STUDY OF SUBSTITUTED FORMYLCHROMONES

Article abstract of DOI:10.1135/cccc19941673

Substituted derivatives of 2- and 3-formylchromones were synthesized and studied by IR, 13C and 1H NMR spetroscopy and the AM1 quantum chemical method.Energy and electron distribution calculation confirm the preference of synplanar conformation in 3-formylchromones.The calculated charges of the carbon atom correlate well with the experimental 13C chemical shifts.Substituents bonded to the aromatic nucleus have only small effect on the electron structure of the pyrone ring.

Full text of DOI:10.1135/cccc19941673

Substituted Formylchromones
1673
STUDY OF SUBSTITUTED FORMYLCHROMONES
a
a
a
b
Hafez M. EL-SHAAER , Pavol ZAHRADNIK , Margita LACOVA and Maria MATULOVA
a Department of Organic Chemistry,
Komensky University, 842 15 Bratislava, The Slovak Republic
b Institute of Chemistry,
Slovak Academy of Sciences, 842 38 Bratislava, The Slovak Republic
Received August 6, 1993
Accepted December 29, 1993
Substituted derivatives of 2- and 3-formylchromone were synthesized and studied by IR, 13C and
1H NMR spectroscopy and the AM1 quantum chemical method. Energy and electron distribution cal-
culations confirm the preference of the synplanar conformation in 3-formylchromones. The calculated
charges on the carbon atoms correlate well with the experimental 13C chemical shifts. Substituents
bonded to the aromatic nucleus have only small effect on the electron structure of the pyrone ring.
Thanks to their biological activity, chromone derivatives are the subject of considerable
pharmaceutical and chemical interest1. In connection with prediction and preparation of
biologically active compounds we synthesized some derivatives of 2- and 3-formyl-
chromone. Whereas the derivatives of 3-formylchromone were prepared by methods
described in the literature2, the hitherto used method of preparation of 2-formyl-
chromones3 was modified to give better results.
Conformational analysis was performed using the AM1 quantum chemical method,
the electron structure was studied by 13C and 1H NMR spectroscopy and the theoretical
AM1 method.
EXPERIMENTAL
The melting points were determined on a Kofler block and are uncorrected. IR spectra were recorded
in Nujol on a Specord 75 IR instrument (Zeiss, Jena) in the region 400 – 4 000 cm−1. NMR spectra
were measured on an FT NMR spectrometer Bruker AM 300 at 298 K in CDCl3 with tetramethyl-
1
silane as internal standard. The resolution in the H NMR spectra was 0.12 Hz per point. Parameters
of the 13C NMR spectra (75.46 MHz) are as follows: relaxation delay 3 s, pulse width 30°, digital
resolution 1.6 Hz per point. The 13C NMR signals were assigned using a semiselective INEPT ex-
periment, optimized to the long-range coupling constant value J = 6 Hz.
Synthesis of Nitrones Ia – If. General Method
A solution of sodium ethoxide (prepared from 44 mg (2 mmol) of Na and 15 ml of ethanol) was
added dropwise to a cold (−5 to 0 °C) solution of the 2-methylchromone derivative (2 mmol) and
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

1674
El-Shaaer, Zahradnik, Lacova, Matulova:
4-nitroso-N,N-dimethylaniline (0.9 g, 6 mmol) in a minimum amount of absolute ethanol and the
mixture was set aside at room temperature for 10 min. The red precipitate was collected, washed
with ethanol and then with ether. The product was purified by column chromatography on silica gel
in chloroform and crystallized from dioxane.
Nitrones Ig and Ih
A solution of sodium ethoxide (prepared from 44 mg (2 mmol) of Na and 15 ml of ethanol) was
added dropwise at 40 °C to a solution of 2-methylchromone (0.42 g, 2 mmol) and 4-nitroso-N,N-di-
methylaniline (0.9 g, 6 mmol) in a minimum amount of absolute ethanol, and the solution was stirred
at 40 °C for 20 min. The precipitate was collected, washed with ethanol and then with ether. The
product was purified by column chromatography on silica gel in chloroform and crystallized from
dioxane.
