Concise access to N9-mono-, N2-mono- and N2,N9-di-substituted guanines via efficient Mitsunobu reactions

Article abstract of DOI:10.1016/j.tet.2010.03.118

Guanine poses several problems to the synthetic chemist owing to its polyfunctional nature and poor solubility. Over the past few decades, synthetic guanines have found applications as anti-cancer and anti-viral agents. Coupled with the ever-growing inter

Full text of DOI:10.1016/j.tet.2010.03.118

Tetrahedron 66 (2010) 4621–4632
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Concise access to N9-mono-, N2-mono- and N2,N9-di-substituted guanines via
efficient Mitsunobu reactions
,
a
b
a,
*
Steven Fletcher a , Vijay M. Shahani , Alan J. Lough , Patrick T. Gunning
*
a Department of Chemistry, University of Toronto, 3359 Mississauga Road N, South Building, Mississauga, ON L5L 1C6, Canada
b X-ray Crystallography Lab, University of Toronto, 80 St. George Street, Toronto, ON M5S 3H6, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Guanine poses several problems to the synthetic chemist owing to its polyfunctional nature and poor
solubility. Over the past few decades, synthetic guanines have found applications as anti-cancer and
anti-viral agents. Coupled with the ever-growing interest in designer PNAs and G-quartets, simple and
efficient synthetic routes to novel guanines would be of significant benefit. We herein report that, upon
simple protection and/or activation step(s), the guanine precursor 2-amino-6-chloropurine is rendered
an excellent substrate for Mitsunobu chemistry, furnishing, after subsequent hydrolytic dechlorination
and appropriate deprotection step(s), the desired N9-mono-, N2-mono- or N2,N9-di-substituted
guanines in excellent yields (ꢀ80%). Importantly, we demonstrate that N9-functionalization proceeds
with very good N9/N7 regioselectivity and with complete inversion of stereochemistry.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 14 February 2010
Received in revised form 28 March 2010
Accepted 30 March 2010
Available online 8 April 2010
Keywords:
Mitsunobu
Guanine
2-Amino-6-chloropurine
Purine
Regioselective
1. Introduction
poses several problems to the synthetic chemist, due to its noto-
rious insolubility in organic solvents, along with its polyfunctional
Nucleotides are arguably the most fundamental biochemicals in
living systems, playing several key roles in the cell. For example, in
addition to serving as the structural units of DNA and RNA, they are
also sources of chemical energy (as ATP and GTP), secondary
messengers (cAMP and cGMP) and are incorporated into enzyme
cofactors (e.g., coenzyme A). It is, therefore, little wonder that
nucleosides and nucleotides have inspired the discovery of new
drugs. In particular, modification of both the carbohydrate pentose
portion and the nucleobase has led to the development of potent
anti-cancer and anti-viral agents, such as the guanosine analogue
acyclovir.1 More recently, nucleobases have featured in Nielsen’s
peptide nucleic acids (PNAs),2 which have attracted considerable
attention owing to their potential for diagnostic as well as phar-
maceutical applications, especially in anti-gene and anti-sense
therapies.3 Indeed, we are currently developing novel PNAs that
incorporate synthetic nucleobases as a new class of inhibitor of the
oncogenic transcription factor protein Stat3.4
nature (imidazole, amide and guanidine sub-structures). Guanine
analogues have found applications as anti-cancer and anti-viral
compounds for decades.1d–f Currently, a very popular area of
research is centred on the ability of guanine to form hydrogen-
bonded cyclic tetramers known as G-quartets,5 which are demon-
strating special significance in ageing,6 cancer7 and genetic
diseases,8 and in the development of nanodevices.9 For all these
reasons, we believe that a study on the synthesis of novel guanine
derivatives would be welcomed. We herein report concise, facile
and efficient syntheses of novel guanines functionalized at the N9
and/or N2 positions, starting from 2-amino-6-chloropurine, and
employing Mitsunobu reactions as key steps.
2. Results and discussion
2.1. N9-Functionalization of guanine
To aid in the further investigation and development of such
nucleobase-containing drugs, synthetic protocols that allow for the
facile and efficient derivatization of nucleobases would be of sig-
nificant benefit. Of the main nucleobases, guanine, in particular,
The most popular approach towards N9-functionalization of
purine bases is direct nucleophilic displacement of halides or
activated alcohols, but considerable competition at the N9 and N7
sites is often reported.10 An alternative procedure takes advantage
of the Mitsunobu reaction,11 in which an alcohol is activated and
then coupled to the purine nucleus in situ.12 The Mitsunobu
reaction has become a very popular chemical transformation,
owing to its mildness, occurring under essentially neutral
* Corresponding authors. Tel.: þ1 905 828 5354; fax: þ1 905 569 4929; e-mail
addresses: steven.fletcher@utoronto.ca (S. Fletcher), patrick.gunning@utoronto.ca
(P.T. Gunning).
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.03.118

4622
S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
conditions, and its stereospecificity, proceeding with complete
Walden inversion of stereochemistry.11 To the best of our knowl-
edge, there are no reports on the N9-functionalization of un-
protected guanine with alcohols via the Mitsunobu reaction, likely
owing to a combination of the limited solubility of the purine base
as well as the potential for competing reactions at the N2, N7 and
O6 positions. However, Lu et al. have recently described the N9-
regioselective synthesis of guanine derivatives by reacting the O6-
diphenylcarbamoyl, N2-acetyl protected analogue of guanine with
a variety of alcohols under modified Mitsunobu conditions, fol-
lowed by treatment with a 1:1 mixture of aqueous ammonia and
methanol to remove the blocking groups.13 Whilst the authors re-
port very good to excellent yields, two rounds of the Mitsunobu
reaction are required in order to consume all of the starting purine,
compounding the main drawback of Mitsunobu chemistry (poor
atom economy). In addition, elevated temperatures are also nec-
essary. We considered if a milder Mitsunobu approach to N9-
functionalized guanines would be possible.
2-Amino-6-chloropurine (1) has become a popular guanine
precursor since the chlorine atom can be effectively displaced with
hydroxyl by nucleophilic aromatic substitution; one of the mildest
of such approaches involves water as the nucleophile in an acid-
catalyzed hydrolysis reaction.14 Although N9-functionalization of
purine 1 has been achieved through Mitsunobu chemistry, its poor
solubility in THF, the solvent of choice for such reactions, and the
competing nucleophilicity of the exocyclic N2 amino group are
likely primary reasons for the generally poor-to-moderate yields
observed.15 In related work on the synthesis of 2,6,9-tri-
substituted purines, we showed that the more soluble and N2-
blocked purine 3, prepared in two steps in near-quantitative yield
from 1 (Scheme 1), is an excellent substrate for the Mitsunobu
reaction, proceeding with very good N9-regioselectivity.16 More-
over, excellent chemoselectivity was also observed in the presence
of the activated (acidic) but hindered NHBoc group. We demon-
strated that hydrolytic dechlorination and Boc deprotection of
subsequent purine 4a (R1¼n-Bu) with a 3:1 mixture of TFA/H2O
for two days at room temperature furnished the corresponding
guanine analogue 5a (R1¼n-Bu) in quantitative yield. Alternatively,
we herein report that the required hydrolytic dechlorination re-
action of purines 4, along with simultaneous Boc removal, can be
accomplished conveniently and in near-quantitative yields by
treatment with 80% formic acid17 at 75 ꢁC for 2 h (Scheme 1).
our previous communication,16 we demonstrate here that this re-
action is also compatible with the increasingly more hindered al-
cohols 2-indanol and (ꢂ)-menthol, and that no reaction occurs
with tert-butanol, owing to steric hindrance at the tertiary carbon.
The Mitsunobu reactions were especially mild and swift, pro-
ceeding to completion within 15 min at room temperature, and
requiring only 1.1 equiv of reagents relative to the alcohol; these
features suggest our protocol should also be compatible with re-
lated but more complex purine substrates. In fact, although yields
are approximately the same, our reaction conditions are signifi-
cantly milder and more cost effective than those reported by
Lu et al.13dlikely due in part to the enhanced solubility of our
purine substratedrendering our protocol particularly attractive.
Moreover, due to the presence of the 6-chloro leaving group, our
work is further appealing since it allows for the preparation of
medicinally important 2,6,9-tri-subtituted purines, as well as
substituted guanines, from a common intermediate by nucleophilic
aromatic substitution with either amines18 or water, respectively.
Complete characterizations for purines 4 and guanines 5 are given
in the Experimental section.
Table 1
R1OH substrate scope for N9 Mitsunobu coupling with guanine precursor 3a
CI
DIAD,
Entry
R1OH
Regioselectivity
N9/N7
Product
Yieldb (%)
1
2
3
4
5
>7.3:1
>9:1
5a
5b
5c
5d
5e
84
85
83
82
81
>5.3:1
>6.1:1
>5.7:1
6
7
8.5:1
5f
82
86
CI
CI
CI
>9:1
5g
8c
9c
>5.3:1
>5.3:1
5h
5i
81
80
CI
10c
>11.5:1
5jd
88
Scheme 1. a) Boc2O, cat. DMAP, DMSO, 0 ꢁC/rt, 30 min, 99%; b) NaH, THF, rt, 2 h, 96%;
c) (1) R1OH, PPh3, THF, rt, 2 min; (2) DIAD, rt, 15 min, 84–92%; d) 80% HCOOH, 75 ꢁC,
2 h, 95–99%.
11
d
5k
0
a
Reaction conditions: (1) To a solution of purine 3 (1 mmol) in anhydrous THF
(7 mL) was added alcohol R1OH (1.1 mmol) and PPh3 (1.1 mmol). After 2 min, DIAD
(1.1 mmol) was added dropwise at rt (over w30 s to 1 min). The reaction was then
stirred for 15 min at rt under a nitrogen atmosphere. (2) Purine 4 (0.5 mmol) was
dissolved in 80% HCOOH (5 mL), and heated at 75 ꢁC for 2 h.
As illustrated in Table 1, purine 3 coupled in very good to
excellent yields (the second step proceeded in yields of ꢀ95% in all
cases) with a broad scope of alcohols, including benzylic, allylic,
propargylic as well as primary and secondary aliphatic alcohols,
under standard Mitsunobu conditions (triphenylphosphine (PPh3)/
diisopropylazodicarboxylate (DIAD) redox system). Elaborating on
b
Isolated overall yields.
Minor modifications to the experimental procedures were observed.
c
d
Structure confirmed by X-ray crystallography.

S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
4623
In order to confirm the N9-regioselectivity of the Mitsunobu
reaction, the NMR spectral data of the N9 and N7 regioisomers of
compound 4d were compared. Since purification of the suspected
N7 isomer of 4d prepared by the Mitsunobu reaction was more
complicated than for the N9 isomer, requiring pedantic flash
chromatography conditions, the two regioisomers of 4d were in-
stead prepared by classical alkylation of purine 3 with benzyl
bromide. Subsequently, comparison of the 1H and 13C NMR data of
the readily separable N9 and N7 isomers of 4d as described in the
literature suggested that the less polar (TLC, silica gel) of the two
products was the N9 regioisomer. This was further supported by
excellent agreement of the NMR data of the Boc-deprotected
regiosiomers of 4d with literature values.19 Furthermore, HMBC
aldehyde and p-thiocresol,23 or classical N-alkylation of the corre-
sponding amide followed by de-acylation.24 Alternatively, we and
others have shown that transformation of the exocyclic N2 amino
group of guanine precursors, as well as the exocyclic N6 amino group
of adenine, to an acetamide or a Boc carbamate renders the NH
sufficiently acidic to participate in the Mitsunobu reaction.17,25–27
The resulting N2-functionalized purines may then undergo suit-
able synthetic transformations to reveal the target guanines. Due to
the strongly electron-withdrawing nature of the trifluoroacetyl
group, converting an amine to its trifluoroacetamide renders the
resultant NH especially acidic. Indeed, Schultz et al. have reported
using the trifluoroacetyl group to activate the N2 amino group of 2-
amino-6-chloropurine derivatives, and demonstrated its reactivity
in the Mitsunobu reaction.28 Although the combined yield for the
Mitsunobu reaction and the deprotection of the trifluoroacetyl
group for their representative example was moderate, their work
was not intended as an optimization study. To the best of our
knowledge, there exists no comprehensive study on the Mitsunobu-
mediated N2-substitution of such functionalized purines. We con-
sidered that, with suitable N9 protection, the trifluoroacetyl group
would prove a convenient and efficient activating group to access
N2-functionalized guanines via Mitsunobu chemistry.
To this end, 2-amino-6-chloropurine (1) was first trityl protected
at the N9 position with TrCl to afford N9-trityl-2-amino-6-chloro-
purine 6, analogous to the previously reported N9-trityl-persilylated
guanine (Scheme 2).29 Employing K2CO3 as the base in this reaction
led to considerable N2-tritylation, but we found that this could be
substantially reduced by forming the anion of 1 with NaH. Sub-
sequently, the exocyclic N2 amino group was acylated with tri-
fluoroacetic anhydride (TFAA) using an excess of pyridine as the base
to ensure no inadvertent Tr cleavage occurred. As anticipated, the
resulting amide of purine 7 was sufficiently acidic to undergo the
Mitsunobu reaction with a wide range of alcohols (Table 2). After
optimization, it was determined that this reaction required only
a slight excess of reagents and proceeded to completion within
15 min at room temperature to furnish the series of N2-substituted
trifluoroacetamides 9. With only three exceptions, yields were very
good to excellent (deprotection steps (1)(ii) and (2) in Table 2 were
near-quantitative). However, compounds 9 slowly decomposed on
the bench after purification by silica gel flash chromatography, likely
initiated by attack of the N7 lone pair of electrons on the particularly
electrophilic trifluoroacetyl carbonyl carbon. Thus, the
NMR experiments of N7-4d and N9-4d showed a long-range 1H–13
C
coupling between H-8 and C-5 (J3¼11.7 Hz) for the less polar (TLC,
silica gel) of the two isomers with a much weaker coupling be-
tween H-8 and C-4 (J3<4 Hz), suggesting the N]CH double bond to
be between N7 and C8, and this product to be the N9 isomer.
Conversely, the more polar product exhibited couplings between
H-8/C-5 and between H-8/C-4 of J3<4 Hz and 13.3 Hz, respectively,
indicating the double bond to be between N9 and C8, and the more
polar product to be the N7 isomer. These relative polarities have
been observed by others.20 With all this data in hand, we thus
deduced that the Mitsunobu reaction conducted on purine 3 with
benzyl alcohol led to the N9 regioisomer of 4d as the major product,
and with a far superior N9/N7 regioselectivity than was observed
with classical alkylation.21 Hence, the Mitsunobu reactions in Table
1 led to the N9 regioisomers of 4a–j, as shown, in yields of 84–92%.
Subsequently, hydrolytic dechlorination and Boc removal were
accomplished almost quantitatively (95–99%) and in one pot by
treatment with 80% formic acid for 2 h at 75 ꢁC to afford the target,
N9-functionalized guanine analogues 5a–j. As a representative
example, X-ray crystallography of compound 5j confirmed the N9-
regiochemical outcome of the Mitsunobu reaction (Fig. 1). More-
over, since a single diastereomer of compound 4j was furnished, the
X-ray crystal structure of 5j (CCDC 771261) also confirmed that this
reaction proceeded with complete inversion of the alcohol stereo-
genic centre of (ꢂ)-menthol. This finding is consistent with the SN2
mechanism of the Mitsunobu reaction, suggesting our methodol-
ogy is suitable for the stereospecific synthesis of non-sugar carbon
guanosines.
CI
CI
CI
c–e
CI
f
h
Figure 1. ORTEP drawing of the crystal structure of compound 5j.
g
2.2. N2-Functionalization of guanine
Scheme 2. a) (1) NaH, DMF, rt, 15 min; (2) TrCl, rt, 16 h, 85%; b) (CF3CO)2O, pyridine,
CHCl3, 0 ꢁC, 30 min, 90%; c) Boc2O, cat. DMAP, DMSO, 0 ꢁC/rt, 30 min. 99%; d) NaH,
THF, rt, 2 h, 96%; e) TrCl, DIPEA, DMF, rt, 4 h, 90%; f) (1) R2OH, PPh3, THF, rt, 2 min; (2)
DIAD, rt, 15 min; g) K2CO3, MeOH/THF, 3:2, rt, 1 h, 35–90% (combined yields for steps
(f) and (e) (g)); h) 80% HCOOH/THF, 4:1, 75 ꢁC, 4 h, 95–99%.
N2-Functionalization of guanines is often accomplished by
reacting a suitable 2-halopurine precursor with primary amines at
elevated temperatures.22 Other approaches, employing suitable
2-aminopurine precursors, include reductive amination with an

