Efficient synthesis of 1,3,7-substituted xanthines by a safety-catch protection strategy

Article abstract of DOI:10.1016/j.tet.2007.09.067

An efficient synthesis of selectively N-substituted xanthine derivatives is described. Cyclocondensation of a suitably protected aminoimidazole with methyl-2-phenylthioethyl carbamate, followed by oxidation of sulfur to the sulfone, provides access to an orthogonally 1,7-protected xanthine, which may then be regioselectively alkylated and deprotected under mild conditions.

Full text of DOI:10.1016/j.tet.2007.09.067

Tetrahedron 63 (2007) 12294–12302
Efficient synthesis of 1,3,7-substituted xanthines by a
safety-catch protection strategy
Matthew B. Allwood,a Booma Cannan,b Daan M. F. van Aaltena and Ian M. Egglestonb,
*
aDivision of Biological Chemistry and Drug Discovery, College of Life Sciences, University of Dundee, Dundee DD1 5EH, UK
bWolfson Laboratory of Medicinal Chemistry, Department of Pharmacy and Pharmacology, University of Bath,
Claverton Down, Bath BA2 7AY, UK
Received 26 July 2007; revised 11 September 2007; accepted 27 September 2007
Available online 29 September 2007
Abstract—An efficient synthesis of selectively N-substituted xanthine derivatives is described. Cyclocondensation of a suitably protected
aminoimidazole with methyl-2-phenylthioethyl carbamate, followed by oxidation of sulfur to the sulfone, provides access to an orthogonally
1,7-protected xanthine, which may then be regioselectively alkylated and deprotected under mild conditions.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
and nematocides, and renewing interest in their use as
asthma therapeutics.
Substituted xanthine derivatives are an important class of
pharmacologically active compounds with well-known ac-
tivity as adenosine receptor antagonists, phosphodiesterase
inhibitors and inducers of histone deacetylase activity.1
This has led to a wide range of medical applications includ-
ing the treatment of asthma, bronchitis and chronic obstruc-
tive pulmonary disease, and their use as diuretics, cardiac
stimulants and renal protective agents.2 Very recently, xan-
thine derivatives such as caffeine, theophylline and pent-
oxifylline (Fig. 1) have been identified as micromolar
inhibitors of bacterial family 18 chitinases,3 thereby opening
up potential applications for such molecules as fungicides
As part of our studies in this area, we required an efficient
route to xanthine derivatives, selectively substituted at the
1-, 3-, or 7-positions. Unfortunately, the synthesis of such
compounds by the classic Traube4 method from 6-aminoura-
cils is both lengthy and incompatible with the introduction of
complex N-substituents. An alternative approach is to use an
imidazole starting material,5 and in an interesting recent
development, described by Zavialov,6 the cyclocondensation
of a metallated 4-amino-5-alkoxycarbonyl imidazole with
in situ generated isocyanates that provide the N-1 substituent
was reported (Scheme 1). Although this gives efficient access
to the heterocyclic skeleton, and provides some scope for sub-
sequent N-3 and N-7 modifications, a key drawback is that the
N-1 substituent is fixed, thereby limiting further elaboration
at this position, and is also restricted to simple alkyl or aralkyl
groups that can withstand the strongly basic reaction con-
ditions. In order to address these issues, we envisaged that a
more flexible and powerful approach would be to generate
an orthogonally protected xanthine via the Zavialov strategy
O
O
O
O
H
N
1
7
N
N
N
N
N
N
N
N
O
N
O
N
O
N
3
Pentoxifylline
Theophylline
Caffeine
Figure 1.
H
N
OR
+
R1
R7
N
O
O
R7
O
O
KOtBu
N
R1
O
EtO
K
N
R1
N
+
R7
N
C O
N
N
N
H
N
EtO
H2N
Scheme 1. Zavialov’s6 synthesis of substituted xanthine derivatives.
R3
N
* Corresponding author. Tel.: +44 01225 383101; fax: +44 01225 386114; e-mail: ie203@bath.ac.uk
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.09.067

M. B. Allwood et al. / Tetrahedron 63 (2007) 12294–12302
12295
that might then be selectively deprotected and decorated at
each nitrogen in turn. To achieve this goal, we have therefore
devised a novel modification of this approach, that utilises
a sacrificial N-1 substituent, which is stable to the basic
cyclocondensation conditions, but may then be activated
and released (safety-catch principle) in the presence of appro-
priate N-3 and N-7 protection or substitution once the ring
system has been formed, and subsequently replaced as re-
quired. We describe herein the realisation of this principle
and its application for the synthesis of various 1,3-, 1,7- and
1,3,7-substituted xanthine derivatives.
This was chosen since it was anticipated that it should be
readily introducible into N-1 of the xanthine core via the
appropriate isocyanate, produced in situ from a carbamate
derivative, but might then be oxidised to the sulfone to pro-
vide a base-labile fragment. This approach, which is sum-
marised in Scheme 2, was inspired by the safety-catch
linker for peptide synthesis reported by Tesser,7 and it was
envisioned that such a protecting group strategy8 would be
entirely compatible with acid-labile N-7 protection such as
PMB or DMB, and allyl-based N-3 protection, cleavable
under neutral conditions by Pd(0) catalysis.
The realisation of the synthetic concept is shown in Scheme
3. Carbamate 19 was generated in 90% yield by a one-pot
procedure, by the treatment of 2-oxazolidinone with thio-
phenol and sodium propoxide in dry propanol to form 2-phe-
nylthioethylamine,10 followed by in situ trapping with
methyl chloroformate. The carbamate 1 and aminoimidazole
211 were then treated with KOtBu in diglyme at 80 ꢀC,
according to the method of Zavialov.6 This gave the desired
2. Results and discussion
2.1. Synthesis of orthogonally protected xanthine
derivatives
A promising candidate for the N-1 substituent in our pro-
jected xanthine synthesis was the 2-phenylthioethyl group.
base stable
base labile
N
C O
Zavialov chemistry
PhS
PhO2S
O
O
PhS
R7
R7
Oxidise
N
N
N
N
+
"Safety-catch
activation"
R7
O
N
H
O
N
H
O
N
N
N
EtO
H2N
N-3 modification,
then base-induced
N-1 modification
N
O
R7
N
HN
N
O
N
R3
Scheme 2. Xanthine synthesis using a 2-phenylthioethyl safety-catch.
O
HN
O
1. Na, dry nPrOH,
PhSH
O
DMB
N
2. ClCO2CH3
PhS
PhO2S
MCPBA,
O
O
EtO
DMB
DMB
N
H
N
N
N
N
N
2
OMe
H2N
PhS
1
KOtBu,diglyme,80 °C
CH2Cl2
N
N
90%
O
N
H
O
N
H
O
4
95%
3
58%
O
N
DMB
N
HN
O
KOtBu, THF,
0 °C, 10 min
6
N
64%
O
PhO2S
PhO2S
O
N
O
H
N
DMB
Allyloxymethyl
chloride
N
TFA-CH2Cl2
(9:1), RT
N
O
N
4
7
K2CO3, DMF
N
N
N
O
O
82%
O
5
95%
Pd(Ph3P)4, DMBA
CH2Cl2, reflux
4
82%
Scheme 3. Safety-catch synthesis and orthogonal protection strategy for the xanthine scaffold.

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M. B. Allwood et al. / Tetrahedron 63 (2007) 12294–12302
1,7-substituted xanthine 3 in 58% yield.12 Subsequent treat-
ment of 3 with m-CPBA in CH2Cl2 then gave the required
N-1 activated intermediate 4 in 95% yield. Finally, treatment
of 4 with allyloxymethyl chloride in DMF in the presence of
K2CO3 gave the triply protected derivative 5, in 95% yield.
Selective removal of each N-protecting group in turn, under
mild conditions, was confirmed by treatment with, respec-
tively, KOtBu (N-1), TFA (N-7) and Pd(PPh3)4, with N,N-di-
methylbarbituric acid (N,N-DMBA) as the allyl scavenger
(N-3).13 In each case the desired partially protected xan-
thines 4, 6 and 7 were generated in good to excellent yields.
PMB was also interchangeable for DMB for the N-7 pro-
tection, when introduced via the corresponding imidazole
derivative, however, its removal required much more forcing
conditions (refluxing TFA, three days14).
2.2. Synthesis of selectively 1,3-substituted xanthine
derivatives
Having established an efficient preparation of doubly pro-
tected xanthine 4, its potential for the preparation of various
1,3-substituted (theophylline-like) xanthine derivatives was
explored. As shown in Scheme 4, alkylation of 4 at N-3
with simple aliphatic and functionalised electrophiles
proceeded in excellent yields, followed by treatment with
KOtBu to give N-1 deprotected derivatives 9. These could
then be alkylated as before, followed by cleavage of DMB
protection by treatment with TFA in CH2Cl2, to give selec-
tively 1,3-substituted compounds 11 in good overall yields.
