1000 (h) (13%)[d]
791ꢁ110 (h)
351ꢁ57 (r)
1510ꢁ031 (h)
322ꢁ129 (r)
>1000 (h) (15%)[d]
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Table 1. (Continued)
Compd
R
Ki ꢁSEM [nm] (h, human; r, rat)
A1 vs. [3H]CCPA[a]
A2A vs. [3H]MSX-2[a]
A2B vs. [3H]PSB-603[b]
A3 vs. [3H]PSB-11[b]
376ꢁ20 (h)
238ꢁ50 (r)
37
3660ꢁ660 (r)
>1000 (h) (ꢀ7%)[d]
>1000 (h) (23%)[d]
38
39
>1500 (r) (20%)[d]
>1000 (r) (34%)[d]
>1000 (r) (32%)[d]
>1000 (h) (10%)[d]
>1000 (h) (15%)[d]
>1000 (h) (28%)[d]
>1000 (h) (14%)[d]
687ꢁ172 (h)
77.1ꢁ18.6 (r)
794ꢁ174 (h)
85.5ꢁ15.5 (r)
40
1220ꢁ220 (r)
>1000 (h) (4%)[d]
>1000 (h) (13%)[d]
41
42
43
44
>1500 (r) (17%)[d]
>1000 (r) (29%)[d]
>1000 (h) (16%)[d]
>1000 (h) (16%)[d]
>1000 (h) (11%)[d]
>1000 (h) (11%)[d]
>1000 (h) (26%)[d]
>1000 (h) (ꢀ4%)[d]
>1000 (h) (15%)[d]
>1000 (h) (18%)[d]
231ꢁ70 (h)
107ꢁ14 (r)
1270ꢁ220 (h)
518ꢁ78 (r)
>1500 (r) (35%)[d]
>1000 (r) (41%)[d]
243ꢁ5 (h)
812ꢁ151 (h)
624ꢁ79 (r)
54.4ꢁ8.8 (r)
186ꢁ36 (h)
1000ꢁ120 (h)
758ꢁ161 (r)
45
>1000 (h) (3%)[d]
>1000 (h) (15%)[d]
99.9ꢁ23.0 (r)
384ꢁ95 (h)
46
47
48
1070ꢁ200 (r)
1330ꢁ50 (r)
>1000 (h) (ꢀ3%)[d]
>1000 (h) (7%)[d]
>1000 (h) (4%)[d]
>10000 (h) (23%)[d]
>10000 (h) (21%)[d]
>1000 (h) (12%)[d]
95.5ꢁ23.0 (r)
1500 (r) (21%)[d]
150ꢁ31 (h)
28.9ꢁ9.4 (r)
1150ꢁ200 (h)
681ꢁ170 (r)
247ꢁ54 (h)
144ꢁ29 (r)
1420ꢁ430 (h)
652ꢁ127 (r)
49
>1000 (h) (4%)[d]
>10000 (h) (17%)[d]
84.3ꢁ17.6 (h)
151ꢁ43 (r)
50
51
1020ꢁ280 (r)
2570ꢁ660 (r)
>1000 (h) (2%)[d]
>1000 (h) (9%)[d]
>1000 (h) (3%)[d]
>1500 (r) (26%)[d]
>1000 (h) (ꢀ3%)[d]
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Table 1. (Continued)
Compd
R
Ki ꢁSEM [nm] (h, human; r, rat)
A1 vs. [3H]CCPA[a]
A2A vs. [3H]MSX-2[a]
A2B vs. [3H]PSB-603[b]
A3 vs. [3H]PSB-11[b]
52
53
>1500 (r) (14%)[d]
1870ꢁ464 (r)
>1000 (h) (29%)[d]
>10000 (h) (22%)[d]
>1500 (r) (5%)[d]
>1500 (r) (10%)[d]
>1000 (r) (35%)[d]
>1000 (r) (33%)[d]
>1000 (h) (17%)[d]
>1000 (h) (14%)[d]
>100 (h) (3%)[d]
54
>10000 (h) (30%)[d]
678ꢁ99 (h)
611ꢁ123 (r)
55
56
57
58
59
60
61
62
>1000 (r) (28%)[d]
1160ꢁ210 (r)
>1000 (h) (13%)[d]
>1000 (h) (10%)[d]
>1000 (h) (12%)[d]
>1000 (h) (10%)[d]
>1000 (h) (8%)[d]
>1000 (h) (12%)[d]
>1000 (h) (1%)[d]
>1000 (h) (13%)[d]
>1000 (h) (17%)[d]
>10000 (h) (18%)[d]
>1000 (h) (17%)[d]
>1000 (h) (ꢀ6%)[d]
>300 (h) (ꢀ4%)[d]
>1000 (h) (16%)[d]
>1000 (h) (12%)[d]
>1000 (h) (17%)[d]
>1500 (r) (9%)[d]
>1500 (r) (31%)[d]
>1000 (r) (36%)[d]
67.5ꢁ10.6 (h)
23ꢁ2 (r)[b]
261ꢁ47 (h)
358ꢁ114 (r)
>1500 (r) (26%)[d]
>1500 (r) (21%)[d]
>1000 (r) (35%)[d]
>1000 (r) (43%)[d]
76.5ꢁ3.0 (h)
36ꢁ8 (r)[b]
4730ꢁ1790 (h)
991ꢁ14 (r)
1910ꢁ560 (h)
729ꢁ125 (r)
>1500 (r) (35%)[d]
51.9ꢁ12.4 (h)
46ꢁ6 (r)[b]
63
64
1100ꢁ100 (r)
>1000 (h) (24%)[d]
>1000 (h) (4%)[d]
>1000 (h) (14%)[d]
>1000 (h) (6%)[d]
>1500 (r) (ꢀ2%)[d]
>1000 (r) (13%)[d]
217ꢁ49 (h)
134ꢁ40 (r)
1120ꢁ310 (h)
236ꢁ24 (r)
65
66
>1000 (h) (ꢀ3%)[d]
>100 (h) (7%)[d]
>10000 (h) (25%)[d]
>10000 (h) (29%)[d]
>1500 (r) (28%)[d]
>1000 (r) (10%)[d]
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Table 1. (Continued)
Compd
R
Ki ꢁSEM [nm] (h, human; r, rat)
A1 vs. [3H]CCPA[a]
A2A vs. [3H]MSX-2[a]
A2B vs. [3H]PSB-603[b]
A3 vs. [3H]PSB-11[b]
67
>1500 (r) (24%)[d]
>1000 (r) (10%)[d]
>300 (h) (17%)[d]
>1000 (h) (12%)[d]
692ꢁ171 (h)
688ꢁ33 (r)
68
1530ꢁ240 (r)
>1000 (h) (8%)[d]
>1000 (h) (ꢀ5%)[d]
Trisubstituted 8-benzyl-1,3-dimethyltetrahydropyrazino[2,1-f]purinediones
69
70
71
>1500 (r) (10%)[d]
1670ꢁ380 (r)
1880ꢁ890 (r)
>1000 (h) (7%)[d]
>1000 (h) (2%)[d]
>1000 (h) (9%)[d]
>100 (h) (ꢀ10%)[d]
>1000 (h) (2%)[d]
>100 (h) (ꢀ10%)[d]
>1500 (r) (19%)[d]
91.4ꢁ14.8 (h)
73ꢁ2 (r)[b]
1080ꢁ30 (h)
897ꢁ266 (r)
217ꢁ64 (h)
111 ꢁ24 (r)
268ꢁ75 (h)
603ꢁ54 (r)
72
73
>1000 (h) (5%)[d]
>1000 (h) (2%)[d]
>300 (h) (8%)[d]
>1500 (r) (22%)[d]
1390ꢁ300 (r)
>300 (h) (ꢀ5%)[d]
74
75
76
77
78
>1500 (r) (17%)[d]
>1000 (r) (28%)[d]
>1000 (h) (21%)[d]
>1000 (h) (14%)[d]
>1000 (h) (9%)[d]
>1000 (h) (5%)[d]
>1000 (h) (5%)[d]
>300 (h) (14%)[d]
>10000 (h) (19%)[d]
>100 (h) (ꢀ3%)[d]
>1000 (h) (21%)[d]
>1000 (h) (5%)[d]
>1500 (r) (ꢀ3%)[d]
2670ꢁ630 (r)
752ꢁ86 (h)
170ꢁ16 (r)
3810ꢁ280 (h)
823ꢁ142 (r)
3040ꢁ80 (h)
343ꢁ108 (r)
1700ꢁ210 (r)
>1500 (r) (3%)[d]
>1000 (r) (20%)[d]
[a] n=3. [b] n=2. [c] Data are from Mꢀller and Jacobson[33] or are unpublished data from our research group. [d] Percent inhibition of radioligand binding
at the indicated concentration. [e] Data are from Drabczynska et al.[47] [f] nd: not determined.
´
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leading to 1-methylbenzyl residues. Our initial goal was to in-
crease the A2AAR affinity of the parent compound 10.
A1 rat: 351 nm, A2A: 322 nm). The introduction of a methyl
group attached to the methylene group of the benzyl residue
of compound 36, resulting in compound 68, which was pre-
dicted to display an increased basicity of the N8 atom, was tol-
erated by A1 and even much better by A2AARs.
All investigated derivatives 10–78 showed no or only negli-
gible affinity for the A2B and A3AR subtypes. Within the mono-
substituted series (11–28), ortho or meta substituents (halogen,
methoxy, trifluoromethyl) were, in most cases, better tolerated
than para substitution (for example, p-F 16, p-Br 19, and
p-OMe 25). Exceptions from this rule were the p-chlorobenzyl
derivative 13, which was similarly potent to its o- and m-chlor-
obenzyl isomers 11 and 12, and the o-trifluoromethyl-substi-
tuted compound 21, which showed decreased affinity. Many of
the compounds of this series retained high A1AR affinity, and
some of them showed the desired increase in A2A affinity. In
particular, o-bromo substitution (compound 17) led to a large
increase in A2A affinity combined with a moderate increase in
A1 affinity. Derivative 17 was the best balanced dual-target A1/
A2A antagonist of the present series with similar potency at
human and rat ARs (Ki human, A1: 85.7 nm, A2A: 291 nm; Ki rat,
A1: 40.1 nm, A2A: 257 nm). The m-trifluoromethyl derivative 22
showed a similar profile.
A series of 10 trisubstituted derivatives 69–78 was studied,
most of which contained para-substituents in addition to two
additional residues. The SARs were in line with those observed
in the other series. All compounds displaying a para-substitu-
ent other than chloro showed low AR affinity. The best A1/A2A
antagonist was compound 72 bearing a 2,4-dichloro-5-fluoro-
benzyl substituent (Ki human A1: 217 nm, A2A: 268 nm; Ki rat A1:
111 nm, A2A: 603 nm).
Moderate to large species differences have previously been
described for different classes of AR antagonists.[50,59,60] As pre-
clinical in vivo evaluation is typically performed in rodents,
mainly rats or mice, we compared affinities of the investigated
1,3-dimethyltetrahydropyrazino[2,1-f]purinediones at rat and
human A1 and A2AARs. Correlation plots of pKi values for both
species are shown in the Supporting Information (Figures S1
and S2). For many compounds large species differences were
observed at the A1AR. Except for compounds 26 and 50 (bear-
ing a 3-(thiazol-2-yl)benzyl or a 2-fluoro-5-(trifluoromethyl)ben-
zyl residue) all compounds of this series investigated in both
species displayed a preference for the rat over the human
A1AR. In particular, compounds 13, 14, 24, 32, 33, 34, 39, and
40 were 5.8–14.1-fold more potent at the rat than at the
human A1AR. The Pearson product–moment correlation coeffi-
cient r was calculated to be 0.617.
As a next step, heterocyclic substituents (thiazolyl 26,
2-thienyl 27, and 1-pyrrolyl 28) were introduced into the m-po-
sition of the benzyl ring. These modifications were also well
tolerated by A1 and A2AARs, and 27 and 28 led to increases in
affinities at both receptor subtypes. The most potent com-
pound was the m-(2-thienyl)benzyl derivative 27, the most
potent A1 antagonist of the present series (Ki human A1:
21.8 nm; rat A1: 10.1 nm) which also showed good, although
~30-fold lower A2A affinity (Ki human A2A: 426 nm; rat A2A:
375 nm).
Bioisosteric replacement of the benzyl moiety of the parent
benzyl derivative 10 by a 2-thienylmethyl moiety (compound
29) was well tolerated. Affinity for both A1 and A2A were even
improved (two- to fourfold, compare 10 and 29).
Within the series displaying a disubstituted 8-benzyl moiety
(31–68), para-substituents at the benzyl ring (38, 41, 43, 47,
51–54, 56, 57, 59, 60, 62, 66, and 67) largely decreased affinity
for both A1 and A2AARs in line with the SARs obtained for the
Species differences of the investigated tetrahydropyrazino-
[2,1-f]purinediones at A2AARs were less pronounced. Although
most of the tested compounds showed somewhat higher affin-
ity for the rat than the human A2AAR, for a few derivatives the
opposite was true (Supporting Information Figure S2). For ex-
ample, the most potent antagonist at the human A2AAR of this
series, compound 31, displayed a tenfold lower affinity at the
rat A2AAR. For correlation of rat and human A2AAR affinity the r
coefficient was calculated to be 0.382.
monosubstituted pyrazinopurinediones. However,
a 2,4-di-
chloro and a 3,4-dichloro substitution pattern, as displayed by
compounds 33 and 36, was tolerated by both rat AR subtypes.
Compound 33 proved to be a dual A1 and A2A antagonist (Ki
human A1: 296 nm, A2A: 662 nm; Ki rat A1: 33.5 nm, A2A:
174 nm). Among the compounds with 2,3-, 2,5-, 2,6-, or 3,5-dis-
ubstitution several derivatives showed relatively high affinity
for both, A1 and A2AARs, mostly with a certain preference for
the A1AR. The affinity for the human receptors was in many
cases lower than for rat ARs. The best compounds of this
series were the 2,3-dichlorobenzyl derivative 31 (Ki human A1:
116 nm, A2A: 94.3 nm), the 2,5-dichlorobenzyl derivative 35 (Ki
human A1: 89.9 nm, A2A: 320 nm; Ki rat A1: 14.1 nm, rat A2A:
484 nm), and the 2-chloro-5-trifluoromethylbenzyl derivative
58 (Ki human 67.5 nm, A2A: 261 nm; Ki rat A1: 23 nm, A2A:
358 nm).
SAR at monoamine oxidases
Test results of 1,3-dimethyltetrahydropyrazino[2,1-f]purine-
diones at MAO-A and MAO-B are collected in Table 2. All of the
tested derivatives were found to be inactive at MAO-A. Com-
pounds from the monosubstituted series (11–28) bearing a hal-
ogen atom in the meta or para position (12, 13, 15, 16, 18, 19,
and 20) or either a trifluoromethyl group (22) or a thienyl
moiety in the meta position (27) inhibited the enzyme with
IC50 values in the low micromolar range. In contrast, substitu-
tion of the ortho-position (11, 14, 17, and 21) or a methoxy
substituent in any position (23–25) abrogated MAO-B inhibito-
ry activity.
Consistent observations were made in the disubstituted
series of 8-benzyl-1,3-dimethyltetrahydropyrazino[2,1-f]purine-
diones (31–68). Compounds with a 2,6-substitution pattern
(32, 37, and 45) or a large ortho substituent, such as a trifluoro-
methyl group (51, 55, and 57), or a methoxy residue in any po-
The 3,4-dichlorobenzyl derivative 36 was found to be a bal-
anced A1/A2A antagonist with moderate potency at human, but
higher potency at rat ARs (Ki human A1: 791 nm, A2A: 1510 nm;
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Table 2. Human (h) MAO-A and human MAO-B inhibitory potencies of
1,3-dimethyltetrahydropyrazino[2,1-f]purinediones and standard com-
pounds.