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S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
trifluoroacetyl group was removed in situ with K2CO3 in MeOH/THF
at room temperature to deliver the N2-functionalized guanine
precursors 10. Primary alcohols typically gave excellent yields for
these two steps (ꢀ85%), and subsequent one pot hydrolytic de-
chlorination and de-tritylation with a 4:1 mixture of 80% HCOOH/
THF proceeded almost quantitatively (ꢀ95%) to furnish the desired
N2-functionalized guanines 12a–f in excellent overall yields. 1H
NMR in DMSO-d6 at 293 K indicated that compounds 12a–f were
present as an approximate 4:1 mixture of the N9H/N7H tautomers.
Both N9H and N7H tautomers of N2-acetylated guanines have also
been observed on the NMR time-scale.30 X-ray crystallography
confirmed the structure of the major N9H tautomer of 12f (Fig. 2;
(CCDC 771260)).
Table 2
R2OH substrate scope for N2 Mitsunobu coupling with guanine precursors 7 (X¼CF3)
or 8 (X¼OtBu)a
Figure 2. ORTEP drawing of the crystal structure of compound 12f.
DIAD,
CI
the Mitsunobu reaction and trifluoroacetyl removal)), whilst
tert-butanol once more did not react in the Mitsunobu reaction due
to steric hindrance. Where R2 was a primary alkyl moiety, removal
of the trifluoroacetyl group in purines 9a–f was typically complete
within 1 h to furnish the corresponding purines 10a–f in excellent
yields. In contrast, where R2 was a secondary alkyl group, 36 h were
apparently required for complete removal of the trifluoroacetyl
groups to afford 10g and 10h. Upon isolation and purification of the
N2-trifluoroacetylated intermediates 9g and 9h, we discovered that
the Mitsunobu reaction had generated a mixture of two inseparable
products, which 1H NMR suggested to be the isomeric N- and
O-alkylated trifluoroacetamides; such an observation has been
reported previously with a related N2-acetylated substrate.31 The
modified trifluoroacetyl group can no longer be removed from the
O-alkylated trifluoroacetamides O-9g and O-9h by basic meth-
anolysis, so presumably the extended reaction time to consume all
of the trifluoroacetamides was due to a slower decomposition
reaction (possibly addition/elimination displacement of the sec-
ondary alcohol by methanol), rather than removal of the modified
trifluoroacetyl group in O-9g and O-9h. The promotion of the
undesired O-alkylated product is probably a consequence of both
steric (the hindered secondary alcohol as well as the nearby bulky
Entry
1
X
R2OH
Product
Yieldb (%)
88
CF3
12a
2
3
4
5
6
CF3
CF3
CF3
CF3
CF3
12b
12c
12d
12e
12ff
87
80
83
83
81
7c
CF3
12g
12g
12h
12h
64
82
33
80
8d,e
Ot-Bu
CF3
C–O
trityl group) and electronic (the amide resonance nN/
sition) factors. Since the resonance energy of a carbamate is less
than that of an amide owing to competition of the nN/
transition with the nO/ C–O transition, we considered that an N2
p
*
tran-
p
*
C–O
9c
p
*
carbamate may generate more of the desired N-alkylated product
with the hindered secondary alcohols. Indeed, we have previously
shown that activation of the N2 amino group of N9-butyl-2-amino-
6-chloropurine as its Boc carbamate allows the Mitsunobu reaction
with iso-propanol to proceed in excellent yield,16 wherein the steric
bulk of the Boc group may also contribute to offset undesired
O-alkylation further still.
Thus, activation of the N2 amino group in 7 was alternatively
achieved with Boc rather than with trifluoroacetyl to give purine 8,
and pleasingly this allowed the couplings with iso-propanol and
cyclopentanol to proceed under modified Mitsunobu conditions
(2.5 equiv of ROH, PPh3 and DIAD, 35 ꢁC, 30 min to 2 h) in much
improved yields (11g: 86%; 11h: 84%). Finally, de-tritylation,
hydrolytic dechlorination and Boc removal were all accomplished
almost quantitatively and in one pot with 80% HCOOH at 75 ꢁC for
4 h, delivering the N2-functionalized guanines 12g and 12h in far
superior overall yields than via the trifluoroacetamide route (Table
2, entries 8 and 10, cf. entries 7 and 9). Whilst 1H NMR of 12h in
DMSO-d6 at 293 K demonstrated the existence of both N9H and
N7H tautomers in a ratio of 81:19, there was minimal resolution of
12g into its corresponding N9H and N7H tautomers on the NMR
time-scale.
10d,e
Ot-Bu
11
CF3
9i
0
a
Reaction conditions: (1) (i) To a solution of purine 7 (1 mmol) in anhydrous THF
(14 mL) were added alcohol R2OH (1.2 mmol) and PPh3 (1.3 mmol). After 2 min,
DIAD (1.3 mmol) was added dropwise at rt (over w30 s to 1 min). The reaction was
then stirred for 15 min at rt under a nitrogen atmosphere. (1) (ii) Crude 9 (1 mmol)
was dissolved in a 3:2 mixture of MeOH/THF (5 mL). K2CO3 (1.2 mmol) was added,
and the reaction mixture was stirred for 1 h at rt. (2) Purine 10 (0.5 mmol) was
dissolved in THF (1 mL). HCOOH (80%, 4 mL) was added, and the reaction was stirred
under reflux at 75 ꢁC for 4 h.
b
Isolated overall yields.
Trifluoroacetyl deprotection conditions: 1.2 equiv K2CO3, rt, 36 h.
Mitsunobu conditions as per method A, Table 3.
Step (e) was omitted and removal of the Boc group was accomplished in the
c
d
e
formic acid hydrolytic dechlorination step.
f
Structure confirmed by X-ray crystallography.
On the other hand, the secondary alcohols iso-propanol and
cyclopentanol generated the respective products 10g and 10h in
much poorer yields (67% and 35%, respectively (combined yields for

S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
4625
2.3. N2,N9-Di-functionalization of guanine
crystallography confirmed that the Mitsunobu-mediated N9-
functionalization of N2-Boc-2-amino-6-chloropurine proceeded
with very good N9/N7 regiochemical control and excellent ste-
reochemical control (complete inversion). We believe that all
these factors render our two- and three-step protocols especially
attractive routes to synthetic guanines. Indeed, we are currently
investigating some novel guanine analogues as potential
anti-cancer compounds (Stat3 inhibitors) and as guanine alter-
natives in PNA oligomers. Finally, it is noteworthy that our
methodology may also be applied to the synthesis of 6,9-di- and
2,6,9-tri-substituted purines, such as the potent and selective
cyclin-dependent kinase (CDK) inhibitor bohemine, through
displacement of the 6-chloro group with amines.16,18
We have previously reported that purine 3 can undergo se-
quential Mitsunobu reactions, first at the endocyclic N9 position
and then at the exocyclic N2 position.16 The second Mitsunobu
reaction requires the use of either the DIAD/PPh3 redox system with
gentle heating or the 1,10-(azodicarbonyl)dipiperidine (ADDP)/PBu3
[CAUTION: PBu3 is pyrophoric] redox system at room temperature,
depending on the alcohol coupling partner (Table 3). Sterically
hindered tert-butanol did not react under either set of conditions.
As anticipated, subjecting the intermediate purines 13a–f to formic
acid at 75 ꢁC effected both Boc deprotection and hydrolytic de-
chlorination. The targets 14a–f were thus furnished in very good
overall yields, illustrating that this facile approach to N2,N9-di-
substituted guanines is also a very efficient approach.
4. Experimental section
4.1. General
Table 3
R2OH substrate scope for N2 Mitsunobu coupling with N9-butylated guanine pre-
cursor 4aa
Anhydrous solvents THF, MeOH, DMSO, CH2Cl2 and DMF were
purchased from Sigma–Aldrich and used directly from their Sure-
Seal bottles. Chemicals were purchased from Sigma–Aldrich of
Alfa–Aesar. All reactions were performed under an atmosphere of
dry nitrogen in oven-dried glassware and were monitored for
completeness by thin-layer chromatography (TLC) using silica gel
(visualized by UV light, or developed by treatment with KMnO4
stain or Hanessian’s stain). 1H and 13C NMR spectra were recorded
on a Bruker 400 MHz spectrometer in either CDCl3 or DMSO-d6.
DIAD,
CI
Entry
1
R2OH
Method/Time
A/30 min
Product
Yieldb (%)
92
Chemical shifts (d) are reported in parts per million after calibration
14a
to residual isotopic solvent (CHCl3: dH¼7.26 ppm, dC¼77.00 ppm;
DMSO: dH¼2.50 ppm, dC¼39.43 ppm). Coupling constants (J) are
reported in hertz.
2
3
B/8 h
B/8 h
14b
14c
83
85
OH
4.2. Typical procedure for Mitsunobu coupling of purine 3
To a stirred solution of purine 3 (270 mg, 1 mmol, 1 equiv) in
anhydrous THF (14 mL) under a nitrogen atmosphere at room
temperature were added the alcohol R1OH (1.1 mmol, 1.1 equiv)
4
5
B/4 h
14d
14e
83
89
A/30 min
and PPh3 (289 mg, 1.1 mmol, 1.1 equiv). After w2 min, DIAD (217 ml,
1.1 mmol,1.1 equiv) was added dropwise (over w30 s to 1 min). The
reaction mixture was stirred at room temperature for 15 min, after
which time TLC analysis indicated the reaction was complete. The
reaction mixture was concentrated in vacuo, then adsorbed onto
silica gel from CH2Cl2, and purified by silica gel flash column
chromatography to afford purine 4.
6
A/2 h
14f
85
0
7
A/16 h
14g
a
Reaction conditions: (1) Method (A) purine 4a (0.5 mmol), R2OH (1.25 mmol)
and PPh3 (1.25 mmol) were dissolved in anhydrous THF (7 mL) at rt. After 2 min,
DIAD (1.25 mmol) was added dropwise (over w30 s), then reaction was heated at
35 ꢁC under a nitrogen atmosphere for time indicated. Method (B) purine 4a
(1 equiv), R2OH (1.25 mmol) and PBu3 (1.25 mmol) [CAUTION: PBu3 is pyrophoric]
were dissolved in anhydrous THF (7 mL). After 2 min, ADDP (1.25 mmol) was added
in one portion. Reaction was stirred under a nitrogen atmosphere at rt for time
indicated. (2) Resultant purine 13 (0.3 mmol) was dissolved in 80% HCOOH (3 mL),
and heated at 75 ꢁC for 2 h.
4.2.1. N2-Boc-9-butyl-6-chloro-9H-purin-2-ylamine (4a). R1OH was
1-butanol. Flash column chromatography (eluent: hex/EtOAc, 2:3)
afforded the title compound 4a as a white foam (287 mg, 88%): Rf
0.36 (hex/EtOAc, 1:2); mp 102–105 ꢁC; IR (KBr, cmꢂ1) 3425, 3257,
3186, 3105, 3011, 2963, 2932, 2875, 1749, 1720, 1608, 1578, 1518,
1473, 1442, 1403; dH (400 MHz, DMSO-d6) 0.90 (t, J¼7.3 Hz, 3H,
CH2CH3), 1.17–1.31 (m, 2H, CH2CH3), 1.47 (s, 9H, C(CH3)3), 1.83 (app
quintet, J¼7.3 Hz, 2H, CH2CH2CH3), 4.17 (t, J¼7.3 Hz, 2H,
CH2CH2CH2CH3), 8.51 (s, 1H, H-8), 10.33 (s, 1H, NHBoc); dC
(100 MHz, CDCl3) 13.4, 19.8, 28.2, 31.6, 43.9, 81.6, 127.6, 144.0, 150.2,
151.1, 152.3, 152.9; HRMS (ESIþ) m/z calcd for C14H20ClN5O2Na
[MþNaþ] 348.1197, obsd 348.1196.
b
Isolated overall yields for two steps.
3. Conclusions
In conclusion, we have demonstrated that, after simple and
high-yielding protection and/or activation steps, 2-amino-6-
chloropurine is rendered a versatile reagent in the Mitsunobu
reaction, allowing for the synthesis of novel guanines function-
alized at the N9 endocyclic position and/or at the N2 exocyclic
position. Reactions were mild and rapid, a large scope of alcohols
was tolerated, and good to excellent yields were observed
(ꢀ84%). Subsequent hydrolytic dechlorination and deprotection
step(s) were accomplished near-quantitatively and in one pot,
delivering the target guanines in overall yields of ꢀ80% (from
the appropriate Mitsunobu substrate). Significantly, X-ray
4.2.2. N2-Boc-9-(2-benzyloxyethyl)-6-chloro-9H-purin-2-ylamine
(4b). R1OH was 2-benzyloxyethanol. Flash column chromatogra-
phy (eluent: hex/EtOAc, 2:3) to afforded 4b as a white foam
(363 mg, 90%): Rf 0.37 (hex/EtOAc, 1:2); mp 86–91 ꢁC; IR (KBr,
cmꢂ1) 3451, 3225, 3089, 2978, 2923, 2853, 2359, 1744, 1607, 1576,
1520, 1445, 1407; dH (400 MHz, DMSO-d6) 1.46 (s, 9H, C(CH3)3), 3.85
(t, J¼5.0 Hz, 2H, CH2CH2OBn), 4.39 (t, J¼5.0 Hz, 2H, CH2CH2OBn),
4.47 (s, 2H, CH2Ph), 7.14–7.28 (m, 5H, Ph), 8.45 (s, 1H, H-8), 10.3 (s,

