Synthesis and transamination of enaminones: Derivatives of 1-phenyl-4-(phenylhydroxymethylidene)-pyrrolidine-2,3,5-trione

Article abstract of DOI:10.1007/PL00010234

The reaction of 1-phenyl-4-(phenylhydroxymethylidene)-pyrrolidine-2,3,5-trione with difunctional bases (α-,β-,-γ-amino acid derivatives or aminoethanol) leads to a mixture of tautomeric Schiff bases and enaminones. Some of the products easily undergo transamination at their enaminone moiety.

Full text of DOI:10.1007/PL00010234

Monatshefte fuÈr Chemie 130, 555±562 (1999)
Synthesis and Transamination of
Enaminones: Derivatives of 1-Phenyl-4-
(phenylhydroxymethylidene)-pyrrolidine-
2,3,5-trione
Katarzyna Ostrowska1;Ã, Maryla Ciechanowicz-Rutkowska2, Tulio Pilati3,
4
Ç
and Grzegorz Zuchowski
1
Â
Department of Organic Chemistry, Jagiellonian University, PL-30060 Krakow, Poland
2
3
4
Â
Regional Laboratory of Physicochemical Analyses and Structural Research, PL-30060 Krakow,
Poland
Centro del C.N.R. per lo Studio delle Relazioni tra Struttura e Reattivita Chimica, Dipartmento di
Á
Chimica Fisica ed Elettrochimica, Universita di Milano, I-20133 Milano, Italy
Â
Collegium Medicum UJ, Department of Pharmacy, PL-30688 Krakow, Poland
Summary. The reaction of 1-phenyl-4-(phenylhydroxymethylidene)-pyrrolidine-2,3,5-trione with
difunctional bases (ꢀ-,ꢁ-,ꢂ-amino acid derivatives or aminoethanol) leads to a mixture of tautomeric
Schiff bases and enaminones. Some of the products easily undergo transamination at their enaminone
moiety.
Keywords. Enaminones; Pyrrolidine-2,3,5-trione; ꢀ-,ꢁ-,ꢂ-Amino acids; Transamination.
Synthese und Transaminierung von Enaminonen:
Derivative des 1-Phenyl-4-(phenylhydroxymethyliden)-pyrrolidin-2,3,5-trions
Zusammenfassung. Die Reaktion von 1-Phenyl-4-(phenylhydroxymethyliden)pyrrolidin-2,3,5-
È
È
trions mit difunktionellen Basen (ꢀ-,ꢁ-ꢂ-Aminosaurederivativen oder Aminoethanol) fuhrt zu einem
Gemisch der tautomeren Schiffschen Basen und Enaminone. Einige dieser Verbindungen werden
leicht am Enaminonfragment transaminiert.
Introduction
Tetramic acid derivatives have found interest because of their biological and
pharmacological properties; thus, their chemistry has been extensively investigated
[1±4]. In a previous study [5], the analogue of tetramic acid, 1-phenyl-4-
(phenylhydroxsymethylidene)-pyrrolidine-2,3,5-trione (1, Scheme 1), has been
synthesized. It exists only in the exo-enol form and is in principle susceptible to
nucleophilic attack at C-2, C-3, C-5, and C-6. Reaction of 1 with amines leads to
Ã
Corresponding author

556
K. Ostrowska et al.
Schiff bases which are known to possess anticancer activity [6,7]. Because of the
biological importance of such tetramic acid derivatives, their synthesis using
different mono-, di-, and triamines varying in basicity has been studied. When
strong bases were used ( pKa > 13.6) only salts were obtained [8±10], whereas with
weaker bases ( pKa 3.43±10.71) condensation took place: for monoamines at C-6,
for diamines in a ®rst step at C-6 and then at C-3 [11,12]. In this work we have
studied the reaction of 1 with weak bases containing two different functional
groups such as ꢀ-, ꢁ-, or ꢂ- amino acid derivatives and aminoethanol.
Results and Discussion
Scheme 1 shows the reaction products 2±5 obtained as a result of condensation of 1
with glycine, ꢁ-alanine, ꢂ-aminobutyric acid, and aminoethanol in ethanolic
solution. Under these conditions, condensation took place at C-6 as inferred from
comparison of the spectroscopic data with those of the analogous compound
4-(phenyl-(phenylamino)-methylidene)-pyrrolidine-2,3,5-trione [5]. The crucial
proof was provided by the absence of signals characteristic for a benzoyl group
1
in their 13C NMR and MS data (Table 1). The H NMR spectra show that
compounds 2 and 3 exist in two tautomeric forms (enol-imino and keto-enamino).
At equilibrium, the enol-imine content amounts to 53% for 2 and 52% for 3.
Scheme 1
In the reaction of 1 with glycine ethyl ester hydrochloride, carried out in
boiling ethanol, condensation unexpectedly occurred at C-3 (Scheme 2). Two
tautomeric forms (enol-imino, 6a; keto-enamino, 6b) were obtained. 1H NMR data
showed that the keto-enamino form predominated at equilibrium (76%).
Scheme 2

