New Tetracyclic Spiro-1,2,4-trioxolanes (Ozonides). Synthesis and Mass Spectrometric Study

Article abstract of DOI:10.1134/S1070428019010123

New tetracyclic dispiro-1,2,4-trioxolanes (ozonides) were synthesized by reactions of 5–7-membered alicyclic 1,5-diketones with 30% hydrogen peroxide in diethyl ether or ethanol in the presence of boron trifluoride-diethyl ether complex or a strong minera

Full text of DOI:10.1134/S1070428019010123

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2019, Vol. 55, No. 1, pp. 101–107. © Pleiades Publishing, Ltd., 2019.
Russian Text © T.I. Akimova, V.G. Rybin, O.A. Soldatkina, 2019, published in Zhurnal Organicheskoi Khimii, 2019, Vol. 55, No. 1, pp. 136–144.
New Tetracyclic Spiro-1,2,4-trioxolanes (Ozonides).
Synthesis and Mass Spectrometric Study
a
†b
a
T. I. Akimova ,* V. G. Rybin , and O. A. Soldatkina
a
Far Eastern Federal University, ul. Sukhanova 8, Vladivostok, 690091 Russia
e-mail: akimova.ti@dvfu.ru
*
b
Zhirmunskii Institute of Marine Biology, Far Eastern Branch, Russian Academy of Sciences,
ul. Pal’chevskogo 17, Vladivostok, 690041 Russia
Received July 25, 2018; revised August 6, 2018; accepted August 17, 2018
Abstract—New tetracyclic dispiro-1,2,4-trioxolanes (ozonides) were synthesized by reactions of 5–7-mem-
bered alicyclic 1,5-diketones with 30% hydrogen peroxide in diethyl ether or ethanol in the presence of boron
trifluoride–diethyl ether complex or a strong mineral acid (HCl, H SO , HClO ). Mass spectrometric study of
2 4 4
the title compounds under atmospheric pressure chemical ionization revealed specificity of fragment ion
decomposition with respect to the ring size, which makes it possible to identify such compounds by their mass
spectra.
Keywords: alicyclic 1,5-diketones, hydrogen peroxide, spiro-1,2,4-trioxolanes, tetracyclic ozonides, mass
spectrometry, atmospheric pressure chemical ionization, quasi-molecular ions.
DOI: 10.1134/S1070428019010123
We previously found [1] that alicyclic 1,5-di-
ketones 1–5 are oxidized with 30% hydrogen peroxide
in diethyl ether or ethanol in the presence of boron
trifluoride–diethyl ether complex or a strong mineral
acid (HCl, H SO , HClO ) to give novel compounds,
tetracyclic dispiro-1,2,4-trioxolanes (ozonides) 6–10
in good yields (70–90%; Scheme 1). The classical
method for the synthesis of ozonides by ozonation of
olefins dates back to the past century. The preparation
of ozonides from 1,5-diketones and hydrogen peroxide
is quite unusual since the products are obtained
without using ozone.
sized from diketone 1 by the action of peroxyformic
acid or hydrogen peroxide in methanol in the presence
of V O [2]. Terent’ev et al. [3, 4] recently reported the
2
5
synthesis of a series of bicyclic ozonides by reaction
of aliphatic 1,5-diketones with hydrogen peroxide in
acetonitrile in the presence of BF ·OEt ; the same
2
4
4
3
2
compounds were obtained previously [5] from β,δ-tri-
ketones in 15–25% yield as mixtures with bridged
tetraoxanes and monoperoxides. The first ozonide of
this series was synthesized from heptane-2,6-dione [6].
Tricyclic ozonides can be synthesized according to
Griesbaum [7, 8] by ozonation of a mixture of ketone
oxime O-alkyl ether and monocarbonyl compound in
pentane at –78°C.
Up to now, only one compound of this type has
been reported, namely ozonide 6 which was synthe-
Scheme 1.
R
R
(
)m
( )n
+
( )m
( )n
3
0% H O , H or BF ·Et O
2 2 3 2
O
O
O
O
O
1
–5
6–10
1
, 6, m = n = 0, R = H; 2, 7, m = n = 1, R = H; 3, 8: m = 1, n = 0, R = H; 4, 9, m = 1, n = 2, R = H; 5, 10, m = n = 1, R = Me.
