4-(Isoxazol-3-yl)pyrimidines from pyrimidinyl nitrile oxides

Article abstract of DOI:10.1055/s-0028-1087346

The 1,3-dipolar cycloaddition reaction of pyrimidinylaldoxime derived nitrile oxides and alkynes delivers 4-(isoxazol-3-yl)pyrimidines. The procedures reported accommodate three points of diversification around this bisheterocyclic scaffold and the resulting library of compounds has been added to the National Institutes of Health repository (ca. 10 mg of each with >90% purity) for pilot-scale biomedical studies with bioassay data available at the National Center for Biotechnology Information PubChem database. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1087346

3036
LETTER
4-(Isoxazol-3-yl)pyrimidines from Pyrimidinyl Nitrile Oxides
S
ynthesisof 4-(Isoxazol
o
-3-yl)pyrimi
n
dines ken Choung,a Beth A. Lorsbach,b Thomas C. Sparks,b James M. Ruiz,b Mark J. Kurth*a
a
b
Received 26 February 2008
5-isoxazolyl series. In both series, we planned to construct
the pyrimidine ring by a methyl carbamimidothioate con-
densation reaction {R1 = Me: with 5,5-diethoxypent-3-
yn-2-one6 in the former and 2-[(dimethylamino)methyl-
Abstract: The 1,3-dipolar cycloaddition reaction of pyrimidinyl al-
doxime derived nitrile oxides and alkynes delivers 4-(isoxazol-3-
yl)pyrimidines. The procedures reported accommodate three points
of diversification around this bisheterocyclic scaffold and the re-
sulting library of compounds has been added to the National Insti- ene]-3-oxobutanoate7 in the latter} and introduce the isox-
tutes of Health repository (ca. 10 mg of each with >90% purity) for
azole ring by a 1,3-dipolar cycloaddition reaction between
an aldoxime-derived nitrile oxide and an alkyne.8 Finally,
pilot-scale biomedical studies with bioassay data available at the
National Center for Biotechnology Information PubChem database.
2-(methylthio)pyrimidine to 2-(methylsulfonyl)pyrimi-
dine oxidation9 followed by methanesulfinate ipso
Key words: nitrile oxide, 2-aminopyrimidine, isoxazoles, isoxazo-
substitution10 would deliver the 4-(isoxazol-3-yl)pyrimi-
lines, ipso substitution, cycloaddition
dine.
Synthetic1 and biological2 studies focused on substituted
2-aminopyrimidines are extensive due to the broad bioac-
O
N
R2
R1
O
R1
N
N
tivity of naturally occurring as well as synthetic com-
pounds containing this functionality. Indeed, the
guanidine-like arrangement of the 2-aminopyrimidine
moiety appears to impart these pharmaceutical properties.
Moreover, the 2-aminopyrimidine is also a prevalent mo-
tif in agrochemicals.2 Based on these reports, we became
intrigued by the biological potential and synthetic oppor-
tunities of isoxazolopyrimidin-2-amine derivatives.
R2
N
N
N
R3
HN
N
R3
H
4-isoxazoylpyrimidines
5-isoxazoylpyrimidines
OH
N
OH
R1
N
N
R2
R2
R1
Generally, 2-aminopyrimidine are prepared by the reac-
tion of a 1,3-diketone and a guanidine compound.3 This
process is not, however, generally adequate for chemical
library production because it necessitates the synthesis of
various substituted guanidines and it establishes diversity
too early in the synthetic scheme. Recently, an amine sub-
stitution reaction in which 2-alkylthiopyrimidines deliver
2-aminopyrimidines has been reported.4 This process fa-
cilitates diversity-oriented synthesis5 by accommodating
the introduction of 2-amino-based diversity late in the
synthesis. We report here a synthetic study focused on the
preparation of isoxazolopyrimidin-2-amine derivatives.