Importantly, this approach removes the need to use isocya-
nates, or precursors thereof, for the introduction of the N-1
grouping for each desired 1,3-decorated analogue at an early
stage of the synthesis. This not only widens the scope of
possible N-1 substituents, but also allows maximum flexibility
for additional modification at other sites on the xanthine
scaffold.
4
R-X
K2CO3, DMF
PhO2S
8a: R3= Me (100%)
O
DMB
8b: = Bn (100)
N
N
8c: = iPr (92)
N
8d: = CH2CH2OH (94)
8e: = CH2CH2NPht (97)
KOtBu
THF, 0 °C
O
N
R3
2.3. Synthesis of selectively 1,7- and 1,3,7-substituted
xanthine derivatives
9a: R3= Me (100%)
9b: = Bn (84)
O
DMB
N
HN
9c: = iPr (94)
The safety-catch protection strategy exemplified above also
provides flexible and efficient routes to a range of disubsti-
tuted xanthines with the less common 1,7-15 and 3,7-substi-
tution16 patterns, as well as 1,3,7-trisubstituted derivatives,
all starting from a single intermediate such as 4, or triply
orthogonally protected derivative 5. Scheme 5 shows how
6, obtained by base-mediated deprotection of 5, was em-
ployed to provide a convenient and direct route to the 1,7-
substituted (paraxanthine-like) derivative 15, which is not
readily accessible by the classic Traube method.
9d: = CH2CH2OH (71)
9e: = CH2CH2NPht (46)
N
O
N
R3
R-X
K2CO3, DMF
10a: R1= Me, R3 = Me (86%)
10b: R1= All, R3 = Me (82)
O
DMB
R1
10c: R1= Bn, R3 = Me (76)
N
N
10d: R1= Me, R3 = Bn (100)
10e: R1= Me, R3 = CH2CH2OH (87)
10f: R1= Me, R3 = CH2CH2NPht (87)
N
O
N
R3
Similarly, sequential elaboration, as previously outlined of
the N-1, N-7-orthogonally protected synthon 8c, derived
from 4, gave rise to the novel 3, 7-decorated intermediates
18a and 18b, with high efficiency in each step (Scheme 6).
Coupling of the two units through N-1 via a three-carbon
spacer was then achieved as follows. Monoalkylation of 18b
with 1,3-dibromopropane to give 19 was effected in quanti-
tative yield under phase transfer conditions using benzyltri-
ethylammonium chloride (BTEAC);17 then 19 was combined
with 18a in the usual way to generate the unsymmetrical
90% TFA-CH2Cl2
O
11a: R1= Me, R3 = Me (87%)
H
N
R1
O
11b: R1= Me, R3 = Bn (83)
N
11c: R1= Me, R3 = CH2CH2OH (41)
11d: R1= Me, R3 = CH2CH2NPht (62)
N
N
R3
Scheme 4. Synthesis of 1,3-xanthines via alkylation and safety-catch cleav-
age of 4 (yields in parentheses).
O
O
H
N
DMB
N
MeI, K2CO3
DMF, 93%
,
90% TFA-CH2Cl2
N
O
N
O
6
60%
N
N
O
N
O
N
12
13
CO2Me
CO2Me
O
O
N
N
N
N
BrCH2CH2CO2Me
K2CO3, DMF, 46%
N
Pd(PPh3)4, N,N-DMBA
CH2Cl2, reflux, 54%
N
O
13
O
N
H
O
N
15
14
Scheme 5. Preparation of 1,7-xanthines.

M. B. Allwood et al. / Tetrahedron 63 (2007) 12294–12302
12297
PhO2S
PhO2S
O
N
O
N
O
N
R7
R7
H
N
KOtBu
THF, 0 °C
N
N
R-I
90% TFA-CH2Cl2
79%
N
N
N
N
HN
8c
K2CO3, DMF
N
O
O
O
16
17a: R7= Me (95%)
17b: = Et (87)
18a: R7= Me (94%)
18b: = Et (89)
Br
1,3-dibromopropane
NaOH, BTEAC, 100 °C
100%
O
O
O
N
N
N
N
N
18a
N
N
18b
K2CO3, DMF
76%
N
N
N
19
N
O
N
O
O
20
Scheme 6. Preparation of novel 1,3,7-substituted xanthines.
dixanthine 20 in 76% yield. Such compounds (so-called
‘dicaffeines’) have previously attracted interest as models
for stacking of nucleic acid bases,18 and have recently also
been identified as potent inhibitors of family 18 chitinases,
showing similar p–p interactions with indolyl moieties in
the protein.3b Preparation of such a compound by existing
methods that do not allow exchange of the xanthine N-1
substituent,4–6would potentially require two successive
cyclocondensations using two different specifically func-
tionalised imidazole or aminopurine derivatives.
and are quoted uncorrected. 1H NMR spectra were recorded
on a Bruker Avance DPX 300 spectrometer at 300 MHz,
or a Bruker Avance 500 spectrometer at 500 MHz. 13C
NMR were recorded on the Avance 500 at 125 MHz. High
resolution electrospray mass spectra were recorded on a
Bruker MicroTOF instrument.
4.1.1. General procedures. N-1 deprotection—General
Procedure A. A solution of the protected xanthine in dry
THF (3 mL/mmol) was cooled in an ice–salt bath, and
KOtBu (2 equiv) was added (the reaction mixture darkens
immediately). When no further starting material was detect-
able by TLC (5–10 min), the reaction mixture was quenched
with saturated aq NH4Cl and the organic layer was separated.
The aqueous layer was re-extracted twice with CH2Cl2, and
the combined organic extracts were dried over MgSO4 and
evaporated under reduced pressure. The crude product was
purified by flash chromatography (0–5% MeOH/CH2Cl2).
3. Summary
We have developed a highly efficient route to selectively N-
functionalised xanthine derivatives, based on an orthogonal
safety-catch protection strategy. The key N-1 2-phenyl-
sulfonylethyl-protected intermediates may be simply and
efficiently alkylated or deprotected in the presence of acid-
labile or other suitable N-3/N-7 protection to generate a
wide range of novel functionalised xanthines, either as syn-
thons for more elaborate purine-like heterocycles,19 or pos-
sible drug leads. In this context, the scope that our method
provides for introducing novel N-1 substituents by standard
alkylation or Mitsunobu chemistry20 should further expand
the range of application of these scaffolds in medicinal
chemistry.
N-7 deprotection—General Procedure B. A solution of the
protected xanthine in 90% TFA/CH2Cl2 (10 mL/mmol)
was stirred at rt for 24 h, or until no more starting material
was detectable by TLC. The reaction mixture was evaporated
under reduced pressure, and the residue was redissolved
in CH2Cl2 (10 mL/mmol) and washed with saturated aq
NaHCO3. The organic layer was dried over MgSO4 (the
purple colouration that appears during the reaction is re-
moved on filtration) and evaporated under reduced pressure.
The crude product was purified by flash chromatography
(0–5% MeOH/CH2Cl2).
4. Experimental
4.1. General
N-3 deprotection—General Procedure C. A solution of the
protected xanthine in dry CH2Cl2 (10 mL/mmol) was treated
with N,N-DMBA (3 equiv), followed by tetrakis(triphenyl-
phosphine)palladium(0) (5 mol %). The reaction mixture
was heated at reflux for 24 h, and then it was diluted with
CH2Cl2 (to 30 mL/mmol) and washed with H2O. The
organic layer was dried over MgSO4 and evaporated under
reduced pressure. The crude product was purified by flash
chromatography (0–80% EtOAc/CH2Cl2).
All the reactions were carried out using oven-dried glass-
ware under an atmosphere of argon. THF was dried by dis-
tillation from sodium/benzophenone under argon. CH2Cl2
and diglyme were dried by distillation from calcium hydride
under argon. All other reagents and solvents were obtained
from commercial sources without further purification. Thin
layer chromatography was performed on silica gel (60 F254
Merck) pre-coated on aluminium sheets. Visualisation was
accomplished by quenching of UV fluorescence at 254 nm,
staining with vanillin or ninhydrin solutions, or iodine. Flash
chromatography was performed on columns of silica gel
(Fluorochem, Silica Gel 60, 40–60 mm).
N-alkylation—General Procedure D. A suspension of the
xanthine in dry DMF (5 mL/mmol) was treated with potas-
sium carbonate (1 equiv), and the reaction mixture was
allowed to stir at rt under Ar for 1 h. The alkyl halide
(1.2 equiv unless otherwise stated) was then added gradually
to the reaction mixture, and stirring was continued
Melting points were recorded on an Electrothermal IA9000
series digital melting point apparatus, using open capillaries,

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M. B. Allwood et al. / Tetrahedron 63 (2007) 12294–12302
overnight. If any starting material still remained, the reaction
mixture was heated at 50 ꢀC for 1 h to complete the reaction.