4626
S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
1H, NHBoc); dC (100 MHz, DMSO-d6) 27.9, 43.3, 66.8, 71.5, 79.5,
126.7, 127.27, 127.32, 128.0, 137.8, 146.4, 148.7, 150.8, 152.2, 152.9;
HRMS (ESIþ) m/z calcd for C19H23ClN5O3 [MþH] 404.1483, obsd
404.1483.
0.48 (EtOAc/MeOH, 98:2); dH (400 MHz, DMSO-d6) 1.46 (s, 9H,
C(CH3)3), 5.40 (s, 2H, CH2Ph), 7.28–7.41 (m, 5H, Ph), 8.58 (s, 1H,
H-8), 10.32 (s, 1, NHBoc); dC (100 MHz, DMSO-d6) 27.8, 46.6, 79.5,
126.7, 127.7, 127.8, 128.6, 136.1, 145.8, 149.0, 150.8, 152.5, 152.7;
HRMS (ESIþ) m/z calcd for C17H18ClN5O2Na [MþNaþ] 382.1041, obsd
382.1041; N7-4f: 35% yield; Rf 0.20 (EtOAc/MeOH, 98:2); dH
(400 MHz, DMSO-d6) 1.46 (s, 9H, C(CH3)3), 5.68 (s, 2H, CH2Ph), 7.16–
7.17 (m, 2H, Ph), 7.30–7.38 (m, 3H, Ph), 8.85 (s, 1H, H-8), 10.21 (s, 1H,
NHBoc); dC (100 MHz, DMSO-d6) 27.8, 49.3, 79.3, 118.2, 126.3, 127.7,
128.7, 136.7, 142.0, 151.0, 151.3, 152.5, 162.9; HRMS (ESIþ) m/z calcd
for C17H18ClN5O2Na [MþNaþ] 382.1041, obsd 382.1025. 1H and 13C
NMR data of the Boc-deprotected compounds (TFA/CH2Cl2, 1:1,
room temperature, 45 min; concentration in vacuo; purine$TFA salt
neutralized and purine extracted into CH2Cl2 by partitioning
against a saturated aqueous solution of NaHCO3) were consistent
with the corresponding compounds 8a and 8b in Ref. 19
Boc-deprotected N9-4f (¼8a in Ref. 19): Rf 0.54 (CH2Cl2/MeOH,
9:1); dH (400 MHz, DMSO-d6) 5.29 (s, 2H, CH2Ph), 6.95 (br s, 2H,
NH2), 7.23–7.36 (m, 5H, Ph), 8.23 (s, 1H, H-8); dC (100 MHz, DMSO-
d6); 46.0, 123.2, 127.1, 127.7, 128.6, 136.6, 143.1, 149.4, 154.0, 159.8;
LRMS (ESIþ) m/z calcd for C12H11ClN5 [MþHþ] 260.07, obsd 260.21;
Boc-deprotected N7-4f (¼8b in Ref. 19): Rf 0.37 (CH2Cl2/MeOH,
9:1); dH (400 MHz, DMSO-d6) 5.56 (s, 2H, CH2Ph), 6.67 (br s, 2H,
NH2), 7.13–7.15 (m, 2H, Ph), 7.29–7.37 (m, 3H, Ph), 8.56 (s, 1H, H-8);
dC (100 MHz, DMSO-d6); 49.1, 114.7, 126.3, 127.6, 128.7, 137.1, 142.3,
149.9, 160.0, 164.3; LRMS (ESIþ) m/z calcd for C12H11ClN5 [MþHþ]
260.07, obsd 260.21.
4.2.3. (N2-Boc-2-amino-6-chloro-purin-9-yl)-acetic acid ethyl ester
(4c). R1OH was ethyl glycolate. Flash column chromatography
(eluent: hex/EtOAc, 1:2) to afforded 4c as a white foam (299 mg,
84%): Rf 0.28 (hex/EtOAc, 1:2); mp 129–136 ꢁC; IR (KBr, cmꢂ1) 3462,
3249, 3166, 3106, 2988, 2948, 2362, 1751, 1693, 1612, 1572, 1523,
1499,1447,1421; dH (400 MHz, DMSO-d6) 1.22 (t, J¼7.1 Hz, 3H, CH3),
1.46 (s, 9H, C(CH3)3), 4.18 (q, J¼7.1 Hz, 2H, CH2CH3), 5.11 (s, 2H,
CH2CO2Et), 8.46 (s, 1H, H-8), 10.33 (s, 1H, NHBoc); dC (100 MHz,
DMSO-d6) 13.9, 27.8, 44.2, 61.6, 79.7,126.3, 146.4, 149.0,150.8,152.6,
152.9, 167.3; HRMS (ESIþ) m/z calcd for C14H18ClN5O4Na [MþNaþ]
378.0939, obsd 378.0945.
4.2.4. N2-Boc-9-allyl-6-chloro-9H-purin-2-ylamine (4d). R1OH was
allyl alcohol. Flash column chromatography (eluent: hex/EtOAc,
2:3) afforded 4d as a white foam (267 mg, 86%): Rf 0.34 (hex/EtOAc,
1:2); mp 106–112 ꢁC; IR (KBr, cmꢂ1) 3428, 3258, 3189, 3091, 3010,
2982, 2933, 2359, 1753, 1721, 1645, 1607, 1578, 1511, 1444, 1400; dH
(400 MHz, DMSO-d6) 1.46 (s, 9H, C(CH3)3), 4.80–4.83 (m, 2H,
CH2CH]CH2), 5.04–5.10 (m, 1H, CH]CH2), 5.21–5.25 (m, 1H,
CH]CH2), 6.05–6.15 (m, 1H, CH]CH2), 8.46 (s, 1H, H-8), 10.29
(s, 1H, NHBoc); dC (100 MHz, DMSO-d6) 27.9, 45.3, 79.5, 117.9, 126.7,
132.5, 145.9, 148.8, 150.8, 152.4, 152.7; HRMS (ESIþ) m/z calcd for
C13H16ClN5O2Na [MþNaþ] 332.0884, obsd 332.0886.
4.2.8. N2-Boc-9-(iso-propyl)-6-chloro-9H-purin-2-ylamine
(4g). R1OH was iso-propanol. Flash column chromatography
(eluent: hex/EtOAc, 2:3) afforded 4g as a white solid (280 mg, 90%):
Rf 0.33 (hex/EtOAc, 1:2); mp 164–165 ꢁC; IR (KBr, cmꢂ1) 3417, 3252,
3021, 2975, 2934, 2360, 1712, 1603, 1570, 1523, 1503, 1482, 1462,
1439; dH (400 MHz, DMSO-d6) 1.47 (s, 9H, C(CH3)3), 1.55 (d,
J¼6.8 Hz, 6H, CH(CH3)2), 4.75 (septet, J¼6.8 Hz, 1H, CH(CH3)2), 8.59
(s, 1H, H-8), 10.27 (s, 1H, NHBoc); dC (100 MHz, DMSO-d6) 21.7, 27.9,
47.3, 79.5, 127.1, 144.3, 148.7, 150.8, 151.9, 152.3; HRMS (ESIþ) m/z
calcd for C13H18ClN5O2Na [MþNaþ] 334.1041, obsd 334.1047.
4.2.5. N2-Boc-9-(prop-2-ynyl)-6-chloro-9H-purin-2-ylamine
(4e). R1OH was propargyl alcohol. Flash column chromatography
(eluent: hex/EtOAc, 2:3) afforded 4e as a white foam (249 mg, 85%):
Rf 0.32 (hex/EtOAc, 1:2); mp 129–136 ꢁC; IR (KBr, cmꢂ1) 3425, 3231,
3154, 3112, 2977, 2358, 2128, 1722, 1591, 1533, 1497, 1450, 1426; dH
(400 MHz, DMSO-d6) 1.47 (s, 9H, C(CH3)3), 3.57 (t, J¼2.4 Hz, 1H,
C^CH), 5.05 (d, J¼2.4 Hz, 2H, CH2C^CH), 8.55 (s, 1H, H-8), 10.38
(s, 1H, H-1); dC (100 MHz, CDCl3) 28.1, 33.5, 75.1, 75.8, 81.8, 127.4,
143.2, 150.2, 151.3, 152.2, 152.6; HRMS (ESIþ) m/z calcd for
C13H14ClN5O2Na [MþNaþ] 330.0728, obsd 330.0731.
4.2.9. N2-Boc-9-cyclopentyl-6-chloro-9H-purin-2-ylamine (4h). To
a stirred solution of purine 3 (269 mg, 1 mmol, 1 equiv) in anhy-
drous THF (14 mL) under an N2 atmosphere at room temperature
4.2.6. N2-Boc-9-benzyl-6-chloro-9H-purin-2-ylamine
(4f). R1OH
was benzyl alcohol. Flash column chromatography (eluent: hex/
EtOAc, 2:3) afforded 4f as a white foam (305 mg, 85%): Rf 0.34 (hex/
EtOAc, 1:2); mp 129–135 ꢁC; IR (KBr, cmꢂ1) 3417, 2979, 2359, 1745,
1610, 1573, 1520, 1444, 1403; dH (400 MHz, DMSO-d6) 1.46 (s, 9H,
C(CH3)3), 5.40 (s, 2H, CH2Ph), 7.28–7.41 (m, 5H, Ph), 8.58 (s, 1H,
H-8), 10.32 (s, 1, NHBoc); dC (100 MHz, DMSO-d6) 27.8, 46.6, 79.5,
126.7, 127.7, 127.8, 128.6, 136.1, 145.8, 149.0, 150.8, 152.5, 152.7;
HRMS (ESIþ) m/z calcd for C17H18ClN5O2Na [MþNaþ] 382.1041, obsd
382.1044.
was added cyclopentanol (118
(340 mg, 1.3 mmol, 1.3 equiv). After w2 min (when all reagents had
dissolved), DIAD (256 l, 1.3 mmol, 1.3 equiv) was added dropwise
ml, 1.3 mmol, 1.3 equiv) and PPh3
m
(over w30 s to 1 min). The reaction mixture was stirred at room
temperature for 15 min, after which time TLC analysis indicated the
reaction was complete. The reaction mixture was concentrated in
vacuo, then dry-loaded onto silica gel from CH2Cl2, and purified by
flash column chromatography (eluent: hex/EtOAc, 2:3) to afford the
title compound as a white foam (284 mg, 84%): Rf 0.36 (hex/EtOAc,
1:2); mp>198 ꢁC (dec); IR (KBr, cmꢂ1) 3452, 3192, 3111, 2979, 2874,
2360, 1736, 1592, 1536, 1490, 1449; dH (400 MHz, DMSO-d6) 1.47
(s, 9H, C(CH3)3), 1.60–1.71 (m, 2H, 2ꢃCH), 1.85–1.94 (m, 2H, 2ꢃCH),
1.95–2.06 (m, 2H, 2ꢃCH), 2.12–2.22 (m, 2H, 2ꢃCH), 4.84 (quintet,
J¼7.5 Hz, 1H, CH(CH2)2), 8.55 (s, 1H, H-8), 10.3 (s, 1H, NHBoc); dC
(100 MHz, DMSO-d6) 23.5, 27.9, 31.7, 55.9, 79.5, 127.2, 144.7, 148.7,
150.8, 151.9, 152.6; HRMS (ESIþ) m/z calcd for C15H20ClN5O2Na
[MþNaþ] 360.1197, obsd 360.1208.
4.2.7. N2-Boc-9-benzyl-6-chloro-9H-purin-2-ylamine (N9-4f) and
N2-Boc-7-benzyl-6-chloro-9H-purin-2-ylamine (N7-4f). Purine
3
(100 mg, 0.307 mmol, 1 equiv) was dissolved in anhydrous DMF
(3 mL). K2CO3 (85 mg, 0.614 mmol, 2 equiv) was added and the
reaction mixture was stirred for 1 h at room temperature, after
which time benzyl bromide (40 ml, 0.337 mmol, 1.1 equiv) was
added dropwise. The reaction was allowed to stir overnight at room
temperature. TLC indicated no starting material remaining and two
clearly distinguishable products. Water (40 mL) was added to the
reaction mixture, and the crude organics were extracted into EtOAc
(3ꢃ10 mL). The combined EtOAc extractions were washed with 5%
NaHCO3 (5ꢃ10 mL), water (10 mL), brine (10 mL), dried (Na2SO4),
filtered and concentrated. The two isomers were separated by silica
gel flash column chromatography (EtOAc/MeOH, 98:2, then EtOAc/
MeOH, 95:5), with the N9 isomer eluting first. N9-4f: 62% yield; Rf
4.2.10. N2-Boc-9-(indan-2-yl)-6-chloro-9H-purin-2-ylamine (4i). To
a stirred solution of purine 3 (269 mg, 1 mmol, 1 equiv) in anhy-
drous THF (14 mL) under an N2 atmosphere at room temperature
was added 2-indanol (201 mg, 1.5 mmol, 1.5 equiv) and PPh3
(394 mg, 1.5 mmol, 1.5 equiv). After w2 min (when all reagents had
dissolved), DIAD (295 ml, 1.5 mmol, 1.5 equiv) was added dropwise

