Synthesis of novel 1-[(1-ethoxymethylene)amino]imidazol-5(4H)-ones and 1,2,4-triazin-6(5H)-ones from optically active α-aminocarboxylic acid hydrazides

Article abstract of DOI:10.1016/j.tetlet.2013.06.052

New derivatives of 1-[(1-ethoxymethylene)amino]imidazol-5(4H)-one and 1,2,4-triazin-6(5H)-one were synthesized via reactions of optically active α-aminocarboxylic acid hydrazides and triethyl orthoesters in xylene. The factors influencing the formation of the unexpected five-membered products and attempts to elucidate the mechanism are discussed.

Full text of DOI:10.1016/j.tetlet.2013.06.052

Tetrahedron Letters 54 (2013) 4637–4640
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of novel 1-[(1-ethoxymethylene)amino]imidazol-5(4H)-
ones and 1,2,4-triazin-6(5H)-ones from optically active
a-aminocarboxylic acid hydrazides
⇑
´
Agnieszka Kudelko , Wojciech Zielinski, Karolina Jasiak
Department of Chemical Organic Technology and Petrochemistry, The Silesian University of Technology, Krzywoustego 4, PL-44100 Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
New derivatives of 1-[(1-ethoxymethylene)amino]imidazol-5(4H)-one and 1,2,4-triazin-6(5H)-one were
Received 22 April 2013
Revised 27 May 2013
Accepted 12 June 2013
Available online 19 June 2013
synthesized via reactions of optically active a-aminocarboxylic acid hydrazides and triethyl orthoesters
in xylene. The factors influencing the formation of the unexpected five-membered products and attempts
to elucidate the mechanism are discussed.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Heterocycles
a-Aminocarboxylic acid hydrazide
Cyclization
Racemization
Heterocyclic compounds containing five-membered azole or
pared from acid hydrazides,19 amides,20 iminoesters,21,22 or from
azirines.23
six-membered azine scaffolds have attracted the attention of many
scientists due to their broad spectrum of biological activity. In
addition, these scaffolds find various industrial applications. One
subgroup of the azole family is imidazole and its derivatives,1 with
many of these attracting interest in medicinal chemistry mainly for
their antifungal, antiprotozoal and antihypertensive activities.2
They have also been utilized as corrosion inhibitors, ionic liquids,
and for the production of thermally stable polymers.3 The first
imidazole synthesis was successfully performed in 1858 by De-
bus.4 The three-component reaction involving dicarbonyl com-
pounds, aldehydes, ammonia, or its salts is still employed for the
formation of this particular group of heterocycles.5,6 The most pop-
Our earlier research on the application of free a-aminocarbox-
ylic acid hydrazides and triethyl orthoesters demonstrated the pos-
sibility of the synthesis of both six-membered 1,2,4-triazines and
five-membered 1,3,4-oxadiazoles.24 We found that the product
formed depended strongly on the structure of the starting hydra-
zide, in particular on the electronic nature and steric hindrance
of substituents adjacent to the
a carbon. In cases where the reac-
tions were conducted with the use of equimolar amounts of tri-
ethyl orthobenzoate and hydrazides containing electron-
withdrawing groups (EWG) or bulky substituents at the
a position,
we observed the formation of mainly 1,3,4-oxadiazoles (1) pos-
ular methods for the preparation of imidazoles make use of
a
-halo-
sessing a free aminomethyl group at position 2 (Scheme 1).
carbonyl compounds and guanidines,7 1,2-diaminoalkanes, and
carboxylic acids8 or comprise transformations of other heterocy-
cles such as azirines,9 4-aminoisoxazoles,10 1,2,4-oxadiazoles,11
and pyrazines.12
N
NH2
N
C6H5
C6H5C(OC2H5)3
C6H5C(OC2H5)3
O
(R)H
CONHNH2
(R)H
1
1,2,4-Triazine and its derivatives constitute another interesting
azine subgroup.13 These are applied in medicine as potential anti-
bacterial and antifungal agents, in the agrochemical industry as
plant protecting materials, and as components of commercial
dyes.14–18 A wide range of synthetic procedures have been re-
ported for 1,2,4-triazin-6-one derivatives. They are commonly pre-
EWG
EWG NH2
H
N
NH2
CONHNH2
EDG
H
C6H5
H
EDG
N
2
O
N
H
Scheme 1. General relationship between the structures of the
acid hydrazides and the products resulting from the reaction with equimolar
amounts of triethyl orthobenzoate.
a-aminocarboxylic
⇑
Corresponding author. Tel.: +48 32 2371729; fax: +48 32 2371021.