N
N
N
MeS
SMe
OEt
OEt
O
OEt
R1
R1
NMe2
NH
O
O
H2N
NH
SMe
H2N
SMe
With amino diversity slated for the 2-position, we consid-
ered placing the isoxazole moiety at either the 4- or 5-po-
sition (Figure 1). Each scaffold provides a unique
arrangement of heteroatoms for subsequent interaction
with biological receptors. Retrosynthetic considerations
suggested two similar but distinct plans. For the 4-isox-
azolyl series, a Weinreb amide starting point would intro-
duce the first diversity input (R1) and was attractive as
these reagents are easily prepared from acyl halides. Ethyl
3-oxoalkanoates would serve as the starting point for the
Me
N
R1
OEt
OEt
OEt
O
R1
OMe
H
NMe2
O
O
O
Figure 1 Pyrimidinylisoxazole retrosynthetic options
Our early efforts were directed toward 5-isoxazolopyrimi-
dine targets. While the preparation of ethyl 4-methyl-2-
(methylthio)pyrimidine-5-carboxylate proved uneventful,
reduction of the carboethoxy group directly to the corre-
sponding aldehyde (or hydroxymethyl) analogue failed
(Figure 2). Despite literature precedence for this type of
reduction,11 a variety of attempts (LiAlH4 and NaBH4 in a
range of solvents and reaction temperatures) gave a com-
plex mixture of products which, because of poor solubili-
SYNLETT 2008, No. 19, pp 3036–3040
x
x
.x
x
.2
0
0
8
Advanced online publication: 12.11.2008
DOI: 10.1055/s-0028-1087346; Art ID: S02108ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of 4-(Isoxazol-3-yl)pyrimidines
3037
ty, were difficult to separate and characterize. Given this overcome, in part, by the addition of excess alkyne and by
setback, focus was shifted to the 4-isoxazolopyrimidine keeping the reaction mixture dilute (0.1 M).
targets.
To introduce the final point of diversity, the 2-(methylth-
io)pyrimidine moiety was oxidized to the corresponding
2-(methylsulfonyl)pyrimidine by treatment with MCPBA
CO2Et
CHO
CH2OH
in CH2Cl2.14 The resulting crude sulfones were then react-
ed with various amines in THF to deliver 5. Following the
chemistry outlined in Scheme 1, sixteen 4-(isoxazol-3-
yl)-6-methylpyrimidin-2-amines were prepared (R1, R2,
and R3 are defined in Scheme 1). Commercially available
inputs for these sixteen compounds were chosen to pro-
duce a small, but diverse set of molecules that would be
suitable for biological screening. They spanned a range of
MW (294–383) and lipophilicity (calcd15 log Kow 3.3–4.9)
that were consistent with Lipinski’s rule of five.16
N
N
N
MeS
N
MeS
N
MeS
N
Figure 2 Attempted reduction of ethyl 4-methyl-2-(methylthio)py-
rimidine-5-carboxylate
As delineated in Scheme 1, treating the appropriate
Weinreb amide with (3,3-diethoxyprop-1-ynyl)lithium12
in THF at –78 °C delivered diethoxyalkyone 1 in excel-
lent yield. Condensation of this alkynone with methyl car-
bamimidothioate (hydrobromide salt and Et3N) produced
the targeted 2-(methylthio)pyrimidine which, upon acetal
hydrolysis, gave pyrimidine-4-carbaldehyde 2. The corre-
sponding pyrimidine-4-carbaldehyde oxime was obtained
by treatment of aldehyde 2 with hydroxylamine hydro-
chloride and Na2CO3. At this point, in situ nitrile oxide
formation following the Huisgen protocol13 (bleach and
ethynylarene in CH2Cl2) delivered 3-[2-(methylthio)pyri-
midin-4-yl]isoxazole 4. However, because of poor al-
doxime solubility in CH2Cl2, these pyrimidinyl nitrile
oxides tended to dimerize – a problem which could be
Target diversity was further expanded as outlined in
Scheme 2 by replacing the ethynylarene employed in cy-
cloaddition 3 → 4 with methyl propiolate. This substitu-
tion delivered 3-[2-(methylthio)pyrimidin-4-yl]isoxazole
6 and set the stage for ester to amide diversification (6 →
7) which was accomplished by simply treating ester 6
with excess amine (2 equiv) in methanol at room temper-
ature. Employing the methylthio → methylsufonyl →
amine sequence delivered the 3-(2-aminopyrimidin-4-yl)-
N-alkylisoxazole-5-carboxamide 8. Thus, 24 analogues
were prepared (R1, R2, and R3 are defined in Scheme 2).
As with the first set of analogues, inputs for these ana-
logues were chosen to be consistent with Lipinski’s rule
of five.
