Synthesis of trifluoromethyl-promoted functional pyrazolo[1,5-a]pyrimidine and pyrazolo[5,1-d][1,2,3,5]tetrazine-4(3H)-ones

Article abstract of DOI:10.1016/j.jfluchem.2006.02.001

Ethyl 5-amino-3-trifluoromethyl-1H-pyrazole-4-carboxylate (1) was efficiently synthesized via the condensation of ethyl cyanoacetate and trifluoroacetic anhydride, followed by chloridization and the condensation with aqueous hydrazine. Its unique reactivi

Full text of DOI:10.1016/j.jfluchem.2006.02.001

Journal of Fluorine Chemistry 127 (2006) 409–416
www.elsevier.com/locate/fluor
Synthesis of trifluoromethyl-promoted functional pyrazolo
[1,5-a]pyrimidine and pyrazolo[5,1-d][1,2,3,5]tetrazine-4(3H)-ones
Yan-Chao Wu a,b, Yong-Jun Chen b, Hui-Jing Li b, Xiao-Mao Zou a,
Fang-Zhong Hu a, Hua-Zheng Yang a,
*
a State Key Laboratory of Element-Organic Chemistry, Institute of Element-Organic Chemistry, Nankai University, Tianjin 300071, China
b Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
Received 31 December 2005; received in revised form 25 January 2006; accepted 1 February 2006
Available online 23 February 2006
Abstract
Ethyl 5-amino-3-trifluoromethyl-1H-pyrazole-4-carboxylate (1) was efficiently synthesized via the condensation of ethyl cyanoacetate and
trifluoroacetic anhydride, followed by chloridization and the condensation with aqueous hydrazine. Its unique reactivity was exploited for the
synthesis of trifluoromethylated pyrazolo[1,5-a]pyrimidine (5) and pyrazolo[5,1-d][1,2,3,5]tetrazine-4(3H)-ones (8a–j). Among them, 5 was
firstly found to be a novel fluorescent molecule that might be exploited as an attractive fluorophore for possessing many binding sites, and its
fluorescence intensity was obviously stronger than its methyl analogue. 8 were found to be a new chemical class of potential monocotyledonous
Echinochloa crus-galli L. Beauv inhibitors, and were more active than their methyl analogues.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Fluorine; Pyrazole; Pyrazolo[1,5-a]pyrimidine; Pyrazolo[5,1-d][1,2,3,5]tetrazine-4(3H)-one; Inhibitor
1. Introduction
methylated compounds has been an important field in both
organofluorine chemistry and organic synthetic chemistry.
In connection with our ongoing project aiming at the
development of new and highly efficient synthetic methods for
various functional pyrazole derivatives and their fused
analogues [15–17], we required a facile and reliable access
to trifluoromethylated 5-amino-1H-pyrazoles. Although some
methods have been developed for the syntheses of 5-amino-1H-
pyrazoles due to their key importance as intermediates in the
syntheses of various pharmaceuticals [15–20], trifluoromethy-
lated 5-amino-1H-pyrazoles are relatively underdeveloped
[21–23]. Herein, we would like to report our recent results
on the convenient preparation of ethyl 5-amino-3-trifluoro-
methyl-1H-pyrazole-4-carboxylate (1) (Scheme 1) and its use
in the synthesis of trifluoromethylated pyrazolo[1,5-a]pyrimi-
dine (5) (Scheme 2) and pyrazolo[5,1-d][1,2,3,5]tetrazine-
4(3H)-ones (8a–j) (Scheme 3).
Organic fluorine compounds have been receiving significant
attention in the materials and pharmaceutical sciences due to
their unique physical and biological properties such as the
increased membrane permeability, enhanced hydrophobic
binding and stability against metabolic oxidation [1–6]. Among
these compounds the trifluoromethyl group-containing mole-
cules are especially important, and continue to attract increasing
attention from various fields [7–10]. Since fluorine is virtually
absent in the living tissue [11], fluorinated pharmaceuticals
mightpossess comparatively less environmental andmammalian
toxicity. Nowadays many trifluoromethylated molecules have
been developed as well-known drugs such as prozac (anti-
depressant), diflucan (anti-fungal agent), casodex (anti-cancer
agent) and desflurane (inhalation anesthetic) [10]. On the other
hand, trifluoromethyl-containing Mosher’s acid and its deriva-
tives are widely used as chiral NMR resolution agents [12–14].
Therefore, the development of synthetic methods for trifluoro-
2. Experimental
2.1. General methods
All reactions were performed using oven-dried glassware
under a positive atmosphere of dry nitrogen. Common reagents
* Corresponding author. Fax: +86 22 23503627.
E-mail address: nk_yanghz@126.com (H.-Z. Yang).
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2006.02.001

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Y.-C. Wu et al. / Journal of Fluorine Chemistry 127 (2006) 409–416
Scheme 1.
Scheme 3.
and materials were purchased from commercial sources and
purified by recrystallization or distillation. Analytical thin layer
chromatography was performed with 0.2 mm coated commer-
cial silica gel plates (Kiesselgel 60 GF254). NMR spectra were
measured on a Bruker XL-300 (1H, 300 MHz and 13C,
trifluorobut-2-enoate (3) as pale yellow liquid in 58.1% yield. 1H
NMR (300 MHz, CDCl3): d 1.37 (3H, t, J = 6.92 Hz, CH2-CH3),
4.39 (2H, q, J = 6.92 Hz, CH2-CH3) [8].