†
Deceased.
1
01

1
02
AKIMOVA et al.
Over the past two decades, tricyclic ozonides like A
The mass spectra of 6–10 showed some specific
features related to the size of the carbocycles and
substituent nature. Taking into account that these
compounds were not described previously, their mass
spectra were analyzed in more detail with a view to
further identification of related structures with differ-
ent sizes of the rings.
have attracted increased interest due to their pro-
nounced antimalarial activity [9–11]. Therefore, they
are considered to be synthetic analogs of the natural
peroxide artemisinin which is the main drug for the
treatment of malaria [12–22]. Tetracyclic ozonides 6–
1
0 synthesized by us possess the same pharmacophoric
fragment as in structure A.
Mass spectrometric study showed that compounds
Me
6–10 under atmospheric pressure chemical ionization
+
(
APCI) give rise to quasi-molecular ions [M + H]
O
which undergo fragmentation via successive elimina-
2
O
O
tion of three water molecules. The MS fragmentation
O
OH
5a
pattern of the quasi-molecular ions of 6–10 differs
from that observed for ozonides derived from linear
olefins [23, 24]. The MS spectra of compounds 6
and 7 containing two five- or six-membered alicyclic
fragments displayed signals arising from elimination
of similar neutral species from the [M + H] ions
A
2
1
,5-Diketones 1–4 exist in the stable diketone form,
whereas compound 5 has cyclic structure 5a which
undergoes thermal ring opening at 180–186°C. Com-
pounds 6–10 are highly resistant to heat; they do not
decompose on melting (110–130°C), burn with a flash
in an open flame, and give a positive test for peroxides
with a solution of potassium iodide.
+
(
Table 1).
However, the spectra of 6 and 7 showed some dif-
ferences which may be related to specific fragmenta-
tions of the five- and six-membered carbocycles. Thus,
The structure of ozonides 6–10 was confirmed by
IR, NMR, and mass spectra. The IR spectra of 6–10
+
the [M + H] ion of 6 loses C H O and C H O species,
2
6
3
6
2
while elimination of 2 H O + C H , C H O, and
(
KBr) showed no carbonyl stretching bands typical of
2
3
6
6
10
2
H O + C H is observed from the quasi-molecular ion
2 5 6
initial diketones, but contained bands in the region
–
1
9
00–970 cm due to peroxide O–O bond. Their
C NMR spectra (DEPT-135) displayed signals typi-
of 7. The C H O species is likely to consist of water
2 6
1
3
and ethylene molecules, and elimination of ethylene is
possible only in combination with water molecule.
cal of quaternary carbon atoms linked to two oxygen
atoms (spiro carbon atoms) at δ 122, 108–110, and
Presumably, joint elimination of C
molecule is responsible for the loss of the C H O
3 6 2
2 6
H O and CO
C
1
12 ppm for cyclopentane, cyclohexane, and cyclo-
heptane derivatives. The number and position of
signals from quaternary, tertiary, secondary, and pri-
mary carbon atoms were consistent with the proposed
structures. The number of signals in the spectra of 6, 7,
and 10 was twice as low as the number of carbons in
their molecules due to their symmetric structure
species. This becomes possible when elimination of
one water molecule due to cleavage of the oxygen–
oxygen bond in the peroxide fragment is followed by
carbonylation of the carbon atom at the remaining
oxygen atom in the resulting ion. Elimination of
propylene in combination with two water molecules, as
well as of cyclopentadiene, also together with two
water molecules, may be noted as specific for the
decomposition of six-membered alicyclic fragments of
1
corresponding to meso form. The H NMR spectra of
6
–10 were low informative; all proton signals appeared
as a multiplet in the region δ 1.07–1.9 ppm [9–11].
O
O
O
O
O
O
O
O
O
6
7
8
Me
O
O
O
O
O
O
9
10
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 55 No. 1 2019

NEW TETRACYCLIC SPIRO-1,2,4-TRIOXOLANES (OZONIDES).
Table 1. Mass spectra (APCI) of compounds 6–10
103
Monoisotopic
molecular weight
2
MS
Comp.
no.