R1
O
OEt
OEt
R1
CHO
O
a, b
c, d
R1
Cl
N
e
N
OH
O
O
1
N
N
SMe
R1
R1
OMe
2
a
N
N
N
N
SMe
SMe
6
3
OH
O
N
N
R2
b
R1
R1
g, h
f
N
N
N
N
O
O
O
O
N
N
R1
R1
SMe
SMe
HN R2
HN R2
c, d
4
3
O
N
N
N
N
N
R2
R1
SMe
HN R3
7
8
N
N
R1 = Me, i-Pr
HN R3
R
R
2 = PMB, Bn, cyclopentyl
3 = (CH2)2OMe, cyclopropylmethyl, cyclopentyl, Pr
5
R1 = Me, i-Pr
R2 = Ph, 4-ClC6H4
R3 = (CH2)2OMe, cyclopropylmethyl, cyclopentyl, Pr
Scheme 2 Reagents and conditions: (a) methylpropiolate, bleach,
Na2CO3, CH2Cl2, overnight, 0 °C to r.t.; (b) NH2R2, MeOH, over-
night, r.t.; (c) MCPBA, CH2Cl2, overnight, 0 °C to r.t.; (d) amine,
THF, overnight, r.t.
Scheme 1 Reagents and conditions: (a) N,O-dimethylhydroxyl-
amine, Et3N, CH2Cl2, overnight, r.t.; (b) propargylaldehyde diethyl-
acetal, n-BuLi, THF, 5 h, –78 °C, then Weinreb amide; (c)
MeSC(=NH)NH2·HBr, Et3N, THF, overnight, 0 °C to r.t.; (d) 4 N aq
H2SO4, THF, 30 h, 50 °C; (e) NH2OH·HCl, Na2CO3, EtOH–H2O
(2:1), overnight, 0 °C to r.t.; (f) aryl acetylene, bleach, Na2CO3,
CH2Cl2, overnight, 0 °C to r.t.; (g) MCPBA, CH2Cl2, overnight, 0 °C
to r.t.; (h) amine, THF, overnight, r.t.
As part of a collaboration with Dow AgroSciences, these
compounds were evaluated using a high-throughput
screening (HTS) assay19 with larvae of Spodoptera exigua
(beet armyworm) and Aedes aegypti (yellow fever mos-
quito) as the test organisms. All 40 compounds were inac-
Synlett 2008, No. 19, 3036–3040 © Thieme Stuttgart · New York

3038
W. Choung et al.
LETTER
tive against larvae of mosquito. In contrast, a high
proportion of the tested compounds showed interesting
levels of activity against beet armyworm, with 10 (28.5%)
of the 40 compounds tested resulting in 66–100% mortal-
ity, and more than half of those showing 83% mortality
(Table 1 and Table 2). Further testing against and Heli-
coverpa zea (corn earworm) larvae in follow-up assays
showed no activity. In addition, poor activity was ob-
served for a majority of the compounds tested against beet
armyworm at lower doses.
Table 2 Biological Activity against Spodoptera exigua (Beet Army-
worm) for Derivatives of Compound 8
Entry R1
R2
R3
Mortalityagainst
beet armyworm
(%)
1
2
Me
Bn
(CH2)2OMe
cyclopentyl
cyclopropylmethyl
Pr
67
50
33
17
33
67
17
67
67
17
33
33
83
83
83
33
67
67
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
Bn
3
Bn
4
Bn
Table 1 Biological Activity against Spodoptera exigua (Beet Army-
worm) for Derivatives of Compound 5
5
PMB
PMB
PMB
PMB
(CH2)2OMe
cyclopentyl
cyclopropylmethyl
Pr
Entry R1
R2
R3
Mortalityagainst
beet armyworm
(%)
6
7
1
2
Me
Ph
(CH2)2OMe
cyclopentyl
cyclopropylmethyl
Pr
20
67
50
50
33
100
67
50
67
67
67
33
67
67
8
Me
Me
Me
Me
Me
Me
Me
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
Ph
9
cyclopentyl (CH2)2OMe
cyclopentyl cyclopentyl
cyclopentyl cyclopropylmethyl
cyclopentyl Pr
3
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
4
Ph
5
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
Ph
(CH2)2OMe
cyclopentyl
cyclopropylmethyl
Pr
6
Bn
(CH2)2OMe
cyclopentylmethyl
cyclopropylmethyl
Pr
7
Bn
8
Bn
9
(CH2)2OMe
cyclopentyl
cyclopropylmethyl
Pr
Bn
10
11
12
13
14
15
16
Ph
PMB
PMB
PMB
PMB
(CH2)2OMe
cyclopentyl
Ph
Ph
cyclopropylmethyl 100
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
(CH2)2OMe
cyclopentyl
Pr
67
17
83
0
cyclopentyl (CH2)2OMe
cyclopentyl cyclopentyl
cyclopentyl cyclopropylmethyl
cyclopentyl Pr
cyclopropylmethyl 100
Pr 67
50
In summary, reliable routes to 3-(pyrimidin-4-yl)isox-
azoles 5 and 8 have been developed. A total of forty novel
examples of these two scaffolds have been successfully
prepared through eight and nine steps, respectively. The
reactions required for these transformations are generally
reliable and quite mild. The one-step ester → amide and
two-step 2-(methylthio)pyrimidine → 2-(amino)pyrimi-
dine reactions are effective combinatorial transformations
which occur near the end of the reaction sequence and, be-
cause a wide variety of amines are suitable for these trans-
formation, allow for broad diversification.