Synthesis of 2-Formylchromone (IIa)
Method A. A mixture of nitrone Ia (0.5 g, 16 mmol) and 5 M H2SO4 (5 ml) was stirred at room
temperature for 15 min. After dilution with water (15 ml), the resulting solution was extracted with
chloroform (3 × 20 ml). The chloroform layer was washed twice with water, dried over calcium
chloride and the solvent was evaporated. Crystallization of the residue from ethyl acetate afforded
0.25 g (89%) of chromone IIa, m.p. 161 – 162 °C (reported3 m.p. 162 – 163 °C).
Method B. A mixture of nitrone Ia (0.3 g, 9.6 mmol), cyclohexane (10 ml) and p-toluenesulfonic
acid (0.4 g, 2 mmol) was stirred at room temperature for 10 min and then at 50 °C for 20 min. Water
(15 ml) was added and the stirred reaction mixture was heated at 50 °C for 1 h. After cooling, the
cyclohexane layer was separated, the aqueous one was extracted with cyclohexane (2 × 10 ml) and
the combined cyclohexane solutions were dried. Evaporation of the solvent afforded yellow crystals
of IIa (0.20 g, 70%) which were crystallized from ethanol; m.p. 161 – 162 °C.
Synthesis of Substituted 2-Formylchromones IIb – IIh
A mixture of the appropriate nitrone (Ib – Ih; 16 mmol) and 5 M H2SO4 (5 ml) was stirred at room
temperature for 15 min, diluted with water (15 ml) and allowed to stand overnight. The precipitate
was collected and crystallized from ethyl acetate. For yields, analyses, melting points and IR data of
the products IIb – IIh see Table I.
RESULTS AND DISCUSSION
Several methods of preparation of 2-formylchromones are described in the literature3–7
.
In our present study we made use of a modified method of Schmutz3. Condensation of
substituted 2-methylchromones with 4-nitrosodimethylaniline in the presence of so-
dium ethoxide, followed by hydrolysis of the obtained nitrones Ia – Ih with sulfuric
acid, afforded the corresponding 2-formylchromones IIa – IIh (Scheme 1).
The analytical data for nitrones Ia – Ih and 2-formylchromones IIb – IIh are given in
Table I. The structure of nitrones Ib – Ih was confirmed by IR spectra (Table I), which
1
exhibit a strong CO band of the pyrone at 1 580 – 1 623 cm−1 in Nujol, by H NMR
spectra, which display a singlet due to H-9 at δ 8.27 – 8.39 ppm and another one due to
H-3 at δ 7.94 – 8.05 ppm, and also by 13C NMR spectra of nitrones Ia, Ib (Table II).
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Substituted Formylchromones
1675
TABLE I
Characteristic data of compounds Ia – Ih and IIb – IIh
Calculated/Found
IR, cm−1
Formula
M.w.
M.p., °C
Yield, %
Compound
ν(C=O) ν(HC=O)
pyrone
% C
% H
% N
% X
Ia
C18H16N2O3
