N-N bond-forming cyclization for the one-pot synthesis of N-aryl[3,4-d]pyrazolopyrimidines

Article abstract of DOI:10.1021/ol301561a

An efficient one-pot synthesis of N-aryl[3,4-d]pyrazolopyrimidines in good yield and under mild reaction conditions is described. By exploiting electron-deficient hydroxylamines, the substituted oxime products were formed with very high E-diastereoselectivity. The key step utilizes a cyclization reaction upon an oxime derived from hydroxylamine-O-sulfonic acid to form the N-N bond of the product.

Full text of DOI:10.1021/ol301561a

ORGANIC
LETTERS
2012
Vol. 14, No. 13
3546–3549
NÀN Bond-Forming Cyclization for the
One-Pot Synthesis of N-Aryl[3,4-d]-
pyrazolopyrimidines
Lindsay E. Evans, Matthew D. Cheeseman, and Keith Jones*
Division of Cancer Therapeutics, The Institute of Cancer Research, 15 Cotswold Road,
Sutton, Surrey SM2 5NG, U.K.
keith.jones@icr.ac.uk
Received June 6, 2012
ABSTRACT
An efficient one-pot synthesis of N-aryl[3,4-d]pyrazolopyrimidines in good yield and under mild reaction conditions is described. By exploiting
electron-deficient hydroxylamines, the substituted oxime products were formed with very high E-diastereoselectivity. The key step utilizes a
cyclization reaction upon an oxime derived from hydroxylamine-O-sulfonic acid to form the NÀN bond of the product.
The discovery of ATP-competitive inhibitors of protein
kinases and ATPases has become one of the most active
areas of research in medicinal chemistry and the pharma-
ceutical industry.1 [3,4-d]Pyrazolopyrimidines are impor-
tant scaffolds in this respect because of their ability
to closely mimic the purine ring of adenosine in ATP
while imparting changes to the electronic properties of the
ring system and creating novel vectors for substitution.
The [3,4-d]pyrazolopyrimidine scaffold has been used in
potent inhibitors of various protein kinases and ATPases,
which has led to a greater understanding of the role of
these enzymes in disease and to the development of novel
therapeutics.2
order to establish structureÀactivity relationships and
modifyphysicochemicalproperties of compounds. A num-
ber of approaches to the synthesis of [3,4-d]pyrazolo-
pyrimidine libraries have previously been described which
focused on the cyclization of hydrazine derivatives with an
appropriate electrophile to incorporate the NÀN bond of
the ring system.3 However, utilizing hydrazines has a
number of disadvantages, including lengthy synthetic se-
quences, the synthesis and handling of hydrazine derivatives,
low overall yields, and harsh reaction conditions. These
synthetic drawbacks have so far limited the application of
the pyrazolopyrimidine scaffold in medicinal chemistry.
During a recent project investigating the function of
chaperonin ATPases in cancer,4 we required the rapid
synthesis of a library of ATP-mimics based on the [3,4-d]-
pyrazolopyrimidine scaffold. To achieve this, we developed
An important factor in a successful drug discovery
program is the ability to synthesize a large number of
structural analogues in a rapid and efficient manner, in
(3) (a) Slavish, P. J.; Price, J. E.; Hanumesh, P.; Webb, T. R. J. Comb.
Chem. 2010, 12, 807. (b) Hanefeld, U.; Rees, C. W.; White, A. J. P.;
Williams, D. J. J. Chem. Soc., Perkin Trans. 1 1996, 1545. (c) Revesz, L.;
Blum, E.; Di Padova, F. E.; Buhl, T.; Feifel, R.; Gram, H; Hiestand, P.;
Manning, U.; Neumann, U.; Rucklin, G. Bioorg. Med. Chem. Lett. 2006,
16, 262. (d) Baraldi, P. G.; Bovero, A.; Fruttarolo, F.; Romagnoli, R.;
Tabrizi, M. A.; Preti, D.; Varani, K.; Borea, P. A.; Moorman, A. R.
