Amino acid derivatives and oximes of flavones

Article abstract of DOI:10.1023/A:1015675018067

4-Ethoxyflavylium tetrafluoroborates with substituents in rings A and B were synthesized. Their reaction with nitrogen-containing nucleophiles was investigated. It was shown that derivatives of flavones at the carbonyl group are formed as a result of thes

Full text of DOI:10.1023/A:1015675018067

Chemistry of Heterocyclic Compounds, Vol. 38, No. 3, 2002
AMINO ACID DERIVATIVES
AND OXIMES OF FLAVONES
V. V. Ishchenko, T. S. Shulyak, and V. P. Khilya
4-Ethoxyflavylium tetrafluoroborates with substituents in rings A and B were synthesized. Their
reaction with nitrogen-containing nucleophiles was investigated. It was shown that derivatives of
flavones at the carbonyl group are formed as a result of these reactions. The major distinctive
physicochemical characteristics of the oximes of flavones and isoxazoles were determined.
Keywords: flavone amino acid derivatives, flavone oximes, 4-ethoxyflavylium tetrafluoroborates.
The synthetic possibilities and biological properties of the chromone system have been investigated in a
fair amount of detail. However, the reliable published information on the characteristics of chromone
derivatives with respect to the carbonyl group is insufficient, although these compounds can be used both for the
synthesis of new heterocyclic systems and are themselves biologically active compounds. The formation of the
C=N fragment at position 4 of the chromone ring promotes elongation of the principal conjugation chain and
increase in the polarity of the molecule as a whole, thereby ensuring that these compounds are soluble in a polar
medium and improving their assimilation by the organism as drugs [1].
The low degree of study of these compounds is explained by the diversity of the reactions of chromones
with nucleophilic reagents, which can result in opening of the pyrone ring followed by recyclization of the
intermediate with the formation of the corresponding five-membered heterocycles. For example, isoxazoles with
structures A and B can be formed during the reaction of flavones with hydroxylamine.
OH
A
O
O
NH2OH
O
N
OH
B
N
O
According to data in [2], the most reliable method for the synthesis of 4-substituted flavones may be the
path through the thioxochromones. In this case, however, the formation of the desired compounds is
accompanied by cleavage of the pyrone ring [3].
__________________________________________________________________________________________
Taras Shevchenko Kiev National University, Kiev, Ukraine; e-mail: ishchenko@mail.univ.kiev.ua,
ishchenko@yahoo.com. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 306-313, March,
2002. Original article submitted December 1, 1999.
274
0009-3122/02/3803-0274$27.00©2002 Plenum Publishing Corporation

Another method for the synthesis of 4-substituted derivatives of flavones is the reaction of
4-ethoxyflavylium salts with nucleophilic reagents [4], but only derivatives with respect to position 4, a mixture
with their recyclization products, or the latter alone may be formed, depending on the medium, the anion, and
the substituents. The structure of the products of this reaction depends on the nucleophilic reagent [5]. For
example, the action of hydrazine gives pyrazoles, while the reaction with phenylhydrazine leads to the
phenylhydrazones of the flavones. Irrespective of the structure of the flavylium salts, their reaction with aniline
in acetic acid leads to substitution of the 4-ethoxy group by an amino group. The reaction with hydroxylamine
gives the flavone oxime, the structure of which was only confirmed by means of its melting point (without
spectral data). Although it agrees with the melting point obtained by Baker [2] it does not provide reliable
evidence for the structure of the obtained products.
On the basis of data [4] for the synthesis of flavone derivatives at the carbonyl group we first used
4-ethoxyflavylium salts containing the perchlorate anion as counterion. However, on account of their thermal
stability, explosion resistance, higher solubility, and somewhat greater activity the tetrafluoroborates are
preferred for practical use. At the beginning of our work, however, the synthesis of 4-ethoxyflavylium
tetrafluoroborates had hardly been described at all. We prepared 4-ethoxyflavylium tetrafluoroborates with
various substituents both in ring A and in ring B.
R1
O
+
OH
Me
HOC
CH(OEt)3
.
+
.