Derivatives of 1-Phenyl-4-(phenylhydroxymethylidene)-pyrrolidine-2,3,5-trione
557
In 6a, the CH2 protons and the NH proton resonated as doublets with a coupling
constant of 6.9 Hz. It should be mentioned that recently [13] a two-step synthesis of
4-acyl-(carbamoyl)-3-amino-pyrrol-2,5-dione, the product of nucleophilic sub-
stitution at C-3, has been reported. As starting material, the endo-enol form of 1
has been used; reaction with SOCl2 afforded the 3-chloro derivative which in turn
underwent nucleophilic substitution with amine.
With the exception mentioned above, reaction of 1 with amines in ethanol
always led to a condensation at C-6. However, further investigation revealed that
upon changing the solvent from ethanol to toluene the reaction of 1 with
aminoethanol led to two products of condensation: 5 (at C-6) and 7 (at C-3)
1
(Scheme 3). The H NMR spectrum of 1 in methanol-d4 at room temperature
showed the presence of 90% exo-enol form, whereas in benzene-d6 only 74% of
this isomer could be detected. Thus, toluene inverted the endo-exo-enol
equilibrium of 1, enabling formation of the C-3 product.
Scheme 3
The structure of 5 was proven by X-ray analysis (Fig. 1) as well as by its NMR
and MS data. Compound 5 exists as the (E)-isomer stabilized by an intramolecular
hydrogen bond with the following geometry: N2Á Á ÁO3 ˆ 2.793(2), N2±H2 ˆ
ꢀ
Ê
0.95(2), H2Á Á ÁO3 ˆ 2.02(2) A and N2±H2Á Á ÁO3 ˆ 138(2) . MS and NMR tech-
Fig. 1. Structure of 5 including the atom numbering scheme corresponding to that used in the
deposited tables (ORTEP-3 [20]; thermal vibration ellipsoids are scaled to enclose 50% probability)

558
K. Ostrowska et al.
Scheme 4
niques were also used to establish the structure of 7, revealing especially the
presence the benzoyl group.
Compounds 2, 3, and 6 were chosen for reactivity studies with benzylamine in
ethanolic solution. As shown in Scheme 4, 6 undergoes transamination.
Simultanously, cleavage of the N1±C2 bond of the succinimide group took place
leading to 8. In case of compounds 2 and 3 (Scheme 5), only transamination
occurred leading to compound 9. The structures of 8 and 9 were elucidated on the
basis of elemental analysis, mass, and NMR spectroscopy (Table 1). For compound
8, the latter method showed the presence of signals speci®c for the benzoyl group.
In conclusion, we have shown that the condensation of 1 with bases can take
place either at C-6 or at C-3 depending on the solvent used. When the reaction of 1
with bases takes place at C-6, two tautomers are formed at the same ratio, whereas
the condensation at C-3 gives a product for which the equilibrium is shifted
towards the enaminone form. With benzyl amine, compounds 2, 3, and 6, presumed
as enaminones, undergo transamination. This means that nucleophilic substitution
at the carbon atom attached to the nitrogen atom of the enaminone moiety is
preferred for nucleophilic addition. That proves the lower reactivity of the latter,
independently of the fact whether it is the benzoyl or a ring carbonyl group. The
statement that the carbon atom attached to the nitrogen atom of the enaminone
Table 1. Selected 13C NMR chemical shifts (ppm) for compounds 1±9
C-5
C-4
C-3
C-2
C-6
1
2 keto-enamino
enol-imino
170.75
169.44
171.35
168.07
167.03
173.80
162.15
168.59
167.73
166.09
161.85
C-1
102.70
93.45
95.46
92.69
92.69
98.11
94.43
97.73
97.73
98.04
120.45
C-2
171.87
175.10
179.88
172.47
174.80
174.81
167.55
150.32
155.68
156.54
166.98
C-3
163.87
161.60
161.89
158.58
158.58
170.52
162.10
166.09
162.51
162.92
158.59
C-4
183.20
166.78
165.96
162.51
161.85
164.63
155.00
189.76
188.91
191.61
160.13
C-5
3 keto-enamino
enol-imino
4
5
6b(enol-imino)
6a(keto-enamino)
7
9
8
164.36
87.95
145.05
162.55
185.06