E-mail address: Agnieszka.Kudelko@polsl.pl (A. Kudelko).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.052

4638
A. Kudelko et al. / Tetrahedron Letters 54 (2013) 4637–4640
Table 1
Optimization of the reaction conditions for the preparation of 1-[(1-ethoxyethyl-
ene)amino]-2-methyl-4-(4-hydroxybenzyl)imidazol-5(4H)-one (7a)
O
O
N
N
HO
Entry
Molar ratio
Solvent 7a
Yielda (%)
2a
H
N
Hydrazide 5a CH3C(OC2H5)3 p-TsOH
Yielda (%)
1
2
3
4
5
1
1
1
1
1
5
5
3
3
1
—
0.07
0.07
0.07
0.07
—
—
48
68
0
0
10
7
Figure 1. The structure of 1-[(1-ethoxypropylene)amino]-2-ethyl-4-(4-hydroxy-
benzyl)imidazol-5(4H)-one.
CH3CN 55
Xylene 75
Xylene
5
66
a
Yield with respect to the starting hydrazide 5a.
Derivatives of 1,2,4-triazine (2) were obtained in satisfactory
yields from the reactions of hydrazides bearing an electron-donat-
ing group (EDG) at the
a position (Scheme 1). These substituents
and in the presence of a catalytic amount of p-toluenesulfonic acid
(p-TsOH) (Table 1, entry 4). Decreasing the molar ratio of hydrazide
5a and CH3C(OC2H5)3 to an equimolar amount changed the reac-
tion course and resulted in the formation of the six-membered
1,2,4-triazin-6(5H)-one 2 (method B) (Table 1, entry 5).
The yields of the reactions leading to the five-membered imida-
zol-5(4H)-ones 7a–i were high and varied from 51% to 78% (Ta-
ble 2). It was found that compounds possessing alkyl groups
(R2 = CH3, C2H5) at position 2 were formed more easily than their
counterparts substituted with a phenyl group. In addition, the ste-
ric hindrance caused by the R1 substituent in the starting hydra-
zide also played an important role. The yields of imidazol-5(4H)-
increase both the nucleophilicity and basicity of the amino group.
In this way, the amino center responsible for the formation of the
six-membered ring becomes more reactive than the competing
carbonyl. Heating
L
-(ꢀ)-tyrosine hydrazide in the presence of an
excess of the orthoester, we obtained another five-membered het-
erocycle identified as 1-[(1-ethoxypropylene)amino]-2-ethyl-4-(4-
hydroxybenzyl)imidazol-5(4H)-one (Fig. 1).25
Inspired by this unexpected result, we decided to investigate
the reactions of selected
der to obtain this interesting class of heterocycles. The starting
hydrazides were obtained from commercially available -amino-
carboxylic acids according to well known procedures. Thus,
-(+)-isoleucine, and
a-aminocarboxylic acid hydrazides in or-
a
L
-
-
ones 7g–i obtained from
L-(+)-isoleucine hydrazide (5c) were
(ꢀ)-tyrosine,
L-(ꢀ)-phenylalanine,
L
D
-(ꢀ)-
a
relatively lower than those prepared from the hydrazides of
phenylglycine (3a–d) were transformed into the corresponding
methyl ester hydrochlorides 4a–d with methanol and thionyl chlo-
ride. The resulting compounds 4a–d were then converted into the
final hydrazides 5a–d by reaction with hydrazine hydrate under
mild conditions (Scheme 2).
L
-(ꢀ)-tyrosine (5a) and
L
-(ꢀ)-phenylalanine (5b). The six-
membered 1,2,4-triazin-6(5H)-ones 2a–i, produced in the reac-
tions with equimolar amounts of orthoester, were also obtained
in high yields of 56–80% (Table 2). The structure of the products
both from the family of imidazol-5(4H)-one 7 and 1,2,4-triazin-
6(5H)-one 2 was confirmed by X-ray analysis,25,26 elemental
analysis, and typical spectroscopic methods.