Et2O
BF3
_
R2
R2
BF4
R1
OEt
O
1a–i
1 a R1 = OMe, R2 = H; b R1 = Me, R2 = H; c R1 = OH, R2 = F; d R1 = H, R2 = F; e R1 = OMe,
R2 = F; f R1 = Me, R2 = F; g R1 = H, R2 = Br; h R1 = OH, R2 = Br; i R1 = OMe, R2 = Br
1
The H NMR spectra of the salts 1a-i (Table 1) contain a triplet in the region of 1.05-1.07 ppm and a
quartet in the region of 3.43-3.46 ppm, corresponding to an ethoxy group at position 4 of the chromone ring.
The presence of signals in the range of 6.96-7.04 ppm (3-H) confirms that the chromone structure is retained.
The synthesized tetrafluoroborates were brought into reaction with hydroxylamine, aniline, and
phenylhydrazine. As a result of these reactions we obtained derivatives of the flavones at position 4 with various
substituents in rings A and B. Their characteristics agreed fully with the characteristics of the analogous
compounds obtained from the perchlorates.
1
1
1
R
R
R
O
O
O
+
_
4
BF
2
2
2
R
R
R
NCHRCOOH
NOH
2a–l
OEt
3a–e
1
1
1
1
а
1 = Н, R2 = Н;
ОН, R2 Н;
ОМе, R2 Н;
Ме, R2 Н;
Н,
2
R
b R =
=
c R =
=
d R =
=
e R =
ОН, R2
ОМе, R2
ОМе, R2
ОМе, R2
2
1
1
1
1
R = F; f R =
= F; g R =
= F; h R =
= Cl; i R =
= Br;
Ме, R2
ОН, R2
= Br; c R = i-C3H7, R
1 = ОМе, R2
а
ОМе, R2 Н;
1
1
1
j R =
R = H, R =
= Br; k R =
= NO2; l R
= NO2; 3 R = H, R =
=
ОМе, R2
1 = ОН, R2 Н;
ОН, R2
1
1
=
d R = i-C3H7, R =
= Br;
ОМе, R2
1
e R = i-C3H7, R =
= Br
275

TABLE 1. The Physicochemical Characteristics of Compounds 1a-i*
Found, %
——————
Com-
pound
Empirical
formula
Calculated, %
mp, °С
184
Characteristic signals in the 1H NMR spectrum (DMSO), δ, ppm (J, Hz)
Yield, %
75.7
C
H
Br
С18Н17ВF4O3
С18Н17ВF4O2
С17Н14ВF5O3
59.1
4.31
4.62
1.05 (3Н, t, 4-ОСН2СН3); 3.43 (2Н, q, 4-ОСН2СН3); 3.83 (3Н, s, 4'-ОСН3);
6.93 (1Н, s, 3-Н); 7.12 (2Н, d, J = 9, 3'-,5'-Н); 8.07 (2H, d, J = 9, 2'-,6'-Н);
1a
1b
1c
58.73
7.2-8.2 (4Н, m, Harom
)
61.55
61.36
5.05
4.83
176
1.06 (3Н, t, 4-ОCН2СН3); 3.46 (2Н, q, 4-ОСН2СН3); 2.39 (3Н, s, 4'-СН3);
65.1
6.98 (1Н, s, 3-Н); 7.4 (2Н, d, J = 9, 3'-,5'-Н); 8.0 (2H, d, J = 9, 2'-,6'-Н);
7.45-8.1 (4Н, m, Harom
)
54.5
4.15
3.76
267
1.05 (3Н, t, 4-ОСН2СН3); 3.46 (2Н, q, 4-ОСН2CН3); 6.89 (1Н, s, 3-Н);
72.3
54.84
6.95 (2Н, d, J = 8.5, 3'-,5'-Н); 7.97 (2Н, d, J = 8.5, 2'-,6'-Н);
7.36-7.9 (4Н, m, Harom
)
С17Н14ВF5O2
С18Н16ВF5O3
56.71
56.98
4.35
3.91
224
212
1.05 (3Н, t, 4-ОСН2СН3); 3.46 (2Н, q, 4-ОСН2CН3); 7.0 (1Н, s, 3-Н);
67.1
82.3
1d
1e
6.55-8.3 (8Н, m, Harom
)
55.61
4.25
1.06 (3Н, t, 4-ОСН2СН3); 3.45 (2Н, q, 4-ОСН2СН3); 3.87 (3Н, s, 4'-ОСН3);
55.96
4.14
6.93 (1Н, s, 3-Н); 7.12 (2Н, d, J = 9, 3'-,5'-Н); 8.01 (2H, d, J = 9, 2'-,6-Н);
7.6-8.0 (4Н, m, Harom
)
С18Н16ВF5O2
59.15
58.98
4.41
4.32
222
1.05 (3Н, t, 4-ОСН2СН3); 3.43 (2Н, q, 4-ОСН2СН3); 2.39 (3Н, s, 4'-СН3);
72.1
1f
7.0 (1Н, s, 3-Н); 7.4 (2Н, d, J = 8.5, 3'-,5'-Н); 8.0 (2Н, d, J = 8.5, 2'-,6'-Н);