APCI, m/z (I, %)
Formula
+
fragment ion [M + H – X]
found calculated
X
m/z (I, %)
composition
+
+
0
0
6
7
[M + H] , 197.1191 (100);
196.1118 196.1099 C11
H
H
16
O
O
3
179.1093 (100) [C11
H
H
H
15
O
2
]
2
H O
+
+
[
[
[
2
M + H – H O] , 179.1096 (42);
M + H – 2H
M + H – 3H
1
1
1
1
1
1
61.0963 (30) [C11
43.0906 (8)
51.0791 (41) [C
33.0630 (83) [C
23.0773 (15) [C
05.0404 (2.8) [C
13O]
2H
3H
2
O
O
+
O] , 161.0983 (11.5);
+
2
[C11
11
]
2
+
2
O] , 143.0904 (1.8)
+
H
11
O
O]
2
+
]
2 6
C H O
9
9
H
9
2H
2
O + C
2
H
H
4
8
+
H
11O]
3 6 2
C H O
8
+
7
H
4
O]
2
2H O + C
4
+
+
[M + H] , 225.1455 (100);
224.1382 224.1412 C13
20
3
207.1357 (100) [C13
H
H
H
H
19
O
2
]
2
H O
+
+
[
[
[
2
M + H – H O] , 207.1355 (72);
M + H – 2H
M + H – 3H
1
1
1
1
1
1
1
89.1232 (67) [C13
71.1183 (6.5) [C13
17O]
2H
3H
2H
2
2
2
O
O
O + C
+
O] , 189.1229 (19);
O] , 171.1172 (3.5)
+
2
15
]
+
2
+
61.0931 (7)
[C11
13O]
2
H
4
+
47.0774 (2.5) [C10
H
11O] 2H
2
O + C
3 6
H
+
33.0648 (0.3) [C H O]
2H
2
O + C H
4 8
9
9
+
27.0751 (0.1) [C
7
H
11
11
17
O
2
]
6 10
C H O
+
23.0774 (2.7) [C H O]
2H
2
O + C
5
H
6
8
+
+
0
0
1
8
9
0
[M + H] , 211.1352 (100);
210.1279 210.1256 C12
238.1549 238.1569 C14
238.1570 238.1569 C14
H
H
H
18
22
22
O
O
O
3
3
3
193.1287 (100) [C12
H
O
2
]
H O
2
+
+
[
[
[
2
M + H – H O] , 193.1287 (35);
M + H – 2H
M + H – 3H
1
1
1
1
1
1
1
1
1
75.1168 (44) [C12
57.1063 (1.5) [C12
H
H
H
H
15O]
2H
3H
2
O
O
+
O] , 175.1165 (10);
O] , 157.1067 (1.3)
+
2
13
13
]
2
+
2
+
65.0951 (1)
[C10
O
2
]
2 6
C H O
+
47.0771 (16) [C10
11O]
2H
2
O + C
2
H
4
+
37.0972 (0.1) [C
33.0681 (0.3) [C
9
9
8
6
7
H
H
H
H
H
13O]
3 6 2
C H O
+
+
9
7
9
O]
O]
2H
2H
2
O + C
O + C
3
H
H
6
19.0545 (1)
13.0685 (3)
[C
[C
2
4
8
+
O
O]
2
]
6 10
C H O
+
09.0661 (0.5) [C
9
2H
2
O + C
5
H
6
+
+
[M + H] , 239.1612 (74);
221.1464 (100) [C14
H
21
O
2
]
2
H O
+
+
[
[
[
2
M + H – H O] , 221.1462 (100);
M + H – 2H
M + H – 3H
2
1
1
1
1
1
1
1
1
03.1385 (45) [C14
85.1273 (15) [C14
H
H
H
H
H
19O]
2H
3H
2H
2H
2H
2
2
2
2
2
O
O
O + C
O + C
O + C
+
O] , 203.1368 (10.5);
O] , 185.1277 (2.5)
+
2
17
]
+
2
+
+
+
75.1041 (8)
[C12
15O]
13O]
2
H
3
H
4
H
4
6
8
61.0932 (0.5) [C11
47.0776 (0.1) [C10
11O]
+
43.0841 (1)
37.0933 (31) [C
23.0771 (2)
[C
19.1085 (3.5) [C
[C11
H
13O]
11
]
3H
2H
2H
2
O + C
O + C
O+C
3 6
H
+
9
H
2
5 6
H
+
8
H
H
11O]
2
6
H
8
+
6
14
O
2
]
C
8
H
8
O
+
+
[M + H] , 239.1643 (45);
221.1566 (100) [C14
H
O
]
H
2
O
21
2
+
+
[
[
[
2
M + H – H O] , 221.1535 (100);
M + H – 2H
M + H – 3H
2
1
1
1
1
1
03.1450 (82) [C14
85.1322 (10.5) [C14
H
H
H
H
19O]
2H
3H
2H
2H
2
2
2
2
O
O
O + C
O + C
+
O] , 203.1410 (9);
O] , 185.1280 (1.2)
+
2
17
]
+
2
+
+
75.1114 (7)
61.1001 (6.5) [C11
59.1205 (0.5) [C12
37.0998 (9) [C
[C12
15O]
2
H
H
4
13O]
3
6
+
H
15
]
3H
2H
O + C
O + C H
2 2
H
2
+
9
H
13O]
2
5 6
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 55 No. 1 2019

1
04
AKIMOVA et al.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 55 No. 1 2019

NEW TETRACYCLIC SPIRO-1,2,4-TRIOXOLANES (OZONIDES).