Table 2. (Continued)
Compd
IC50 ꢁSEM [nm][a]
hMAO-A
hMAO-B
Compd
IC50 ꢁSEM [nm][a]
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
>10000 (ꢀ1%)[c]
>10000 (3%)[c]
nd
687ꢁ89
hMAO-A
hMAO-B
2060ꢁ600
>10000 (18%)[c]
>10000 (24%)[c]
>10000 (22%)[c]
371ꢁ76
Standard compounds
selegiline (1a)
lazabemide (1b)
safinamide (1c)
caffeine (2a)
nd[b]
nd
6.13ꢁ0.85
17.6ꢁ4.2
nd
nd
nd
7.67ꢁ1.81
>10000 (5%)[c]
>10000 (4%)[c]
nd
>500000 (33%)[c]
>10000 (18%)[c]
>10000 (23%)[c]
>500000 (16%)[c]
>10000 (20%)[c]
18.1ꢁ3.3
223ꢁ37
istradefylline (4a)
CSC (4b)
>10000 (30%)[c]
>10000 (41%)[c]
>10000 (7%)[c]
>10000 (6%)[c]
>10000 (2%)[c]
nd
>10000 (4%)[c]
>10000 (5%)[c]
nd
508ꢁ23 (rat: 3000ꢁ60)
1,3-Dimethyltetrahydropyrazino[2,1-f]purinediones
3370ꢁ450
>10000 (14%)[c]
~10000 (48%)[c]
~10000 (45%)[c]
>10000 (28%)[c]
>10000 (0%)[c]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
nd
>10000 (36%)[c]
~10000 (58%)[c]
1270ꢁ270
>10000 (5%)[c]
>10000 (4%)[c]
>10000 (8%)[c]
nd
1690ꢁ130
>10000 (43%)[c]
4840ꢁ340
nd
>10000 (5%)[c]
>10000 (ꢀ10%)[c]
>10000 (4%)[c]
>10000 (1%)[c]
>10000 (2%)[c]
>10000 (1%)[c]
nd
[a] n=3. [b] nd: not determined. [c] Percent inhibition at the indicated
concentration.
2660ꢁ660
~10000 (58%)[c]
2140ꢁ270
4390ꢁ950
2460ꢁ130
sition (59, 65, and 66), proved to be inactive at MAO-B.
Tetrahydropyrazino[2,1-f]purinediones with 3,4- or 3,5-dihalo-
gen substitution at the benzyl moiety yielded MAO-B inhibitors
with nanomolar to low micromolar IC50 values.
>10000 (40%)[c]
2670ꢁ530
>10000 (3%)[c]
nd
>10000 (8%)[c]
>10000 (29%)[c]
>10000 (39%)[c]
~10000 (45%)[c]
4730ꢁ690
nd
nd
nd
nd
nd
nd
Among these compounds, the 3,4- and 3,5-dichlorobenzyl-
substituted derivatives 34 and 36 were the most potent MAO-
B inhibitors of the present series (34: IC50 =250 nm; 36: IC50 =
197 nm). The introduction of a methyl group at the methylene
spacer of the benzyl moiety was well tolerated by MAO-B
(compare 68 and 36). Among the trisubstituted derivatives
only compound 69 bearing a 3,4,5-trifluorobenzyl moiety at
position 8 (IC50 =223 nm) and compound 72 with a 2,4-di-
chloro-5-fluorobenzyl residue (IC50 =508 nm) showed notable
MAO-B inhibition.
~10000 (60%)[c]
>10000 (28%)[c]
1440ꢁ180
>10000 (7%)[c]
>10000 (7%)[c]
nd
>10000 (12%)[c]
>10000 (7%)[c]
>10000 (8%)[c]
>10000 (5%)[c]
3350ꢁ620
>10000 (42%)[c]
3810ꢁ560
250ꢁ6
1060ꢁ30
197ꢁ25
(rat: 260ꢁ38)
>10000 (13%)[c]
838ꢁ57
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
nd
>10000 (3%)[c]
>10000 (0%)[c]
>10000 (1%)[c]
>10000 (ꢀ4%)[c]
>10000 (5%)[c]
>10000 (ꢀ10%)[c]
>10000 (2%)[c]
nd
Dual- and triple-target drugs
802ꢁ30
840ꢁ133
The goal of the present study was to obtain dual- and triple-
target drugs for the potential treatment of neurodegenerative
diseases including PD and AD. We initially investigated the
SARs of a set of 8-benzyltetrahydropyrazino[2,1-f]purinediones
at A2AARs and at MAO-B to study the requirements of each of
the main targets (see Figure 4, top). As expected, the SARs at
the very different target structures were quite different. How-
ever, there was a common chemical space accepted by both
targets (see Figure 4, bottom), which allowed us to obtain
dual-target drugs. Some of those compounds showed addi-
tional blockade of A1ARs by chance. Dual A1/A2A antagonists
were identified including compound 17 (o-bromobenzyl deriv-
ative; Ki human A1: 85.7 nm, rat A1: 40.1 nm; human A2A:
291 nm, rat A2A: 257 nm). We discovered adenosine A1 and
A2AAR antagonists with ancillary MAO-B inhibitory activity. For
example, compound 34 was found to be an equipotent A1 an-
tagonist and inhibitor of MAO-B (m,m-dichlorobenzyl deriva-
tive; Ki human A1: 257 nm, IC50 human MAO-B: 250 nm), where-
as 61 was more potent at A1 (o-F,m-Br-benzyl derivative; Ki
663ꢁ80
685ꢁ106
711ꢁ75
932ꢁ33
>10000 (22%)[c]
652ꢁ103
>10000 (ꢀ3%)[c]
>10000 (3%)[c]
>10000 (5%)[c]
>10000 (7%)[c]
>10000 (2%)[c]
nd
>10000 (1%)[c]
>10000 (1%)[c]
>10000 (ꢀ4%)[c]
nd
>10000 (4%)[c]
>10000 (10%)[c]
>10000 (5%)[c]
>10000 (4%)[c]
>10000 (3%)[c]
>10000 (8%)[c]
>10000 (3%)[c]
548ꢁ65
3560ꢁ340
2050ꢁ300
3350ꢁ1160
>10000 (25%)[c]
1190ꢁ90
1530ꢁ15
2650ꢁ880
>10000 (33%)[c]
1060ꢁ60
~10000 (48%)[c]
1330ꢁ230
~10000 (50%)[c]
306ꢁ4
512ꢁ65
1020ꢁ130
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Figure 4. SAR of 8-benzyltetrahydropyrazino[2,1-f]purinediones at individual targets, A2AAR, and MAO-B (main differences are indicated), and deduction of
common SARs at both targets.
human A1: 76.5 nm) than at MAO-B (IC50 human MAO-B:
512 nm). Compound 72 (o-Cl,m-F,p-Cl-benzyl derivative) was
developed as a potent triple-target drug blocking human A1 (Ki
217 nm) and A2AARs (Ki 268 nm), and inhibiting human MAO-B
(IC50 508 nm) in the same concentration range. Compound 36
(m,p-dichlorobenzyl derivative) was nearly equipotent at rat
A1AR (Ki 351 nm), rat A2AAR (Ki 322 nm) and rat MAO-B (IC50
260 nm), and will therefore be a useful tool for proof-of-princi-
ple studies to confirm this multitarget approach in rodent
models of neurodegenerative diseases.
Our data indicate that the nitrogen atom N8 of the
8-benzyltetrahydropyrazino[2,1-f]purinediones can be proton-
ated under physiological conditions in the stomach. This prop-
erty may be important for oral bioavailability of this new class
of AR antagonists and MAO-B inhibitors. The basicity of N8 can
be fine-tuned by substitution of the benzyl residue. Appropri-
ate substitution can lead to compounds which are sufficiently
soluble even at pH 7.4.
Pharmacokinetic studies
For the best triple-target compound 36, which showed drug-
like physicochemical properties, preliminary pharmacokinetic
evaluation was performed (Supporting Information Table S2). It
was found to show oral bioavailability in rat (50%), CNS pene-
tration with a brain–plasma ratio of 0.8, and a half-life in rat of
1.5 h. Clearance in rat and human liver microsomes was in an
acceptable range (see Table S2). Measurements of plasma–pro-
tein binding showed a free fraction of 2–6% in the plasma of
mouse, rat, and human, and in mouse brain.
Water solubility
The water solubility of selected compounds was determined
by thermodynamic solubility measurements at physiologically
relevant pH values, namely pH 1, pH 4, and pH 7.4 (Supporting
Information Table S1). All tested compounds showed good sol-
ubility at pH 1. Especially certain substituents at the ortho-posi-
tion of the benzyl ring had an impact on solubility. Introduc-
tion of a trifluoromethyl group at that position resulted in less
soluble compounds (21 and 55). In contrast, compounds 65
and 73, both bearing a methoxy group in the ortho-position of
the benzyl ring, displayed high solubility (>1.5 mgmLꢀ1) and
were also soluble at a higher pH value of 4 (0.04 to
0.05 mgmLꢀ1). Excellent solubility was also measured for the
o-fluorobenzyl derivative 14, which showed good solubility at
all three pH values (>1.5 mgmLꢀ1 at pH 1, 0.006 mgmLꢀ1 at
pH 4, 0.002 mgmLꢀ1 at pH 7.4). The 3,4-dichlorobenzyl deriva-
tive 36 displayed higher solubility than the 2,6-dichlorobenzyl
derivative 32 and the 2,5-dichlorobenzyl derivative 35. The
lowest solubility in the series of dichlorobenzyl derivatives was
observed for the 3,5-dichlorobenzyl derivative 34. For com-
pound 36 a logD (octanol–buffer pH 7.4, RT) was determined
to be 3.1.
Conclusions
A
library of 69 novel 8-benzyl-substituted 1,3-dimethyl-
tetrahydropyrazino[2,1-f]purinediones has been synthesized
and optimized as multitarget drugs for the potential treatment
of neurodegenerative diseases such as Parkinson’s and Alz-
heimer’s disease. We identified compounds that act on two or
three different targets which are thought to be relevant for
the treatment of neurodegenerative diseases. Such com-
pounds are expected to be superior to single-target drugs for
the therapy of complex diseases. Compounds potently block-
ing A1 and A2AARs and inhibiting MAO-B, all at sub-micromolar
concentrations, were discovered. The thus far poorly investi-
gated class of tricyclic xanthine derivatives has the advantage
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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General procedure for the synthesis of tetrahydropyrazino[2,1-
f]purinediones 10–78: 7-(2-Bromoethyl)-8-(hydroxymethyl)-1,3-di-
methyl-1H-purine-2,6(3H,7H)-dione (8) (1000 mg, 3.2 mmol) was
dissolved in dry CH2Cl2 (30 mL) and cooled to 08C. A solution of
PBr3 (900 mL, 9.4 mmol) in dry CH2Cl2 (20 mL) was added dropwise.
The solution was allowed to warm to RT and stirred for 1 h. Then it
was cooled to 08C and excess PBr3 was carefully hydrolyzed by
slow addition of cold saturated aqueous NaHCO3 (~10 mL), setting
the solution to pH 8. The organic layer was then separated, and
the aqueous layer extracted with CH2Cl2 (2ꢃ50 mL). The combined
organic extracts were dried (Na2SO4), and the solvent was removed
in vacuo. The crude 7-(2-bromoethyl)-8-bromo-1,3-dimethylpurine-
2,4-dione (9) was used directly in the next step. The residue was
dissolved in a mixture of dimethoxyethane (100 mL) and DIPEA
(5 mL). The solution was split into ten vessels. Then, the appropri-
ate amine (0.64 mmol) was added and the reactions were stirred
overnight at RT. The volatiles were removed in vacuo and
tetrahydropyrazino[2,1-f]purinediones 10–78 precipitated upon ad-
dition of H2O (20 mL). For purification, the compounds were either
filtered off and washed with H2O (3ꢃ5 mL) and Et2O (3ꢃ10 mL), or
subjected to flash chromatography (silica gel, gradient of CH2Cl2/
MeOH 100:0 to 40:1). The yields of compounds 10–78 were calcu-
lated over two steps from 8.
of improved water solubility in comparison with the well-inves-
tigated isomeric tricyclic 1,3-dimethyltetrahydropyrimido[2,1-
f]purinediones due to increased basicity. Preliminary pharmaco-
kinetic studies of one of the most potent compounds have
been promising indicating oral bioavailability and brain pene-
tration. The present work represents the first extensive SAR
study of this new class of AR antagonists and MAO-B inhibi-
tors. Further modification, for example, at positions 1 and 3,
and more variations at N8 of the tetrahydropyrazino[2,1-f]puri-
nedione core are currently being explored.
Experimental Section
Chemistry
All commercially available reagents and solvents were used with-
out further purification. The reactions were monitored by thin layer
chromatography (TLC) using aluminum sheets coated with silica
gel 60 F254 (Merck). Melting points were determined on a Bꢀchi 530
melting point apparatus and are uncorrected. Column chromatog-
raphy was performed on silica gel 0.040–0.063 mm (Fluka) using
a Sepacore flash chromatography system (Bꢀchi). 1H NMR and
13C NMR data were recorded on a Bruker Advance spectrometer at
500 MHz for proton and 125 MHz for carbon at ambient tempera-
ture. Shifts are given in ppm relative to the remaining protons of
the deuterated solvents. Mass spectra were recorded on an API
2000 mass spectrometer (electron spray ion source, Applied Biosys-
tems, Darmstadt, Germany) coupled with an Agilent 1100 HPLC
system using a Phenomenex Luna HPLC C18 column (50ꢃ2.00 mm,
particle size 3 mm). The purity of the tested compounds was deter-
mined by HPLC–UV obtained on an LC–MS instrument (Applied
Biosystems API 2000 LC–MS/MS, HPLC Agilent 1100) using the fol-
8-Benzyl-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-f]purine-
1
2,4(1H,3H)-dione (10): Yield: 53%; mp: 1538C; H NMR (CDCl3): d=
7.33–7.28 (m, 5H, phe), 4.32 (t, 3J=5.4 Hz, 2H, C6-H2), 3.73–3.71
(m, 4H, N8-CH2, C9-H2), 3.50 (s, 3H, N1-CH3), 3.35 (s, 3H, N3-CH3),
2.93 ppm (t, 3J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0
(C9a), 151.7 (C4), 148.5 (C2), 147.9 (C10a), 136.5 (C1, phe), 129.1
(C2/C6, phe), 128.6 (C3/C5, phe), 127.9 (C4, phe), 106.5 (C4a), 61.9
(N8-CH2), 51.2 (C7), 48.7 (C9), 44.2 (C6), 29.7 (N1-CH3), 27.8 ppm
(N3-CH3); ESI-MS: positive mode 326.3 [M+H]+; HPLC: 99.3% (C).
8-(2-Chlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (11): Yield: 70%; mp: 2338C; 1H NMR
(CDCl3): d=7.46–7.45 (m, 1H, C3-H, phe), 7.40–7.38 (m, 1H, C6-H,
lowing procedure: the compounds at
a concentration of
1.0 mgmLꢀ1 were dissolved in MeOH, and if necessary, sonication
was used to complete dissolution. Then, 10 mL of the substance so-
lution was injected into a Phenomenex Luna C18 HPLC column
(50ꢃ2.00 mm, particle size 3 mm) and elution was performed for
30 min at a flow rate of 250 mLminꢀ1 with a gradient of H2O/MeOH
either containing 2 mm ammonium acetate from 90:10 up to
0:100, starting the gradient after 10 min (system A) or containing
2 mm ammonium acetate and 0.1% formic acid from 90:10 up to
0:100, starting the gradient after 10 min (system B) or containing
2 mm ammonium acetate from 60:40 up to 0:100 for 30 min, start-
ing the gradient after 0 min and ending after 20 min (system C).