Derivatives of 1-Phenyl-4-(phenylhydroxymethylidene)-pyrrolidine-2,3,5-trione
559
Scheme 5
moiety is more reactive towards nucleophiles is thus in accordance with earlier
reports [14±17].
Experimental
The 1H NMR and 13C NMR spectra were recorded in DMF-d7, DMSO-d6, and CDCl3 with a Bruker
AMX 500 NMR spectrometer using TMS as internal standard. The IR spectra were measured in
Nujol and hexachlorobutadiene (HCBN) with a Bruker IFS 48 spectrometer. The MS spectra were
obtained on an LKB 9000 S and a Finnigan TSQ 700 triple quadruple mass spectrometer. Elemental
analyses were carried out with a Perkin Elmer analyser 240 in the regional Laboratory of Physico-
Â
Chemical Analyses and Structural Research in Krakow. Their results were in good agreement with
the calculated values. X-Ray intensities were collected at room temperature on an Enraf-Nonius
Ê
CAD-4 diffractometer using graphite monochromated MoKꢀ radiation (ꢃ ˆ 0.71073 A).
General procedure for the preparation of enaminones 2±6
To a solution of 2.9 g of 1 (0.01 mol) in ethanol, 0.012 mol of the corresponding amine were added.
The solution was re¯uxed for 2.5 h and left overnight. The precipitate was ®ltered off and
crystallized from ethanol. Compound 6 was puri®ed by radial chromatography (aluminum oxide,
CHCl3:CH2Cl2 ˆ 2:1).
1-Phenyl-4-(phenyl-(carboxymethylamino)-methylidene)-pyrrolidine-2,3,5-trione (2;C19H14N2O5)
Yield: 33.7%; m.p.: 238ꢀC; IR (HCBN): v ˆ 3177 (NH), 3062 (C±H arom), 2926 (CH2), 1746, 1669
(C=O), 1635 (C=N) cmꢁ1; 1H NMR (DMSO-d6, ꢄ, 500.13 MHz): 3.9 (s, 2H, CH2), 4.39 (s, 2H, CH2),
7.3±7.56 (m, arom), 10.29 (s, 1H, NH), 10.85 (s, 1H, OH), 13.2 (s, 1H, COOH) ppm; 13C NMR
(DMSO-d6, ꢄ, 125.77 MHz): 46.05 (CH2), 56.01 (CH2), 119.34, 124.11, 127.0, 127.31, 127.68,
128.04, 128.31, 128.50, 128.756, 128.976, 129.61, 129.82,130.48, 130.48, 131.55, 131.80 (C-arom.),
‡
‡
‡
171.31 (COOH) ppm; MS: m/z (%) ˆ 350 (67) M , 322 (75) [M±CO] , 278 (33) [M±CO±COO] ,
‡
77(100) C6H5 .
1-Phenyl-4-(phenyl-(2-carboxyethylamino)-methylidene)-pyrrolidine-2,3,5-trione (3;C20H16N2O5)
Yield: 21.5%; m.p.: 225ꢀC; IR (HCBN): v ˆ 3104 (NH), 1767 (CO), 1726 (CO), 1707 (CO), 1663
1
(CO), 1634 (C=N) cmꢁ1; H NMR (DMSO-d6, ꢄ, 500.13 MHz): 2.6 (t, 2H, CH2±CO), 2.85 (t, 2H,
CH2±CO), 3.87 (t, 3H, CH2±N), 4.11 (t, 2H, CH2±N), 7.16±7.90 (m, arom), 10.30 (s, 1H, NH), 10.88
(s, 1H, OH), 12.60 (s, 1H, COOH) ppm; 13C NMR (DMSO-d6, ꢄ, 125.77 MHz): 32.52 (CH2±N),
33.64 (CH2±N), 40.56 (CH2±CO), 45.81 (CH2±CO), 120.42, 126.95, 127.19, 127.52, 127.58, 127.72,
128.04, 128.59, 129.45, 129.70, 130.31, 130.53, 131.68, 132.23, 133.90, 137.31, (C-arom), 172.47
‡
‡
‡
(COOH) ppm; MS: m/z (%) ˆ 364 (30.5) M , 336 (35.6) [M±CO] , 292 (11) [M±CO±COO] ,
‡
77(100) C6H5 .