The reaction mixture was then poured into H2O (15 mL/
mmol) and extracted with CH2Cl2 (2ꢁ10 mL/mmol). The
organic extracts were dried over MgSO4 and evaporated
under reduced pressure, co-evaporating with toluene to re-
move residual DMF. The crude product was then purified
by flash chromatography (0–5% MeOH/CH2Cl2).
(2.45 g, 55.9 mmol) in dry CH2Cl2 (200 mL) was treated
with m-CPBA (2.42 g, 140 mmol). The reaction mixture
was allowed to stir at rt for 15 min, after which time no
starting material was visible by TLC. The reaction mixture
was quenched by washing with saturated aq NaHCO3
(100 mL), and the aqueous layer was then extracted with
CH2Cl2 (3ꢁ100 mL). The organic extracts were dried over
MgSO4 and evaporated under reduced pressure to give
a white solid. Recrystallisation from CH2Cl2/hexane gave
4 as fine white needles (2.51 g, 95%). Mp 218 ꢀC; dH
(CDCl3, 500 MHz): 3.59 (2H, t, J 7.1, SCH2), 3.83 (3H, s,
OMe), 3.87 (3H, s, OMe), 4.39 (2H, t, J 7.1, N-1 CH2N),
5.39 (2H, s, DMB CH2), 6.49–6.52 (2H, m, DMB Ar–H),
7.46–7.65 (5H, m, SO2Ph), 7.71 (1H, s, H-8), 7.98 (2H, dt,
J 8.2, 1.9, SO2Ph Ar–H), 11.27 (1H, br s, N-3 H); dC
(CDCl3, 125 MHz): 34.6, 45.5, 52.9, 55.5, 55.6, 98.8,
104.5, 115.2, 128.1 (two signals), 129.3, 132.2, 133.8,
139.0, 141.7, 146.8, 150.8, 154.9, 158.7, 161.9; found
(ES+) 493.1155 [M+Na]+, C22H22N4O6SNa requires
493.1152.
4.1.1.1. Methyl-2-phenylthioethyl carbamate (1).9 A
solution of sodium propoxide was prepared by the addition
of sodium (1.94 g, 84 mmol) to dry nPrOH (250 mL). To the
resulting solution was added thiophenol (13.1 g, 119 mmol),
and the mixture was stirred at rt for 0.5 h. 2-Oxazolidinone
(3.62 g, 41.6 mmol) was then added, and the resulting mix-
ture was heated under reflux for 5 h, over which time a fine
white precipitate formed. The reaction mixture was allowed
to cool to rt with stirring and methyl chloroformate (5 mL,
64.5 mmol) was added. After 2 h stirring at rt, additional
methyl chloroformate (5 mL, 64.5 mmol) and sodium meth-
oxide (2.25 g, 41.6 mmol) were added and stirring was con-
tinued for a further 1.5 h. The reaction mixture was then
evaporated under reduced pressure, and the residue was sus-
pended in water (100 mL) and extracted with Et2O
(2ꢁ75 mL). The organic extracts were combined and dried
over Na2SO4 and evaporated under reduced pressure. The
crude product obtained was purified by flash chromatography
(0–50% EtOAc/hexane), with positive fractions being re-
crystallised from hexane in two batches. This gave 19 as a
white crystalline solid (7.89 g, 90%). Mp 64–66 ꢀC (lit.,9
66–67 ꢀC); dH (CDCl3, 300 MHz): 3.06 (2H, t, J 6.2, CH2),
3.39 (2H, q, J 6.2, CH2), 3.66 (3H, s, COOMe), 5.06 (1H,
br s, NH), 7.14 (1H, t, J 7.5, Ar–H), 7.30 (2H, t, J 7.5, Ar–
H), 7.38 (2H, d, J 7.5, Ar–H); dC (CDCl3, 125 MHz): 33.9,
34.0, 52.2, 126.6, 129.1, 129.8, 134.9, 156.9; found (ES+)
212.0743 [M+H]+, C10H14NO2S requires 212.0740.
Compound 4 was also obtained by N-3 deprotection of 6 on
a 0.024 mmol scale, according to General Procedure C. The
sample obtained in this way (9.3 mg, 82%) was indistin-
guishable from that described above.
4.1.1.4. 3-Allyloxymethyl-1-(2-phenylsulfonylethyl)-7-
(2,4-dimethoxybenzyl)-3,7-dihydropurine-2,6-dione (5).
A suspension of 14 (200 mg, 0.43 mmol) and potassium
carbonate (59 mg, 0.43 mmol) in dry DMF (2 mL) was
stirred under Ar at rt for 1 h. After this time, allyloxymethyl
chloride12 (54.3 mg, 0.51 mmol) was added and stirring was
continued for a further 1.5 h, when no more 4 was detectable
by TLC. The reaction mixture was poured into water
(50 mL) and extracted with CH2Cl2 (2ꢁ50 mL). The or-
ganic extracts were dried over MgSO4 and evaporated under
reduced pressure. Purification of the residue by flash chro-
matography (0–30% EtOAc/CH2Cl2) gave 5 as a foaming
syrup (220 mg, 95%). dH (CDCl3, 500 MHz): 3.49 (2H, t,
J 7.2, SCH2), 3.73 (3H, s, OMe), 3.76 (3H, s, OMe), 4.07
(2H, dt, J 5.6, 1.3, OCH2C]C), 4.32 (2H, t, J 7.2, N-1
CH2N), 5.08 (1H, dt, J 7.8, 1.5, CH]CH2), 5.22 (1H, dt,
J 16.8, 1.5, CH]CH2), 5.30 (2H, s, NCH2OAll, DMB
CH2), 5.41 (2H, s, NCH2OAll, DMB CH2), 5.78–5.82 (1H,
m, CH]CH2), 6.37–6.43 (2H, m, DMB Ar–H), 7.41 (1H,
d, J 8.3, DMB Ar–H), 7.44 (2H, t, J 7.5, SO2Ph Ar–H),
7.55 (1H, t, J 7.5, SO2Ph Ar–H), 7.57 (1H, s, H-8), 7.87–
7.89 (2H, m, SO2Ph Ar–H); dC (CDCl3, 125 MHz): 35.2,
45.3, 52.9, 55.4, 55.5, 71.0, 72.2, 98.7, 104.3, 106.7,
115.7, 117.6, 128.1, 129.3, 132.1, 133.8, 133.9, 139.0,
142.2, 148.0, 151.1, 154.4, 158.6, 161.7; found (ES+)
541.1749 [M+H]+, C26H29N4O7S requires 541.1751.
4.1.1.2. 7-(2,4-Dimethoxybenzyl)-1-(2-phenylthioethyl)-
3,7-dihydropurine-2,6-dione (3). A solution of 1 (773 mg,
3.65 mmol) and 2 (930 mg, 3.05 mmol) in dry diglyme
(13 mL) at 80 ꢀC was treated with a solution of KOtBu
(476 mg, 4.25 mmol) in diglyme (2 mL), added dropwise.
The reaction mixture was stirred at 80 ꢀC overnight, and
then it was cooled to rt, poured into 5% citric acid (20 mL),
and extracted with CH2Cl2. The organic extracts were
washed with saturated aq NaHCO3, dried over MgSO4 and
evaporated under reduced pressure. Purification of the crude
material by flash chromatography (0–90% EtOAc/CH2Cl2)
gave 3 as a white solid (780 mg, 58%). Mp 155 ꢀC; dH
(CDCl3, 500 MHz): 3.16 (2H, t, J 6.5, SCH2), 3.72 (3H, s,
OMe), 3.77 (2H, s, OMe), 4.21 (2H, t, J 6.5, N-1 CH2N),
5.32 (2H, s, DMB CH2), 6.39–6.42 (2H, m, DMB Ar–H),
7.06 (1H, t, J 7.4, SPh Ar–H), 7.09 (1H, d, J 8.1, DMB
Ar–H), 7.18 (2H, t, J 7.4, SPh Ar–H), 7.37 (2H, d, J 7.4,
SPh Ar–H), 7.39 (2H, d, J 8.1, DMB Ar–H), 7.61 (1H, s,
C8 Ar–H), 11.62 (1H, br s, N-3 H); dC (CDCl3, 125 MHz):
30.3, 40.3, 45.4, 55.5, 55.6, 98.8, 104.4, 115.4, 125.8,
128.2, 128.5, 128.9, 129.1, 132.2, 135.8, 137.9, 151.4,
155.5, 158.7, 161.8; found (ES+) 439.1439 [M+H]+,
C22H23N4O4S requires 439.1435.