UV absorption was detected from l 220 to 400 nm using a diode
array detector.
3
phe), 7.27–7.23 (m, 2H, C4-/C5-H, phe), 4.37 (t, J=5.3 Hz, 2H, C6-
H2), 3.89 (s, 2H, N8-CH2), 3.84 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3),
3
3.38 (s, 3H, N3-CH3), 3.04 ppm (t, J=5.2 Hz, 2H, C7-H2); 13C NMR
(CDCl3): d=154.9 (C9a), 151.7 (C4), 148.4 (C2), 147.7 (C10a), 134.5
(C1, phe), 133.7 (C2, phe), 130.9 (C3, phe), 129.8 (C5, phe), 129.1
(C6, phe), 127.0 (C4, phe), 106.6 (C4a), 58.3 (N8-CH2), 51.1 (C7), 48.9
(C9), 44.2 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive
mode 360.1 [M+H]+; HPLC: 97.4% (A) and 97.7% (B).
8-(3-Chlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (12): Yield: 68%; mp: 2148C; 1H NMR
(CDCl3): d=7.35 (s, 1H, C2-H, phe), 7.28–7.26 (m, 2H, C4-/C5-H,
3
phe), 7.23–7.22 (m, 1H, C6-H, phe), 4.35 (t, J=5.5 Hz, 2H, C6-H2),
3.74 (s, 2H, N8-CH2), 3.72 (s, 2H, C9-H2), 3.52 (s, 3H, N1-CH3), 3.38
(s, 3H, N3-CH3), 2.94 ppm (t, 3J=5.5 Hz, 2H, C7-H2); 13C NMR
(CDCl3): d=154.9 (C9a), 151.7 (C4), 148.4 (C2), 147.8 (C10a), 138.7
(C1, phe), 134.6 (C3, phe), 129.9 (C5, phe), 128.9 (C2, phe), 128.0
(C4, phe), 127.0 (C6, phe), 106.6 (C4a), 61.3 (N8-CH2), 51.2 (C7), 48.8
(C9), 44.3 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive
mode 360.3 [M+H]+; HPLC: 98.1% (A) and 98.0% (B).
7-(2-Bromoethyl)-8-(hydroxymethyl)-1,3-dimethyl-1H-purine-
2,6(3H,7H)-dione (8): 8-(Hydroxymethyl)-1,3-dimethyl-1H-purine-
2,6(3H,7H)-dione (7) (1.05 g, 5.0 mmol) was dissolved in DMF
(10 mL), and NEt3 (2.1 mL, 15.0 mmol) and 1,2-dibromoethane
(2.82 g, 15.0 mmol) were added. The solution was stirred for 6 h at
808C. Then, the volatiles were removed in vacuo and the residue
was purified by column chromatography (silica gel, CH2Cl2/MeOH
1
40:1 to 20:1). Yield: 0.90 g, 55%; mp: 2038C; H NMR (MeOD): d=
8-(4-Chlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (13): Purification by column chromatog-
raphy. Yield: 50%; mp: 1918C; 1H NMR (CDCl3): d=7.31 (d, 3J=
3
3
4.81 (t, J=6.6 Hz, 2H, N7-CH2), 4.74 (s, 2H, CH2), 3.88 (t, J=6.7 Hz,
2H, N7-CH2-CH2), 3.59 (s, 3H, N3-CH3), 3.40 ppm (s, 3H, N1-CH3);
13C NMR (MeOD): d=156.6 (C9a), 155.7 (C4), 153.7 (C2), 150.0
(C10a), 108.6 (C4a), 58.8 (CH2), 50.7 (N7-CH2), 31.7 (N7-CH2-CH2),
30.7 (N1-CH3), 28.7 ppm (N3-CH3); ESI-MS: negative mode 315.2,
317.2 [MꢀH]ꢀ, positive mode 317.0, 319.0 [M+H]+.
3
8.5 Hz, 2H, C3-/C5-H, phe), 7.28 (d, J=8.5 Hz, 2H, C2-/C6-H, phe),
3
4.35 (t, J=5.4 Hz, 2H, C6-H2), 3.73 (s, 1H, N8-CH2), 3.71 (s, 2H, C9-
H2), 3.51 (s, 3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 2.94 ppm (t, 3J=
5.3 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0 (C9a), 151.7 (C4),
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemMedChem 2014, 9, 1704 – 1724 1715

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3
148.5 (C2), 147.6 (C10a), 133.7 (C1, phe), 130.3 (C4, phe), 130.3 (C3/
C5, phe), 128.9 (C2/C6, phe), 106.6 (C4a), 61.2 (N8-CH2), 51.2 (C7),
48.8 (C9), 44.2 (C6), 29.7 (N1-CH3), 27.9 ppm (N3-CH3); ESI-MS: posi-
tive mode 360.1 [M+H]+; HPLC: 99.9% (A) and 99.8% (B).
4.55 (t, J=5.4 Hz, 2H, C6-H2), 3.93 (brs, 4H, N8-CH2, C9-H2), 3.52 (s,
3
3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 3.18 ppm (t, J=5.1 Hz, 2H, C7-
H2); 13C NMR (CDCl3): d=154.9 (C9a), 151.7 (C4), 148.4 (C2), 147.7
(C10a), 132.7 (C1, phe), 132.2 (C2, phe), 130.7 (C4, phe), 128.5 (C5,
phe), 128.4 (C6, phe), 123.1 (C3, phe), 106.8 (C4a), 60.7 (N8-CH2),
50.1 (C7), 48.5 (C9), 42.9 (C6), 29.8 (N1-CH3), 27.9 ppm (N3-CH3);
ESI-MS: positive mode 404.0 and 406.1 [M+H]+; HPLC: 97.5% (A)
and 97.3% (B).
8-(2-Fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (14): Yield: 55%; mp: 1988C; 1H NMR
(CDCl3): d=7.40–7.36 (m, 1H, C4-H, phe), 7.32–7.27 (m, C3-H, phe),
3
4
7.16–7.13 (dd, J=6.3 Hz, J=1.3 Hz, 1H, C6-H, phe), 7.10–7.06 (m,
3
2H, C5-H, phe), 4.36 (t, J=5.4 Hz, 2H, C6-H2), 3.84 (s, 2H, N8-CH2),
8-(4-Bromobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (19): Purification by column chromatog-
raphy. Yield: 44%; mp: 2318C; 1H NMR (CDCl3): d=7.45 (d, 3J=
3.80 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3),
3
3.00 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=161.4 (d,
1JC,F =245.4 Hz, C2, phe), 155.0 (C9a), 151.7 (C4), 148.5 (C2), 148.0
8.2 Hz, C3-/C5-H, 2H, phe), 7.44 (d, J=8.2 Hz, 2H, C2/C6-H, phe),
3
3
3
3
(C10a), 131.4 (d, JC,F =4.0 Hz, C6, phe), 129.7 (d, JC,F =8.2 Hz, C4,
phe), 124.3 (d, 4JC,F =3.5 Hz, C5, phe), 123.1 (d, 2JC,F =14.4 Hz, C1,
4.32 (t, J=5.4 Hz, 2H, C6-H2), 3.72 (s, 2H, N8-CH2), 3.67 (s, 2H, C9-
H2), 3.51 (s, 3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 2.92 ppm (t, 3J=
5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=154.9 (C9a), 151.7 (C4),
148.4 (C2), 147.7 (C10a), 135.6 (C1, phe), 131.0 (C3/C5, phe), 130.6
(C2/C6, phe), 121.7 (C4, phe), 106.6 (C4a), 61.2 (N8-CH2), 51.0 (C7),
48.8 (C9), 44.2 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: posi-
tive mode 404.0 and 406.1 [M+H]+; HPLC: 96.4% (A) and 98.3%
(B).
phe), 115.6 (d, 2JC,F =22.0 Hz, C3, phe), 106.6 (C4a), 54.4 (d, JC,F
=
1.5 Hz, N8-CH2), 51.0 (C7), 48.8 (C9), 44.3 (C6), 29.7 (N1-CH3),
27.8 ppm (N3-CH3); ESI-MS: positive mode 344.3 [M+H]+; HPLC:
99.9% (A) and 99.9% (B).
8-(3-Fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (15): Purification by column chromatog-
raphy. Yield: 41%; mp: 2078C; 1H NMR (CDCl3): d=7.32–7.29 (m,
1H, C5-H, phe), 7.12–7.10 (m, 1H, C4-H, phe), 7.09–7.07 (m, 1H, C6-
8-(3-Iodobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (20): Purification by column chromatog-
raphy. Yield: 38%; mp: 2188C; H NMR (CDCl3): d=7.79 (s, 1H, C2-
H, phe), 7.73 (d, J=7.9 Hz, 1H, C4-H, phe), 7.55 (d, J=6.7 Hz, 1H,
3
1
H, phe), 7.00–6.97 (m, 1H, C2-H, phe), 4.35 (t, J=5.4 Hz, 2H, C6-
3
3
H2), 3.75 (s, 2H, N8-CH2), 3.74 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3),
3
3
3.38 (s, 3H, N3-CH3), 2.96 ppm (t, J=5.5 Hz, 2H, C7-H2); 13C NMR
C6-H, phe), 7.17–7.13 (m, C5-H, phe), 4.60 (t, J=5.4 Hz, 2H, C6-H2),
(CDCl3): d=160.0 (d, 1JC,F =246.5 Hz, C3, phe), 155.0 (C9a), 151.7
3.99 (s, 2H, N8-CH2), 3.98 (s, 2H, C9-H2), 3.52 (s, 3H, N1-CH3), 3.37
(s, 3H, N3-CH3), 3.25 ppm (t, 3J=5.1 Hz, 2H, C7-H2); 13C NMR
(CDCl3): d=154.9 (C9a), 151.7 (C4), 148.4 (C2), 147.7 (C10a), 139.1
(C2, phe), 138.7 (C1, phe), 130.9 (C4, phe), 129.3 (C5, phe), 128.7 (C6,
phe), 106.8 (C4a), 94.0 (C3, phe), 60.7 (N8-CH2), 50.1 (C7), 48.5 (C9),
42.9 (C6), 29.8 (N1-CH3), 27.9 ppm (N3-CH3); ESI-MS: positive mode
404.0 and 406.1 [M+H]+; HPLC: 97.5% (A) and 97.3% (B).
3
(C4), 148.4 (C2), 147.8 (C10a), 139.2 (d, JC,F =7.0 Hz, C1, phe), 130.1
3
4
(d, JC,F =8.2 Hz, C5, phe), 124.4 (d, JC,F =2.6 Hz, C6, phe), 115.7 (d,
2JC,F =21.5 Hz, C2, phe), 114.8 (d, 2JC,F =21.1 Hz, C4, phe), 106.5
(C4a), 61.3 (N8-CH2), 51.3 (C7), 48.8 (C9), 44.3 (C6), 29.7 (N1-CH3),
27.8 ppm (N3-CH3); ESI-MS: positive mode 344.3 [M+H]+; HPLC:
97.7% (A) and 96.8% (B).
8-(4-Fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (16): Purification by column chromatog-
raphy. Yield: 31%; mp: 1888C; 1H NMR (CDCl3): d=7.31–7.29 (m,
8-(2-(Trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (21): Purification by
1
column chromatography. Yield: 60%; mp: 2368C; H NMR (CDCl3):
3
3
3
2H, C2-/C6-H, phe), 7.03–7.00 (m, 2H, C3-/C5-H, phe), 4.34 (t, J=
d=7.75 (d, J=7.5 Hz, 1H, C6-H, phe), 7.67 (d, J=7.8 Hz, 1H, C2-
3
3
5.1 Hz, 2H, C6-H2), 3.72 (s, 2H, N8-CH2), 3.71 (s, 2H, C9-H2), 3.50 (s,
H, phe), 7.54 (dd, J=7.5 Hz, J=7.5 Hz, 1H, C5-H, phe), 7.39 (dd,
3
3
3
3H, N1-CH3), 3.36 (s, 3H, N3-CH3), 2.94 ppm (t, J=4.8 Hz, 2H, C7-
3J=7.6 Hz, J=7.6, 1H, C4-H, phe), 4.37 (t, J=5.4 Hz, 2H, C6-H2),
3.92 (s, 2H, N8-CH2), 3.80 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.38
(s, 3H, N3-CH3), 2.99 ppm (t, 3J=5.4 Hz, 2H, C7-H2); 13C NMR
(CDCl3): d=155.0 (C9a), 151.7 (C4), 148.4 (C2), 147.7 (C10a), 135.6
H2); 13C NMR (CDCl3): d=162.4 (d, 1JC,F =246.5 Hz, C4, phe), 155.0
(C9a), 151.7 (C4), 148.4 (C2), 147.7 (C10a), 132.0 (C1, phe), 130.6 (d,
3JC,F =7.8 Hz, C2/C6, phe), 115.4 (d, 2JC,F =21.4 Hz, C3/C5, phe),
106.6 (C4a), 61.1 (N8-CH2), 51.1 (C7), 48.7 (C9), 44.2 (C6), 29.7 (N1-
CH3), 27.8 ppm (N3-CH3); ESI-MS: positive mode 344.3 [M+H]+;
HPLC: 97.5% (A) and 97.4% (B).
(C1, phe), 132.1 (C5, phe), 130.5 (C6, phe), 128.9 (q, JC,F =30.3 Hz,
2
3
C2, phe), 127.7 (C4, phe), 126.1 (q, JC,F =5.7 Hz, C3, phe), 124.2 (q,
1JC,F =273.9 Hz, CF3), 106.6 (C4a), 57.4 (N8-CH2), 51.3 (C7), 49.0 (C9),
44.3 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive mode
394.4 [M+H]+; HPLC: 98.7% (A) and 99.0% (B).
8-(2-Bromobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (17): Yield: 61%; mp: 2198C; 1H NMR
3
4
3
(CDCl3): d=7.55 (dd, J=8.2 Hz, J=1.3 Hz, 1H, C3-H), 7.44 (d, J=
8-(3-(Trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
3
7.3 Hz, 1H, C6-H, phe), 7.30–7.27 (m, 1H, C4-H, phe), 7.14 (dd, J=
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (22): Yield: 48%; mp:
7.6 Hz, 3J=7.6 Hz, 1H, C5-H, phe), 4.35 (t, 3J=5.4 Hz, 2H, C6-H2),
3.86 (s, 2H, N8-CH2), 3.83 (s, 2H, C9-H2), 3.51 (s, 3H, N1-CH3), 3.36
(s, 3H, N3-CH3), 3.02 ppm (t, 3J=5.4 Hz, 2H, C7-H2); 13C NMR
(CDCl3): d=154.9 (C9a), 151.7 (C4), 148.4 (C2), 147.7 (C10a), 135.6
(C1, phe), 133.1 (C3, phe), 131.0 (C6, phe), 129.4 (C4, phe), 127.6
(C5, phe), 124.8 (C2, phe), 106.6 (C4a), 60.8 (N8-CH2), 51.0 (C7), 48.8
(C9), 44.2 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive
mode 404.0 and 406.1 [M+H]+; HPLC: 97.5% (A) and 97.3% (B).