worm, have a broad host range and, as such, are adapted
to metabolize a wide range of xenobiotics.20
General Procedure for the Preparation of 1
To a suspension of N,O-dimethylhydroxylamine hydrochloride (1
equiv) in CH2Cl2 (0.5 M) at 0 °C was slowly added Et3N (2.1
equiv). The requisite acid chloride (1 equiv) was added dropwise to
this slurry, the ice bath was removed, and reaction mixture stirred
for 2 h at r.t. The precipitated Et3N·HCl was removed by filtration
and the filtrate concentrated in vacuo. Ethyl acetate was added, the
additional Et3N·HCl was removed by filtration, and the filtrate was
washed with 1 N HCl, dried over anhyd Na2SO4, filtered, and con-
centrated in vacuo. The resulting Weinreb amide was purified by
column chromatography (silica gel, n-hexane–EtOAc) and obtained
in ca. 85% yield.
These compounds were evaluated as potential insecti-
cides. The poor activity of these molecules could be attrib-
uted in part to metabolism, as there are several metabolic
sites. Many species of armyworms, such as the beet army-
Synlett 2008, No. 19, 3036–3040 © Thieme Stuttgart · New York

LETTER
Synthesis of 4-(Isoxazol-3-yl)pyrimidines
3039
n-Butyllithium (1.6 M, 1.2 equiv) was added dropwise to a solution
of propargylaldehyde diethylacetal (1 equiv) in THF (1.5 M) cooled
to –78 °C. After stirring for 30 min, the solution was warmed
to –30 °C for 2 h, recooled to –78 °C, and treated slowly dropwise
with a THF solution (ca. 35 M) of the Weinreb amide (1.2 equiv).
The reaction mixture was slowly warmed to –30 °C for 3 h at which
time EtOAc was added. The resulting mixture was washed with aq
NH4Cl, dried over anhyd Na2SO4, filtered, and concentration in vac-
uo to afford crude 1. Purification by column chromatography (silica
gel, n-hexane–EtOAc) afforded pure 1 in 90% yield.
Acknowledgment
MJK would like to thank Dow AgroSciences and the National Sci-
ence Foundation (CHE-0614756) for support of this work.
References and Notes
(1) (a) Kryl’skii, D. V.; Shikhaliev, K. h. S.; Shestakov, A. S.;
Liberman, M. M. Russian J. Gen. Chem. 2005, 75, 303.
(b) Hurst, D. T.; Atcha, S.; Marshall, K. L. Aust. J. Chem.
1991, 44, 129. (c) Crawley, L. S.; Fanshawe, W. J.
J. Heterocycl. Chem. 1977, 14, 531.
General Procedure for the Preparation of 2
To a THF solution (0.6 M) of alkynone acetal 1 (1 equiv) cooled to
0 °C was added MeSC(=NH)NH2·HBr (1.1 equiv) and Et3N (2.2
equiv). The reaction was warmed to r.t. with stirring overnight, di-
luted with EtOAc, washed with aq NH4Cl, dried over anhyd
Na2SO4, filtered, and concentrated in vacuo to afford crude product
as a white solid. Purification by column chromatography (silica gel,
n-hexane–EtOAc) afforded the pyrimidine acetal in 75–80% yield.
(2) (a) Sabat, M.; VanRens, J. C.; Laufersweiler, M. J.; Brugel,
T. A.; Maier, J.; Golebiowski, A.; De, B.; Easwaran, V.;
Hsieh, L. C.; Walter, R. L.; Mekel, M. J.; Evdokimov, A.;
Janusz, M. J. Bioorg. Med. Chem. Lett. 2006, 16, 5973.