4.1.1.5. 3-Allyloxymethyl-7-(2,4-dimethoxybenzyl)-
3,7-dihydropurine-2,6-dione (6). Prepared from 5, using
General Procedure A. Scale: 0.317 mmol. Off-white solid
(118 mg, 64%). Mp 156 ꢀC; dH (CDCl3, 500 MHz): 3.82
(3H, s, OMe), 3.86 (3H, s, OMe), 4.21 (2H, dt, J 5.6, 1.3,
CH2C]C), 5.18 (1H, dq, J 10.5, 1.5, All C]CH2), 5.33
(1H, dq, J 17.2, 1.5, All C]CH2), 5.40 (2H, s, DMB CH2,
N-3 CH2O), 5.56 (2H, s, DMB CH2, N-3 CH2O), 5.88–
5.93 (1H, m, CH]CH2), 6.47–6.50 (2H, m, DMB Ar–H),
7.53 (1H, d, J 8.3, DMB Ar–H), 7.71 (1H, s, H-8), 8.83
4.1.1.3. 1-(2-Phenylsulfonylethyl)-7-(2,4-dimethoxy-
benzyl)-3,7-dihydropurine-2,6-dione (4). A solution of 3

M. B. Allwood et al. / Tetrahedron 63 (2007) 12294–12302
12299
(1H, s, N-1 H); dC (CDCl3, 125 MHz): 45.4, 55.4, 55.5, 71.0,
71.5, 98.6, 104.3, 107.2, 115.7, 117.6, 132.2, 133.9, 142.5,
149.9, 151.0, 154.6, 158.6, 161.7; found (ES+) 373.1504
[M+H]+, C18H21N4O5 requires 373.1506.
(2H, dt, J 7.1, 1.0, SO2Ph Ar–H); dC (CDCl3, 125 MHz):
19.7, 35.1, 36.5, 44.9, 48.4, 53.1, 55.4, 55.5, 98.6, 104.3,
107.1, 116.0, 128.1, 129.2, 132.2, 133.7, 139.1, 141.5,
148.2, 150.3, 154.5, 158.6, 161.6; found (ES+) 535.1602
[M+Na]+, C25H28N4O6SNa requires 535.1622.
4.1.1.6. 3-Allyloxymethyl-1-(2-phenylsulfonylethyl)-
3,7-dihydropurine-2,6-dione (7). Prepared from 5, using
General Procedure B. Scale 0.127 mmol. White amorphous
solid (41 mg, 82%). Mp 181 ꢀC; dH (CDCl3, 500 MHz): 3.57
(2H, t, J 6.6, SCH2), 4.19 (2H, dt, J 5.6, 1.4, N-3 CH2C]C),
4.51 (2H, t, J 6.6, N-1 CH2), 5.19 (1H, dq, J 10.5, 1.5, N-3
C]CH2), 5.31 (1H, dq, J 14.3, 1.6, N-3 C]CH2), 5.58
(2H, s, NCH2O), 5.87–5.93 (1H, m, CH]CH2), 7.57 (2H,
t, J 7.4, SO2Ph Ar–H), 7.66 (1H, t, J 7.4, SO2Ph Ar–H),
7.92–7.96 (3H, m, SO2Ph Ar–H, H-8); dC (CDCl3,
125 MHz): 35.2, 53.0, 71.2, 72.7, 106.9, 117.8, 127.9,
129.4, 133.8, 134.0, 139.2, 141.5, 148.5, 150.9, 155.5; found
(ES+) 413.0889 [M+Na]+, C17H18N4O5Na requires
413.0890.
4.1.1.10. 1-(2-Phenylsulfonylethyl)-7-(2,4-dimethoxy-
benzyl)-3-(2-hydroxyethyl)-3,7-dihydropurine-2,6-dione
(8d). Using 2-iodoethanol. Scale: 0.956 mmol. Foaming
syrup (464 mg, 94%). dH (CDCl3, 500 MHz): 3.08 (1H, t,
J 5.7, OH), 3.49 (2H, t, J 6.9, CH2S), 3.74 (3H, s, OMe),
3.78 (3H, s, OMe), 3.83 (2H, q, J 5.7, CH2OH), 4.19 (2H,
t, J 5.7, CH2N), 4.33 (2H, t, J 6.9, N-1 CH2N), 5.31 (2H,
s, DMB CH2), 6.39–6.43 (2H, m, DMB Ar–H), 7.40–7.46
(3H, m, DMB Ar–H, SO2Ph Ar–H), 7.53–7.56 (2H, m,
SO2Ph Ar–H), 7.55 (1H, s, H-8), 7.89 (2H, dt, J 7.2, 1.3,
SO2Ph Ar–H); dC (CDCl3, 125 MHz): 22.7, 31.4, 35.3,
45.3, 46.2, 52.9, 55.5 (two signals) 98.7, 104.3, 106.8,
115.6, 128.1, 129.2, 132.2, 133.8, 139.0, 141.8, 148.5, 151.4,
154.3, 158.6, 161.8; found (ES+) 537.1390 [M+Na]+,
C24H26N4O7SNa requires 537.1414.
The following compounds were prepared from 4 according
to General Procedure D.
4.1.1.11. 1-(2-Phenylsulfonylethyl)-7-(2,4-dimethoxy-
benzyl)-3-[3-(1,3-dioxo-1,3-di-hydroisoindol-2-yl)-propyl]-
3,7-dihydropurine-2,6-dione (8e). Using 3-bromopro-
pylphthalimide. Scale: 0.956 mmol. White solid (611 mg,
97%). Mp 60 ꢀC; dH (CDCl3, 500 MHz): 2.17 (2H, quintet,
J 7.1, CH2CH2CH2), 3.59 (2H, t, J 7.1, SCH2), 3.77 (2H, t,
J 7.1, N-3 CH2N), 3.82 (3H, s, OMe), 3.86 (3H, s, OMe),
4.11 (2H, t, J 7.1, N-3 CH2N), 4.38 (2H, t, J 7.1, N-1
CH2N), 5.36 (2H, s, DMB CH2), 6.47–6.51 (2H, m, DMB
Ar–H), 7.49–7.53 (3H, m, DMB Ar–H, SO2Ph Ar–H),
7.56 (1H, s, H-8), 7.62 (1H, t, J 7.5, SO2Ph Ar–H), 7.73
(2H, dd, J 5.4, 3.0, Phth Ar–H), 7.84 (2H, dd, J 5.4, 3.0,
Phth Ar–H), 7.97 (2H, d, J 7.5, SO2Ph Ar–H); dC (CDCl3,
125 MHz): 15.3, 27.0, 35.2, 35.6, 41.2, 45.2, 52.9, 55.4,
65.9, 98.7, 104.3, 106.7, 115.8, 123.2, 128.1, 129.2, 132.1,
132.2, 133.7, 134.0, 139.1, 142.0, 148.3, 150.6, 154.4,
158.6, 161.7, 168.3; found (ES+) 658.1973 [M+H]+,
C33H32N5O8S requires 658.1966.
4.1.1.7. 1-(2-Phenylsulfonylethyl)-7-(2,4-dimethoxy-
benzyl)-3-methyl-3,7-dihydropurine-2,6-dione (8a). Using
methyl iodide (3 equiv). Scale: 1.70 mmol. White solid
(816 mg, quantitative). Mp 142 ꢀC; dH (CDCl3, 500 MHz):
3.39 (3H, s, N-3 Me), 3.51 (2H, t, J 7.2, N-1 CH2), 3.73
(3H, s, OMe), 3.77 (3H, s, OMe), 4.32 (2H, t, J 7.2, N-1
CH2N), 5.30 (2H, s, DMB CH2), 6.38–6.42 (2H, m, DMB
Ar–H), 7.38–7.44 (3H, m, DMB Ar–H, SO2Ph Ar–H), 7.53
(1H, t, J 7.4, SO2Ph Ar–H), 7.55 (1H, s, H-8), 7.87 (2H, d,
J 7.4, SO2Ph Ar–H); dC (CDCl3, 125 MHz): 29.6, 35.2,
45.3, 52.9, 55.4, 55.5, 98.7, 104.3, 106.7, 115.9, 128.1,
129.2, 132.1, 133.7, 139.1, 142.0, 148.8, 151.0, 154.4,
158.6, 161.7; found (ES+) 485.1492 [M+H]+, C23H25N4O6S
requires 485.1489.