1658C; H NMR (CDCl3): d=7.67–7.64 (m, 2H, C2-/C6-H, phe), 7.60–
1
3
7.59 (m, 1H, C5-H, phe), 7.52–7.49 (m, 1H, C4-H, phe), 4.46 (t, J=
5.4 Hz, 2H, C6-H2), 3.91 (s, 2H, N8-CH2), 3.85 (s, 2H, C9-H2), 3.52 (s,
3
3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 3.09 ppm (t, J=5.3 Hz, 2H, C7-
H2); ESI-MS: positive mode 394.4 [M+H]+; HPLC: 97.5% (A) and
97.2% (B).
8-(2-Methoxybenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (23): Yield: 65%; mp: 1818C;
1H NMR (CDCl3): d=7.29 (1H, d, 3J=7.2 Hz, C6-H, phe), 7.14–7.11
(m, 1H, C4-H, phe), 6.89–6.86 (m, 1H, C5-H, phe), 6.83 (d, 3J=
8-(3-Bromobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-
f]purine-2,4(1H,3H)-dione (18): Yield: 74%; mp: 2048C; 1H NMR
3
4
3
(CDCl3): d=7.58 (s, 1H, C2-H, phe), 7.51 (dd, J=8.2 Hz, J=1.2 Hz,
8.2 Hz, 1H, C3-H, phe), 4.60 (t, J=5.4 Hz, 2H, C6-H2), 3.99 (s, 2H,
3
1H, C4-H, phe), 7.45 (d, J=6.9 Hz, 1H, C6-H, phe), 7.30–7.27 (m,
N8-CH2), 3.85 (brs, 5H, C9-H2, OCH3), 3.52 (s, 3H, N1-CH3), 3.37 (s,
1H, C5-H, phe), 7.14 (dd, 3J=7.6 Hz, 3J=7.6 Hz, 1H, C5-H, phe),
3H, N3-CH3), 3.25 ppm (t, J=5.1 Hz, 2H, C7-H2); 13C NMR (CDCl3):
3
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4
3
4
d=165.8 (C2, phe), 155.0 (C9a), 151.7 (C4), 148.5 (C2), 147.6 (C10a),
130.2 (C4 and C6, phe), 127.5 (C1, phe), 121.2 (C5, phe), 111.0 (C3,
phe), 106.5 (C4a), 55.5 (N8-CH2), 55.6 (OCH3), 50.1 (C7), 48.5 (C9),
42.9 (C6), 29.8 (N1-CH3), 27.9 ppm (N3-CH3); ESI-MS: positive mode
355.9 [M+H]+; HPLC: 99.6% (A) and 99.7% (B).
2.2 Hz, J=1.3 Hz, 1H, C4-H, phe), 7.22 (ddd, J=7.6 Hz, J=1.3 Hz,
4J=1.3 Hz, 1H, C6-H, phe), 7.09–7.08 (m, 2H, C2-/C5-H, pyrrolyl),
3
6.35–6.34 (m, 2H, C3-/C4-H, pyrrolyl), 4.37 (t, J=5.5 Hz, 2H, C6-H2),
3.79 (s, 2H, N8-CH2), 3.78 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.40
(s, 3H, N3-CH3), 3.00 ppm (t, 3J=5.5 Hz, 2H, C7-H2); 13C NMR
(CDCl3): d=155.0 (C9a), 151.8 (C4), 148.5 (C2), 148.0 (C10a), 141.1
(C3, phe), 138.4 (C1, phe), 129.8 (C5, phe), 126.0 (C6, phe), 120.8 (C4,
phe), 119.9 (C2, phe), 119.3 (C2/C5, pyrrolyl), 110.6 (C3/C4, pyrrolyl),
106.6 (C4a), 61.7 (N8-CH2), 51.3 (C7), 48.9 (C9), 44.3 (C6), 29.7 (N1-
CH3), 27.9 ppm (N3-CH3); ESI-MS: positive mode 391.4 [M+H]+;
HPLC: 97.7% (A) and 97.5% (B).
8-(3-Methoxybenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (24): Yield: 73%; mp: 1418C;
3
3
1H NMR (CDCl3): d=7.22 (dd, J=7.2 Hz, J=7.6 Hz, 1H, C5-H, phe),
3
6.91 (s, 1H, C2-H, phe), 6.90 (d, J=6.8 Hz, 1H, C4-H, phe), 6.82 (dd,
3J=7.3 Hz, J=1.6 Hz, 1H, C6-H, phe), 4.35 (brs, 2H, C6-H2), 3.78–
4
3.74 (m, 7H, N8-CH2, C9-H2, OCH3), 3.52 (s, 3H, N1-CH3), 3.37 (s, 3H,
N3-CH3), 2.96 ppm (brs, 2H, C7-H2); 13C NMR (CDCl3): d=159.9 (C3,
phe), 155.0 (C9a), 151.7 (C4), 148.4 (C2), 147.7 (C10a), 136.1 (C1,
phe), 129.7 (C5, phe), 121.4 (C6, phe), 114.6 (C4, phe), 113.4 (C2,
phe), 106.6 (C4a), 61.8 (N8-CH2), 55.2 (OCH3), 51.1 (C7), 48.7 (C9),
44.0 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive mode
355.9 [M+H]+; HPLC: 99.0% (A) and 99.0% (B).
1,3-Dimethyl-8-(thiophen-3-yl)-6,7,8,9-tetrahydropyrazino[2,1-f]-
purine-2,4(1H,3H)-dione (29): Yield: 66%; mp: 2128C; 1H NMR
3
(CDCl3): d=7.30 (d, J=4.8 Hz, 1H, C5-H, thienyl), 7.16 (s, 1H, C2-H,
3
3
thienyl), 7.06 (d, J=4.8 Hz, 1H, C4-H, thienyl), 4.33 (t, J=5.4 Hz,
2H, C6-H2), 3.75 (s, 2H, N8-CH2), 3.72 (s, 2H, C9-H2), 3.52 (s, 3H, N1-
CH3), 3.37 (s, 3H, N3-CH3), 2.93 ppm (t, 3J=5.4 Hz, 2H, C7-H2);
13C NMR (CDCl3): d=155.0 (C9a), 151.8 (C4), 148.5 (C2), 148.0
(C10a), 137.4 (C3, thienyl), 128.0 (C4, thienyl), 126.3 (C5, thienyl),
123.5 (C2, thienyl), 106.8 (C4a), 61.7 (N8-CH2), 50.1 (C7), 48.5 (C9),
42.9 (C6), 29.8 (N1-CH3), 27.9 ppm (N3-CH3); ESI-MS: positive mode
408.4 [M+H]+; HPLC: 99.9% (A) and 99.1% (B).
8-(4-Methoxybenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (25): Purification by column chro-
matography. Yield: 33%; mp: 1648C; 1H NMR (CDCl3): d=7.30 (d,
3J=8.5 Hz, 2H, C2-/C6-H, phe), 6.87 (d, 3J=8.5 Hz, 2H, C3-/C5-H,
phe), 4.35 (brs, 2H, C6-H2), 3.78–3.74 (brs, 7H, N8-CH2, C9-H2,
OCH3), 3.52 (s, 3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 2.96 ppm (brs,
2H, C7-H2); 13C NMR (CDCl3): d=159.9 (C4, phe), 154.9 (C9a), 151.7
(C4), 148.4 (C2), 147.7 (C10a), 130.9 (C2/C6, phe), 128.9 (C1, phe),
114.3 (C3/C5, phe), 105.9 (C4a), 61.8 (N8-CH2), 55.2 (OCH3), 51.1
(C7), 48.7 (C9), 44.0 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS:
positive mode 355.9 [M+H]+; HPLC: 99.0% (A) and 99.0% (B).
(R,S)-8-(1-(4-Fluorophenyl)ethyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (30): Purification by
1
column chromatography. Yield: 50%; mp: 2088C; H NMR (CDCl3):
d=7.33–7.30 (m, 2H, C2-/C6-H, phe), 7.06–7.03 (m, 2H, C3-/C5-H,
phe), 4.32 (brs, 2H, C6-H2), 3.92–3.90 (m, 1H, N8-CH), 3.73–3.65 (m,
2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3), 2.93 (brs, 2H,
C7-H2), 1.54 ppm (brs, 3H, CH3); 13C NMR (CDCl3): d=162.3 (d,
1JC,F =246.6 Hz, C4, phe), 154.9 (C9a), 151.7 (C4), 148.5 (C2), 148.0
1,3-Dimethyl-8-(3-(thiazol-2-yl)benzyl)-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (26): Yield: 61%; mp:
1718C; 1H NMR (CDCl3): d=7.98 (s, 1H, C2-H, phe), 7.89–7.87 m,
2H, C4-H, thiazol, C4-H, phe), 7.45–7.40 (m, 2H, C5-H, Phe, C5-H,
3
(C10a), 130.9 (C1, phe) 129.0 (d, JC,F =7.6 Hz, C2/C6, phe), 115.6 (d,
4JC,F =21.2 Hz, C3/C5, phe), 106.5 (C4a), 63.4 (N8-CH), 48.9 (C7), 46.5
(C9), 44.3 (C6), 29.7 (N1-CH3), 27.8 (N3-CH3), 19.5 ppm (CH3); ESI-
MS: positive mode 358.3 [M+H]+; HPLC: 97.0% (A) and 97.7% (B).
3
3
thiazol), 7.35 (d, J=3.3 Hz, 1H, C6-H, phe), 4.37 (t, J=5.5 Hz, 2H,
C6-H2), 3.81 (s, 2H, N8-CH2), 3.79 (s, 2H, C9-H2), 3.53 (s, 3H, N1-
CH3), 3.39 (s, 3H, N3-CH3), 3.00 ppm (t, 3J=5.5 Hz, 2H, C7-H2);
13C NMR (CDCl3): d=168.0 (C2, thiazol), 155.0 (C9a), 151.8 (C4),
148.5 (C2), 148.0 (C10a), 143.7 (C4, thiazol), 137.7 (C1, phe), 134.0
(C3, phe), 130.5 (C6, phe), 129.3 (C5, phe), 126.9 (C2, phe), 126.2 (C4,
phe), 119.0 (C5, thiazol), 106.8 (C4a), 61.6 (N8-CH2), 50.3 (C7), 48.5
(C9), 44.4 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive
mode 409.4 [M+H]+; HPLC: 99.0% (A) and 97.3% (B).
8-(2,3-Dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (31): Yield: 59%; mp: 2218C;
1H NMR (CDCl3): d=7.44–7.40 (m, 2H, C4-/C6-H, phe), 7.20–7.18 (m,
3
1H, C5-H, phe), 4.38 (t, J=5.4 Hz, 2H, C6-H2), 3.91 (s, 2H, N8-CH2),
3.83 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.36 (s, 3H, N3-CH3),
3.02 ppm (t, 3J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0
(C9a), 151.7 (C4), 148.5 (C2), 147.6 (C10a), 136.3 (C1, phe), 133.6
(C2, phe), 132.7 (C3, phe), 130.0 (C4, phe), 128.8 (C6, phe), 127.4 (C5,
phe), 106.5 (C4a), 58.9 (N8-CH2), 51.1 (C7), 49.0 (C9), 44.1 (C6), 29.7
(N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive mode 394.1 [M+H]+
; HPLC: 97.3% (A) and 97.2% (B).
1,3-Dimethyl-8-(3-(thiophen-2-yl)benzyl)-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (27): Purification by
1
column chromatography. Yield: 66%; mp: 1958C; H NMR (CDCl3):
d=7.59 (s, 1H, C2-H, phe), 7.57–7.55 (m, 1H, C4-H, phe), 7.36 (dd,
3J=7.7 Hz, 3J=7.7 Hz, 1H, C5-H, phe), 7.32 (dd, 3J=3.6 Hz, 4J=
1.1 Hz, 1H, C3-H, thienyl), 7.27–7.26 (m, 1H, C5-H, thienyl), 7.25
(brs, 1H, C6-H, phe), 7.08 (dd, 3J=5.1 Hz, 3J=3.6 Hz, 1H, C4-H,
8-(2,6-Dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (32): Yield: 55%; mp: 2578C;
3
1H NMR (CDCl3): d=7.33 (d, J=8.4 Hz, 2H, C3-H/C5-H, phe), 7.18
3
3
3
thienyl), 4.35 (t, J=5.4 Hz, 2H, C6-H2), 3.77 (s, 2H, N8-CH2), 3.76 (s,
(dd, J=8.4 Hz, 1H, C4-H, phe), 4.38 (t, J=5.4 Hz, 2H, C6-H2), 3.91
2H, C9-H2), 3.52 (s, 3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 2.97 ppm (t,
3J=5.1 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0 (C9a), 151.8 (C4),
148.5 (C2), 148.0 (C10a), 144.0 (C1, thienyl), 137.3 (C1, phe), 134.8
(C3, phe), 129.2 (C2, phe), 128.0 (C5/C6, phe), 126.4 (C5, thienyl),
125.4 (C4, thienyl), 125.0 (C3, thienyl), 123.3 (C4, phe), 106.6 (C4a),
61.7 (N8-CH2), 50.1 (C7), 48.5 (C9), 42.9 (C6), 29.8 (N1-CH3),
27.9 ppm (N3-CH3); ESI-MS: positive mode 408.4 [M+H]+; HPLC:
99.9% (A) and 99.1% (B).
(s, 2H, N8-CH2), 3.83 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.36 (s, 3H,
3
N3-CH3), 3.02 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=
155.0 (C9a), 151.7 (C4), 148.5 (C2), 147.6 (C10a), 137.0 (C2/C6, phe),
133.4 (C1, phe), 129.7 (C4, phe), 128.6 (C3/C5, phe), 106.5 (C4a),
55.4 (N8-CH2), 51.1 (C7), 49.0 (C9), 44.1 (C6), 29.7 (N1-CH3),
27.8 ppm (N3-CH3); ESI-MS: positive mode 394.1 [M+H]+; HPLC:
97.3% (A) and 97.2% (B).
8-(2,4-Dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
8-(3-(1H-Pyrrol-1-yl)benzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (28): Yield: 58%; mp:
[2,1-f]purine-2,4(1H,3H)-dione (33): Yield: 58%; mp: 1498C;
3
4
1H NMR (CDCl3): d=7.58 (d, J=7.9 Hz, 1H, C6-H, phe), 7.43 (d, J=
1
4
1888C; H NMR (CDCl3): d=7.42–7.41 (m, 1H, C2-H, phe), 7.40 (dd,
1.9 Hz, 1H, C3-H, phe), 7.28 (dd, 3J=8.2 Hz, J=1.9 Hz, 1H, C5-H,
3J=7.7 Hz, 3J=7.7 Hz, 1H, C5-H, phe), 7.34 (ddd, 3J=8.0 Hz, 4J=
phe), 4.33 (t, 3J=5.4 Hz, 2H, C6-H2), 3.99 (s, 2H, N8-CH2), 3.93 (s,
ChemMedChem 2014, 9, 1704 – 1724 1717
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2H, C9-H2), 3.51 (s, 3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 3.15 ppm (t,
3J=5.7 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0 (C9a), 151.7 (C4),
148.5 (C2), 147.6 (C10a), 137.7 (C2, phe), 135.3 (C1, phe), 132.5 (C4,
phe), 129.8 (C3/C6, phe), 127.8 (C5, phe), 106.5 (C4a), 57.2 (N8-CH2),
51.2 (C7), 48.8 (C9), 44.3 (C6), 29.8 (N1-CH3), 27.9 ppm (N3-CH3);
ESI-MS: positive mode 394.3 [M+H]+; HPLC: 98.1% (A) and 98.6%
(B).
