Synthesis of 3-substituted 2-trifluoro(trichloro)methyl-2H-chromenes by reaction of salicylaldehydes with activated trihalomethyl alkenes

Article abstract of DOI:10.1002/hc.20146

The reaction of activated trihalomethylsubstituted alkenes with salicylaldehydes in the presence of triethylamine gives 3-substituted 2-trifluoromethylchroman-4-ols and 2-trifluoro(trichloro)methyl-2H-chromenes in high yields.

Full text of DOI:10.1002/hc.20146

Heteroatom Chemistry
Volume 16, Number 6, 2005
Synthesis of 3-Substituted
2-Trifluoro(trichloro)methyl-2H-chromenes
by Reaction of Salicylaldehydes with Activated
Trihalomethyl Alkenes
Vladislav Yu. Korotaev, Igor B. Kutyashev,
and Vyacheslav Ya. Sosnovskikh
Department of Chemistry, Ural State University, Lenina 51, Ekaterinburg 620083,
Russian Federation
Received 20 March 2005
cal activity and are potential medicinal agents [1,8].
ABSTRACT: The reaction of activated trihalome-
Because of their reactivity and relative stability, 2-
methyl- and 2,2-dimethyl-2H-chromenes have an
important role and are valuable intermediates for
synthetic purposes in chroman chemistry [9–14].
thylsubstituted alkenes with salicylaldehydes in the
presence of triethylamine gives 3-substituted 2-trifluo-
romethylchroman-4-ols and 2-trifluoro(trichloro)-
ꢀ
C
methyl-2H-chromenes in high yields.
2005 Wiley
The introduction of fluorine in place of hydro-
gen to modify the bioactivity of organic molecules
is a well-established practice [15–18]. As a result,
considerable efforts have been made in the develop-
ment of trifluoromethylated analogues of precocenes
[19–21], cromakalim [22–24], and lactarochromal
[19,20,25], in which both or one of the methyl groups
in the gem-dimethyl moiety are replaced by the
CF3 group. In spite of advances in this area, pub-
lished data on the synthesis of 2-(trihalomethyl)-
2H-chromenes are lacking. To our knowledge, there
has been only one report on the preparation of
2-(trifluoromethyl)chroman-4-one [26], which may
be regarded as a precursor for the synthesis of
2-trifluoromethyl-2H-chromene.
Periodicals, Inc. Heteroatom Chem 16:492–496, 2005;
Published online in Wiley InterScience (www.interscience.
wiley.com). DOI 10.1002/hc.20146
INTRODUCTION
Substituted 2,2-dimethylchromans and chromenes
are common natural products which are widely dis-
tributed among many plants [1]. Furthermore, they
have considerable biological importance, especially
as potentially useful pesticides (antijuvenile hor-
mones precocene I and precocene II [2,3]) and drug
candidates in the field of potassium channel openers
(for example, cromakalim, a highly potent antihyper-
tensive drug [4–7]). Several analogues of 2-methyl-
2H-chromene also show interesting pharmaceuti-
RESULTS AND DISCUSSION
It is well known that the reactions of salicylaldehydes
with acrylonitrile [27–29], alkyl vinyl ketones [30–
33], and nitro alkenes [34–39] give ꢀ3-chromenes
containing electron-withdrawing substituents at the
3-position. Although these compounds are not found
in nature, their derivatives are reported to be useful
Correspondence to: Vyacheslav Ya. Sosnovskikh; e-mail:
vyacheslav.sosnovskikh@usu.ru.
Contract grant sponsor: Russian Foundation for Basic
Research.
Contract grant number: 04-03-32463.
2005 Wiley Periodicals, Inc.
c
ꢀ
492

Synthesis of 3-Substituted 2-Trifluoro(trichloro)methyl-2H-chromenes by Reaction of Salicylaldehydes 493
from industrial and medical points of view [40,41].
in refluxing toluene for 3 h in the presence of p-
toluene sulfonic acid as a catalyst in excellent yields
(Scheme 1).
