Ring-ring tautomerism in the series of isoxazolidine and δ2-isoxazoline derivatives

Article abstract of DOI:10.1023/A:1019981606118

We have studied the reaction of 5-hydroxy-3,3,5-trimethylisoxazolidine and 5-hydroxy-3,5-dimethyl-Δ²isoxazoline with derivatives of thiosemicarbazide and also thiocarbonohydrazine. Both reactions serve as a method for synthesis of the previousl

Full text of DOI:10.1023/A:1019981606118

Chemistry of Heterocyclic Compounds, Vol. 38, No. 6, 2002
RING–RING TAUTOMERISM
IN THE SERIES OF ISOXAZOLIDINE
AND ∆2-ISOXAZOLINE DERIVATIVES
A. Yu. Ershov
We have studied the reaction of 5-hydroxy-3,3,5-trimethylisoxazolidine and 5-hydroxy-3,5-dimethyl-∆2-
isoxazoline with derivatives of thiosemicarbazide and also thiocarbonohydrazine. Both reactions serve
as a method for synthesis of the previously unknown 5-thiosemicarbazido(thiocarbonohydrazino)-
isoxazolidines and -∆2-isoxazolines. 1H and 13C NMR spectroscopy revealed a tendency of the indicated
compounds toward ring–chain and ring–ring tautomeric conversions in solutions involving the
1,2,4-triazolidine, ∆2-pyrazoline, and 1,2,4,5-tetrazine rings.
Keywords: isoxazolidines, ∆2-isoxazolines, ∆2-pyrazolines, tetrahydro-1,2,4,5-tetrazine-2-thiones,
1,2,4-triazolidine-3-thiones, ring–ring tautomerism.
Interest in 5-substituted isoxazolidines and ∆2-isoxazolines in which the substituent, most often OH or
NHR, is directly bonded to the isoxazole ring, is due to the possibility of novel isomeric (tautomeric)
conversions, since the presence of the cyclic hemiacetal moiety in such molecules suggests an elevated tendency
toward rupture of the C(5)–O bond to form linear structures (ring–chain isomerism or tautomerism) [1]. In a
number of cases, we may also observe subsequent intramolecular addition of functional nucleophilic moieties at
the polar bonds C=O or C=N contained in the resulting linear molecules, and formation of novel cyclic
structures (ring–ring isomerism or tautomerism) [2]. This may be illustrated by the examples we observed
recently of recyclization of 1-arylpyrimidin-2(1H)-ones to 5-arylureido-∆2-isoxazolines when they are treated
with hydroxylamine [3] (a typical variant of the ANRORC process [4]), and also ring–ring tautomeric equilibria
in solutions of the 5-hydrazino-∆2-isoxazoline – 5-hydroxyamino-∆2-pyrazoline [5] or isoxazolidine –
tetrahydropyrimidine-2(1H)-thione [6] type.
Introduction of a thiosemicarbazide or thiocarbonohydrazine functional group at the 5 position of the
isoxazole ring suggests the possibility of complex tautomeric (isomeric) conversions involving additional cyclic
forms in addition to isoxazoline (∆2-isoxazoline) and linear forms. The tendency toward cyclization of
2(4)-substituted thiosemicarbazones and thiocarbohydrazones to derivatives of 1,2,4-triazolidine-3-thione and
tetrahydro-1,2,4,5-tetrazine-2-thione respectively is a general property of compounds in these classes [7-9].
13
The basic criterion for selecting a specific cyclic structure is the C NMR spectroscopy data. Thus for
the isoxazolidine (∆2-isoxazoline) form, the appearance of a signal from the sp3-hybridized C(5) atom at 100 ppm
(N,C,O-environment) is characteristic [5]. In the five-membered triazolidine ring, the analogous C(5) atom has a
chemical shift at 85 ppm (N,C,N-environment) [7], while in the six-membered tetrazine ring, the signal from the
__________________________________________________________________________________________
Institute of High-Molecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russia;
e-mail: ershov@hq.macro.ru. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 828-836,
June, 2002. Original article submitted March 6, 2000.
730
0009-3122/02/3806-0730$27.00©2002 Plenum Publishing Corporation