(b) Laughlin, S. K.; Clark, M. P.; Djung, J. F.; Golebiowski,
A.; Brugel, T. A.; Sabat, M.; Bookland, R. G.;
Laufersweiler, M. J.; VanRens, J. C.; Townes, J. A.; De, B.;
Hsieh, L. C.; Xu, S. C.; Walter, R. L.; Mekel, M. J.; Janusz,
M. J. Bioorg. Med. Chem. Lett. 2005, 15, 2399. (c) Nagata,
T.; Masuda, K.; Maeno, S.; Miura, I. Pest Manage. Sci.
2004, 60, 399. (d) Wang, S.; Meades, C.; Wood, G.;
Osnowski, A.; Anderson, S.; Yuill, R.; Thomas, M.; Mezna,
M.; Jackson, W.; Midgley, C.; Griffiths, G.; Fleming, I.;
Green, S.; McNae, I.; Wu, S.-Y.; McInnes, C.; Zheleva, D.;
Walkinshaw, M. D.; Fischer, P. M. J. Med. Chem. 2004, 47,
1662. (e) Wagner, O.; Harries, V.; De Kramer, J.-J. DE
19827611 A1, 1982.
To a THF solution (0.6 M) of this pyrimidine acetal was added 5 M
H2SO4 (1:1, v:v with THF) and the reaction mixture was warmed to
50 °C with stirring overnight. Upon cooling to r.t., the reaction mix-
ture was diluted with EtOAc and poured onto cold aq NaHCO3. Ex-
traction with EtOAc (3×), drying over anhyd Na2SO4, filtering, and
concentration in vacuo afford crude 2 as a white solid. Purification
by column chromatography (silica gel, n-hexane–EtOAc) afforded
pure 2 in 95% yield.
General Procedure for the Preparation of 3
(3) Shikhaliev, Kh. S.; Kryl’skii, D. V.; Kovygin, Yu. A.;
Verezhnikov, V. N. Russian J. Gen. Chem. 2005, 75, 294.
(4) (a) Zanatta, N.; Flores, D. C.; Madruga, C. C.; Faoro, D.;
Flores, A. F. C.; Bonacorso, H. G.; Martins, M. A. P.
Synthesis 2003, 894. (b) Adlington, R. M.; Baldwin, J. E.;
Catterick, D.; Pritchard, G. J. J. Chem. Soc., Perkin Trans. 1
1999, 855.
To a solution of pyrimidine-4-carbaldehyde 2 (1 equiv) in EtOH
(0.25 M) was added a solution of hydroxylamine hydrochloride
(1.05 equiv) and aq Na2CO3 (0.25 M, 1.1 equiv). This mixture was
vigorously stirred overnight at r.t. The product was collected by fil-
tration, washed with a small portion of cold EtOH, and air dried to
give 3 in >90% yield.
(5) Malherbe, P.; Masciadri, R.; Prinssen, E.; Spooren, W.;
Thomas, A. W. US 2005197337, 2005.
(6) Baldwin, J. E.; Pritchard, G. J.; Rathmell, R. F. J. Chem.
Soc., Perkin Trans. 1 2001, 2906.
(7) (a) Gabbutt, C. D.; Hepworth, J. P.; Heron, B. M. J. Chem.
Soc., Perkin Trans. 1 1992, 2603. (b) Sansebastiano, L.;
Mosti, L.; Menozzi, G.; Schenone, P.; Muratore, O.; Petta,
A.; Debbia, F.; Schito, A. P.; Schito, G. C. Farmaco 1993,
48, 335.
General Procedure for the Preparation of 4
Bleach (10 equiv) was added dropwise to a 0 °C solution of al-
doxime 3 (1 equiv) and excess alkyne (5 equiv) in CH2Cl2 (0.1 M).
Upon completion of this addition, the reaction mixture was warmed
to r.t. with stirring overnight. Additional CH2Cl2 was added, and the
resulting mixture was washed with H2O, dried over anhyd Na2SO4,
filtered, and concentrated in vacuo to afford crude cycloadduct. Pu-
rification by column chromatography (silica gel, n-hexane–EtOAc)
afforded 4 as a white solid in 45–75% yield.