Acid hydrazides 5a–c were heated with excess amounts of tri-
ethyl orthoesters: orthoacetate, orthopropionate, and orthobenzo-
ate (R2 = CH3, C2H5, C6H5, Scheme 3) yielding the derivatives of
imidazol-5(4H)-one 7 (method A). The optimization of the reaction
conditions revealed that the formation of the desired imidazol-
5(4H)-ones 7 proceeded smoothly in a non-polar solvent (xylene)
However, regardless of the presence or absence of a catalyst, we
did not obtain any acyclic intermediates 6 or 8, which accompa-
nied the reactions starting from
aminocarboxylic acids and orthoesters.27,28 Thus, the compounds
possessing a free amino group at the position, were more reac-
tive than their -hydroxy or N-protected -amino counterparts
and underwent cyclization immediately after their formation.
Hydrazides of -aminocarboxylic acids constitute materials pos-
sessing at least two nucleophilic sites susceptible to attack by
orthoesters: the free amino group adjacent to the carbon and
a-hydroxy- and N-protected a-
a
a
a
NH2
NH2*HCl
NH2
ii
i
H
H
H
COOH
3a-d
R1
R1
COOCH3
4a-d
R1
CONHNH2
5a-d
R1
a
a
a
b
c
d
4-HOC6H4CH2
C6H5CH2
C2H5(CH3)HC
C6H5
4a
5a
(95%)
(85%)
the hydrazine group. Generally, the amino group is more reactive
than the hydrazine that neighbors the carbonyl functionality. The
4b (90%)
4c (93%)
5b (80%)
5c (78%)
reaction of
D
-(ꢀ)-
a-phenylglycine methyl ester (9) with triethyl
4d
5d
(80%)
(95%)
orthoacetate leading to N-(1-ethoxyethylene) derivative (10) pro-
ceeded smoothly with heating for about 30 min, while the trans-
formation of phenylacetic acid hydrazide 5e into the
Scheme 2. Synthesis of the starting a-aminocarboxylic acid hydrazides 5. Reagents
and conditions: (i) MeOH, SOCl2, 0 °C, 6 h; (ii) N2H4ꢁH2O, MeOH, rt, 24 h.
R2
N
R2
C
O
O
R1
method A
O
N
H
N
O
C
N
H
C
O
N
R2
H
N
N
R1
NH2
R2C(OC2H5)3
R2
6
7
H
CONHNH2
R1
R2
C
H
N
5
R1
H
R2
method B
N
O
R1 = 4-HOC6H4CH2, C6H5CH2, C2H5(CH3)HC
R2 = CH3, C2H5, C6H5
H
O
N
H
CONHNH2
R1
2
8
Scheme 3. Reactions of
a-aminocarboxylic acid hydrazides 5 with triethyl orthoesters: method A – conducted with excess orthoester (3 equiv), xylene, p-TsOH (0.07 equiv),
reflux 2–20 h; method B—conducted with equimolar amounts of orthoester (1 equiv), xylene, p-TsOH (0.07 equiv), reflux 2–6 h.

A. Kudelko et al. / Tetrahedron Letters 54 (2013) 4637–4640
4639
Table 2
Imidazol-5(4H)-ones 7 and 1,2,4-triazin-6(5H)-ones 2 obtained from the reactions of
a
-aminocarboxylic acid hydrazides 5 with triethyl orthoesters29
Entry
R1
R2
Product 7
Product 2
method A
method B
Time (h)
Yielda (%)
Mp (°C)
Time (h)
Yielda (%)
Mp (°C)
a
b
c
d
e
f
g
h
i
4ꢀHOC6H4CH2
4ꢀHOC6H4CH2
4ꢀHOC6H4CH2
C6H5CH2
C6H5CH2
C6H5CH2
C2H5(CH3)HC
C2H5(CH3)HC
C2H5(CH3)HC
CH3
20
20
8
4
4
2
8
6
2
75
78
51
78
74
62
59
65
57
164–16524
173–175
79–81
—
—
95–96
—
—
—
6
6
3
4
4
2
6
6
4
66
74
68
80
78
56
70
66
63
58-60
63–65
C2H5
C6H5
CH3
C2H5
C6H5
CH3
106–10723
118–120
176–178
208–210
142–144
81–83
C2H5
C6H5
158–161
a
Yield with respect to the starting hydrazide 5.
adjacent to the
a carbon in hydrazides 5a–c attacked the electro-
NH2
C
philic carbon atom of the orthoester yielding iminoether
8
N
O
i
+
CH3C(OC2H5)3
H
(Scheme 3). When the transformation was conducted with equi-
molar amounts of reagents, the resulting iminoether 8 underwent
further cyclization to form 1,2,4-triazin-6(5H)-one derivatives 2.