Bioorg. Med. Chem. 2003, 11, 4161. (e) Chapman, E.; Farr, G. W.;
Furtak, K.; Horwich, A. L. Bioorg. Med. Chem. Lett. 2009, 19, 811.
(f) Boyd, S.; Campbell, L.; Liao, W.; Meng, Q.; Peng, Z.; Wang, X.;
Waring, M. J. Tetrahedron Lett. 2008, 49, 7395. (g) Tan, T. M. C.; Yang,
F.; Fu, H.; Raghavendra, M. S.; Lam, Y. J. Comb. Chem. 207, 9, 210.
(4) For a review of the function of chaperonins, see: Horwich, A. L.;
Farr, G. W.; Fenton, W. A. Chem. Rev. 2006, 106, 1917.
(1) (a) Cohen, P. Nat. Rev. Drug Discov. 2002, 1, 309. (b) Zhang, J.;
Yang, P. L.; Gray, N. S. Nat. Rev. Cancer 2009, 9, 28. (c) Chene, P. Nat.
Rev. Drug Discov. 2002, 1, 665.
ꢀ
(2) (a) Staerk, J.; Lyssiotis, C. A.; Medeiro, L. A.; Bollong, M.;
Foreman, R. K.; Zhu, S.; Garcia, M.; Gao, Q.; Bouchez, L. C.; Lairson,
L. L.; Charette, B. D.; Supekova, L.; Janes, J.; Brinker, A.; Cho, C. Y.;
Jaenisch, R.; Schultz, P. G. Angew. Chem., Int. Ed. 2011, 50, 5734.
(b) Smalley, T. L., Jr.; Peat, A. J.; Boucheron, J. A.; Dickerson, S.;
Garrido, D.; Preugschat, F.; Schweiker, S. L.; Thomson, S. A.; Wang,
T. Y. Bioorg. Med. Chem. Lett. 2006, 16, 2091. (c) Honigberg, L. A.;
Smith, A. M; Sirisawad, M.; Verner, E.; Loury, D.; Chang, B.; Li, S.;
Pan, Z.; Thamm, D. H.; Miller, R. A.; Buggy, J. J. Proc. Nat. Sci. 2010,
107, 13075.
r
10.1021/ol301561a
Published on Web 06/26/2012
2012 American Chemical Society

a novel one-pot synthesis that is readily applicable to
producinganalogues usingcommerciallyavailable starting
materials and does not require metal-based catalysts or
hydrazines.
decomposition during attempted chromatographic separa-
tion, the mixture was taken on to the next step without
further purification.9 Treatment of oximes 3 and 4 with
mesyl chloride and N,N-diisopropylethylamine gave the
desired pyrazolopyrimidine 6 in 41% isolated yield. It has
previously been reported that only E-oximes can undergo
NÀN bond-forming cyclizations with amines to give 5-mem-
bered aromatic rings.10 The nitrile byproduct 5 was isolated
in 36% yield, presumably as a result of elimination of the O-
mesyl-Z-oxime isomer intermediate, since this isomer could
not undergo cyclization to form the desired product.11
Rather than incorporating the NÀN bond of the aro-
matic heterocycle as a preformed unit prior to cyclization,
this functionality can be introduced in the key synthetic
step via a cyclization reaction using an appropriately
activated nitrogen source.5 This approach negates the
requirement for the synthesis and handling of hydrazines
and exploits the greater number of commercially available
amine derivatives, expanding the diversity of pyrazolopyr-
imidine libraries that could be used in drug discovery. A
related NÀN bond-forming cyclization reaction was pre-
viously observed by Fenniri and co-workers during their
attempted syn-elimination of an E-oxime derivative to give
a nitrile.6 The undesired pyrazolopyrimidine byproduct
was isolated in 28% yield.