105
2
compound 7. Furthermore, the MS spectrum of 7
are typical of fragmentation of the quasi-molecular ion
of 7 with two cyclohexane rings. In the MS spectra of
2
showed elimination of C H O species which can be
6
10
interpreted as joint expulsion of water and cyclo-
hexadiene.
9 and 10, relatively abundant ions are those arising
from decomposition of the six-membered ring. How-
ever, the quasi-molecular ion of 9 loses a neutral
+
The tandem mass spectrum of the [M + H] ion
C H O species which, as noted above, is specific for
of 8 contained signals both common and specific for
8
8
2
seven-membered alicyclic fragment. On the other
compounds 6 and 7. The MS fragmentation pattern of
+
hand, fragmentation of the [M + H] ion of 10 involves
the quasi-molecular ion of 9 was characterized by
formation of all ions typical of decomposition of the
six-membered ring and ions specific for this com-
pound, namely those resulting from simultaneous loss
of two water molecules and cyclohexadiene and of
three water molecules in combination with propylene.
In addition, a ion peak corresponding to elimination
of a C H O species was observed, which is likely to
elimination of acetylene molecule together with three
water molecules, indicating the presence of a methyl
group in the bridging moiety.
Thus, peculiar features of fragmentation of the
quasi-molecular ions of compounds 6–10 (Table 2)
provide the possibility of identifying ozonides con-
taining five-, six-, and seven-membered rings.
8
8
indicate the presence of a seven-membered ring in
the parent ion.
EXPERIMENTAL
2
Unlike compounds 6–9, the MS fragmentation of
+
The IR spectra were recorded in KBr on a Bruker
the [M + H] ion of 10 involved joint elimination of
1
13
Vertex-70 spectrometer. The H and C NMR spectra
were measured on a Bruker Avance II 400 spectrom-
eter (400 and 125 MHz, respectively) at 30°C using
three water molecules and acetylene. Presumably, this
is determined by the presence of a methyl group in the
bridging fragment connecting the cyclohexane rings.
Elimination of acetylene molecule provides the pos-
sibility of directly linking the rings to each other, and
the loss of three water molecule leads to the formation
of a stable conjugated bicyclic structure.
CDCl as solvent and tetramethylsilane as internal
3
1
3
standard. Signals in the C NMR spectra were
assigned using DEPT-135 pulse sequence. The mass
spectra were obtained on a Shimadzu LCMS-IT-TOF
instrument (atmospheric pressure chemical ionization;
simultaneous positive and negative ion detection; ion
source temperature 300°C; nebulizer gas nitrogen,
flow rate 2 L/min; drying gas pressure 20 kPa; source
voltage +4.5 kV for positive ions and –3.8 kV for
negative ions; a.m.u. range 100–1500). The melting
points were measured in sealed capillaries with
a Buchi Melting Point V-540 apparatus. The progress
of reactions was monitored by TLC on Silufol plates
Comparison of the compositions of ions formed by
2
+
the MS fragmentation of the [M + H] ion of 6–10
Table 2) showed preferential formation (apart from
(
+
the dehydration processes) of [C H O] ions from 6,
, and 9, [C H O] ions from 6, 7, and 8, and
C H O] species from 8, 9, and 10. Thus, we can
8
11
+
7
[
9 9
+
9 13
exclude ions resulting from simultaneous loss of two
water molecules and ethylene, which are highly abun-
dant in the spectra of all compounds but are not
structure-specific, as well as ions resulting from loss of
two water molecules and butene.
