Rapid synthesis of 1,3,5-substituted 1,2,4-triazoles from carboxylic acids, amidines, and hydrazines

Article abstract of DOI:10.1021/jo1023393

A general method for the synthesis of 1,3,5-trisubstituted 1,2,4-triazoles has been developed from reaction of carboxylic acids, primary amidines, and monosubstituted hydrazines. This highly regioselective and one-pot process provides rapid access to highly diverse triazoles.

Full text of DOI:10.1021/jo1023393

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SCHEME 1. Carbonylative and Noncarbonylative Access to
Primary Acylamidines en route to 1,2,4-Triazoles
Rapid Synthesis of 1,3,5-Substituted 1,2,4-Triazoles
from Carboxylic Acids, Amidines, and Hydrazines
Georgette M. Castanedo,* Pamela S. Seng,
Nicole Blaquiere, Sean Trapp, and Steven T. Staben*
Discovery Chemistry Group, Genentech, Inc., 1 DNA Way,
South San Francisco, California 94080, United States
castanedo.georgette@gene.com; staben.steven@gene.com
Received November 30, 2010
TABLE 1. Coupling Reagent and Base Screen for Acylamidine For-
mation
A general method for the synthesis of 1,3,5-trisubstituted
1,2,4-triazoles has been developed from reaction of carbo-
xylic acids, primary amidines, and monosubstituted hydra-
zines. This highly regioselective and one-pot process pro-
vides rapid access to highly diverse triazoles.
entry coupling agent
base
% conversiona % formation of 3
1b
2
HATU
DIPEA
>99
97
>99
>99
53
>99
90
91
86
4d
HATU/HOBt DIPEA
HATU DIPEA
HATU/HOBt DIPEA
3c
4c
5
EDC
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
NMM
Cs2CO3
K2CO3
Na2CO3
NaHCO3
6
7
8
9
EDC/HOBt
HBTU
TFFH
CD
75
59
97
89
0
1,2,4-Triazoles are an important class of heterocycles that
are found in a variety of pharmaceutically active molecules.1
Although a variety of methods exist for reliable and regio-
selective synthesis of 3,4,5-substituted 1,2,4-triazoles,2 direct
access to nonsymmetrical 1,3,5-substituted regioisomers re-
mains difficult as limits to substituent identity and regios-
electivity exist.3 Meckler’s method4 involves reaction of
primary hydrazides with primary amidines to form 1H-
1,2,4-triazoles. However, subsequent alkylation is not regio-
selective, giving mixtures of 3,4,5 and 1,3,5 isomers, creating
purification and structural charaterization difficulties.5
3-Methyl-1,5-substituted triazoles can be obtained by reac-
tion of a primary amide with dimethylacetamide dimethyla-
cetal (DMA-DMA) followed by reaction with a mono-
substituted hydrazine.6 Unfortunately, this rapid and highly
>99
92
33
10e
11
12
13b
14
15
PYBOP
HATU
HATU
HATU
HATU
HATU
97
94
94
91
96
97
85
71f
96
91
87
48g
aPercent conversions determined by LC-MS integration (254 nm)
against an anthracene-9-carbonitrile internal standard. bAverage of
three runs on a similar scale. Preactivation of the carboxylate before
c
amidine addition. dNo other discernible byproduct observed by LC-MS.
eSignificant pyrrolidine amide formation observed by LC-MS. Seven-
teen percent formation of 4-propyloxybenzamide. Forty-four percent
formation of 4-propyloxybenzamide.
f
g
regioselective synthesis is constrained by the lack of com-
mercially available orthoaminals.7,8
(1) For a review, see: Al-Masoudi, A.; Al-Soud, Y. A.; Al-Salihi, N. J.;
Al-Masoudi, N. A. Chem. Heterocycl. Compd. 2006, 42, 1377.
(2) For a recent one-pot method, see: (a) Stocks, M. J.; Cheshire, D. R.;
Reynolds, R. Org. Lett. 2004, 6, 2969. For a comprehensive review, see: (b)
Moulin, A.; Bibian, M.; Blayo, A.-L.; Habnouni, S. E.; Martinez, J.;
Fehrentz, J.-A. Chem. Rev. 2010, 110, 1809.
(3) (a) Temple, C., Jr. The Chemistry of Heterocyclic Compounds, Vol. 37;
Montgomery, J. A., Ed.; Wiley-Interscience: New York, 1981; pp 62-95.
(b) Potts, K. T. Chem. Rev. 1961, 61, 87.
(4) (a) Francis, J. E.; Gorczyca, L. A.; Mazzeuga, G. C.; Meckler, H.
Tetrahedron Lett. 1987, 28, 5133. One-pot primary hydrazide and triazole
formation: (b) Funabiki, K.; Noma, N.; Kuzuya, G.; Matsui, M.; Shibata, K.
J. Chem. Res. (S) 1999, 300. Using imidoyl benzotriazoles: (c) Katritzky,
A. R.; Khashab, N. M.; Kirichenko, N.; Singh, A. J. Org. Chem. 2006, 71,
9051.
Our group recently reported (Scheme 1) a four-component
synthesis of fully substituted 5-aryl-1,2,4 triazoles by carbo-
nylative heterocyclization of aryl halides.9 This method
allows regioselective access to a 5-triazolyl heterobiaryl
linkage without generation of organometallic reagents and
proceeds through reactive acyl amidine intermediates II. An
alternative and experimentally convenient strategy to access
(7) DMF-DMA and DMA-DMA are the only commercially available
orthoaminals allowing only access to 3-H and 3-Me 1,2,4 triazoles. Alter-
ꢀ
natively, multistep synthesis of an acylimidate is a viable intermediate: Perez,
M. A.; Dorado, C. A.; Soto, J. L. Synthesis 1983, 483.
(8) For a recent synthesis of 5-carboxyl-3H-1,2,4-triazoles, see: Xu, Y.;
McLaughlin, M.; Bolton, E. N.; Reamer, R. A. J. Org. Chem. 2010, 75, 8666.
(9) Staben, S. T.; Blaquiere, N. Angew. Chem., Int. Ed. 2010, 49, 325.
(5) Katritzky, A. R.; Qi, M.; Feng, D.; Zhang, G.; Griffith, M. C.;
Watson, K. Org. Lett. 1999, 1, 1189.
(6) Lin, Y.; Lang, S. A.; Lovell, M. F.; Perkinson, N. A. J. Org. Chem.
1979, 44, 4160.
DOI: 10.1021/jo1023393
r
Published on Web 01/14/2011
J. Org. Chem. 2011, 76, 1177–1179 1177
2011 American Chemical Society