4.1.1.8. 1-(2-Phenylsulfonylethyl)-3-benzyl-7-(2,4-di-
methoxybenzyl)-3,7-dihydro-purine-2,6-dione (8b). Using
benzyl bromide. Scale: 0.956 mmol. White solid (560 mg,
quantitative). Mp 58 ꢀC; dH (CDCl3, 500 MHz): 3.50 (2H,
t, J 7.2, SCH2), 3.73 (3H, s, OMe), 3.77 (3H, s, OMe), 4.32
(2H, t, J 7.2, CH2N), 5.09 (2H, s, Bn CH2, DMB CH2),
5.29 (2H, s, Bn CH2, DMB CH2), 6.38–6.42 (2H, m, DMB
Ar–H), 7.18–7.24 (3H, m, Ar–H), 7.34–7.38 (3H, m, Ar–H),
7.41 (1H, d, J 12.3, Ar–H), 7.49 (1H, t, J 7.5, Bn Ar–H,
SO2Ph Ar–H), 7.55 (1H, s, H-8), 7.92–7.84 (2H, m, Ar–H);
dC (CDCl3, 125 MHz): 35.3, 45.1, 46.5, 52.9, 55.4, 55.5,
98.7, 104.3, 106.7, 115.8, 127.9, 128.1, 128.6, 128.7, 129.2,
132.2, 133.7, 136.2, 139.0, 142.0, 148.4, 150.8, 154.4,
158.6, 161.7; found (ES+) 561.1810 [M+H]+, C29H29N4O6S
requires 561.1802.
The following compounds were prepared according to
General Procedure A.
4.1.1.12. 7-(2,4-Dimethoxybenzyl)-3-methyl-3,7-dihy-
dropurine-2,6-dione (9a). Using 8a. Scale: 1.44 mmol.
Pale yellow crystals, after recrystallisation from hot MeOH
(468 mg, quantitative). Mp 182 ꢀC; dH (CDCl3, 500 MHz):
3.55 (3H, s, N–Me), 3.82 (3H, s, OMe), 3.86 (3H, s,
OMe), 5.41 (2H, s, DMB CH2), 6.47–6.50 (2H, m, DMB
Ar–H), 7.52 (1H, d, J 8.2, DMB Ar–H), 7.69 (1H, s, H-8),
8.58 (1H, br s, N-1 H); dC (CDCl3, 125 MHz): 29.0, 45.4,
55.4, 55.5, 98.7, 104.3, 107.1, 115.8, 132.2, 142.2, 150.6,
151.1, 154.6, 158.6, 161.7; found (ES+) 339.1068
[M+Na]+, C15H16N4O4Na requires 339.1064.
4.1.1.9.
1-(2-Phenylsulfonylethyl)-7-(2,4-dimethoxy-
benzyl)-3-isopropyl-3,7-di-hydropurine-2,6-dione (8c).
Using 2-iodopropane. Scale: 0.425 mmol. Colourless syrup
(201 mg, 92%). dH (CDCl3, 500 MHz): 1.44 (6H, d, J 7.0,
(CH3)2CH), 3.50 (2H, t, J 7.2, SCH2), 3.73 (3H, s, OMe),
3.78 (3H, s, OMe), 4.30 (2H, t, J 7.2, N-1 CH2N), 5.01
(1H, q, J 7.0, N-3 C–H), 5.31 (2H, s, DMB CH2), 6.37–
6.43 (2H, m, DMB Ar–H), 7.39–7.45 (3H, m, DMB Ar–H,
SO2Ph Ar–H), 7.53–7.55 (2H, m, SO2Ph Ar–H, H-8), 7.87
4.1.1.13. 7-(2,4-Dimethoxybenzyl)-3-benzyl-3,7-dihy-
dropurine-2,6-dione (9b). Using 8b. Scale: 0.914 mmol.
Fine white crystalline solid (301 mg, 84%). Mp 192 ꢀC; dH
(CDCl3, 500 MHz): 3.82 (3H, s, OMe), 3.86 (3H, s, OMe),
5.24 (2H, s, Bn CH2, DMB CH2), 5.39 (2H, s, Bn CH2,
DMB CH2), 6.47–6.50 (2H, m, DMB Ar–H), 7.25–7.28
(3H, m, Bn Ar–H), 7.30–7.53 (3H, m, Bn Ar–H, DMB

12300
M. B. Allwood et al. / Tetrahedron 63 (2007) 12294–12302
Ar–H), 7.69 (1H, s, H-8), 8.50 (1H, br, N1-H); dC (CDCl3,
125 MHz): 45.2, 45.8, 55.4, 55.5, 98.6, 104.3, 107.2,
115.8, 127.8, 128.5, 128.7, 132.3, 136.3, 142.2, 150.2,
150.8, 154.6, 158.6, 161.7; found (ES+) 415.1385
[M+Na]+, C21H20N4O4Na requires 415.1377.
impurity. White solid (92 mg, 82%). Mp 112 ꢀC; dH
(CDCl3, 500 MHz): 3.49 (3H, s, N-3 Me), 3.72 (3H, s,
OMe), 3.76 (3H, s, OMe), 4.57 (2H, dt, J 5.7, 1.4,
CH2C]C), 5.15 (1H, dt, J 9.6, 1.4, CH]CH2), 5.19 (1H,
dt, J 17.2, 1.4, CH]CH2), 5.35 (2H, s, Bn CH2), 5.84–
5.88 (1H, m, CH]CH2), 6.38–6.40 (2H, m, DMB Ar–H),
7.41 (1H, d, J 8.1, DMB Ar–H), 7.56 (1H, s, H-8); dC
(CDCl3, 125 MHz): 29.7, 43.4, 45.2, 55.4 (two signals),
98.6, 104.3, 106.9, 116.0, 117.4, 132.1, 132.4, 141.8,
148.8, 151.3, 154.9, 158.6, 161.6; found (ES+) 357.1558
[M+H]+, C18H21N4O4 requires 357.1557.
4.1.1.14. 3-Isopropyl-7-methyl-3,7-dihydropurine-2,6-
dione (9c). Using 8c. Scale: 0.307 mmol. Off-white solid
(60 mg, 94%). Mp 230 ꢀC; dH (CDCl3, 500 MHz): 1.60
(6H, d, J 6.9, (CH3)2CH), 3.99 (3H, s, N-7 Me), 5.15 (1H,
septet, J 6.9, (CH3)2CH), 7.53 (1H, s, H-8), 8.42 (1H, br s,
N-1 H); dC (CDCl3, 125 MHz): 19.7, 33.5, 47.8, 108.6,
141.3, 150.5, 154.8, 174.2; found (ES+) 231.0855
[M+Na]+, C9H12N4O2Na requires 231.0852.
4.1.1.19. 1-Benzyl-7-(2,4-dimethoxy-benzyl)-3-meth-
yl-3,7-dihydropurine-2,6-dione (10c). Using 9a and benzyl
bromide. Scale: 0.379 mmol. White amorphous solid
(98 mg, 76%). Mp 126 ꢀC; dH (CDCl3, 500 MHz): 3.57
(3H, s, N-3 Me), 3.82 (3H, s, OMe), 3.84 (3H, s, OMe),
5.23 (2H, s, Bn CH2, DMB CH2), 5.45 (2H, s, Bn CH2,
DMB CH2), 6.47–6.50 (2H, m, DMB Ar–H), 7.27 (1H, t,
J 6.3, Bn Ar–H), 7.32 (2H, t, J 6.3, Bn Ar–H), 7.50–7.53
(3H, m, Bn Ar–H, DMB Ar–H), 7.63 (1H, s, H-8); dC
(CDCl3, 125 MHz): 29.7, 44.4, 45.2, 55.4, 55.5, 98.6,
104.3, 107.0, 116.0, 127.5, 128.4, 128.8, 132.0, 137.5,
141.8, 148.8, 151.6, 155.2, 158.6, 161.6; found (ES+)
407.1695 [M+H]+, C22H23N4O4 requires 407.1714.
4.1.1.15. 7-(2,4-Dimethoxybenzyl)-3-(2-hydroxyethyl)-
3,7-dihydropurine-2,6-dione (9d). Using 8d. Scale:
0.519 mmol. An additional equivalent of KOtBu was
used due to the acidic hydroxyl proton. Off-white solid
(128 mg, 71%). Mp 222 ꢀC; dH (CDCl3, 500 MHz): 3.31
(1H, t, J 5.3, OH), 3.73 (3H, s, OMe), 3.78 (3H, s, OMe),
3.87 (2H, q, J 5.3, N-3 CH2OH), 4.27 (2H, t, J 5.3,
CH2N), 5.35 (2H, s, DMB CH2), 6.38–6.42 (2H, m, DMB
Ar–H), 7.42 (1H, d, J 8.3, DMB Ar–H), 7.56 (1H, s, H-8);
dC (CDCl3, 125 MHz): 28.1, 45.3, 46.3, 55.5, 62.0, 98.7,
104.3, 107.0, 115.8, 132.1, 141.5, 148.3, 152.2, 155.1,
158.6, 161.7; found (ES+) 347.1360 [M+H]+, C16H19N4O5
requires 347.1350.
