Synthesis of N-substituted -alkoxy-3-aryl-4-methyl-2,5-dihydro-2-pyrrolones

Article abstract of DOI:10.1023/B:COHC.0000037310.26204.24

5-Alkoxy-1-aralkyl-3-aryl-4-methyl-2,5-dihydro-2-pyrrolones and the corresponding alkylthio derivatives were synthesized for the first time through the intermediate formation of unsymmetrical maleimides. The possibility of wide variation of the substituen

Full text of DOI:10.1023/B:COHC.0000037310.26204.24

Chemistry of Heterocyclic Compounds, Vol. 40, No. 5, 2004
SYNTHESIS OF N-SUBSTITUTED
5-ALKOXY-3-ARYL-4-METHYL-
2,5-DIHYDRO-2-PYRROLONES
K. V. Nikitin and N. P. Andryukhova
5-Alkoxy-1-aralkyl-3-aryl-4-methyl-2,5-dihydro-2-pyrrolones and the corresponding alkylthio
derivatives were synthesized for the first time through the intermediate formation of unsymmetrical
maleimides. The possibility of wide variation of the substituents at positions 1, 3, and 5 of the
2-pyrrolones was demonstrated.
Keywords: amido alcohols, herbicides, pyrrolones, ethers.
The interest in substituted 2,5-dihydro-2-pyrrolones 1 is due primarily to their application as components
of herbicides [1-5]. However, in spite of the fact that many 3,4-homosubstituted pyrrolones 1 (R1 = R2) have
been described [3-5], there is little information on compounds with different substituents at positions 3 and 4 (R1
≠ R2) and a functional group at position 5 [6].
Two main methods are usually employed for the production of such pyrrolones. The first involves
successive acylation of the amine RNH2 2 by the action of R1CH2COCl followed by alkylation of the obtained
amide with the α-halo ketone R2COCHR3X and, finally, condensation of the product of the last reaction to the
corresponding pyrrolone.
H
R1CH2COCl
R2COCHR3X
N
R1
R
RNH2
O
2
O
R1
O
R1
MeONa
N
R
R N
R3
O
R2
R3
R2
1
However, the method cannot be used for the synthesis of 5-alkoxy- or 5-alkylthio-substituted pyrrolones.
The second method is used for the production of these compounds [3-5], i.e., the symmetrical cyclic anhydride 3
is converted into the symmetrical imide 4 by the action of the amine RNH2 2. The obtained imide 4 is then
reduced to the hydroxypyrrolone 5, and the 5-hydroxy group of the latter is esterified with the corresponding
alcohol (X = O) or thiol (X = S) R3XH 6.
__________________________________________________________________________________________
M. V. Lomonosov Moscow State University, Moscow, Russia; e-mail: newscientist@mtu-net.ru.
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 670-678, May, 2004. Original article
submitted January 24, 2000; revision submitted April 9, 2001.
0009-3122/04/4005-0561©2004 Plenum Publishing Corporation
561

O
O
O
R1
R2
R1
R2
NaBH4
2
O
N R
O
3
4
O
O
N
R1
R2
R1
R2
6
N
R
R
XR3
OH
5
1
X = O, S
The production of pyrrolones 1 through the stage of 3,4-heterosubstituted pyrroles [6] is probably less
widely used.
In the present work we describe a new convenient and more general method for the synthesis of
3,4-heterosubstituted pyrrolones 1 that makes it possible to vary widely the substituents at positions 1, 3, and 5
of this heterocycle. The synthesis was conducted by successive arylation of the monosubstituted cyclic
anhydride 7a,b, imidation of the obtained unsymmetrical anhydride 8 with the primary amine 2, reduction of the
obtained imide 9 to the N-substituted 3-aryl-5-hydroxy-4-methyl-2,5-dihydro-2-pyrrolone 10, and finally
substitution of the hydroxyl in the latter by the OR3 or SR3 group by the action of alcohol or thiol 6 on the
hydroxypyrrolone 5 with the formation of the final pyrrolone 1.
O
O
O
R1
R2
R1
R2
1. R1N2Cl
2. Ac2O
2
NaBH4
N R
O
O
R2
O
O
O
9a–j
8a–f
7a,b
O
R1
+
6, H
N R
1
R2
OH
10
7 a R2 = H, b R2 = Me
The unsymmetrical anhydrides 8a-f were obtained in the Meerwein reaction [7] by the reaction of
arenediazonium chloride with monosubstituted anhydrides 7a,b in acetone with copper salts as catalysts
(Table 1). The intermediately formed 3-aryl-4-chloro-4-methylsuccinic anhydrides were not isolated but were
subjected directly to elimination of HCl in boiling acetic anhydride. The obtained unsymmetrical maleic
anhydrides 8a-f were purified by vacuum distillation. It should be noted that alternative methods for the
synthesis of the anhydrides 8 have been described in the literature. One of them is based on pyrolysis of the
relatively inaccessible 1-ethoxyalkenyl esters of α-keto acids [8]. In our case methods based on the condensation
of arylacetonitriles R1CH2CN and α-keto acids [9] or phenylacetic acids with α-keto acids [10] did not give the
desired results.
562