These facts and our continuing interest in the chem-
istry of trifluoromethyl analogues of natural chro-
mans and chromenes [19,20,25] led us to investigate
the synthesis of the hitherto unknown 2-CX3-3-R-
2H-chromenes (X = F, Cl; R = COPh, NO2). In this
work, we report a simple and convenient synthesis
of these compounds involving the condensation of
salicylaldehydes 1 with trihaloethylidene derivatives
of acetophenone and nitromethane 2a–c, prepared
from trifluoro(trichloro)acetaldehyde hydrates [42–
45]. Although much attention has been paid to the
chemistry of alkenes 2 mainly due to the possibil-
ity of using them as an excellent building blocks for
the preparation of a variety of CX3-containing com-
pounds [46–53], their reactions with salicylaldehy-
des were not described in the literature.
We have found that salicylaldehyde and 5-
bromosalicylaldehyde react with (E)-4,4,4-trifluoro-
1-phenyl-2-buten-1-one (2a) in the presence of tri-
ethylamine in dichloromethane for 1–3 days at room
temperature to afford chromanols 3a,b in 71%
and 56% yields, respectively. In all cases, only one
regio- and stereo-isomer was obtained (Scheme 1).
A plausible mechanism for the reaction involves
triethylamine catalyzed tandem conjugate addition/
aldol-type reaction [38]. When triethylamine was re-
placed by DABCO, the reaction did not occur and
only resinification was observed.
Next, we investigated the reaction of salicylalde-
hydes 1 with (E)-3,3,3-trifluoro- and 3,3,3-trichloro-
1-nitroprop-1-enes (2b,c), for this, it was antici-
pated, would give a range of new trihalomethylated
3-nitro-2H-chromenes as precursors to a variety of
medicinally important chroman derivatives [40,41].
It turned out that unlike alkene 2a, the reaction
of trihaloethylidene nitromethanes 2b,c with sal-
icylaldehydes 1 is much faster, reaching comple-
tion within 10 min to 2 h. Moreover, under the
conditions used, additional step to affect dehydra-
tion of the corresponding chromanols 3 was not
necessary since the reaction proceeded smoothly
to give 2-trifluoromethyl- and 2-trichloromethyl-3-
nitro-2H-chromenes 4c–f directly in 76–99% yields
(Scheme 2). It seems that the CX3 group favors the
initial Michael addition reaction due to its electron-
withdrawing character, which lowers the LUMO
level of the molecule [54]. However, the halogenated
substituents do not exercise significant control over
the reaction: the regiochemistry is determined by the
nitro or acyl group. In the light of the present interest
in fluoro-containing compounds as pharmaceutical
intermediates [15–18], this novel entry to fluorinated
analogues of 2-methyl-2H-chromene is noteworthy.
The structures of compounds 4 compare well
with the results of elemental analysis, 1H, 19F NMR,
and IR spectroscopy. A characteristic feature of the
1H NMR spectra of 4c–f is the appearance of singlet
at δ 8.03–8.13 ppm for the H-4 proton and quartet at
δ 6.09 ppm with JH,F = 6.2–6.3 Hz for the H-2 pro-
ton (singlet at δ 6.32–6.33 ppm in the case of 4e,f).
All reactions are clean, easy to perform, and proceed
at room temperature; however, trichloroethylidene
acetonitrile did not react under similar reaction con-
ditions. The results obtained by using two salicyl-
aldehydes 1 and three activated alkenes 2a–c are
summarized in Table 1.