4.1.1.20. 3-Benzyl-7-(2,4-dimethoxybenzyl)-1-methyl-
3,7-dihydropurine-2,6-dione (10d). Using 9b and methyl
iodide (3 equiv). Scale: 0.204 mmol. White powder
(108 mg, quantitative). Mp 179 ꢀC; dH (CDCl3, 500 MHz):
3.47 (3H, s, N-1 Me), 3.82 (3H, s, OMe), 3.90 (3H, s,
OMe), 5.28 (2H, s, Bn CH2, DMB CH2), 5.43 (2H, s, Bn
CH2, DMB CH2), 6.47–6.49 (2H, m, DMB Ar–H), 7.25–
7.33 (3H, m, Bn Ar–H), 7.49–7.53 (3H, m, Bn Ar–H,
DMB Ar–H), 7.68 (1H, s, H-8); dC (CDCl3, 125 MHz):
28.1, 45.1, 46.6, 55.4, 55.5, 98.6, 104.2, 106.9, 116.1,
127.8, 128.5, 128.7, 132.1, 136.5, 141.8, 148.3, 151.5,
155.3, 158.6, 161.6; found (ES+) 429.1543 [M+Na]+,
C22H22N4O4Na requires 429.1533.
4.1.1.16. 7-(2,4-Dimethoxybenzyl)-3-[3-(1,3-dioxo-1,3-
dihydroisoindol-2-yl)-propyl]-3,7-dihydropurine-2,6-dione
(9e). Using 8e. Scale: 0.826 mmol. Off-white amorphous
solid (187 mg, 46%). Mp 164 ꢀC; dH (CDCl3, 500 MHz):
2.09 (1H, quintet, J 7.4, CH2CH2CH2), 3.68–3.73 (5H, m,
N-3 CH2N, OMe), 3.76 (3H, s, OMe), 4.06 (2H, t, J 7.4,
N-3 CH2N), 5.28 (2H, s, DMB CH2), 6.37–6.40 (2H, m,
DMB Ar–H), 7.42 (1H, d, J 8.3, DMB Ar–H), 7.51 (1H, s,
H-8), 7.63 (2H, dd, J 5.4, 3.0, Phth Ar–H), 7.75 (2H, dd, J
5.4, 3.0, Phth Ar–H), 8.09 (1H, br s, N-1 H); dC (CDCl3,
125 MHz): 27.3, 35.8, 40.5, 45.3, 55.4, 55.5, 98.7, 104.3,
107.2, 115.8, 123.3, 132.1, 132.2, 133.9, 142.2, 150.1,
150.5, 154.5, 158.6, 161.7, 168.3; found (ES+) 490.1716
[M+H]+, C25H24N5O6 requires 490.1721.
4.1.1.21. 7-(2,4-Dimethoxybenzyl)-3-(2-hydroxyethyl)-
1-methyl-3,7-dihydropurine-2,6-dione (10e). Using 9d
and methyl iodide (3 equiv). Scale: 0.331 mmol. White solid
(103 mg, 87%). Mp 174 ꢀC; dH (CDCl3/MeOD, 500 MHz):
3.33 (3H, s, N-1 Me), 3.73 (3H, s, OMe), 3.78 (3H, s, OMe),
3.80 (2H, t, J 5.5, CH2N), 4.17 (2H, t, J 5.5, N-3 CH2O), 5.32
(2H, s, DMB CH2), 6.40–6.42 (2H, m, DMB Ar–H), 7.39
(1H, d, J 8.1, DMB Ar–H), 7.57 (1H, s, H-8); dC (CDCl3,
125 MHz): 28.0, 45.3, 45.8, 55.3, 55.4, 60.2, 98.6, 104.3,
107.0, 115.6, 132.0, 141.5, 155.2, 158.6; found (ES+)
361.1496 [M+H]+, C17H21N4O5 requires 361.1506.
The following compounds were prepared according to
General Procedure D.
4.1.1.17. 7-(2,4-Dimethoxybenzyl)-1,3-dimethyl-3,7-
dihydropurine-2,6-dione (10a). Using 9a and methyl io-
dide (3 equiv). Scale: 0.316 mmol. White crystalline rosettes
(90 mg, 86%). Mp 152 ꢀC; dH (CDCl3, 500 MHz): 3.43 (3H,
s, N–Me), 3.57 (3H, s, N–Me), 3.81 (3H, s, OMe), 3.86 (3H,
s, OMe), 5.44 (2H, s, DMB CH2), 6.46–6.50 (2H, m, DMB
Ar–H), 7.50 (1H, d, J 8.2, DMB Ar–H), 7.66 (1H, s, H-8); dC
(CDCl3, 125 MHz): 28.0, 29.7, 45.2, 55.4 (two signals),
76.8, 77.1, 77.3, 98.6, 104.2, 106.9, 116.0, 132.0, 141.8,
148.6, 151.7, 155.3, 158.6, 161.6; found (ES+) 353.1234
[M+Na]+, C16H18N4O4Na requires 353.1220.
4.1.1.22. 7-(2,4-Dimethoxybenzyl)-3-[3-(1,3-dioxo-1,3-
dihydroisoindol-2-yl)-propyl]-1-methyl-3,7-dihydropur-
ine-2,6-dione (10f). Using 9e and methyl iodide (3 equiv).
Scale: 0.204 mmol. Foaming syrup (89 mg, 87%). dH
(CDCl3, 500 MHz): 2.19 (2H, quintet, J 7.2, CH2CH2CH2),
3.40 (3H, s, N–Me), 3.78–3.83 (5H, m, OMe, N-3 CH2N),
3.86 (3H, s, OMe), 4.20 (2H, t, J 7.2, N-3 CH2N), 5.41 (2H,
s, DMB CH2), 6.46–6.50 (2H, m, DMB Ar–H), 7.51 (1H, d,
J 8.3, DMB Ar–H), 7.59 (1H, s, H-8), 7.72 (2H, dd, J 5.4,
2.4, Phth Ar–H), 7.84 (2H, dd, J 5.4, 2.4, Phth Ar–H); dC
(CDCl3, 125 MHz): 27.3, 28.0, 35.8, 41.3, 45.1, 55.4 (two
4.1.1.18. 1-Allyl-7-(2,4-dimethoxybenzyl)-3-methyl-
3,7-dihydropurine-2,6-dione (10b). Using 9a and allyl
bromide. Scale: 0.379 mmol. Additional chromatography
(10–20% EtOAc/CH2Cl2) was required to remove a polar

M. B. Allwood et al. / Tetrahedron 63 (2007) 12294–12302
12301
signals), 98.6, 104.2, 106.9, 116.0, 123.2, 132.0, 132.1, 133.9,
141.7, 148.1, 151.3, 155.3, 158.6, 161.6, 168.3; found (ES+)
504.1879 [M+H]+, C26H26N5O6 requires 504.1878.
(20.2 mg, 46%), after further chromatography (0–20%
EtOAc/CH2Cl2). dH (CDCl3, 500 MHz): 2.89 (2H, t, J 6.0,
N-7 CH2CO), 3.34 (3H, s, N–Me), 3.61 (3H, s, COOMe),
4.14 (2H, d, J 5.5, OCH2C]C), 4.49 (2H, t, J 6.0, CH2N),
5.11 (1H, dt, J 10.4, 1.2, C]CH2), 5.25 (1H, dt, J 17.2,
1.2, C]CH2), 5.52 (2H, s, NCH2O), 5.80–5.86 (1H, m,
CH]CH2), 7.61 (1H, s, H-8); dC (CDCl3, 125 MHz): 28.0,
34.7, 42.6, 52.1, 71.1, 72.3, 76.8, 77.1, 77.3, 106.6, 117.6,
134.0, 142.4, 148.4, 151.7, 155.2, 171.2; found (ES+)
323.1340 [M+H]+, C14H19N4O5 requires 323.1350.
The following compounds were prepared according to
General Procedure B.
4.1.1.23. Theophylline (11a).21 Using 10a. Scale:
0.0908 mmol. White solid (14 mg, 87%). Mp 275 ꢀC (lit.,21
276 ꢀC).
4.1.1.24. 3-Benzyl-1-methyl-3,7-dihydropurine-2,6-di-
one (11b).22 Using 10d. Scale: 0.0738 mmol. White solid
(16 mg, 83%). Mp 262 ꢀC (lit.,22 263 ꢀC).
4.1.1.30.
3-(1-Methyl-2,6-dioxo-1,2,3,6-tetrahydro-
purin-7-yl)-propionic acid methyl ester (15). Using 14,
according to General Procedure C. Scale: 0.053 mmol. Col-
ourless syrup (7.2 mg, 54%). dH (CDCl3, 300 MHz): 2.90
(2H, t, J 6.2, CH2CO2CH3), 3.25 (3H, s, COOMe, N-1
Me), 3.62 (3H, s, COOMe, N-1 Me), 4.49 (2H, t, J 6.2,
N-7 CH2N), 7.65 (1H, s, H-8), 10.18 (1H, br s, N-3 H); dC
(CDCl3, 125 MHz): 27.5, 34.7, 42.7, 52.1, 106.5, 142.3,
147.3, 151.5, 155.6, 171.2; found (ES+) 275.0746 [M+Na]+,
C10H12N4O4Na requires 275.0751.