To a solution of aqueous hydrazine (0.63 g, 85%) in 30 mL
ethanol at À5 8C, 3 (2.46 g, 10.8 mmol) was added slowly.
Then Et3N (1.09 g, 10.8 mmol) was added dropwise. After this
dropping, the mixture was stirred until the reaction was
completed as monitored by TLC (ethyl acetate:petroleum
ether = 2:1, v/v, Rf (1) = 0.6). The solution was evaporated
under reduced pressure, dissolved in 50 mL ethyl acetate,
washed with 10 mL water, dried over anhydrous Na2SO4,
filtered, and evaporated under reduced pressure. The crude
product was washed by CH2Cl2 to give pure yellow solid 1.
1
75 MHz). Data for H are reported as follows: chemical shift
(ppm), integration, multiplicity (s, singlet; d, doublet; t, triplet;
q, quartet; m, multiplet; br, broad), coupling constant (Hz) and
number. Data for 13C NMR are reported in terms of chemical
shift (ppm). Infrared (IR) spectra were recorded on a Perkin
Elmer 500 FT-IR spectrophotometer and are reported in terms
of frequency of absorption (cmÀ1). Elemental analyses were
performed on Yanaco-CHN CORDER elementary analyzer.
Mass spectra were obtained from VG ZAB-HS instrument or
GCT-MS Micromass UK (TOF MS EI+). The fluorescence
spectra were measured with a Hitachi F-4500 fluorescence
spectrophotometer at 25 8C (lex = 299.74 nm, the excitation and
emission slits widths were 10 nm). Melting points were mea-
sured on a Thomas–Hoover apparatus and were not corrected.
Yield: 56.4%; mp = 170–171 8C.
FTIR (KBr): y 1718 (CO), 1552, 1130, 775 cmÀ1
.
1H NMR (300 MHz, d6-Me2SO): d 1.23 (3H, t, J = 7.15 Hz,
CH2-CH3), 4.39 (2H, q, J = 7.15 Hz, CH2-CH3), 6.36 (2H, s,
br, NH2), 12.5 (1H, s, br, NH).
13C NMR (75 MHz, d6-Me2SO): d 164.10 (s, CO), 160.94,
2.2. Preparation of ethyl 5-amino-3-trifluoromethyl-1H-
pyrazole-4-carboxylate (1)
145.75
(q,
2JCF = 37.76 Hz,
C-3),
120.65
(q,
1JCF = 269.10 Hz, CF3), 100.09, 60.76 (s, CH2-CH3),
13.95 (s, CH2-CH3).
To a solution of ethyl cyanoacetate (2) (6.41 g, 56.7 mmol)
and trifluoroacetic anhydride (14.3 g, 68.0 mmol) in 50 mL
CH2Cl2 at À10 8C, the solution of Et3N (14.3 g, 141.8 mmol)
was added dropwise. After this dropping, the mixture was stirred
at À10 8C for 10 h, washed with 20 mL cold water, dried over
anhydrous Na2SO4, filtered and then decreased the obtained
CH2Cl2 solution temperature to zero. oxalyl chloride (36.0 g,
284 mmol) was added dropwise. After this dropping, the mixture
was added five drops of pyridine, and then refluxed for 6 h. The
solution was evaporated under reduced pressure, and then
distilled (36–39 8C/Pa) to give ethyl 3-chloro-2-cyano-4,4,4-
Anal. calcd. for C7H8F3N3O2: C, 37.68; H, 3.61; N, 18.83.
Found: C, 37.79; H, 3.68; N, 18.82.
2.3. Preparation of ethyl 2-(trifluoromethyl)-5,7-
dimethylpyrazolo[1,5-a]pyrimidine-3-carboxylate (5)
To a solution of 1 (0.22 g, 1.0 mmol) in 20 mL acetic acid,
pentane-2,4-dione (4) (0.12 g, 1.2 mmol) and two drops of
concentrated H2SO4 was added, and then stirred at room
temperature until the reaction was completed as monitored by
TLC (ethyl acetate:petroleum ether = 2:1, v/v, Rf (1) = 0.6).
The solution was added with 10 mL water, and then 10 mL
CH2Cl2. The organic layer was separated, and the aqueous
solution was extracted with 20 mL CH2Cl2. The combined
organic layer was dried over anhydrous Na2SO4, filtered, and
evaporated under reduced pressure. Pure product 5 was
obtained via recrystallization from CH2Cl2/Et2O.
Scheme 2.
Yield: 94.0% yield; white solid; mp 115–116 8C.

Y.-C. Wu et al. / Journal of Fluorine Chemistry 127 (2006) 409–416
411
FTIR (KBr): y 1713 (CO), 1629, 1554, 1512, 1437, 1379,
1343, 1245, 1180, 1046, 878, 846, 795, 712 cmÀ1
1H NMR (300 MHz, CDCl3): d 1.43 (3H, t, J = 7.13 Hz,
CH2-CH3), 4.51 (2H, q, J = 7.13 Hz, CH2-CH3), 7.39 (1H, t,
J = 8.50 Hz, Ar-H), 7.43 (1H, t, J = 7.96 Hz, Ar-H), 7.57–
7.66 (2H, m, Ar-H).