308.3
202 – 203
58
70.12
70.23
5.23
5.28
9.08
9.12
–
–
1 587
1 621
1 623
1 613
1 623
1 589
1 580
1 587
1 659
1 641
1 640
1 639
1 649
1 633
1 644
–
–
Ib
C18H14Cl2N2O3
377.2
220
86
57.31
57.62
3.74
3.75
7.43
7.37
18.80
18.90
Ic
C19H17ClN2O3
356.8
224 – 225
78
63.96
64.11
4.80
4.65
7.85
7.42
9.94
9.81
–
Id
C18H15BrN2O3
387.2
229 – 230
63
55.83
55.60
3.90
3.89
7.23
7.11
20.63
20.40
–
Ie
C18H14Cl2N2O3
377.2
224 – 225
86
57.31
57.76
3.74
3.75
7.43
6.98
18.80
18.65
–
If
C19H17ClN2O3
356.8
239 – 240
72
63.96
63.85
4.80
4.70
7.85
7.82
9.94
9.85
–
Ig
C22H18N2O3
358.4
219 – 221
92
73.73
73.61
5.06
5.01
7.82
7.74
–
–
–
Ih
C22H18N2O3
358.4
220 – 221
100
73.73
73.65
5.06
5.02
7.82
7.84
–
–
–
IIb
IIc
IId
IIe
IIf
IIg
IIh
C10H4Cl2O3
243.1
146 – 147
87
49.42
49.43
1.66
1.62
–
–
29.17
29.08
1 708
1 700
1 699
1 700
1 700
1 707
1 707
C11H7ClO3
222.6
169 – 170
81
59.35
59.44
3.17
3.20
–
–
15.92
15.69
C10H5BrO3
253.1
196 – 197
51
47.46
47.52
1.99
1 98
–
–
31.58
31.83
C10H4Cl2O3
243.1
184 – 186
77
49.42
49.89
1.66
1.61
–
–
29.17
28.91
C11H7ClO3
222.6
190 – 191
78
59.35
59.55
3.17
3.19
–
–
15.92
15.84
C14H8O3
224.2
196 – 197
87
75.00
74.87
3.60
3.50
–
–
–
–
C14H8O3
224.2
216 – 217
91
75.00
75.17
3.60
3.64
–
–
–
–
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

1676
El-Shaaer, Zahradnik, Lacova, Matulova:
SCHEME 1
SCHEME 2
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Substituted Formylchromones
TABLE II
1677
1H NMR spectra of the nitrones Ia – Ih, 2-formylchromones IIb – IIh and 13C NMR spectra of the
nitrones Ia, Ib
Compound
Ia
δ, ppm (J, Hz)
1H NMR: 8.27 s, 1 H (H-9); 8.19 dd, 1 H, J5,6 = 7.6, J5.7 = 1.6 (H-5); 7.95 s, 1 H
(H-3); 7.69 d, 2 H, J12,13 = 9.3, (H-12 and H-16); 7.66 dd, 1 H, J6,5 = 7.8, J6,7 = 7.1
(H-6); 7.41 d, 1 H, J8,7 = 8.2 (H-8); 7.38 ddd, 1 H, J7,6 = 7.1, J7,8 = 8.2, J7,5 = 1.6
(H-7); 6.66 d, 2 H, J13,12 = 9.3 (H-13 and H-15); 3.06 s, 6 H (CH3)
13C NMR: 155.8 (C-2), 117.7 (C-3), 179.1 (C-4), 124.4 (C-4a), 125.6 (C-5), 124.9
(C-6), 133.8 (C-7), 110.6 (C-8), 152.0 (C-8a), 124.3 (C-9), 137.4 (C-11), 122.3
(C-12), 111.0 (C-13), 155.8 (C-14), 111.0 (C-15), 122.3 (C-16), 40.2 (N(CH3)2)
Ib
1H NMR: 8.27 s, 1 H (H-9); 8.10 d, 1 H, J5,7 = 2.1 (H-5); 8.05 s, 1 H (H-3); 7.73 d,
1 H, J7,5 = 2.1 (H-7); 7.73 d, 2 H, J12,13 = 9.1 (H-12 and H-16); 6.71 d, 2 H, J13,12 = 9.1
(H-13 and H-15); 3.08 s, 6 H (CH3)
13C NMR: 156.2 (C-2), 110.2 (C-3), 177.2 (C-4), 126.2 (C-4a), 123.9 (C-5),
130.6 (C-6), 133.8 (C-7), 124.2 (C-8), 150.1 (C-8a), 123.6 (C-9), 137.4 (C-11),
122.4 (C-12), 111.1 (C-13), 152.2 (C-14), 111.1 (C-15), 122.4 (C-16), 40.3 (N(CH3)2)
Ic