Table 1. Substituted Hydroxylamines
entry
hydroxylamine
E:Z ratiob
Scheme 1. Stepwise Pyrazolopyrimidine Synthesis
1
2
3
4
NH2OH HCl
1:1
1:1
2:3
4:1
3
NH2OBn HCl
3
NH2Ot-Bu HCl
3
NH2OTMSa
a Product observed as free oxime. b Determined by examination of
crude 1H NMR spectrum.
To improve the overall yield of the cyclization strategy it
was necessary to synthesize the E-oxime 3 with high
diastereoselectivity. The stereoselectivity of oxime forma-
tion can be difficult tocontrol for aromatic aldehydes, with
results often depending on catalyst and temperature.12
However, in this case, changes to the reaction conditions
gave little improvement in diastereoselectivity of this reac-
tion. It had previously been proposed that E-oxime dia-
stereoselectivity could be improved by increasing the steric
hindranceof the O-substituent on the hydroxylamine.13 To
investigate this rationale, a number of oxime derivatives
weregeneratedusingsubstitutedhydroxylamines(Table1).
We began our synthesis with a nucleophilic substitution
reaction on the commercially available dichloropyrimidine
aldehyde 1 with aniline to give 2 in 96% yield under
standard conditions (Scheme 1).7 No products resulting
from imine formation at the aldehyde functionality were
observed. The resulting pyrimidine 2 was then treated with
hydroxylamine hydrochloride in ethanol to give oximes 3
and 4 as a 1:1 mixture of E- and Z-isomers.8 Owing to
Scheme 2. Oxime Stereoselectivity
(5) For examples of NÀN bond formation in the synthesis of aro-
matic heterocycles, see: (a) Wray, B. C.; Stambuli, J. P. Org. Lett. 2010,
12, 4576. (b) Clayton, K. A.; Black, D. StC.; Harper, J. B. Tetrahedron
2008, 64, 3183. (c) Sugimoto, T.; Itagaki, K.; Irie, K. Bioorg. Med. Chem.
2008, 16, 650. (d) O’Dell, D. K.; Nicholas, K. M. Heterocycles 2004, 63,
373. (e) Reddy, A. C. S.; Narsaiah, B.; Venkataratnam, R. V. Synth.
Commun. 1997, 27, 2217. (f) Buscemi, S.; Vivona, N.; Caronna, T. J. Org.
Chem. 1996, 61, 8397. (g) Ardakani, M. A.; Smalley, R. K.; Smith, R. H.
J. Chem. Soc., Perkin. Trans. 1 1983, 2501.
(6) Beingessner, R. L.; Deng, B. L.; Fanwick, P. E.; Fenniri, H.
J. Org. Chem. 2008, 73, 931.
(7) Zheng, L.; Yang, F.; Dang, Q.; Bai, X. Org. Lett. 2008, 10, 889.
(8) Diastereomeric ratio was determined by examination of the crude
1H NMR spectrum. Configuration was determined by NOE correlation;
see: Heinisch, G.; Holzer, W. Tetrahedron Lett. 1990, 31, 3109.
(9) All attempts at chromatographic separation resulted in decom-
position. The yield refers to recovered mass and reflects a clean 1H NMR
spectrum of crude product.
Org. Lett., Vol. 14, No. 13, 2012
3547

It was clear from our screen that the diastereoselectivity
of oxime formation was not determined by the relative
steric hindrance of the O-hydroxylamine substituent. In-
creasing steric hindrance from hydroxylamine to O-tert-
butylhydroxylamine actually resulted in a small preference
for the formation of the Z-diastereomer.14 In contrast,
O-TMS-hydroxylaminegaveincreased diastereoselectivity
for the desired E-oxime isomer. To explain the observed
stereoselectivity, we proposed that the electron-withdrawing
nature of the silicon substituent in O-TMS-hydroxylamine15
slowed the stereodefining elimination step of oxime forma-
tion (Scheme 2). This allowed intermediate 7 to rearrange to
the more stable conformer 8, which contains an intramolec-
ular NHN-hydrogen bond and the OR group in a pseudoe-
quatorial conformation.16 Conformational change prior to
elimination forces the hydroxylamine derivative away from
the aromatic ring so that the elimination step occurs with
high E-stereoselectivity.
minor isomer could not be observed by 1H NMR analysis,
see the Supporting Information). Unfortunately, treat-
ment of intermediate 9 under both acidic and basic condi-
tions in various organic solvents either led to syn-elimination
to give nitrile 5 or a complex mixture of products through
decomposition.