(8) (a) Giguère, D.; Patnam, R.; Bellefleur, M.-A.; St-Pierre, C.;
Sato, S.; Roy, R. Chem. Commun. 2006, 2379. (b) Mosher,
M. D.; Natale, N. R. J. Heterocycl. Chem. 1995, 32, 779.
(c) Han, X.; Natale, N. R. J. Heterocycl. Chem. 2001, 38,
415. (d) Han, X.; Li, C.; Rider, K. C.; Blumenfeld, A.;
Twamley, B.; Natale, N. R. Tetrahedron Lett. 2002, 43,
7673. (e) Han, X.; Twamley, B.; Natale, N. R. J. Heterocycl.
Chem. 2003, 40, 539. (f) Li, C.; Twamley, B.; Natale, N. R.
J. Heterocycl. Chem. 2008, 45, 259.
General Procedure for the Preparation of 7
The 1°-amine (5 equiv) was added to a MeOH solution (1.4 M) of
ester 6 (1 equiv) and the mixture was stirred at r.t. The reaction mix-
ture was concentrated in vacuo, diluted with CH2Cl2, washed with
aq NH4Cl, dried over anhyd Na2SO4, filtered, and concentrated in
vacuo to afford crude product as a solid. Purification by column
chromatography (silica gel, n-hexane–EtOAc) afforded 7 in 85–
90% yield.
(9) Herrera, A.; Martínez-Alvarez, R.; Ramiro, P.; Almy, J.;
Molro, D.; Sánchez, A. Eur. J. Org. Chem. 2006, 3332.
(10) Adlington, R. M.; Baldwin, J. E.; Catterick, D.; Pritchard,
G. J. J. Chem. Soc., Perkin Trans. 1 1999, 855.
(11) Angiolini, M.; Bassini, D. F.; Gude, M.; Menichincher, M.
Tetrahedron Lett. 2005, 46, 8749.
(12) (a) Calvo, L.; González-Ortega, A.; Navarro, R.; Pérez, M.;
Sañudo, M. C. Synthesis 2005, 3152. (b) Labeeuw, O.;
Phansavath, P.; Genêt, J.-P. Tetrahedron Lett. 2004, 45,
7107. (c) Hegde, V. B.; Renga, J. M.; Owen, J. M.
Tetrahedron Lett. 2001, 42, 1847.
General Procedure for the Preparation of 5 (or 8)
MCPBA (2.2 equiv) was added to a CH2Cl2 solution (0.1 M) of thio
ether 4 (1 equiv, or 7) and the resulting mixture was stirred at r.t.
overnight. The reaction was washed with H2O, dried over anhyd
Na2SO4, filtered, and concentrated in vacuo to afford crude product
as a white solid. The resulting sulfone (1 equiv) was dissolved in
THF (0.1 M), treated with 1°-amine (4 equiv), and stirred at r.t.
overnight. The reaction mixture was washed with aq NH4Cl, dried
over anhyd Na2SO4, filtered, and concentrated in vacuo to afford
crude product. Purification by column chromatography (silica gel,
n-hexane–EtOAc) delivered 517 (or 8)18 in 75–95% yield.
(13) Christl, M.; Huisgen, R. Chem. Ber. 1973, 106, 3345.
Synlett 2008, No. 19, 3036–3040 © Thieme Stuttgart · New York

3040
W. Choung et al.
LETTER
(14) Note that bleach did not result in oxidation of the 2-(methyl-
thio)pyrimidine moiety. For a related report, see: Tene
Ghomsi, J. N.; Ahabchane, H. H.; Essassi, E. M.
Phosphorus, Sulfur Silicon Relat. Elem. 2004, 179, 353.
(15) Demeter, D., log Kow model developed at Dow
AgroSciences, unpublished work
(16) Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J.
Adv. Drug Delivery Rev. 2001, 46, 3.
(17) Representative Examples of 5
1H NMR (300 MHz, CDCl3): d = 8.46 (br s, 1 H), 7.46 (s,
1 H), 7.30 (d, J = 8.7 Hz, 2 H), 7.09 (s, 1 H), 6.90 (d, J = 8.7
Hz, 2 H), 5.59 (br s, 1 H), 4.59 (d, J = 5.4 Hz, 2 H), 3.80 (s,
3 H), 3.69–3.56 (m, 4 H), 3.38 (s, 3 H), 2.39 (s, 3 H). 13
C
NMR (75 MHz, CDCl3): d = 169.6, 164.0, 163.8, 162.7,
159.6, 155.7, 154.9, 129.7, 129.2, 114.5, 107.1, 106.5, 70.4,
59.0, 55.6, 43.4, 41.4, 24.4.