S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
4627
(over w30 s to 1 min). The reaction mixture was stirred at room
temperature for 15 min, after which time TLC analysis indicated the
reaction was complete. The reaction mixture was concentrated in
vacuo, then dry-loaded onto silica gel from CH2Cl2, and purified by
flash column chromatography (eluent: hex/EtOAc, 2:3) to afford the
title compound as a white foam (324 mg, 84%): Rf 0.34 (hex/EtOAc,
1:2); mp 161–167 ꢁC; IR (KBr, cmꢂ1) 3458, 3282, 3010, 2981, 2926,
1752, 1614, 1584, 1522, 1481, 1460, 1428; dH 1.44 (s, 9H, C(CH3)3),
3.43–3.54 (m, 4H, CH2CHCH2), 5.33 (quintet, J¼7.5 Hz, 1H,
CH(CH2)2), 7.20–7.25 (m, 2H, Ph), 7.28–7.33 (m, 2H, Ph), 8.43 (s, 1H,
H-8), 10.30 (s, 1H, NHBoc); dC 27.8, 38.0, 55.4, 79.5, 124.4, 126.9,
127.1, 139.9, 144.4, 148.9, 150.8, 152.0, 152.7; HRMS (ESIþ) m/z calcd
for C19H20ClN5O2Na [MþNaþ] 408.1197, obsd 408.1179.
2H, CH2CH2OBn), 4.14 (t, J¼5.4 Hz, 2H, CH2CH2OBn), 4.47 (s, 2H,
CH2Ph), 6.48 (s, 2H, NH2), 7.21–7.33 (m, 5H, Ph), 7.66 (s, 1H, H-8),
10.60 (s, 1H, H-1); dC (100 MHz, DMSO-d6) 42.9, 67.9, 72.0, 116.7,
127.7, 127.8, 128.5, 138.1, 138.4, 151.5, 153.8, 157.1; HRMS (ESIþ) m/z
calcd for C14H16N5O2 [MþHþ] 286.1298, obsd 286.1290.
4.3.3. (2-Amino-6-oxo-1,6-dihydro-purin-9-yl)-acetic acid ethyl ester
(5c). The substrate purine was 4c. Flash column chromatography
(eluent: CH2Cl2/MeOH, 7:1/4:1) furnished 5c as a white solid
(117 mg, 99%): Rf 0.33 (CH2Cl2/MeOH, 5:1); mp>260 ꢁC (dec); IR
(KBr, cmꢂ1) 3442, 3136, 2988, 2733, 1749, 1690, 1634, 1604, 1542,
1481, 1425; dH (400 MHz, DMSO-d6) 1.20 (t, J¼7.2 Hz, 3H, CH2CH3),
4.15 (q, J¼7.2 Hz, 2H, CH2CH3), 4.85 (s, 2H, CH2CO2CH2CH3), 6.55
(s, 2H, NH2), 7.66 (s,1H, H-8),10.65, (s,1H, H-1); dC (100 MHz, DMSO-
d6) 13.9, 43.7, 61.2,116.0,137.7,151.4,153.7,156.7,167.9; HRMS (ESIþ)
m/z calcd for C9H12N5O3 [MþHþ] 238.0934, obsd 238.0942.
4.2.11. N2-Boc-9-([10S,20S,50R]-20-isopropyl-50-methylcyclohexyl)-6-
chloro-9H-purin-2-ylamine (4j). To a stirred solution of purine 3
(269 mg, 1 mmol, 1 equiv) in anhydrous THF (14 mL) under an N2
atmosphere at room temperature was added (ꢂ)-menthol (390 mg,
2.5 mmol, 2.5 equiv) and PPh3 (658 mg, 2.5 mmol, 2.5 equiv). After
4.3.4. 9-Allyl-2-amino-1,9-dihydro-purin-6-one (5d).13 The substrate
purine was 4d. Flash column chromatography (eluent: CH2Cl2/MeOH,
9:1/5:1) furnished 5d as a white solid (91 mg, 95%): Rf 0.43 (CH2Cl2/
MeOH, 5:1); mp>260 ꢁC (dec); IR (KBr, cmꢂ1) 3452, 3398, 3165, 2923,
2879, 2697, 1689, 1663, 1634, 1606, 1573, 1541, 1523, 1487, 1475; dH
(400 MHz, DMSO-d6) 4.56–4.59 (m, 2H, CH2CH]CH2), 4.92–4.98
(m, 1H, CH]CH2), 5.14–5.18 (m, 1H, CH]CH2), 5.95–6.04 (m, 1H,
CH]CH2), 6.51 (s, 2H, NH2), 7.64 (s, 1H, H-8), 10.34 (v br, H-1); dC
(100 MHz, DMSO-d6) 44.5,116.4,116.8,133.5,137.2,151.0,153.6,156.7;
HRMS (ESIþ) m/z calcd for C8H10N5O [MþHþ] 192.0879, obsd
192.0877.
w2 min (when all reagents had dissolved), DIAD (492 ml, 2.5 mmol,
2.5 equiv) was added dropwise (over w30 s to 1 min). The reaction
mixture was stirred for 1 h at 35 ꢁC, after which time TLC analysis
indicated the reaction was completed. The reaction mixture was
concentrated in vacuo, then dry-loaded onto silica gel from CH2Cl2,
and purified by flash column chromatography (eluent: hex/EtOAc,
2:3) to afford the title compound as a white foam (375 mg, 92%): Rf
0.65 (hex/EtOAc, 1:2); mp 170–173 ꢁC; IR (KBr, cmꢂ1) 3414, 3252,
3191, 3145, 2964, 2917, 2883, 2841, 1728, 1609, 1573, 1511, 1479,
1463, 1444, 1416; dH (400 MHz, DMSO-d6) 0.69 (d, J¼6.4 Hz, 3H,
CH3), 0.74 (d, J¼6.8 Hz, 3H, CH3), 0.81 (d, J¼6.0 Hz, 3H, CH3),
0.97–1.12 (m, 2H, 2ꢃCH), 1.42–1.57 (m, 11H, 2ꢃCH, C(CH3)3), 1.62–
1.78 (m, 3H, 3ꢃCH), 1.81–1.93 (m, 2H, 2ꢃCH), 5.13–5.17 (m, 1H, N9-
CH), 8.73 (s, 1H, H-8), 10.29 (s, 1H, NHBoc); dC (100 MHz, DMSO-d6)
20.3, 20.7, 22.0, 24.9, 25.7, 27.9, 29.0, 33.8, 39.9, 44.6, 51.8, 79.5,
125.8, 145.6, 149.1, 150.8, 152.3, 153.2; HRMS (ESIþ) m/z calcd for
C20H31ClN5O2 [MþHþ] 408.2160, obsd 408.2175.
4.3.5. 2-Amino-9-(prop-2-ynyl)-1,9-dihydro-purin-6-one (5e).13
The substrate purine was 4e. Flash column chromatography (elu-
ent: CH2Cl2/MeOH, 9:1/5:1) furnished 5e as a white solid (90 mg,
95%): Rf 0.41 (CH2Cl2/MeOH, 5:1); mp>260 ꢁC (dec); IR (KBr, cmꢂ1
)
3459, 3194, 3112, 2851, 2721, 2100, 1698, 1636, 1608, 1579, 1543,
1531, 1482, 1428; dH (400 MHz, DMSO-d6) 3.44 (t, J¼2.5 Hz, 1H,
C^CH), 4.80 (d, J¼2.5 Hz, 2H, CH2C^CH), 6.77 (s, 2H, NH2), 7.73 (s,
1H, H-8), 10.85 (s, 1H, H-1); dC (100 MHz, DMSO-d6) 31.9, 75.6, 78.4,
116.3, 136.5, 150.7, 153.9, 156.6; HRMS (ESIþ) m/z calcd for C8H8N5O
[MþHþ] 190.0723, obsd 190.0730.
4.3. Typical procedure for hydrolytic dechlorination and Boc
deprotection of purines 4
A solution of purine 4 (0.5 mmol) in 80% HCOOH (5 mL; 4:1
mixture of HCOOH/H2O) was stirred at 75 ꢁC for 2 h, by which
time TLC indicated all starting material and Boc-deprotected
intermediate had been consumed. The reaction mixture was con-
centrated under reduced pressure to dryness, adsorbed onto silica
gel from CH2Cl2/MeOH, and then purified by silica gel flash column
chromatography.
4.3.6. 2-Amino-9-benzyl-1,9-dihydro-purin-6-one (5f).13 The sub-
strate purine was 4f. Flash column chromatography (eluent:
CH2Cl2/MeOH, 9:1/5:1) furnished 5f as a white solid (117 mg,
97%): Rf 0.42 (CH2Cl2/MeOH, 5:1); mp>260 ꢁC (dec); IR (KBr, cmꢂ1
)
3453, 3134, 2954, 1694, 1651, 1607, 1584, 1538, 1480, 1412; dH
(400 MHz, DMSO-d6) 5.18 (s, 2H, CH2Ph), 6.51 (s, 2H, NH2),
7.21–7.36 (m, 5H, Ph), 7.78 (s, 1H, H-8), 10.65 (s, 1H, H-1); dC
(100 MHz, DMSO-d6) 45.7, 116.4, 127.0, 127.5, 128.6, 137.2, 137.4,
151.2, 153.6, 156.7; HRMS (ESIþ) m/z calcd for C12H12N5O [MþHþ]
242.1036, obsd 242.1039.
4.3.1. 2-Amino-9-butyl-1,9-dihydro-purin-6-one (5a). The substrate
purine was 4a. Flash column chromatography (eluent: CH2Cl2/
MeOH, 9:1/5:1) furnished 5a13 as a white solid (98 mg, 95%): Rf
0.42 (CH2Cl2/MeOH, 5:1); mp>260 ꢁC (dec); IR (KBr, cmꢂ1) 3474,
3150, 2930, 2873, 2670, 1688, 1630, 1572, 1543, 1477; dH (400 MHz,
DMSO-d6) 0.87 (t, J¼7.3 Hz, 3H, CH3), 1.23 (app sextet, J¼7.3 Hz, 2H,
CH2CH3), 1.69 (app quintet, J¼7.3 Hz, 2H, CH2CH2CH3), 3.92
(t, J¼7.3 Hz, 2H, CH2CH2CH2CH3), 6.49 (s, 2H, NH2), 7.68 (s, 1H, H-8),
10.59 (s, 1H, H-1); dC (100 MHz, DMSO-d6) 13.3, 19.1, 31.4, 42.3,
116.4, 137.3, 151.0, 153.4, 156.7; HRMS (ESIþ) m/z calcd for C9H14N5O
[MþHþ] 208.1198, obsd 208.1195.
4.3.7. 2-Amino-9-(iso-propyl)-1,9-dihydro-purin-6-one (5g).13 The
substrate purine was 4g. Flash column chromatography (eluent:
CH2Cl2/MeOH, 9:1/5:1) furnished 5g as a white solid (93 mg,
96%): Rf 0.41 (CH2Cl2/MeOH, 5:1); mp>260 ꢁC (dec); IR (KBr, cmꢂ1
)
3416, 3145, 2976, 2739, 1689, 1620, 1604, 1574, 1533, 1475, 1405; dH
(400 MHz, DMSO-d6) 1.42 (d, J¼6.8 Hz, 6H, CH(CH3)2), 4.48 (septet,
J¼6.8 Hz, 1H, CH(CH3)2), 6.44 (s, 2H, NH2), 7.79 (s, 1H, H-8), 10.57, (s,
1H, H-1); dC (100 MHz, DMSO-d6) 22.1, 45.6, 116.7, 134.9, 150.5,
153.2, 156.8; HRMS (ESIþ) m/z calcd for C8H12N5O [MþHþ]
194.1036, obsd 194.1028.
4.3.2. 2-Amino-9-(2-benzyloxyethyl)-1,9-dihydro-purin-6-one
(5b). The substrate purine was 4b. Flash column chromatography
(eluent: CH2Cl2/MeOH, 9:1/5:1) furnished 5b as a white solid
(135 mg, 95%): Rf 0.44 (CH2Cl2/MeOH, 5:1); mp>260 ꢁC (dec); IR
(KBr, cmꢂ1) 3327, 3161, 3034, 2859, 2744, 1686, 1647, 1602, 1576,
1543, 1489, 1436, 1416; dH (400 MHz, DMSO-d6) 3.72 (t, J¼5.4 Hz,
4.3.8. 2-Amino-9-cyclopentyl-1,9-dihydro-purin-6-one (5h). The sub-
strate purine was 4h. Flash column chromatography (eluent:
CH2Cl2/MeOH, 9:1/5:1) furnished 5h as a white solid (105 mg,
96%): Rf 0.44 (CH2Cl2/MeOH, 5:1); mp>260 ꢁC (dec); IR (KBr, cmꢂ1
)