7.65-7.92 (4Н, m, Harom
)
С17Н14BВrF4O2
С17Н14BВrF4O3
С18Н16BВrF4O3
18.95
19.18
196
261
231
1.07 (3Н, t, 4-ОСН2СН3); 3.43 (2Н, q, 4-ОСН2CН3); 7.04 (1Н, s, 3-Н);
65.4
78.2
83.1
1g
1h
1i
8.12 (1Н, d, J = 2.8, 5-Н); 7.76 (1Н, d, J = 8.8, 8-Н);
18.35
1.06 (3Н, t, 4-ОСН2СН3); 3.46 (2Н, q, 4-ОСН2СН3); 6.85 (1Н, s, 3-Н);
18.48
6.93 (2Н, d, J = 9, 3'-, 5'-Н); 7.95 (2Н, d, J = 9, 2'-,6'-Н); 7.7 (1Н, d, J = 9, 8-Н)
18.05
17.8
1.07 (3Н, t, 4-ОСН2СН3); 3.43 (2Н, q, 4-ОСН2СН3); 3.87 (3Н, s, 4'-ОСН3);
6.95 (1Н, s, 3-Н); 7.11 (2Н, d, J = 8, 3'-,5'-Н); 8.05 (2Н, d, J = 8, 2'-,6'-Н);
7.92 (1Н, d, J = 2.9, 5-Н); 7.75 (1Н, d, J = 8.9, 8-Н)
_______
*In the IR spectra of the salts 1a-i there were absorption bands for the symmetric and asymmetric vibrations of the
pyrilium ring in the region of 1550-1510 cm-1.

We examined in greater detail the reaction of the 4-ethoxyflavylium tetrafluoroborates with amino acids
in acetic acid and with hydroxylamine in pyridine and in acetic acid in the presence of sodium acetate.
Derivatives of flavones at the carbonyl group 2 and 3 were formed in acetic acid irrespective of the nature of the
substituents in the molecules of the flavylium salts. In pyridine under analogous conditions about 30% of
5-(o-hydroxyphenyl)-3-phenylisoxazole, the constants of which agreed fully with published data [3], was
obtained in addition to the flavone oxime.
All the synthesized oximes 2 gave a negative reaction with an alcohol solution of ferric chloride and did
not dissolve in a 2 N solution of alkali or sodium carbonate either in the cold or on heating. This indicates
retention of the pyrone ring and the absence of recyclization products containing a phenolic hydroxyl group and
therefore soluble in alkali and sodium carbonate. In addition, when the oximes 2 and amino acid derivatives 3
that we obtained were boiled in acidified methanol they changed into the corresponding flavones. This provides
another argument in favor of retention of the flavone structure.
The presence of a signal for the 3-H proton of the chromone ring in the region of 6.96-7.65 ppm in the
1H NMR spectra of compounds 2 and 3 (Table 2) indicates retention of the flavone ring. In the region of 11 ppm
there is a signal for the proton of the NOH group of the oximes, while in the region of 10.9-11.8 there is a signal
for the proton of the COOH group in amino acids. Under the influence of the unshared electron pair of the
nitrogen atom the signal of the 5-H proton (Table 2) is shifted downfield compared with the spectrum of the
flavone. In the IR spectra of these derivatives there is a band for the stretching vibrations of the C=N bond in the
region of 1600-1625 cm-1. In the 13C NMR spectra of the glycine derivatives 3a,b there is a signal for the carbon
atom of the COOH group in the region of 169 ppm.