4.1.1.25. 3-(2-Hydroxyethyl)-1-methyl-3,7-dihydro-
purine-2,6-dione (11c).23 Using 10e. Scale: 0.259 mmol.
Pale yellow solid (22 mg, 41%). Mp 278 ꢀC (lit.,23
268 ꢀC, dec).
4.1.1.26.
3-[3-(1,3-Dioxo-1,3-dihydroisoindol-2-yl)-
propyl]-1-methyl-3,7-dihydropurine-2,6-dione (11d). Us-
ing 10f. Scale: 0.0596 mmol. White solid (13.1 mg, 62%).
Mp 198 ꢀC; dH (CDCl3, 500 MHz): 2.18 (2H, quintet, J
7.1, CH2CH2CH2), 3.39 (3H, s, N-1 Me), 3.75 (2H, t, J
7.1, N-3 CH2N), 4.21 (2H, t, J 7.1, N-3 CH2N), 7.64 (2H,
dd, J 5.5, 3.1, Phth Ar–H), 7.70 (1H, s, H-8), 7.76 (2H, dd,
J 5.5, 3.1, Phth Ar–H), 12.49 (1H, br s, N-7 H); dC
(CDCl3, 125 MHz): 27.2, 28.5, 35.7, 41.7, 107.0, 123.3,
132.1, 134.0, 140.2, 148.5, 151.1, 156.1, 168.3; found
(ES+) 354.1183 [M+H]+, C17H16N5O4 requires 354.1197.
4.1.1.31. 1-(2-Phenylsulfonylethyl)-3-isopropyl-3,7-di-
hydropurine-2,6-dione (16). Using 8c, according to Gen-
eral Procedure B. Scale: 1.01 mmol. White solid (290 mg,
79%). Mp 278 ꢀC; dH (DMSO-d6, 500 MHz) 1.52 (6H, d,
J 6.9, (CH3)2CH), 3.69 (2H, t, J 7.4, SCH2), 4.22 (2H, t, J
7.4, N-1 CH2N), 5.07 (1H, septet, J 6.9, (CH3)2CH), 7.68–
7.98 (5H, m, SO2Ph), 8.10 (1H, s, H-8), 13.63 (1H, br s,
N-7 H); dC (DMSO-d6, 125 MHz): 19.4, 34.7, 47.4, 51.9,
107.2, 127.6, 129.4, 133.8, 138.7, 140.3, 147.5, 149.9,
153.7; found (ES+) 385.0927 [M+Na]+, C16H18N4O4SNa
requires 385.0941.
4.1.1.27. 3-Allyloxymethyl-7-(2,4-dimethoxybenzyl)-
1-methyl-3,7-dihydropurine-2,6-dione (12). Using 6 and
methyl iodide (3 equiv), according to General Procedure
D. Scale: 0.298 mmol. White solid (107 mg, 93%). Mp
128 ꢀC; dH (CDCl3, 500 MHz): 3.43 (3H, s, N–Me), 3.82
(3H, s, OMe), 3.86 (3H, s, OMe), 4.20 (2H, dt, J 5.6, 1.4,
OCH2C]C), 5.19 (1H, dt, J 11.7, 1.3, C]CH2), 5.33 (1H,
dt, J 7.2, 1.3, C]CH2), 5.43 (2H, s, NCH2O, DMB CH2),
5.59 (2H, s, NCH2O, DMB CH2), 5.88–5.94 (1H, m,
CH]C), 6.47–6.50 (2H, m, DMB Ar–H), 7.51 (1H, d, J
8.3, DMB Ar–H), 7.68 (1H, s, H-8); dC (CDCl3, 125 MHz):
27.9, 45.2, 55.4, 71.0, 72.3, 98.7, 104.4, 107.0, 116.2, 117.3,
132.0, 134.1, 141.9, 148.0, 151.8, 155.4, 158.8, 161.7; found:
(ES+) 387.1654 [M+H]+, C19H23N4O5 requires 387.1663.
4.1.1.32.
1-(2-Phenylsulfonylethyl)-3-isopropyl-7-
methyl-3,7-dihydropurine-2,6-dione (17a). Using 16 and
methyl iodide, according to General Procedure D. Scale:
0.359 mmol. Granular white solid (128 mg, 95%). Mp
134 ꢀC; dH (CDCl3, 500 MHz): 1.57 (6H, d, J 6.5,
(CH3)2CH), 3.56 (2H, t, J 7.1, SCH2), 3.97 (3H, s, N-7
Me), 4.38 (2H, t, J 7.1, CH2N), 5.14 (1H, septet, J 6.5,
(CH3)2CH), 7.50 (1H, s, H-8), 7.58 (2H, t, J 8.5, SO2Ph
Ar–H), 7.66 (1H, t, J 8.5, SO2Ph Ar–H), 8.00 (2H, d,
J 8.5, SO2Ph Ar–H); dC (CDCl3, 125 MHz): 19.7, 33.6,
35.1, 48.5, 53.0, 108.0, 128.2, 129.3, 133.8, 139.0, 141.2,
148.4, 150.3, 154.7; found (ES+) 377.1264 [M+H]+,
C17H21N4O4S requires 377.1278.
4.1.1.28. 3-Allyloxymethyl-1-methyl-3,7-dihydropur-
ine-2,6-dione (13). Using 12, according to General Proce-
dure B. Scale: 0.262 mmol. Glassy solid (37 mg, 60%).
Mp 165 ꢀC; dH (CDCl3, 500 MHz): 3.52 (3H, s, N-1 Me),
4.26 (2H, dt, J 5.7, 1.2, CH2C]C), 5.21 (1H, dq, J 10.5,
1.5, C]CH2), 5.35 (1H, dq, J 17.3, 1.5, C]CH2), 5.69
(2H, s, NCH2O), 5.90–5.96 (1H, m, CH]CH2), 7.93 (1H,
s, H-8), 12.64 (1H, br s, N-7 H); dC (CDCl3, 125 MHz):
28.5, 71.1, 72.7, 107.0, 117.7, 133.9, 140.5, 148.3, 151.6,
156.2; found (ESꢂ) 235.0834 [MꢂH]ꢂ, C10H11N4O3
requires 235.0826.
4.1.1.33. 1-(2-Phenylsulfonylethyl)-3-isopropyl-7-eth-
yl-3,7-dihydropurine-2,6-dione (17b). Using 16 and ethyl
iodide, according to General Procedure D. Scale: 0.359 mmol.
Fluffy white solid (122 mg, 87%). Mp 148 ꢀC; dH (CDCl3,
500 MHz): 1.53 (3H, t, J 7.2, CH2CH3), 1.57 (6H, d, J 6.9,
(CH3)2CH), 3.57 (2H, t, J 7.1, SCH2), 3.43 (2H, q, J 7.2,
CH2CH3), 4.40 (2H, t, J 7.1, N-1 CH2N), 5.15 (1H, septet,
J 6.9, N-3 (CH3)2CH), 7.55 (1H, s, H-8), 7.57–8.02 (5H,
m, SO2Ph); dC (CDCl3, 125 MHz): 16.5, 19.7, 35.1, 42.3,
48.5, 53.1, 107.3, 128.2, 129.3, 133.8, 139.1, 134.0, 148.7,
150.3, 154.3; found (ES+) 391.1453 [M+H]+, C18H23N4O4S
requires 391.1435.
4.1.1.29. 3-(3-Allyloxymethyl-1-methyl-2,6-dioxo-1,2,3,6-
tetrahydropurin-7-yl)-propionic acid methyl ester (14).
Using 13 and methyl 3-bromopropionate, according to
General Procedure D. Scale: 0.137 mmol. Colourless oil
4.1.1.34. 3-Isopropyl-7-methyl-3,7-dihydropurine-2,6-
dione (18a). Using 17a, according to General Procedure A.

12302
M. B. Allwood et al. / Tetrahedron 63 (2007) 12294–12302
J. Med. Chem. 2006, 49, 3682–3692; (b) Lin, R.-Y.; Wu,
B.-N.; Lo, Y.-C.; An, L.-M.; Dai, Z.-K.; Lin, Y.-T.; Tang, C.-S.;
Chen, I.-J. J. Pharmacol. Exp. Ther. 2006, 316, 709–717; (c)
Ito, K.; Lim, S.; Caramori, G.; Cosio, B.; Chung, K. F.;
Adcock, I. M.; Barnes, P. J. Proc. Natl. Acad. Sci. U.S.A. 2002,
99, 8921–8926.
Scale: 0.307 mmol. Off-white solid (60 mg, 94%).