4628
S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
3475, 3406, 3175, 3099, 2954, 2920, 2873, 2717, 1685, 1626, 1601,
1568, 1538, 1479, 1413; dH (400 MHz, DMSO-d6) 1.59–1.70 (m, 2H,
2ꢃCH), 1.77–1.91 (m, 4H, 4ꢃCH), 2.01–2.09 (m, 2H, 2ꢃCH), 4.59
(quintet, J¼7.2 Hz, 1H, CH(CH2)2), 6.45 (s, 2H, NH2), 7.75 (s, 1H, H-8),
10.58 (s, 1H, H-1); dC (100 MHz, DMSO-d6) 23.3, 31.9, 54.6, 116.7,
135.4, 150.9, 153.2, 156.7; HRMS (ESIþ) m/z calcd for C10H14N5O
[MþHþ] 220.1192, obsd 220.1190.
3027, 2961, 2932, 2869, 1784, 1632, 1580, 1552, 1490, 1446; dH
(400 MHz, DMSO-d6) 0.69 (t, J¼6.8 Hz, 3H, CH3), 0.92–1.10 (m, 4H,
CH2CH2CH3), 2.55–2.64 (m, 2H, NHCH2), 7.19–7.23 (m, 6H, Trt),
7.28–8.28 (m, 10H, Trt, NH), 7.81 (s, 1H, H-8); dC (100 MHz,
DMSO-d6) 13.6, 19.5, 30.3, 40.6, 75.0, 124.0, 127.5, 127.9, 129.3, 141.0,
141.8, 149.7, 154.6, 157.8; HRMS (ESIþ) m/z calcd for C28H27ClN5
[MþHþ] 468.1949, obsd 468.1942.
4.3.9. 2-Amino-9-(indan-2-yl)-1,9-dihydro-purin-6-one (5i).13 The
substrate purine was 4i. Flash column chromatography (eluent:
CH2Cl2/MeOH, 9:1/5:1) furnished 5i as a white solid (127 mg,
4.4.2. (2-Benzyloxyethyl)-(6-chloro-9-trityl-9H-purin-2-yl)-amine
(10b). R2OH was 2-benzyloxyethanol. Flash column chromatogra-
phy (eluent: hex/EtOAc, 3:1) yielded 10b as a white, sticky foam
(490 mg, 90%): Rf 0.39 (hex/EtOAc, 1:1); dH (400 MHz, DMSO-d6)
2.80–2.88 (m, 2H, NHCH2), 3.01–3.08 (m, 2H, CH2CH2OBn), 4.24
(s, 2H, CH2Ph), 7.17–7.21 (m, 6H, Trt), 7.22–7.35 (m, 14H, Trt, Ph),
7.43–7.50 (m, 1H, NH), 7.82 (s, 1H, H-8); dC (100 MHz, DMSO-d6)
67.4, 67.7, 71.6, 74.9, 124.0, 127.2, 127.5, 127.9 (2), 128.0, 129.2, 138.4,
140.8, 141.9, 149.8, 154.3, 157.5; HRMS (ESIþ) m/z calcd for
C33H29ClN5O [MþHþ] 546.2055, obsd 546.2043. Product was
contaminated with a small amount of reduced DIAD (DIAD-H2);
yield was determined by analysis of the 1H NMR spectrum.
95%): Rf 0.43 (CH2Cl2/MeOH, 5:1); mp>260 ꢁC (dec); IR (KBr, cmꢂ1
)
3485, 3147, 2637, 1703, 1633, 1604, 1537, 1480; dH (400 MHz,
DMSO-d6) 3.36 (d, J¼7.7 Hz, 4H, 2ꢃCH2), 5.11 (quintet, J¼7.7 Hz,
1H, CH(CH2)2), 6.52 (s, 2H, NH2), 7.19–7.23 (m, 2H, Ar), 7.26–7.29
(m, 2H, Ar), 7.60 (s, 1H, H-8), 10.62 (s, 1H, H-1); dC (100 MHz,
DMSO-d6) 38.3, 54.5, 116.9, 124.4, 126.8, 135.5, 140.2, 151.0, 153.3,
156.7; HRMS (ESIþ) m/z calcd for C14H14N5O [MþHþ] 268.1192, obsd
268.1189.
4.3.10. 2-Amino-9-([10S,20S,50R]-20-isopropyl-50-methylcyclohexyl)-
1,9-dihydro-purin-6-one (5j). The substrate purine was 4j. Flash
column chromatography (eluent: CH2Cl2/MeOH, 10:1/7:1)
furnished 5j as a white solid (139 mg, 96%): Rf 0.51 (CH2Cl2/MeOH,
5:1); mp>260 ꢁC (dec); IR (KBr, cmꢂ1) 3416, 3134, 2953, 2844,
2729, 1698, 1644, 1603, 1567, 1536, 1474; dH (400 MHz, DMSO-d6)
0.70 (d, J¼6.8 Hz, 3H, CH3), 0.77 (d, J¼6.4 Hz, 3H, CH3), 0.81 (d,
J¼6.0 Hz, 3H, CH3), 0.96–1.05 (m, 1H, CH), 1.11–1.16 (m, 1H, CH),
1.33–1.44 (m, 2H, 2ꢃCH), 1.53–1.66 (m, 3H, 3ꢃCH), 1.81–1.88 (m,
2H, 2ꢃCH), 4.86 (m, 1H, N9-CH), 6.46 (s, 2H, NH2), 7.91 (s, 1H, H-8),
10.57 (s, 1H, H-1); dC (100 MHz, DMSO-d6) 20.4, 20.6, 22.0, 25.2,
25.6, 28.9, 33.9, 39.9, 44.8, 50.2, 115.5, 137.0, 151.3, 153.3, 156.8;
HRMS (ESIþ) m/z calcd for C15H24N5O [MþHþ] 290.1975, obsd
290.1972. Slow vapour diffusion of ethyl ether into a solution of
5j in DMF gave a single crystal suitable for X-ray diffraction
analysis.
4.4.3. (6-Chloro-9-trityl-9H-purin-2-ylamino)-acetic acid ethyl ester
(10c). R2OH was ethyl glycolate. Flash column chromatography
(eluent: hex/EtOAc, 3:1) yielded 10c as a white powder (413 mg,
83%): Rf 0.34 (hex/EtOAc, 1:1); mp 195–201 ꢁC; IR (KBr, cmꢂ1) 3451,
3283, 2937, 2355, 1751, 1613, 1585, 1550, 1489, 1449; dH (400 MHz,
DMSO-d6) 1.05 (t, J¼6.7 Hz, 3H, CH2CH3), 3.30–3.32 (m, 2H, NHCH2),
3.77 (br q, J¼6.7 Hz, 2H, CH2CH3), 7.15–7.19 (m, 6H, Trt), 7.28–7.37
(m, 9H, Trt), 7.64–7.70 (m, 1H, NH), 7.88 (s, 1H, H-8); dC (100 MHz,
DMSO-d6) 13.8, 42.2, 59.8, 75.0, 124.5, 127.4, 127.8, 129.2, 140.8,
142.3, 149.8, 154.0, 157.3, 169.6; HRMS (ESIþ) m/z calcd for
C28H25ClN5O2 [MþHþ] 498.1691, obsd 498.1678.
4.4.4. Allyl-(6-chloro-9-trityl-9H-purin-2-yl)-amine (10d). R2OH was
allyl alcohol. Flash column chromatography (eluent: hex/EtOAc,
3:1) yielded 10d as a white solid (390 mg, 86%): Rf 0.37 (hex/EtOAc,
1:1); mp 145–151 ꢁC; IR (KBr, cmꢂ1) 3449, 3288, 3136, 3058, 2923,
2361, 1611, 1578, 1547, 1492, 1448, 1414; dH (400 MHz, DMSO-d6)
3.23–3.32 (m, 2H, NHCH2), 4.77–4.85 (m, 2H, CH]CH2), 5.36–5.50
(m, 1H, CH]CH2), 7.20–7.25 (m, 6H, Trt), 7.26–7.36 (m, 9H, Trt),
7.46–7.56 (m, 1H, NH), 7.85 (s, 1H, H-8); dH (400 MHz, DMSO-d6)
43.1, 74.9, 114.9, 124.1, 127.4, 127.8, 129.2, 135.0, 140.9, 142.0, 149.7,
154.4, 157.4; HRMS (ESIþ) m/z calcd for C27H22ClN5Na [MþNaþ]
474.1455, obsd 474.1461.
4.4. Typical procedure for Mitsunobu coupling of purine 7
and trifluoroacetyl deprotection of subsequent purines
To a stirred solution of purine 7 (507 mg, 1 mmol, 1 equiv) in
anhydrous THF (14 mL) under a nitrogen atmosphere at room
temperature was added the alcohol R2OH (1.2 mmol, 1.2 equiv)
and PPh3 (341 mg, 1.3 mmol, 1.3 equiv). After w2 min, DIAD
(256 ml, 1.3 mmol, 1.3 equiv) was added dropwise (over w30 s to
1 min). The reaction mixture was stirred at room temperature for
15 min, after which time TLC analysis indicated the reaction was
complete. The reaction mixture was concentrated in vacuo. The
crude residue was taken up in a 3:2 mixture of MeOH/THF
(10 mL). K2CO3 (1.2 mmol) was added, and the reaction mixture
was stirred at room temperature for 1 h, when TLC analysis
suggested all intermediate trifluoroacetamide (e.g., 9a: Rf 0.40
(hex/EtOAc, 2:1)) had been deprotected (e.g., 10a: Rf 0.34 (hex/
EtOAc, 2:1)). The reaction mixture was diluted with water
(150 mL) along with a small amount of brine (10 mL), then
extracted into EtOAc (3ꢃ30 mL). The combined EtOAc extractions
were washed with water (20 mL), brine (20 mL), dried (Na2SO4),
filtered and concentrated. The crude residue was adsorbed onto
silica gel from CH2Cl2 at room temperature (heating was avoided
in order to prevent trityl cleavage), then purified by flash column
chromatography.
4.4.5. (6-Chloro-9-trityl-9H-purin-2-yl)-(prop-2-ynyl)-amine
(10e). R2OH was propargyl alcohol. Flash column chromatography
(eluent: hex/EtOAc, 3:1) yielded 10e as a white solid (386 mg, 86%):
Rf 0.37 (hex/EtOAc, 1:1); dH (400 MHz, DMSO-d6) 2.89 (br t, 1H,
C^CH), 3.37–3.52 (m, 2H, CH2C^CH), 7.26–7.37 (m, 15H, Trt), 7.58–
7.72 (m, 1H, NH), 7.97 (s, 1H, H-8); dC (100 MHz, DMSO-d6) 30.0,
71.9, 75.1, 81.3, 124.5, 127.3, 127.9, 129.1, 141.0, 142.7, 149.8, 154.2,
156.9; HRMS (ESIþ) m/z calcd for C27H20ClN5Na [MþNaþ] 472.1299,
obsd 472.1308. Product was contaminated with a small amount of
reduced DIAD (DIAD-H2); yield was determined by analysis of the
1H NMR spectrum.
4.4.6. Benzyl-(6-chloro-9-trityl-9H-purin-2-yl)-amine (10f). R2OH
was benzyl alcohol. Flash column chromatography (eluent: hex/
EtOAc, 3:1) yielded 10f as a white solid (420 mg, 84%): Rf 0.37 (hex/
EtOAc, 1:1); dH (400 MHz, DMSO-d6) 3.82–3.90 (m, 2H, CH2Ph),
6.80–6.90 (m, 2H, Ph), 7.14–7.30 (m, 18H, Trt, Ph), 7.89 (s, 1H, H-8),
7.85–7.98 (m, 2H, H-8, NH); dC (100 MHz, DMSO-d6) 44.0, 74.9,
124.2, 126.3, 127.1, 127.3, 127.8, 127.9, 129.1, 139.8, 141.0, 142.2, 149.8,
154.3, 157.6; HRMS (ESIþ) m/z calcd for C31H25ClN5 [MþHþ]
502.1793, obsd 502.1779. Product was contaminated with a small
4.4.1. 9-Butyl-(6-chloro-9-trityl-9H-purin-2-yl)-amine (10a). R2OH
was 1-butanol. Flash column chromatography (eluent: hex/EtOAc,
3:1) yielded 10a as an off-white solid (416 mg, 89%): Rf 0.39 (hex/
EtOAc, 1:1); mp>230 ꢁC (dec); IR (KBr, cmꢂ1) 3472, 3297, 3063,