C(6) atom is observed upfield at about 70 ppm [9]. Transition of the cyclic structures to linear structures should
lead to disappearance of the "hemiacetal" signals in the 70-100 ppm region and the appearance in the carbon
spectra downfield of a signal at 150 ppm (C=N).
Guided by these considerations, we studied the reaction of 5-hydroxy-3,3,5-trimethylisoxazolidine (1)
with 2(4)-substituted thiosemicarbazides, and also thiocarbonohydrazine.
We found that reaction of the indicated compounds occurs only after prolonged boiling of the reagents
in ethanol in the presence of catalytic amounts of acetic acid (see Experimental and Table 1) and leads to
formation of the compounds 2a-c in good yield.
H
N
Me
S
Me
NH
Me
CH2CMe2
Me
HO
R2 = NH2
NH2NR1CSNHR2
Me
HN
NH
N
H
O
1
HNOH
D
Me
Me
HNOH
R2HNCR1N
S
N
Me
R1
S
Me
H
N
N
Me
R1NHN
Me
NH
Z,E-B
O
N
CH2CMe2
R2
NHR2
2a–c
S
HNOH
A
C
2 a R1 = H, R2 = Ph; b R1 = Me, R2 = Ph; c R1 = H, R2 = NH2
Compound 2a exists in the crystalline state in the cyclic isoxazolidine form A, which can be decided
from the appearance of a signal from the C(5) atom at 100 ppm in the 13C NMR spectrum taken in the solid phase
(Table 2). Dissolution of compound 2a in DMSO-d6 leads to the appearance of (in addition to signals from the
cyclic form A) signals corresponding to the linear form B, which is represented by two Z,E-configuration
isomers. The content of the isoxazolidine form A increases as we go from polar solvents to the low-polar CDCl3
(Table 3).
TABLE 1. Physical and Chemical Characteristics of Compounds 2a-c and 6a-b
Found, %
Calculated, %
H
Form in
crystalline
state
——————
Com-
pound
Empirical
formula
Yield,
%
mp, °С
C
N
С13H20N4OS
С14H22N4OS
С14H22N4OS
С7H17N5OS
С6H12N4OS
С6H13N5OS
55.73
55.69
7.22
7.19
20.05
19.98
134-136
155-157
151-153
142-144
123-125
178-180
45
95
40
65
50
60
2а
А
А
C
А
А
А
57.16
7.49
19.07
2b
57.11
7.53
19.03
57.07
57.11
7.58
7.53
18.98
19.03
38.30
7.76
31.87
2c
6а
6b
38.34
7.81
31.93
38.32
38.28
6.37
6.43
29.80
29.76
35.41
6.48
34.39
35.45
6.45
34.45
731