(
eluent hexane–ethyl acetate, 5:1), following the dis-
appearance of the initial diketone.
General procedure for the synthesis of ozonides
–8 and 10. A solution of 0.001 mol of diketone 1, 2,
, or 5 in 2 mL of diethyl ether was cooled in an ice
bath, 0.18 mL (0.015 mol) of 30% aqueous hydrogen
peroxide was added with stirring, and a solution of
.15 mL (0.001 mol) of boron trifluoride–diethyl ether
complex in 2 mL of diethyl ether was added dropwise,
maintaining the temperature at 0°C. The mixture was
left to stand for 12 h at 20°C and neutralized with
a saturated solution of sodium carbonate, and the
organic layer was separated, washed with water (3×
It is seen that the presence in the mass spectra of
intense ion peaks due to elimination of one water
molecule and ethylene both jointly with CO molecule
and without it indicates that the quasi-molecular ion
contains a five-membered carbocycle. Analogous com-
pounds with a cyclohexane fragment are characterized
by preferential elimination of two water molecules and
propylene, as well as of two water molecules and
6
3
0
+
cyclopentadiene, from the [M + H] ion. Compound 8
containing five- and six-membered alicyclic fragments
shows reduced intensities of ring size-specific ion
peaks (see above). Simultaneously, increased intensity
is observed for ion peaks corresponding to joint
elimination of water molecule and cyclohexadiene that
3
mL), dried over MgSO , and evaporated at 20°C.
4
1
,4 4,8
2,3,14-Trioxatetracyclo[8.3.0.1 .0 ]tetradecane
(6). Yield 86%, white powder, mp 122–124°C (from
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 55 No. 1 2019

1
06
AKIMOVA et al.
–
1
EtOH). IR spectrum, ν, cm : 2939, 2860, 1149, 1137,
was then treated according to the general procedure.
Yield 168 mg (70%), white powder, mp 121–123.5°C
1
1
072, 922. H NMR spectrum: δ 1.18–2.11 ppm, m.
C NMR spectrum, δ , ppm: 122.42 (OCO), 42.36
1
3
–1
(from EtOH). IR spectrum, ν, cm : 2956, 2935, 2863,
C
1
(
CH); 32.05, 31.13, 22.40 (CH ). Mass spectrum:
1205, 1190, 1126, 1062, 977, 923. H NMR spectrum,
2
+
m/z 197.1191 [M + H] . C H O . Calculated:
δ, ppm: 0.90–0.91 d (3H, CH ), 1.07–1.90 m (19H).
1
1
16
3
3
1
3
M 196.1099.
,3,16-Trioxatetracyclo[9.4.0.1 .0 ]hexadecane
7). Yield 92%, white plates, mp 118.5–120°C (from
EtOH); published data [2]: mp 119–120°C (from
C NMR spectrum, δ
C
, ppm: 108.24 (OCO); 41.21,
1
,4 4,9
35.51 (CH); 32.92, 28.17, 25.29, 23.86 (CH ); 15.98
2
2
+
(
CH ). Mass spectrum: m/z 239.1643 [M + H] .
3
(
C H O . Calculated: M 238.1569.
1
4
22
3
–
1
MeOH). IR spectrum, ν, cm : 2938, 2857, 1269, 1157,
1
CONFLICT OF INTERESTS
1
1
138, 1066, 972, 923. H NMR spectrum: δ 1.18–
.91 ppm, m. C NMR spectrum, δ , ppm: 108.5
OCO), 41.21 (CH); 32.56, 31.59, 30.75, 25.11, 23.96
CH ). Mass spectrum: m/z 225.1455 [M + H] .
1
3
C
The authors declare the absence of conflict of
interests.
