Convenient substitution of hydroxypyridines with trifluoroacetaldehyde ethyl hemiacetal

Article abstract of DOI:10.1002/jhet.5570380103

Thermal substitution of 4-hydroxypyridine 1 with trifluoroacetaldehyde ethyl hemiacetal (TFAE) leads to a moderate yield of 3-(1-hydroxy-2,2,2-trifluoroethyl)-4-hydroxypyridine 7 in the presence of a catalytic amount of anhydrous potassium carbonate. Under similar conditions, several α-trifluoromethyl hydroxypyridinemethanols 8-15 are easily prepared from 2- or 3-hydroxypyridines 2-6.

Full text of DOI:10.1002/jhet.5570380103

Jan-Feb 2001
Convenient Substitution of Hydroxypyridines with
Trifluoroacetaldehyde Ethyl Hemiacetal
Yuefa Gong,* Katsuya Kato, Hiroshi Kimoto
25
National Industrial Research Institute of Nagoya, Hirate-cho, Kita-ku, Nagoya 462-8510
Received June 22, 2000
Thermal substitution of 4-hydroxypyridine 1 with trifluoroacetaldehyde ethyl hemiacetal (TFAE) leads
to a moderate yield of 3-(1-hydroxy-2,2,2-trifluoroethyl)-4-hydroxypyridine 7 in the presence of a catalytic
amount of anhydrous potassium carbonate. Under similar conditions, several α-trifluoromethyl hydroxy-
pyridinemethanols 8-15 are easily prepared from 2- or 3-hydroxypyridines 2-6.
J. Heterocyclic Chem., 38, 25 (2001).
In the last two decades, considerable attention has been
tion was carried out below 90 °C. However, a moderate
yield of 3-(1-hydroxy-2,2,2-trifluoroethyl)-4-hydroxy-
pyridine 7 was obtained when the reaction was carried out
at 130 °C for 20 hours (Scheme 1). Under similar condi-
tions, the reaction of 2-hydroxypyridine 2 with TFAE
gave two substituted products, 2-hydroxy-5-(1-hydroxy-
2,2,2-trifluoroethyl)pyridine 8 and 2-hydroxy-3-(1-
hydroxy-2,2,2-trifluoroethyl)pyridine 9 (runs 4-5), and
that of 2-hydroxy-5-carbethoxypyridine 3 gave
2-hydroxy-3-(1-hydroxy-2,2,2-trifluoroethyl)-5-
carbethoxypyridine 10 (run 6). The chemical structures
for all the substituted products were easily distinguished
according to the chemical shifts and the coupling
constants of the protons in the pyridine ring. The low
isolated yields of products 7-10 were mainly attributed to
the recovery of the corresponding hydroxypyridines used.
In contrast, a high yield (89 %) of 2-(1-hydroxy-2,2,2-
trifluoroethyl)-3-hydroxy-6-methylpyridine 11 was
afforded from 3-hydroxy-6-methylpyridine 4 (run 7).
given to the preparation of α-trifluoromethyl carbinols
because of their unique physical, chemical and biological
properties [1]. A number of synthetic methods have been
established, mainly via reduction of α-trifluoromethyl
ketones [2], addition of nucleophiles to trifluoroacetalde-
hyde [3] and addition of trifluoromethyl anion, such as
trifluoromethyltrimethylsilane (CF TMS), to carbonyl
3
compounds [4]. Trifluoroacetaldehyde ethyl hemiacetal
TFAE), as a synthetic equivalent of trifluoroacetalde-
(
hyde, has been widely used for this purpose [5]. In our
work, its thermal substitution with a variety of electron-
rich heteroarenes, such as indole, pyrrole, furan, thio-
phene, phenols and N,N-dimethylaniline, has proved to be
an efficient way of preparing the corresponding
α-trifluoromethyl arylmethanols [6].
As a continuous work, we are interested in the reaction
of TFAE with pyridine because of the important bioact-
ivities of their fluorinated derivatives. In fact, α-trifluoro-
methyl-3-pyridinemethanols, as fungicidal pyridine deriv-
atives, have been prepared by addition of 3-pyridyllithium
to α-trifluoromethyl ketone or by addition of alkyllithium
to 2,2,2-trifluoro-1-(3-pyridyl)ethanone [7]. Pyridine
derivatives are electron-deficient and generally resistent
to electrophilic substitution under mild conditions,
although trifluoroacetylation of dimethylamino-substi-
tuted pyridines using trifluoroacetic anhydride has been
successful [8]. Actually in our experiments, pyridine and
its hydroxy-substituted derivatives did not undergo substi-
tution with TFAE, even in the presence of zinc chloride.