C2, phe), 161.4 (dd, 1JC,F =249.6 Hz, 3JC,F =11.9 Hz, C4, phe), 154.9
3
(C9a), 151.7 (C4), 148.4 (C2), 147.3 (C10a), 132.4 (dd, JC,F =7.8 Hz,
3JC,F =5.7 Hz, C6, phe), 118.6 (C1, phe), 111.6 (dd, 2JC,F =21.2 Hz,
2
2
4JC,F =2.9 Hz, C5, phe), 106.6 (C4a), 104.4 (dd, JC,F =25.7 Hz, JC,F
=
25.7 Hz, C3, phe), 53.8 (N8-CH2), 50.7 (C7), 48.6 (C9), 44.1 (C6), 29.7
(N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive mode 362.1 [M+H]+
; HPLC: 99.9% (A) and 99.9% (B).
8-(3,5-Dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
8-(2,5-Difluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (39): Yield: 31%; mp: 2168C;
1H NMR (CDCl3): d=7.24–7.20 (m, 1H, C3-H, phe), 7.08–6.98 (m,
[2,1-f]purine-2,4(1H,3H)-dione (34): Yield: 58%; mp: 2408C;
4
1H NMR (CDCl3): d=7.28 (d, J=1.9 Hz, 2H, C2-/C6-H, phe), 7.24 (s,
3
3
1H, C4-H, phe), 4.36 (t, J=5.4 Hz, 2H, C6-H2), 3.73 (s, 2H, N8-CH2),
2H, C4-/C6-H, phe), 4.45 (t, J=5.3 Hz, 2H, C6-H2), 3.90 (s, 2H, N8-
3.69 (s, 2H, C9-H2), 3.51 (s, 3H, N1-CH3), 3.37 (s, 3H, N3-CH3),
2.96 ppm (t, 3J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0
(C9a), 151.7 (C4), 148.5 (C2), 147.6 (C10a), 140.0 (C1, phe), 135.3
(C3/C5, phe), 128.1 (C4, phe), 127.2 (C2/C6, phe), 106.6 (C4a), 60.8
(N8-CH2), 51.2 (C7), 48.9 (C9), 44.1 (C6), 29.7 (N1-CH3), 27.8 ppm
(N3-CH3); ESI-MS: positive mode 394.3 [M+H]+; HPLC: 99.7% (A)
and 99.6% (B).
CH2), 3.88 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3),
3
3.10 ppm (t, J=5.1 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=158.8 (dd,
1JC,F =243.7 Hz, 4JC,F =1.6 Hz, C2, phe), 157.2 (dd, 1JC,F =242.5 Hz,
4JC,F =2.3 Hz, C5, phe), 154.9 (C9a), 151.7 (C4), 148.5 (C2), 147.5
(C10a), 123.8–123.7 (m, C1, phe)117.7 (d, 2JC,F =26.2 Hz, C6, phe),
2
3
116.8 (dd, JC,F =25.2 Hz, JC,F =8.4 Hz, C3/C4, phe), 106.7 (C4a), 53.8
(N8-CH2), 50.6 (C7), 48.7 (C9), 43.8 (C6), 29.8 (N1-CH3), 27.9 ppm
(N3-CH3); ESI-MS: positive mode 362.1 [M+H]+; HPLC: 99.9% (A)
and 99.9% (B).
8-(2,5-Dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (35): Yield: 74%; mp: 2088C;
4
3
1H NMR (CDCl3): d=7.49 (d, J=2.5 Hz, 1H, C6-H, phe), 7.30 (d, J=
8-(3,5-Difluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (40): Yield: 71%; mp: 2098C;
1H NMR (CDCl3): d=6.92–6.89 (m, 1H, C4-H, phe), 6.77–6.73 (m,
8.5 Hz, 1H, C3-H, phe), 7.21 (dd, 3J=8.5 Hz, J=2.5 Hz, 1H, C4-H,
4
phe), 4.36 (t, 3J=5.4 Hz, 2H, C6-H2), 3.86 (s, 2H, N8-CH2), 3.84 (s,
2H, C9-H2), 3.52 (s, 3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 3.04 ppm (t,
3J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0 (C9a), 151.7 (C4),
148.5 (C2), 147.6 (C10a), 135.6 (C1, phe), 133.1 (C2, phe), 132.5 (C5,
phe), 130.9 (C6, phe), 130.6 (C3, phe), 129.2 (C4, phe), 106.6 (C4a),
57.9 (N8-CH2), 51.0 (C7), 48.9 (C9), 44.1 (C6), 29.7 (N1-CH3),
27.9 ppm (N3-CH3); ESI-MS: positive mode 394.3 [M+H]+; HPLC:
98.6% (A) and 99.1% (B).
3
2H, C2-/C6-H, phe), 4.38 (t, J=5.5 Hz, 2H, C6-H2), 3.76 (s, 2H, N8-
CH2), 3.73 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3),
3
2.98 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=163.2 (dd,
1
1JC,F =249.3 Hz, JC,F =12.7 Hz, C3/C5, phe), 155.0 (C9a), 151.7 (C4),
148.5 (C2), 147.6 (C10a), 140.7 (dd, 3JC,F =8.9 Hz, 3JC,F =8.9 Hz, C1,
2
3
phe), 111.4 (dd, JC,F =19.5 Hz, JC,F =5.9 Hz, C2/C6, phe), 106.5 (C4a),
2
2
103.3 (dd, JC,F =25.3 Hz, JC,F =25.3 Hz, C4, phe), 61.0 (N8-CH2), 51.3
(C7), 48.9 (C9), 44.3 (C6), 29.7 (N1-CH3), 27.9 ppm (N3-CH3); ESI-MS:
positive mode 362.1 [M+H]+; HPLC: 99.9% (A) and 99.9% (B).
8-(3,4-Dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (36): Yield: 81%; mp: 2018C;
4
3
1H NMR (CDCl3): d=7.44 (d, J=1.9 Hz, 1H, C2-H, phe), 7.40 (d, J=
8-(3,4-Difluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (41): Yield: 59%; mp: 2208C;
1H NMR (CDCl3): d=7.23–7.19 (m, 1H, C6-H, phe), 7.16–7.11 (m, C5-
8.2 Hz, 1H, C5-H, phe), 7.17 (dd, 3J=8.2 Hz, J=2.4 Hz, 1H, C6-H,
4
phe), 4.33 (t, 3J=5.4 Hz, 2H, C6-H2), 3.72 (s, 2H, N8-CH2), 3.68 (s,
2H, C9-H2), 3.51 (s, 3H, N1-CH3), 3.36 (s, 3H, N3-CH3), 2.93 ppm (t,
3J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0 (C9a), 151.7 (C4),
149.5 (C2), 147.6 (C10a), 137.0 (C1, phe), 132.8 (C3, phe), 131.9 (C4,
phe), 130.7 and 130.6 (C2 and C5, phe), 128.1 (C6, phe), 106.5 (C4a),
60.7 (N8-CH2), 51.3 (C7), 48.8 (C9), 44.3 (C6), 29.7 (N1-CH3),
27.9 ppm (N3-CH3); ESI-MS: positive mode 394.3 [M+H]+; HPLC:
98.0% (A) and 97.9% (B).
3
H, phe), 7.08–7.05 (m, 1H, C2-H, phe), 4.36 (t, J=5.4 Hz, 2H, C6-
H2), 3.74 (s, 2H, N8-CH2), 3.71 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3),
3
3.39 (s, 3H, N3-CH3), 2.96 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR
1
2
(CDCl3): d=155.0 (C9a), 151.7 (C4), 150.5 (dd, JC,F =248.9 Hz, JC,F
=
1
2
12.6 Hz, C3, phe), 149.4 (dd, JC,F =248.5 Hz, JC,F =12.7 Hz, C4, phe),
148.5 (C2), 147.6 (C10a), 133.6 (d, JC,F =4.5 Hz, C1, phe), 124.7 (dd,
3
3JC,F =5.9 Hz, 4JC,F =3.6 Hz, C6, phe), 117.6 (d, 2JC,F =17.4 Hz, C2,
phe), 117.4 (d, 2JC,F =17.2 Hz, C5, phe), 106.6 (C4a), 60.8 (N8-CH2),
51.2 (C7), 48.8 (C9), 44.2 (C6), 29.7 (N1-CH3), 27.9 ppm (N3-CH3);
ESI-MS: negative mode 360.0 [MꢀH]ꢀ, positive mode 362.1 [M+
H]+; HPLC: 99.9% (A) and 99.9% (B).
8-(2,6-Difluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
[2,1-f]purine-2,4(1H,3H)-dione (37): Yield: 45%; mp: 2208C;
1H NMR (CDCl3): d=7.35–7.29 (m, 1H, C4-H, phe), 6.99–6.93 (m,
3
2H, C3-/C5-H, phe), 4.40 (t, J=5.4 Hz, 2H, C6-H2), 3.98 (s, 2H, N8-
CH2), 3.86 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.38 (s, 3H, N3-CH3),
8-(2-Chloro-4-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
3
3.04 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=162.0 (d,
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (42): Yield: 70%; mp:
1JC,F =249.2 Hz, phe), 161.9 (d, 1JC,F =249.3 Hz, phe), 154.9 (C9a),
2238C; H NMR (CDCl3): d=7.46–7.43 (m, 1H, C3-H, phe), 7.16–7.14
1
3
3
151.7 (C4), 148.4 (C2), 147.4 (C10a), 130.4 (dd, JC,F =10.5 Hz, JC,F
=
(m, 1H, C6-H, phe), 7.01–6.98 (m, 1H, C5-H, phe), 4.37 (t, 3J=
10.5 Hz, C4, phe), 111.5 (d, 2JC,F =25.9 Hz, C3/C5, phe), 111.5 (d,
2JC,F =15.7 Hz, C1, phe), 106.5 (C4a), 50.1 (N8-CH2), 48.4 (C7), 47.9
(C9), 44.0 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive
mode 362.8 [M+H]+; HPLC: 97.4% (A) and 97.8% (B).
5.3 Hz, 2H, C6-H2), 3.85 (s, 2H, N8-CH2), 3.82 (s, 2H, C9-H2), 3.53 (s,
3
3H, N1-CH3), 3.38 (s, 3H, N3-CH3), 3.03 ppm (t, J=5.3 Hz, 2H, C7-
H2); 13C NMR (CDCl3): d=161.9 (d, 1JC,F =250.4 Hz, C4, phe), 155.0
3
(C9a), 151.7 (C4), 148.4 (C2), 147.6 (C10a), 135.2 (d, JC,F =10.2 Hz,
4
C2, phe), 132.0 (d, JC,F =8.2 Hz, C6, phe), 130.0 (C1, phe), 117.1 (d,
8-(2,4-Difluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino-
3JC,F =24.6 Hz, C3, phe), 114.3 (d, 3JC,F =21.0 Hz, C5, phe), 106.6
(C4a), 57.7 (N8-CH2), 51.0 (C7), 48.8 (C9), 44.2 (C6), 29.7 (N1-CH3),
27.9 ppm (N3-CH3); ESI-MS: positive mode 378.1 [M+H]+; HPLC:
99.9% (A) and 99.9% (B).
[2,1-f]purine-2,4(1H,3H)-dione (38): Purification by column chro-
1
matography. Yield: 43%; mp: 2418C; H NMR (CDCl3): d=7.41–7.37
3
(m, 1H, C3-H, phe), 6.91–6.82 (m, 2H, C5-/C6-H, phe), 4.38 (t, J=
5.1 Hz, 2H, C6-H2), 3.82 (s, 2H, N8-CH2), 3.80 (s, 2H, C9-H2), 3.53 (s,
3
3H, N1-CH3), 3.38 (s, 3H, N3-CH3), 3.02 ppm (t, J=4.9 Hz, 2H, C7-
8-(4-Chloro-2-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (43): Yield: 53%; mp:
H2); 13C NMR (CDCl3): d=162.7 (dd, 1JC,F =249.8 Hz, 3JC,F =12.0 Hz,
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemMedChem 2014, 9, 1704 – 1724 1718

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1
1978C; H NMR (CDCl3): d=7.44–7.41 (m, 1H, C3-H, phe), 7.18–7.12
phe), 106.6 (C4a), 60.8 (N8-CH2), 51.3 (C7), 48.8 (C9), 44.2 (C6), 29.7
(N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive mode 378.6 [M+H]+
; HPLC: 99.9% (A) and 99.9% (B).
3
(m, 2H, C5-/C6-H, phe), 4.42 (t, J=4.8 Hz, 2H, C6-H2), 3.88 (s, 2H,
N8-CH2), 3.85 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.38 (s, 3H, N3-
CH3), 3.04 ppm (brs, 2H, C7-H2); 13C NMR (CDCl3): d=161.1 (d,
1JC,F =250.6 Hz, C2, phe), 154.9 (C9a), 151.7 (C4), 148.5 (C2), 146.7
(C10a), 135.1 (d, JC,F =6.2 Hz, C4, phe), 132.4 (d, JC,F =2.3 Hz, C6,
phe), 125.0 (d, 4JC,F =2.9 Hz, C5, phe), 120.7 (d, 2JC,F =15.4 Hz, C1,
phe), 116.5 (d, 2JC,F =25.6 Hz, C3, phe), 106.6 (C4a), 53.7 (N8-CH2),
50.6 (C7), 48.6 (C9), 43.8 (C6), 29.8 (N1-CH3), 27.9 ppm (N3-CH3);
ESI-MS: positive mode 378.0 [M+H]+; HPLC: 99.9% (A) and 99.9%
(B).
8-(3-Fluoro-5-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (48): Yield:
3
3
1
55%; mp: 2268C; H NMR (CDCl3): d=7.43 (s, 1H, C6-H, phe), 7.33
(d, 3JH,F =8.4 Hz, 1H, C4-H, phe), 7.29 (d, 3JH,F =8.3 Hz, 1H, C2-H,
phe), 4.41 (t, 3J=5.4 Hz, 2H, C6-H2), 3.83 (s, 2H, N8-CH2), 3.79 (s,
2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.40 (s, 3H, N3-CH3), 3.02 ppm (t,
1
3J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=162.7 (d, JC,F
=
249.8 Hz, C3, phe), 155.0 (C9a), 151.7 (C4), 148.5 (C2), 147.1 (C10a),
3
140.5 (d, 3JC,F =7.3 Hz, C1, phe), 132.9 (dq, 2JC,F =33.2 Hz, JC,F
=
8-(2-Chloro-5-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
1
8.2 Hz, C5, phe), 123.1 (q, JC,F =272.7 Hz, CF3), 121.3–121.2 (m, C6,
phe), 119.1 (d, 2JC,F =21.7 Hz, C2, phe), 112.5 (dq, 2JC,F =24.5 Hz,
3JC,F =3.5 Hz, C4, phe), 106.7 (C4a), 60.9 (N8-CH2), 51.2 (C7), 48.9
(C9), 44.1 (C6), 29.8 (N1-CH3), 27.9 ppm (N3-CH3); ESI-MS: positive
mode 412.4 [M+H]+; HPLC: 99.9% (A) and 99.8% (B).
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (44): Purification by
column chromatography. Yield: 40%; mp: 2428C; H NMR (CDCl3):
1
d=7.36–7.34 (m, C3-H, phe), 7.27–7.24 (m, 1H, C4-H, phe), 6.99–
3
6.95 (m, 1H, C6-H, phe), 4.40 (t, J=5.4 Hz, 2H, C6-H2), 3.88 (s, 2H,
N8-CH2), 3.86 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-
3
CH3), 3.06 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=161.5
8-(2-Fluoro-3-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
(d, 1JC,F =247.2 Hz, C5, phe), 155.0 (C9a), 151.7 (C4), 148.4 (C2),
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (49): Yield:
3
3
147.3 (C10a), 136.1 (d, JC,F =6.0 Hz, C1, phe), 131.0 (d, JC,F =8.1 Hz,
C3, phe), 129.0 (d, 4JC,F =3.0 Hz, C2, phe), 117.5 (d, 2JC,F =23.7 Hz,
C6, phe), 116.1 (d, 2JC,F =22.9 Hz, C4, phe), 106.6 (C4a), 58.0 (N8-
CH2), 51.1 (C7), 49.0 (C9), 44.2 (C6), 29.8 (N1-CH3), 27.9 ppm (N3-
CH3); ESI-MS: positive mode 378.6 [M+H]+; HPLC: 99.9% (A) and
99.9% (B).