TABLE 2. 13C NMR Spectra of Compounds 2a-c and 6a,b
13C NMR spectrum, δ, ppm (25°C, 72 h after dissolution)
С(3); С–N or С=NOH C(4) or СН2 С(5); С(6) or C=N
Compound
Solvent
Form
СН3
С=S or C(3)
Solid phase
DMSO-d6
22.9 (2СН3); 27.1
22.4; 24.9; 27.7
19.2; 23.8; 25.2
23.1; 26.4*
23.3 (2СН3); 28.6; 43.9
21.5 (2СН3); 29.4; 38.0
21.6; 23.7; 27.0; 42.4
23.4; 25.7; 27.8; 41.7
23.6 (2СН3); 29.7
22.4; 24.1; 27.9
18.4; 25.2*
24.7; 26.5*
15.1; 22.1
14.6; 21.0
15.3; 20.5
14.5; 24.5
13.1; 22.7
12.0; 21.8
62.4 (С(3)
61.4 (С(3)
57.3 (C–N)
56.5 (C–N)
)
)
52.7 (C(4)
51.9 (C(4)
46.3 (CH2)
37.6 (CH2)
)
)
97.2 (C(5)
99.2 (C(5)
154.3 (C=N)
154.0 (C=N)
)
)
178.1 (C=S)
180.7 (C=S)
179.5 (C=S)
176.8 (C=S)
178.3 (C=S)
2a
A
A
E-B
Z-B
A
Solid phase
Solid phase
DMSO-d6*2
62.4 (С(3)
67.3 (C–N)
61.0 (С(3)
66.4 (C–N)
)
53.8 (C(4)
53.0 (CH2)
53.3 (C(4)
52.6 (CH2)
)
99.7 (C(5)
88.4 (C(5)
)
)
2b
2c
175.0 (C(3)
181.6 (C=S)
179.3 (C(3)
)
C
)
)
100.3 (C(5)
87.3 (C(5)
100.0 (C(5)
100.1 (C(5)
153.2 (C=N)
)
A
)
)
C
Solid phase
DMF-d7
61.8 (С(3)
62.0 (С(3)
)
)
54.1 (C(4)
52.7 (C(4)
)
)
)
181.9 (C=S)
182.9 (C=S)
178.1 (C=S)
A
)
A
E-B
D
58.1 (C–N)
62.2 (C–N)
162.8 (С(3)
154.5 (С(3)
154.1 (С(3)
156.4 (С(3)
156.7 (С(3)
46.6 (CH2)
40.4 (CH2)
69.5 (C(6)
96.4 (C(5)
97.0 (C(5)
86.7 (C(5)
97.2 (C(5)
98.8 (C(5)
75.3 (C(6)
)
)
)
)
)
)
)
172.6 (C(3))
Solid phase
DMF-d7
)
)
)
)
)
46.2 (C(4)
44.2 (C(4)
46.8 (C(4)
46.5 (C(4)
45.7 (C(4)
)
)
)
)
)
182.2 (C=S)
182.8 (C=S)
178.7 (C=S)
179.9 (C=S)
180.1 (C=S)
6a
6b
A
A
E
Solid phase
DMF-d7
A
A
153.6 (С=NOH)
45.3 (CH2)
170.6 (C(3))
D
_______
* Obscured by solvent signals or by signals from the major form.
*2 Isomer composition of forms A and C.

TABLE 3. Tautomeric Composition and 1H NMR Spectra of Compounds 2a-c and 6a,b
Tautomeric
1H NMR spectrum, δ, ppm (spin–spin coupling constant, J, Hz) (25°C, 72 h after dissolution)
Compound
Solvent
CDCl3
composition, %
СН3, s
H(4), AB system or CH2, s
NH or ОН, br. s
1.18 (2СН3); 1.47
1.09 (2СН3); 2.01
1.11; 1.17;1.38
1.01 (2СН3); 2.03
1.08 (2СН3); 2.03
1.24 (2СН3); 1.46
1.59; 1.67; 1.98
1.08; 1.13; 1.31
1.01 (2CH3); 1.93
1.03 (2CH3); 1.93
1.15; 1.22; 1.40
1.10 (2СН3); 2.12
1.39; 1.42; 1.47
1.52; 1.93
1.83; 2.01 (J = 13)
2.47 s
1.79; 1.94 (J = 12)
2.41 s
5.53; 7.79; 9.01
2а
А
(90)
E-B (10)
А (65)
E-B (25)
Z-B (10)
А (40)
C (60)
А (70)
E-B (20)
Z-B (10)
A (75)
E-B (10)
D (15)
A (90)
E (10)
4.95; 6.21; 9.14
5.96; 8.75; 9.69
6.39; 7.01; 9.83
7.91;10.25*
4.83; 5.51; 9.35
4.76; 8.37; 9.22
5.51; 8.66; 9.83
6.45; 9.02*
DMSO-d6
2.48 s
CDCl3*2
1.85; 2.12 (J = 13)
2.39; 2.54 (J = 13)
1.69; 1.87 (J = 13)
2.32 s
2.40 s
1.81; 2.02 (J = 13)
2.47 s
2b
2c
DMSO-d6
7.25*
4.38; 5.87; 8.61
6.78*
DMF-d7
2.51 s
5.63*
DMF-d7
2.82; 2.97 (J = 18)
3.02 (J = 18)*
2.81; 3.06 (J = 18)
2.70; 3.29 (J = 19)
2.81; 2.93 (J = 17)
3.27 s
6.11; 7.63; 9.87
5.78; 7.86; 8.12
6.34; 9.22; 10.36
5.84; 8.61; 9.71
4.39; 5,91; 8.85
5.94; 9.24
6а
1.84; 2.06
1.63; 1.78
1.74; 1.93
1.46; 1.87
Pyridine-d5
DMF-d7
A (90)
E (10)
A (85)
D (15)
A (90)
D (10)
6b
1.38; 1.79
Pyridine-d5
1.57; 1.74
1.54; 1.84
2.74; 3.01 (J = 18)
3.45 s
5.25; 7.08; 10.38
5.70; 9.76
_______
* Obscured by solvent signals or signals from the major form.
*2 Singlet signals from protons of the CH3N group of forms A and C are located at 3.68 ppm and 3.73 ppm respectively.