The configuration and the conformational pref-
erences of chromanols 3a,b have been assigned on
the basis of 1H NMR data. In particular, two coupling
constants, the J2,3 = 10.5–10.6 Hz (axial–axial) and
J3,4 = 9.7 Hz (axial–pseudoaxial), indicate an equa-
torial position for the 2-CF3 and 3-COPh substituents
and a pseudoequatorial position for the 4-OH group
in the mobile dihydropyran fragment of 3a,b, which
are therefore trans-trans products [34]. The subse-
quent dehydration of the chromanols 3a,b to the
corresponding 2H-chromenes 4a,b was performed
In conclusion, the reaction of salycilaldehydes
with activated trihalomethyl substituted alkenes pro-
vides convenient preparative process from readily
available starting materials to 2-CF3- and 2-CCl3-
2H-chromenes, which may be considered as a new
precursors in the synthesis of other useful chroman
derivatives.
SCHEME 1
SCHEME 2

494 Korotaev, Kutyashev, and Sosnovskikh
TABLE 1 Synthesis of Chromanols 3a,b and Chromenes
4a–f by Reaction of Salicylaldehydes 1 with Trihalomethyl
Substituted Alkenes 2a–c
9.7 Hz), 4.90 (dq, 1H, H-2, J = 10.6, 5.7 Hz), 5.27
(dd, 1H, H-4, J = 9.7, 7.2 Hz), 6.88 (d, 1H, H-8,
J = 8.7 Hz), 7.38 (ddd, 1H, H-7, J = 8.7, 2.4, 1.0 Hz),
7.50–7.55 (m, 2H, H-3ꢁ, H-5ꢁ), 7.63 (dd, 1H, H-5,
J = 2.4, 0.8 Hz), 7.65 (tt, 1H, H-4ꢁ, J = 7.4, 1.1 Hz),
8.00–8.03 (m, 2H, H-2ꢁ, H-6ꢁ). 19F NMR (376 MHz,
CDCl3/C6F6): δ 85.58 (d, CF3, J = 5.7 Hz). Anal.
Calcd for C17H12BrF3O3: C, 50.90; H, 3.01. Found: C,
51.03; H, 3.01.
R
X
Compound
Yield (%)
Mp ( ◦C)
H
Br
H
Br
H
Br
H
F
F
F
F
F
F
Cl
Cl
3a
3b
4a
4b
4c
4d
4e
4f
71
56
98
80
76
93
94
85
195–196
205–206
98–100
110–112
82–83
97–98
105–106
123–125
3-Benzoyl-2-(trifluoromethyl)-2H-chromene (4a).
A mixture of 3a (0.11 g, 0.3 mmol) and a catalytic
amounts of TsOH in toluene (5 mL) was refluxed for
3 h. The resulting solution was concentrated under
reduced pressure, and the precipitate that formed
was recrystallized from hexane as a colorless powder.
Br
EXPERIMENTAL
1
IR (KBr): ν 1637, 1603, 1571, 1482 cm−1. H NMR
Melting points obtained were uncorrected. IR spec-
tra were recorded on an Perkin-Elmer Spectrum
BX-II instrument as KBr disks. 1H and 19F NMR
spectra were recorded on a Bruker DRX-400 spec-
trometer (1H at 400 MHz and 19F at 376 MHz) with
TMS and C6F6 as internal standards. All solvents
used were dried and distilled per standard proce-
dures. The starting trihalomethyl substituted alkenes
2a–c were prepared by direct condensation of the
appropriate trihaloacetaldehyde hydrates with ace-
tophenone and nitromethane according to described
procedures [42–45].
(400 MHz, CDCl3/TMS): δ 6.09 (q, 1H, H-2, J = 7.1
Hz), 6.99 (td, 1H, H-6, J = 7.5, 1.0 Hz), 7.02 (d, 1H,
H-8, J = 8.2 Hz), 7.17 (dd, 1H, H-5, J = 7.5, 1.6 Hz),
7.29 (s, 1H, H-4), 7.35 (ddd, 1H, H-7, J = 8.2, 7.5,
1.6 Hz), 7.50–7.55 (m, 2H, H-3ꢁ, H-5ꢁ), 7.62 (tt, 1H,
H-4ꢁ, J = 7.4, 1.3 Hz), 7.75–7.78 (m, 2H, H-2ꢁ, H-6ꢁ).