Scheme 3. One-Pot Synthesis
We were able to exploit this observation by utilizing the
commercially available hydroxylamine-O-sulfonic acid as
an electron-deficient hydroxylamine equivalent. Treatment
of aldehyde 2 with hydroxylamine-O-sulfonic acid in etha-
nol resulted in the formation of E-oxime 3, in 87%yield and
>9:1 dr (Scheme 1).17 The intermediate O-sulfonic acid
oxime was not observed, presumably due to ethanolysis
under the reaction conditions. Reaction of E-oxime 3 with
mesyl chloride and N,N-diisopropylethylamine resulted in
clean cyclization to give the desired pyrazolopyrimidine 6 in
74% yield (Scheme 1).18
In Kemp and Woodward’s seminal publication on the
use of hydroxylamine-O-sulfonic acid for the synthesis of
benzisoxazole, biphasic reaction conditions were em-
ployed to facilitate the cyclization reaction.20 Presumably,
aqueous conditions stabilize the highly charged sulfonate
leaving group, promoting the cyclization process. A solu-
tion of intermediate 9 in acetonitrile was therefore diluted
with dichloromethane and aqueous 1 M sodium hydroxide
solution. Clean cyclization proceeded to give the desired
pyrazolopyrimidine 6 in 59% yield from 2, and no nitrile
byproduct 5 was observed. Finally, we were able to
combine all three steps of this synthesis into a one-pot
procedure by carrying out the nucleophilic substitution
reaction of 1 with aniline in acetonitrile. Hydroxylamine-
O-sulfonic acid was then added followed by dilution with
dichloromethane and aqueous 1 M sodium hydroxide
solution to give a biphasic mixture, which resulted in the
formation of the desired pyrazolopyrimidine 6 in 51%
yield from 1 (Scheme 3).
Hydroxylamine-O-sulfonic acid has previously been
exploited as a source of electrophilic nitrogen to synthesize
NÀN and NÀO bonds.19 Under our reaction conditions,
no spontaneous cyclization of the O-sulfonic acid oxime
intermediate 9 was observed. When the oxime synthesis
was carried out in acetonitrile, intermediate 9 was suffi-
1
ciently stable to be observed by H NMR spectroscopy,
confirming the high diastereoselectivity of the reaction (the
(10) The stereochemical requirement of this type of NÀN bond-
forming cyclization reaction has previously been described: Counceller,
C. M.; Eichman, C. C.; Wray, B. C.; Stambuli, J. P. Org. Lett. 2008, 10,
1021.
(11) E-Oxime mesyl ethers can also undergo syn-elimination to give
nitriles: Al-Awadi, N. A.; Elnagdi, M. H.; Kaul, K.; Ilingovan, S.; El-
Dusouqui, O. M. E. Tetrahedron 1998, 54, 4633.
To explore the scope of our one-pot pyrazolopyrimidine
synthesis, several analogues were synthesized incorporat-
ing both electron-donating and -withdrawing groups
on the aromatic ring (Table 2). The targets were produced
in good yield from this three-step, one-pot protocol.
Of note are the mild reaction conditions used which allow
for potentially sensitive functionality, labile protecting
groups, and unprotected groups to be used while still
affording good yields. Unfortunately, the use of aliphatic
amines failed to induce formation of the desired pyrazo-
lopyrimidine 17, despite the success of the first two steps of
this protocol.21 Treatment of the O-sulfonic acid oxime
(12) Sharghi, H.; Sarvari, M. H. Synlett 2001, 99.