Castanedo et al.
JOCNote
TABLE 2. Substrate scope
aYields reported in this table are isolated yields after silica gel chromatography or reverse phase HPLC.1,3,5- substituted regioisomers were
confirmed by 15N HMBC and/or 1H nOe measurements for compounds 8, 16, 17 and 18; the remainder of products were assigned by analogy.
b
Hydrazines and amidines generally added as HCl salts. Product was not analyzed for potential racemization. Yield of ∼90% pure 9, an analytical
sample for characterization was obtained on repurification. cDue to poor solubility of carboxylic acid, preactivation with HATU was necessary.
dPotassium carbonate was used instead of DIPEA.
acyl amidines II would be reaction of primary amidines (I)
with activated carboxylic acids (III) that would obviate the
need for using carbon monoxide and also allow synthesis of
triazoles with nonaryl 5-substitution. Herein we report reac-
tion optimization and substrate scope studies for this one-
pot heterocycle synthesis.
bases, we noticed a trend that bases with lower pKa values
such as N-methylmorpholine and sodium bicarbonate lead
to formation of the primary amide byproduct, presumably
due to hydrolysis of protonated acylamidine intermediate.13
Thus, HATU/DIPEA (entry 1) was selected as a standard
condition for substrate scope studies.
Using 4-propyloxybenzoic acid and benzamidine as test
substrates,10 we began with optimization for formation of
the acylamidine intermediate through a screen of standard
peptide coupling reagents11 using diisopropylethylamine
(DIPEA) as a base and DMF as the reaction solvent. The
coupling reactions were monitored over 3 h by LC-MS for
the percent consumption of acid 1 and conversion to acyla-
midine intermediate 3 as determined by comparison to an
internal standard. The results (Table 1) indicate that a variety
of coupling agents worked well in the formation of acylami-
dine 3.12 HATU (2-(7-Aza-1H-benzotriazole-1-yl)-1,1,3,
3-tetramethyluronium hexafluorophosphate) was selected
for further evaluation based on availability and ease of
removal of coupling byproducts. While screening alternative
As reported in our previous communication, after addi-
tion of isopropylhydrazine (1.5 equiv) and acetic acid
(10 equiv) to the crude solution of acyl amidine 3, cyclization
was complete after 3 h at 80 °C to give triazole 7 (Table 2). On
the basis of these optimized reaction conditions, we commenced
exploration of our substrate scope. Results are summarized
in Table 2. In addition to incorporation of aryl and hetero-
aryl substitutents at the 5-position (5-7, 9, 16-18, 22, 25),
substituted alkyl (8, 10-12, 19, 21, 26), and heterocycloalkyl
(13-15, 20, 24) carboxylic acids are viable substrates for this
one-pot process. We were pleased to see that the presence of
less nucleophilic nitrogen and oxygen atoms on the reactants
are not a problem in triazole formation (11,17,18,21,22,25,26).
Notable is the synthesis of 18, demonstrating selective
reaction of a nonaromatic amidine over an aminopyridine
substructure. Importantly, clean synthesis of antipodes 14
and 15 demonstrate that the reaction conditions are mild
enough to avoid racemization of enantiomerically pure amino
(10) To the best of our knowledge, a systematic study for the acylation of
amidines has not been reported, see: (a) Baker, P. L.; Gendler, P. L.;
Rapoport, H. J. Org. Chem. 1981, 46, 2455. (b) Gupta, S.; Agarwal, P. K.;
Kundu, B. Tetrahedron Lett. 2010, 51, 1887.
(11) Montalbetti, C. A. G. N.; Falque, V. Tetrahedron 2005, 61, 10827.
(12) In contrast, reaction of 4-propyloxybenzoyl chloride with benzami-
dine gave a complex mixture of products.
(13) MoKa estimated pKa of the conjugate acid of 3 = 7.13.
1178 J. Org. Chem. Vol. 76, No. 4, 2011