560
K. Ostrowska et al.
1-Phenyl-4-(phenyl-(3-carboxypropylamino)-methylidene)-pyrrolidine-2,3,5-trione
(4; C21H18N2O5)
Yield: 50%; m.p.: 200±202ꢀC; IR (HCBN): v ˆ 3143 (NH), 3050 (C±H arom), 2900±2485 (COOH),
1755, 1701, 1690 (CO), 1630 (C=N) cmꢁ1; 1H NMR (DMSO-d6,ꢄ, 500.13 MHz): 1.78 (m, 2H, CH2),
2.35 (t, 2H, CH2N), 2.85 (t, 2H, CH2CO), 3.45 (s, 1H, NH), 7.29±7.87 (m, 10H, Ph-H), 12.3 (s, 1H,
COOH) ppm; 13C NMR(DMSO-d6, ꢄ, 125.77 MHz): 22.57 (C-30), 30.48 (CH2±N), 38.53 (CH2±CO),
‡
‡
‡
187.48 (COOH) ppm; MS: m/z (%) ˆ 378 (5.1) M , 350 (5.6) [M±CO] , 306 (2) [M±CO±COO] ,
‡
77(100) C6H5 .
(E)-1-Phenyl-4-(phenyl-(2-hydroxyethylamino)-methylidene)-pyrrolidine-2,3,5-trione
(5; C19H16N2O4)
Yield: 36%; m.p.: 237ꢀC; IR (HCBN): v ˆ 3525 (OH), 3193 (NH), 1767, 1710, 1695 (CO), 1662
(C=C) cmꢁ1; 1H NMR (DMSO-d6, ꢄ, 500.13 MHz): 3.26 (d, 2H, CH2±N), 3.52 (d, 2H, CH2±O), 5.11
(s, 1H, OH), 7.71±7.66 (m, 10H, arom), 10.30 (s, 1H, NH) ppm; 13C NMR (DMSO-d6, ꢄ,
125.77 MHz): 46.92 (CH2±N), 56.10 (CH2±O), 126.96, 127.69 128.33, 128.47, 128.61, 129.79,
‡
‡
130.25, 131.70, (C-arom) ppm; MS: m/z (%) ˆ 336(29.1) M , 308 (21.6) [M±CO] , 104 (100)
‡
‡
C6H5CNH , 77 (40.5) C6H5 .
Crystal data: C19H16N2O4, MW ˆ 336.34, monoclinic, P21/n, a ˆ 6283(5), b ˆ 10.946(2),
Ê
Ê 3
ꢀ
ꢁ3
,
c ˆ 23.858(3) A, ꢀ ˆ 90.0, ꢁ ˆ 96.652, ꢂ ˆ 90.0 , V ˆ 1629.6(4) A , Z ˆ 4, D ˆ 1.371 Mgm
ꢃ(MoKꢀ) ˆ 0.71073 A, ꢅ ˆ 0.098 mmꢁ1, F(000) ˆ 704, T ˆ 293 K.
Ê
Crystals of 5 suitable for X-ray analysis were grown from ethanol as gold yellow prisms. Crystal
of dimensions 0.84Â0.44Â0.10 mm were used for the measurements. Cell parameters were
determined using 48 re¯ections in the range 4.1 < Â < 21.5ꢀ. Intensity measurements were carried out
in the range 2 ꢂ Â ꢂ 25ꢀ with the !/2Â scan mode. The ranges of indices were 0 ꢂ h ꢂ 6, 0 ꢂ k ꢂ 11,
ꢁ25 ꢂ l ꢂ 24. Three standared re¯ections were monitored every 197 re¯ections; no decay in