3-[2-(Cyclopentylamino)-6-isopropylpyrimidin-4-yl]-N-
(4-methoxybenzyl)isoxazole-5-carboxamide
4-Methyl-6-(5-phenylisoxazol-3-yl)-N-propylpyrimidin-
2-amine
1H NMR (300 MHz, CDCl3): d = 7.48 (s, 1 H), 7.29 (d,
J = 7.5 Hz, 2 H), 7.08 (s, 1 H), 6.97 (br s, 1 H), 6.88 (d,
J = 8.7 Hz, 2 H), 5.23 (br s, 1 H), 4.58 (d, J = 5.1 Hz, 2 H),
4.30 (m, 1 H), 3.79 (s, 3 H), 2.84 (m, 1 H), 2.06 (m, 2 H),
1.73–1.61 (m, 4 H), 1.45 (m, 2 H), 1.25 (d, J = 5.1 Hz, 6 H).
13C NMR (75 MHz, CDCl3): d = 178.2, 164.1, 163.9, 162.5,
159.6, 155.8, 155.0, 129.7, 129.3, 114.5, 106.6, 104.2, 55.6,
53.3, 43.4, 36.3, 33.5, 24.0, 21.8.
1H NMR (300 MHz, CDCl3): d = 7.80 (m, 2 H), 7.42 (m,
3 H), 7.10 (s, 1 H), 7.00 (s, 1 H), 3.42 (m, 2 H), 2.31 (s, 3 H),
2.18 (s, 1 H), 1.62 (m, 2 H), 0.93 (m, 3 H). 13C NMR (75
MHz, CDCl3): d = 170.9, 162.9, 162.7, 156.3, 130.6, 129.3,
127.5, 126.1, 106.8, 105.8, 98.7, 43.5, 23.3, 23.1, 11.8.
N-(Cyclopropylmethyl)-4-isopropyl-6-(5-phenylisoxazol-
3-yl)pyrimidin-2-amine
(19) General Procedure for Bioassays (a) The primary screen
involved two high-throughput insect assays, both
1H NMR (300 MHz, CDCl3): d = 7.60 (m, 2 H), 7.24 (m,
3 H), 6.91 (s, 1 H), 6.84 (s, 1 H), 5.22 (br s, 1 H), 3.11 (m,
2 H), 2.64 (m, 1 H), 1.27 (d, J = 7.0 Hz, 6 H), 0.82 (m, 1 H),
0.30 (m, 2 H), 0.05 (m, 2 H). 13C NMR (75 MHz, CDCl3):
d = 178.2, 170.9, 163.6, 162.8, 156.2, 130.6, 129.3, 127.5,
126.1, 104.4, 98.7, 46.8, 36.3, 21.9, 11.9, 3.7.
incorporating 96-well microtiter plates. The first assay was a
96-well high-throughput bioassay for beet armyworm
(Spodoptera exigua: Lepidoptera) larvae, based on
modifications of Lewer et al.(2006). Eggs of S. exigua were
placed on top of artificial diet (100 mL) in each well of 96-
well microtiter plate. The diet had been pretreated with test
compounds (12 mg dissolved in 30 mL of DMSO–acetone–
H2O mixture) layered on top of the diet using a Sagian
(Bechman-Coulter, Fullerton, CA) liquid handling system
and then allowed to dry for several hours. Infested plates
were then covered with a layer sterile cotton batting and the
plate lid, and then held in the dark at 29 °C (Lewer et al.
2006). Mortality was recoded at 6 d post-treatment. Each
plate had six replicates. The second assay used the yellow
fever mosquito (Aedes aegypti: Diptera). One-day old larvae
of A. aegypti were pippetted into each well of a 96-well
microtiter plate that had been pretreated with 6 mg
(dissolved in 15 mL of DMSO–acetone–H2O mixture, which
had been allowed to dry in the well) of the test compound.
The microtiter plates, covered with a lid of prevent
evaporation, were held 3 d at r.t., and then graded for
mortality. There were six replicates per treatment.