However, in excess orthoester, the less reactive hydrazine group
of intermediate 8 reacted with another molecule of the orthoester
giving the appropriate diiminoether 6. This acyclic compound cy-
clized to form imidazol-5(4H)-ones 7 (Scheme 3) in the presence
COOCH3
C6H5
H
COOCH3
C6H5
10
9
(95%)
H
ii
N
C
O
C6H5
CONHNH2 + CH3C(OC2H5)3
C6H5
N
C
O
11
5e
(76%)
of a catalytic amount of p-TsOH. The bulky phenyl group at the
position of the starting -phenylglycine hydrazide (5d) pre-
-(ꢀ)-
vented the reaction with sterically crowded triethyl orthoester at
the reactive -amino site. Thus, the hydrazine group attacked the
a
Scheme 4. Synthesis of N-(1-ethoxyethylene)-a-phenylglycine methyl ester (10)
D
a
and N0-(1-ethoxyethylene)-phenylacetic acid hydrazide (11) in the reaction with
triethyl orthoacetate. Reagents and conditions: (i) CH3C(OC2H5)3, xylene, 30 min,
reflux; (ii) CH3C(OC2H5)3, xylene, reflux, 6 h.
a
electrophilic carbon atom of the orthobenzoate to form the inter-
mediate 12 (Scheme 5). This underwent cyclization in the presence
of the catalyst yielding both 1,3,4-oxadiazole 13 and 1,2,4-triazine
2.
The optical rotations of imidazol-5(4H)-ones 7 (method A) and
1,2,4-triazin-6(5H)-ones 2 (method B), derived from optically ac-
corresponding N0-(1-ethoxyethylene) hydrazide (11) required
heating for a few hours with the orthoester (Scheme 4). Such
behavior is probably caused by the difference in basicity of the
two nucleophilic sites. The pKa ionization constants of
phenylglycine hydrazide (5d) determined by the potentiometric
D
-(ꢀ)-
a-
tive hydrazides of:
nine (5b,
L
-(+)-tyrosine (5a, ½a D20
ꢂ
+80.5),
-(+)-isoleucine (5c,
L
-(+)-phenylala-
+27.8),
½
a 2D0
ꢂ
+33.0), and
L
½ ꢂ
a 2D0
method in aqueous-methanol solution being equal to:
showed that in the reactions with triethyl orthoacetate, orthopro-
pionate, and orthobenzoate, racemization at the asymmetric car-
bon occurred although this atom was not directly involved in the
formation of the products. We believe that this phenomenon can
be explained by tautomerism, where the mobile hydrogen atom
pKa1 = 6.53 0.08 for the amino group adjacent to the
a carbon
and pKa2 = 2.45 0.10 for the terminal hydrazine group, remain
in accordance with observed trends.
Different results were obtained when
D-(ꢀ)-a-phenylglycine
hydrazide (5d) was heated with an equimolar amount of triethyl
orthobenzoate. This time no traces of the imidazol-5(4H)-one 7
were found in the post-reaction mixture, but another five-mem-
bered product: 2-(1-amino-1-phenylmethyl)-5-phenyl-1,3,4-oxa-
diazole (13) (Scheme 5).24 It was concluded that this outcome
was caused not only by the decrease in the nucleophilicity of the
amino group, but mainly due to the steric hindrance of the phenyl
groups both from hydrazide 5d and orthobenzoate. In this way, the
competing hydrazine site reacted readily yielding N0-eth-
oxybenzylidenehydrazide 12, which underwent cyclization with
the formation of two different products: the six-membered 1,2,4-
triazine 2j and the five-membered 1,3,4-oxadiazole 13.
situated on the stereogenic carbon atom at the
a position of the
intermediate iminoethers 6 or 8 could be envisioned to migrate
to the imino or carbonyl groups with a simultaneous shift of the
double bond and loss of the optical activity.