However, we found that the substitution between hydroxy-
pyridines and TFAE readily took place when a catalytic
amount of anhydrous potassium carbonate was added. The
details for the reaction are described as follows.
Results and Discussion.
Hydroxypyridines 1-6 used in this work are given in
Scheme 1. We first tried the reaction of 4-hydroxypyridine
1
with TFAE without any solvents in the presence of 10
mmol % of potassium carbonate, and the results are given
in Table 1 (runs 1-3). It was found that the reaction was
greatly dependent upon the reaction temperature, and no
detectable substituted product was formed when the reac-

26
Y. Gong, K. Kato, H. Kimoto
Vol. 38
Table 1
Reaction of hydroxypyridines 1-6 with TFAE
Run Substrate Molar Ratio Temperature Time
Products
(
S)
(TFAE/S)
1.1
(°C)
100
130
130
130
130
130
130
100
90
(h) (isolated yields, %)
1
2
3
4
5
6
7
8
9
1
1
1
2
2
3
4
5
5
5
6
16
6
7 (11)
7 (34)
1.1
1.1
1.1
1.5
2.0
1.1
1.1
1.0
20
8
20
36
8
7 (49)
8 (14) 9 (4)
8 (23) 9 (7)
10 (9)
EXPERIMENTAL
11 (89)
1
The H nmr spectra were recorded with tetramethylsilane
TMS) as an internal standard at 90 MHz on a Hitachi R-90H FT
spectrometer. The F nmr spectra were recorded with hexafluoro-
benzene as an internal standard at 84.7 MHz on the same spectro-
6
8
8
8
12 (33) 13 (35) 14 (11)
12 (27) 13 (29) 14 (5)
12 (10) 13 (13) 14 (62)
15 (93)
(
19
1
0
3.0
1.1
130
130
11
meter. All nmr spectra were measured in acetone-d . Mass spectra
6
Apart from the marked difference in the reactivity of
(70 eV) were measured on a Hitachi M-80 instrument. The ir
spectra were recorded on a Shimadzu FTIR-8600PC instrument.
Melting points were measured in a glass capillary on a heating
block, and are uncorrected. High-resolution mass spectra were
measured on a JEOL JMS-SX102A MS spectrometer.
these substrates, the important orientation problem arises
in the substitution reactions of 2- and 3-hydroxypyridines.
In the case of 2-hydroxypyridine 2, the formation of
5
-substituted product 8 was obviously favored (runs 4-5).
In the case of 3-hydroxy-6-methylpyridine 4, an unex-
pected result was observed that no 4-substituted product
was detected except 2-substituted product 11. The specific
orientation was also observed in other 3-hydroxy-
pyridines. In fact, no detectable amount of 4-substituted
product was formed in the reaction of 3-hydroxypyridine
3-(1-Hydroxy-2,2,2-trifluoroethyl)-4-hydroxypyridine (7).
A mixture of 1.90 g (0.020 mole) of 4-hydroxypyridine (1),
3
.17 g (0.022 mole) of ethyl trifluoroacetaldehyde hemiacetal
and 0.276 g (0.002 mole) of anhydrous potassium carbonate was
heated with continuous stirring at 130 °C for 6 hours. After
cooled, the mixture was dissolved in 30 ml of ethyl acetate and
5
, although the product distribution formed was greatly
2
0 ml of distilled water. The organic layer was separated, and
dependent upon the molar ratio of 5/TFAE.
Monosubstituted compounds, 2-(1-hydroxy-2,2,2-
trifluoroethyl)-3-hydroxypyridine 12 and 3-hydroxy-6-(1-
hydroxy-2,2,2-trifluoroethyl) pyridine 13, were mainly
generated when equivalent amounts of 1a and TFAE were
caused to react at 90 °C (run 9), whereas disubstituted
compound, 2,6-bis(1-hydroxy-2,2,2-trifluoroethyl)-3-
hydroxypyridine 14, was the main product when an
excess amount of TFAE was used (run 10). Moreover,
blocking the 2-site of 3-hydroxypyridine with a chlorine
atom 6 led to only the formation of 6-substituted product,
the aqueous layer extracted twice with 20 ml of ethyl acetate.