TABLE 1. The Characteristics of the Unsubstituted Maleic Anhydrides 8a-f
Found, %
Calculated, %
——————
IR spectrum
Com-
pound*
Empirical
formula
bp, °C
(0.01 mm Hg)
R1
mp, °С
1H NMR spectrum, δ, ppm
Yield, %
ν, cm-1 (CO)
C
H
8a
8b
Ph
С11Н8О3
—
—
130-135
120
92
(94 [8])
1750
1758
2.3 (3H, s); 7.6 (5H, m)
32
41
2-FC6H4
С10Н5FO3
62.30
62.51
2.77
2.62
68
7.18-7.38 (3H, m);
7.50-7.65 (1H, m); 8.34 (1H, s)
8c
8d
8e
8f
2-FC6H4
C11H7FO3
64.42
64.08
63.88
64.08
55.09
54.91
55.34
54.91
3.13
3.42
3.44
3.42
2.70
2.51
2.66
2.51
127
125
45
44
—
—
1755
1750
1758
1760
2.18 ( 3H, s); 7.2-7.6 (4H, m)
2.33 (3H, s); 7.1-7.6 (4H, m)
2.33 (3H, s); 7.0-7.8 (3H, m)
2.27 (3H, s); 7.0-7.7 (3H, m)
65
58
31
38
3-FC6H4
C11H7FO3
3-Cl-4-FC6H3
4-Cl-2F-C6H3
C11H6ClFO3
C11H6ClFO3
145-150
148-155
_______
* 8 a,c-f R2 = Me, b R2 = H.

TABLE 2. The Characteristics of the Unsubstituted Maleimides 9a-j, Obtained from the Anhydrides 8a-f and Amines 2
Found, %
Calculated, %
H
1H NMR spectrum,
δ, ppm (J, Hz)
——————
Com-
pound*
Empirical
formula
Yield, %
(method)*2
R1
R
mp, °С
C
N
3
9a
9b
9c
9d
H
PhCH2
PhCH2
PhCH2
C12H11NO2
—
—
—
*
2.03 (3H, s); 4.6 (2H, s);
6.28 (1H, s); 7.3 (5H, m)
90 (A)
82 (A)
86 (A)
Ph
C18H15NO2
77.68
77.96
5.60
5.45
4.97
5.05
70
74
2.03 (3H, s); 4.73 (2H, s);
7.1-7.5 (10H, m)
2-FC6H4
2-FC6H4
C18H14FNO2
C20H17ClFNO2
73.31
73.21
4.89
4.78
4.66
4.74
2.06 (3H, s); 4.73 (2H, s);
7.1-7.50 (9H, m)
3-ClC6H4CMe2
67.28
67.14
5.00
4.79
3.97
3.91
1.96 (6H, s); 2.02 (3H, s);
7.05-7.5 (8H, m)
45 (B),
78 (В)
55 (B)
9e
9f
2-FC6H4
3-FC6H4
3,5-Cl2C6H3CMe2
3,5-Cl2C6H3CMe2
C20H16Cl2FNO2
C20H16Cl2FNO2
C20H15Cl3FNO2
C20H17Cl2FNO2
60.93
61.24
4.50
4.11
3.55
3.57
106
108
1.94 (6H, s); 2.03 (3H, s);
7.2 ( 5Н, m); 7.6 (2H, m)
60.99
61.24
4.24
4.11
3.43
3.57
1.95 (6H, s); 2.02 (3H, s);
7.2-7.6 (7H, m)
50 (B)
48 (B)
84 (В)
75 (A)
9g
9h
9i
4-Cl-2FC6H3 3,5-Cl2C6H3CMe2
56.12
56.30
3.44
3.54
3.23
3.28
1.93 (6H, s); 2.02 (3H, s);
7.2-7.5 (6H, m)
Ph
H
3,5-Cl2C6H3CMe2
64.00
64.18
4.90
4.58
4.02
3.74
1.93 (6H, s); 2.15 (3H, s);
7.22 (3H, s); 7.4-7.6 (5H, m)
2-F-4-Cl-5-HOC6H2 C11H7ClFNO3
2-F-4-Cl-5-HOC6H2 C17H11Cl3FNO3
51.60
51.66
2.69
2.76
5.26
5.48
171
166
2.18 (3H, s); 5.72 (1H, s);
6.53 (1H, s); 6.90 (1H, d,
J = 6.5); 7.23 (1H, d, J = 9.1)
9j
Ph
61.61
61.55
3.51
3.34
4.27
4.22
2.30 (3H, s); 5.7 (1H, br. s);
6.99 (1H, d, J = 6.5);
7.33 (1H, d, J = 7.9);
7.5-7.7 (5H, m)
50 (A)
_______
* 9a-j R2 = Me.
*2 A is the method described in [1]; B is 2 mol of acetic acid in dioxane; C is 1 mol of acetic acid and 1 mol of
triethylamine in benzene.
*3 bp 130°C (1 mm Hg); the boiling point was not given in [11].