intensities was detected. Lorentz and polarization, but no absorption corrections were applied. The
number of re¯ections was 2223, and the number of symmetry independent re¯ections 2003. The
structure was solved by direct methods using the program SIR92 [18] and re®ned against F2 by full-
matrix least squares using SHELXL-93 [19] with all hydrogens except those of enamine and
hydroxyl group placed geometrically using the AFIX routine of the program. In the course of the
re®nement, their coordinates were kept riding on the atoms to which the hydrogens were attached.
The temperature factors of the hydrogen atoms were set to 1.2 times U(eq) of the respective heavy
atom. Hydrogens of the enamine and hydroxyl group were localized on the difference Fourier
map and re®ned. The non-hydrogen atoms were re®ned anisotropically. The re®nement of 235
parameters converged at R ˆ 0.0303 and wR ˆ 0.0674 for 1447 re¯ections with I > 2ꢆ(I) and
2
2
2
R ˆ 0.0482 and wR ˆ 0.0723 for all data. w ˆ (ꢆ2(Fo )‡(0.0377p)2)ꢁ1 ( p ˆ (Fo ‡2Fc )/3); extinction
Ê ꢁ3
coef®cient ˆ 0.0061; max. and min. peak on ®nal difference Fourier map: 0.104 and ꢁ0.101 eA
.
Additional material to the structure determination may be ordered from Fachinformationszentrum
Karlsruhe, Gesellschaft fuÈr wissenschaftlich-technische Information mbH, D-76344 Eggenstein-
Leopolshafen, Federal Republic of Germany, referring to the deposition number CSD-410246 and
the citation of the present paper.
1-Phenyl-3-((ethoxycarbonylmethylene)-imine)-4-(phenyl-(hydroxy)-methylidene)-
pyrrolidine-2,5-dione (6:C21H18N2O5)
Yield: 53%; m.p.: 175ꢀC; IR (HCBN): v ˆ 3460 (OH), 3251 (NH), 1763 (CO), 1743 (CO), 1709
(CO), 1640 (C=N), 1612 (C=C) cmꢁ1; 1H NMR (DMSO-d6, ꢄ, 500.13 MHz): 0.92 (t, 3H, CH3), 1.21
(t, 3H, CH3), 3.8 (q, 2H, CH2±O), 4.16 (q, 2H, CH2±O),4.36 (s, 2H, CH2±N), 4.68 (d, J ˆ 6.9 Hz, 2H,
CH2±N), 7.31±7.75 (m, 10H, arom), 9.24 (s, 1H, OH), 10.56 (d, J ˆ 6.9 Hz, 1H, NH) ppm; 13C NMR