13C NMR (75 MHz, CDCl3): d 158.41 (s, CO), 147.84 (q,
2JCF = 40.06 Hz, C-7), 145.10 (s, CO), 141.42, 137.93,
133.12, 132.75, 128.50, 125.87, 125.28, 119.23 (q,
1JCF = 270.38 Hz, CF3), 109.38, 62.65 (s, CH2-CH3),
13.94 (s, CH2-CH3).
MS (TOF EI): m/z 387 (M+) (2%), 234 (2%), 208 (4%), 180
(4%), 163 (5%), 153 (100%), 125 (22%), 90 (12%), 63 (4%).
Anal. calcd. for C14H9ClF3N5O3: C, 43.37; H, 2.34; N,
18.06. Found: C, 43.44; H, 2.31; N, 18.14.
.
1H NMR (300 MHz, CDCl3): d 1.35 (3H, t, J = 7.13 Hz,
CH2-CH3), 2.64 (3H, s, CH3), 2.72 (3H, s, CH3), 4.37 (2H, q,
J = 7.13 Hz, CH2-CH3), 6.81 (s, 1H).
13C NMR (75 MHz, CDCl3): d 164.12 (s, CO), 160.96 (s,
2
C N), 148.81, 146.58, 145.80 (q, JCF = 39.50 Hz, C-2),
1
120.65 (q, JCF = 270.70 Hz, CF3), 111.93, 60.78 (s, CH2-
CH3), 25.27 (s, CH3), 16.73 (s, CH3), 13.88 (s, CH2-CH3).
Anal. calcd. for C12H12F3N3O2: C, 50.18; H, 4.21; N, 14.63.
Found: C, 50.05; H, 4.39; N, 14.61.
2.4. Preparation of ethyl 5-diazo-3-trifluoromethyl-
pyrazole-4-carboxylate (6)
2.5.2. Ethyl 3-(3-chlorophenyl)-7-(trifluoromethyl)-3,4-
dihydro-4-oxopyrazolo[5,1-d][1,2,3,5]tetrazine-8-
carboxylate (8b)
To a solution of 1 (0.53 g, 2.4 mmol) and HCl (0.52 g,
36.5%) in 10 mL EtOH at À10 8C, 10 mL 2% aqueous
NaNO2 was added slowly. After the reaction was completed
as monitored by TLC (ethyl acetate:petroleum ether = 2:1,
v/v, Rf (1) = 0.6), 40 mL CH2Cl2 was added and then the
mixture was neutralized with saturated aqueous Na2CO3
until pH 8. The organic layer was separated, and the
aqueous solution was extracted with 40 mL CH2Cl2. The
combined organic layer was dried over anhydrous Na2SO4,
filtered, and the solvent was removed in vacuum. The crude
product was washed by Et2O to give pure pale yellow
crystals 6.
White solid; yield: 74.9%; mp = 106–107 8C.
FTIR (KBr): y 1764 (CO), 1716 (CO), 1580, 1468, 1300,
1228, 1196, 1153, 1045, 796 cmÀ1
.
1H NMR (300 MHz, CDCl3): d 1.41 (3H, t, J = 7.13 Hz,
CH2-CH3), 4.45 (2H, q, J = 7.13 Hz, CH2-CH3), 7.48–7.57
(3H, m, Ar-H), 7.65 (1H, s, Ar-H).
13C NMR (75 MHz, CDCl3): d 158.19 (s, CO), 147.73 (q,
2JCF = 39.98 Hz, C-7), 145.12 (s, CO), 138.49, 137.19,
135.22, 130.52, 130.50, 125.99, 123.91, 119.25 (q,
1JCF = 270.38 Hz, CF3), 109.32, 62.63 (s, CH2-CH3),
13.88 (s, CH2-CH3).
Yield: 76.9%; mp = 86–88 8C.
FTIR (KBr): y 2224 (N2 ), 1727 (CO), 1549, 1456, 1326,
MS (TOF EI): m/z 387 (M+) (2%), 342 (3%), 234 (3%), 208
(4%), 180 (7%), 163 (10%), 153 (100%), 125 (25%), 90
(12%), 63 (4%).
+
1279, 1134, 1104, 778 cmÀ1
.
1H NMR (300 MHz, CDCl3): d 1.38 (3H, t, J = 7.26 Hz,
CH2-CH3), 4.38 (2H, q, J = 7.26 Hz, CH2-CH3).
Anal. calcd. for C14H9ClF3N5O3: C, 43.37; H, 2.34; N,
18.06. Found: C, 43.27; H, 2.28; N, 18.06.
13C NMR (75 MHz, CDCl3): d 164.14 (s, CO), 160.97,
2
145.78 (q, JCF = 37.76 Hz, C-3), 120.69 (q, JCF
1
=
2.5.3. Ethyl 3-(4-chlorophenyl)-7-(trifluoromethyl)-3,4-
dihydro-4-oxopyrazolo[5,1-d][1,2,3,5]tetrazine-8-
carboxylate (8c)
270.00 Hz, CF3), 101.12, 60.78 (s, CH2-CH3), 13.96 (s,
CH2-CH3).
Anal. calcd. for C7H5F3N4O2: C, 35.91; H, 2.15; N, 23.93.
Found: C, 35.85; H, 2.10; N, 24.04.
White solid; yield: 78.2%; mp = 130–131 8C.
FTIR (KBr): y 1772 (CO), 1731 (CO), 1576, 1491, 1306,
1226, 1189, 1153, 1110, 1040 cmÀ1
.