19F NMR (376 MHz, CDCl3/C6F6): δ 82.76 (d, CF3,
J = 7.1 Hz). Anal. Calcd for C17H11F3O2: C, 67.11; H,
3.64. Found: C, 67.10; H, 3.63.
3-Benzoyl-6-bromo-2-(trifluoromethyl)-2H-chro-
mene (4b). This compound was prepared analo-
gously to 4a as a colorless powder. IR (KBr): ν
1
Trans-trans-3-Benzoyl-2-(trifluoromethyl)chro-
man-4-ol (3a). To a solution of salicylaldehyde
(0.64 g, 5.2 mmol) and alkene 2a (1.05 g, 5.2 mmol)
in dichloromethane (10 mL) was added triethyl-
amine (0.10 g, 1.0 mmol). The mixture was allowed
to stand for 72 h at r.t. After partial evaporation of
the solvent, the residue was diluted with hexane
(6 mL) and the crystalline material was collected
by filtration to give 1.2 g (71%) of 3a as a color-
less powder. IR (KBr): ν 3485, 3420, 1668, 1583,
1645, 1598, 1564, 1473 cm−1. H NMR (400 MHz,
CDCl3/TMS): δ 6.09 (q, 1H, H-2, J = 7.0 Hz), 6.92
(d, 1H, H-8, J = 8.7 Hz), 7.20 (s, 1H, H-4), 7.32 (d,
1H, H-5, J = 2.4 Hz), 7.43 (dd, 1H, H-7, J = 8.7,
2.4 Hz), 7.50–7.55 (m, 2H, H-3ꢁ, H-5ꢁ), 7.64 (tt, 1H,
H-4ꢁ, J = 7.5, 1.3 Hz), 7.74–7.77 (m, 2H, H-2ꢁ, H-6ꢁ).
19F NMR (376 MHz, CDCl3/C6F6): δ 83.03 (d, CF3,
J = 7.0 Hz). Anal. Calcd for C17H10BrF3O2: C, 53.29;
H, 2.63. Found: C, 53.43; H, 2.60.
1
1485 cm−1. H NMR (400 MHz, CDCl3/TMS): δ 2.27
General Procedure for the Synthesis of
3-Nitrochromenes 4c–f
(d, 1H, OH, J = 7.4 Hz), 4.15 (dd, 1H, H-3, J = 10.5,
9.7 Hz), 4.90 (dq, 1H, H-2, J = 10.5, 5.8 Hz), 5.30
(dd, 1H, H-4, J = 9.7, 7.4 Hz), 6.99 (dd, 1H, H-8,
J = 8.2, 1.0 Hz), 7.08 (td, 1H, H-6, J = 7.5, 1.0 Hz),
7.29 (br t, 1H, H-7, J = 7.8 Hz), 7.45-7.55 (m, 3H,
H-5, H-3ꢁ, H-5ꢁ), 7.64 (tt, 1H, H-4ꢁ, J = 7.4, 1.0 Hz),
8.01–8.04 (m, 2H, H-2ꢁ, H-6ꢁ). 19F NMR (376 MHz,
CDCl3/C6F6): δ 85.63 (d, CF3, J = 5.8 Hz). Anal.
Calcd for C17H13F3O3: C, 63.36; H, 4.07. Found: C,
63.00; H, 3.95.
To a solution of salicylaldehyde 1 (10 mmol) and
nitroalkene 2 (10 mmol) in a minimal volume of
dichloromethane (1.5–5 mL) was added triethy-
lamine (0.15 g, 1.5 mmol). The mixture was stirred
for 2 h (10 min in the case of 4d) at r.t. After evap-
oration of the solvent, the residue was recrystallized
from hexane to give compound 4 as yellow needles.
3-Nitro-2-(trifluoromethyl)-2H-chromene (4c). IR
(KBr): ν 1651, 1608, 1571, 1525, 1457, 1327 cm−1.