The combined organic layer was dried over sodium sulfate and
evaporated to yield an oily residue. The residue was purified on
a silica gel column with ethyl acetate:ethanol (98:2, v/v) as the
elution to give a white solid, which was recrystallized from ethyl
acetate to yield 1.30 g (34 %) of 3-(1-hydroxy-2,2,2-trifluoro-
ethyl)-4-hydroxypyridine (7), white plates, mp 170-171 °C; ir:
(
1
potassium bromide): 3200 (OH), 1645 (pyridinol, CO), 1598,
520, 1452, 1404, 1328, 1258, 1173, 1151, 1130, 1079, 847
-
1 1
cm ; H nmr: δ 2.98 (br s, 1H, OH), 5.26 (q, 1H, CHCF ,
3
J = 7.7 Hz), 6.43 (d, 1H, 5-H, J = 7.3 Hz), 7.92 (d, 1H, 6-H,
19
J = 7.3 Hz), 8.00 (s, 1H, 2-H), 11.20 (br s, 1H, PyOH); F nmr:
2
-chloro-3-hydroxy-6-(1-hydroxy-2,2,2-trifluoroethyl)-
+
δ 84.96 (d, 3F, CF , J = 7.7 Hz); ms: m/z 193 (M , 20), 124
3
pyridine 15, in an excellent yield (93 %, run 11).
(
M-CF , 100), 95 (M-CF CHO, 34).
3 3
Based on these observations, we believe that the
removal of the phenolic proton of hydroxypyridine by
potassium carbonate, which increases the negative charge
on the ring, greatly favors the substitution with TFAE.
The low reactivity for 2-hydroxypyridine and 4-hydroxy-
pyridine can be easily interpreted from the markedly
important contribution of the pyridone canonical forms
Anal. Calcd. for C H NO F : C, 43.52; H, 3.13; N, 7.25.
7
6
2 3
Found: C, 43.47; H, 3.12; N, 7.07.
The same procedure was taken for other hydroxy-substituted
pyridines 2-6.
2
-Hydroxy-5-(1-hydroxy-2,2,2-trifluoroethyl)pyridine (8).
This compound was separated by column chromatography
silica gel-ethyl acetate) and recrystallized from ethyl aceate as a
white solid, mp 205-206 °C; ir: (potassium bromide): 3257
(
1
6a and 16b (Fig. 1), which decrease the electron density
on the carbons (C and C ) of the pyridine ring to a large
3
5
(
1
OH), 1667, 1626 (pyridone, CO), 1554, 1470, 1419, 1365,
extent. On the other hand, it seems to be difficult to
understand why no 4-substituted product was generated in
the reaction of 3-hydroxypyridines, though the regioselec-
tive substitution was commonly rationalized from the
different electron density delocalized at each atom of the
pyridine ring. We therefore suggested the possibility that
-1 1
262, 1199, 1161, 1130, 1101, 826, 702 cm ; H nmr: δ 2.80 (br
s, 1H, OH), 5.05 (q, 1H, CHCF , J = 7.3 Hz), 6.43 (d, 1H, 3-H,
3
J = 10.3 Hz), 7.57 (dd, 1H, 4-H, J = 10.3 Hz, 2.6 Hz), 7.63 (d,
19
1
(
H, 6-H, J = 2.6 Hz), 11.30 (br s, 1H, PyOH); F nmr: δ 85.43
+
d, 3F, CF , J = 7.3 Hz); ms: m/z 193 (M , 33), 124 (M-CF ,
3
3
100), 95 (M-CF CHO, 27), 69 (CF +, 87).
3
3
2
- and 6-substituted products were given through a five-
Anal. Calcd. for C H NO F : C, 43.52; H, 3.13; N, 7.25.
7 6 2 3
membered transition state outlined as 17 (Figure 1).
Found: C, 43.66; H, 3.14; N, 7.06.

Jan-Feb 2001
Convenient Substitution of Hydroxypyridines
27
2-Hydroxy-3-(1-hydroxy-2,2,2-trifluoroethyl)pyridine (9).
3-Hydroxy-6-(1-hydroxy-2,2,2-trifluoroethyl)pyridine (13).