The transition from the unsymmetrical anhydrides 8a-f to the maleimides 9 is usually realized with good
yields [3-5, 11] by refluxing them with amines 2 in acetic acid. Direct alkylation of the maleimides 9 (R = H)
with alcohols in the presence of triphenylphosphine also gives good results. It is surprising that the authors of
[11] used alkylation of the silver salt of citraconimide for the synthesis of the N-benzylimide of citraconic acid
9a with a yield of 24%, whereas we obtained the imide 9a by the usual method (with a yield of 88%). The
imidation of arylmethylmaleic anhydrides 8a-c with benzylamine also does not give rise to any complications
(Table 2), but the corresponding acetanilide is formed as side product if substituted anilines are used in the
reaction. In the reaction of the anhydride 8c with α,α-dimethyl-3-chlorobenzylamine in acetic acid (method A
[11]) the corresponding imide is hardly formed at all, and the reaction takes place with a satisfactory yield only if
2 eq. of acetic acid in dioxane is used (method B). With the use of 1 mol of triethylamine and 1 mol of acetic
acid to 1 mol of the anhydride in benzene (method C) the yield of the imide becomes high. The restricted
formation of the imides from amines in which the NH2 group is attached to a tertiary carbon atom can probably
be explained by steric hindrances to nucleophilic attack by such an amine on the carbonyl groups of the
anhydride.
The reduction of the imides 9a-j to the corresponding hydroxypyrrolones was realized with sodium
borohydride in methanol [3-5], since it is known that the same reaction in THF [11] leads to the formation of
side products. Reduction of the imides 9 under our conditions leads to the formation of a mixture of two
regioisomeric cyclic N-substituted amido alcohols: 3-Aryl-4-methyl-1,5-dihydro-2H-pyrrol-2-one 10 and 4-aryl-
3-methyl-1,5-dihydro-2H-pyrrol-2-one 11. During reduction of the imide 9a the succinimide 12 is formed in
addition.
O
OH
R1
R2
R1
R2
NaBH4
N
R
9a
N
R
+
10a
+
O
O
12
11a
The structure of the individual isomeric pyrrolones 10a and 11a was established by 2D 1H NMR and 1H
NOE. The acyclic aldehyde form was not found in the reaction mixture.
TABLE 3. The Reduction of the Imides 9a-j by Sodium Borohydride in
Methanol
Yield, %
Imide
NaBH4, mol
10 : 11
10a–j
11a–j
9a
9b
9c
9d
9e
9f
0.5
1
8:1
1:1
76
43
61
62
58
66
53
76
70
55
10*
53
34
18
16
17
15
23
14
25
0.5
1
2:1
3.5:1
3.5:1
4:1
1
1
9g
9h
9i
1
3.5:1
3.5:1
5:1
1
1
9j
1.5
2:1
_______
* In addition, 12% of N-benzylmethylsuccinimide is formed.
565