Derivatives of 1-Phenyl-4-(phenylhydroxymethylidene)-pyrrolidine-2,3,5-trione
561
(DMSO-d6, ꢄ, 125.77 MHz): 13.59 (CH3), 14.02 (CH3), 45.58 (CH2±CO), 45.6 (CH2±CO), 60.84
(CH2±O), 61.14 (CH2±O), 126.92, 127.19, 127.46, 127.72, 127.79, 127.87, 128.46, 128.67, 129.40,
‡
‡
131.30, 131.34, 131.52, 132.46 (C-arom) ppm; MS: m/z (%) ˆ 378 (37.4) M , 350 (12.5) [M±CO] ,
‡
‡
‡
332 (12.5) [M±CH3±CH2O] , 304 (87.5) [M±COOCH2CH3±H] , 105 (23.2) [C6H5CO] , 77 (100)
‡
C6H5 .
General procedure for the preparation of 5 and 7
To a solution of 2.9 g of 1 (0.01 mol) in toluene, 0.012 mol of 2-hydroxyethanolamine were added.
The solution was re¯uxed for 5 h and left overnight. The excess of toluene was evaporated, and the
products were ®ltered and washed with boiling ethanol. The ®ltrate was left for crystallization
(compound 7). The precipitate (compound 5) was ®ltered off and crystallized from ethanol.
1-Phenyl-3-(2-hydroxyethylimino)-4-(phenyl(hydroxy)-methylidene)-pyrrolidine-2,5-dione
(7:C19H16N2O4)
Yield: 14%; m.p.: 125ꢀC; IR (HCBN): v ˆ 3387 (broad, OH), 3250 (broad, OH), 3230 (NH), 1764
(CO), 1709 (CO), 1638(C=N) cmꢁ1; 1H NMR (CDCl3, ꢄ, 500.13 MHz) 2.33 (s, 1H, OH), 3.78 (t, 2H,
CH2±N), 4.23 (q, 2H, CH2±O) 7.18±7.81 (m. 10H, arom), 10.76 (s, 1H, OH) ppm; 13C NMR
(DMSO-d6, ꢄ, 125.77 MHz): 45.24 (CH2±N), 61.48 (CH2±OH), 126.64, 127.62, 128.11, 128.72,
‡
128.96, 131.03, 131.76, 138.46 (C-arom) ppm; MS: m/z (%) ˆ 336.1 (31.8) M , 318 (54.7)
‡
‡
‡
[M±H2O] , 317(56) [M±H2O±H] , 105 (100) [C6H5CO] .
General procedure for the preparation of 8 and 9
To a solution of 0.01 mol of 2 (3) or 6 in ethanol, 0.012 mol of benzylamine were added. The solution
was re¯uxed for 2.5 h and left overnight. The precipitate was ®ltered off and crystallized from
ethanol.
((N-Phenyl-N0-benzyl)-3-(phenylhydroxymethylidene)-2-(iminobenzyl))-succinic diamide
(8; C31H27N3O3)
Yield: 31%; m.p.: 140ꢀC; IR(HCBN): v ˆ 3381 (OH), 3280 (NH), 3272 (NH), 1671 (CO), 1665
(CO), 1644 (C=N), 1611 (C=C) cmꢁ1; 1H NMR (DMSO-d6, ꢄ, 500.13 MHz): 4.05 (d, 2H, CH2), 4.29
(d, 2H, CH2), 5.08 (m, 2H, 2NH), 6.98±7.53 (m, 20H, arom), 7.93 (s, 1H, NH), 8.13 (s, 1H, OH)
ppm; 13C NMR (DMSO-d6, ꢄ, 125.77 MHz): 42.36 (CH2±Ph), 45.23 (CH2±Ph), 107, 118.64, 123.2,
125.69, 126.61, 126.97, 127.35, 127.79, 128.35, 128.55, 128.88, 137.24, 139.35, 140.36 (C-arom)
‡
‡
‡
ppm; MS: m/z(%) ˆ 471 [M±H2O] , 364 (34) [M±H2O±C6H5CH2NH2] , 105 (95) C6H5CO , 77
‡
(100) C6H5 .
1-Phenyl-4-(phenyl-(benzylamine)-methylidene)-pyrrolidine-2,3,5-trione (9; C24H18N2O3)
Yield: from 2, 28%; from 3, 70%; m.p.: 195ꢀC; IR (HCBN): v ˆ 3470 (v. weak, OH), 3282 (NH),
1
1763 (CO), 1720 (CO), 1658 (CO) cmꢁ1; H NMR (DMSO-d6, ꢄ, 500.13 MHz): 4.36 (s, 2H, CH2),
7.18±7.89 (m, 15H, arom), 10.5 (s, 1H, NH) ppm; 13C NMR (DMSO-d6, ꢄ, 125.77 MHz):
47.68(CH2Ph), 126.97, 127.60, 127.74, 128.28, 128.49, 128.63, 128.69, 130.42, 130.66, 131.71,
‡
‡
136.72, 138.11, (C-arom) ppm; MS: m/z(%) ˆ 382 (36.2) M , 354 (20.6) [M±CO] , 77 (100)
‡
C6H5 .

562
K. Ostrowska et al.: Derivatives of 1-Phenyl-4-(phenylhydroxymethylidene)
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È
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Received July 14, 1998. Accepted (revised) October 12, 1998.