Id
Ie
If
1H NMR: 8.30 s, 1 H (H-9); 8.02 s, 1 H (H-3); 7.90 d, 1 H, J5,7 = 2.5 (H-5); 7.71 d,
2 H, J12,13 = 9.3 (H-12 and H-16); 7.55 d, 1 H, J7,5 = 2.5 (H-7); 6.68 d, 2 H,
J
13,12 = 9.3 (H-13 and H-15); 3.06 s, 6 H (CH3); 2.43 s, 3 H (CH3)
1H NMR: 8.32 s, 1 H (H-9); 8.28 d, 1 H, J5,7 = 2.4 (H-5); 7.94 s, 1 H (H-3); 7.74 dd,
1 H, J7,8 = 8.9, J7,5 = 2.4 (H-7); 7.69 d, 2 H, J12,13 = 7.3 (H-12 and H-16); 7.30 d, 1 H,
J = 8.9 (H-8); 6.68 d, 2 H, J13,12 = 7.3 (H-13 and H-15); 3.06 s, 6 H (CH3)
1H NMR: 8.27 s, 1 H (H-9); 8.25 s, 1 H (H-5); 8.22 s, 1 H (H-8); 7.92 s, 1 H (H-3);
7.69 d, 2 H, J12,13 = 9.3 (H-12 and H-16); 6.67 d, 2 H, J13,12 = 9.27 (H-13 and H-15);
3.05 s, 6 H (CH3)
1H NMR: 8.34 s, 1 H (H-9); 8.22 brs, 2 H (H-5 and H-8); 7.92 s, 1 H (H-3); 7.69 d,
2 H, J12,13 = 9.3 (H-12 and H-16); 6.68 d, 2 H, J13,12 = 9.3 (H-13 and H-15);
3.07 s, 6 H (CH3)
Ig
1H NMR: 10.08 dd, 1 H, J9,10 = 7.9, J9,11 = 1.0 (H-9); 8.39 s, 1 H (H-13); 8.08 d,
1 H, J7,8 = 9.0 (H-7); 8.01 s, 1 H (H-3); 7.90 dd, 1 H, J12,11 = 8.1, J12,10 = 1.2 (H-12);
7.76 ddd, 1 H, J10,9 = 7.9, J10,11 = 6.9, J10,12 = 1.2 (H-10); 7.72 d, 2 H, J16,17 = 9.3
(H-16 and H-20); 7.61 ddd, 1 H, J11,9 = 1.0, J11,12 = 8.1 (H-11); 7.47 d, 1 H,
J8,7 = 9.0 (H-8); 6.68 d, 2 H, J17,16 = 9.3 (H-17 and H-19); 3.08 s, 6 H (CH3)
Iha
IIb
IIc
IId
1H NMR: 8.62 – 7.50 m, 9 H (Ar-H); 8.37 s, 1 H (H-13); 6.70 d, 2 H, J17,16 = 9.3
(H-17 and H-19); 3.06 s, 6 H (CH3)
1H NMR: 9.87 s, 1 H (CHO); 8.08 d, 1 H, J5,7 = 2.5 (H-5); 7.82 d, 1 H, J7,5 = 2.5
(H-7); 6.96 s, 1 H (H-3)
1H NMR: 9.86 s, 1 H (CHO); 7.89 d, 1 H, J5,7 = 2.5 (H-5); 7.67 d, 1 H, J7,5 = 2.5
(H-7); 6.92 s, 1 H (H-3); 2.46 s, 3 H (CH3)
1H NMR: 9.80 s, 1 H (CHO); 8.34 d, 1 H, J5,7 = 2.5 (H-5); 7.86 dd, 1 H, J7,8 = 8.9,
J7,5 = 2.5 (H-7); 7.52 d, 1 H, J8,7 = (H-8); 6.94 s, 1 H (H-3)
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

1678
El-Shaaer, Zahradnik, Lacova, Matulova:
TABLE II
(Continued)
Compound
δ, ppm (J, Hz)
IIe
IIf
1H NMR: 9.76 s, 1 H (CHO); 8.25 s, 1 H (H-5); 8.15 s, 1 H (H-8); 7.16 s, 1 H (H-3)
1H NMR: 9.76 s, 1 H (CHO); 7.97 s, 1 H (H-5); 7.84 s, 1 H (H-8); 7.11 s, 1 H (H-3);
2.48 s, 3 H (CH3)
IIga
1H NMR: 9.97 dd, 1 H, J9,10 = 8.7, J9,11 = 1.0 (H-9); 9.84 s, 1 H (CHO); 8.19 d, 1 H,
J7,8 = 9.0 (H-7); 7.95 dd, 1 H, J12,11 = 8.1, J12,10 = 1.2 (H-12); 7.80 ddd, 1 H,
J10,9 = 8.7, J10,11 = 7.0, J10,12 = 1.2 (H-10); 7.67 ddd, 1 H, J11,9 = 1.0, J11,10 = 7.0,
J11,12 = 8.1 (H-11); 7.64 d, 1 H, J8,7 = 9.0 (H-8); 7.06 s, 1 H (H-3)
IIh
1H NMR: 9.90 s, 1 H (CHO); 8.10 – 7.75 m, 6 H (Ar-H); 7.25 s, 1 H (H-3)
a
Numbering of atoms is the same as in Ig.