1H NMR (400 MHz, CDCl3/TMS): δ 6.09 (q, 1H, H-2,
J = 6.3 Hz), 7.07 (d, 1H, H-8, J = 8.2 Hz), 7.11 (td,
1H, H-6, J = 7.5, 1.0 Hz), 7.37 (dd, 1H, H-5, J = 7.6,
1.6 Hz), 7.46 (ddd, 1H, H-7, J = 8.2, 7.5, 1.6 Hz), 8.12
(s, 1H, H-4). 19F NMR (376 MHz, CDCl3/C6F6): δ 83.92
Trans-trans-3-Benzoyl-6-bromo-2-(trifluorome-
methyl)chroman-4-ol (3b). This compound was
prepared analogously to 3a for 24 h as a color-
less powder. IR (KBr): ν 3483, 3431, 1673, 1633,
1
1475 cm−1. H NMR (400 MHz, CDCl3/TMS): δ 2.32
(d, 1H, OH, J = 7.2 Hz), 4.12 (dd, 1H, H-3, J = 10.6,

Synthesis of 3-Substituted 2-Trifluoro(trichloro)methyl-2H-chromenes by Reaction of Salicylaldehydes 495
[14] Murugesh, M. G.; Subburaj, K.; Trivedi, G. K. Tetra-
(d, CF3, J = 6.3 Hz). Anal. Calcd for C10H6F3NO3: C,
hedron 1996, 52, 2217–2228.
48.99; H, 2.47; N, 5.71. Found: C, 49.03; H, 2.49; N,
5.61.
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6-Bromo-3-nitro-2-(trifluoromethyl)-2H-chro-
mene (4d). IR (KBr): ν 1650, 1599, 1564, 1520, 1473,
1
1331 cm−1. H NMR (400 MHz, CDCl3/TMS): δ 6.09
(q, 1H, H-2, J = 6.2 Hz), 6.98 (d, 1H, H-8, J =
8.7 Hz), 7.50 (d, 1H, H-5, J = 2.4 Hz), 7.54 (dd, 1H,
H-7, J = 8.7, 2.4 Hz), 8.04 (s, 1H, H-4). 19F NMR
(376 MHz, CDCl3/C6F6): δ 84.02 (d, CF3, J = 6.2 Hz).
Anal. Calcd for C10H5BrF3NO3: C, 37.07; H, 1.56; N,
4.32. Found: C, 37.16; H, 1.45; N, 4.24.
3-Nitro-2-(trichloromethyl)-2H-chromene (4e).
1
IR (KBr): ν 1642, 1605, 1526, 1453, 1328 cm−1. H
NMR (400 MHz, CDCl3/TMS): δ 6.33 (s, 1H, H-2),
7.09 (td, 1H, H-6, J = 7.5, 1.0 Hz), 7.11 (d, 1H, H-8,
J = 8.2 Hz), 7.36 (dd, 1H, H-5, J = 7.5, 1.6 Hz),
7.46 (ddd, 1H, H-7, J = 8.2, 7.5, 1.6 Hz), 8.13 (s, 1H,
H-4). Anal. Calcd for C10H6Cl3NO3: C, 40.78; H, 2.05;
N, 4.76. Found: C, 40.76; H, 1.87; N, 4.75.
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1
1335 cm−1. H NMR (400 MHz, CDCl3/TMS): δ 6.32
(s, 1H, H-2), 7.01 (dd, 1H, H-8, J = 8.7, 0.4 Hz),
7.49 (d, 1H, H-5, J = 2.4 Hz), 7.54 (dd, 1H, H-7,
J = 8.7, 2.4 Hz), 8.03 (s, 1H, H-4). Anal. Calcd for
C10H5BrCl3NO3: C, 32.17; H, 1.35; N, 3.75. Found:
C, 32.34; H, 1.25; N, 3.72.
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