To obtain more rigorous evidence for the formation of the 4-amino acid derivatives 3 in the reaction of
the 4-ethoxyflavylium salts with amino acids we brought glycine diethyl ester into this reaction. The presence of
signals for the protons of the ethoxy group (4.25, q, CH2; 1.28, t, CH3) and the N–CH2 group (4.87, s) in the 1H
NMR spectrum of the product favors the proposed structure.
Earlier [5] the structure of 4'-methylflavone oxime was proved conclusively on the basis of the mass
spectra. We obtained the compound both from the perchlorate and from 4-ethoxy-4'-methylflavylium
tetrafluoroborate (1b) and by an alternative synthesis from thioxochromone. The physicochemical
characteristics of the oxime 4d, which we synthesized by various methods, agreed completely with published
data. The samples did not give a melting point depression in a mixed melting test.
In order to study the chemical characteristics of the derivatives of flavones with respect to the carbonyl
group we obtained the acetyl derivatives of the oximes 2i and 2l, the structure of which was proved by elemental
analysis and spectroscopy. The 1H NMR spectra of the acetates of the oximes 4a,b contain a signal for the three
protons of the acetyl group at 2.28 ppm. The signal of the 3-H proton (~7 ppm) confirms the retention of the
chromone system. The IR spectra of the acetates 4a,b contain bands for the stretching vibrations of the carbonyl
bond in the acetyl group in the region of 1750 cm-1 and the band of the C=N bond at 1620 cm-1.
The possibility of the formation of oximes and isoxazoles as a result of the reactions created the
problem of assigning the obtained compounds. By analyzing and collating data [3, 6, 7] on the reaction of
chromones, thioxochromones, and 4-ethoxychromylium salts with hydroxylamine we determined criteria by
means of which it was possible to assign the obtained products instantly to one of these classes of compound
(Table 3).
EXPERIMENTAL
The IR spectra were recorded in tablets of potassium bromide on a Pye Unicam SP-300 instrument. The
1H NMR spectra were recorded on a Bruker WP-100 spectrometer (100 MHz) with TMS as internal standard.
The reaction and the purity of the synthesized compounds were monitored by TLC (Silufol UV-254, 9:1
benzene–ethanol and 85:15 chloroform–methanol).
277

TABLE 2. The Characteristics of Compounds 2a-l, 3a-e
Found, %
IR spectrum (KBr), ν, cm-1
Characteristic signals in 1H NMR spectra (DMSO),
——————
Com-
pound
Empirical
formula
Yield,
%
Calculated, %
mp, °С
δ, ppm (J, Hz)
N
3
Hal
4
CN
6
OH
7
NO
8
1
2a
2
5
9
10
C15H11NO2
5.83
5.90
184
236
1610
1604
3200
3100
1010
1020
7.14 (1Н, s, 3-Н ); 11.01 (1Н, s, NOH); 7.15-7.8 (9Н, m, Harom
)
81.0
97.4
C15H10NO3
5.66
10.95 (1Н, s, NOH); 10.0 (1Н, s, 4'-ОН); 6.94 (1Н, s, 3-Н);
6.9 (2Н, d, J = 8.6, 3'-,5'-H); 7.78 (2H, d, J = 8.6, 3'-,6'-Н);
7.88 (1Н, dd, J = 2.1, 5-Н)
2b
5.55
C16H13NO3
C16H13NO2
5.18
5.24
207
213
1602
1605
3100
3150
1020
1010
11.04 (1Н, s, NOH); 3.8 (3Н, s, 4'-ОСН3); 6.97 (1Н, s, 3-Н);
72.8
65.3
2c
7.1 (2Н, d, J = 8.5, 3'-,5'-H); 7.92 (2H, d, J = 8.5, 2'-,6'-Н)
5.61
10.94 (1Н, s, NOH); 2.37 (3Н, s, 4'-СН3); 7.07 (1Н, s, 3-Н);
7.3 (2Н, d, J = 8.0, 3'-,5'-H); 7.82 (2H, d, J = 8.0, 2'-,6'-Н)
2d
5.57
C15H10FNO2
C15H10FNO3
5.38
5.49
145
263
1610
1600
2980
3100
1019
1040
11.14 (1Н, s, NOH); 7.08 (1Н, s, 3-Н)
75.0
62.5
2e
2f
5.31
11.0 (1Н, s, NOH); 10.02 (1Н, s, 4'-ОН); 6.93 (1Н, s, 3-Н);
6.88 (2Н, d, J = 8.5, 3'-,5'-H); 7.75 (2H, d, J = 8.5, 2'-,6'-Н)
5.16
C16H12FNO3
C16H12ClNO3
4.88
4.91
209
1604
1600
3100
3100
1020
1020
11.05 (1Н, s, NOH); 3.83 (3Н, s, 4'-ОСН3); 6.98 (1Н, s, 3-Н);
93.3
89.5
2g
2h
7.05 (2Н, d, J = 8.5, 3'-,5'-H); 7.86 (2H, d, J = 8,5, 2',6'-Н);
4.88
11.67
11.75
210
11.0 (1Н, s, NOH); 3.82 (3Н, s, 4'-ОСН3); 7.31 (1Н, s, 3-Н);
7.1 (2Н, d, J = 9.0, 3'-,5'-H); 7.89 (2H, d, J = 9.0, 2'-,6'-Н);
7.77 (1Н, d, J = 3.0, 5-Н); 7.45 (1Н, dd, J = 9.0; 3,0, 7-H);
7.08 (1Н, d, J = 9.0, 8-Н)
4.64
decomp.