The structure of compound 2b proved to be interesting: this is the product of condensation of
5-hydroxyisoxazolidine 1 with 2-methyl-4-phenylthiosemicarbazide. Thus during the synthesis, we can isolate a
product that has the 1,2,4-triazolidine structure C in the crystalline state. This is unambiguously indicated by the
presence of a signal at 88.4 ppm (C(5)) in the 13C NMR spectrum taken in the solid phase (Table 2). As is
completely consistent with the triazolidine structure C of compound 2b, signals are also found in the carbon
spectrum for the model 2,5,5-trimethyl-4-phenyl-1,2,4-triazolidine-3-thione (2-methyl-4-phenylthio-
semicarbazone of acetone) 3 (see Experimental). The isoxazolidine form A, to which correspond two
asymmetric H(4) doublets at 1.85-2.12 ppm (AB system) in the NMR spectrum (Table 3 and Fig. 1), accumulates
gradually in a solution of compound 2b in CDCl3. After 14 days, form A becomes the only one in solution, and
it can be isolated in pure form by removing the solvent. In other words, we observed the recyclization of
1,2,4-triazolidine C → isoxazolidine A occurring over time.
5-(Thiocarbonohydrazino)isoxazolidine 2c in a freshly prepared solution in DMSO-d6, is represented by
a tautomeric mixture of cyclic A and linear B forms, and the latter exists in the form of steric Z,E-isomers. The
existence in solutions of compounds 2a,c in the linear form B, as generally occurs for most hydrazones tending
toward ring–chain tautomeric processes, is explained by stabilization of the hydrazone moiety as a result of
p,π-conjugation [10]. After 3 days, additional singlet signals from methyl protons at 1.39 ppm, 1.42 ppm, and
1.47 ppm appear in the NMR spectrum of compound 2c in DMF-d7, corresponding to one more cyclic form
(Table 3) to which, based on comparison of the spectral characteristics with literature analogs [8, 9], should be
assigned the tetrahydro-1,2,4,5-tetrazine-3-thione structure D. The 13C NMR spectroscopy data (69.5 ppm (C(6))
and 172.6 ppm (C(2))) are fully consistent with the proposed cyclic structure of form D for compound 2c
(Table 2).
Me
Me
H
N
Me
NH
Me
HO
Me
Me
N
Me
Me
Me
HOHN
S
N
N
O
HNOH
A
Ph
B
Z,E-
4
3
Thus we should note an elevated tendency of compounds 2a-c toward existence in cyclic forms. In this
they are radically different from 4-hydroxyamino-4-methyl-2-pentanone oxime (4), their closest structural
analog. Compound 4, as we know [11], in solution completely exists in the linear form B, represented by two
4.0
3.5
3.0
2.5
2.0
1.5
, ppm
δ
Fig. 1. 1H NMR spectrum of compound 2b in CDCl3 (72 h after dissolution):
1) signals from form A; 2) signals from form C.
734