This compound was separated by silica gel column chro-
matography (ethyl acetate:hexane, 3:1, v/v) and recrystallized
from chloroform:ethyl aceate as white plates, mp 165.5-166.5
This compound was separated by silica gel column chro-
matography (hexane:ethyl acetate, 3:5, v/v) and recrystallized
from chloroform:ethyl aceate as white plates, mp 135-136 °C; ir:
(potassium bromide): 3442 (OH), 1617, 1577, 1496, 1339, 1287,
°
C; ir: (potassium bromide): 3300 (OH), 1652, 1598 (pyridone,
-1 1
CO), 1570, 1472, 1440, 1390, 1282, 1157, 1126, 1090, 1062,
1250, 1214, 1171, 1119, 1103, 840, 816, 701, 655 cm ; H nmr:
δ 2.85 (br s, 1H, OH), 5.10 (q, 1H, CHCF , J = 6.9 Hz), 7.27
-1 1
7
(
1
78, 707 cm ; H nmr: δ 5.28 (q, 1H, CHCF , J = 7.0 Hz), 6.41
3
3
(
8
dd, 1H, 4-H, J = 8.7 Hz, 2.2 Hz), 7.58 (d, 1H, 5-H, J = 8.7 Hz),
dd, 1H, 5-H, J = 6.8 Hz, 6.6 Hz), 6.48 ( br s, 1H, OH), 7.56 (dd,
19
.21 (d, 1H, 2-H, J = 2.2 Hz), 9.10 (br s, 1H, PyOH); F nmr: δ
H, 4-H, J = 6.6 Hz, 1.8 Hz), 7.69 (dd, 1H, 6-H, J = 6.8 Hz, 1.8
+
1
9
86.10 (d, 3F, CF , J = 6.9 Hz); ms: m/z 193 (M , 15), 124
(M-CF , 100), 94 (M-CF CHOH, 43), 69 (CF , 85).
Hz), 11.20 (br s, 1H, PyOH); F nmr: δ 85.52 (d, 3F, CF3,
3
+
+
J = 7.0 Hz); ms: m/z 193 (M , 22), 124 (M-CF , 100), 96
3
3
3
3
Anal. Calcd. for C H NO F : C, 43.52; H, 3.13; N, 7.25.
(
M-CF CO, 13).
7
6
2 3
3
Found: C, 43.32; H, 3.08; N, 7.13.
Anal. Calcd. for C H NO F : C, 43.52; H, 3.13; N, 7.25.
7
6
2 3
Found: C, 43.55; H, 3.13; N, 6.93.
2,6-Bis(1-hydroxy-2,2,2-trifluoroethyl)-3-hydroxypyridine (14).
2
-Hydroxy-3-(1-hydroxy-2,2,2-trifluoroethyl)-5-carbethoxy-
This compound was separated by silica gel column chro-
matography (hexane:ethyl acetate, 1:3, v/v) and recrystallized
from chloroform:ethyl aceate as white plates, mp 129-130 °C; ir:
pyridine (10).
This compound was separated by silica gel column chro-
matography (hexane:ethyl acetate, 1:1, v/v) and recrystallized
from chloroform:ethyl aceate as white solid, mp 178.5-179 °C;
(
1
potassium bromide): 3400 (OH), 1588, 1489, 1434, 1353, 1271,
158, 1126, 1066, 995, 985, 835, 701, 638 cm ; H nmr: δ 3.12
-1
1
(
br s, 1H, OH), 5.23 (q, 1H, CHCF , J = 6.6 Hz), 5.47 (q, 1H,
3
ir: (potassium bromide): 3406 (OH), 1700 (CO Et), 1660, 1649
2
CHCF , J = 6.6 Hz), 5.94 (br s, 1H, OH), 7.52 (s, 2H, 4-H, 5-H),
3
(
pyridone, CO), 1624, 1437, 1376, 1284, 1253, 1218, 1170,
19
9
.6 (br s, 1H, PyOH); F nmr: δ 86.63 (d, 3F, CF , J = 6.6 Hz),
86.10 (d, 3F, CF , J = 6.6 Hz); ms: m/z 291 (M , 14), 222
(M-CF , 62), 204 (23), 184 (18), 128 (24), 69 (CF , 100).
3 3
3
-1 1
1
4
6
6
3
1
2
127, 762, 657 cm ; H nmr: δ 1.33 (t, 3H, CH , J = 7.2 Hz),
+
3
3
.32 8q, 2H, CH , J = 7.2 Hz), 5.41 (q, 1H, CHCF , J = 6.2 Hz),
+
2
3
.08 ( br s, 1H, OH), 8.18 (d, 1H, 4-H, J = 2.2 Hz), 8.26 (d, 1H,
-H, J = 2.2 Hz), 11.30 (br s, 1H, PyOH); F nmr: δ 85.66 (d,
F, CF , J = 6.2 Hz); ms: m/z 265 (M , 18), 196 (M-CF , 100),
Anal. Calcd. for C H NO F : C, 37.11; H, 2.42; N, 4.81.