TABLE 4. Characteristics of the Synthesized Substituted 4-Methyl-1,5-dihydro-2H-pyrrol-2-ones 1a-l*
Found, %
——————
Calculated, %
Com-
Empirical
formula
Yield,
%
R
R3
1H NMR spectrum, δ, ppm (J, Hz)
pound*2
C
H
N
1a
1b
1c
1d
1e
1f
PhCH2
PhCH2
PhCH2
Me
C13H15NO2
1.95 (3H, s); 2.98 (3H,s); 4.04 (1H, d, J = 14.8);
4.95 (1H, d, J = 14.8); 5.02 (1H, s); 5.95 (1H, s); 7.29 (5H, s)
92
97
90
92
90
87
71.55
71.87
7.07
6.96
6.33
6.45
CH2CH2Cl
Me
C14H16ClNO2
C19H19NO2
1.92 (3H, s); 3.2–3.5 (4H, m); 4.12 (1H, d, J = 15);
4.8 (1H, d, J = 15); 5.06 (1H, s); 5.89 (1H, s); 7.25 (5H, s)
63.40
63.28
6.22
6.07
5.20
5.27
2.07 (3H, s); 3.03 (3H, s); 4.13 (1H, d, J = 14.8);
5.05 (1H, d, J = 14.8); 5.11 (1H, s); 7.2–7.6 (10H, m)
1.74 (3H, s); 1.84 (3H, s); 2.14 (3H, s); 3.17 (3H, s);
5.6 (1H, s); 7.2-7.6 (8H, m)
77.59
77.79
6.68
6.53
4.85
4.77
3,5-Cl2C6H3CMe2 Me
3,5-Cl2C6H3CMe2 Et
3,5-Cl2C6H3CMe2 i-Pr
C21H21Cl2NO2
C22H23Cl2NO2
C23H25Cl2NO2
64.77
64.62
5.30
5.42
3.60
3.59
1.26 (3H, m); 1.7 (3H, s); 1.85 (3H, s); 2.15 (3H, s);
3.2-3.5 (2H, m); 5.6 (1H, s); 7.2-7.6 (8H, m)
65.60
65.35
5.49
5.73
3.64
3.46
65.91
66.03
6.17
6.02
3.29
3.35
1.18 (3H, d, J = 6.1); 1.27 (3H, d, J = 6.1); 1.75 (3H, s);
1.87 (3H, s); 2.19 (3H, s); 3.97 (1H, m); 5.6 (1H, s);
7.15-7.6 (8H, m)
1g
1h
1i
PhCH2
CH2CH2Cl
C20H20ClNO2
C22H23Cl2NOS
C26H29Cl2NOS
C24H25Cl2NO4
2.11 (3H, s); 3.3-3.6 (4H, m); 4.30 (1H, d, J = 14.5);
4.98 (1H, d, J = 14.5); 5.22 (1H, s); 7.2-7.6 (10H, m)
1.16 (3H, t, J = 7.5); 1.87 (3H, s); 1.94 (3H, s); 2.14 (2H, q,
J = 7.5); 2.23 (3H, s); 5.18 (1H, s); 7.2-7.5 (8H, m)
95
95
96
70
70.03
70.27
6.11
5.90
3.88
4.10
3,5-Cl2C6H3CMe2 Et
3,5-Cl2C6H3CMe2 C6H11-cyclo
3,5-Cl2C6H3CMe2 CH2COOEt
63.00
62.85
5.45
5.51
3.25
3.33
1.1-1.5 (6H, m); 1.5-1.8 (4H, m); 1.87 (3H, s); 1.95 (3H, s);
2.24 (3H, s); 2.25 (1H, m); 5.20 (1H, s); 7.2-7.4 (8H, m)
66.08
65.81
6.06
6.16
3.00
2.95
1j
62.28
62.34
5.62
5.45
2.80
3.03
1.30 (3H, t, J = 7); 1.80 (3H, s); 1.87 (3H, s); 2.17 (3H, s);
3.76 (1H, d, J = 14); 4.02 (1H, d, J = 14); 5.79 (1H, s);
7.2-7.5 (8H, m)
1k
1l
2F-4Cl-5HOC6H2 CH2CH2Br
C13H12BrClFNO3
C19H18FNO2
2.15 (3H, s); 3.3-3.7 (4H, m); 5.85 (1H, s); 6.1 (1H, s);
7.0-7.2 (2H, m); 7.4 (1H, s)
1.94 (3H, s); 3.06 (3H, s); 4.16 (1H, d, J = 14.6);
91
90
42.44
42.83
3.10
3.32
3.91
3.84
PhCH2
Me
72.92
73.29
5.63
5.83
4.26
4.50
5.04 (1H, d, J = 14.6); 5.16 (1H, s); 7.05-7.50 (9H, m)
_______
* Compounds 1a-l are thick oils.
*2 1a,b,k R1 = H, c-j R1 = Ph, l R1 = 2-FC6H4; a-l R2 = Me.