2.5. Preparation of ethyl 3-aryl-7-(trifluoromethyl)-3,4-
dihydro-4-oxopyrazolo[5,1-d][1,2,3,5]tetrazine-8-
carboxylates (8)
1H NMR (300 MHz, CDCl3): d 1.45 (3H, t, J = 7.14 Hz,
CH2-CH3), 4.52 (2H, q, J = 7.14 Hz, CH2-CH3), 7.56–7.63
(4H, m, Ar-H).
13C NMR (75 MHz, CDCl3): d 158.21 (s, CO), 147.94 (q,
2JCF = 40.13 Hz, C-7), 145.13 (s, CO), 138.51, 136.54,
The mixture of 2 mmol aryl isocyanates (7) and 2 mmol
6 in 5 mL CH2Cl2 was stirred at room temperature. After
the reaction was completed as monitored by TLC (ethyl
acetate:petroleum ether = 2:1, v/v, Rf (6) = 0.8), the solvent was
removed in vacuum. The pure product was isolated by flash
chromatography on silica gel (200–300 mesh).
1
134.76, 129.88, 126.9, 119.26 (q, JCF = 270.38 Hz, CF3),
109.44, 62.66 (s, CH2-CH3), 13.97 (s, CH2-CH3).
MS (TOF EI): m/z 387 (M+) (2%), 342 (3%), 208 (5%), 180
(8%), 163 (14%), 153 (100%), 125 (24%), 90 (12%), 63
(4%).
Anal. calcd. for C14H9ClF3N5O3: C, 43.37; H, 2.34; N,
18.06. Found: C, 43.26; H, 2.30; N, 18.15.
2.5.1. Ethyl 3-(2-chlorophenyl)-7-(trifluoromethyl)-3,4-
dihydro-4-oxopyrazolo[5,1-d][1,2,3,5]tetrazine-8-
carboxylate (8a)
2.5.4. Ethyl 3-(2,4-dichlorophenyl)-7-(trifluoromethyl)-3,4-
dihydro-4-oxopyrazolo[5,1-d][1,2,3,5]tetrazine-8-
carboxylate (8d)
White solid; yield: 70.3%; mp = 114–115 8C.
FTIR (KBr): y 1768 (CO), 1724 (CO), 1578, 1480, 1303,
1227, 1193, 1152, 1078 cmÀ1
.
White solid; yield: 81.0%; mp = 131–132 8C.

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Y.-C. Wu et al. / Journal of Fluorine Chemistry 127 (2006) 409–416
FTIR (KBr): y 1782 (CO), 1723 (CO), 1582, 1479, 1371,
1298, 1231, 1158, 1124, 1100, 1042, 955, 824, 736 cmÀ1
Anal. calcd. for C15H9F6N5O3: C, 42.77; H, 2.15; N, 16.63.
Found: C, 42.65; H, 2.11; N, 16.45.
.
1H NMR (200 MHz, CDCl3): d 1.44 (3H, t, J = 7.16 Hz,
CH2-CH3), 4.49 (2H, q, J = 7.14 Hz, CH2-CH3), 7.50–7.67
(3H, m, Ar-H).
13C NMR (75 MHz, CDCl3): d 158.15 (s, CO), 147.84 (q,
2JCF = 40.05 Hz, C-7), 145.25 (s, CO), 138.24, 138.01,
133.34, 132.20, 130.86, 130.23, 128.70, 119.24 (q,
1JCF = 270.60 Hz, CF3), 109.93, 62.72 (s, CH2-CH3),
13.95 (s, CH2-CH3).
MS (TOF EI): m/z 376 (M+-OEt)(4%), 329(1%), 289 (2%),
269 (4%), 260 (3%), 208 (10%), 187 (100%), 180 (20%),
161 (54%), 124 (34%), 97 (4%), 73 (5%), 61 (2%).
Anal. calcd. for C14H8Cl2F3N5O3: C, 39.83; H, 1.91; N,
16.59. Found: C, 39.74; H, 2.00; N, 16.42.
2.5.7. Ethyl 7-(trifluoromethyl)-3,4-dihydro-3-(2-
nitrophenyl)-4-oxopyrazolo[5,1-d][1,2,3,5]tetrazine-8-
carboxylate (8g)
Yellow solid; yield: 85.4%; mp = 144–145 8C.
FTIR (KBr): y 3494, 3354, 3240, 1746 (CO), 1712 (CO),
1614, 1504, 1441, 1344, 1217, 1166, 750 cmÀ1
.
1H NMR (300 MHz, CDCl3): d 1.36 (3H, t, J = 7.12 Hz,
CH2-CH3), 4.33 (2H, q, J = 7.12 Hz, CH2-CH3), 7.30 (1H,
dd, J = 1.09, 7.89 Hz, Ar-H), 7.30 (1H, dd, J = 1.09,
7.89 Hz, Ar-H), 7.73 (1H, td, J = 1.30, 7.90 Hz, Ar-H),
8.30 (1H, dd, J = 1.38, 8.44 Hz, Ar-H), 8.64 (1H, d,
J = 8.47 Hz, Ar-H).