S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
4629
amount of reduced DIAD (DIAD-H2); yield was determined by
analysis of the 1H NMR spectrum.
153.2; HRMS (ESIþ) m/z calcd for C34H35ClN5O2 [MþHþ] 580.2473,
obsd 580.2472.
4.4.7. (6-Chloro-9-trityl-9H-purin-2-yl)-(iso-propyl)-amine
(10g). R2OH was iso-propanol. Flash column chromatography
(eluent: hex/EtOAc, 3:1) yielded 10g as a white solid (304 mg, 67%):
Rf 0.38 (hex/EtOAc, 1:1); mp>140 ꢁC (dec); IR (KBr, cmꢂ1) 3417,
3260, 3134, 3053, 2967, 2927, 2871, 2360, 1610, 1575, 1543, 1495,
1448; dH (400 MHz, DMSO-d6) 0.77 (d, J¼6.4 Hz, 6H, CH(CH3)2),
3.08–3.18 (m, 1H, CH(CH3)2), 7.18–7.22 (m, 7H, Trt, NH), 7.28–7.38
(m, 9H, Trt), 7.80 (s, 1H, H-8); dC (100 MHz, DMSO-d6) 21.6, 42.2,
74.9, 123.7, 127.4, 127.8, 129.2, 140.9, 141.7, 149.7, 154.5, 157.0; HRMS
(ESIþ) m/z calcd for C27H25ClN5 [MþHþ] 454.1793, obsd 454.1783.
4.5. Typical procedure for hydrolytic dechlorination and trityl
deprotection of purines 10, 11g and 11h
To a solution of purine 10 or 11 (0.5 mmol) in THF (1 mL) was
added 80% HCOOH (4 mL; 4:1 mixture of HCOOH/H2O). The
resulting mixture was stirred at 75 ꢁC for 4 h, by which time TLC
indicated all starting material and de-tritylated intermediate had
been consumed. The reaction mixture was concentrated under
reduced pressure to dryness, adsorbed onto silica gel from CH2Cl2/
MeOH, and purified by silica gel flash column chromatography.
4.4.8. (6-Chloro-9-trityl-9H-purin-2-yl)-cyclopentyl-amine
(10h). R2OH was cyclopentanol. Flash column chromatography
(eluent: hex/EtOAc, 3:1) yielded 10h as a white solid (168 mg, 35%):
Rf 0.39 (hex/EtOAc, 1:1); mp 133–139 ꢁC; IR (KBr, cmꢂ1) 3454, 3252,
3131, 3058, 2956, 2867, 2361, 1610, 1578, 1541, 1495, 1450; dH
(400 MHz, DMSO-d6) 1.10–1.20 (m, 2H, 2ꢃCH), 1.24–1.52 (m, 6H,
6ꢃCH), 3.20–3.29 (m, 1H, NHCH), 7.19–7.23 (m, 6H, Trt), 7.27–7.38
(m, 10H, Trt, NH), 7.81 (s, 1H, H-8); dC (100 MHz, DMSO-d6) 23.1,
31.3, 52.5, 74.9, 123.7, 127.4, 127.8, 129.2, 140.9, 141.7, 149.5, 154.4,
157.3; HRMS (ESIþ) m/z calcd for C29H27ClN5 [MþHþ] 480.1949,
obsd 480.1954.
4.5.1. 2-Butylamino-1,9-dihydro-purin-6-one (12a). The substrate
purine was 10a. Flash column chromatography (eluent: CH2Cl2/
MeOH, 7:1/4:1) afforded the title compound 12a as a 78:22
mixture of N9H/N7H tautomers (103 mg, 99%): white solid; Rf 0.23
(CH2Cl2/MeOH, 5:1); mp>220 ꢁC (dec); IR (KBr, cmꢂ1) 3457, 2955,
2824, 2360, 1695, 1616, 1505, 1472; dH (400 MHz, DMSO-d6) 0.89
(t, J¼7.2 Hz, 3H, CH3), 1.32 (app sextet, J¼7.2 Hz, 2H, CH2CH3), 1.49
(app quin, J¼7.2 Hz, 2H, CH2CH2CH3), 3.23 (app q, J¼7.2 Hz, 2H,
NHCH2), 6.14–6.25 (m, 0.22H, NHCH2), 6.40–6.50 (m, 0.78H,
NHCH2), 7.61 (s, 0.78H, H-8), 7.89 (s, 0.22H, H-8), 10.42–10.64
(m, 1H, H-1), 12.46 (s, 0.78H, imidazole NH), 12.86 (s, 0.22H,
imidazole NH); dC (100 MHz, DMSO-d6; only signals for major N9H
tautomer given) 13.6, 19.5, 30.8, 40.0, 116.3, 135.1, 151.5, 152.5,
156.8; HRMS (ESIþ) m/z calcd for C9H14N5O [MþHþ] 208.1198, obsd
208.1198.
4.4.9. N2-Boc-(6-chloro-9-trityl-9H-purin-2-yl)-(iso-propyl)-amine
(11g). To a stirring solution of 8 (256 mg, 0.5 mmol, 1 equiv) in
anhydrous THF (7 mL) was added iso-propanol (95
2.5 equiv) and PPh3 (328 mg, 1.25 mmol, 2.5 equiv). After w2 min
(when all reagents had dissolved), DIAD (246 l, 1.25 mmol,
ml, 1.25 mmol,
m
4.5.2. 2-(2-Benzyloxyethyl)amino-1,9-dihydro-purin-6-one
(12b). The substrate purine was 10b. Flash column chromatogra-
phy (eluent: CH2Cl2/MeOH, 7:1/4:1) afforded the title compound
12b as a 78:22 mixture of N9H/N7H tautomers (138 mg, 97%):
white solid; Rf 0.29 (CH2Cl2/MeOH, 5:1); mp>250 ꢁC (dec); IR (KBr,
cmꢂ1) 3477, 3277, 3116, 3061, 2937, 2872, 2806, 2705, 1694, 1612,
1539, 1507, 1468, 1435; dH (400 MHz, DMSO-d6) 3.47 (m, 2H,
NHCH2), 3.58 (t, J¼5.2 Hz, 2H, CH2CH2OBn), 4.51 (s, 2H, CH2Ph),
6.20–6.28 (m, 0.22H, NHCH2), 6.42–6.49 (m, 0.78H, NHCH2),
7.25–7.35 (m, 5H, Ph), 7.63 (s, 0.78H, H-8), 7.91 (s, 0.22H, NHCH2),
10.55 (br s, 0.78H, H-1), 10.69 (br s, 0.22H, H-1), 12.47 (br s, 0.78H,
imidazole NH), 12.88 (br s, 0.22H, imidazole NH); dC (100 MHz,
DMSO-d6; only signals for major N9H tautomer given) 40.2, 68.1,
71.9, 116.5, 127.4, 127.5, 128.1, 135.3, 138.2, 151.3, 152.4, 156.8; HRMS
(ESIþ) m/z calcd for C14H16N5O2 [MþHþ] 286.1298, obsd 286.1288.
2.5 equiv) was added dropwise (over w30 s). The reaction mixture
was then heated at 35 ꢁC for 30 min, after which time TLC
confirmed the reaction was complete. All THF was removed in
vacuo, then the residue was dry-loaded onto silica gel from CH2Cl2
at room temperature, and purified by flash column chromatogra-
phy (eluent: hex/EtOAc, 5:1) to give the title compound 11g as
a
white foam (237 mg, 86%): Rf 0.55 (hex/EtOAc, 1:1); mp
172–177 ꢁC; IR (KBr, cmꢂ1) 3414, 3132, 3057, 3026, 2973, 2927, 1711,
1590, 1553, 1495, 1470, 1447, 1427; dH (400 MHz, DMSO-d6) 0.75
(d, J¼6.8 Hz, 6H, CH(CH3)2), 1.23 (s, 9H, C(CH3)3), 4.05 (septet,
J¼6.8 Hz, 1H, CH(CH3)2), 7.20–7.25 (m, 6H, Trt), 7.27–7.37 (m, 9H,
Trt), 8.35 (s, 1H, H-8); dC (100 MHz, DMSO-d6) 20.0, 27.6, 48.6, 75.8,
79.7, 127.7, 128.0, 129.1, 129.8, 140.5, 146.8, 149.1, 151.9, 152.7, 153.1;
HRMS (ESIþ) m/z calcd for C32H33ClN5O2 [MþHþ] 554.2317, obsd
554.2326.
4.5.3. (6-Oxo-6,9-dihydro-1H-purin-2-ylamino)-acetic acid ethyl ester
(12c). The substrate purine was 10c. Flash column chromatography
(eluent: CH2Cl2/MeOH, 7:1/4:1) afforded the title compound 12c
as a 71:29 mixture of N9H/N7H tautomers (114 mg, 96%): white
4.4.10. N2-Boc-(6-chloro-9-trityl-9H-purin-2-yl)-cyclopentyl-amine
(11h). To a stirring solution of 8 (256 mg, 0.5 mmol, 1 equiv) in
anhydrous THF (7 mL) was added cyclopentanol (113
2.5 equiv) and PPh3 (328 mg, 1.25 mmol, 2.5 equiv). After w2 min
(when all reagents had dissolved), DIAD (246 l, 1.25 mmol,
ml, 1.25 mmol,
solid; Rf 0.23 (CH2Cl2/MeOH, 5:1); mp>190 ꢁC (dec); IR (KBr, cmꢂ1
)
m
3452, 3283, 3124, 3069, 2935, 2674, 1726, 1708, 1613, 1584, 1515,
1475; dH (400 MHz, DMSO-d6) 1.19 (t, J¼7.1 Hz, 3H, CH2CH3), 4.04
(d, J¼6.0 Hz, 2H, CH2CO2Et), 4.11 (q, J¼7.1 Hz, 2H, CH2CH3),
6.53–6.62 (m, 0.29H, NHCH2), 6.76–6.85 (m, 0.71H, NHCH2), 7.63
(s, 0.71H, H-8), 7.91 (s, 0.29H, H-8), 10.85–11.20 (m, 1H, H-1), 12.55
(br s, 0.71H, imidazole NH), 12.94 (br s, 0.29H, imidazole NH); dC
(100 MHz, DMSO-d6; only signals for major N9H tautomer given)
14.0, 42.3, 60.4, 116.8, 135.5, 150.9, 152.3, 156.7, 170.2; HRMS (ESIþ)
m/z calcd for C9H12N5O3 [MþHþ] 238.0934, obsd 238.0939.
2.5 equiv) was added dropwise (over w30 s). The reaction mixture
was then heated at 35 ꢁC for 2 h, after which time TLC confirmed
the reaction was complete. All THF was removed in vacuo, then the
residue was dry-loaded onto silica gel from CH2Cl2 at room tem-
perature, and purified by flash column chromatography (eluent:
hex/EtOAc, 5:1) to give the title compound 11h as a white solid
(243 mg, 84%): Rf 0.56 (hex/EtOAc, 1:1); mp 176–180 ꢁC; IR (KBr,
cmꢂ1) 3453, 3124, 3062, 3027, 2970, 2869, 2360, 1712, 1589, 1554,
1494, 1469, 1449, 1431; dH (400 MHz, DMSO-d6) 1.10–1.19 (m, 3H,
3ꢃCH (cyclopentyl)), 1.20–1.27 (m, 12H, 3ꢃCH (cyclopentyl),
C(CH3)3), 1.39–1.50 (m, 2H, 2ꢃCH (cyclopentyl)), 4.07 (quintet,
J¼8.4 Hz, 1H, CH(CH2)2), 7.17–7.22 (m, 6H, Trt), 7.29–7.38 (m, 9H,
Trt), 8.34 (s, 1H, H-8); dC (100 MHz, DMSO-d6) 22.8, 27.6, 28.6, 58.0,
75.8, 79.8, 127.7, 128.1, 129.1, 129.9, 140.5, 147.0, 149.2, 152.1, 153.0,
4.5.4. 2-Allylamino-1,9-dihydro-purin-6-one (12d). The substrate
purine was 10d. Flash column chromatography (eluent: CH2Cl2/
MeOH, 7:1/4:1) afforded the title compound 12d as a 78:22
mixture of N9H/N7H tautomers (93 mg, 97%): white solid; Rf 0.23
(CH2Cl2/MeOH, 5:1); mp>250 ꢁC (dec); IR (KBr, cmꢂ1) 3475, 3437,