In order to avoid inaccuracies in the separation of the isomeric pyrrolones 10 and 11 the ratio of the
1
reduction products was determined by H NMR spectroscopy from the ratio of the signals for the protons at
position 5 of the pyrrolone ring. For compounds 10 the position of this signal is 0.5-0.6 ppm downfield from the
signal for compounds 11. The ratio of the hydroxypyrrolones 10 and 11, determined by NMR, agrees well with
the yields of the individual 5-alkoxypyrrolones 1 isolated from mixtures of the isomers 10 and 11 (Table 3).
During investigation of this reaction we established that the pyrrolones 10 are mostly formed, but the
selectivity of reduction is low, and sometimes the pyrrolone 11 (reduction of the imide 9b) predominates in the
mixture. The reduction of the citraconimides 9a and 9i is most selective. These imides, which do not have
substituents at position 3, probably have the most symmetric distribution of electron density of the π-system,
characterized by a relatively low electron density at position 5, to which attack by the reducing agent is mostly
directed. Conversely, reduction of N-benzyl-3-methyl-4-phenylmaleimide 9b, where the phenyl substituent has a
+M effect, gives mostly the pyrrolone 11b. Nevertheless, all the imides with tertiary benzyl or aromatic
substituents at the nitrogen atom 9d-j are reduced fairly selectively with the formation of the isomeric pyrrolones
10d-j and 11d-j in ratios (3:1)-(5:1).
The target products – the pyrrolones 1 (Table 4) – are easily formed during treatment of the respective
compounds 10 with an excess of the primary or secondary alcohol with protic acids (HCl, TsOH) as catalysts.
Tertiary alcohols do not enter into the reaction, as we demonstrated for the case of tert-butyl alcohol. The
transformation of the pyrrolones 11 into the alkylated derivatives 1 takes place readily under the same
conditions. It is possible, therefore, not to separate the highly polar compounds 10 and 11 but to separate their
isomeric alkoxylation products 1. An interesting feature of the NMR spectra of compounds 1a-c,g,l should be
mentioned (Table 4). The signals of the diastereotopic protons of the methylene group of the benzyl radical in
these compounds are separated from each other by 0.8-1.0 ppm with spin–spin coupling constant 14-15 Hz in
spite of the fact that the asymmetric carbon atom is three bonds away from these protons. A possible explanation
of the such a strong effect of the asymmetry on the chemical shift may be that the nitrogen atom in these systems
has a nonplanar configuration and itself becomes asymmetric.
The formation of the alkylthiopyrrolones 1 in the reaction of hydroxypyrrolones 10 with thiols is similar
to the reaction of the latter with alcohols but takes place noticeably more readily. A small excess of the thiol is
sufficient to complete the reaction.
The ease of substitution of the hydroxyl at position 5 in the investigated 1,5-dihydro-2H-pyrrol-2-ones is
probably due to the formation an intermediate carbenium-immonium cation during the action of the acids on
them. Nevertheless, in a number of cases alkylation of the free hydroxyl of the 5-hydroxy-1,5-dihydro-2H-
pyrrol-2-one with the respective halide (ClCH2COOEt) proved more successful for the introduction of a
substituent at position 5 (for example OCH2COOEt).
Thus, it is possible by the proposed method to produce a wide range of compounds 1 by varying the
substituents at positions 1, 3, and 5.
EXPERIMENTAL
The 1H NMR spectra were measured on a Varian GEMINI-200 instrument (200 MHz) in
deuterochloroform with TMS as internal standard. The mass spectra were obtained on a GCMS-OP 1000
instrument. The IR-FT spectra were recorded on a Mattson Genesis II spectrophotometer. Chromatographic
separation of the mixtures was performed on Merck silica gel 40/63 µ. The eluant was hexane–ethyl acetate. The
melting points were determined on Thomas-Hoover capillary equipment and were not corrected. The commercial
reagents and solvents were used without further purification.
The characteristics of the synthesized compounds are given in Tables 1-4.
567