Castanedo et al.
JOCNote
acids. As noted in our previous report,9 acylamidines created
from alkyl, aryl and heteroaryl amidines react cleanly with
widely varied hydrazines. Interestingly, ester functionality is
also tolerated as amidine and hydrazine substituents, as in the
synthesis of 22 and 23. Yields were generally low when bulky
carboxylic acids and/or hydrazines were used (i.e., PivOH for
11 and tBuNHNH2 for 12).14
In summary, we have developed an efficient and experi-
mentally convenient synthesis of 1,3,5-trisubstituted 1,2,4-
triazoles from carboxylic acids. This synthesis allows greater
diversity at the 5-position compared to our previously
reported palladium-catalyzed carbonylative route. Yields
range from 25-84%, or 63-94% per bond for this one pot
reaction. The wide-substrate scope and high functional
group compatibility allows generation of considerable
diversity.15 The speed, experimental ease and regioselectivity
of this process are improvements on existing methods for
access to this important substructure.
4.2 mmol, 1.5 equiv) and HATU (1.16 g, 3.1 mmol, 1.1 equiv) in
a flask. The reaction was stirred at room temperature and moni-
tored by LC-MS for consumption of acid 1 and formation of
acylamidine intermediate 3. After 3 h, isopropylhydrazine
hydrochloride (0.460 g, 4.2 mmol, 1.5 equiv) then acetic acid
(1.58 mL, 27.7 mmol, 10 equiv) were added and the reaction
mixture was heated to 80 °C and monitored for consump-
tion of acylamidine intermediate. After 3 h, the reaction was
cooled to room temperature, diluted with 200 mL EtOAc
and extracted once with a saturated NaHCO3 (200 mL). The
organic layer was dried (MgSO4), filtered and concentrated.
The crude material was purified by flash column chromatog-
raphy (0-100% EtOAc in heptanes) to afford pure compound
7 as a colorless crystalline solid (68% yield).
1H NMR (500 MHz, DMSO) δ 8.05 (d, J = 7.0 Hz, 2H), 7.62
(d, J = 7.7 Hz, 2H), 7.51-7.40 (m, 3H), 7.13 (d, J = 8.8 Hz, 2H),
4.68 (dt, J = 13.1, 6.5 Hz, 1H), 4.02 (t, J = 6.5 Hz, 2H), 1.82 -
1.73 (m, 2H), 1.48 (d, J = 6.6 Hz, 6H), 1.01 (t, J = 7.4 Hz, 3H).
13C NMR (126 MHz, DMSO) δ 159.9, 159.8, 154.2, 131.2,
130.3, 129.0, 128.7, 125.7, 120.0, 114.8, 69.1, 50.3, 22.6, 21.9,
10.4.
Experimental Section
HREIMS calcd for C20H24N3O: 322.1919, found 322.2159.
Representative procedure for the synthesis of 7: DMF (10 mL)
and DIPEA (1.45 mL, 8.3 mmol) were added to 4-propyloxybenzoic
acid 1 (0.500 g, 2.8 mmol, 1.0 equiv), benzamidine 2 (500 mg,
Acknowledgment. We thank Deven Wang, Yutao Jiang,
Steve Huhn, and Yanzhou Liu for analytical support. P.S.
thanks the Genentech Internship Program
(14) Common byproduct observed by LC-MS include primary amide
formation, reaction of HATU with amidine to form guanidinyl by-product,
acid/amidine dimer and reaction of hydrazine with carboxylic acid
(indicating incomplete formation of acylamidine intermediate). Isolated
yields seem highly dependent on the solubility of the triazole products.
(15) Notably, all reagents utilized in this publication are commercially
available.
Supporting Information Available: Experimental proce-
dures, tabulated spectroscopic data, images of 1H and 13C
NMR spectra and HREIMS data for all final products. This
material is available free of charge via the Internet at http://
pubs.acs.org.
J. Org. Chem. Vol. 76, No. 4, 2011 1179