Compounds found to be active were further evaluated a
larger format diet-based bioassay using 128-well diet trays
(Bio-Serv, Frenchtown, NJ). Three-to-five-second instar
larvae of either S. exigua or Helicoverpa zea (corn earworm:
Lepidoptera) were placed in each well (3 mL) of the diet tray
that had been previously filled with 1 mL of artificial diet to
which 25 mg the test compound [dissolved in 50 mL of an
acetone–H2O mixture (90:10)] had been applied (to each of
eight wells) and then allowed to dry. Trays were covered
with a clear self-adhesive cover, and held at 25 °C, 14:10
light/dark for 6 d. Percent mortality was recorded for the
larvae in each well; activity in the eight wells was then
averaged. (b) Lewer, P.; Graupner, P. R.; Hahn, D. R.; Karr,
L. L.; Duebelbeis, D. O.; Lira, J. M.; Anzeveno, P. B.;
Fields, S. C.; Gilbert, J. R.; Pearce, C. J. Nat. Prod. 2006, 69,
1506.
4-Isopropyl-6-(5-phenylisoxazol-3-yl)-N-propyl-
pyrimidin-2-amine
1H NMR (300 MHz, CDCl3): d = 7.87 (m, 2 H), 7.50 (m,
3 H), 7.19 (s, 1 H), 7.11 (s, 1 H), 3.50 (m, 2 H), 2.91 (m, 1
H), 1.70 (m, 2 H), 1.30 (d, J = 7.2 Hz, 6 H), 1.02 (t, J = 6.2
Hz, 3 H); note: NH proton not detected. 13C NMR (75 MHz,
CDCl3): d = 170.9, 163.8, 162.6, 156.4, 130.6, 130.2, 129.3,
127.5, 126.1, 104.1, 98.7, 43.6, 36.1, 23.1, 21.9, 11.9.
4-[5-(4-Chlorophenyl)isoxazol-3-yl]-6-isopropyl-N-propyl-
pyrimidin-2-amine
1H NMR (300 MHz, CDCl3): d = 7.80 (d, J = 8.7 Hz, 2 H),
7.48 (d, J = 8.7 Hz, 2 H), 7.16 (s, 1 H), 7.09 (s, 1 H), 5.90 (br
s, 1 H), 3.48 (br s, 2 H), 2.90 (m, 1 H), 1.70 (m, 2 H), 1.32
(d, J = 7.2 Hz, 6 H), 1.03 (t, J = 7.5 Hz, 3 H). 13C NMR (75
MHz, CDCl3): d = 178.2, 170.9, 163.6, 162.8, 156.2, 130.6,
129.3, 127.5, 126.1, 104.4, 98.7, 46.8, 36.3, 21.9, 11.9, 3.7.
(18) Representative Examples of 8
N-Cyclopentyl-3-[2-(2-methoxyethylamino)-6-methyl-
pyrimidin-4-yl]isoxazole-5-carboxamide
1H NMR (300 MHz, CDCl3): d = 7.42 (s, 1 H), 7.10 (s, 1 H),
6.55 (br s, 1 H), 5.53 (br s, 1 H), 4.41 (m, 1 H), 3.69–3.55 (m,
4 H), 3.38 (s, 3 H), 2.39 (s, 3 H), 2.13–2.04 (m, 2 H), 1.85–
1.52 (m, 6 H). 13C NMR (75 MHz, CDCl3): d = 169.7, 164.3,
163.9, 162.8, 155.5, 154.9, 107.1, 106.2, 71.5, 59.0, 51.9,
41.4, 33.3, 24.4, 24.0.
N-Benzyl-3-[2-(cyclopentylamino)-6-methylpyrimidin-
4-yl]isoxazole-5-carboxamide
1H NMR (300 MHz, CDCl3): d = 7.49 (s, 1 H), 7.35 (m, 5 H),
7.06 (br s, 2 H), 5.22 (d, J = 6.9 Hz, 1 H), 4.64 (d, J = 6.9 Hz,
2 H), 4.33 (m, 1 H), 2.37 (s, 3 H), 2.04 (m, 2 H), 1.72–1.24
(m, 6 H). 13C NMR (75 MHz, CDCl3): d = 169.5, 163.9,
162.6, 162.4, 155.9, 154.8, 137.2, 129.1, 129.0, 128.2,
106.7, 106.5, 53.2, 43.8, 33.5, 24.5, 23.9.
N-(4-Methoxybenzyl)-3-[2-(2-methoxyethylamino)-6-
methyl-pyrimidin-4-yl]isoxazole-5-carboxamide
(20) Krieger, R. L.; Feeny, P. P.; Wilkinson, C. F. Science 1971,
172, 579.
Synlett 2008, No. 19, 3036–3040 © Thieme Stuttgart · New York