Unsymmetrical Maleic Anhydrides (8a-f) (General Procedure). To conc. hydrochloric acid (8 ml, 0.1
mol) we added water (15 ml) and substituted aniline (50 mmol). The mixture was cooled to 3°C, and a saturated
aqueous solution of sodium nitrite (3.45 g, 50 mmol) was added drop by drop. Then a solution of copper
chloride (1.34 g, 10 mmol) and the anhydride 7 (5.6 g, 50 mmol) in a mixture of water (2 ml) and acetone (20
ml) was added. The reaction mixture was stirred at 5°C for 2 h and then at room temperature for 20 h. The
bottom layer was rejected, and the top layer was evaporated under vacuum, and 30 ml of acetic anhydride was
added to the residue. The mixture was refluxed for 12 h and submitted to fractional distillation under vacuum,
and the last fraction was collected.
Unsymmetrical Maleimides (9a-j). A. To a solution of the anhydride 8 (1 mmol) in acetic acid (1 ml)
we added the amine 2 (1 mmol). The mixture was refluxed for 12 h, and the reaction product was isolated by
vacuum distillation or chromatography.
B. To a solution of the anhydride 8 (1 mmol) in dioxane (2 ml) we added acetic acid (2 mmol) and then
the amine 2 (1 mmol). The mixture was refluxed for 12 h, and the reaction product was isolated by vacuum
distillation or chromatography.
C. To a solution of the anhydride 8 (1 mmol) in benzene (2 ml) we added acetic acid (1 mmol),
triethylamine (1 mmol), and then the amine 2 (1 mmol). The mixture was refluxed for 12-24 h, and the reaction
product was isolated by vacuum distillation or chromatography.
3,4-Disubstituted 5-Hydroxy-1,5-dihydro-2H-pyrrol-2-ones (10a-j, 11a-j) (General Procedure). In
methanol (80 ml) in an atmosphere of nitrogen at 40°C we dissolved the hydroxypyrrolone 9 (10 mmol). With
stirring we then added slowly sodium borohydride (0.38 g, 10 mmol). The reaction mixture was evaporated
under vacuum, and water (10 ml) was added. The product was extracted with ethyl acetate (for compounds 9i,j
the pH of the aqueous phase was previously brought to 6-7), washed with ammonium chloride solution, dried,
and evaporated. The mixture of isomers 10 and 11 was used without further purification. They can be isolated in
the individual form by chromatography on silica gel.
3,4-Disubstituted 5-Alkoxy-1,5-dihydro-2H-pyrrol-2-ones and 3,4-Disubstituted 5-Alkylthio-1,5-
dihydro-2H-pyrrol-2-ones (1a-l). General Procedure for the Volatile Alcohols. To compound 10 (1 mmol)
(containing the isomer 11 as impurity) we added the respective alcohol (5 ml) and p-toluenesulfonic acid
(0.019 g, 0.1 mmol). The mixture was refluxed for 1 h, and triethylamine (0.1 ml, 0.72 mmol) was added. The
mixture was evaporated under vacuum, and ethyl acetate (10 ml) was added. The mixture was washed with
water, dried, and evaporated, and the reaction product (1) was purified by chromatography.
General Procedure for the Nonvolatile Alcohols and Thiols. To compound 10 (1 mmol) we added
benzene (5 ml), the respective alcohol (2 mmol) (1.1 mmol of the thiol), and p-toluenesulfonic acid (0.019 g,
0.1 mmol). The mixture was refluxed for 4 h, and triethylamine (0.1 ml, 0.72 mmol) and ethyl acetate (10 ml)
were added. The product was washed with water, dried, evaporated, and purified by chromatography.
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