9 7 3 6
1
9
Found: C, 36.93; H, 2.49; N, 4.83.
-Chloro-3-hydroxy-6-(1-hydroxy-2,2,2-trifluoroethyl)pyridine
15).
This compound was purified on a silica gel column with
+
3
3
2
(
68 (M-CF CO, 90). HRMS Calcd. for C10H10NO F :
3
4 3
65.0562. Found: 265.0561.
2
-(1-Hydroxy-2,2,2-trifluoroethyl)-3-hydroxy-6-methylpyridine
hexane:ethyl aceate (4:1, v/v) as the elution, and recrystallized
from chloroform:ethyl aceate as white solid, mp 150-151 °C; ir:
(11).
(
potassium bromide): 3500 (OH), 1568, 1493, 1429, 1320, 1290,
258, 1216, 1183, 1120, 1092, 839, 733, 686 cm ; H nmr: δ
This compound was purified on a silica gel column with
hexane:ethyl aceate (3:5, v/v) as the elution, and recrystallized
from chloroform:ethyl aceate as white needles, mp 137.5-138.5
-1 1
1
4
2
.27 (br s, 1H, OH), 5.09 (q, 1H, CHCF , J = 7.0 Hz), 7.49 (s,
3
1
9
H, 4-H, 5-H), 9.96 (br s, 1H, PyOH); F nmr: δ 86.39 (d, 3F,
°
1
2
C; ir: (potassium bromide): 3400 (OH), 1480, 1385, 1344,
+
CF , J = 7.0 Hz); ms: m/z 229 (M+2, 3.6), 227 (M , 10), 160
-1
1
3
259, 1236, 1164, 1132, 1098, 938, 697, 662 cm ; H nmr: δ
.44 (s, 3H, CH ), 2.89 (br s, 1H, OH), 5.34 (q, 1H, CHCF ,
(36), 158 (M-CF , 100).
3
3
3
Anal. Calcd. for C H ClNO F : C, 36.93; H, 2.22; N, 6.16.
7
5
2 3
J = 7.8 Hz), 7.16 (d, 1H, 4-H, J = 8.3 Hz), 7.35 (d, 1H, 5-H,
Found: C, 36.86; H, 2.37; N, 5.88.
1
9
J = 8.3 Hz), 9.0 (br s, 1H, PyOH); F nmr: δ 86.69 (d, 3F, CF3,
+
J = 7.8 Hz); ms: m/z 207 (M , 39), 138 (M-CF , 100), 108
3
Acknowledgement.
+
(
M-CF CHOH, 23), 69 (CF , 40).
3
3
Anal. Calcd. for C H NO F : C, 46.37; H, 3.89; N, 6.76.
Y.G. thanks the Science and Technology Agency of Japan
8
8
2 3
(
STA) for the award of the Fellowship Program, which is
managed by the Research Development Corporation of Japan
JRDC) in cooperation with Japan International Science and
Found: C, 46.45; H, 3.84; N, 6.60.
2
-(1-Hydroxy-2,2,2-trifluoroethyl)-3-hydroxypyridine (12).
(
Technology Exchange Center (JISTEC). This research program
is partially supported by Chiiki Consortium Project (ID 12G
This compound was separated by silica gel column chro-
matography (hexane:ethyl acetate, 1:3, v/v) and recrystallized
from chloroform:ethyl aceate as white plates, mp 116-116.5 °C;
ir: (potassium bromide): 3528 (OH), 1607, 1582, 1469, 1423,
4005) from NEDO.
-1
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1
302, 1259, 1185, 1162, 1138, 1075, 892, 804, 700, 642 cm ;
1
H nmr: 2.87 (br s, 1H, OH), 5.40 (q, 1H, CHCF , J = 6.4 Hz),
3
[
7
9
.36 (m, 2H, 4-H, 5-H), 8.17 (dd, 1H, 6-H, J = 3.7 Hz, 2.2 Hz),
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9
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3
+
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7
6
2 3
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28
Y. Gong, K. Kato, H. Kimoto
Vol. 38
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