13C NMR (75 MHz, CDCl3): d 162.69 (s, CO), 154.55,
2
149.21 (s, CO), 143.84 (q, JCF = 40.05 Hz, C-7), 137.34,
2.5.5. Ethyl 7-(trifluoromethyl)-3-(2-fluorophenyl)-3,4-
dihydro-4-oxopyrazolo[5,1-d][1,2,3,5]tetrazine-8-
carboxylate (8e)
135.84, 132.80, 126.20, 124.39, 121.45, 119.24 (q,
1JCF = 270.00 Hz, CF3), 92.36, 62.66 (s, CH2-CH3), 13.98
(s, CH2-CH3).
Orange solid; yield: 71.6%; mp = 117–118 8C.
FTIR (KBr): y 1772 (CO), 1727 (CO), 1306, 1242, 1224,
MS (TOF EI): m/z 342 (2%), 223(53%), 177 (100%), 164
(20%), 158 (11%), 120 (15%), 106 (5%), 90 (19%), 63 (4%),
52 (2%).
1190, 1154, 1099 cmÀ1
.
1H NMR (300 MHz, CDCl3): d 1.46 (3H, t, J = 7.14 Hz,
CH2-CH3), 4.54 (2H, q, J = 7.14 Hz, CH2-CH3), 7.38 (1H, t,
J = 8.54 Hz, Ar-H), 7.40 (1H, t, J = 7.92 Hz, Ar-H), 7.58–
7.67 (2H, m, Ar-H).
Anal. calcd. for C14H9F3N6O5: C, 42.22; H, 2.28; N, 21.10.
Found: C, 42.19; H, 2.27; N, 20.95.
13C NMR (75 MHz, CDCl3): d 158.46 (s, CO), 157.34 (d,
2.5.8. Ethyl 7-(trifluoromethyl)-3,4-dihydro-3-(4-
nitrophenyl)-4-oxopyrazolo[5,1-d][1,2,3,5]tetrazine-8-
carboxylate (8h)
3
1JCF = 250.50 Hz), 155.39 (d, JCF = 9.75 Hz), 147.87 (q,
2JCF = 40.06 Hz, C-7), 145.30 (s, CO), 137.98, 133.04 (d,
2
3JCF = 8.25 Hz), 128.81 (d, JCF = 39.75 Hz), 125.28 (d,
Yellow solid; yield: 92.3%; mp = 139–140 8C.
FTIR (KBr): y 1740 (CO), 1716 (CO), 1578, 1497, 1332,
4JCF = 3.75 Hz), 119.23 (q, 1JCF = 270.00 Hz, CF3), 117.26
(d, 2JCF = 18.75 Hz), 109.33, 63.09 (s, CH2-CH3), 14.07 (s,
CH2-CH3).
1302, 1231, 1203, 1159, 1110, 1010, 850 cmÀ1
.
1H NMR (200 MHz, d6-Me2SO): d 1.44 (3H, t, J = 7.06 Hz,
CH2-CH3), 4.53 (2H, q, J = 7.06 Hz, CH2-CH3), 7.51–7.67
(4H, m, Ar-H).
MS (TOF EI): m/z 371 (M+) (2%), 326 (2%), 208 (3%), 180
(8%), 163 (14%), 137 (100%), 109 (27%), 89 (6%), 82 (8%),
62 (2%), 57 (2%).
13C NMR (75 MHz, d6-Me2SO):
d
158.43 (q,
Anal. calcd. for C14H9F4N5O3: C, 45.29; H, 2.44; N, 18.86.
Found: C, 45.33; H, 2.58; N, 18.74.
2JCF = 34.95 Hz, C-7), 147.26 (s, CO), 142.01 (s, CO),
137.85 (q, 1JCF = 270.60 Hz, CF3), 128.12, 126.83, 125.59,
3
125.38, 118.39 (d, JCF = 5.85 Hz, C-8), 112.79, 62.72 (d,
JCF = 6.98 Hz, CH2-CH3), 13.95 (d, JCF = 21.90 Hz, CH3).
2.5.6. Ethyl 7-(trifluoromethyl)-3-(2-
(trifluoromethyl)phenyl)-3,4-dihydro-4-oxopyrazolo[5,1-
d][1,2,3,5]tetrazine-8-carboxylate (8f)
MS (TOF EI): m/z 344 (2%), 302(3%), 223 (1%), 177 (3%),
164 (100%), 138 (79%), 134 (45%), 122 (5%), 118 (12%),
108 (32%), 90 (47%), 65 (25%), 63 (12%).
Orange solid; yield: 96.8%; mp = 122–123 8C.
FTIR (KBr): y 3457, 3345, 2232, 1734 (CO), 1689 (CO),
Anal. calcd. for C14H9F3N6O5: C, 42.22; H, 2.28; N, 21.10.
Found: C, 42.14; H, 2.35; N, 21.14.
1620, 1545, 1459, 1326, 1288, 1237, 1142, 1036, 764 cmÀ1
.
1H NMR (300 MHz, CDCl3): d 1.45 (3H, t, J = 7.14 Hz,
CH2-CH3), 4.47 (2H, q, J = 7.14 Hz, CH2-CH3), 7.23–8.20
(4H, m, Ar-H).
2.5.9. Ethyl 3-(4-(ethoxycarbonyl)phenyl)-7-
(trifluoromethyl)-3,4-dihydro-4-oxopyrazolo[5,1-
d][1,2,3,5]tetrazine-8-carboxylate (8i)
13C NMR (75 MHz, CDCl3): d 162.63 (s, CO), 158.65,
3
154.37, 149.14 (s, CO), 132.95 (q, JCF = 10.50 Hz),
White solid; yield: 94.1%; mp = 58–59 8C.