1
60%; mp: 1798C; H NMR (CDCl3): d=7.78–7.75 (m, 1H, C4-H, phe),
7.63–7.61 (m, 1H, C5-H, phe), 7.31–7.28 (m, 1H, C6-H, phe), 4.47 (t,
3J=5.2 Hz, 2H, C6-H2), 3.99 (s, 2H, N8-CH2), 3.90 (s, 2H, C9-H2), 3.54
(s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3), 3.13 ppm (t, 3J=5.2 Hz, 2H,
C7-H2); 13C NMR (CDCl3): d=158.3 (d, 1JC,F =256.6 Hz, C2, phe),
154.9 (C9a), 151.7 (C4), 148.5 (C2), 146.2 (C10a), 135.6 (C6, phe),
4
127.5–127.4 (m, C4, phe), 124.4 (d, JC,F =4.1 Hz, C5, phe), 122.4 (q,
8-(2-Chloro-6-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
1JC,F =272.8 Hz, CF3), 123.3 (d, 2JC,F =14.8 Hz, C1, phe), 112.4–111.8
(m, C3, phe), 106.5 (C4a), 53.5 (N8-CH2), 50.5 (C7), 48.8 (C9), 43.7
(C6), 29.8 (N1-CH3), 27.9 ppm (N3-CH3); ESI-MS: positive mode
413.0 [M+H]+; HPLC: 98.4% (A) and 98.4% (B).
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (45): Yield: 60%; mp:
1
2408C; H NMR (CDCl3): d=7.28–7.22 (m, 2H, C3-/C5-H, phe), 7.05–
3
4
7.01 (m, 1H, C4-H, phe), 4.35 (t, J=5.4 Hz, 2H, C6-H2), 3.96 (d, J=
2.0 Hz, 2H, N8-CH2), 3.87 (s, 2H, C9-H2), 3.52 (s, 3H, N1-CH3), 3.37
(s, 3H, N3-CH3), 3.07 ppm (t, 3J=5.4 Hz, 2H, C7-H2); 13C NMR
(CDCl3): d=162.0 (d, 1JC,F =249.8 Hz, C6, phe), 154.9 (C9a), 151.7
8-(2-Fluoro-5-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (50): Yield:
48%; mp: 1858C; 1H NMR (CDCl3): d=7.75–7.73 (m, C6-H, phe),
3
(C4), 148.4 (C2), 147.7 (C10a), 136.5 (d, JC,F =5.3 Hz, C2, phe), 130.1
3
4
3
(d, JC,F =9.7 Hz, C4, phe), 125.7 (d, JC,F =3.2 Hz, C3, phe), 122.2 (d,
1JC,F =17.4 Hz, C1, phe), 114.2 (d, 2JC,F =23.2 Hz, C5, phe), 106.5
(C4a), 51.5 (N8-CH2), 50.7 (C7), 48.8 (C9), 44.3 (C6), 29.7 (N1-CH3),
27.8 ppm (N3-CH3); ESI-MS: positive mode 378.6 [M+H]+; HPLC:
99.9% (A) and 99.9% (B).
7.61–7.58 (m, C4-H, phe), 7.22–7.19 (m, 1H, C3-H, phe), 4.40 (t, J=
5.4 Hz, 2H, C6-H2), 3.89 (s, 2H, N8-CH2), 3.83 (s, 2H, C9-H2), 3.54 (s,
3
3H, N1-CH3), 3.39 (s, 3H, N3-CH3), 3.05 ppm (t, J=5.4 Hz, 2H, C7-
H2); 13C NMR (CDCl3): d=163.0 (d, 1JC,F =252.9 Hz, C2, phe), 155.0
(C9a), 151.7 (C4), 148.4 (C2), 147.1 (C10a), 128.7–128.6 (m, C6, phe),
127.5–126.8 (m, C4/C5, phe), 123.6 (q, 1JC,F =272.1 Hz, CF3), 124.2
8-(3-Chloro-5-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (46): Yield: 74%; mp:
1948C; 1H NMR (CDCl3): d=7.17 (s, 1H, C2-H, phe), 7.05–7.02 (m,
2
2
(d, JC,F =15.4 Hz, C1, phe), 116.3 (d, JC,F =23.5 Hz, C3, phe), 106.5
(C4a), 53.9 (N8-CH2), 50.9 (C7), 48.6 (C9), 44.2 (C6), 29.7 (N1-CH3),
27.8 ppm (N3-CH3); ESI-MS: positive mode 412.0 [M+H]+; HPLC:
99.7% (A) and 98.9% (B).
3
C4-/C6-H, phe), 4.40 (t, J=5.3 Hz, 2H, C6-H2), 3.78 (s, 2H, N8-CH2),
3.75 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3),
3
3.00 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=162.8 (d,
8-(4-Fluoro-2-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
1JC,F =250.4 Hz, C5, phe), 155.0 (C9a), 151.7 (C4), 148.4 (C2), 147.2
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (51): Yield:
3
3
1
(C10a), 140.1 (d, JC,F =5.2 Hz, C1, phe), 135.3 (d, JC,F =10.6 Hz, C3,
phe), 124.8 (d, 4JC,F =2.0 Hz, C2, phe), 115.8 (d, 2JC,F =24.8 Hz, C6,
phe), 114.2 (d, 2JC,F =21.7 Hz, C4, phe), 106.6 (C4a), 60.8 (N8-CH2),
51.1 (C7), 48.9 (C9), 44.1 (C6), 29.8 (N1-CH3), 27.9 ppm (N3-CH3);
ESI-MS: positive mode 378.6 [M+H]+; HPLC: 99.9% (A) and 99.9%
(B).
73%; mp: 2488C; H NMR (CDCl3): d=7.95–7.92 (m, 1H, C6-H, phe),
3
7.42–7.40 (m, 1H, C5-H, phe), 7.32–7.28 (m, C3-H, phe), 4.47 (t, J=
5.4 Hz, 2H, C6-H2), 4.00 (s, 2H, N8-CH2), 3.89 (s, 2H, C9-H2), 3.54 (s,
3
3H, N1-CH3), 3.40 (s, 3H, N3-CH3), 3.10 ppm (t, J=5.4 Hz, 2H, C7-
H2); ESI-MS: positive mode 412.0 [M+H]+; HPLC: 98.8% (A) and
98.9% (B).
8-(4-Chloro-3-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
8-(4-Fluoro-3-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (52): Yield:
66%; mp: 1808C; 1H NMR (CDCl3): d=7.61–7.59 (m, C6-H, phe),
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (47): Yield: 71%; mp:
1
2018C; H NMR (CDCl3): d=7.36–7.32 (m, 1H, C5-H, phe), 7.15 (d,
3JH,F =9.2 Hz, 1H, C2-H, phe), 7.05 (d, 3J=8.0 Hz, 1H, C6-H, phe),
7.56–7.53 (m, C2-H, phe), 7.21–7.17 (m, 1H, C5-H, phe), 4.36 (t, J=
3
3
4.33 (t, J=5.2 Hz, 2H, C6-H2), 3.72 (s, 2H, N8-CH2), 3.70 (s, 2H, C9-
5.5 Hz, 2H, C6-H2), 3.76 (s, 2H, N8-CH2), 3.74 (s, 2H, C9-H2), 3.53 (s,
H2), 3.51 (s, 3H, N1-CH3), 3.36 (s, 3H, N3-CH3), 2.93 ppm (t, 3J=
5.3 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=158.2 (d, 1JC,F =249.6 Hz,
C3, phe), 154.9 (C9a), 151.7 (C4), 148.4 (C2), 147.6 (C10a), 137.7 (d,
3JC,F =6.2 Hz, C1, phe), 130.7 (C6, phe), 125.0 (d, 3JC,F =3.3 Hz, C5,
3H, N1-CH3), 3.38 (s, 3H, N3-CH3), 2.97 ppm (t, J=5.5 Hz, 2H, C7-
3
1
3
H2); 13C NMR (CDCl3): d=159.3 (dd, JC,F =256.7 Hz, JC,F =1.5 Hz, C4,
phe), 155.0 (C9a), 151.7 (C4), 148.4 (C2), 147.5 (C10a), 134.1 (d,
4
3JC,F =8.3 Hz, C6, phe), 133.0 (d, JC,F =3.7 Hz, C1, phe), 127.4–127.3
2
2
1
2
phe), 120.3 (d, JC,F =17.7 Hz, C4, phe), 116.8 (d, JC,F =21.3 Hz, C2,
(m, C2, phe), 122.4 (q, JC,F =272.5 Hz, CF3), 118.6 (qd, JC,F =33.1 Hz,
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2
2JC,F =12.8 Hz, C3, phe), 117.2 (d, 2JC,F =20.7 Hz, C5, phe), 106.5
(C4a), 60.7 (N8-CH2), 51.2 (C7), 48.9 (C9), 44.1 (C6), 29.8 (N1-CH3),
27.9 ppm (N3-CH3); ESI-MS: positive mode 412.0 [M+H]+; HPLC:
99.9% (A) and 99.8% (B).
(C1, phe), 132.2 (C5, phe), 131.9 (C6, phe), 130.3 (q, JC,F =31.1 Hz,
3
1
C2, phe), 126.4 (q, JC,F =9.4 Hz, C3, phe), 123.4 (q, JC,F =274.4 Hz,
CF3), 106.6 (C4a), 56.8 (N8-CH2), 51.4 (C7), 49.0 (C9), 44.4 (C6), 29.7
(N1-CH3), 27.9 ppm (N3-CH3); ESI-MS: positive mode 428.0 [M+H]+
; HPLC: 98.9% (A) and 99.4% (B).
8-(3-Fluoro-4-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (53): Yield:
8-(2-Chloro-5-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
1
69%; mp: 2118C; H NMR (CDCl3): d=7.59–7.56 (m, 1H, C5-H, phe),
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (58): Yield:
3
1
7.26–7.23 (m, C2-/C6-H, phe), 4.37 (t, J=5.4 Hz, 2H, C6-H2), 3.79 (s,
79%; mp: 2028C; H NMR (CDCl3): d=7.75 (s, 1H, C6-H, phe), 7.53–
3
2H, N8-CH2), 3.76 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.38 (s, 3H,
7.49 (m, 2H, C3-/C4-H, phe), 4.39 (t, J=5.4 Hz, 2H, C6-H2), 3.90 (s,
3
N3-CH3), 2.98 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=
2H, N8-CH2), 3.84 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H,
1
3
3
159.0 (dd, JC,F =256.9 Hz, JC,F =1.5 Hz, C3, phe), 155.0 (C9a), 151.7
(C4), 148.4 (C2), 147.4 (C10a), 143.1 (d, 3JC,F =7.4 Hz, C1, phe),
N3-CH3), 3.04 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=
155.0 (C9a), 151.7 (C4), 148.4 (C2), 147.5 (C10a), 138.1 (C2, phe),
135.6 (C1, phe), 130.3 (C3, phe), 129.6 (q, 2JC,F =33.0 Hz, C5, phe),
127.3 (q, 3JC,F =3.4 Hz, C6, phe), 125.7 (q, 3JC,F =3.5 Hz, C4, phe),
1
126.5–126.4 (m, C5 Phe), 123.1 (d, JC,F =3.3 Hz, C6, phe), 121.5 (q,
1JC,F =272.1 Hz, CF3), 116.9 (qd, 1JC,F =33.1 Hz 1JC,F =12.5 Hz, C4,
phe), 115.9 (d, 2JC,F =21.0 Hz, C2, phe), 105.6 (C4a), 60.8 (N8-CH2),
51.3 (C7), 48.9 (C9), 44.2 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3);
ESI-MS: positive mode 411.9 [M+H]+; HPLC: 99.9% (A) and 99.9%
(B).
1
123.6 (q, JC,F =272.3 Hz, CF3), 106.6 (C4a), 58.0 (N8-CH2), 51.2 (C7),
49.0 (C9), 44.2 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: posi-
tive mode 427.9 [M+H]+; HPLC: 99.3% (A) and 99.5% (B).
8-(3-Fluoro-4-methoxybenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
8-(2-Fluoro-4-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (59): Purification by
1
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (54): Purifica-
column chromatography. Yield: 43%; mp: 1848C; H NMR (CDCl3):
1
tion by column chromatography. Yield: 43%; mp: 1858C; H NMR
d=7.10 (dd, 4J=2.0 Hz and 3JH,F =11.9 Hz, 1H, C2-H, phe), 7.04–
3
(CDCl3): d=7.18–7.14 (m, 2H, C5-/C6-H, phe), 6.99–6.96 (m, 1H, C3-
7.02 (m, 1H, C6-H, phe), 6.93–6.90 (m, 1H, C5-H, phe), 4.35 (t, J=
3
H, phe), 4.40 (t, J=5.1 Hz, 2H, C6-H2), 3.86 (s, 2H, N8-CH2), 3.83 (s,
5.5 Hz, 2H, C6-H2), 3.89 (s, 3H, OCH3), 3.73 (s, 2H, N8-CH2), 3.67 (s,
2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.38 (s, 3H, N3-CH3), 2.94 ppm (t,
3J=5.5 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0 (C9a), 152.5 (d,
1JC,F =246.5 Hz, C3, phe), 151.7 (C4), 148.5 (C2), 147.6 (C10a), 147.3
2H, C9-H2), 3.51 (s, 3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 3.04 ppm (t,
3J=5.1 Hz, 2H, C7-H2); ESI-MS: positive mode 412.0 [M+H]+;
HPLC: 99.1% (A) and 98.8% (B).
2
3
(d, JC,F =10.7 Hz, C4, phe), 129.6 (d, JC,F =5.9 Hz, C1, phe), 124.6 (d,
4JC,F =3.3 Hz, C6, phe), 116.6 (d, 2JC,F =18.4 Hz, C2, phe), 113.3 (d,
4JC,F =1.5 Hz, C5, phe), 106.6 (C4a), 61.0 (N8-CH2), 56.3 (OCH3), 51.2
(C7), 48.7 (C9), 44.3 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS:
positive mode 374.0 [M+H]+; HPLC: 99.7% (A) and 98.8% (B).