The structure of 2-formylchromones IIa – IIh was confirmed by IR spectra which
show an aldehyde band at 1 699 – 1 708 cm−1 and a strong CO band of the pyrone at
1
1 633 – 1 659 cm−1; the H NMR spectra show an aldehyde proton signal at δ 9.76 –
9.90 ppm whereas the H-3 signal is located at δ 6.92 – 7.25 ppm (Table II). The 13C
NMR spectral data for aldehydes IIa – IId are given in Table III; the data for compound
IIa were taken from the literature8.
Substituted 3-formylchromones IIIa – IIId were prepared according to Nohara2 by
reaction of the corresponding o-hydroxyacetophenones with the Wilsmeier reagent. The
structure of the obtained compounds and their 13C NMR spectral data are listed in
Table IV, the data for compound IIIa being again taken from the literature8.
Compounds IIa – IId and IIIa – IIId were studied by semiempirical quantum chemi-
cal method AM1 (ref.9). We studied both planar arrangements of the formyl group
relative to the pyrone C=C double bond: the synperiplanar, sp, and antiperiplanar, ap,
arrangement (Scheme 2). We performed full optimization of the geometry for both
conformations of unsubstituted 2- as well as 3-formylchromone. The thus-obtained ge-
ometries were used as input for the calculation of electron structure and energy of the
substituted derivatives. The calculated charges Q on the carbon atoms are listed in
Tables III and IV. As seen, comparison with the observed values of 13C NMR
chemical shifts shows a good accord. For all the carbon atoms there is a linear relation-
ship Q = a δ(13C) + b with regression coefficient r = 0.910 – 0.945. A change of
substituent on the aromatic ring results in a relatively small shift of the δ values in 13C
NMR spectra and thus in a relatively small change of the calculated charge on the
carbon atoms of the pyrone ring and the formyl group.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Substituted Formylchromones
1679
The calculated heats of formation for 3-formylchromones are about 32 kJ/mol lower
than those for the corresponding isomeric 2-formylchromones.
The dependence of the heat of formation on the dihedral angle about the single bond
C−CHO in 3-formylchromone and its derivatives shows that the planar sp conformation
is more stable than the ap rotamer. Tsukerman and coworkers10 on the basis of IR
spectra and dipole moments of 3-formylchromones also assume the preference of sp
conformation that may be stabilized by hydrogen bonding between the pyrone carbonyl
and the hydrogen atom of the formyl group. Experimental values of dipole moments of
3-formylchromones10 are also closer to those calculated for the sp conformation (Table IV).
TABLE III
Calculated and experimental characteristics of 2-formylchromone derivatives IIa – IId
Parameter
∆∆Hfa
IIa
IIb
IIc
IId
3.5
4.0
−3.3
−3.9
µ, Db
4.28 0.96
3.23 0.42
3.82 1.29
3.93 1.05
Charge (chemical shift)c
C-2
−17(156.7)
−219(117.8)
301(178.3)
−198(124.4)
−44(126.4)
−164(125.3)
−74(135.8)
−158(119.2)
100(155.3)
228(187.7)
−134
−15(155.9)
−219(115.1)
302(176.4)
−178(125.3)
−46(124.0)
−99(132.1)
−78(135.2)
−94(126.5)
97(149.9)
228(184.5)
−120
−14(155.7)
−221(115.2)
302(177.6)
−184(125.7)
−47(124.0)
−98(136.8)
−77(136.4)
−100(123.4)
93(149.5)
228(185.0)
−122
−18(156.0)
−216(116.8)
301(176.9)
−198(126.0)
−17(128.6)
−210(119.8)
−51(138.2)
−157(120.7)
111(154.3)
228(185.0)
−132
C-3
C-4
C-4a
C-5
C-6
C-7
C-8
C-8a
CHO
O-1
O-4
−287
−278
−284
−280
OCH
H-3d
HCOd
C=Oe
HC=Oe
−251
−248
−250
−248
186(6.91)
127(9.80)
1.856
189(6.96)
131(9.87)
1.863
187(6.92)
130(9.86)
1.858
188(6.94)
128(9.80)
1.861
1.950
1.954
1.952
1.953
a
b
Energy difference (kJ/mol) between ap and sp conformation by AM1. Calculated values for sp
and ap conformation. Values of AM1 charge (Q . 103) for sp conformation, 13C NMR shift in
c
parentheses. d 1H NMR shift in parentheses. e Bond orders for sp conformation.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

1680
El-Shaaer, Zahradnik, Lacova, Matulova:
The relatively large energy difference between sp and ap conformation in 3-formyl-
chromones (Table IV) can be explained by destabilization of the ap conformation by
strong repulsion between the carbonyl oxygen of the pyrone ring and the formyl group.