FTIR (KBr): y 3438, 2989, 1780 (CO), 1730 (CO), 1705
(CO), 1575, 1477, 1371, 1296, 1223, 1166, 1116, 1044, 771,
697 cmÀ1
.
1H NMR (300 MHz, CDCl3): d 1.40–1.47 (6H, m, 2 CH2-
CH3), 4.42 (2H, q, J = 7.13 Hz, CH2-CH3), 4.51 (2H, q,
J = 7.13 Hz, CH2-CH3), 7.76 (2H, d, J = 8.71 Hz, Ar-H),
8.26 (2H, d, J = 8.68 Hz, Ar-H).
2
3
126.58(q, JCF = 15.75 Hz), 126.52 (q, JCF = 5.25 Hz),
2
125.41 (q, JCF = 18.83 Hz), 130.23, 123.68, 119.81 (q,
1JCF = 268.88 Hz, CF3), 118.53 (q, 1JCF = 268.13 Hz, CF3),
117.63, 62.77 (s, CH2-CH3), 13.71 (s, CH2-CH3).
MS (TOF EI): m/z 333 (12%), 305 (13%), 285 (2%), 266
(2%), 216 (8%), 187 (41%), 173 (38%), 145 (100%), 141
(11%), 114 (11%), 95 (6%), 75 (2%), 63 (1%).

Y.-C. Wu et al. / Journal of Fluorine Chemistry 127 (2006) 409–416
413
13C NMR (75 MHz, CDCl3): d 165.17 (s, CO), 158.18 (s,
reasonable because of the easy formation of polymers if the
high nucleophilic hydrazine was excess [27–29]. But in the
above addition order, this reaction cannot yield the desired
product 1 in acceptable yield (<10%). However, the reaction
went smoothly (56.4% yield for 1) if 3 was added to the solution
of aqueous hydrazine, followed by the neutralization with
triethylamine. This phenomenon might come from the high
electrophilic activity of 3, and two molecules of 3 might couple
with one molecule of hydrazine if 3 was excess.
2
CO), 147.92 (q, JCF = 40.20 Hz, C-7), 145.07 (s, CO),
139.71, 138.44, 132.11, 130.83, 125.45, 119.25 (q,
1JCF = 270.38 Hz, CF3), 109.49, 62.66 (s, CH2-CH3),
61.62 (s, CH2-CH3), 14.27 (s, CH2-CH3), 13.96 (s, CH2-
CH3).
MS (TOF EI): m/z 397(M+-N2) (5%), 323 (1%), 286 (8%),
258 (4%), 241 (5%), 208 (10%), 191 (15%), 180 (31%), 163
(100%), 146 (64%), 137 (12%), 120 (90%), 92 (10%), 65
(8%).
Scheme 2 shows the synthesis of ethyl 2-(trifluoromethyl)-
5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-carboxylate
Anal. calcd. for C17H14F3N5O5: C, 48.01; H, 3.32; N, 16.47.
Found: C, 47.85; H, 3.14; N, 16.50.
(5)
from 1. The condensation of 1 with pentane-2,4-dione (4) in
acetic acid (AcOH) with addition of two drops of concentrated
vitriol (H2SO4) at room temperature gave 5 [30] in high isolated
yield (94.0%). Since pyrazolo[1,5-a]pyrimidines have been
found as a variety of biologically active compounds such as
estrogen receptor ligands [19], KDR kinase inhibitors [31],
COX-2-selective inhibitors [32] and CRF-1 receptor antago-
nists [33]. The synthesis of trifluoromethylated pyrazolo[1,5-
a]pyrimidines is possible to not only enlarge the structure–
activity relationship (SAR) studies of parent compounds, but
also induce a different physical and pharmacological profile.
For example, 5 was firstly found to be a novel fluorescent
molecule that might be exploited as an attractive fluorophore
for possessing many binding sites.
2.5.10. Ethyl 7-(trifluoromethyl)-3-(4-fluorophenyl)-3,4-
dihydro-4-oxopyrazolo[5,1-d][1,2,3,5]tetrazine-8-
carboxylate (8j)
Orange solid; yield: 76.5%; mp = 133–134 8C.
FTIR (KBr): y 3073, 2988, 1775 (CO), 1730 (CO), 1309,
1241, 1221, 1187, 1156, 1110, 1039, 850 cmÀ1
.
1H NMR (300 MHz, CDCl3): d 1.45 (3H, t, J = 7.14 Hz,
CH2-CH3), 4.51 (2H, q, J = 7.14 Hz, CH2-CH3), 7.26–7.32
(2H, m, Ar-H), 7.62–7.66 (2H, m, Ar-H).