8-(5-Fluoro-2-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (55): Yield:
78%; mp: 2208C; 1H NMR (CDCl3): d=7.67 (dd, 3J=8.7 Hz, JH,F
=
4
4
3
5.3 Hz, 1H, C3-H, phe), 7.49 (dd, J=2.4 Hz, JH,F =9.6 Hz, 1H, C6-H,
3
phe), 7.09–7.06 (m, 1H, C4-H, phe), 4.39 (t, J=5.5 Hz, 2H, C6-H2),
3.91 (s, 2H, N8-CH2), 3.81 (s, 2H, C9-H2), 3.55 (s, 3H, N1-CH3), 3.40
(s, 3H, N3-CH3), 3.00 ppm (t, 3J=5.5 Hz, 2H, C7-H2); 13C NMR
(CDCl3): d=164.9 (d, 1JC,F =253.3 Hz, C5, phe), 155.0 (C9a), 151.8
8-(3-Bromo-4-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (60): Purification by
1
column chromatography. Yield: 45%; mp: 2488C; H NMR (CDCl3):
3
4
4
(C4), 148.5 (C2), 147.5 (C10a), 139.5 (d, JC,F =7.9 Hz, C1, phe), 128.6
d=7.57 (dd, J=2.1 Hz, JH,F =6.6 Hz, 1H, C2-H, phe), 7.30–7.27 (m,
1H, C6-H, phe), 7.11–7.08 (m, 1H, C5-H, phe), 4.38 (t, 3J=5.4 Hz,
2H, C6-H2), 3.76 (s, 2H, N8-CH2), 3.72 (s, 2H, C9-H2), 3.52 (s, 3H, N1-
CH3), 3.37 (s, 3H, N3-CH3), 2.99 ppm (t, 3J=5.4 Hz, 2H, C7-H2);
3
3
2
(dq, JC,F =11.6 Hz, JC,F =5.6 Hz, C3, phe), 124.8 (qd, JC,F =30.9 Hz,
4JC,F =5.1 Hz, C2, phe), 124.0 (q, JC,F =273.1 Hz, CF3), 117.2 (d, JC,F
=
1
2
2
23.4 Hz, C4, phe), 114.6 (d, JC,F =22.0 Hz, C6, phe), 106.7 (C4a), 57.0
(N8-CH2), 51.4 (C7), 49.1 (C9), 44.4 (C6), 29.7 (N1-CH3), 27.9 ppm
(N3-CH3); ESI-MS: positive mode 412.0 [M+H]+; HPLC: 99.9% (A)
and 99.9% (B).
1
13C NMR (CDCl3): d=158.8 (d, JC,F =248.2 Hz, C2, phe), 155.0 (C9a),
4
151.7 (C4), 148.5 (C2), 147.2 (C10a), 134.0 (C2, phe), 133.5 (d, JC,F
=
=
2
3.4 Hz, C1, phe), 129.5 (d, 3JC,F =6.7 Hz, C6, phe), 116.7 (d, JC,F
1
22.4 Hz, C5, phe), 109.4 (d, JC,F =21.0 Hz, C3, phe), 106.5 (C4a), 60.5
(N8-CH2), 51.0 (C7), 48.8 (C9), 44.0 (C6), 29.8 (N1-CH3), 27.9 ppm
(N3-CH3); ESI-MS: positive mode 422.0 and 424.0 [M+H]+; HPLC:
99.9% (A) and 99.9% (B).
8-(4-Chloro-3-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (56): Yield:
1
65%; mp: 1938C; H NMR (CDCl3): d=7.67 (s, 1H, C2-H, phe), 7.50–
3
7.49 (m, 2H, C5-/C6-H, phe), 4.37 (t, J=5.3 Hz, 2H, C6-H2), 3.78 (s,
2H, N8-CH2), 3.75 (s, 2H, C9-H2), 3.51 (s, 3H, N1-CH3), 3.38 (s, 3H,
8-(5-Bromo-2-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (61): Yield: 71%; mp:
2098C; 1H NMR (CDCl3): d=7.54 (dd, 4J=2.5 Hz, 4JH,F =6.3 Hz, 1H,
C6-H, phe), 7.42–7.39 (m, 1H, C4-H, phe), 7.00–6.96 (m, 1H, C3-H,
3
N3-CH3), 2.98 ppm (t, J=5.3 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=
155.0 (C9a), 151.7 (C4), 148.4 (C2), 147.3 (C10a), 135.9 (C1, phe),
133.1 (C4/C6, phe), 131.8 (C5, phe), 128.7 (q, 2JC,F =31.3 Hz, C3,
3
1
3
phe), 127.8 (q, JC,F =4.9 Hz, C2, phe), 122.7 (q, JC,F =273.4 Hz, CF3),
106.6 (C4a), 60.7 (N8-CH2), 51.2 (C7), 48.9 (C9), 44.2 (C6), 29.7 (N1-
CH3), 27.8 ppm (N3-CH3); ESI-MS: positive mode 428.0 [M+H]+;
HPLC: 98.9% (A) and 99.2% (B).
phe), 4.38 (t, J=5.5 Hz, 2H, C6-H2), 3.79 (brs, 4H, N8-CH2, C9-H2),
3
3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3), 3.00 ppm (t, J=5.5 Hz,
1
2H, C7-H2); 13C NMR (CDCl3): d=160.3 (d, JC,F =247.4 Hz, C2, phe),
155.0 (C9a), 151.7 (C4), 148.5 (C2), 147.6 (C10a), 133.8 (C6, phe),
132.5 (d, 3JC,F =8.4 Hz, C4, phe), 125.6 (d, 2JC,F =15.6 Hz, C1, phe),
117.4 (d, 2JC,F =23.8 Hz, C3, phe), 116.9 (d, 4JC,F =3.6 Hz, C5, phe),
106.5 (C4a), 53.9 (N8-CH2), 51.0 (C7), 48.9 (C9), 44.3 (C6), 29.7 (N1-
CH3), 27.9 ppm (N3-CH3); ESI-MS: positive mode 422.0 and 424.0
[M+H]+; HPLC: 99.9% (A) and 99.9% (B).
8-(4-Chloro-2-(trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (57): Yield:
1
3
62%; mp: 1748C; H NMR (CDCl3): d=7.71 (d, J=8.4 Hz, 1H, C6-H,
4
3
4
phe), 7.66 (d, J=2.1 Hz, 1H, C3-H, phe), 7.52 (dd, J=8.4 Hz, J=
2.1 Hz, 1H, C5-H, phe), 4.36 (t, J=5.4 Hz, 2H, C6-H2), 3.88 (s, 2H,
3
N8-CH2), 3.79 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-
8-(3-Fluoro-4-methylbenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
3
CH3), 2.98 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (62): Yield: 70%; mp:
1
(C9a), 151.7 (C4), 148.5 (C2), 147.6 (C10a), 134.4 (C4, phe), 133.7
2128C; H NMR (CDCl3): d=7.16–7.12 (m,1H, C5-H, phe), 7.02–6.99
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ChemMedChem 2014, 9, 1704 – 1724 1720

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3
(m, C2-/C6-H, 2H, phe), 4.34 (t, J=5.4 Hz, 2H, C6-H2), 3.74 (s, 2H,
48.6 (C9), 44.3 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: posi-
N8-CH2), 3.70 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-
tive mode 394.3 [M+H]+; HPLC: 98.1% (A) and 98.6% (B).
CH3), 2.95 (t, 3J=5.4 Hz, 2H, C7-H2), 2.24 ppm (C4-H3, phe);
13C NMR (CDCl3): d=161.4 (d, JC,F =245.3 Hz, C3, phe), 155.0 (C9a),
1
8-(Benzo[d][1,3]dioxol-5-ylmethyl)-1,3-dimethyl-6,7,8,9-tetra-
hydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (67): Yield: 71%;
3
151.8 (C4), 148.5 (C2), 148.0 (C10a), 136.3 (d, JC,F =7.1 Hz, C1, phe),
1
131.5 (d, 3JC,F =5.4 Hz, C5, phe), 124.4 (d, 2JC,F =17.2 Hz, C4, phe),
124.2 (d, 4JC,F =3.1 Hz, C6, phe), 115.4 (d, 2JC,F =22.5 Hz, C2, phe),
106.6 (C4a), 54.0 (N8-CH2), 51.2 (C7), 48.8 (C9), 44.3 (C6), 29.7 (N1-
mp: 2008C; H NMR (CDCl3): d=6.85 (s, 1H, C3-H, phe), 6.77–6.76
(m, 2H, C5-/C6-H, phe), 5.95 (s, 2H, O-CH2-O, phe), 4.34 (t, 3J=
5.4 Hz, 2H, C6-H2), 3.72 (s, 2H, N8-CH2), 3.66 (s, 2H, C9-H2), 3.53 (s,
3
4
3H, N1-CH3), 3.39 (s, 3H, N3-CH3), 2.94 ppm (t, J=5.5 Hz, 2H, C7-
CH3), 27.8 (N3-CH3), 14.3 ppm (d, JC,F =3.4 Hz, CH3); ESI-MS: posi-
tive mode 358.0 [M+H]+; HPLC: 99.9% (A) and 99.9% (B).
H2); 13C NMR (CDCl3): d=155.0 (C9a), 151.7 (C4), 148.5 (C2), 148.0
(C3, phe), 147.9 (C10a), 147.2 (C4, phe), 130.3 (C1, phe), 122.3 (C6,
phe), 109.2 (C5, phe), 108.1 (C2, phe), 106.6 (C4a), 101.0 (O-CH2-O),
61.7 (N8-CH2), 51.2 (C7), 48.7 (C9), 44.4 (C6), 29.7 (N1-CH3),
27.8 ppm (N3-CH3); ESI-MS: positive mode 384.3 [M+H]+; HPLC:
96.5% (A) and 96.7% (B).
8-(3-Fluoro-5-methylbenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (63): Yield: 75%; mp:
1628C; H NMR (CDCl3): d=6.92 (s, 1H, C6-H, phe), 6.89 (d, JH,F
9.3 Hz, 1H, C4-H, phe), 6.81 (d, JH,F =9.5 Hz, 1H, C2-H, phe), 4.36 (t,
3J=5.5 Hz, 2H, C6-H2), 3.74 (s, 2H, N8-CH2), 3.69 (s, 2H, C9-H2), 3.54
(s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3), 2.96 (t, J=5.5 Hz, 2H, C7-H2),
2.32 ppm (C5-H3, phe); 13C NMR (CDCl3): d=163.0 (d, JC,F
245.8 Hz, C3, phe), 155.0 (C9a), 151.7 (C4), 148.5 (C2), 147.9 (C10a),
140.6 (d, 3JC,F =8.0 Hz, C5, phe), 138.8 (d, 3JC,F =7.7 Hz, C1, phe),
125.2 (d, 4JC,F =2.2 Hz, C6, phe), 115.4 (d, 2JC,F =21.0 Hz, C2, phe),
112.7 (d, 2JC,F =21.6 Hz, C4, phe), 106.5 (C4a), 61.5 (N8-CH2), 51.3
(C7), 48.9 (C9), 44.3 (C6), 29.7 (N1-CH3), 27.9 (N3-CH3), 21.3 ppm
(CH3); ESI-MS: positive mode 412.0 [M+H]+; HPLC: 99.9% (A) and
99.8% (B).
1
3
=
3
3
(R,S)-8-(1-(3,4-Dichlorophenyl)ethyl)-1,3-dimethyl-6,7,8,9-tetra-
hydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (68): Yield: 62%;
1
=
1
4
mp: 2088C; H NMR (CDCl3): d=7.44 (d, J=1.9 Hz, 1H, C2-H, phe),
3
3
4
7.40 (d, J=8.2 Hz, 1H, C5-H, phe), 7.17 (dd, J=8.2 Hz, J=2.4 Hz,
1H, C6-H, phe), 4.33 (t, J=5.4 Hz, 2H, C6-H2), 3.88 (brs, 1H, N8-
3
CH), 3.68 (s, 2H, C9-H2), 3.51 (s, 3H, N1-CH3), 3.36 (s, 3H, N3-CH3),
2.93 (t, 3J=5.4 Hz, 2H, C7-H2), 1.58 ppm (brs, 3H, CH3); ESI-MS:
positive mode 408.1 [M+H]+; HPLC: 98.7% (A) and 99.3% (B).
8-(3,4,5-Trifluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (69): Yield: 61%; mp:
8-(3,5-bis(Trifluoromethyl)benzyl)-1,3-dimethyl-6,7,8,9-tetra-
hydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (64): Yield: 60%;
mp: 2288C; 1H NMR (CDCl3): d=7.84–7.83 (m, 3H, C2-/C4-/C6-H,
phe), 4.39 (t, 3J=5.4 Hz, 2H, C6-H2), 3.87 (s, 2H, N8-CH2), 3.78 (s,
2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3), 3.01 ppm (t,
3J=5.4 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.0 (C9a), 151.7 (C4),
1
2208C; H NMR (CDCl3): d=7.03–7.00 (m, 2H, C2-/C6-H, phe), 4.37
3
(t, J=5.5 Hz, 2H, C6-H2), 3.72 (s, 2H, N8-CH2), 3.69 (s, 2H, C9-H2),
3
3.54 (s, 3H, N1-CH3), 3.40 (s, 3H, N3-CH3), 2.97 ppm (t, J=5.5 Hz,
2H, C7-H2); 13C NMR (CDCl3): d=155.0 (C9a), 151.7 (C4), 151.4 (ddd,
2
3
1JC,F =248.1 Hz, JC,F =10.1 Hz, JC,F =3.7 Hz, C3/C5, phe), 148.5 (C2),
147.4 (C10a), 139.5 (dd, 1JC,F =252.1 Hz, 2JC,F =15.4 Hz, C4, phe),
148.5 (C2), 147.2 (C10a), 139.6 (C1, phe), 132.1 (q, 2JC,F =33.4 Hz,
2
3
1
C3/C5, phe), 128.8 (q, 3JC,F =2.0 Hz, C2/C6, phe), 123.2 (q, JC,F
=
133.2–133.0 (m, C1, phe), 112.2 (dd, JC,F =16.3 Hz, JC,F =5.0 Hz, C2/
C6, phe), 106.6 (C4a), 60.6 (N8-CH2), 51.2 (C7), 48.8 (C9), 44.2 (C6),
29.8 (N1-CH3), 27.9 ppm (N3-CH3); ESI-MS: positive mode 380.0
[M+H]+; HPLC: 99.6% (A) and 99.3% (B).
272.8 Hz, CF3), 122.0–121.9 (m, C4, phe), 106.6 (C4a), 60.8 (N8-CH2),
51.3 (C7), 49.0 (C9), 44.1 (C6), 29.7 (N1-CH3), 27.9 ppm (N3-CH3);
ESI-MS: positive mode 462.0 [M+H]+; HPLC: 99.7% (A) and 99.6%
(B).
8-(2,4,6-Trifluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (70): Yield: 65%; mp:
8-(2,3-Dimethoxybenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
1
2688C; H NMR (CDCl3): d=6.73–6.70 (m, 2H, C2-/C6-H, phe), 4.34
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (65): Purification by
3
1
(t, J=5.5 Hz, 2H, C6-H2), 3.87 (s, 2H, N8-CH2), 3.79 (s, 2H, C9-H2),
column chromatography. Yield: 34%; mp: 1728C; H NMR (CDCl3):
3
d=6.87 (dd, 3J=7.9 Hz, 3J=7.9 Hz, 1H, C5-H, phe), 6.92 (dd, 3J=
3.53 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3), 3.00 ppm (t, J=5.4 Hz,
1
3
2H, C7-H2); 13C NMR (CDCl3): d=162.6, (ddd, JC,F =250.6 Hz, JC,F
=
=
4
3
4
7.6 Hz, J=1.0 Hz, 1H, C4-H, phe), 6.86 (dd, J=8.2 Hz, J=1.3 Hz,
15.7 Hz, 3JC,F =15.7 Hz, C4, phe), 162.2 (ddd, 1JC,F =249.9 Hz, JC,F
3
3
1H, C6-H, phe), 4.30 (t, J=5.4 Hz, 2H, C6-H2), 3.85 (s, 3H, OCH3),
14.8 Hz, 3JC,F =11.1 Hz, C2/C6, phe), 154.9 (C9a), 151.7 (C4), 148.4
3.81 (s, 3H, OCH3), 3.77 (s, 4H, N8-CH2, C9-H2), 3.51 (s, 3H, N1-CH3),
(C2), 147.1 (C10a), 107.9 (ddd, 2JC,F =20.2 Hz, 2JC,F =20.0 Hz, JC,F
=
=
4
3
3.36 (s, 3H, N3-CH3), 2.95 ppm (t, J=5.4 Hz, 2H, C7-H2); 13C NMR
4.4 Hz, C1, phe), 106.5 (C4a), 100.4 (ddd, 2JC,F =25.4 Hz, JC,F
2
(CDCl3): d=155.0 (C9a), 152.9 (C3, phe), 151.7 (C4), 148.5 (C2),
148.3 (C2, phe), 147.9 (C10a), 130.2 (C1, phe), 124.0 (C6, phe), 122.4
(C5, phe), 111.9 (C4, phe), 106.5 (C4a), 61.0 (N8-CH2), 55.7 (2ꢃ
OCH3), 55.6 (N8-CH2), 51.3 (C7), 48.9 (C9), 44.3 (C6), 29.7 (N1-CH3),
27.9 ppm (N3-CH3); ESI-MS: positive mode 485.8 [M+H]+; HPLC:
99.9% (A) and 99.9% (B).