C16H12BrNO3
3.91
4.05
23.22
23.08
216
1605
2980
1020
11.1 (1Н, s, NOH); 3.87 (3Н, s, 4'-ОСН3); 6.95 (1Н, s, 3-Н);
7.07 (2Н, d, J = 9.0, 3'-,5'-H); 8.12 (2H, d, J = 9.0, 2'-,6'-Н);
7.92 (1Н, d, J = 3.0, 5-Н); 7.12 (1Н, d, J = 9.0, 8-Н)
87.7
2i

TABLE 2 (continued)
1
2
3
4
5
6
7
8
9
10
C16H12BrNO2
3.81
4.24
24.25
24.20
193
1610
3080
1028
11.05 (1Н, s, NOH); 2.37 (3Н, s, 4'-СН3); 7.6 (1Н, s, 3-Н);
7.35 (2Н, d, J = 9.0, 3'-,5'-H); 7.8 (2H, d, J = 9.0, 2'-,6'-Н)
12.32 (1Н, s, NOH); 9.91 (1Н, s, 4'-ОН); 7.32 (1Н, s, 3-Н);
7.78 (2Н, d, J = 9.0, 3'-,5'-H); 8.22 (2H, d, J = 9.0, 2'-,6'-Н);
8.6 (1Н, d, J = 3.0, 5-Н); 7.12 (1Н, d, J = 9.5, 8-Н)
78.0
2j
C15H10N2O5
9.28
253
1600
3100
1040
93.3
2k
9.39
C16H12N2O5
9.01
8.96
268
1604
3000
1020
12.32 (1Н, s, NOH); 3.84 (3Н, s, 4'-ОСН3); 7.4 (1Н, s, 3-Н);
7.08 (2Н, d, J = 9.0, 3'-,5'-H); 7.87 (2H, d, J = 9.0, 2'-,6'-Н);
8.6 (1Н, d, J = 3.0, 5-Н); 8.23 (1Н, dd, J = 9.0; 3.0, 7-Н);
7.23 (1Н, d, J = 9.0, 8-Н)
73.8
2l
C18H15NO4
4.21
4.54
290
285
1620
1625
1615
1620
1625
1600
1610
3.92 (3Н, s, 4'-ОСН3); 4.79 (2Н, s, CH2);
85.2
89.1
83.2
84.5
88.4
72.5
69.2
3a
3b
3c
3d
3e
4a
4b
7.20 (2Н, d, J = 8.5, 3'-,5'-H); 8.36 (2H, d, J = 8.5, 2'-,6'-Н)
C18H14BrNO4
C20H19NO4
3.31
20.65
20.59
3.93 (3Н, s, 4'-ОСН3); 4.78 (2Н, s, CH2); 7.0 (2Н, d, J = 8.5,
3.61
3'-,5'-H); 8.37 (2H, d, J = 8.5, 2'-,6'-Н); 7.73 (1Н, s, 3-Н)
4.01
4.15
274
5.13 (1H, d, J = 6.5, NСН); 7.05 (2Н, d, J = 8.5, 3'-,5'-H);
8.3 (2H, d, J = 8.5, 2'-,6'-Н); 7.68 (1Н, s, 3-Н)
C20H18BrNO4
C21H20BrNO4
C18H14BrNO4
C18H15NO4
3.25
19.01
19.20
268
5.1 (1H, d, J = 6.5, NСН); 7.05 (2Н, d, J = 8.5, 3'-,5'-H);
3.36
8.1 (2H, d, J = 8.5, 2'-,6'-Н); 7.3 (1Н, s, 3-Н)
3.61
3.26
18.42
272
3.93 (3H, s, 4'-ОСН3); 5.22 (1H, d, J = 6.5, NСН); 7.3 (2Н, d,
J = 8.5, 3'-,5'-H); 8.4 (2H, d, J = 8.5, 2'-,6'-Н); 7.8 (1Н, s, 3-Н)
18.57
3.88
20.35
20.58
145.5
150
1020
1025
3.95 (3Н, s, 4'-ОСН3); 7.15 (1Н, s, 3-Н);
3.61
2.28 (3Н, s, 4-CNOCOCH3)
4.48
4.53
3.92 (3Н, s, 4'-OCH3); 7.10 (1H, s, 3-H);
2.25 (3H, s, 4-CNOCOCH3)

TABLE 3. The Principal Distinctive Characteristics of the Oximes of
Flavones and Isoxazoles
Characteristics
Oximes
Isoxazoles
Reaction with FeCl3
Does not form colored
complex
A. Does not form colored complex
B. Forms colored complex
Solubility in alkalis
Solubility in acids
Does not dissolve
Dissolves