13C NMR (75 MHz, CDCl3): d 163.32 (d, 1JCF = 250.50 Hz),
158.24 (s, CO), 147.89 (q, 2JCF = 39.75 Hz, C-7), 145.20 (s,
4
CO), 138.66, 132.28 (d, JCF = 3 Hz), 127.80 (d,
1
3JCF = 9.75 Hz), 119.28 (q, JCF = 270.00 Hz, CF3),
The fluorescence spectrum of 5 is easy to be detached. With
excitation of 5 at 299.74 nm in extremely low concentration
(1 mM) of acetonitrile solution, its strong fluorescence emission
at 396 nm could be observed as shown in Fig. 1. The interesting
property of trifluoromethylated pyrazolo[1,5-a]pyrimidine (5)
triggered us to synthesize various pyrazolo[1,5-a]pyrimidines
and their analogues, and to study their structure–fluorescence
intensity relationship. The preliminary results indicated that the
fluorescence intensity of trifluoromethylated pyrazolo[1,5-
a]pyrimidine (5) is obviously stronger than its methyl analogue
(Fig. 1). Based on these pyrazolo[1,5-a]pyrimidine derivatives,
some highly selective fluorescent chemosensors were also
developed, which are particularly attractive for their high
simplicity, high sensitivity and high detection limits for trace
2
116.78 (d, JCF = 23.25 Hz), 109.36, 62.64 (s, CH2-CH3),
13.96 (s, CH2-CH3).
MS (TOF EI): m/z 371 (M+) (2%), 326 (3%), 208 (3%), 180
(7%), 163 (12%), 137 (100%), 109 (30%), 89 (5%), 82
(10%), 62 (2%), 57 (2%).
Anal. calcd. for C14H9F4N5O3: C, 45.29; H, 2.44; N, 18.86.
Found: C, 45.15; H, 2.21; N, 19.05.
3. Results and discussion
Firstly, our efforts were focused on the efficient synthesis of
1. Scheme 1 shows its synthetic route from the easily available
materials of ethyl cyanoacetate (2) and trifluoroacetic
anhydride (TFAA) in three steps. The condensation of 2 and
TFAA in dichloromethane (DCM) with the presence of
triethylamine (Et3N) at zero, followed by chloridization with
oxalyl chloride (DCM, reflux) provided ethyl 3-chloro-2-
cyano-4,4,4-trifluorobut-2-enoate (3) in 58.1% yield (two steps
based on 2) [24,25]. Then the condensation of 3 with aqueous
hydrazine in ethanol afforded the desired trifluoromethylated
product 1 [26] in 56.4% yield. On one hand, since
deprotonating cyano acetic esters at low temperature followed
by addition of methyl trifluoroacetate gave predominantly
mixture of various by-products [25], the condensation of 2 and
TFAA in the presence of triethylamine is an efficient and highly
reliable way to introduce trifluoromethyl group to cyano acetic
esters. On the other hand, it was found that the yield of the
condensation of 3 with aqueous hydrazine was dependent on
the addition order of the starting materials. According to the
usual addition order in the syntheses of various 5-amino-1H-
pyrazoles, the addition of aqueous hydrazine to 3 seemed to be
Fig. 1. Fluorescence spectra of 5 and its methyl analogue in acetonitrile (1 mM,
lex = 299.74 nm), lmax = 396 nm.

414
Y.-C. Wu et al. / Journal of Fluorine Chemistry 127 (2006) 409–416
chemicals detection. These results will be published separately
in due course.
Scheme 3 shows the synthesis of trifluoromethylated
pyrazolo[5,1-d][1,2,3,5]tetrazine-4(3H)-ones (8a–j) from 1 in
two steps. The diazotization of 1 using nitrous acid under À5 8C
followed by neutralization with saturated aqueous sodium
carbonate proceeded smoothly to provide ethyl 5-diazo-3-
trifluoromethyl-pyrazole-4-carboxylate (6) [34] in excellent
yield (76.9%), and pure product could be obtained by washing
with diethylether. Thecondensation of 6 with various substituted
aryl isocyanates (7a–j) at room temperature could efficiently
give a series of trifluoromethylated pyrazolo[5,1-d][1,2,3,5]tet-
razine-4(3H)-ones (8a–j) [35] in good to high yields.
Scheme 4.
Subsequently, the scope and limitation of the cycloaddition
was examined with various inputs. Solvent effect was firstly
screened. Since aryl isocyanates are unstable in proton solvents,
the selected solvents must be non-proton solvents. When using
non-polar solvents (CCl4, PhH, etc.) or weak polar solvents
(PhMe, EtOEt, etc.), 6 is insoluble and the reaction is slow.
When using strong polar solvents such as ethyl acetate
(CH3CO2Et), tetrahydrofuran (THF) and chloroform (CHCl3),
the reaction is also slow that might come from the stable effect
of these strong polar solvents to 6. Therefore, dichloromethane
(CH2Cl2), a soluble solvent for 6 with weak polarity, is the most
suitable reaction solvent in the screened media. Reaction
temperature was the next considered factor. After screened
various temperature conditions, room temperature was adopted
considering both the reaction yields and the reaction time.
Finally, aryl isocyanates were also examined. As Table 1
indicated, the yields were good when aryl isocyanates bearing
weak electron-attracting groups such as halogen are used
(Table 1, Entries 1–5 and 10). High yields were obtained when
using aryl isocyanates bearing strong electron-withdrawing
groups such as ethoxycarbonyl, nitro and trifluoromethyl
(Table 1, Entries 6–9). There was a decrease in the yield when
using 2-substituted or 3-substituted aryl isocyanate in
comparison with its 4-substituted analogue (Table 1, Entries
1–3), and the reaction failed to give any product at all when aryl
isocyanates bearing electron-donating group such as methoxy
is used (Table 1, Entry 11).