4630
S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
3133, 2979, 2858, 1677, 1618, 1518, 1461; dH (400 MHz, DMSO-d6)
3.90 (app t, J¼5.2 Hz, 2H, CH2CH]CH2), 5.06–5.11 (m, 1H,
CH]CH2), 5.17–5.24 (m,1H, CH]CH2), 5.86–5.95 (m,1H, CH]CH2),
6.48–6.60 (s, 0.22H, NHCH2), 6.74–6.88 (s, 0.78H, NHCH2), 7.63
(s, 0.78H, H-8), 7.89 (s, 0.22H, H-8), 10.66–10.92 (m, 1H, H-1), 12.51
(br s, 0.78H, imidazole NH), 12.89 (br s, 0.22H, imidazole NH); dC
(100 MHz, DMSO-d6; only signals for major N9H tautomer given)
42.5, 115.2, 116.5, 135.1, 135.3, 151.3, 152.4, 156.8; HRMS (ESIþ) m/z
calcd for C8H10N5O [MþHþ] 192.0879, obsd 192.0877.
220.1192, obsd 220.1191. Hydrolytic dechlorination and depro-
tection of the Trt and Boc groups of 11h in an identical manner gave
a compound identical to 12h.
4.6. Typical procedure for Mitsunobu coupling of purine 4a:
condition A
To a stirred solution of purine 4a (163 mg, 0.5 mmol, 1 equiv) in
anhydrous THF (7 mL) under an N2 atmosphere at room tempera-
ture was added the alcohol R2OH (1.25 mmol, 2.5 equiv) and PPh3
4.5.5. 2-(Prop-2-ynyl)amino-1,9-dihydro-purin-6-one (12e). The sub-
strate purine was 10e. Flash column chromatography (eluent:
CH2Cl2/MeOH, 7:1/4:1) afforded the title compound 12e as
a 78:22 mixture of N9H/N7H tautomers (92 mg, 97%): white solid;
Rf 0.24 (CH2Cl2/MeOH, 5:1); mp>190 ꢁC (dec); IR (KBr, cmꢂ1) 3448,
2857, 2361, 2328, 2127, 1689, 1611, 1520, 1463; dH (400 MHz, DMSO-
d6) 3.13 (t, J¼2.1 Hz, 1H, C^CH), 4.05 (dd, J¼5.6, 2.1 Hz, 2H,
CH2C^CH), 6.79–6.88 (m, 0.22H, NHCH2), 7.12 (s, 0.78H, NHCH2),
7.65 (s, 0.78H, H-8), 7.92 (s, 0.22H, H-8), 10.84–11.10 (m, 1H, H-1),
12.62 (br s, 0.78H, imidazole NH), 12.95 (br s, 0.22H, imidazole NH);
dC (100 MHz, DMSO-d6; only signals for major N9H tautomer given)
29.9, 73.1, 81.2, 116.7, 135.5, 150.9, 152.0, 156.7; HRMS (ESIþ) m/z
calcd for C8H8N5O [MþHþ] 190.0723, obsd 190.0727.
(328 mg, 1.25 mmol, 2.5 equiv). After w2 min, DIAD (246 ml,
1.25 mmol, 2.5 equiv) was added dropwise (over w30 s). The
reaction mixture was stirred at 35 ꢁC for the specified time, at
which point TLC analysis indicated the reaction was complete.
The reaction mixture was concentrated in vacuo, then adsorbed
onto silica gel from CH2Cl2, and purified by flash column
chromatography.
4.6.1. N2-Boc-N2-butyl-9-butyl-6-chloro-9H-purin-2-yl-amine
(13a). R2OH was n-butanol, and the reaction time was 30 min.
Flash column chromatography (eluent: hex/EtOAc, 3:2) furnished
13a as a pale yellow gum: (177 mg, 93%): Rf 0.50 (hex/EtOAc, 1:1);
mp>101–103 ꢁC; IR (KBr, cmꢂ1) 3453, 3094, 2962, 2935, 2874, 1713,
1608, 1561, 1509, 1451, 1429, 1403; dH (400 MHz, CDCl3) 0.91
(t, J¼7.4 Hz, 3H, CH2CH3), 0.97 (t, J¼7.2 Hz, 3H, CH2CH3), 1.30–1.44
(m, 4H, 2ꢃCH2CH3), 1.51 (s, 9H, C(CH3)3), 1.61–1.69 (m, 2H,
CH2CH2CH3), 1.86–1.94 (m, 2H, CH2CH2CH3), 3.94 (t, J¼7.4 Hz, 2H,
N2-CH2), 4.21 (t, J¼7.2 Hz, 2H, N9-CH2), 8.00 (s, 1H, H-8); dC
(100 MHz, CDCl3) 13.2, 13.6, 19.6, 19.8, 28.0, 30.7, 31.6, 43.8, 48.0,
81.1, 127.9, 144.6, 150.0, 152.4, 153.8, 155.1; HRMS (ESIþ) m/z calcd
for C18H28ClN5O2Na [MþNaþ] 404.1823, obsd 404.1824.
4.5.6. 2-Benzylamino-1,9-dihydro-purin-6-one (12f). The substrate
purine was 10f. Flash column chromatography (eluent: CH2Cl2/
MeOH, 7:1/4:1) afforded the title compound 12f as a 78:22
mixture of N9H/N7H tautomers (116 mg, 96%): white solid; Rf 0.28
(CH2Cl2/MeOH, 5:1); mp>220 ꢁC (dec); IR (KBr, cmꢂ1) 3444, 2966,
2360, 1673, 1614, 1515, 1456; dH (400 MHz, DMSO-d6) 4.49
(d, J¼6.0 Hz, 2H, CH2Ph), 6.56–6.67 (m, 0.22H, NHCH2), 6.79–6.60
(m, 0.78H, NHCH2), 7.22–7.28 (m, 1H, Ph), 7.30–7.36 (m, 4H, Ph),
7.63 (s, 0.78H, H-8), 7.90 (s, 0.22H, H-8), 10.55–10.76 (m, 1H, H-1),
12.49 (br s, 0.78H, imidazole NH), 12.90 (br s, 0.22H, imidazole NH);
dC (100 MHz, DMSO-d6; only signals for major N9H tautomer given)
43.8, 116.6, 126.8, 127.0, 128.3, 135.3, 139.1, 151.3, 152.4, 156.8; HRMS
(ESIþ) m/z calcd for C12H12N5O [MþHþ] 242.1036, obsd 242.1040.
Slow vapour diffusion of ethyl ether into a solution of 12f in DMF
gave a single crystal suitable for X-ray diffraction analysis.
4.6.2. N2-Boc-N2-(iso-propyl)-9-butyl-6-chloro-9H-purin-2-yl-
amine (13e). R2OH was iso-propanol, and the reaction time was
30 min. Flash column chromatography (hex/EtOAc, 3:2) furnished
13e as a white solid (169 mg, 92%): Rf 0.50 (hex/EtOAc, 1:1); mp
85–89 ꢁC; IR (KBr, cmꢂ1) 3446, 3114, 2976, 2954, 2871, 1693, 1600,
1559, 1504, 1473, 1435, 1405; dH (400 MHz, DMSO-d6) 0.89
(t, J¼7.2 Hz, 3H, CH3), 1.16–1.30 (m, 8H, CH2CH3, CH(CH3)2), 1.35
(s, 9H, C(CH3)3), 1.84 (app quintet, J¼7.2 Hz, 2H, CH2CH2CH3), 4.26
(t, J¼7.2 Hz, 2H, N9-CH2), 4.46 (sextet, J¼6.8 Hz, 1H,
CH(CH3)2CH(CH3)2), 8.69 (s, 1H, H-8); dC (100 MHz, CDCl3) 13.4,
19.8, 20.9, 28.2, 31.8, 44.1, 49.9, 80.9, 128.8,145.0, 150.3, 152.4, 153.9,
154.2; HRMS (ESIþ) m/z calcd for C17H26ClN5O2Na [MþNaþ]
390.1667, obsd 390.1669.
4.5.7. 2-(iso-Propyl)amino-1,9-dihydro-purin-6-one (12g). The sub-
strate purine was 10g. Flash column chromatography (eluent:
CH2Cl2/MeOH, 7:1/4:1) afforded the title compound 12g as awhite
solid (92 mg, 95%): Rf 0.23 (CH2Cl2/MeOH, 5:1); mp>220 ꢁC (dec); IR
(KBr, cmꢂ1) 3460, 2974, 2802, 1674, 1608, 1512, 1464; dH (400 MHz,
DMSO-d6) 1.15 (d, J¼6.5 Hz, 6H, CH(CH3)2), 3.93 (apparent octet,
J¼6.5 Hz, 1H, CH(CH3)2), 6.22–6.33 (m, 1H, NHCH(CH3)2), 7.67 (br
s, 1H, H-8), 10.35 (s, 1H, H-1), 12.37–12.89 (m 1H, imidazole NH);
dC aromatic signals too weak to allow reliable reporting, 13C NMR
spectrum is included in the Supplementary data 3; HRMS (ESIþ) m/z
calcd for C8H12N5O [MþHþ] 194.1036, obsd 194.1033. Hydrolytic
dechlorination and deprotection of the Trt and Boc groups of 11g
in an identical manner gave a compound identical to 12g.
4.6.3. N2-Boc-N2-cyclopentyl-9-butyl-6-chloro-9H-purin-2-yl-amine
(13f). R2OH was cyclopentanol, and the reaction time was 2 h. Flash
column chromatography (hex/EtOAc, 3:2) furnished the title
compound 13f as a white solid (175 mg, 89%): Rf 0.53 (hex/EtOAc,
1:1); mp 96–99 ꢁC; IR (KBr, cmꢂ1) 3549, 3414, 3114, 2977, 2960,
2873, 2363, 1691, 1638, 1598, 1558, 1506, 1466, 1439, 1403; dH
(400 MHz, DMSO-d6) 0.87 (t, J¼7.3 Hz, 3H, CH3), 1.24 (app sextet,
J¼7.3 Hz, 2H, CH2CH3), 1.33 (s, 9H, C(CH3)3), 1.40–1.74 (m, 6H, 6ꢃCH
(cyclopentyl)), 1.78–1.92 (m, 4H, CH2CH2CH3, 2ꢃCH (cyclopentyl)),
4.25 (t, J¼7.3 Hz, 2H, N9-CH2), 4.54 (app quintet, J¼8.3 Hz, 1H,
N(Boc)CH), 8.70 (s,1H, H-8); dC (100 MHz, DMSO-d6) 13.2, 19.1, 23.6,
27.7, 29.4, 30.9, 43.4, 58.4, 80.0, 128.6, 147.9, 148.4, 152.6, 153.1,
153.4; HRMS (ESIþ) m/z calcd for C19H29ClN5O2 [MþHþ] 394.2004,
obsd 394.2003.
4.5.8. 2-Cyclopentylamino-1,9-dihydro-purin-6-one (12h). The sub-
strate purine was 10h. Flash column chromatography (eluent:
CH2Cl2/MeOH, 7:1/4:1) afforded the title compound 12h as
a 81:19 mixture of N9H/N7H tautomers (52 mg, 95%): white solid;
Rf 0.26 (CH2Cl2/MeOH, 5:1); mp>220 ꢁC (dec); IR (KBr, cmꢂ1) 3457,
2961, 2876, 2358, 1677, 1607, 1514, 1454; dH (400 MHz, DMSO-d6)
1.38–1.70 (m, 6H, 6ꢃCH), 1.86–1.94 (m, 2H, 2ꢃCH), 4.03–4.12 (m,
1H, NHCH(CH3)2), 6.24–6.60 (m, 1H, NHCH(CH3)2), 7.52–8.00 (m,
1H, H-8), 10.20–10.44 (m, 1H, H-1), 12.49 (br s, 0.81H, imidazole
NH), 12.86 (br s, 0.19H, imidazole NH); dC (100 MHz, DMSO-d6; only
signals for major N9H tautomer given) 23.1, 32.4, 51.9, 116.3, 135.2,
151.5, 152.0, 156.7; HRMS (ESIþ) m/z calcd for C10H14N5O [MþHþ]
4.7. Typical procedure for Mitsunobu coupling of purine 4a:
condition B
To a stirred solution of purine 4a (163 mg, 0.5 mmol, 1 equiv)
in anhydrous THF (7 mL) under a nitrogen atmosphere at room