Mp 230 ꢀC; dH (CDCl3, 500 MHz): 1.60 (6H, d, J 6.9,
(CH3)2CH), 3.99 (3H, s, N-7 Me), 5.15 (1H, septet, J 6.9,
(CH3)2CH), 7.53 (1H, s, H-8), 8.42 (1H, br s, N-1 H); dC
(CDCl3, 125 MHz): 19.7, 33.5, 47.8, 108.6, 141.3, 150.5,
154.8, 174.2; found (ES+) 231.0855 [M+Na]+, C9H12N4O2Na
requires 231.0852.
2. (a) Caramori, G.; Adcock, I. Pulm. Pharmacol. Ther. 2003, 16,
247–277; (b) Dal Piaz, V.; Giavannoni, M. P. Eur. J. Med.
Chem. 2000, 35, 463–480; (c) Kiesman, W. F.; Zhao, J.;
Conlon, P. R.; Dowling, J. E.; Petter, R. C.; Lutterodt, F.; Jin,
X.; Smits, G.; Fure, M.; Jayaraj, A.; Kim, J.; Sullivan, G.;
Linden, J. J. Med. Chem. 2006, 49, 7119–7131.
3. (a) Rao, F. V.; Andersen, O. A.; Vora, K. A.; DeMartino, J. A.;
van Aalten, D. M. F. Chem. Biol. 2005, 12, 973–980; (b)
Schuttelkopf, A. W.; Andersen, O. A.; Rao, F. V.; Allwood,
M.; Lloyd, C.; Eggleston, I. M.; van Aalten, D. M. F. J. Biol.
Chem. 2006, 37, 27278–27285.
4.1.1.35. 3-Isopropyl-7-ethyl-3,7-dihydropurine-2,6-
dione (18b). Using 17b, according to General Procedure A.
Scale: 0.218 mmol. Off-white solid (56 mg, 89%). Mp
160 ꢀC; dH (CDCl3, 500 MHz): 1.46 (3H, t, J 7.3,
CH2CH3), 1.51 (6H, d, J 7.0, (CH3)2CH), 4.25 (2H, q, J
7.3, CH2CH3), 5.07 (1H, septet, J 7.0, (CH3)2CH), 7.49
(1H, s, H-8), 8.38 (1H, br s, N1-H); dC (CDCl3, 125 MHz):
16.5, 19.7, 42.4, 47.8, 107.8, 140.1, 150.5, 154.5, 165.2; found
(ES+) 245.1016 [M+Na]+, C10H14N4O2Na requires 245.1009.
4. Traube, W. Chem. Ber. 1900, 33, 3035–3056.
5. Bridson, P. K.; Wang, X. D. Synthesis 1995, 8, 855–858.
6. Zavialov, I. A.; Dahanukar, V. H.; Nguyen, H.; Orr, C.;
Andrews, D. R. Org. Lett. 2004, 6, 2237–2240.
7. Tesser, G. I.; Buis, J. T. W. A. R. M.; Wolters, E. T. M.;
Bothehelmes, E. G. A. M. Tetrahedron 1976, 32, 1069–1072.
8. For recent examples of a related safety-catch, see: (a) Fridkin,
G.; Lubell, W. D. J. Comb. Chem. 2005, 7, 977–986; (b)
Rambouts, F. J. R.; Fridkin, G.; Lubell, W. D. J. Comb.
Chem. 2005, 7, 589–598.
9. Heine, H. W. J. Am. Chem. Soc. 1963, 85, 2743–2746.
10. Ishibashi, H.; Uekagi, M.; Sakai, M.; Takeda, Y. Tetrahedron
2001, 57, 2115–2120.
11. He, R. J.; Lam, Y. L. J. Comb. Chem. 2005, 7, 916–920.
12. Use of the corresponding tert-butyl carbamate (Annedi, S. C.;
Li, W.; Samson, S.; Kotra, L. P. J. Org. Chem. 2003, 68, 1043–
1049) gave a slightly improved yield in the cyclocondensation
(59%). Methyl carbamate 1 was however preferred, as it is
a crystalline compound, and more readily purified on a large
scale.
13. Harding, S. J.; Jones, J. H. J. Pept. Sci. 1999, 5, 399–402.
14. Buckle, D. R.; Rockell, C. J. M. J. Chem. Soc., Perkin Trans. 1
1982, 627–630.
15. Muller, C. E.; Shi, M.; Manning, M., Jr.; Muller, C. E.; Shi, M.;
Manning, M., Jr.; Daly, J. W. J. Med. Chem. 1993, 36, 3341–
3349.
16. Muller, C. E.; Thorand, M.; Qurishi, R.; Diekmann, M.;
Jacobson, K. A.; Padgett, W. L.; Daly, J. W. J. Med. Chem.
2002, 45, 3440–3450.
17. Kalcheva, V. B.; Apostolova, T. M.; Anakieva, V. Z. J. Prakt.
Chem. 1985, 327, 165–168.
4.1.1.36. 1-(3-Bromopropyl)-7-ethyl-3-isopropyl-3,7-
dihydropurine-2,6-dione (19). A stirred suspension of
18b (30.3 mg, 0.136 mmol), NaOH (10.9 mg, 0.272 mmol)
and BTEAC (0.9 mg, 0.0041 mmol) in 1,3-dibromopropane
(1 mL) was heated at 100 ꢀC for 3 h. The reaction mixture
was subjected to a hot filtration, and the filtrate was evapo-
rated under reduced pressure to give a yellow syrup. Purifi-
cation by flash chromatography (0–1% MeOH/CH2Cl2)
gave 19 as a pale yellow syrup (47.4 mg, quantitative). dH
(CDCl3, 500 MHz): 1.45 (3H, t, J 7.2, CH2CH3), 1.51 (6H,
d, J 6.9, (CH3)2CH)), 2.18 (1H, quintet, J 7.0, CH2CH2CH2),
3.39 (2H, t, J 7.0, N-1 CH2N), 4.07 (2H, t, J 7.0, N-1 CH2Br),
4.28 (2H, q, J 7.2, N-7 CH2CH3), 5.11 (1H, septet, J 6.9,
(CH3)2CH), 7.49 (1H, s, H-8); dC (CDCl3, 125 MHz):
16.6, 19.7, 30.7, 31.3, 40.3, 42.3, 48.4, 107.4, 139.8,
148.6, 150.7, 154.9; found (ES+) 365.0577 [M+Na]+,
C13H19N4O2BrNa requires 365.0584.
4.1.1.37. C3-dicaffeine N-3, N-30]CH(CH3)2; N-7]
CH3, N-70]CH2CH3 (20). Using 18a and 19, according to
General Procedure D. Scale: 0.115 mmol. Colourless syrup
(40.9 mg, 76%). dH (CDCl3, 500 MHz): 1.44 (3H, d, J 7.2,
CH2CH3, 1.49 (6H, d, J 6.9, (CH3)2CH), 1.50 (6H, d, J 6.9,
(CH3)2CH), 1.98 (2H, quintet, J 7.1, CH2CH2CH2), 3.89
(3H, s, N-7 Me), 4.02–4.06 (4H, m, N-1 CH2N), 4.26 (2H,
q, J 7.2, CH2CH3), 5.06–5.12 (2H, m, (CH3)2CH), 7.40
(1H, s, H-8), 7.45 (1H, s, H-8); dC (CDCl3):16.5, 19.7,
27.1, 33.5, 39.4 (2 signals), 42.2, 48.0, 48.2, 53.4, 107.5,
108.2, 139.6, 140.8, 148.2, 148.5, 150.7 (2 signals), 155.0,
155.4; [Found: (ES+) 493.2288 [M+Na]+, C21H28N8O4Na
requires 479.2282].
18. Itahara, T.; Imamura, K. Bull. Chem. Soc. Jpn. 1994, 67,
203–209.
19. Weyler, S.; Hayallah, A. M.; Muller, C. E. Tetrahedron 2003,
59, 47–54.
Acknowledgements
20. Beer, D.; Bhalay, G.; Dunstan, A.; Glen, A.; Haberthuer, S.;
Moser, H. Bioorg. Med. Chem. Lett. 2002, 12, 1973–1976.
21. Ebisuzaki, Y.; Boyle, P. D.; Smith, J. A. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1997, 53, 777–779.
22. Merlos, M.; Gomez, L.; Vericat, M. L.; Bartroli, J.;
Garciarafanell, J.; Forn, J. Eur. J. Med. Chem. 1990, 25,
653–658.
We thank the Wellcome Trust for a studentship (M.B.A.).
D.M.F.vA. is a Wellcome Trust Senior Research Fellow.
References and notes
1. (a) Kalla, R. V.; Elzein, E.; Perry, T.; Li, X.; Palle, V.;
Varkhedkar, V.; Gimbel, A.; Maa, T.; Zeng, D.; Zablocki, J.
23. Wells, J. N.; Garst, J. E.; Kramer, G. L. J. Med. Chem. 1981,
24, 954–958.