(13) Emami, S.; Falahati, M.; Banifatemi, A.; Shafiee, A. Bioorg.
Med. Chem. 2004, 12, 5881.
(14) The configuration of isolated O-substituted oximes could be
determined by HN-HMBC NMR spectroscopy; see: (a) Huang, X. S.;
Liu, X.; Constantine, K. L.; Leet, J. E.; Roongta, V. Magn. Reson.
Chem. 2007, 45, 447. (b) Jansma, A.; Zhang, Q.; Li, B.; Ding, Q.; Uno,
T.; Bursulaya, B.; Liu, Y.; Furet, P.; Gray, N. S.; Geierstanger, B. H.
J. Med. Chem. 2007, 50, 5875.
(15) For a discussion of the SiON motif as an electron-withdrawing
group, see: Mitzel, N. W.; Blake, A. J.; Rankin, D. W. H. J. Am. Chem.
Soc. 1997, 119, 4143.
(16) For a discussion on the stability of this type of intramolecular
hydrogen bond, see: Kuhn, B.; Mohr, P.; Stahl, M. J. Med. Chem. 2010,
53, 2601.
(17) This ratio was found to vary between 9:1 and 19:1 depending on
the dryness of the ethanol used.
(18) The base used in the cyclization of the E-oxime 3 had a
significant effect on the yield of this reaction. Triethylamine gave much
lower yields of pyrazolopyrimidine 6 and larger amounts of nitrile 5.
(19) For a review of O-sulfonic acid hydroxylamine, see: Wallace,
R. G. Aldrichimica Acta 1980, 13, 3.
(20) Kemp, D. S.; Woodward, R. B. Tetrahedron 1965, 21, 3019.
(21) Under the conditions reported, a variety of aliphatic amines
failed to give the desired pyrazolopyrimidine product and resulted in
decompostition of starting material.
3548
Org. Lett., Vol. 14, No. 13, 2012

Although the mechanism of this unusual NÀN bond-
forming reaction is unclear, a plausable mechanism is
shown in Scheme 4.22 This resembles the mechanism of
the Neber reaction used to synthesize indoles.23 A further
discussion of possible mechanisms is presented in the
Supporting Information.
Table 2. Three-Step One-Pot Pyrazolopyrimidine Synthesis
Scheme 4. Plausable Mechanism
In summary, we have described a novel one-pot synth-
esis of N-aryl[3,4-d]pyrazolopyrimidines in good yield and
under mild reaction conditions. By exploiting electron-
deficient hydroxylamines the oxime product was formed
withveryhigh E-diastereoselectivity. The keysteputilizes a
hydroxylamine-O-sulfonic acid derived oxime in a cycliza-
tion reaction to form the NÀN bond of the pyrazolopyr-
imidine ring. Carrying out the reaction under biphasic
aqueous conditions promoted clean ring closure to afford
the pyrazolopyrimidines with a range of functionality in
good yield. We believe our one-pot protocol is a significant
addition to the methodologies available for the synthesis of
these important pharmaceutical compounds.
Acknowledgment. We thank Cancer Research UK
(grant no. C309/A11566) and The Wellcome Trust for
funding. We also thank Dr Maggie Liu of The Institute of
Cancer Research for assistance with NMR analysis.
Supporting Information Available. Detailed spectro-
scopic data for new compounds and representative ex-
perimental procedures. This material is available free of
charge via the Internet at http://pubs.acs.org.
a Key: (i) amine, NEt3, MeCN, À15 °C to rt, 1À16 h; (ii) NH2O-
SO3H, rt, 16 h; (iii) DCM, 1 M NaOH, 6 h.
(22) Greszler, S.; Stevens, K. L. Org. Synth. 2009, 86, 17.
(23) Ooi, T.; Takahashi, M.; Doda, K.; Maruoka, K. J. Am. Chem.
Soc. 2002, 124, 7641.
intermediate derived from allylamine with base resulted
only in a complex mixture of products.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 13, 2012
3549