S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
4631
temperature was added the alcohol R2OH (1.25 mmol, 2.5 equiv)
and PBu3 (308 l, 1.25 mmol, 2.5 equiv); CAUTION: PBu3 is pyro-
(dec); IR (KBr, cmꢂ1) 3465, 3273, 3063, 2957, 2930, 2872, 2688,
1712, 1608, 1576, 1514, 1470; dH (400 MHz, DMSO-d6) 0.87–0.92
(m, 6H, 2ꢃCH3), 1.22 (app sextet, J¼7.0 Hz, 2H, CH2CH3), 1.32 (app
sextet, J¼7.3 Hz, 2H, CH2CH3), 1.51 (app quintet, J¼7.0 Hz, 2H,
CH2CH2CH3),1.72 (app quintet, J¼7.3 Hz, 2H, CH2CH2CH3), 3.27 (app
quartet, J¼7.0 Hz, 2H, NHCH2), 3.95 (t, J¼7.3 Hz, 2H, N9-CH2), 6.33
(t, J¼5.2 Hz, 1H, NHCH2), 7.68 (s, 1H, H-8), 10.34 (s, 1H, H-1); dC
(100 MHz, CDCl3þfew drops MeOH-d4) 13.3, 13.7, 19.6, 20.0, 31.2,
31.7, 40.7, 43.3, 114.5, 137.3, 152.0, 152.6, 158.7; HRMS (ESIþ) m/z
calcd for C13H22N5O [MþHþ] 264.1818, obsd 264.1816.
m
phoric. After w2 min, ADDP (315 mg, 1.25 mmol, 2.5 equiv) was
added. The reaction mixture was stirred at room temperature for
the time indicated, at which point TLC analysis indicated the re-
action was complete. The reaction mixture was concentrated in
vacuo, then adsorbed onto silica gel from CH2Cl2, and purified by
flash column chromatography.
4.7.1. N2-Benzyl-N2-Boc-9-butyl-6-chloro-9H-purin-2-yl-amine
(13d). R2OH was benzyl alcohol, and the reaction time was 4 h.
Flash column chromatography (eluent: hex/EtOAc, 3:2) furnished
13d as a pale yellow gum (181 mg, 87%): Rf 0.47 (hex/EtOAc, 1:1);
mp>105–107 ꢁC; IR (KBr, cmꢂ1) 3416, 3089, 3032, 2963, 2933,
2874, 1712, 1606, 1561, 1508, 1493, 1444, 1427, 1401; dH (400 MHz,
CDCl3) 0.93 (t, J¼7.3 Hz, 3H, CH2CH3), 1.31 (app sextet, J¼7.3 Hz, 2H,
CH2CH3), 1.47 (s, 9H, C(CH3)3), 1.83 (app quintet, J¼7.3 Hz, 2H,
CH2CH2CH3), 4.15 (t, J¼7.3 Hz, 2H, N9-CH2), 5.18 (s, 2H, CH2Ph),
7.16–7.27 (m, 3H, Ph), 7.36–7.38 (m, 2H, Ph), 7.96 (s, 1H, H-8); dC
(100 MHz, CDCl3) 13.4, 19.8, 28.1, 31.7, 44.0, 51.5, 81.8, 126.9, 127.5
(2), 128.2, 138.4, 144.5, 150.3, 152.4, 153.9, 155.0; HRMS (ESIþ) m/z
calcd for C21H26ClN5O2Na [MþNaþ] 438.1667, obsd 438.1675.
4.8.2. 2-Allylamino-9-butyl-1,9-dihydro-purin-6-one (14b). The pu-
rine substrate was 13b. Flash column chromatography (eluent:
CH2Cl2/MeOH, 10:1/7:1) delivered 14b as a white powder (72 mg,
97%): Rf 0.54 (CH2Cl2/MeOH, 5:1); mp>300 ꢁC (dec); IR (KBr, cmꢂ1
)
3449, 2958, 2927, 2865, 2363, 1719, 1609, 1516, 1473; dH (400 MHz,
DMSO-d6) 0.88 (t, J¼7.3 Hz, 3H, CH3), 1.22 (app sextet, J¼7.3 Hz, 2H,
CH2CH3), 1.71 (app quin, J¼7.3 Hz, 2H, CH2CH2CH3), 3.91–3.97
(m, 4H, N9-CH2, CH2CH]CH2), 5.08–5.13 (m, 1H, CH]CH2),
5.17–5.24 (m, 1H, CH]CH2), 5.87–5.96 (m, 1H, CH]CH2), 6.49
(t, J¼5.6 Hz, 1H, NHCH2), 7.70 (s, 1H, H-8), 10.52 (s, 1H, H-1); dC
(100 MHz, CDCl3þfew drops MeOH-d4) 13.3, 19.5, 31.7, 43.3, 43.5,
115.2, 116.0, 134.4, 137.4, 151.7, 152.3, 158.8; HRMS (ESIþ) m/z calcd
for C12H18N5O [MþHþ] 248.1505, obsd 248.1496.
4.7.2. N2-Allyl-N2-Boc-9-butyl-6-chloro-9H-purin-2-yl-amine
(13b). R2OH was allyl alcohol, and the reaction time was 8 h. Flash
column chromatography (hex/EtOAc, 3:2) furnished 13b as a white
solid (157 mg, 86%): Rf 0.48 (hex/EtOAc, 1:1); mp 87–92 ꢁC; IR (KBr,
cmꢂ1) 3448, 3089, 3010, 2978, 2958, 2933, 2873, 1822, 1701, 1643,
1604, 1560, 1510, 1441, 1401; dH (400 MHz, DMSO-d6) 0.89(t, J¼7.3 Hz,
3H, CH3),1.25 (app sextet, J¼7.3 Hz, 2H, CH2CH3),1.44 (s, 9H, C(CH3)3),
1.83 (app quintet, J¼7.3 Hz, 2H, CH2CH2CH3), 4.21 (t, J¼7.3 Hz, 2H,
N9-CH2), 4.42–4.45 (m, 2H, CH2CH]CH2), 5.05–5.10 (m, 1H,
CH]CH2), 5.12–5.18 (m, 1H, CH]CH2), 5.87–5.96 (m, 1H, CH]CH2),
8.62 (s, 1H, H-8); dC (100 MHz, CDCl3) 13.4, 19.8, 28.1, 31.7, 44.0, 50.5,
81.6, 116.7, 127.9, 133.8, 144.6, 150.2, 152.4, 153.6, 154.9; HRMS (ESIþ)
m/z calcd for C17H24ClN5O2Na [MþNaþ] 388.1510, obsd 388.1525.
4.8.3. 9-Butyl-2-(prop-2-ynyl)-amino-1,9-dihydro-purin-6-one
(14c). The purine substrate was 13c. Flash column chromatography
(eluent: CH2Cl2/MeOH, 10:1/7:1) delivered 14c as a white powder
(71 mg, 97%): Rf 0.50 (CH2Cl2/MeOH, 5:1); mp>250 ꢁC (dec); IR
(KBr, cmꢂ1) 3584, 3421, 3060, 2928, 2876, 2663, 1729, 1607, 1518,
1472; dH (400 MHz, DMSO-d6) 0.88 (t, J¼7.3 Hz, 3H, CH3), 1.23 (app
sextet, J¼7.3 Hz, 2H, CH2CH3), 1.74 (quin, J¼7.3 Hz, 2H, CH2CH2CH3),
3.14 (t, J¼2.4 Hz, 1H, CH2C^CH), 3.99 (t, J¼7.3 Hz, 2H, N9-CH2), 4.08
(dd, J¼5.6, 2.4 Hz, 2H, CH2C^CH), 6.87 (t, J¼5.6 Hz, 1H, NHCH2),
7.83 (s, 1H, H-8), 10.83 (s, 1H, H-1); dC (100 MHz, DMSO-d6) 13.3,
19.0, 30.0, 31.2, 42.3, 73.1, 81.0, 116.2, 137.6, 150.3, 151.8, 156.4;
HRMS (ESIþ) m/z calcd for C12H16N5O [MþHþ] 246.1349, obsd
246.1347.
4.7.3. N2-Boc-N2-(prop-2-ynyl)-9-butyl-6-chloro-9H-purin-2-yl-
amine (13c). R2OH was propargyl alcohol, and the reaction time
was 8 h. Flash column chromatography (hex/EtOAc, 3:2) furnished
13c as a white solid (160 mg, 88%): Rf 0.47 (hex/EtOAc, 1:1); mp
104–108 ꢁC; IR (KBr, cmꢂ1) 3416, 3218, 3096, 2973, 2932, 2867,
2363, 2114, 1821, 1697, 1602, 1557, 1509, 1441, 1418; dH (400 MHz,
DMSO-d6) 0.90 (t, J¼7.3 Hz, 3H, CH3), 1.27 (app sextet, J¼7.3 Hz, 2H,
CH2CH3), 1.46 (s, 9H, C(CH3)3), 1.85 (app quintet, J¼7.3 Hz, 2H,
CH2CH2CH3), 3.12 (t, J¼2.4 Hz, 1H, C^CH), 4.23 (t, J¼7.3 Hz, 2H, N9-
CH2), 4.61 (d, J¼2.4 Hz, 2H, CH2C^CH), 8.65 (s, 1H, H-8); dC
(100 MHz, CDCl3) 13.4, 19.8, 28.1, 31.7, 37.6, 44.1, 70.9, 79.7, 82.4,
144.7, 128.1, 150.4, 152.4, 152.8, 154.1; HRMS (ESIþ) m/z calcd for
C17H22ClN5O2Na [MþNaþ] 386.1354, obsd 386.1355.
4.8.4. 2-Benzylamino-9-butyl-1,9-dihydro-purin-6-one (14d). The
purine substrate was 13d. Flash column chromatography (eluent:
CH2Cl2/MeOH, 10:1/5:1) delivered 14d as a white powder (85 mg,
95%): Rf 0.56 (CH2Cl2/MeOH, 5:1); mp>280 ꢁC (dec); IR (KBr, cmꢂ1
)
3456, 2923, 2357, 1705, 1609, 1514, 1470; dH (400 MHz, DMSO-d6)
0.84 (t, J¼7.2 Hz, 3H, CH3) 1.18 (app sextet, J¼7.2 Hz, 2H, CH2CH3),
1.66 (app quintet, J¼7.2 Hz, 2H, CH2CH2CH3), 3.93 (t, J¼7.2 Hz, 2H,
N9-CH2), 4.47 (d, J¼6.0 Hz, 2H, CH2Ph), 6.90–6.97 (m, 1H, NHCH2),
7.21–7.38 (m, 5H, Ph), 7.68 (s, 1H, H-8), 10.54 (br s, 1H, H-1); dC
insufficiently soluble; HRMS (ESIþ) m/z calcd for C16H20N5O
[MþHþ] 298.1662, obsd 298.1667.
4.8. Typical procedure for hydrolytic dechlorination and Boc
deprotection of purines 13
4.8.5. 2-(iso-Propyl)amino-9-butyl-1,9-dihydro-purin-6-one
(14e). The purine substrate was 13e. Flash column chromatography
(eluent: CH2Cl2/MeOH, 10:1/7:1) delivered 14e as a white powder
(72 mg, 97%): Rf 0.56 (CH2Cl2/MeOH, 5:1); mp>275 ꢁC (dec); IR
(KBr, cmꢂ1) 3452, 3260, 3072, 2967, 2930, 2863, 2750, 1699, 1599,
1578, 1519, 1465; dH (400 MHz, DMSO-d6) 0.88 (t, J¼7.3 Hz, 3H,
CH3), 1.20 (d, J¼6.4 Hz, 6H, CH(CH3)2), 1.22 (app sextet, J¼7.3 Hz, 2H,
CH2CH3), 1.72 (quintet, J¼7.3 Hz, 2H, CH2CH2CH3), 3.92–4.00
(m, 3H, N9-CH2, CH(CH3)2), 6.22 (d, J¼7.2 Hz, 1H, NHCH), 7.68 (s, 1H,
H-8), 10.21 (s, 1H, H-1); dC (100 MHz, CDCl3þfew drops MeOH-d4)
13.3,19.5, 22.2, 31.7, 43.0, 43.3,115.1,137.4, 151.7,152.0,158.8; HRMS
(ESIþ) m/z calcd for C12H20N5O [MþHþ] 250.1662, obsd 250.1655.
A solution of purine 13 (0.3 mmol) in 80% HCOOH (3 mL; 4:1
mixture of HCOOH/H2O) was stirred at 75 ꢁC for 2 h, by which time
TLC indicated all starting material and Boc-deprotected in-
termediate had been consumed. The reaction mixture was con-
centrated under reduced pressure to dryness, adsorbed onto silica
gel from CH2Cl2/MeOH, and then purified by silica gel flash column
chromatography.
4.8.1. 9-Butyl-2-butylamino-1,9-dihydro-purin-6-one (14a). The pu-
rine substrate was 13a. Flash column chromatography (eluent:
CH2Cl2/MeOH, 10:1/7:1) delivered the title compound as a white
powder (78 mg, 99%): Rf 0.58 (CH2Cl2/MeOH, 5:1); mp>310 ꢁC
4.8.6. 9-Butyl-2-cyclopentylamino-1,9-dihydro-purin-6-one
(14f). The purine substrate was 13f. Flash column chromatography

4632
S. Fletcher et al. / Tetrahedron 66 (2010) 4621–4632
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(eluent: CH2Cl2/MeOH, 10:1/7:1) delivered 14f as a white powder
(78 mg, 95%): Rf 0.58 (CH2Cl2/MeOH, 5:1); mp>320 ꢁC (dec); IR
(KBr, cmꢂ1) 3430, 3418, 2960, 2861, 2696, 1696, 1597, 1512, 1464; dH
(400 MHz, DMSO-d6) 0.88 (t, J¼7.4 Hz, 3H, CH3), 1.22 (app sextet,
J¼7.4 Hz, 2H, CH2CH3), 1.44 (m, 2H, 2ꢃCH (cyclopentyl)), 1.50–1.77
(m, 6H, CH2CH2CH3, 4ꢃCH (cyclopentyl)), 1.95 (m, 2H, 2ꢃCH
(cyclopentyl)), 3.96 (t, J¼6.8 Hz, 2H, N9-CH2), 4.09 (app sextet,
J¼6.7 Hz, 1H, NHCH), 6.40 (d, J¼6.7 Hz, 1H, NHCH), 7.68 (s, 1H, H-8),
10.13 (s, 1H, H-1); dC (100 MHz, CDCl3þfew drops MeOH-d4) 13.1,
19.4, 23.5, 31.6, 32.4, 43.2, 52.7, 115.0, 137.3, 151.8, 152.0, 158.3;
HRMS (ESIþ) m/z calcd for C14H22N5O [MþHþ] 276.1818, obsd
276.1824.
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We thank NSERC, the Canadian Foundation for Innovation and
the Ontario Research Foundation for financial support of this work.
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Supplementary data
Experimental procedures for the syntheses of 2, 3, 6, 7, 8, 9g
and 9h, copies of 1H and 13C NMR spectra for all compounds are
provided. Crystallographic data (excluding structure factors) for
the structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publi-
cation numbers CCDC 771261 (compound 5j) and CCDC 771260
(compound 12f). Copies of the data can be obtained, free of charge,
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK,
(fax: þ44 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk). Sup-
plementary data associated with this article can be found in the
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