23.3 Hz, 4JC,F =2.6 Hz, C3/C5, phe), 50.3 (N8-CH2), 48.4 (C7), 47.7
(C9), 44.2 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive
mode 380.0 [M+H]+; HPLC: 99.9% (A) and 98.9% (B).
8-(2,3-Dichloro-6-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (71): Yield: 53%; mp:
2548C; 1H NMR (CDCl3): d=7.43 (dd, 3J=8.9 Hz, 4JH,F =5.4 Hz, 1H,
C4-H, phe), 7.03–6.99 (m, 1H, C5-H, phe), 4.33 (t, 3J=5.5 Hz, 2H,
8-(2,4-Dimethoxybenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (66): Purification by
1
4
column chromatography. Yield: 38%; mp: 1628C; H NMR (CDCl3):
C6-H2), 3.96 (d, JH,F =2.4 Hz, 2H, N8-CH2), 3.85 (s, 2H, C9-H2), 3.52
d=7.21–7.19 (m, 1H, C6-H, phe), 7.43 (d, 4J=1.9 Hz, 1H, C3-H,
(s, 3H, N1-CH3), 3.37 (s, 3H, N3-CH3), 3.06 ppm (t, 3J=5.4 Hz, 2H,
3
phe), 6.49–6.47 (m, 1H, C5-H, phe), 4.34 (t, J=5.0 Hz, 2H, C6-H2),
C7-H2); 13C NMR (CDCl3): d=160.1 (d, 1JC,F =249.5 Hz, C6, phe),
3
3.82 (s, 3H, OCH3), 3.81 (s, 3H, OCH3), 3.79 (brs, 2H, N8-CH2), 3.75
(brs, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3),
154.9 (C9a), 151.7 (C4), 148.4 (C2), 147.7 (C10a), 134.9 (d, JC,F
=
=
=
4
5.6 Hz, C2, phe), 130.5 (d, 3JC,F =9.4 Hz, C4, phe), 129.1 (d, JC,F
3
2
2
2.98 ppm (t, J=5.0 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=160.6 (C4,
3.7 Hz, C3, phe), 124.4 (d, JC,F =18.3 Hz, C1, phe), 114.9 (d, JC,F
phe), 159.0 (C4, phe), 155.1 (C9a), 151.7 (C4), 149.8 (C2), 148.4
(C10a), 131.7 (C6, phe), 116.5 (C1, phe), 106.5 (C4a), 104.2 (C5, phe),
98.6 (C3, phe), 55.5 (OCH3), 55.4 (OCH3), 54.9 (N8-CH2), 50.9 (C7),
24.7 Hz, C5, phe), 106.5 (C4a), 52.4 (N8-CH2), 50.8 (C7), 48.9 (C9),
44.3 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive mode
412.0 [M+H]+; HPLC: 99.9% (A) and 99.9% (B).
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8-(2,4-Dichloro-5-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (72): Yield: 54%; mp:
2308C; 1H NMR (CDCl3): d=7.45 (d, 4JH,F =6.6 Hz, 1H, C3-H, phe),
(C7), 48.8 (C9), 44.4 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS:
positive mode 392.0 [M+H]+; HPLC: 99.9% (A) and 99.7% (B).
8-(2,4,6-Trimethoxybenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
3
3
7.32 (d, JH,F =9.4 Hz, 1H, C6-H, phe), 4.38 (t, J=5.5 Hz, 2H, C6-H2),
3.83 (s, 2H, N8-CH2), 3.82 (s, 2H, C9-H2), 3.55 (s, 3H, N1-CH3), 3.39
(s, 3H, N3-CH3), 3.02 ppm (t, 3J=5.5 Hz, 2H, C7-H2); 13C NMR
(CDCl3): d=157.0 (d, 1JC,F =249.8 Hz, C5, phe), 155.0 (C9a), 151.7
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (77): Purification by
1
column chromatography. Yield: 27%; mp: 1718C; H NMR (CDCl3):
d=6.13 (s, 2H, C3-/C5-H, phe), 4.35 (brs, 2H, C6-H2), 3.78–3.74 (m,
13H, N8-CH2, C9-H2, 3ꢃOCH3), 3.52 (s, 3H, N1-CH3), 3.37 (s, 3H, N3-
CH3), 2.96 ppm (brs, 2H, C7-H2); 13C NMR (CDCl3): d=160.1 (C2/C6,
phe), 159.9 (C4, phe), 154.9 (C9a), 151.7 (C4), 148.4 (C2), 147.7
(C10a), 105.9 (C4a), 93.8 (C1, phe), 90.6 (C3/C5, phe), 56.0 (OCH3),
55.5 (OCH3), 55.2 (OCH3), 54.2 (N8-CH2), 51.1 (C7), 48.7 (C9), 44.0
(C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3); ESI-MS: positive mode
416.0 [M+H]+; HPLC: 99.9% (A) and 99.9% (B).
3
(C4), 148.4 (C2), 147.4 (C10a), 135.1 (d, JC,F =6.2 Hz, C1, phe), 131.1
(C3, phe), 129.1 (d, 4JC,F =3.6 Hz, C2, phe), 121.0 (d, 2JC,F =19.2 Hz,
C6, phe), 117.9 (d, 2JC,F =23.2 Hz, C4, phe), 106.6 (C4a), 57.6 (N8-
CH2), 51.2 (C7), 49.0 (C9), 44.2 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-
CH3); ESI-MS: positive mode 412.0 [M+H]+; HPLC: 99.9% (A) and
99.5% (B).
8-(4,5-Difluoro-2-methoxybenzyl)-1,3-dimethyl-6,7,8,9-tetra-
8-(3,4,5-Trimethoxybenzyl)-1,3-dimethyl-6,7,8,9-tetrahydro-
hydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (73): Yield: 45%;
pyrazino[2,1-f]purine-2,4(1H,3H)-dione (78): Purification by
column chromatography. Yield: 31%; mp: 1728C; H NMR (CDCl3):
1
3
4
1
mp: 2088C; H NMR (CDCl3): d=7.21–7.17 (dd, JH,F =10.7 Hz, JH,F
=
3
4
9.1 Hz 1H, C3-H, phe), 6.71 (dd, JH,F =12.0 Hz, JH,F =6.5 Hz, 1H, C6-
d=6.57 (s, 2H, C2-/C6-H, phe), 4.36 (brs, 2H, C6-H2), 3.84 (s, 9H,
3ꢃOCH3), 3.76 (brs, 2H, N8-CH2), 3.69 (brs, 2H, C9-H2), 3.52 (s, 3H,
N1-CH3), 3.37 (s, 3H, N3-CH3), 2.97 ppm (brs, 2H, C7-H2); 13C NMR
(CDCl3): d=154.9 (C9a), 153.4 (C3/C5, phe), 151.7 (C4), 148.4 (C2),
147.7 (C10a), 137.1 (C4, phe), 133.5 (C1, phe), 105.9 (C4a), 93.8 (C1,
phe), 90.6 (C-3/C5, phe), 62.1 (N8-CH2), 60.8 (OCH3), 56.2 (2ꢃOCH3),
51.1 (C7), 48.7 (C9), 44.2 (C6), 29.7 (N1-CH3), 27.8 ppm (N3-CH3);
ESI-MS: positive mode 416.0 [M+H]+; HPLC: 99.9% (A) and 99.9%
(B).
3
H, phe), 4.36 (t, J=5.5 Hz, 2H, C6-H2), 3.80 (s, 3H, OCH3), 3.78 (s,
2H, N8-CH2), 3.72 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H,
3
N3-CH3), 2.98 ppm (t, J=5.5 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=
155.0 (C9a), 153.8 (dd, 3JC,F =7.2 Hz, 4JC,F =1.6 Hz, C2, phe), 151.7
(C4), 149.7 (d, 1JC,F =247.8 Hz, 2JC,F =13.6 Hz, C4, phe), 148.4 (C2),
148.0 (C10a), 144.4 (dd, 1JC,F =240.8 Hz, 2JC,F =12.5 Hz, C5, phe),
3
120.7 (dd, 3JC,F =4.4 Hz, 4JC,F =4.1 Hz, C1, phe), 118.5 (d, JC,F
=
3
18.6 Hz, C6, phe), 106.6 (C4a), 100.7 (d, JC,F =21.1 Hz, C3, phe), 56.2
(N8-CH2), 54.2 (OCH3), 51.0 (C7), 48.9 (C9), 44.3 (C6), 29.7 (N1-CH3),
27.8 ppm (N3-CH3); ESI-MS: positive mode 392.0 [M+H]+; HPLC:
99.9% (A) and 99.9% (B).
Biological evaluation
Radioligand binding assays at adenosine receptors: The radioligands
were obtained from the following sources: [3H]CCPA from Amer-
sham Biosciences (58 Cimmolꢀ1), [3H]MSX-2 from Amersham Biosci-
ences (84 Cimmolꢀ1), [3H]PSB-603 from Amersham Biosciences
(73 Cimmolꢀ1), and [3H]PSB-11 (53 Cimmolꢀ1) from Quotient Biore-
search. The nonradioactive precursors of [3H]MSX-2,[56] [3H]PSB-
603,[57] and [3H]PSB-11[58] were synthesized in our laboratory. Mem-
brane preparations and radioligand binding assays at rat A1 (rat
brain cortex) and rat A2AARs (rat brain striatum) were performed as
previously described.[61,62] For assays at human A1, A2A, A2B, and
A3ARs, CHO cell membranes expressing one of the human AR sub-
types were used as previously reported.[57]
8-(2,3-Difluoro-4-methoxybenzyl)-1,3-dimethyl-6,7,8,9-tetra-
hydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (74): Purification by
1
column chromatography. Yield: 36%; mp: 2088C; H NMR (CDCl3):
d=7.06–7.02 (m, 1H, C5-H, phe), 6.76–6.73 (m, 1H, C6-H, phe),
3
4.35 (t, J=5.4 Hz, 2H, C6-H2), 3.91 (s, 3H, OCH3), 3.79 (s, 2H, N8-
CH2), 3.78 (s, 2H, C9-H2), 3.54 (s, 3H, N1-CH3), 3.39 (s, 3H, N3-CH3),
2.98 ppm (t, 3J=5.4 Hz, 2H, C7-H2); ESI-MS: positive mode 392.0
[M+H]+; HPLC: 99.9% (A) and 99.9% (B).
8-(3,5-Difluoro-4-methoxybenzyl)-1,3-dimethyl-6,7,8,9-tetra-
hydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (75): Yield: 24%;
1
3
mp: 1998C; H NMR (CDCl3): d=6.91 (d, JH,F =8.4 Hz, 2H, C2-/C6-H,
3
phe), 4.36 (t, J=5.1 Hz, 2H, C6-H2), 3.98 (s, 3H, OCH3), 3.73 (s, 2H,
Monoamine oxidase assays: The determination of MAO-A and
MAO-B inhibition was performed using commercially available re-
combinant human MAO-A and MAO-B enzymes expressed in bacu-
lovirus-infected insect cells (Sigma–Aldrich, M7316 and M7441) ap-
plying the commercially available Amplex Red monoamine oxidase
assay kit (Invitrogen A12214). The assays were performed as previ-
ously described.[59] The determination of rat MAO-B inhibition was
performed using mitochondrial-enriched fractions from male Spra-
gue–Dawley rat livers. The assays were conducted as previously de-
scribed.[56]
N8-CH2), 3.65 (s, 2H, C9-H2), 3.53 (s, 3H, N1-CH3), 3.38 (s, 3H, N3-
3
CH3), 2.95 ppm (t, J=5.2 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.7
(dd, 1JC,F =247.3 Hz, 3JC,F =6.0 Hz, C3/C5, phe), 155.0 (C9a), 151.7
2
(C4), 148.4 (C2), 147.6 (C10a), 135.8 (dd, 2JC,F =14.2 Hz, JC,F
=
14.2 Hz, C4, phe), 132.0 (dd, 3JC,F =8.1 Hz, 3JC,F =8.1 Hz, C1, phe),
2
4
112.3 (dd, JC,F =17.4 Hz, JC,F =5.7 Hz, C2/C6, phe), 106.6 (C4a), 61.8
(N8-CH2), 60.7 (OCH3), 51.2 (C7), 48.8 (C9), 44.2 (C6), 29.7 (N1-CH3),
27.8 ppm (N3-CH3); ESI-MS: positive mode 392.0 [M+H]+; HPLC:
99.9% (A) and 99.9% (B).
8-(2-Chloro-6-fluoro-3-methoxybenzyl)-1,3-dimethyl-6,7,8,9-
tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (76): Yield:
Determination of water solubility
1
55%; mp: 2388C; H NMR (CDCl3): d=7.03–6.99 (m, 1H, C5-H, phe),
6.90–6.87 (m, 1H, C4-H, phe), 4.34 (t, 3J=5.5 Hz, 2H, C6-H2), 3.97
(d, 4JH,F =244.4 Hz 2H, N8-CH2), 3.89 (s, 2H, C9-H2), 3.87 (s, 3H,
Water solubility (in mgmLꢀ1) was determined using the following
procedure: at RT, 500 mL of buffer was added to 1 mg of precisely
weighed compound and sonicated for 30 sec. The pH value was
checked and adjusted if deviating more than 0.2 pH units from the
target value. The solution was left for 1 h at RT and then filtered
through a 0.45 mm filter. A reference solution of precisely weighed
1 mg of the same compound dissolved in methanol was prepared.
Both solutions were injected into a Chromatographic Acquity UPLC
3
OCH3), 3.53 (s, 3H, N1-CH3), 3.38 (s, 3H, N3-CH3), 3.06 ppm (t, J=
5.5 Hz, 2H, C7-H2); 13C NMR (CDCl3): d=155.9 (d, 1JC,F =241.8 Hz,
C6, phe), 155.0 (C9a), 152.0 (d, 4JC,F =2.5 Hz, C3, phe), 151.7 (C4),
3
148.4 (C2), 148.0 (C10a), 124.7 (d, JC,F =5.5 Hz, C2, phe), 123.5 (d,
2
2JC,F =18.7 Hz, C1, phe), 113.6 (d, JC,F =24.6 Hz, C4, phe), 112.0 (d,
1JC,F =9.2 Hz, C4, phe), 106.6 (C4a), 56.7 (N8-CH2), 51.9 (OCH3), 50.8
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The authors thank Sabine Terhart-Krabbe and Annette Reiner for
determining NMR spectra, Marion Schneider for LC–MS analyses,
and Nicole Florin, Stephanie Biebersdorf, and Christin Vielmuth
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Keywords: adenosine receptors · antagonists · inhibitors ·
monoamine oxidases · multitarget drugs · neurodegenerative
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