(
(
+
2
C H O . Calculated: M 224.1412
1
3
20
3
REFERENCES
1
,4 4,9
2
,3,15-Trioxatetracyclo[9.3.0.1 .0 ]penta-
–
1
decane (8). Yield 81%, oil. IR spectrum, ν, cm :
1. Akimova, T.I. and Soldatkina, O.A., RU Patent
1
2
940, 2867, 1446, 1349, 1099, 1045, 928. H NMR
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1
3
spectrum: δ 1.18–2.1 ppm, m. C NMR spectrum, δ ,
C
ppm: 123.62, 111.21 (OCO); 47.79, 43.57 (CH);
3
4.52, 34.15, 33.43, 32.52, 30.44, 25.58, 23.66, 22.58
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Nikishin, G.I., Alabugin, I.V., and Terent’ev, A.O.,
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+
(
CH ). Mass spectrum: m/z 211.1352 [M + H] .
2
C H O . Calculated: M 210.1256.
1
2
18
3
1
,4 4,10
2
,3,17-Trioxatetracyclo[10.4.0.1 .0 ]hepta-
4
5
6
. Yaremenko, I.A., Gomes, G.P., Radulov, P.S.,
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Eur. J., 2014, vol. 20, p. 10160.
decane (9). Diketone 4, 100 mg (0.45 mmol), was
dissolved in 2 mL of ethanol, 0.2 mL (1.64 mmol)
of 30% aqueous hydrogen peroxide and a drop of
3
3% H SO (0.4 mmol) were added, and the mixture
2 4
was left to stand for 12 h at room temperature. The
product was isolated according to the general proce-
dure (see above). Yield 60 mg (56%), needles, mp 60–
6
1
–
1
1°C (from EtOH). IR spectrum, ν, cm : 2938, 2858,
. Criegee, R. and Lohaus, G., Chem. Ber., 1953, vol. 86,
1
157, 1138, 1066, 967, 923. H NMR spectrum:
p. 1.
1
3
δ 1.18–1.91 ppm, m. C NMR spectrum, δ , ppm:
C
7. Griesbaum, K., Liu, X., and Dong, Y., Tetrahedron,
1
3
12.8, 109.6 (OCO); 44.89, 41.50 (CH); 35.99, 33.11,
1997, vol. 53, p. 5463.
. Griesbaum, K., Liu, X., Kassiaris, A., and Scherer, M.,
Justus Liebigs Ann. Chem., 1997, p. 1381.
2.34, 30.36, 30.15, 30.15, 29.06, 25.10, 23.89, 22.53
8
+
(
CH ). Mass spectrum: m/z 239.1612 [M + H] .
2
C H O . Calculated: M 238.1569
1
4
22
3
9. Barton, V., Ward, S.A., Chadwick, J., Hill, A., and
1
,4 4,9
O’Neill, P.M., J. Med. Chem., 2010, vol. 53, p. 4555.
1
0-Methyl-2,3,16-trioxatetracyclo[9.4.0.1 .0 ]-
1
1
0. Dong, Y., Chollet, J., Matile, H., Charman, S.A.,
Chiu, F.C.K., Charman, W.N., Scorneaux, B.,
Urwyler, H., Tomas, J.S., Scheurer, C., Snyder, C.,
Dorn, A., Wang, X., Karle, J.M., Tang, Y., Wittlin, S.,
Brun, R., and Vennerstrom, J.L., J. Med. Chem., 2005,
vol. 48, p. 4953.
1. Dong, Y., Wittlin, S., Sriraghavan, K., Chollet, J.,
Charman, S.A., Charman, W.N., Scheurer, C.,
Urwyler, H., Tomas, J.S., Snyder, C., Creek, D.J.,
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Padmanilayam, M., Tang, Y., Dorn, A., Brun, R., and
Vennerstrom, J.L., J. Med. Chem., 2010, vol. 53, p. 481.
hexadecane (10). Ketol 5a, 200 mg (0.9 mmol), was
melted at 180–186°C on a metal bath and was kept for
3
0 min at that temperature. Diketone 5 thus obtained
was dissolved in 2 mL of diethyl ether, and the
undissolved material (ketol 5a, 44 mg) was filtered off.
The filtrate was placed in an ice bath, 0.18 mL
(
15 mmol) of 30% aqueous hydrogen peroxide was
added, and a solution of 0.15 mL (1 mmol) of boron
trifluoride–diethyl ether complex in 2 mL of diethyl
ether was added dropwise, maintaining the temperature
at 0°C. The mixture was stirred for 12 h at 20°C and
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 55 No. 1 2019

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