configuration isomers, and does not exhibit tendencies toward transition to the alternative
5-hydroxyaminoisoxazolidine form A. An analogous tendency toward retention of the oxime moiety has also
been noted, by the way, for 1,3-hydroxyamino oximes of other carbonyl compounds [12].
In this regard, 1,3-hydroxyamino oximes are similar to the bisoximes of β-dicarbonyl compounds we
studied earlier [13], which can be considered as 5-hydroxyamino-∆2-isoxazolines only under special conditions
and within very narrow limits. Also note that other 1,3-bifunctional derivatives of β-dicarbonyl compounds
(bishydrazones, iminohydrazones (oximes), and hydrazone oximes) exist exclusively in cyclic 5-amino-
(hydrazino)-∆2-isoxazoline or -∆2-pyrazoline forms [3, 5, 14].
Accordingly we should turn our attention to the unanswered question concerning the structure of one
more class of bifunctional derivatives of β-dicarbonyl compounds: 1,3-thiosemicarbazone
(-thiocarbonohydrazone) oximes. Study of this question is especially important because in this case, it is
possible to realize additional tetrahydro-1,2,4,5-tetrazine D and ∆2-pyrazoline E forms in addition to the
∆2-isoxazoline form A.
For synthesis of 1,3-thiosemicarbazone(thiocarbonohydrazone) oximes, we carried out the reaction of
3,5-dimethyl-5-hydroxy-∆2-isoxazoline 5 with thiosemicarbazide and thiocarbonohydrazide, as a result of which
compounds 6a,b are formed in high yield (see Experimental and Table 1).
H
S
N
R = NH2
NH
Me
Me
HN
Me
NOH
Me
N
H
CH2CMe
Me
HO
NH2NHCSNHR
N
O
NOH
RHNCHNN
S
D
5
Me
B
Me
HOHN
Me
N
Me
HNHN
N
N
O
S
NHR
E
S
NHR
A
6a,b
6 a R = H, b R = NH2
In the crystalline state, compounds 6a,b exist in isoxazoline form A, which is indicated by the signals
from the C(5) atoms at 96 ppm and 97 ppm respectively in the 13C NMR spectra taken in the solid phase
1
(Table 2). 3 days after dissolution in pyridine, in the H NMR spectrum of compound 6a, along with two
asymmetric doublets for the H(4) proton forming a typical AB system at 2.80-3.06 ppm, we see an additional AB
system indicating the presence of one more cyclic form in solution. The existence of a second cyclic form in
solution is also confirmed by the 13C NMR spectrum (Table 2).
1
13
The appearance of doubled signals in the H and C NMR spectra of compound 6a can be associated
only with a ring–ring tautomeric equilibrium in solution between the isoxazoline form A and the pyrazoline
form E. We already observed recently [5] an analogous variant of tautomeric equilibrium for the example of
1,3-alkanoylhydrazone oximes of acetylacetone, the closest structural analogs of compound 6a. The spectral
characteristics of the previously studied [15] series of 3,5-dimethyl-5-thiosemicarbazido-∆2-pyrazolines are also
.
completely consistent with pyrazoline structure E of compound 6a. The tautomeric equilibrium A
E is
shifted strongly toward the isoxazoline form A side, and the percentage of the minor pyrazoline form E in all the
solvents used (pyridine-d5, acetone-d6, DMSO, DMF, acetone, pyridine) does not exceed 5% to 10% (Table 3).
We found that compound 6b also tends toward ring–ring tautomerism in solution. All the spectral
parameters indicate the presence in DMF solution, as in the case of compound 2c, of the 1,2,4,5-tetrazine-3-
thione form D. We should note that we did not observe formation in solution of one more possible cyclic form,
735