Is transformed
11.0-11.3 (N–OH);
6.8-7.2 (3-Н);
7.8-8.2 (5-Н);
7.3-7.6 (8-Н)
1630-1650 (С=N);
1615-1645 (С=С);
1120-1125 (С–О–С);
3000-3300 (ОН)
Dissolves
Does not dissolve
Is not transformed
10.2-10.8 (ОН),
А. 7.3-7.5 (4-Н);
B. 7.4-7.8 (4-Н),
7.1-7.8 (6-H)
1610-1615 (C=N, С=С);
1270-1275 (С–О–N);
3100-3170 (ОН)
Transformation to flavone
1Н NMR spectrum
(DMSO), ppm
IR spectrum, cm-1
4-Ethoxyflavylium Tetrafluoroborates (1a-i). A solution of the respective acetophenone (2 mmol)
and substituted benzaldehyde (6 mmol) in orthoformic ester (8.3 ml, 40 mmol) was brought to boiling, and
boron trifluoride etherate (2.8 mmol) was added drop by drop. The reaction mixture was left at room
temperature for 12-24 h. The precipitate was filtered off, washed with isopropyl alcohol, and recrystallized from
acetic acid.
Flavone Oximes (2a-l). A mixture of 4-ethoxyflavylium tetrafluoroborate, hydroxylamine
hydrochloride, and sodium acetate in molar ratios 1:2:1 was boiled in glacial acetic acid for 1 h. At the end of
the reaction (TLC) the reaction mixture was poured onto ice. The precipitate was filtered off, washed repeatedly
with water, and recrystallized from isopropyl alcohol.
5-(o-Hydroxyphenyl)-3-phenylisoxazole. A mixture of flavylium perchlorate and hydroxylamine
hydrochloride in molar ratio 1:2 was boiled in pyridine for 1 h. After the reaction (chromatographic control) the
reaction mixture was poured onto ice. The precipitated mixture of products (oxime and isoxazole) was filtered
off, washed repeatedly with water and separated by fractional crystallization from isopropyl alcohol. The yield
of 5-(o-hydroxyphenyl)-3-phenylisoxazole was 30%; mp 234-235°C.
Amino Acid Derivatives of Flavones (3a-e). A mixture of 4-ethoxyflavylium tetrafluoroborate 1
(2.6 mmol) and the respective amino acid (5 mmol) was boiled in glacial acetic acid (monitored by TLC). The
reaction mixture was cooled, and the precipitate was filtered off and recrystallized from acetic acid.
Acetates of Flavone Oximes (4a, b). A solution of the oxime 2i,f (0.1 mmol) in acetic anhydride (6 ml)
and pyridine (1 ml) was heated for 0.5-1 h. The reaction mixture was poured onto ice, and the precipitate was
filtered off, washed thoroughly with water, and recrystallized from acetic acid.
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