The cycloaddition of 6 with 7 can be understood as a [7 + 2]
cycloaddition consisting of the ring nitrogen acylation to form a
1,9-dipole, followed by the intramolecular coupling (Scheme 4)
[15]. Before the cycloaddition, 6 might be in its ‘‘polymer’’
pattern that came from intermolecular and intramolecular ion-
anion interactions, which make 6 relatively inactive in
comparing with its free molecular pattern. So the relatively
inactive aryl isocyanates such as 4-methoxylphenyl isocyanate
might have not enough ability to make 6 ‘‘depolymerize’’, and
then could not triggered the cycloaddition to take place
(Table 1, Entry 11). When aryl isocyanates borne strong
electron-drawing groups, they became active and were easy to
be nucleophilic attacked, and then had enough ability to induce
the cycloaddition to take place (Table 1, Entries 6–9).
Pyrazolo[5,1-d][1,2,3,5]tetrazine-4(3H)-ones have attracted
increasing attention and been reported to exhibit various
bioactivities such as bactericidal [36], medicinal fungicidal
[36], antitumor [37], antineoplastic [38] and anti-rape
Table 2
Table 1
Herbicide activity against monocotyledonous Echinochloa crus-galli L. Beauv
at 100 mg/mL (percent inhibition)
The cycloaddition of 6 with 7 to synthesize 8
Entry
Ar
Time (days)
8
Yielda (%)
Entry
X
Ar
Activity
1
2
2-ClC6H4
10
9
8a
8b
8c
8d
8e
8f
8g
8h
8i
70.3
74.9
78.2
81.0
71.6
96.8
85.4
92.3
94.1
76.5
0
1
2
CF3
CF3
CF3
CF3
CF3
CF3
CF3
Me
2-ClC6H4
3-ClC6H4
4-ClC6H4
2,4-Cl2C6H3
4-NO2C6H4
4-CO2EtC6H4
4-FC6H4
23.6
29.0
59.1
44.7
36.0
20.7
54.8
39.7
0
3-ClC6H4
3
4-ClC6H4
8
3
4
2,4-Cl2C6H3
2-FC6H4
7
4
5
10
5
5
6
2-CF3C6H4
2-NO2C6H4
4-NO2C6H4
4-CO2EtC6H4
4-FC6H4
6
7
5
7
8
4
8
4-ClC6H4
4-ClC6H4
4-ClC6H4
–
9
4
9
MeS(O)
MeSO2
–
10
11
8
8j
–
10
11
0
4-MeOC6H4
10
100
a
Isolated yields.

Y.-C. Wu et al. / Journal of Fluorine Chemistry 127 (2006) 409–416
415
(dicotyledonous Brassica campestris L.) activities [15,16].
Table 2 showed that the trifluoromethylated ones (8) have some
herbicide activity against monocotyledonous Echinochloa
crus-galli L. Beauv (barnyard grass), which was valued by
using the reported procedure (the inhibition of the seeding
growth of barnyard grass) [39,40]. Due to the continuing
evolution of herbicide resistance in agriculture, discovery of
new herbicides such as potent barnyard grass inhibitors is
important to widen its structural diversity that might possess
new mode of action and then lack resistance to the currently
used chemicals [41,42]. Moreover, it also helps to confirm the
perspectives of further investigations in this area.
important in both organofluorine and organic synthetic area,
and should be exploited for the syntheses of various other
interesting trifluoromethylated compounds.
Acknowledgements
We thank the National Key Project for Basic Research
(2003CB114400), the National Natural Science Foundation of
China (20572054), and the Chinese Research Fund for the
Doctoral Program of High Education for financial supports. We
also appreciate the suggestions from the reviewers.
As Table 2 showed, the barnyard grass inhibitive activities of
8 depended on the nature of their ‘‘Ar’’ and ‘‘X’’ (Table 2,
Entries 1–10). As to the ‘‘Ar’’, on one hand, 4-chlorophenyl
substituted analogue (8c) is the most active one in the selected
(di)chlorophenyl substituted analogues (8a–8d) (Table 2,
Entries1–4). On the other hand, when the ‘‘Ar’’ borne strong
electron-drawing groups such as nitro and ethoxycarboryl (8h
and 8i), their activities decreased in comparing with 4-
chloro(fluoro)phenyl substituted ones (8c and 8j) (Table 2,
Entries 3 and 5–7). 8c was the most active one in all these 7-
trifluoromethyl-substituted pyrazolo[5,1-d][1,2,3,5]tetrazine-
4(3H)-ones (Table 2, Entries 1–7). ‘‘X’’ was also examined
to investigate their SAR, and was found to be more important
than the ‘‘Ar’’. When the ‘‘X’’ was methyl, it (8k) possessed
some herbicide activity against barnyard grass (39.7% at
100 mg/mL) [17] (Table 2, Entry 8). When the ‘‘X’’ was
methylsulfinyl (8l) [15] or methylsulfonyl (8m) [16], both 8l
and 8m had no activities at the same concentration as 8k
(Table 2, Entries 9–10). These results indicated it might be
disadvantageous for ‘‘X’’ that borne strong electron-drawing
groups, to possess herbicide activities against barnyard grass.
Then 8c ‘‘should’’ have no herbicide activity for trifluor-
omethyl is a strong electron-drawing group. But 8c is more
active than its methyl analogue 8k (Table 2, Entries 3 and 8).
This indicated trifluoromethyl is an interesting substituted
group that possesses its unique properties, which often gave the
world certain surprising results.
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