1
the pyrazoline form E, which was monitored by taking the H NMR spectra of compound 6b in different
solvents with variation of the temperature and time parameters over a broad range.
Thus ring–ring tautomerism is a characteristic feature of 1,3-thiosemicarbazone(thiocarbonohydrazone)
oximes of acetylacetone. It is appropriate to recall that the 1,3-thiobenzoylhydrazone oxime of acetylacetone 7
that we studied earlier [16] also is represented in solutions in polar solvents by a tautomeric equilibrium
between the isoxazoline form A and the 1,3,4-thiadiazoline form F. We should note that in all cases, form A is
predominant in solution. We noted earlier an analogous stability of the isoxazoline ring in an investigation of
other functional derivatives of acetylacetone oxime [3, 5, 6].
Me
H
Me
NOH
Me
N
N
Me
Me
N
Ph
S
HNHN
O
CH2CMe
NOH
PhCHNN
S
F
B
S
Ph
A
7
Thus we have determined synthesis methods for and studied the structure of two new classes of
compounds: 5-thiosemicarbazido(thiocarbonohydrazino)isoxazolidines and -∆2-isoxazolines. We have noted a
tendency of the indicated compounds toward ring–chain and ring–ring tautomeric conversions in solutions
involving the 1,2,4-triazolidine, tetrahydro-1,2,4,5-tetrazine, and ∆2-pyrazoline rings. The data obtained expand
our ideas concerning recyclizations (including reversible recyclizations) in the isoxazole derivative series, and
supplement our previous research in [3, 5, 6, 17].
EXPERIMENTAL
The 1H NMR spectra were taken on a Bruker AC 200 spectrometer at a frequency of 200 MHz, and the
13C NMR spectra were taken in solutions on a Bruker AM 500 at a frequency of 125 MHz. The high resolution
13C NMR spectra for the crystalline samples were obtained with "magic angle" spinning at a frequency of
3.6 kHz and nuclear cross polarization on a Bruker CXP 100 spectrometer at a frequency of 25 MHz. The
number of accumulations of free induction signals was equal to 300. The quantitative composition of the
tautomeric forms was determined by integration of the corresponding signals in the 1H NMR spectra. The course
of the reactions and the purity of the compounds obtained were monitored by TLC on Silufol UV-254 plates,
eluent benzene–acetone, 2:1. The 2-methyl-4-phenylthiosemicarbazide, thiocarbonohydrazine, and also
compounds 1 and 5 were obtained by the familiar procedures in [18-21].
5-Thiosemicarbazido(thiocarbonohydrazino)-3,3,5-trimethylisoxazolidines (2a-c) and -3,5-Dimethyl-
∆2-isoxazolines (6a,b). A mixture of compound 1 (or 5) (0.02 mol), thiosemicarbazide or thiocarbonohydrazine
(0.01 mol), a few drops of acetic acid, and ethanol (50 ml) was boiled for 3 days. After the solvent was removed
under reduced pressure, the residue was washed with ether and recrystallized from an ethanol–water mixture,
4:1.
2,5,5-Trimethyl-4-phenyl-1,2,4-triazolidine-3-thione (3). A mixture of acetone (3.0 g, 0.05 mol) and
2-methyl-4-phenylthiosemicarbazide (4.5 g, 0.025 mol) in methanol (50 ml) was boiled for 5 h. After the solvent
was removed under vacuum, the residue was recrystallized from benzene. Yield, 4.2 g (75%); mp 132-134°C.
1H NMR spectrum (CDCl3), δ, ppm: 1.37 (6H, s, 2CH3); 3.41 (3H, s, CH3N); 4.45 (1H, br. s, NH); 7.25-7.41
(5H, m, H arom.). 13C NMR spectrum (solid phase), δ, ppm: 21.8 (2CH3); 34.7 (CH3N); 85.6 (C(5)); 127.6-142.5
(C arom.); 180.5 (C(3)). Found, %: C 59.73; H 6.78; N 19.04. C11H15N3S. Calculated, %: C 59.69; H 6.83;
N 18.99.
736

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