In summary, the hydrazides of free a-aminocarboxylic acids are
valuable starting materials in the direct synthesis of five-mem-
bered imidazol-5(4H)-ones, 1,3,4-oxadiazoles, and six-membered
1,2,4-triazin-6(5H)-ones. The electronic effects of the substituents
adjacent to the
a carbon in the starting hydrazide, the steric hin-
drance of both the orthoester and hydrazide, and finally the molar
ratio of the reagents were the most important factors impacting on
the reaction course and the products. Electron-donating substitu-
ents and stoichiometric amounts of orthoester promoted the for-
mation of 1,2,4-triazin-6(5H)-ones, while the use of excess
Analyzing the mechanism of the reaction between orthoesters
and
a-aminocarboxylic acid hydrazides, possessing a methylene
linker at the
a
position (5a–c), we assumed that the amino group
H
N
C6H5
H
NH2
C6H5
NH2
N
NH2
C
H
N
O
C
C6H5C(OC2H5)3
H
N
H
+
N
CONHNH2
C6H5
H
C6H5
O
N
H
O
N
C6H5
C6H5
5d
C6H5
O
12
13
2j (52%)
(45%)
Scheme 5. Formation of 1,2,4-triazin-6(5H)-one 2j and 2-aminomethyl-1,3,4-oxadiazole 13 from D-(ꢀ)-
a-phenylglycine hydrazide (5d) and triethyl orthobenzoate.

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A. Kudelko et al. / Tetrahedron Letters 54 (2013) 4637–4640
12. Watanabe, T.; Nishiyama, J.; Hirate, R.; Uehara, K.; Inoue, M.; Matsumoto, K.;
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drawing and bulky groups favoured the reaction at the opposite
hydrazine group. In this case, the transformations were carried
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the formation of derivatives of 1,3,4-oxadiazole.
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Supplementary data (experimental procedures and character-
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can be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2013.06.052.
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29. Representative procedure—(method A): L-(ꢀ)-Phenylalanine hydrazide (5b,
1.79 g, 10 mmol) was added to a mixture of triethyl orthobenzoate (6.87 g,
30 mmol, 7.0 mL), dry xylene (40 mL) and p-TsOH (0.12 g) and heated at reflux
temperature for 2 h (TLC). After cooling, the mixture was washed with H2O
(30 mL), dried over MgSO4 and then concentrated under reduced pressure. The
crude solid was subjected to column chromatography (silica gel, eluent:
benzene/AcOEt,
1:3 v/v)
to
give
2.46 g
(62%)
of
1-[(1-
ethoxybenzylidene)amino]-2-phenyl-4-benzylimidazol-5(4H)-one (7f) as
yellow crystals with a melting point of 95–96 °C; Rf (benzene/AcOEt, 1:3 v/v)
0.62; [Found: C, 75.52; H, 5.87; N, 10.62. C25H23N3O2 requires C, 75.54; H, 5.83;
N, 10.57%]. 1H NMR (400 MHz, DMSO-d6):
d 1.32 (3H, t, J 6.8 Hz,
Ph(OCH2CH3)C@N–N1), 2.61 (1H, dd, J 4.0 and 13.6 Hz, CH-C4), 2.92 (1H, dd,
J 4.0 and 13.6 Hz, CH-C4), 4.30 (1H, t, J 4.0 Hz, H-C4), 4.37 (2H, q, J 6.8 Hz,
Ph(OCH2CH3)C@N–N1), 7.17–7.25 (7H, m, HAr), 7.40–7.51 (6H, m, HAr), 7.82
(2H, d, J 8.0 Hz, HAr). 13C NMR (DMSO-d6): d 13.9, 36.7, 64.4, 66.4, 126.3, 126.8,
127.9, 128.1, 128.2, 128.4, 128.5, 128.7, 129.3, 130.3, 130.8, 131.3, 159.8, 170.2,
176.2. UV–vis: kmax (MeOH) 201.0 nm (
IR (ATR)
1075, 936, 815, 779, 736, 711, 694 cmꢀ1
e
ꢁ10ꢀ3 14.42 cmꢀ1 LMꢀ1), 242.0 (8.58);
10. Reiter, L. A. J. Org. Chem. 1987, 52, 2714.
11. Milcent, R.; Barbier, G. J. Heterocycl. Chem. 1992, 29, 1081.
m
: 3058, 1731, 1611, 1595, 1514, 1446, 1370, 1323, 1263, 1227, 1122,
.