On the other hand, the great negative charge on the formyl oxygen atom as well as the
considerable positive charge on H-2 of the pyrone ring may stabilize the sp conforma-
. . .
tion by an intramolecular hydrogen bond H-2
O(formyl). The enhanced acidity of
the H-2 hydrogen atoms in 3-formylchromones is also evident from comparison of their
TABLE IV
Calculated and experimental characteristics of 3-formylchromone derivatives IIIa – IIId
Parameter
∆∆Hfa
IIIa
IIIb
20.7
IIIc
IIId
24.3
19.7
23.3
µ, Db
4.44 5.58
(4.76)
3.14 4.72
–
4.67 5.59
(4.97)
5.26 8.59
(6.44)
Charge (chemical shift)c
C-2
122(160.7)
−359(120.3)
324(175.8)
−187(125.3)
−48(124.1)
−157(126.6)
−79(134.8)
−148(118.6)
79(156.2)
243(188.5)
−123
119(160.3)
−355(120.2)
325(174.1)
−169(125.0)
−46(124.1)
−84(132.5)
−77(134.9)
−78(127.2)
84(150.5)
244(187.5)
−109
118(160.4)
−358(120.0)
325(175.8)
−182(124.8)
−50(125.3)
−98(136.7)
−83(135.8)
−145(118.2)
76(154.3)
244(188.5)
−123
115(160.8)
−345(120.5)
322(174.4)
−203(125.6)
23(122.6)
−132(145.6)
−19(129.1)
−159(120.5)
127(158.8)
241(187.3)
−117
C-3
C-4
C-4a
C-5
C-6
C-7
C-8
C-8a
CHO
O-1
O-4
−304
−293
−304
−282
OCH
H-2d
HCOd
C=Oe
HC=Oe
OC−He
−308
−298
−304
−296
203(8.90)
130(10.16)
1.847
206(8.60)
129(10.34)
1.855
200(8.45)
127(10.29)
1.846
211(8.60)
136(10.36)
1.864
1.904
1.910
1.910
1.915
0.906
0.905
0.912
0.905
a
b
Energy difference (kJ/mol) between ap and sp conformations. Calculated values for sp and ap
conformation, in parentheses the experimental values10. Values of AM1 charge (Q . 103) for sp
c
conformation, 13C NMR shift in parentheses. 1H NMR shift in parentheses. Bond orders for sp
d
e
conformation.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Substituted Formylchromones
1681
1H NMR shifts in compounds IIIa – IIId with those of the H-3 atoms in substituted
2-formylchromones IIa – IId.
In the case of 2-formylchromone derivatives, the difference between the heats of
formation for the sp and ap conformations is too small to allow determination of the
preferred form (Table III). Whereas the sp conformation is slightly preferred in com-
pounds IIa and IId, the presence of chlorine atom in position 8 (compounds IIb, IIc)
probably reverses the energetic situation in favour of the ap conformer.
The authors are indebted to Dr E. Greiplova for the elemental analyses, Dr A. Perjessy for measure-
ment of IR spectra and Dr E. Solcanyova for obtaining the 1H NMR spectra.
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Translated by M. Tichy.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)